Double structure of ionospheric conductivity in the midnight auroral oval during a substorm

Double structure of ionospheric conductivity in the midnight auroral oval during a substorm

Journalo/Atnm~pheri¢ ond Terrevtrial Physics, Vol 57. N o 2. p p 177 186. 1995, Elsevier Science Lid Prinlcd in Great Brilairt 0021 9169'9q Y,9.5[)4 0...

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Journalo/Atnm~pheri¢ ond Terrevtrial Physics, Vol 57. N o 2. p p 177 186. 1995, Elsevier Science Lid Prinlcd in Great Brilairt 0021 9169'9q Y,9.5[)4 0.00

'~ Pergamon

0021-9169 (93) E002 %9

Double structure of ionospheric conductivity in the midnight auroral oval during a substorm A. L. KOTIKOV,* E. M. SHISHKINA,*O. A. TROSHICHEV* and T. 1. SERG1ENKO+ *Arctic and Antarctic Research Institute, St.-Petersburg, Russia ; +Polar Geophysical Institute. Apatyti, Russia (Received in.[inaljbrm 1 November 1993: accepted I December 1993)

Abstract- Measurements of precipitating particles on board DMSP F7 spacecraft are used to analyze the distribution of ionospheric conductance in the midnight auroral zone during substorms. The distribution is compared wilh the meridional profile of ionospheric currents calculated from magnetic data frmn the Kara meridional chain. Two regions of high Hall conductance are found, one of them is the traditional auroral zone, at latitudes 64~68", and the other is a narrow band at latitudes 70 73. The position of high conductance zones is in agreement with the location of the intense westward currents. The accelerated particle population is typical of electrons E~ > 5 keV in the high conductance region.

!. I N T R O D U C T I O N

Estimates of ionospheric conductivity are essential in calculations of such electrodynamic parameters of the auroral ionosphere as electric fields, ionospheric and field-aligned currents and Joule heating. Averaged global models of conductivity, taking into account the seasonal and U T variations of U V radiation and the averaged dependence, of corpuscular radiation on magnetic activity ( V A N J A N and OSIPOVA, 1975 ; SPIRO et al., t 982), are usually used. Calculations of Hall and Pedersen conductivities using auroral images fi-om D E and V I K I N G satellites have been carried out by GORNEY et al. (1985), KAMIDE et al. (1986), MARKLUND et al. (1987), and KAM~DE (1988a,b). The analyses of AnN (1989) and KAMIDE et al. (1989) have shown that it is necessary to deal with the actual distribution of the conductivity in that specific M L T sector for that actnal time to investigate specific events in the auroral zone. To solve this problem, AHN (1989) and KAMIDE et al. (1989) tried to adjust the averaged model taking into account the auroral observations from D E and V I K I N G satellites axtd particle measurements from rockets and D M S P satellites. KIRKWOOD et al. (1988) have used measurements of the E I S C A T radar for the same purpose. The conclusion is that precisely adjusted models of conductivity are required to study dynamic processes occurring in the auroral zone. In this paper, we, use the D M S P F7 satellite measurements of precipitating particles to analyze the distribution of ionospheric conductance in the midnight auroral zone during a substorm. We compare this distribution with the filamentary structure of the

westward electrojet (KOTIKOV et al., 1987, 1991 ) which is also typical of substorm conditions.

2. EXPERIMENTAL DATA AND PROCEDURE OF CALCULATION The measurements of precipitating particles on board D M S P F7 for five crossings of the nighttime auroral zone have been used in the analysis. The list of crossings is given in Table 1. During the crossings on 5 and 9 April 1986, the Kara meridional chain of magnetic stations is situated near the midnight meridian and this opportunity has been employed to compare ground magnetic observations with satellite measurements. The coordinates of the magnetic stations are listed in Table 2. The satellite tracks and appropriate locations of the meridional chain are shown in Fig. 1. To calculate the latitudinal profiles of electron concentration n,,, the time-independent balance equation

Table 1. List of satellite crossings of the midnight auroral oval

177

Data 1 April 1 9 8 6 5 April 1 9 8 6 9April1986 9 April 1 9 8 6 7May 1986

Start time, UT 16.41.02 18.44.28 17.21.30 19.04.44 14.34.47

Time of crossing auroral zone, UT 16.54 17,01 18.55 19.02 17.34 17.41 19.16- 19.22 14.50 14.55

178

A. L. KOTIKOVeta[. Table 2. Coordinates of magnetic stations constituting the Kara meridional chain Geographic coordinates Latitude, Longitude,'

Station 1. Heiss lsl. 2. Vize lsl. 3. Uedynenia lsl. 4. lzvestiy Isl. 5. Dixon 6. Sopochnaya 7. Norilsk

80.62 79.48 77.52 75.87 73.53 71.87 69.43

N N N N N N N

58.05 76.98 82.22 83.08 80.28 82.70 88.08

is solved at heights of the ionospheric E- and lower Fregions :

Q-

~n~ = 0 ,

where Q is the rate of ionization caused by the auroral electrons, and ~ is the recombination coefficient. In this case, the rate of ionization is calculated by the method of SERGIENKO and IVANOV (1993), the recombination coefficient being taken from VICKREY et al. (1982). The MS|S-86 atmosphere model has been used in the calculation. The electron and ion spectrometers on board the D M S P satellite determine the particle spectra once a second in the local satellite zenith direction in 20 channels, spanning the energy range from 32 eV to 30 keV. Knowing ne, we obtain the altitudinal profile of Hall conductivity : "VleC f

tfOe ?

E E E E E E E

Corrected geomagnetic coordinates Latitude,' • Longitude, 74.5 73.0 71.3 69.9 67.9 66.5 63.4

N N N N N N N

144.9 E 156.2 E 158.8 E 158.8 E 155.8 E 157.4 E 161.8 E

Hall conductivity at h > 115 km is negligible. Bearing this experimental fact in mind, we calculate Z , only in layer 90--115 km instead of calculating the integral conductivity for the total thickness of the ionosphere. One-minute data from the Kara meridional chain magnetometers have been used to calculate meridional profiles of the strength of the ionospheric currents responsible for the magnetic disturbances. The method of calculation put forward by KOTIKOV et al., (1987) is described in detail in KOTIKOV et al. (1991). Figure 3 shows variations of the geomagnetic H and Z components at stations of the Kara chain for the substorms on 5 and 9 April 1986. Figure 4 gives the appropriate meridional profiles of the ionospheric current intensity for some fixed U T times. The altitude of the ionospheric currents is taken to be fixed at h = 100 km in these calculations.

(Di 2

3. THE RESULTS OF THE ANALYSES where : e is the electronic charge ; B is the magnetic field strength ; vi, is the ion-neutral collision frequency ; Yen is the electron-neutral collision frequency ; oJ~, ~oc are the ion and electron gyrofrequencies. Magnetic disturbances in the auroral zone are caused mainly by Hall currents because the magnetic effects of the Pedersen and field-aligned currents compensate each other on the ground (TROSHICHEVet al., 1979). It is also known that Hall conductivity is a maximum in a narrow layer of the ionosphere at altitudes o f 9 0 - I 15 km (BREKKE et al., 1974). As an example, the altitudinal distribution of Hall conductivity calculated by us from D M S P F7 particle measurements along the spacecraft track on 5 April 1986 is shown in Fig. 2. We can see from Fig. 2 that appreciable values of Hall conductivity (~H > 10 -4 mhos/m) are found only at altitudes of 90 115 km, whereas the

In the case of the 5 April 1986 event the satellite crossed the auroral zone (corrected geomagnetic latitude, q~ = 60-70') near the midnight meridian during a substorm expansion phase. Figure 5 shows the behaviour of the calculated Hall conductance along the satellite track. We can see a wide maximum of conductance at latitudes 64-71", with a local minim u m at q~ ~ 6 8 . The same regularity is seen in the meridional profile of current intensity [see Fig. 4(a)] : a strong westward electrojet is at latitudes 6 4 - 7 1 , with a local minimum at • ~ 6 7 . A small increase of Hall conductance at @ = 7 3 is deduced from the satellite data at about 18.56 U T and is also displayed in the current profile for 18.50 UT. In the case of the 9 April 1986 event the satellite crossed the midnight auroral zone at 17.35-17.41 U T (growth phase of substorm) and at 19.1(~19.22 U T (recovery phase). The calculated profiles of conductance Z , for these crossings are presented in Fig.

179

Double structure of ionospheric conductivity 0F,..04,1986 DMSP F7 TI = 1B,44.2B UT (3

12.00 60

18.00

OB.O0

19.02

09.04.1988 DMSF' F7 Ts b

on.o0

09.04,1988 DMSP F-/ T~ -

17,21.30 LIT

12.oo

C

12,oo

60

60

7o

8

o

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07.(15. t 1 ~

e

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,fly

vwl=

17,01

~

,/

~.1~ F7 T m =

14.34.47 LIT

12.00

60

iiii

vl

"

: 19.22

12.00

"~

8f

,;

d

19.04.44 l i t

le

IIwl

06.00

IB,O0

UT

00,00

Fig. 1. The tracks of the DMSP F7 satellite and the appropriate position of the Kara meridional chain for the events of 5 April (a), 9 April (b,c), 1 April (d) and 7 May 1986 (e), in corrected geomagnetic coordinates.

~,00

A. L. KOTIKOV et al.

180 61

63

65

i i i i i i i

67

69

71

73

i i i i i i i i ~ ~ I ~ i i i ~ i i

75

iI 190

190

April

05,

1986

Hall

conductivity

(mhos/rn)

170

170

-'~ 150

2

~

o

1 3 0

110

90

I I I I I

61

63

i I i I I ] I I t i

65

67 69 COM Iotitude

-

150

-

130

04,

110

i i

I I I

71

i i

i [ t I

73

90

75

Fig. 2. Altitudinal distribution of Hall conductivity calculated using the particle measurements made along the DMSP F7 trajectory on 5 April 1986.

6(a) a n d (b). In the first crossing, the high conductance zone is seen at latitudes 62-66 ° a n d a local small increase of Z . is seen at (1) ~ 68". The current profiles also d e m o n s t r a t e the increasing westward electrojet at (I) = 63 66 ~' a n d a local m a x i m u m in the westward currents at (I)= 68-69~L The c o n f o r m i t y between satellite a n d g r o u n d based data vanishes for latitudes higher t h a n 71°. This is quite explicable since the satellite crossed latitudes (I) > 71 '~ in the post midnight sector, whereas the meridional chain of the stations was in the p r e - m i d n i g h t sector at t h a t time. In the second crossing, the satellite crossed the meridional chain just at latitude (I) = 70 °, a n d we c a n see a certain c o r r e s p o n d e n c e between the locations of m a x i m u m c o n d u c t a n c e ZH a n d westward currents in the poleward p a r t of the auroral oval a n d not only at latitudes 64-69 ~.

In the three cases examined, the satellite crossed the poleward part o f the auroral oval away from the m i d n i g h t meridian, a n d the second m a x i m u m of YH at latitudes (I) > 70' was either only outlined [Figs 5 a n d 6(b)] or absent [Fig. 6(a)]. The closer the satellite was to m i d n i g h t at this part o f the trajectory, the higher is the secondary m a x i m u m of Hall c o n d u c t a n c e at latitudes higher t h a n 70 ° . This regularity is clearly seen in the case of the s u b s t o r m s on 1 April 1986 [Fig. 7(a)] a n d 7 M a y 1986 [Fig. 7(b)]. On 1 April, the satellite crossed latitude 70 ° where the second m a x i m u m of ZH was seen just after the m i d n i g h t and, in this case, the values o f EH at (I)= 67-68 c' a n d (1) > 70' were o f the same magnitude. O n 7 M a y 1986 the satellite crossed the m i d n i g h t meridian at latitude (I) = 73 ~, a n d the m a g n i t u d e of the second m a x i m u m o f Y'H turned out to be higher t h a n that of Y.. at

181

Double structure of ionospheric conductivity 05.04.1986

nT

09.04.1986

400. -1 HEISS ISL.

HEISS ISL.

200i 0 -200

-4oo--

J, ~VIZE

0 -200 - 4 0 0 :_ 2OO 0 -200-~

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i

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'

'

i

i

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i

,

7

l

I

~

18

I

19

20

UT

16 . - . .-. .

17

18

UT

7 '

19

20

FIG

Fig. 3. Variations of the geomagnetic H and Z components at stations of the Kara chain for the substorms on 5 and 9 April 1986.

latitudes 64-68". Unfortunately, the Kara meridional chain was outside the midnight sector in both of these cases, and the comparison of the satellite and groundbased data was not possible,

4. DISCUSSION

Thus, the particle precipitation data obtained in crossings of the midnight auroral zone indicate the existence of two regions of high Hall conductance during substorm activity. One of them is a traditional wide zone, with local inhomogeneities at latitudes 64~ 6 8 , and the other is a narrow region at latitudes 707 3 . The positions of these regions agree with the location of the main westward electrojet and the additional westward current filament on the poleward

edge of the auroral zone. Both phenomena are observed in the same longitudinal intervals ; the inten-sity of the westward currents seems to be proportional to the conductance when both values are observed simultaneously. This means that westward currents in the midnight sector are associated with the regions of high ionospheric conductance. This result is in con-. trast with the conclusion of KAMIDE e t al. (1986) and KAMIDE (1988a,b) that the locations of the maximum westward electrojet and maximum integral Hall con-. ductivity do not coincide, and that the westward electrojet in its poleward part is mainly due to the electric: field. This discrepancy in the results may be caused by some difference in the methods of calculation of current parameters used in our study and in the works of

KAMIDE

e t al.

Observations of the aurora by the UV-Imager

182

A . L . KoTIKOvetal. 05.04.1986

09.04.1986

,5~0-

18.50

0 -5000

09.04.1986

rlllJl~

iiJ

17.55

19.16

ill

17.36

0

.,._/J

19.17

-5000

18.57

0

17.37

19.18

-,50~

17.38

0"

-5000 O-

18.59

17.39

19.00

17.40

19.01

17.41

-500e -~

°i

-5000

0 -5000

w I I I i w t I II~l/t'i 65 67 69

71

73

19.22 l,,,,,,

75

, . . . .

,,I

',s

Fig. 4. Meridional profiles of the ionospheric currents calculated using magnetic data for the substorms on 5 and 9 April 1986.

onboard the V I K I N G satellite indicate a stable auroral emission on the poleward edge of the auroral oval (MuRPHREE et al., 1990). According to KOTIKOV et al. (1991, 1993), the aurora and westward currents develop both within the main body of the oval and on the poleward edge of oval during the substorm expansion phase. Assuming that the latitudinal changes in Hall conductance are stimulated by different spectra of precipitating particles, we have examined the spectra obtained by the spacecraft while crossing the midnight auroral zone. Examples of such spectra are given in Fig. 8(a) for the event on 5 April 1986 and in Fig. 8(b) and (c) for the events on 9 April 1986. The spectra in the left column with pronounced spikes of precipitating electrons at energies above some keV are typical of regions with maximum values of ZH. The smooth spectra at high energies in the right column are typical of regions with a m i n i m u m conductance. To obtain quantitative characteristics of these differences, the spectra have been treated as a superposition of Maxwellians. The following Maxwellian parameters have been calculated : density, thermal energy

April 05, 1986 (18.55-19.02 UT)

?-

10

E o

5 o

21

i i i i i F i i 60 t 611 612 613 ~ 61 615 616 t 617 I 6~ 619 t 710 711 i 7j 2 t 713 714 7r5 CGM

latitude

Fig. 5. Hall conductance calculated along the DMSP F7 trajectory on 5 April 1986.

and bulk flow velocity. Plasma with parameters Epe,ak_ 5keV, density n = 0 . 1 1 cm 3 thermal energy E,h..... ~ 0.2 1.0 keV and bulk flow velocity

183

Double structure of ionospheric conductivity

April 09, 1986 (17.34-17.41

UT)

lO v

0 rE 09 U o

"5 "O c O O o "r

]

60

I t I I I I I I I I D I ~ I J I J I J I ~ I I I I 1 I I J I 61 6 2 6 3 6 4 6 5 6 6 6 7 6 8 6 9 7 0 71 7 2 7 3 7 4 7 5 CGM

latitude

April 09, 1986 (18.55-19.02

"Z v

UT)

10

0 rE

c: o

O o

g "1"

60

J l S l l l Z l l l l l l l Z l ~ t l l J l l l 61 6 2 6 3 6 4 6 5 6 6 6 7 6 8 6 9 7 0

71

72

J i r l ~ l 73 74

75

CGM latitude Fig. 6. The same as Fig. 5 but for 9 April 1986.

co ~ 3.104 km/s may be identified as a b o u n d a r y plasma sheet (BPS) p o p u l a t i o n , whereas the other plasma type with E p e a k ~ 3 keV, n ~ 10 cm ~,

E t h ..... ~ 2 3 keV, ~,,, ~ 0 km/s m a y be regarded as a central plasma sheet (CPS) population. We can see t h a t b o t h types of plasma p o p u l a t i o n are presented in

184

A.L. KOTIKOVet al.

01 April 1986 16.54-17.01

UT

10

07

Moy

1986

14.50-14.55

UT

10

%o

g ¢I

8 --r

10 -i

60

rl I Illl"ll Jl Illl 62 6466

IIt 68

Ill 70

tilr 72

I=l I Ill 74 76

10 .I

60

i l I I ;111 l l l l l l l l l l l l l = = l I I r 62 64 66 68 70

lJ t t l 74 76

72

Latitude

Lotitude

1 0 =. •~

70.0

=

I0" 10 =.

10 ~1~

~•



*

12:1

X

2

o

i

10 =.

= 70.0

4'

1 0 ~'.







.,\. ,,, •

7"*

q>

,o

LJJ 1 0 '

i i

IO I

IIIII I

10 '

I

I llllll[

I

10 =

I IlIIIli

I

I I

10"

Energy

'lilll

I

10 "

I

I IIIIll

I

I

I 11111t I

lO =

I

I I

10"

Energy

Fig. 7. Hall conductance calculated along the DMSP F7 trajectory on l April and 7 May 1986 (upper panel) and the appropriate energy spectra of electrons precipitating at latitude 70 (bottom panel).

regions of maximum and minimum conductance. This experimental result supports the conclusion of SAND A n e et al. (1990) that the exact differentiation of particles precipitating in the nighttime auroral oval into two plasma regimes BPS and CPS is not real. Only such a parameter as the bulk flow velocity turns out to be different in regions of maximum and minim u m conductance. Undisplaced Maxwellian distributions with bulk velocity v, = 0 are typical of regions of low ZH, and the accelerated Maxwellian distribution with v, ~ 1 3.104km/s takes place for energies Ee > 5 keV in regions with high 2H. This implies the action of specific mechanisms of particle acceleration in regions of maximum Hall conductance.

5. CONCLUSION 1. TWO regions of high Hall conductance are detected in the midnight auroral oval during substorm activity. One of them is a wide zone at latitudes 6 4 ~ 8 ' , and the other is a narrow band at latitudes 7(~73 . 2. The position of the high conductance regions agrees with the location of the intense westward currents. 3. The accelerated particle population is typical of electrons E~ > 5 keV in the high conductance region. Acknowledgement The DMSP particle detector data were provided by the U.S. Air Force Geophysical Laboratory, and the data were released to the National Data Center (Boulder) for scientific use.

X

101

10'

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10"

.

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1

10"

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10+

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Energy

10.

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~

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"

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electrons at different latitudcs for the substorm

10 ¢

+

lO =

lO'

+o

;o

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,o ,,-.,m

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09.04.1986 17.21.30

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x ~'10"

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lO' to"

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05.04.1986 i8.44.28

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° *

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lol

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70.0

101

= 73.0

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101

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10"

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5 =

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186

A. L. KOTIKOV et al. REFERENCES

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1989

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1988b

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1993

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1990

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1982

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1975

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1982

[ 987

1990

1993

Estimation of ionospheric electrodynamic parameters using ionospheric conductance deduced from bremsstrahlung X-ray image data. ,I. ,qeophys. Res. 94, 2565. Incoherent scatter measurements of E-region conductivities and currents in the auroral zone. J. 9eophys. Res. 79, 3773. A maximum-entropy technique for deconvolution of atmospheric bremsstrahlung spectra. Space Sei. Lah. Rep. No. SSL-86(6940-06)-06, Inferring field-aligned current systems and other ionospheric quantities from ground-based arrays: a review. Ash,. Space Res. 8, no. 9 10, 333. Recent issues in studies of magnetosphere-ionosphere coupling. J. yenmogn. Geoeleet. 40, 131. Modeling substorm current systems using conductance distributions inferred from DE auroral imagers. J, .qeophys. Res. 91, 11235. Combining electric field and aurora observation from DE I and 2 with ground magnetometer records to estimate ionospheric electromagnetic quantities. J. yeophys. Res. 94, 6723. Ionospheric conductivities, electric fields and currents associated with auroral substorms measured by the EISCAT radar. Phmet. Space Sei. 36, 1359. Structure of auroral electrojets by the data from a meridional chain of magnetic stations. Geophysica 23, 143. Structure of the auroral zone phenomena by the data of meridional chains of stations: magnetic disturbances in the nighttime auroral zone and auroras. J. atmos, terr. Phys. 53, 265. Filamentary structure of the westward electrojet in the midnight sector auroral distribution during substorms: comparison with Viking aurora/ observations. J. atmos, terr. Phys. 55, 1763. A new method to derive "instantaneous" high-latitude potential distributions from satellite measurements including auroral imager data. Geophys. Res. Lett. 14, 439. The large-scale conjugacy of the auroras: a first look as part of the C D A W - 9 effort. EOS71, no. 17, 593. Distribution of auroral precipitation at midnight during a magnetic storm. J. qeophys. Res. 95, 6051.

A new approach to calculate the excitation of atmospheric gases by auroral electron impact. Ann. Geoph.ls. I 1, 717. Precipitating electron energy flux and auroral zone conductances- an empirical model. J. #eophys. Res. 87, 8215. Role of field-aligned currents in generation of highlatitude magnetic disturbances. Planet. Space Sci. 27, 1451. Conductivity of polar ionosphere. Geoma#n. Aeron. 15, 847. Energy deposition by precipitating particles and Joule dissipation in the auroral ionosphere. J. 9eophys. Res. 87, 5184.