Journal o, Armosphsrr~ and T~vrr,rrral Printed m Great Britain.
$h.OO+ .oO Press Ltd
The aurora1 distribution and its mapping according to suhstorm phase R. D. ELPHINSTONE,*J. S. MURPHREE,* D. J. HEARN,* W. HEIKK1LA.t M. G. HENDERSON,* L. L. COGGER*
*Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada ; i University of Texas at Dallas. Richardson, Texas, U.S.A. ; $Swedish Institute of Space Physics. Kiruna, Sweden (Receirrrl
in final form
17 March I992 ; uc~cc~pted I I Murch 1992)
Abstract-An attempt is made to reconcile two competing views as to where the aurora1 distribution maps from in the magnetosphere. The structure of the aurora is shown to have two distinctive parts which vary according to the magnetic activity. The low latitude portion of the structured distribution may be a nearEarth central plasma sheet phenomenon while the high latitude portion is linked more closely to boundary layer processes. During quiet times, the polar arcs may be the ionospheric signature of a source region in the deep tail low latitude boundary layer/cool plasma sheet. The structured portion of the ‘oval’ has a dominantly near-Earth nightside source and corresponds to an overlap region between isotropic I-10 keV electrons and 0. I-1 keV structured electrons. The ionospheric local time sector between 13 and I8 MLT is the meeting point between the dayside boundary layer source region and this near-Earth nightside source. Late in the substorm expansion phase and/or start of the substorm recovery phase, the nightside magnetospheric boundaries (both the low latitude and Plasma Sheet Boundary Layers) begin to play an increasingly important role, resulting in an aurora1 distribution specific to the substorm recovery phase. These aurora1 observations provide a means of inferring important information concerning magnetospheric topology.
times emphasized the mapping of the discrete aurora to the more distant boundary layers. This is scheIt seems that if one reads aurora] literature from the matically shown in Fig. l(A) and (B). Figure I (A) time period previous to the mid-l970s, the reader illustrates an idealized aurora1 intensity profile (in the will be struck by the prevailing view that the discrete ultraviolet) as a function of latitude. (The vertical and aurora1 oval is a manifestation of processes orighorizontal scales are arbitrary.) Under quiet coninating in the central plasma sheet. For example, FAIR- ditions the structured aurural zone at midnight is narFIELD(1968) mapped the Feldstein nightside oval to row whereas at dusk/dawn it is quite broad. The the near-Earth nightside region well inside the openreverse is generally true at active times. closed field line boundary (at midnight the 90% occurThe traditional aurora1 oval shown in Fig. 1, conrence region was between xGSM = -7 and - I6 &). sisting of structured auroras, has sometimes been The discrete aurora associated with substorm onset associated with the Plasma Sheet Boundary Layer was thought to originate near the inner edge of the [Fig. l(B)]. This way of thinking about the magcentral plasma sheet as can be seen in various schenetospheric projections might be summarized by the matics of the substorm process (e.g. UNTI and ATKINfollowing quotation from ROBINSONand VONDRAK SON, 1968 ; KROPOTKIN, 1972 ; DRIATSKY and SHUMI(1990) : ‘An aurora] substorm begins with the tranLOV, 1972; HOFFMANNand BURCH, 1973). KROPOTKIN sient brightening of the most equatorward arc, i.e., (1972), referring to the work of FELDSTEINand STARthe arc that is closest to the central plasma sheet KOV (1970), takes as relatively well established that precipitation region’. This is a randomly chosen exam‘the aurora1 disturbances, fundamental in substorm ple illustrating a common current concept of associprocesses, have been shown to take place on closed ating the structured aurora solely with boundary layer field lines stretching into the magnetotail plasma sheet processes. This change of thought might in part be but adjacent to its inner edge and not on those field traced to the mid-1970s when WINNINGHAM et (11. lines stretching to the tail neutral line.. .‘. Models (1975) wrote an excellent article showing two disfor aurora1 precipitation from this inner edge were tinctive types of particle precipitation and how they developed (KENNEL, 1969) and substorm theories changed as a function of substorm phase. The names built based on this assumed mapping or association chosen for these were BPS (boundary plasma sheet) (PARKS et al., 1972). and CPS (central plasma sheet). These authors made In contrast, somewhat more recent work has someno explicit attempt to project these low altitude sig1. INTRODUCTION
A. IONOSPHERIC OBSERVATIONS I
Tradiiionai AuroraI Oval
Aurora1 intensity Peak in UV Oval
2nd UV Peak
Latitude Quiet time: Scale compressed near midnight expanded towards dusk I dawn. Recovery Phase: Scale expanded near midnight compressed towards dusk /dawn.
B. ORIGINAL VIEW
C. ALTERNATIVE VIEW
inner edge CPS
Dusk I Dawn: COOL CPS I LLBC
Dusk / Dawn: LLEL
Midnight: COOL CPS
HOT CORE CPS ~~~~~o~~n
density fluctuations cold 0’ tonics and beams
t -10 keV isotropic electrons
E .f-1 keV kT -200400
Lobe and I or MA>1
kT -5 eV O*f.a beams Midnight: ion beams a10 keV BEN
Fig, 1. Schematic of the mapping between aurora1 regions and their magnetospheric sources. (A) Amoral observations; (B) one view of the mappmg; (C) view adopted in this paper (see also Table 1).
natures into the magnetosphere (although certain regions do appear to have been implied from the names which were chosen). Other researchers did, however, explicitly link the discrete auroras to the
layers (e.g. ROSTOKER and EASTMAN, 1987; an association between the discrete aurora and isotropic ion pr~ipitation. They then associated this with
EASTMAN et al., 1988). LYONS et al. (1988) found
Mapping the auroraf distribution regions in the tail where the guiding center approximation for ion motion was violated. FELD~TEINand GALPERIN(1985) gave considerable evidence that an actual identification of these law altitude signatures (BPS and CPS) with the corresponding magnetospheric domains (Plasma Sheet Boundary Layer and central plasma sheet) may not be correct. After this paper, one begins to commonly see the old substorm schematics again where breakup aurora1 arcs and the westward traveiling surge are associated with the inner edge of the central plasma sheet (e.g. LUI, 199t ; LOPEZ 62 d,,
In this paper, an attempt is made to reconcife the two apparently confiicting views. During the late expansion and recovery phases of a substorm, boundary layer processes may be very active (at least at the inner edge of the boundary layer), thereby giving rise to observations supporting the westward travelling surge being near the boundary of closed field lines (BYTHROWand POTEMRA,1987 ; LYONS, 1991; LYONS atal.,1990).In this interpretation then, the phase of the substorm (and in fact the definition of a substorm) becomes very important and, because of this, there is a need to describe the aurora1 substorm on a global scaie rather than focussing on locat surge-like forms embedded in the larger substorm morphology. Ln Section 2, particle definitions given by SANDAH~ and LINDQVIST(1990) are compared to Viking UV aurora1 images. Working definitions are given for the terms structured aurora, the amoral oval, and diffuse aurora. Tentative relationships between magnetospheric and ionospheric signatures are discussed (e.g. Fig. I and Table I). In Section 3 the mapping of the quiet time am-oral distribution is investigated. A summary ofthe results is given on the left side of Fig. 2. (This iIh.tstrates the compIete local time dependence of the proposed mapping.) It is shown that the crosstaii current may play a fundamental role in determining the main ‘oval’ of structured amoral precipitation. This oval corresponds to the low latitude ‘peak in the UV oval’ shown in Fig. l(A). Poleward of this region there is frequently found diffuse aurora, in which polar arcs are often embedded (e.g. the arcs in Fig. 2 which point towards 12 MLT). In the high latitude dawn and dusk regions this precipitation is attributed to either the low latitude boundary layers (LLBL) or to the cooler outer edge of the central plasma sheet. Section 4 then outlines some of the arguments concerning mapping from the innosphere to the magnetosphere (or vice versa) and why the structured UV ‘oval’ might be considered as a near-Earth phenomenon, The arguments in the above sections are supported in Sections 5 and 6 with observations
of the aurora1 distribution
recovery phase. In these sections two ‘ovals’ are described. The one at high latitudes (a more distant source regian) shown in Fig. i(A) as the ‘2nd UV peak’ develops poleward of the preexisting low latitude system providing direct evidence for the mapping proposed in the previous sections. Substorm onsets and omega bands, seen at the poleward region of the equatorward system, indicate a linkage of discrete aurora to a region well within the closed field line region. The right side of Fig. 2 summarizes the proposed mapping under active times. The higher latitude aurora, being far poleward of the other system, is often not observable by singte ground-based observers. This can sometimes lead to the mistaken interpretation that features such as the omega bands are at the most poleward edge ofthe aurora. In Section 7, amoral observations are presented which help resolve how the boundary regions from the dayside and nightside systems connect (e.g. the 15MLT region on the right side of Fig. 2) and their importance in understanding both dayside morphology and the electrodynamics OFthis portion of the ionosphere. It may be that different researchers studying different aurora1 morphology reach contradictory conclusions based on both incomplete observations and semantic difhcutties regarding what they are in fact observing.
Perhaps one of the most confusing aspects of auroral morphology and hence its interpretation are the definitions used for discrete and diffuse aurora, the aurora) oval, and structured aurora. Traditionally, the structured aurora1 oval consists of visual, aurora1 arcs (containing structure) as observed from the ground [see Fig. I(A)]. However, excluded from this are the polar arc phenomena. From a satellite imaging viewpoint it is difficult to distinguish the small scale structures used in the above definition. Also, any optical technique cannot definitively determine if acceleration has taken place along the field hne. Thus, structured aurora as viewed by optical means above should not necessarily be equated with acceleration processes. The following shall be used in this paper as working definitions of various aurora1 terms. Discrete aurora : the UV aurora which by its association with substorm expansion phase can be reasonably linked to acceleration processes along the magnetic field line. S~~~~tu~~~UUYUM:the UV aurora which consists of arc-like systems. This is a ~on~tudjna~ly elongated region of enhanced emission with very steep latitudinal gradient. In most, but not all, fine scale struc-
R. D. ELPHINSTONE er al.
Table 1. Possible associations Aurora1 ~_
Diffuse most equatorward aurora
Low altitude signature Trupprd electron distributions* Isotropic ions > 10 keV
High altitude Ring current
Main UV aurora1
Structured aurora poleward of main UV ‘oval’ and polar arcs
Midnight : most poleward diffuse aurora dawn/dusk polar arc region
Isofropic: I-10 keV electrons* n > I cm13 Structurrd: 0.1--I keV electrons kT, c 50 eV Maxwellian, SO-300 eV accelerated n z 0.4-5 cm- 3 In poleward half: inverted Vs (peak E > I keV) outflowing ions (> 100 eV)
Structured electrons 0.1 I keV* kT,, < 50 eV Maxwellian kT,, 5C300 eV accelerated )I z 0.4 ~5 cm ’ Inverted Vs (peak > I keV) Outflowing ions (3 100 eV)
Midnighrt : high latitude ion dispersion signature (> IO keV) Convection reversal Counter streaming electrons (E < 100eV) Downward field-aligned current D~~~n/Du.~k*: kT, i 50 eV Maxwcll~an n * 0.45 cm-’
Region of high cross-tail volume current density: Hot core of central plasma sheet9 Isotropic fluxes Supressed wave activity Hot oxygen fluxes (z I keV) kT, 3/ I keV nzlcm--’
Tailward of _ro,, z - 15REX Midnight/j : Plasma Sheet Weak cross-tail current Boundary Layer Cool portion of central Ion beams (> 10 keV) plasma sheet and LLBL Broadband electrostatic noise Waves and density Possible separatrix signature : fluctuations§ plasma wave burst with Cold O+ conical and fieldfrequency >.& h f, aligned beams (active Duwn/dusk : warm envelope times) of plasma sheet/LLBL in kT,, z 20&500 eV deep tailf Cold 0 + and field-aligned beams kT,.< fOOeV,~=O.l-1 keV
* SANDAHLand LIN~QVIST(1990) ; t YAMAMOTOet al. (1993). ZELENYIet al, (1990) ; 2 ELPHINSTONE ef a/. (199la) ; (iZW~LAKOWSKAand POPIELAWSKA ( 1991) ; //SCHRIVERef al. ( 1990).
tures such as spiral forms are observed on these systems. As it is not possible to definitively say whether or not acceleration along the field line has taken place, we prefer to avoid labelling this type of aurora as discrete. [email protected]
~tirora: the UV aurora which shows no structure and extends over many degrees of latitude. This definition is strongly dependent on image spatial and temporal resolution. A schematic UV latitude profile of aurora1 intensities is represented in Fig. 1(A). Figure 2 shows how such profiles are translated into the amoral morphology for quiet and active time periods. At the lowest latitudes is a region of diffuse very faint luminosity. Poleward of this, the main UV amoral oval forms a familiar ring surrounding the magnetic pole. This ring is a mixture of structured and diffuse amoral emissions. At higher latitudes the morphology is governed by the local time and the level of magnetic activity. At quiet times the dawn and/or dusk sectors
show structured aurora many degrees poleward of the oval shaped region of brighter aurora (top of Fig. 2). At more active times this form of aurora is usually less apparent and the midnight sector shows a much broader region of structured aurora (bottom of Fig. 2). The relationship of these aurora1 regions to Viking particle pr~ipitation regions in the evening sector is shown in Fig. 3. The left panel illustrates a morphology with structured aurora extending to the geomagnetic pole (corrected geomagnetic coordinates are used in this plate). The right panel shows a typical structured distribution consisting dominantly of a circular or oval morphology. The colored points in the figure (near 21 or 22 MLT) illustrate particle precipitation boundaries consistent with those described by SANDAHL and LINDQVIST (1990). The yellow dots in the equatorward region of faint aurora1 emissions correspond to trapped electron distributions between I and IO keV (double empty loss cones). Poleward of
Mapping the aurora1 distribution
Cusp [email protected]
Ffanks of tail/ boundary layer (strongly IMP dependent)
06 Deep tail flanks Interface region between aear Earth and deep tail
-_ Near Earth / cross-tail current (discrete arcs embedded in diffuse emission)
DISTURBED Upward f.a.c. region I
plasma sheet boundary layer
Fig. 2. A schematic showing a possible relation between the ionospheric aurora1 rno~ho~o~ spheric regions for quiet (top) and disturbed (bottom) time periods.
this, up to the white dot in each panel, is the region of the main UV aurora1 oval. In terms of electron precipitation this region is related to an overlap between isotropic I-10 keV electrons (single upgoing empty loss cone) and lower energy electrons which show signs of spatial structure and possible acceleration (SANDAHL and LINDQVIST, 1990). Upflowing ions can be found in this region and inverted Vs are occasionally found here. The start of ion isotropy
begins within this region. Poleward of this oval lies the region of diffuse pr~ipitation where polar arcs are embedded. In both examples shown in Fig. 4, there exist diffuse emissions poleward of the structured oval. In this high latitude region the higher energy isotropic electrons are no longer found, leaving only the lower energy 0.1-l keV structured electron precipitation. This region of structured precipitation is bounded by the pink dots in the panels (labelled by
R. D. ELPHINSTONE et al.
3s in the left panel). When associated with Maxwellian distributions this precipitation has a low temperature (kT, < 50 eV) and when accelerated can range up to about 1 keV (SANDAHL and LINDQVIST, 1990). Using this information as well as recent results by ZW~LAKOWSKA and POPIELAWSKA(1991), FELDSTEIN and GALPERIN (1985) YAMAMOTO et al. (1993), SCHRIVER rt al. (1990) and ZELENYI et al. (1990), Table 1 and Fig. 1 summarize a possible association between the low altitude aurora1 emissions, the low altitude particle precipitation and their high altitude equivalent. This mapping will be supported by results presented in the rest of this paper. It is important to note in the context of Fig. 1 that the projections at higher latitudes are dependent on local time. Thus at the highest latitudes near dawn and dusk the diffuse emissions are linked to the deep tail low latitude boundary layer (LLBL) and the ‘warm envelope of the Plasma Sheet’ described by ZWOLAKOWSKA and POPIELAWSKA (1991). Within this region are higher temperature areas (20&500 eV) associated with polar arc phenomena. In the evening and midnight sectors this region is less evident and instead one has a characteristic signature of the Plasma Sheet Boundary Layer (ZELENYIet al., 1990; SCHRIVER et a/., 1990). The low altitude signature of this can be quite distinctive and is described here as a double oval forming in association with substorm recovery phase. The reader will probably find it useful to refer to Figs 1 and 2 and Table I as an orientation for the rest of the paper.
3.THE AURORAL CONFIGURATION BEFORE THE SUBSTORM PROCESS In the time period prior to the substorm growth phase and generally associated with prolonged periods of interplanetary magnetic field (IMF) B_ zero or northward, the region poleward of the structured auroral oval contains high latitude arc systems embedded in diffuse emissions. This is schematically shown on the left side of Fig. 2. The teardrop shape that results under certain conditions was first observed by the ISIS-II satellite (MURPHREE et al., 1982). This symmetry in the polar arc distribution has been attributed to periods when the IMF pointed dominantly towards the Sun, that is, B, positive, B, zero, B_ zero (ELPHINSTONE et al., 1990). LUNDIN et (11. (1991) have associated these arc systems with the inner edge of the low latitude boundary layer. BIRN etul.(1991) later noted the similarity of this teardrop shape to the configuration of the model ‘open-closed’ field line boundary in the TSYCANENKO(1987) long model. In addition to this teardrop morphology, substantial variations in the shape of this high latitude aurora1 distribution take place. Large scale polar arcs resembling ‘theta’ aurora can propagate back and forth across the ‘polar cap’ in only 15 min (MURPHREE et al., 1989) and arcs appear in different sectors dependent on the interplanetary magnetic field (MURPHREE et (II., 1982; TROSHICHEVet ul., 1988; ELPHINSTONE et d., 1990). That these arcs are likely to lie on closed field lines associated with an expanded oval can be deduced
Fig. 3. A comparison between the Viking UV amoral data and Viking particle observations for events on 18 April 1986 near 20 UT and 2 April 1986 near 20 UT. The images are average configurations during the orbit transformed into a polar corrected geomagnetic coordinate system. The false color scale is shown to the left of the panels (S-255 data numbers is the full scale). The upper right descriptions show the coloring used to represent the particle boundaries. In the left panel numbers (in the appropriate color) have also been placed in the regions of interest. The yellow small circles represent the limits of unstructured trapped electrons (double empty loss cones) between I and 10 keV. The white dots show the poleward boundary of isotropic I-10 keV electrons (upgoing empty loss cone) and the mauve dots delineate the regions of structured 0.1-I keV electron precipitation. Fig. 4. A mapping of the TSYCANENKO (1987) long magnetospheric model (+IGRF 1985) from the minimum B surface (bottom panels) into the ionosphere (top panels) for 18 April 1986 at 06 UT. The right panels show the model volume current density (low to high densities are represented by colors from blue to red) while the left panels are colored to illustrate different magnetospheric regions. The ionospheres are shown in eccentric dipole coordinates (1985) with 12 MLT at the top center, 18 MLT to the middle left. Magnetic latitudes 60,70 and 80” are also shown. The minimum B surfaces have xosM positive toward the top and positive yes, to the left. The small solid white square represents the Earth and each large square is 10 R, in width. The white oval in the ionospheres correlates very well with the peak in UV emissions seen by the Viking spacecraft and represents the peak in volume current density at each local time. The teardrop shaped region resembles polar arc morphologies and maps to the deep nightside flanks of the magnetotail.
the aurora1 distribution
Fig. 5. Two aurora1 images separated by 1 min taken by the Viking imager on 19 October 1986. The false color scale used to show intensity variatinns is shown on the far right. A low intensity threshold enables a faint polar arc to be observed as well as emissions which appear poleward of the substorm onset region at 1132:OlUT. This illustrates that the open field line region may be smaller than commonly believed. Fig. 6. Thr~-dimensional schematics of the quiet magnetotail (left) and the projections (right) into the ionosphere (ionospheric~oordinates are the same as in Fig. 3). The top panels are for a very quiet symmetric magnetotail while the bottom panels show a magnetosphere perturbed by a northward away sector interplanetary field (superposition field of B, = - 1, 3, = + 1, 3: = + 1 nT). While the high latitude boundary regions are greatly affected, in agreement with observations of both polar arcs and convective patterns, the shape of the amoral oval remains intact. This argues for a near-Farth source for the quiet ring of structured auroras. Each large square represents IO RE by 10 RE. The Sun is to the left and dusk at the bottom.
Fig. 7. Similar to Fig. 4. In this case, however, a different color scheme is used to illustrate the complexity of mapping to the ionosphere (corrected geomagnetic coordinates (1980) are used). The left panels are static mappings for a Kp 3 TSYGANENKO(1987) model. The middle and right panels are for a model slightly modified from SERGEEVet al. (1990) to illustrate two growth phase possibilities. The middle panels have an intensified and thinned cross-tail current while the far left panels include magnetopause current variations which yield a neutral line formation in the mid-tail. The event modelled was the one presented in Fig. 4. The observed onset location is marked by a red circle near 22.5 MLT, 67 Mlat. Severe modifications to the magnetotail consistent with the substorm growth phase do not seem to be sufficient to move the onset to the boundary layers.
Fig. 8. Amoral images presented in the coordinates described in Fig. 3. A double ‘oval’ during substorm recovery phase is present illustrating that the main ‘oval’ (the equatorward system) occurs well within the closed field line region. New substorm onsets are visible in the top middle, top left and lower right panels showing that discrete aurora and onsets can occur in what is probably the near-Earth region. Omega bands on the poleward edge of the equatorward system which illustrate these patterns are also a near-Earth phenomena.
Fig. 9. The DMSP-F7 particle precipitation boundaries associated with the double ‘oval’ event on 4 May 1986. The trajectory has been plotted on the Viking image and color coded according to the scheme on the right side of the plate. The boundaries are low altitude definitions and are not meant to correspond to magnetospheric regions. Fig. 10. The development of an aurora1 substorm on 30 July 1986 showing the formation of the double ‘oval’ in a period of 30 min. The observation illustrates that the high latitude system appears poleward of the main structured oval after the expansion of the substorm bulge. This is evidence for two source regions for the amoral luminosity separated by the mid-tail plasma sheet.
Fig. 12. Viking UV images illustrating the overlap of the dayside and nightside aurora1 distributions (near 15 MLT) with the dayside system occurring equatorward of the nightside. The co-location of the dayside hot spot indicates the possible importance of the ionospheric meeting place of two different magnetospheric regimes.
Fig. 11. An illustration that the poleward arc system develops in conjunction with the substorm recovery phase. The top panel shows a keogram from the Viking data on 3 May I986 (i.e. a coior plot of intensity as a function of magnetic latitude and universal time at 23 kO.5 MLT). The xs mark times of ground magnetometer H bay changes. The middle panel shows that the X trace recovers at the lower latitude station coincident with the fading of the auroras there. The poleward station (bottom panel) begins a recovery when the double oval has formed.
the aurora1 distribution
from their particle characteristics (ELIASSON et al., 1987) as well as the statistical relationship they have to the region equatorward of them (AUSTIN, 1991). These arcs are embedded in the region of low energy electron precipitation poleward of the structured ‘oval’ (MAKITA et al., 1988). This more equatorward ‘structured oval’ forms a narrow ring during quiet times (FELDSTEINand GALPERIN, 1985). This ring has been related by ELPHINSTONE et al. (1991a) to the inner edge of the central plasma sheet and in particular to the magnetotail region where an enhanced cross-tail volume current density is found. These relationships between the aurora1 ‘oval’. polar arcs and the magnetosphere can be illustrated qualitatively using the TSYGANENKO(1987) long external field model (Fig. 4). Details of the mapping used in this paper are given in ELPHINSTONEet al. (199la) (paper I in what follows) and so shall not be discussed here. The upper panels in Fig. 4 illustrate the ionosphere at 120 km altitude in eccentric dipole (1985) coordinates (HEARN ef (il., 1993). Noon is at the top of the panels, dusk to the left and 60. 70 and 80 magnetic latitudes (Mlat) are shown. The lower panels show the minimum B surface (i.e. the equatorial plane) for zero dipole tilt (18 April 1986 at 6 UT) and a Kp level of 0. Each large square represents 10 Earth radii (R,), the Sun is towards the top of each panel and dusk (positive jlGSM) to the left. The left-hand panels have been colored (somewhat arbitrarily) according to regions in the equatorial plane (see paper 1 for the exact definitions). The yellow region, which in the ionosphere appears linked to the above quoted ‘teardrop’ shape. is associated with the flanks of the magnetotail. This region is most likely either the low latitude boundary layer or the cooler portion of the plasma sheet. These results are consistent with the particle observations in Fig. 3 which link this high latitude region to structured electron precipitation poleward of the main oval. The teardrop morphology has been linked to a requirement of a strong positive B, component in the flanks of the magnetotail (BIRN et al.. 1991). Field lines which are normally open become closed under this B, influence. Some evidence for slightly enhanced values of Br in the flanks of the tail can be found in SLAVIN et al. (1985). One reason for choosing the T87 model over the newer TSYGANENKO(1989) model (referred to as T89 in what follows) is that the B; in the T89 model goes unrealistically to zero in the vicinity of the low latitude magnetopause. Neither model was constrained by observations at high 1~1values in the magnetotail flanks (Fairfield, private commun., 1991). The results concerning the high latitude dayside region are heavily dependent on the relative magnetic
field components in the flanks of the magnetotail. The discussion of Fig. 6 later in this paper outlines some of the potential changes to the mapping which would occur by altering this field. The oval shaped white line in the model ionosphere shown in Fig. 4 was found in paper I to correlate well with the ‘observed UV aurora1 oval’. This line was derived by numerically evaluating the volume current density (curl B) and, for each ionospheric local time, finding the ionospheric latitude at which the peak current density occurred. The oval shaped line in the figure is the locus of points associated with these maxima. The right-hand panels in the figure show in false color (blue weakest and red strongest) the volume current density. In this T87 model the cross-tail current density peaks earthward of sosIM = - 10 RF. Using the T89 model, the peak in the UV aurora1 oval also maps to this same region near the Earth. The T89 model, however, has the drawback that there are negative B, values in the mid-tail at high tilt angles (DONOVAK et al.. 1992). It also has an unusual crosstail current distribution. For these reasons it was felt that the T87 model represented a more realistic large scale model magnetosphere. The cross-tail current illustrated in the bottom right panel of Fig. 4 near _+;sM = - 8 R, merges smoothly with the ring current (at _r(;sM= 0), and then flows to the boundary layers along the flanks. The cross-tail current density maximum which occurs near the inner edge of the central plasma sheet maps to an ‘oval’ shaped region in the ionosphere. At larger radial distance where the ring current falls off, the cross-tail current continues to the boundary layers and this marks the transition region in the ionosphere to dayside magnetic local times (MLT). The closer a point is to the boundary region, the closer to 12 MLT in the ionosphere it will project. How close depends on how open the magnetosphere actually is. Figure 4 should be taken as illustrative of a nearly closed magnetosphere. If one assumes that the electron distributions in this region arc isotropic (consistent with the low altitude observations on the right side of Fig. 4) and if the directional flux within the loss cone is proportional to the volume current density, then there is a natural relationship between the upper right panel of Fig. 4 and that of the UV aurora1 oval. The light blue region in the upper left panel of Fig. 4 (black in the upper right) represents field lines which do not close before exiting the model boundaries (chosen here to be 60 RF). Consider a magnetospheric region similar to this light blue region somewhere in the deep tail (whether it begins at 60 or 150 R, does not matter for this discussion) where the Mach number of the convecting plasma in the equatorial plane exceeds
R. D. ELPHINST~NE e/ al,
unity (SLAVINet al., 1985). Current systems beyond this Mach boundary cannot manifest themselves (via AlfvCn waves) in the ionopshere and the ionosphere loses touch with the plasma there. When this boundary is coincident with the distant tail separatrix then it coincides with the boundary between open and closed field lines. Suppose that this Mach boundary instead occurred within a region of positive Br (i.e. closed field lines). If such a configuration was possible, it would help explain the rapid appearance and disappearance of polar arc systems. These can occur too rapidly to be explained by convection alone (e.g. MU~PH~E ef af., 1989). The observations of SLAVIN et al. (1985) support the possibility that the Mach number could exceed one in a region where B_ is still positive. (They suggest that it is not possible, however, for closed field lines to support a super Alfvknic flow and that these observations are related to the averaging used rather than being a real effect.) Future studies might investigate whether on some occasions a mismatch between super-AlfvCnic flow and the separatrix might exist.
THEY BE TRUSTED?
While the above statistical model can provide a more intuitive view of magnetospheric topology, there are numerous effects it does not account for. It is unclear for instance to what extent the field-aligned currents in the model reflect physical currents (ELPHIC et al. (1987) have shown where these model fieldaligned currents were located when projected into the equatorial plane). This is a serious problem for local field line mappings. Another important effect which cannot be included in a statistical model is the changing current distributions during a substorm. One clue to these changes can be found by studying Kp changes in the model. In paper I it was shown that the increase in the magnitude of the current systems as well as the earthward displacement of the peak in the near-Earth cross-tail current density could account for the gross large scale changes to the aurora1 oval during substorm growth phase (i.e. the polar region bounded by it increases in size with increasing activity). This result, which is independent of whether one uses the T87 or the 7’89 model, then implies a less direct connation between this area and the amount of open flux in the tail lobes. In this interpretation there would be a direct linkage between the strength and movements of the cross-tail current and location of the main UV aurora1 oval. Only indirectly would this also imply a connection to the amount of open flux. The association of the main UV aurora1 oval to a peak in the near-
Earth cross-tail current as well as to a region of overlap between low energy structured electrons and higher energy isotropic electrons points to its source region being near the inner edge of the central plasma sheet. Figure 5 illustrates the difficulty in equating open ffux directly with the main UV aurora1 oval (see Fig. 1). First. the peak in intensity of the UV aurora1 oval occurs at least a couple of degrees equatorward of the poleward boundary of fairly intense emission. Further, depending on the intensity threshold used, there appear to be diffuse emissions several degrees poleward of this point. The high latitude polar arc in the dawn sector shows up using a lower threshold (pink or mauve). A still lower intensity threshold (grey) leaves a smaller polar region that is still devoid of emission but the dawn sector polar arc becomes an extension of the dawn side oval. If one accepts that this precipitation lies on closed field lines then the substorm (seen at near 23 MLT in the left pane1 of Fig. 5) is occurring well within the closed field line region, as suggested by M~RPHRE~ et al. (1991). Regardless of where one would put the open-closed field line boundary, it appears that the latitudinal motion of the peak of UV aurora1 intensity is not a direct consequence of the changing boundary of open flux. Further, considering the excellent direct relationship and correlation between this peak and the crosstail current density maximum (paper I) it seems reasonable (until some better means of estimating the oval is found) to assume the peak in UV aurora1 emissions is strongly coupled to this nightside ‘cusp’ region. This view is then consistent with the substorm onset being in this region (Lur and B~JRROWS,1978). It is quite important that such a high degree of correlation was found even though this statistical model contains no IMF effects or substorm changes. This is probably related to the nightside cusp region being the start of the distortion of the magnetotail from a dipolar-like configuration. Drastic changes and distortions of the magnetic fields in the tail can occur without affecting this basic aurora1 shape. This argument accounts for why the corrected geomagnetic coordinate system has been so successful in ordering the aurora1 oval (HEARNet al., 1993). For a moment, assume that, instead of the aurora1 ‘oval’ being a nearEarth phenomena occurring in regions of reiatively high magnetic field, its origin is somewhere near the magnetopause. What does this assumption imply about the aurora1 distribution? Since there is growing evidence that there is at least some penetration of the IMF into the magnetosphere (e.g. SIBECK et al., 1985; TSURUTANIet ul., 1984; FAIRFIELD, 1979; C~WLEY and HUGHES,1983) it is useful to consider the impli-
the aurora1 distribution
of such a penetration. Numerous authors have superposed IMF onto various magnetospheric models in order to explain polar arc asymmetries (BIRN et ul., 1991; JANKOWSKA et al., 1990; AKASOFU, 1985). Besides implying that the polar arcs lie on closed field lines associated with a twisted plasma sheet (MURPHREE et al.,1982) and/or low latitude boundary layer, the results of such a superposition imply that large distortions in the ionospheric projection of the boundary layers can and probably do occur. If the aurora1 oval were purely a magnetopause/boundary layer phenomena these distortions should play a dominant role in governing its shape. Instead, however, the internal field plays a governing role (HEARN et al., 1991). This is illustrated in Fig. 6 by extending the Tsyganenko model out to 200 RF and representing it three-dimensionally (upper panels of Fig. 6). This plate is meant to be a semi-quantitative cartoon. That is, one does not have to believe exact aspects of this presentation in order to appreciate the general implications of it. The left panels represent the northward portion of a closed magnetosphere and the right panels the projections to the ionosphere. The introduction of a I nT superposition field in each of negative B,, positive B,. and positive B; directions implies a large scale change in the tail (bottom left panel), twisting the plasma sheet to the dusk sector (compare its thickness at .Y<;~~= -45 RF at dawn and dusk) and creating ‘overdraped’ open field lines (green) in the dawn sector. This simple addition of fields adds open flux in a manner consistent with the development of a convection throat directed towards dawn for IMF B,. positive. This can be seen in Fig. 6 (lower right panel) near noon at about 78 Mlat where the green ‘open’ region extends equatorward in the dawn sector and creates the desired ‘throat’ region. The expanded dusk sector (e.g. the red arc-like segment pointing towards noon in the dusk sector in the lower right panel) is also consistent with the preferential locations of polar arc phenomena under IMF B, positive conditions. The above example shows how small systematic magnetic field changes can dramatically alter the ionospheric projection of boundary layer phenomena. Note, however, that the projection of the maximum in the cross-tail volume current density (the white oval shape in the right panels and the white line in the left panels near .Y<;~~= -8 RE) remains relatively unchanged. The qualitative aspects of these results are mirrored in the observations of polar arcs and the aurora1 ‘oval’ and are what will always occur when comparing small systematic magnetic field changes near the Earth and near the boundaries of the magnetosphere. This general effect is model indecations
pcndent. Thus the following points all support the idea that the peak emissions in the UV aurora1 ‘oval’ (shown schematically in Fig. 1) originate in a magnetospheric region of relatively ‘high’ magnetic field presumably near the inner edge of the cross-tail current :
(1) The corrected geomagnetic coordinate system is very successful in ordering the aurora1 ‘oval’ obscrvations. (2) The magnetospheric topology near the boundary layers can change drastically with relatively small magnetic field perturbations. Such changes are observed in polar arc phenomena but the aurora1 ‘oval’ does not seem to respond as dramatically. (3) The correlations found in paper I are unlikely to occur unless a near-Earth region is involved in structured aurora1 processes. Another argument commonly used against static magnetic field mapping of the aurora1 oval is linked to the above argument. Large scale current changes during substorm growth phase lead to distortions of the magnetotail and the inability to accurately know the ionospheric projections of the magnetospheric source regions. SERGEEVef al. (1990) developed modifications to the TSYGANENKO (1987) long model in order to duplicate high altitude satellite magnetic field observations. This has been repeated in a very similar manner by PULKKINEN et al. (1991) using Tsyganenko’s 1989 model. Other works by Lur (1978), KAUFMANN (1987) and MCILWAIN (1991) further illustrate some of the aspects of introducing current systems in the nightside near-Earth region. Adopting the approach of SERCEEV et nl. (1990), several alterations to Tsyganenko’s 1987 mode1 were attempted and the results are shown in Fig. 7. The left panels represent the static model for Kp level 3, the middle and right panels represent two different modifications. The upper panels are projections to the ionosphere and the lower panels show the minimum B surface (for I9 October 1986 at 1132 UT). Various regions in the minimum B surface have been colored in the same manner as in ELPHINS~ONEet al. (1991 b). The ionospheric coordinates used in Fig. 7 are corrected geomagnetic (GUSTAFSSON,1984; HEARN et al., 1991). The red ‘oval-like’ line in the upper panels corresponds to the peak current density, and the dark blue and white areas represent regions of high crosstail current density between _xGSM= -6 RF, and ~~~~~~ = - I2 RF. The middle panels show the changes to the projections associated with the introduction of a current sheet thinning from about 3 to 1 R, (D = I), and an increase in both the magnetopause and cross-
D. ELPHINSTONE et al.
tail currents by a factor of 1.5 (J‘). The other parameters used from the SERGEEV et al. (1990) paper were x, = -2.8, B,, = -45, and DA4 = 1. The right panels in Fig. 7 show the effect of increasing the magnetopause currents by a factor of 2.25 and leaving the other parameters the same as those in the middle panels. The dark blue boundaries near noon (just equatorward of the red current density maximum curve) outline the open flux from the dayside boundary regions and show how the above alterations open the dayside ionosphere. On the nightside, however, it is striking how little the above changes affect the bluewhite ‘oval’ projection. This is the case even though in the bottom right panel a neutral line has formed nearxGs, = - 20 to - 25 &. There is an equatorward motion of the red line or oval shape which is consistent with the growth phase activity for this particular event (ELPHINSTONE et al., 1991b). In the middle panels it is clear that fairly substantial current density changes to the cross-tail current still leave the basic topology discussed above intact. The change on the right, however, creates a neutral line in the mid-tail region and this brings the open-closed field line region relatively close to the poleward edge of the blue-white oval shaped region. The small red circles in the ionospheres of Fig. 7 (near 22 MLT) represent the region of onset found for this event from the Viking images. In all the above calculations this region of onset maps to between 6 and 12 R, from the Earth. The results of the middle panel are in agreement with aurora1 observations which put the onset locations well within the closed field line region (MURPHREE et al., 1991 ; ELPHINSTONE et a/., 1991b). Accepting that this situation sometimes occurs, then other instances of the form shown in the far right panel (i.e. a neutral line forming) take on a different meaning. If the onset mechanism is the same in both cases then the relatively close proximity of the near-Earth neutral line to the onset (in the far right panel) is simply a geometric coincidence of having a certain current configuration. (This does not, however, imply that a neutral line forming does not have an effect on the substorm evolution.) The calculations shown in Figs 4-7 and the results of paper I suggest that statements implying that too little is known about the magnetosphere to say anything meaningful about mapping may be too strong. One should, of course, be cautious in making statements about where exactly specific ionospheric points for particular events map to and one should be very cautious in making anything more than general statements about how the boundary layers map. Nevertheless, it appears that at least during the growth phase one can begin to see rather broad relationships
between the near-Earth central plasma sheet and the quiet time narrow band of structured auroras. The next section attempts to further substantiate these results directly using aurora1 data, and to include boundary layer aurora1 signatures during the more disturbed period of the substorm recovery phase. 5. THE ACTIVE
An aurora1 distribution peculiar to the recovery phase of a substorm exists. A few examples of this are shown in Fig. 8. In all these cases, the main aurora1 ‘oval’ described in the previous sections corresponds to the equatorward region of emissions in the 21-03 MLT sector. Poleward of this emission is a diffuse band which is terminated at the poleward edge by a discrete arc system. The two events in the lower middle (30 July 1986 at 0551 UT) and lower right (20 September 1986 at 1847 UT) panels of Fig. 8 occurred near the start of the recovery. Still visible during these times is a spiral form at the westward tip (near 21 MLT) of the expanded bulge. As well, the most poleward arc systems are relatively intense and limited to the dawn and midnight local time sectors. The upper middle (27 July 1986 at 0811 UT), upper right (3 May 1986 at 0112 UT) and lower left (23 September 1986 at 2044 UT) events are snapshots of the distributions well into recovery and just before or at a new substorm onset (which begins from the most equatorward system). In these cases the far westward spiral form has disappeared and the most poleward arc system extends well into both the dawn and dusk local time sectors. Using the mappings presented in Fig. 6 as a general guide, these high latitude forms can be related to a magnetospheric region well separated from the near-Earth origin of the main structured oval. A natural choice for their origin would be the outer edge of the central plasma sheet near the velocity shear region in the equatorial plane. These distributions illustrate an important distinction between the nearEarth central plasma sheet, the mid-tail and the outer regions. This distinction is not included in previous aurora1 schematics such as those found in FELDSTEIN and GALPERIN (1985). It is also evident in the upper right panel that a series of aurora1 structures resembling ‘omega bands’ exist at the poleward edge of the more equatorward ‘oval’. The aurora1 distribution during this interval resembles two aurora1 ‘ovals’, the most equatorward system being relatively continuous through midnight and the poleward systems being somewhat more broken up and variable (shown schematically in the right panel of Fig. 2). MURPHREE et al. (1981) have previously noted that
Mapping the aurora1 distribution these two distributions exist and that a minimum in 3914 8, luminosity exists between them. They also showed that embedded in the diffuse emissions, structured arc systems could occur and that the most intense arc systems could appear anywhere in the poleward half of the pre-existing distribution (whether quiet or disturbed). This is consistent with the particle observations of HOFFMAN and BURCH (1973) and HOFFMANand LIN (198 1) who observed that inverted Vs were seen inside a region of diffuse precipitation which they termed central plasma sheet precipitation. HOFFMAN and BURCH (1973) found that inverted V events were located equatorward of a 0.7 keV precipitation boundary during the beginning of the substorm expansion phase. They concluded that the lower energy precipitation boundary established by the end of the growth phase was the open/closed field line boundary up to which but not beyond, inverted V precipitation was located. They inferred from this that an aurora1 substorm begins in the near-Earth region and propagates outward to the separatrix (their fig. 12). It is interesting to note in this context that WINNINGHAM et ul.‘s (1975) schematic (fig. 19) can be interpreted in a similar manner if one does not concern oneself with the particle region definitions employed there. The third panel (i.e. recovery phase) in that figure shows what is termed a ‘collapsing BPS’. In the alternative interpretation (see Fig. 1) this could bc associated with the mid-tail central plasma sheet. On a global scale, Fig. 8 illustrates various forms of this aurora1 distribution. For the example shown in the upper left panel of Fig. 8 (4 May 1986 at 0729 UT), a DMSP-F7 trajectory and the corresponding particle signature derived from a neural network have been found and plotted in Fig. 9 (NEWELL et al., 1991). The satellite passed over the nightside amoral region between 0729 and 0732 UT (the region labeled BPS occurred between 0728:59 and 0730: 13 UT) indicating a nearly exact timing for the crossing of the high latitude arc system. The average energy of the electron precipitation in the region labeled ‘BPS’ was 1.1 keV with peak and average electron fluxes of 16.7 and I .5 ergs crn~-’ so I, respectively. The average ion energy was 7.5 keV with peak and average fluxes of 0.17 and 0.08 ergs cm-’ s- ‘. The existence of particle precipitation (BPS type) even further poleward of the intense poleward arc system implies that if this arc does lie in the Plasma Sheet Boundary Layer then it lies closer to its inner edge. Note that in this paper BPS and CPS are meant to be arbitrary low altitude definitions of particle signatures which may have nothing to do with the actual magnetospheric source region. Further, the neural
network definitions are not necessarily identical to WINNINGHAM et al.‘s (1975) definition since there is not an explicit test for acceleration along a field line. Nevertheless, observations of similar systems support that this arc system lies in a region of field-aligned acceleration (YAMAMOTOet al., 1993). A logical place to put this high latitude evening arc system would be at the central plasma sheet’s outer edge where velocity shear could imply an upward field-aligned current and precipitating electrons. This interpretation would be relatively consistent with the view of FELDSTEIN and GALPERIN (1985) and then would put the main Plasma Sheet Boundary Layer further poleward. It would then be associated with the ion dispersion signature described by ZELENYI et 01. (1990). The right column of Table 1 outlines other observations consistent with this dispersion signature (SCHRIVERet al., 1990; YAMAMOTOet al., 1993). The broad region of aurora1 emission equatorward was identified as ‘CPS’ by the neural network. In this region the average ion and electron energies were 9.8 and 2.1 keV ; the average fluxes were 0.19 (ions) and 0.88 (electrons) ergs*cm-’ s- ’ and the peak fluxes were 0.3 (ions) and 1.77 (electrons) ergs-cm-’ s- ‘. From previous work (e.g. HOFFMAN and BURCH, 1973; WINNINGHAM et al., 1975) it is apparent that within the region labeled CPS in Fig. 9 inverted V events can occur. WINNINGHAM et ~1. (1975) would then describe that region as BPS. Discrete arcs do occur in this inner region which is apparent from the substorm onset arc system found in the upper middle panel of Fig. 8 (extending from dusk to 23 MLT), and also in the upper right panel (i.e. the onset arc system near 21 MLT and the ‘omega’ like bands in the morning sector). SANDAHLand LINDQVIST(1990) proposed that at least during quiet times there is an ionospheric overlap region between structured and unstructured electron precipitation and that this is the region of the bright auroras (see Fig. 3 and Section 2). This is consistent with the first equatorward ring being a mix of diffuse and discrete aurora1 emissions. It appears from the above that during recovery phase two distinctive aurora1 distributions exist, one at high latitude, and one at low latitude. The equatorward system also exists during quiet times and is a relatively continuous diffuse band of emission with discrete arc systems embedded within the poleward portion of it (see Fig. 1). This corresponds to the overlap region between isotropic high energy (unstructured) and structured low energy precipitation (see Fig. 1 and Table 1). The peak intensity in this emission region may be the demarcation between purely diffuse and a mix of diffuse and discrete emission. It appears associated in some instances
with the onset arc system of a substorm and is very well correlated with the near-Earth (xosM > - 15 RE) nightside cross-tail current system. Poleward of this region during recovery phase is a relatively dark inactive region which could be interpreted in this scheme as the main central plasma sheet, and would (during recovery phase) be a source for weak diffuse emissions. This dark region is then terminated by a discrete arc system (intense and active at the start of recovery and fainter at the end of this phase). This system is less spatially systematic and more temporally varying than the more equatorward system. Surge or spiral development in the poleward region could be one reason for the existence of contradictory conclusions regarding the source region of the aurora1 substorm. 6. THE DEVELOPMENT OF THE MOST POLEWARD AURORAL
The previous sections have dealt primarily with static aurora1 configurations at two stages of a substorm (i.e. the quiet time and recovery phases). The evolution from one phase to the other contains many important details. This large scale evolution is illustrated in Fig. 10 for an event on 30 July 1986. The middle of the expansion phase is shown in the upper left pane1 of Fig. 10 (at 0523:35 UT). As indicated by MURPHREE et al. (1991) and CRAVEN and FRANK (1991) the region of onset is in the premidnight sector. The panel at 0523:35 UT shows a well developed substorm bulge between 2 1 and about 02 MLT skewed relative to eccentric dipole (1985) magnetic latitude. It is filled with intense aurora] emissions which peak in its center (near 23 MLT). The bulge expands eastward, westward and poleward (ROSTOKER et al., 1987) over the next 30 min during which the poleward portion of the ‘double oval’ has developed. Between 0525 and 0542 UT the emissions fade within the bulge. At very low intensity levels a faint high latitude arc system is apparent in the morning sector at 0525:36 UT (middle upper panel) which intensifies by 0554:50 UT and gives rise to the ‘double oval’ morphology. These high latitude arc systems, while being statistically linked to a northward interplanetary field also appear consistently during the recovery phase of the substorm. An example of this association with the recovery phase is shown in Fig. I I where a keogram of the UV data has been constructed and compared to ground based magnetometer data. The top panel is a keogram constructed from the CDAW-9 event on 3 May 1986 and studied in some detail by HONES et al. (1987). Ultraviolet intensity profiles as a function of latitude
were derived from the Viking images and backgrounds were corrected for using a computer automated iterative procedure. The profile was obtained from a one hour local time average centered on 23 MLT. The algorithm fitted each profile to a cubic polynomial, determined the brightest portion above this initial background, removed this ‘aurora’, and refitted the cubic to the revised profile. This was done iteratively until convergence of the cubic fit was obtained. This fit was used to remove the background contamination due to sunlight and dark current. For each universal time that an image was available (typically one minute) this procedure was performed. The resulting profiles were then stacked together, interpolated and plotted in a keogram format with magnetic latitude vs universal time. The keogram in Fig. IO shows the end of the expansion phase and beginning of recovery phase of an aurora1 substorm. The xs marked on the keogram show the magnetic latitudes of two key ground stations (Fort Yukon and Inuvik) at a few important times in the substorm. [Additional magnetometer data for this event can be found in HONES et al. (1987).] HONES ef al. (1987) reported the plasma sheet thickening at 1003 UT (as seen by ISEE-1) and seemed puzzled by the observations that showed a ‘poleward leap’ of the auroras well before the aurora1 bays recovered and well before this plasma sheet thickening. It is apparent from Fig. I1 that what those authors were attributing to a ‘poleward leap’ of the auroras was an expansion of the substorm bulge before the recovery. This poleward motion is clearly evident in the keogram and is completely unrelated to the substorm recovery phase. The plasma sheet thickening occurs approximately coincident with the complete fading of the aurora] emissions within the bulge. More locally, each ground station bay recovers in accordance with its location relative to the fading aurora] emission. The middle panel of Fig. I1 shows the X trace of the magnetometer at Fort Yukon. That bay begins to recover precisely with the fading of aurora1 forms at that location (about 0952 UT). It has more or less completely recovered by the time the plasma sheet at ISEE-I has thickened. The more poleward station, Inuvik (bottom panel) shows a negative X bay onset approximately when the auroras have moved poleward to that location. The negative X bay then remains until the recovery at about 1005 UT and then partly recovers coincident with the slight equatorward motion of the most poleward arc system. The formation of the ‘double oval’ structure occurs at this time. This figure shows that there is no poleward leap of the auroras associated with the recovery of this substorm and although there is a poleward motion of
the aurora1 distribution
the bulge earlier, that is an expansion phase phenomena. Figures 10 and 11 help establish that the formation of this ‘double oval’ structure is linked to the recovery of the substorm and the thickening of the plasma sheet. If one accepts the view that plasma is ejected down the tail with a plasmoid during the late expansion or recovery phase of a substorm (e.g. SLAVINet al., 1989) then the fading of aurora1 luminosity within the bulge may be related to the topological change associated with the release of the piasmoid. The release of the plasma, the topological change and the requirement of refilling the plasma sheet all imply modifications to the boundary layers. It seems reasonable that changes to the boundary layers could be manifested in the ionosphere through the development of the high iatitude arc systems. The recovery phase can then be seen as a time when the nightside boundary layers play a crucial role in governing nightside ionospheric phenomena. Events occurring in this poieward region could and have been interpreted as ‘mini substorms’ and some researchers would use this as clear evidence that the substorm process is identical with a low latitude boundary layer process. An example of such a ‘mini substorm’ is the surge form seen in the 3 May event at the start of the recovery phase (HONESet al., 1987). it appears possible that two discrete nightside aurora1 source regions exist, one Iinked to the near-Earth nightside ‘cusp’ region and another to the boundary layers. Which one dominates depends on the substorm phase and the pre-existing magnetospheric configuration.
7. THE MAPPING
The dayside aurora1 region of the ionosphere is very difficult to map accurately using any static magnetic field model. This is essentially because the main region of structured dayside aurora is very dynamic and typically will map near the CLISPor magnetopause regions. Under such conditions, a small error in mapping can lead one to interpret a feature as originating from the nightside deep tail when in actuality it originated from the subsolar point (or vice versa). In such circumstances, general mapping knowledge combined with the large-scale aurora1 morphology can substantially aid interpretation. For instance, fan arcs found in the morning sector and resembling the model arcs shown in the upper left panel of Fig. 7 could reasonably be attributed to boundary layer processes with the most equatorward arc o~ginating closest to the subsolar point and the most p&ward arc system
originating along the flanks somewhere in the tail. Such an inte~retat~on would be relatively model independent. Other clues can be found by investigating the implications of a closed magnetosphere. In such a case the entire magnetopause maps to a point near the noon meridian. If one begins neary,,, = 0 R, and assumes this region maps to near magnetic midnight, then moving towards the dusk magnetopause along a line of constant .yoSM will move the ionospheric projection of the point in question towards lOCd noon. As shown in Fig. 7 a nearly closed magnetosphere gives a similar result. The dayside ionosphere equatorward of such a line of constant negative xGsM will map sunward of it in the n~agnetosphere (see Fig. 7 near 15 MLT in the ionosphere). ff one opens the magnetosphere some amount then this line of constant _yosMwill not necessarily stretch all the way to 12 MLT in the ionosphere (e.g. a dipole field is the extreme case). These considerations imply a distinctive signature may exist in the ionosphere consisting of an overlapping set of arc systems. Such morphologies have been reported by VOROBJEV et al. (1976), MENG and LUNIXN (1986) and ELPHINSTONEet al. (1991 b). These overlapping arc systems may be the afternoon sector aurora1 manifestation of such a topological overlap. Figure 12 illustrates four such events with the nightside arc system overlapping poleward of the system originating from the dayside. These forms are transient features typically lasting only a few minutes: they can be associated with breakups in the evening sector and with eastward wave propagation of greater than 5 km/s (ELPHINSTONE et al., 1991 b). The transient nature of these features helps explain why global imagers previous to Viking did not observe them. Figure 12 also illustrates an important possible relation between this overlap region and the intense aurora1 luminosity found in the 14 MLT sector (e.g. Lur et ul.. 1987). The region on the dayside arc system to which the nightside system points seems to correspond to an aurora1 spiral form. This is particularly clear in the lower left panel of Fig. 12 (24 September 1986 at 1348 UT). In this panel two clear spiral forms are apparent between 12 and 18 MLT which are ‘wound up’ anticlockwise (viewed down in the direction of the magnetic field). If the dayside arc system originates in the dayside boundary region then an intense upward field-aligned current sheet there could, if perturbed, lead to a spiral in the manner described by HALLINAN (1976). The perturbation could in part be supplied via the ionospheric coupling of the two distant magnetospheric regimes. The above arguments outline the potential of using certain aurora! morphology to describe possible
ELPHINSTONE et al.
aspects of magnetospheric topology. Both the ionospheric coupling between distant magnetospheric regimes as well as the field-aligned current distortions or wrapping up of field lines may play crucial roles in defining ionospheric signatures of key magnetospheric processes. 8. SUMMARY This paper is an attempt to help reconcile two competing views of the mapping of aurora1 structures in the ionosphere to the magnetosphere. Figures 1 and 2 summarize the mapping results presented here. The first figure shows a possible composite view of the relationship between the aurora and its magnetospheric source regions. This is explored in more detail in Table 1 showing low and high altitude signatures and how they might correspond. Figure 2 shows two aurora1 morphologies in polar coordinates, one for quiet times and one for active periods. It serves to illustrate the importance of local time and substorm phase when discussing where a feature maps. Some further points made in this paper are outlined below. (1) The poleward boundary of the quiet aurora1 distribution is an ill-defined boundary strongly based on which intensity thresholds and imaging wavelengths are used. To equate some particular threshold with a separatrix in the magnetosphere could and probably has led to incorrect interpretations of where the aurora1 distribution maps to. (2) The high latitude aurora1 region is highly variable in contrast to the low latitude distribution. This observation combined with modelling results supports the existence of two structured aurora1 source regions, one near the Earth on the nightside and the other probably related to the interface between the central plasma sheet and the boundary regions. Auroral observations during recovery phase give special importance to the inner and outer regions of the central plasma sheet.
(3) A near-Earth source region for structured aurora is likely to be earthward of an xGSMlocation of - 15 R,. (4) The two dist~butions are most clearly evident during the substorm recovery phase during which a ‘double oval’ appears. The dynamical evolution of this double oval definitively shows that the main structured UV aurora1 ‘oval’ has a source region well within the open-closed field line region. It is probably associated with the hot core of the central plasma sheet and an overlap region between high energy isotropic electron distributions and a lower energy structured popuIation (see Table 1). (5) Regions mapping from vastly different places in the magnetosphere can project to a small region in the dayside portion of the ionosphere. This can play an important role in understanding dayside aurora1 morphology. (6) Some portion of the region called the ‘polar cap’ may be a result of plasma and current systems on closed field lines in the distant tail being unable to transmit (via AlfvCn waves) info~atjon to the ionosphere. This interpretation may help explain some aspects of polar arc dynamics and evolution. (7) High latitude polar arc phenomena represent an extension of structured 0.1-I keV electron precipitation beyond the ‘main UV aurora1 oval’. These morphologies are probably linked to the deep tail low latitude boundary layer and to the warm envelope of plasma surrounding the plasma sheet. Arknoll/e~~emPnt,s-The
authors would like to thank
Marie Morris for typing the manuscript and P. T. Newell for the use of the neural network employed to determine the particle boundaries shown in Fig. 9. This paper was first presented at IAGA, Vienna, 1991. The Viking project was managed by the Swedish Space Corporation under contract to the Swedish Board for &ace Activities. The UV imager was built as a project of the National Research Counciiof Canada and this work was supported under grants from the Natural Sciences & Engineering Research Council of Canada.
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