Response of the ionosphere–thermosphere system to magnetospheric processes

Response of the ionosphere–thermosphere system to magnetospheric processes

ARTICLE IN PRESS Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 2358–2373 Contents lists available at ScienceDirect Journal of Atmos...

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ARTICLE IN PRESS Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 2358–2373

Contents lists available at ScienceDirect

Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp

Response of the ionosphere–thermosphere system to magnetospheric processes R.W. Schunk , L. Zhu Center for Atmospheric and Space Sciences, Utah State University, Logan, UT 84322-4405, USA

a r t i c l e i n f o

abstract

Article history: Accepted 8 July 2008 Available online 19 July 2008

The magnetosphere–ionosphere–thermosphere system at high latitudes is strongly coupled via electric fields, particle precipitation, plasma and neutral outflows, and fieldaligned currents. Although the climatology of the coupled system is fairly well established, our understanding of the variability of the disturbed state (weather) is rudimentary. This variability is associated with magnetic storms and substorms, nonlinear processes that operate over a range of spatial scales, time delays, and feedback mechanisms between the different domains. The variability and resultant structure of the ionosphere can appear in the form of propagating plasma patches and polar wind jets, pulsing ion and neutral polar winds, auroral and boundary blobs, and ionization channels associated with polar cap arcs, discrete auroral arcs, and storm-enhanced densities (SEDs). The variability and structure of the thermosphere can appear in the form of propagating atmospheric holes, neutral gas fountains, neutral density patches, and transient neutral jets. In addition, during periods of enhanced plasma convection, the neutral winds can become supersonic in relatively narrow regions of the polar cap. The spatial structure in the ionosphere–thermosphere system not only affects the local environment, but the cumulative effect of multiple structures may affect the global circulation and energy balance. A focused topical review of recent results in our modeling the variability and structure of the high-latitude ionosphere–thermosphere system is presented. This review was given at the Greenland Space Science Symposium (May 2007). & 2008 Elsevier Ltd. All rights reserved.

Keywords: Thermosphere Ionosphere High latitudes Simulations

1. Introduction In addition to solar EUV and UV radiation, magnetosphere electric fields, particle precipitation, field-aligned currents, heat flows, and ion and neutral escape fluxes also affect the ionosphere–thermosphere system at high latitudes (cf. Schunk and Nagy, 2000). The strength and form of the effect is determined to a large degree by the solar wind dynamic pressure and the orientation of the interplanetary magnetic field (IMF). The various driving forces act in concert to determine the state of the ionosphere–thermosphere system. Because the driving

 Corresponding author. Tel.: +1 435 797 2978; fax: +1 435 797 2992.

E-mail address: [email protected] (R.W. Schunk). 1364-6826/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2008.07.003

forces display characteristic features, the ionosphere– thermosphere system also displays characteristic features, and these can be regarded as the climatology of the system. When the IMF is southward, the characteristic features include a tongue of ionization, a polar hole, a main trough, and an overall electron density enhancement in the auroral oval. However, the ionosphere–thermosphere system at high latitudes typically exhibits a significant variation from hour to hour, and it contains a considerable amount of structure. These weather features occur because the driving forces can be localized, spatially structured, and unsteady (Fig. 1), and because there are time delays in the system with regard to when a driving force acts and when the system responds. The structure in the system varies from less than a meter to 1000 km. For the ionosphere, it can appear in the form of propagating

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Fig. 2. Electron densities measured by the Chatanika radar on March 1, 1982 when it was in the aurora oval. The two altitude–latitude scans are separated by 10 min. The electron density structures are due to ongoing electron precipitation (from Tsunoda, 1988).

Fig. 1. An example of a highly structured plasma convection pattern. Shown are measured ion drift velocities in the high-latitude F-region in a magnetic latitude-MLT reference frame (from Frank, 1986).

plasma patches and polar wind jets (Fukui et al., 1994; Demars and Schunk, 2005), pulsing ion and neutral polar winds (Gardner and Schunk, 2008), auroral and boundary blobs (Tsunoda, 1988), and ionization channels associated with polar cap arcs, discrete auroral arcs, and stormenhanced densities (SEDs) (Zhu et al., 2005; Foster et al., 2005) (Fig. 2). The structure in the thermosphere can appear in the form of propagating atmospheric holes (Ma and Schunk, 1995), neutral gas fountains (Luhr et al., 2004; Demars and Schunk, 2007), neutral density patches (Schlegel et al., 2005), and transient neutral jets (Luhr et al., 2004). In addition, during periods of enhanced plasma convection, the neutral winds can become supersonic in relatively narrow regions of the polar cap (Balthazor and Bailey, 2006). The article is a focused topical review of our recent modeling of the variability and structure of the highlatitude ionosphere–thermosphere system. This review was given at the Greenland Space Science Symposium in May 2007. The review covers selected topics concerning geomagnetic storms and substorms, the cusp neutral fountain, propagating plasma patches, and the ion and neutral polar winds, with the emphasis on the mesoscale (100–1000 km) structures.

2. Storms Geomagnetic storms can be caused by a number of transient plasma processes on the Sun and in the solar

wind. These processes include solar flares, interplanetary magnetic clouds, shock waves in the solar wind, coronal mass ejection events, etc. (McPherron, 1991). More recently, the Corotating Interaction Region (CIR)-driven storms, which are associated with the corotating interaction regions on the Sun and have an approximate 27-day period, have drawn increasing attention in the space science community (Borovsky and Denton, 2007). The typical representation of a geomagnetic storm on the Earth’s surface recorded by magnetometers at lowand mid-latitudes is an increase in the magnetic field lasting a few hours, a large decrease in the H component, and a slow recovery. The magnitude of the reduction is about several hundreds nT. The physical processes connected with the reduction of the B field on the Earth’s surface include a significant enhancement of the ring current, enhanced convection in the magnetotail, compression of the magnetosphere, and an earthward move of plasmapause (McPherron, 1991). Even though the main manifestations of storms are the B field variations at lowand mid-latitudes and the enhancement of the ring currents, they actually produce significant variability and various structures at high latitudes. First of all, during most of the storm periods, there are always substorm activities at high latitudes (the variability of the ionosphere caused by substorms will be discussed in Section 3). In addition, geomagnetic storms significantly enhance the convection field in the high-latitude regions, expand the size of polar cap, increase the precipitation in the auroral oval, and produce various subauroral plasma phenomena, including SEDs (Foster et al., 2005). As an example, Fig. 3 shows the expansion of auroral oval and the aurora activities during the Bastille Day Storm (July 14–15, 2000) recorded by the IMAGE satellite (Burch et al., 2001). These images are 12-min snapshots over a 1-h period. Over the years, by using global time-dependent models of the ionosphere, the group at the Utah State University

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Fig. 3. Snapshots of the IMAGE satellite images over a 1-h period. From top-left to bottom-right, the times are 14:37:27, 14:39:29, 14:47:40, 14:59:56, 15:12:12, and 15:36:44 (from Burch et al., 2001).

(USU) has performed numerous theoretical modeling, model-observation comparisons, and data assimilation studies of the response of the ionosphere to storms and the variability and structures associated with them (Schunk, 1988; Sojka, 1989). Figs. 4 and 5 show the results from a sample of these studies. Fig. 4 shows the comparison of NmF2 for a storm period and a quiet period. It is clear that during a storm time, NmF2 decreases significantly over a large region on the nightside, while the oval expands. The decrease of NmF2 is connected with a downward ExB drift and an O/N2 composition decrease. For a southward IMF, the electric field points from dawn-to-dusk and in combination with the tilt of the geomagnetic field a downward component of the electromagnetic drift occurs (cf. Schunk and Nagy, 2000; Fig. 12.6). Since the ions and electrons exhibit the same electromagnetic drift, no current is associated with it. The expansion of the oval is related to a significant enhancement of convection field during the storm. Fig. 5 shows the comparison of the O+/Ne ratio at an altitude of 300 km for the storm period and quiet time. During the storm, O+ decreases significantly over a region including both the polar cap and the oval. Due to the enhanced convection field during the storm, frictional heating between the ions and neutrals increases, which leads to an increase of the ion temperature (not shown). Associated with the elevated Ti is a substantial O+ to NO+ composition change. This composition change, an O/N2 decrease, and a lowering of the ionosphere due to a

Fig. 4. NmF2 distributions in a magnetic latitude–MLT coordinate system for storm and quiet periods. The color scale is log10 (NmF2).

downward ExB drift eventually cause the decrease of O+ at 300 km.

3. Substorms The magnetospheric substorm (Akasofu, 1964, 1968) is another magnetospheric process that causes significant

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Fig. 5. O+/Ne ratios at 300 km in a magnetic latitude–MLT coordinate system for storm and quiet periods.

variability of the ionosphere–thermosphere system and produces various unique mesoscale plasma and electrodynamic structures. The response of the ionosphere– thermosphere system to substorms is significantly different from that during quiet times. A substorm is a transient process in which a significant amount of energy derived from interactions between the solar wind and the Earth’s magnetosphere is suddenly released into the ionosphere and thermosphere (Zhu, 2005). To rigorously study the variability of the ionosphere during substorm periods, we coupled the USU time-dependent ionospheric model (TDIM) (Schunk, 1988; Sojka, 1989) and a high-latitude M–I electrodynamic model (Zhu et al., 1993) so that we could self-consistently study the development of mesoscale electrodynamic and plasma structures in the high-latitude ionosphere during substorm times. A unique strength of the M–I electrodynamic model is the adoption of the Alfven wave approach. Alfven waves are launched in association with the enhanced magnetospheric convection during substorms. When the Alfven waves arrive in the ionosphere, they can be partially reflected, and the features of the reflected Alfven waves depend on the conductivity distribution in the ionosphere. At the same time, precipitation associated with the Alfven waves enhances the ionosphere conductivity, and the temporal change of the ionospheric conductivity launches new Alfven waves toward the magnetosphere. These Alfven waves bring the characteristic spatial and temporal scales of the ionosphere into the M–I coupling, which reflect the active role of the ionosphere in the M–I system. The Alfven waves are responsible for the generation of many mesoscale plasma and electrodynamic structures during substorms (Streltsov and Lotko, 2004; Zhu et al., 2005). Coupling of our global ionosphere model to the M–I electrodynamic model with such a unique strength allows us to study both the global and mesoscale plasma and electrodynamic features of substorms in a self-consistent way. The example of a substorm run we will show in this topical review is for the condition of winter solstice and low solar activity. Before the substorm simulation started,

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the TDIM was run for 24 h using the quiet-time convection and precipitation patterns provided by the M–I electrodynamic model. This procedure ensured that all quiettime physical quantities in both the models are selfconsistent and diurnally reproducible. The substorm started at 1200 UT with an enhanced convection carried by Alfven waves. Fig. 6 shows snapshots of two of the magnetospheric drivers from the M–I–TDIM coupled model, the energy flux of the precipitating electrons and the magnitude of the convection electric field. In addition, Fig. 6 shows snapshots of the time-varying Hall conductance patterns calculated by the coupled model. The growth phase started at 1200 UT and the expansion phase started at 1224 UT. The expansion maximum was achieved at around 1235 UT. The top panels of Fig. 7 show the field-aligned current distributions for a presubstorm time and the substorm expansion. The bottom panels show the ratio of the fieldaligned currents that are closed by the Hall currents to the total field-aligned currents and the ground magnetic disturbances caused by the substorm currents. From Figs. 6 and 7, we can see that mesoscale-structured aurora and precipitation, as well as channeled field-aligned currents, develop in the aurora breakup region during the expansion phase. The scale size of the channeled precipitation or the upward–downward current pairs is less than 100 km, and such a characteristic spatial scale is not intrinsically contained in the magnetospheric drivers during substorms. It is the M–I coupling process via propagating Alfven waves, in which the temporal and spatial scales of the ionosphere are added in, that leads to the development of these mesoscale structures and this reflects the active role of the ionosphere in the substorm processes. The regions with strong precipitation (Fig. 6; bottom-left dial) and strong electric field (Fig. 6; bottommiddle dial) are spatially separated by a distance of about 1000 km. In the aurora breakup region, the majority of the field-aligned currents are closed by the Hall currents, which is different from the situation for presubstorm or quiet times, where the field-aligned currents are almost solely closed by the Pedersen currents. The localized closure of the field-aligned currents to the Hall currents causes a significant distortion and rotation of the electric field in the substorm onset regions. The sharp gradients and great distortions of the ground magnetic disturbance patterns collocate with the regions of strong electric field distortion. Connected to the mesoscale electrodynamic structures shown above, various mesoscale plasma and temperature structures develop during the substorm. Fig. 8 shows the changes of ion temperature at 200 km and electron temperature at 600 km. Corresponding to the regions with strong precipitation and electric field, there are a Te hot spot (in the aurora breakup region) and a Ti hot spot (near midnight), where Te (600 km) increases to 6000 K and Ti (200 km) increases to 2000 K. The increase of Ti is caused by a strong frictional heating between the ions and neutrals due to a strong electric field. The increase of Te in the substorm onset region is directly connected to a hard auroral precipitation, which acts to increase Te at all altitudes.

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Fig. 6. Snapshots of distributions of the energy flux, the magnitude of the electric field, and the Hall conductance during a simulated substorm (from Zhu et al., 2005).

During the substorm, there is a significant lowering of the ionosphere in the substorm onset region. This feature is shown in the top panels in Fig. 9. Due to a downward ExB drift, the HmF2 decreases from 280 km (presubstorm) to 200 km (expansion maximum). Also, TEC decreases in this large region and the decrease of the TEC is caused by the lowering of the ionosphere and the consequent increased recombination. During the substorm, the plasma density distribution in the ionosphere also changes significantly and various mesoscale plasma structures

develop. The bottom panels of Fig. 9 show the changes of plasma density at an altitude of 800 km. In an extended region near midnight, the Ne at 800 km decreases when the substorm develops and the decrease of plasma density at altitudes above 300 km in this region is due to a largescale downward ExB drift. But for the plasma density at altitudes below 300 km, especially in the aurora breakup region, the ionization associated with hard auroral precipitation overwhelms the effect of the lowering of the ionosphere and actually causes the increase of plasma

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Fig. 7. Distributions of the field-aligned currents for the presubstorm and substorm expansion phase (top), the Hall current contribution to the fieldaligned currents, and the Z component of ground magnetic disturbances (bottom). The distributions are shown in a magnetic latitude–MLT coordinate system.

density. The strong spatial variability of the plasma density in both the horizontal and vertical directions is one of the characters of the response of the ionosphere during substorms. These mesoscale electrodynamic and plasma structures in the ionosphere during substorms have a sig-

nificant theoretical and observational importance. They not only elucidate the multiscale ionospheric responses to substorms and the active role played by the ionosphere, but also provide testable theoretical substorm predictions as well as cautions for the interpretation of various substorm observational data.

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Fig. 8. Variations of Ti at 200 km and Te at 600 km for the prestorm (left) and expansion phase (right) of the substorm. The coordinate system is the same as for Fig. 6 (from Schunk et al., 1997).

4. Cusp neutral fountain CHAMP satellite observations at 400 km indicate that the neutral density in the cusp can be nearly a factor of two larger than that in the adjacent regions (Luhr et al.,

2004). It was suggested that Joule heating of the neutral gas at lower altitudes caused upwelling, which led to the density enhancement measured by CHAMP. In a follow-up study with a more comprehensive CHAMP data set (Schlegel et al., 2005), numerous neutral density peaks

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Fig. 9. Variations of HmF2 and electron density at 800 km for the prestorm (left) and expansion phase (right) of the substorm.

and troughs were observed in the polar region. The width of the structures varied from a few hundred km to 2000 km and the amplitudes of the structures approached 50% of the background density. The density peaks clustered around the cusp, while the troughs tended to

be located near the pole. In comparisons of the observed density peaks and troughs with the predictions of global ionosphere–thermosphere models, it was concluded that these models do not produce neutral density structures at the CHAMP satellite altitude, and therefore, are not

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consistent with the CHAMP measurements (Schlegel et al., 2005). A possible problem with the previous global ionosphere–thermosphere simulations could simply be that the spatial resolution used in the simulations was not adequate and that the Joule heating was not strong enough in the vicinity of the cusp. To test this hypothesis,

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Fig. 10. Neutral wind vectors as a function of altitude and geographic colatitude from the pole (left side), through the cusp, to 401 on the dayside (right side) for a longitude of 1251 east in the northern hemisphere. The region of imposed cusp heating lies between the two arrows shown at the top of the figure (from Demars and Schunk, 2007).

a global, time-dependent thermosphere–ionosphere model was used to simulate the thermosphere’s response to ion heating in the dayside cusp (Demars and Schunk, 2007). The thermosphere model that was used is based on a numerical solution of the neutral gas continuity, momentum, and energy equations. In the model, the coordinate system is fixed to the rotating Earth, and the equations are solved versus altitude (non-hydrostatic equilibrium is assumed). The numerical resolution was 31 in latitude, 51 in longitude, and 49 altitude layers from 97 to 500 km. This model was used to conduct a series of relatively simple simulations, whereby the frictional heating in the cusp was increased during a 3-h period until a factor of 2 neutral density enhancement at 400 km relative to background densities was attained. It was found that increased ion-neutral frictional heating in the cusp results in the formation of a ‘neutral fountain’, with upwelling of the neutral gas in the heated region and a divergence and gradual subsidence of the gas outside of the heated region at higher altitudes. Figs. 10–12 show wind vectors, wind magnitudes, and neutral temperatures for the case where a factor of 2 neutral density enhancement at 400 km in the cusp was obtained. These figures correspond to snapshots 3 h after the cusp heating was imposed and they show neutral distributions versus altitude and co-latitude across the cusp along the midnight to noon meridian. The cusp heating results in a 200 m/s vertical wind in the cusp, an enhanced antisunward wind in the polar cap, and a sunward wind near the cusp on the dayside. The neutral temperature in the cusp reaches 1500 K on the poleward side of the cusp, which is where anti-sunward convection and frictional

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Fig. 11. Magnitude of the upward neutral wind as a function of altitude and geographic co-latitude from the pole (left side), through the cusp, to 401 on the dayside (right side) for a longitude of 1251 east. The region of imposed cusp heating lies between the two arrows shown at the top of the figure. The upward neutral wind has been normalized by dividing by the mean radius of the Earth. The upper end of the color scale (0.12) corresponds to 212 m/s. The calculated downward drifts are small and are not shown (from Demars and Schunk, 2007).

Fig. 12. Neutral temperature as a function of altitude and geographic colatitude from the pole (left side), through the cusp, to 401 on the dayside (right side) for a longitude of 1251 east in the northern hemisphere. The region of imposed cusp heating lies between the two arrows shown at the top of the figure. The color scale is K (from Demars and Schunk, 2007).

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Fig. 13. Propagating plasma patches observed at Qaanaaq, Greenland, on October 29, 1989. The dials show digitized all-sky images taken at 2-min intervals. The shaded areas show plasma patches moving in an anti-sunward direction (from Fukui et al., 1994).

heating are elevated due to the adopted convection pattern. There is also an enhanced neutral temperature on the sunward side of the cusp, which is where the cuspgenerated sunward wind meets the sun-driven antisunward wind. The slowdown of the wind on the sunward side of the cusp leads to increased ion-neutral frictional heating. Although ion-neutral frictional heating in the cusp can account for the CHAMP measurements, it is possible that this is not the only mechanism responsible for the elevated neutral densities in the cusp. Other possible mechanisms include heating via particle precipitation, field-aligned currents, solar EUV radiation, and the dissipation of atmospheric gravity waves (Schlegel et al., 2005).

5. Propagating plasma patches As noted above, plasma structures cause perturbations in the neutral gas. To illustrate this process, the effect that propagating plasma patches have on the thermosphere will be shown. Plasma patches are regions of enhanced ionization that appear when the IMF turns southward (Fukui et al., 2004). They are created either in or near the cusp and then drift in an anti-sunward direction across the dark polar cap with the prevailing convection speed. The size of the patches varies from 100 to 1000 km, and their density varies from 10% to a factor of 10 above background densities. The patches can appear as single, isolated, plasma structures or a series of plasma structures, and the patches can be nearly circular or highly elongated (cigar-shaped) in the horizontal direction perpendicular to their drift (cf. Schunk and Nagy, 2000). Global thermosphere–ionosphere simulations involving single, isolated, propagating plasma patches indicate that a propagating plasma patch acts as a collisional snowplow, creating a hole in the thermosphere in and behind the plasma patch and a neutral density enhancement at the front of the patch (Ma and Schunk, 1995). For a plasma

Fig. 14. Snapshots of the electron density distribution at 300 km at selected times after propagating plasma patches are imposed on the ionosphere. The time t ¼ O+ corresponds to the instant after t ¼ 0 that the first plasma patch is applied (from Ma and Schunk, 2001).

patch with a peak density that is a factor of 10 greater than the background plasma density, there is a 30–35% perturbation in neutral mass density due to the presence

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of the propagating plasma patch. The neutral disturbance that is induced by the propagating plasma patch moves along with the plasma patch and is characterized by an increased wind speed (Du4100 m/s), a temperature enhancement (DTE100–300 K), neutral gas upwelling, and O/N2 composition changes (Ma and Schunk, 1995, 1997). Multiple plasma patches are typically elongated in a direction perpendicular to their generally anti-sunward motion, with horizontal dimensions of about 200 km  2000 km (Fig. 13). The spacing between the plasma patches is approximately equal to the width of the patches. Ma and Schunk (2001) used a global time-

dependent thermosphere–ionosphere model to calculate the thermosphere’s response to a series of cigar-shaped propagating plasma patches. The model used in this study is the same one discussed above in connection with the cusp neutral fountain. In the study, a series of propagating plasma patches was imposed on the ionosphere, but the width, length, direction of propagation, and density of the patches in the series were determined from optical measurements of the 630 nm emissions associated with the patches. In the simulation, a diurnally reproducible, fully global, thermosphere was first calculated, and then at 2.65 UT (t ¼ 0), which is when the IMF turned southward and convection increased, the series of propagating

Fig. 15. Effect of multiple propagating plasma patches on the thermosphere at t ¼ 3 h. The upper-left panel shows the ne distribution at 300 km in units of log10 ne(cm3), the upper-right panel shows the neutral density perturbation at 300 km, and the lower panel shows the neutral density perturbation as a function of altitude and latitude across the polar cap (from Ma and Schunk, 2001).

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plasma patches was introduced in the southern polar region. The patch dimensions were 200 km  1000 km and the plasma density in the patches was a factor of 10 greater than the background density. The patches were introduced in the vicinity of the cusp at a half-hour interval, as observed in the measurements, and then they propagated in an anti-sunward direction with the prevailing convection speed (Fig. 14). Thermosphere simulations were conducted both with and without the propagating plasma patches and the two simulations were subtracted so that the disturbance induced by the plasma patches could be clearly seen. Fig. 15 shows the simulation results for moderate solar activity (F10.7 ¼ 150), a polar cap potential of 100 kV and winter conditions. The results correspond to snapshots 3 h after the first plasma patch was introduced in the vicinity of the cusp. The top-left panel shows the electron density distribution at 300 km, the top-right panel shows the neutral density perturbation at 300 km, and the bottom panel shows the neutral density perturbation as a function of altitude and latitude. At this time (3 h), the neutral density perturbation across the plasma patches is about 30% (20%– +10%), the neutral temperature perturbation is about 150 K (not shown), and the disturbance covers the bulk of the polar cap. For other cases, the neutral density perturbation can be greater than 30%, the neutral temperature perturbation can be as large as 400 K, and the entire polar region can be affected by the presence of multiple propagating plasma patches. The bottom panel of Fig. 15 shows the neutral density perturbation via a 2-dimensional cut across the polar cap from the dayside to the nightside and through the center of the series of patches. The plasma patches are located in the F-region and above, but their effects on the neutral density and temperature propagate to lower altitudes. However, there is a time delay; the neutral perturbations at the E-region altitudes lag behind those at the F-region altitudes.

6. Ion polar wind There is a continuous and substantial ion outflow from both the northern and southern polar regions of the ionosphere. The outflow consists of light thermal ions (H+ and O+) as well as energized ions (NO+, O+2, N+2, O+, N+, He+, and H+). The ion outflow that is driven by pressure gradients in the F-region and topside ionosphere is called the ‘classical’ polar wind, while the ion outflow that is driven by energization processes in the auroral oval and at high altitudes in the polar cap is called the ‘generalized’ polar wind. The ion energization is associated with photoelectrons, hot magnetosphere electrons and ions, wave–particle interactions in the oval, electromagnetic wave turbulence above the polar cap, and centrifugal acceleration (Fig. 16). However, the ion outflow occurs in conjunction with magnetosphere convection, which causes the high-latitude plasma to drift into and out of the dayside ionosphere, cusp, polar cap, nocturnal auroral oval, and subauroral nightside ionosphere. Because of this horizontal motion, the drifting plasma is only subjected to a given energization process for a limited

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Fig. 16. Schematic diagram showing the near-Earth space environment of the polar wind (from Schunk and Sojka, 1997).

time, which acts to produce ionosphere structures at all altitudes and regions at high latitudes. Therefore, 3-dimensional, time-dependent modeling is needed to properly describe ionosphere-polar wind structures. There are several papers on the polar wind that provide an update on our current understanding (Barakat and Schunk, 2006; Banerjee and Gavrishchaka, 2007; Lemaire et al., 2007; Schunk, 2007; Tam et al., 2007; Yau et al., 2007). A substantial plasma structuring occurs even for the classical polar wind and even when relatively simple time-dependent plasma convection and particle precipitation patterns are adopted (cf. Schunk and Sojka, 1997; Demars and Schunk, 2001, 2002; Schunk et al., 2005). In Schunk and Sojka (1997), an ‘idealized’ geomagnetic storm was simulated by varying empirical (smooth) models of convection (Heppner and Maynard, 1987) and precipitation (Hardy et al., 1985) in time. The model used was a global, 3-dimensional, multi-ion ionosphere-polar wind model that covered magnetic latitudes greater than 501 and altitudes from 90 to 9000 km. The storm had a 1-h growth phase, a 1-h main phase, and a 4-h decay phase. During increasing magnetic activity, the plasma convection and electron precipitation patterns expanded, convection speeds increased, and precipitation became more intense. The reverse occurred during decreasing magnetic activity. The geophysical conditions were for winter solstice and solar maximum. Fig. 17 shows a snapshot of the O+ density versus altitude and magnetic latitude across the polar cap at the end of the main phase. The latitude varies from 451 on the dayside through the magnetic pole to 451 on the nightside. Several peaks of elevated O+ density are evident; e.g., in the dayside ionosphere, in the cusp, in the polar

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cap, and in the nocturnal oval. All of the peaks are associated with the upwelling of O+ due to increased plasma pressure at low altitudes. The increased pressure, in turn, results from enhanced ion and electron temperatures, and plasma densities. The corresponding H+ density distribution at the end of the main phase is shown in Fig. 18. Note that the H+ density distribution exhibits more spatial structure than the O+ distribution, which is due to the fact that H+ is lighter than O+ and, hence, can respond more rapidly to changing storm conditions. Also note that

there are H+ density cavities at certain altitudes. One occurs between 1400 and 2000 km on the nightside at latitudes between 801 and 651. Another occurs in the same altitude range on the dayside near 801 latitude. These density cavities occur because of the complex interplay between chemistry, vertical upwelling, and horizontal convection. A specific cavity could be the result of processes that occurred at an earlier time in a different polar region.

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The global ionosphere-polar wind simulations discussed above indicated that the magnitudes of the upward H+ and O+ fluxes increase markedly during geomagnetic storms, and that they are highly non-uniform and time dependent. This implies that during geomagnetic storms large fluxes of H and O could be created via charge exchange in the polar wind. Specifically, as the H+ and O+ ions flow up and out of the topside ionosphere, they also drift horizontally across the polar region, moving into and out of sunlight, the auroral oval, and the polar cap. During this motion, the ions can undergo charge exchange 50°°

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Log10 O+ Density Fig. 17. Snapshot of the O+ density distribution versus altitude and latitude at the end of the main phase of a geomagnetic storm for winter, solar maximum conditions. The latitudes extend along the noon–midnight meridian through the magnetic pole. Densities greater than 103 cm3 are pink and those below 1 cm3 are dark blue (from Schunk and Sojka, 1997).

Fig. 19. Schematic diagram showing the production of ion and neutral streams as the polar wind ions undergo charge exchange reactions with the background neutral atoms (from Gardner and Schunk, 2004).

Fig. 18. Snapshot of the H+ density distribution versus altitude and latitude at the end of the main phase of a geomagnetic storm for winter, solar maximum conditions. The latitudes extend along the noon–midnight meridian through the magnetic pole. Densities greater than 103 cm3 are pink and those below 1 cm3 are dark blue (from Schunk and Sojka, 1997).

Fig. 20. Schematic diagram of the neutral polar wind. At low altitudes, the neutrals produced by charge exchange (H, O) flow in all directions, because of horizontal plasma convection and ion gyration. At high altitudes, the charge-exchange neutrals primarily flow in the vertical direction because of ion outflow (from Gardner and Schunk, 2004).

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reactions with the background neutral atmosphere, which is composed of both thermal neutrals and a hot neutral geocorona (Fig. 19). An up-flowing O+ ion can undergo a charge exchange reaction with either H or O, and this would yield an up-flowing O atom. This reaction would also produce a non-flowing H+ or O+ ion, and subsequently, this ion would be accelerated upwards by the polarization electric field in the polar wind. The initial velocity of a neutral particle created in the polar wind is equal to the velocity of the H+ or O+ parent ion just before the charge exchange process. Consequently, at high

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altitudes, neutral streams of H and O are created that predominantly flow in the vertical direction (the neutral polar wind), while at low altitudes the neutrals tend to move in all directions owing to ion gyration and plasma convection, as shown in Fig. 20 (Gardner and Schunk, 2004, 2005, 2006, 2008). This general behavior of the neutral stream particles has been observed by the LENA instrument on the NASA IMAGE satellite. This instrument measured large neutral outflow fluxes (1–4  109 cm2 s1), but the neutral particles appeared to be moving in all directions (Wilson et al., 2003).

Fig. 21. Snapshots of ion and neutral flux distributions versus altitude and latitude along the noon–midnight meridian from the dayside to the nightside during a simulated geomagnetic storm. The flux magnitudes are color coded and the arrows show flux directions (from Gardner and Schunk, 2005).

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The energetic neutral atoms produced in the polar wind were simulated with the aid of a global, ion-neutral, transport model that solves the time-dependent continuity, momentum, and simplified energy equations for five species (O+, H+, electrons, H stream, and O stream), with charge exchange, plasma convection, and particle precipitation taken into account (Gardner and Schunk, 2005). This numerical model is similar to the ionosphere-polar wind model developed by Schunk and Sojka (1997), except that the neutral O and H streams are included. Fig. 21 shows snapshots of the flux distributions of the H+ and O+ ions and the HS and OS neutral stream particles versus altitude and latitude across the polar cap during a simulated geomagnetic storm. The latitude extends along the noon–midnight meridian from the dayside (left) to the nightside (right). The color scale provides the flux magnitudes and the arrows show the directions. The H+ and O+ fluxes are clearly structured, especially in the nocturnal oval. The neutral streams particles display much less structure, because the neutral particles are not affected by the magnetic field and can move in all directions. The H+ and HS fluxes are upward across the whole polar region. However, the O+ and OS fluxes are upward in the cusp and nocturnal oval and downward in the polar cap and subauroral region. 8. Summary This brief review has been focused on our recent modeling of the variability and structure that occurs in the ionosphere–thermosphere system at high latitudes, with the emphasis on mesoscale (100–1000 km) structures. The variability and structure in the ionosphere can appear in the form of propagating plasma patches, propagating and stationary polar wind jets, pulsating ion and neutral polar winds, auroral and boundary blobs, ion and electron temperature hot spots, plasma density cavities, and ionization channels associated with polar cap arcs, discrete auroral arcs, and storm-enhanced densities. For the thermosphere, the variability and structure can appear in the form of propagating atmospheric holes, neutral gas fountains, neutral density patches, channels of supersonic flow, and transient neutral jets. With regard to modeling, the main challenge will be to self-consistently incorporate both mesoscale and smallscale (instability) processes in the coupled global models. This will probably require a new generation of global models involving nested grid, adaptive grid, and nested model approaches. These models will be needed to further elucidate the production, evolution, and decay of the wide range of structures and irregularities that occur in the ionosphere–thermosphere system at high latitudes.

Acknowledgements This research was supported by the NASA Sun–Earth Connection Theory Program (SECTP) via Grant NNG05GJ48G and by NSF Grant N0001407-0053 to Utah State University.

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