Subauroral Polarization Streams Research Articles

Multipoint MMS observations of fine‐scale SAPS structure in the inner magnetosphere

AbstractWe present detailed observations of dynamic, fine‐scale inner magnetosphere‐ionosphere coupling at ∼3.9 RE in the Region 2 Birkeland field‐aligned current (FAC). We find that observed electrodynamic spatial/temporal scales are primarily characteristic of magnetically mapped ionospheric structure. On 15 September 2015, conjugate Magnetospheric Multiscale (MMS) spacecraft and Millstone Hill radar observations show plasmasphere boundary region subauroral polarization stream (SAPS) electric fields at L = 4.0–4.2 near 21 MLT. MMS observations reveal high‐altitude ∼1 mV/m fine‐scale radial and azimuthal electric field perturbations over ≤0.15 L with high spatial coherence over ≥2–3 min and show outward motion within a broader FAC of ∼0.12 μA/m2. Our analysis shows that MMS electric field fluctuations are most likely reflective of SAPS ionospheric structure at scales of ∼22 km and with ionospheric closure of small‐scale filamentary FAC perturbations. The results highlight the ionosphere's importance in regulating fine‐scale magnetosphere‐ionosphere structure.

SAPS onset timing during substorms and the westward traveling surge

AbstractWe present multispacecraft observations in the magnetosphere and conjugate ionosphere of the onset time of subauroral polarization streams (SAPS) and tens of keV ring current injections on the duskside in three individual substorms. This is probably the first unequivocal determination of the substorm SAPS onset timing. The time lag between the SAPS and substorm onsets is much shorter than the gradient‐curvature drift time of ∼10 keV ions in the plasmasphere. It seemingly depends on the propagation time of substorm‐injected plasma from the dipolarization onset region to the plasmasphere, as well as on the SAPS position. These observations suggest that fast onset SAPS and ring current injections are causally related to the two‐loop system of the westward traveling surge.

Large‐amplitude electric fields in the inner magnetosphere: Van Allen Probes observations of subauroral polarization streams

AbstractThe subauroral polarization stream (SAPS) is an important magnetosphere‐ionosphere (MI) coupling phenomenon that impacts a range of particle populations in the inner magnetosphere. SAPS studies often emphasize ionospheric signatures of fast westward flows, but the equatorial magnetosphere is also affected through strong radial electric fields in the dusk sector. This study focuses on a period of steady southward interplanetary magnetic field (IMF) during the 29 June 2013 geomagnetic storm where the Van Allen Probes observe a region of intense electric fields near the plasmapause over multiple consecutive outbound duskside passes. We show that the large‐amplitude electric fields near the equatorial plane are consistent with SAPS by investigating the relationship between plasma sheet ion and electron boundaries, associated field‐aligned currents, and the spatial location of the electric fields. By incorporating high‐inclination DMSP data we demonstrate the spatial and temporal variability of the SAPS region, and we suggest that discrete, earthward propagating injections are driving the observed strong electric fields at low L shells in the equatorial magnetosphere. We also show the relationship between SAPS and plasmasphere erosion, as well as a possible correlation with flux enhancements for 100s keV electrons.

A subauroral polarization stream driven by field‐aligned currents associated with precipitating energetic ions caused by EMIC waves: A case study

AbstractDuring the energetic ion injection event observed by the Los Alamos National Laboratory geosynchronous spacecraft, observations of the NOAA 15 satellite and Finnish network of search coil magnetometers have shown that a sharp enhancement of precipitating ring current (RC) ion flux is contributed to the pitch angle scattering caused by electromagnetic ion cyclotron (EMIC) waves. At subauroral latitudes, lower than the equatorward edge of precipitating electrons from the plasma sheet, the DMSP F13 satellite observed a subauroral polarization stream (SAPS) with a peak velocity of 688 m/s. When passing the region of EMIC waves derived by the Finnish network of search coil magnetometers and NOAA 15 satellite, the DMSP F13 satellite simultaneously observed field‐aligned currents (FACs) flowing into the ionosphere and precipitating RC ions in the region of the SAPS. The peak of the SAPS accords to the minimum of the ion density in the region of the SAPS. Our result suggests that loss of RC ions caused by EMIC waves would possibly lead to FACs flowing into the ionosphere and drive the SAPS in the evening sector.

Structured subauroral polarization streams and related auroral undulations occurring on the storm day of 21 January 2005

AbstractWe investigate structured subauroral polarization streams (SAPS) and their impacts on the midlatitude trough and auroral regions during the 21–22 January 2005 geomagnetic storm. This was a storm with two sudden commencements occurring under varying interplanetary magnetic field (IMF) conditions and three main phases, two of them unfolding during northward IMF. Its onset at ~1700 UT allowed us to investigate SAPS wave structures (WS) and their impacts during the local evening hours under both southward and northward IMF conditions in the American sector. Results suggest that during southward IMF, SAPS‐WS might be related with standing (toroidal) Alfven waves and structured most intensively the stagnation trough. During northward IMF, the trough was created by SAPS electric (E) field effects only and became less structured by SAPS‐WS that were of Alfvenic origin. Auroral wave structures and undulations occurred in the structured and unstructured oval regions, respectively, and triggered the subauroral region's response to produce SAPS‐WS. Spectrogram images (only one shown) confirmed ring current injections implying magnetotail reconnections during the four SAPS‐WS events investigated. Periodic tail connections are also evidenced by the periodic increases seen in both the solar wind‐magnetosphere coupling function (ε) and the energy input efficiency. Finally, we conclude for the time period investigated that (1) the nature of SAPS‐WS was informative of IMF orientation, (2) plasma stagnation enhanced SAPS E field and SAPS‐WS development, and (3) more efficient energy input into the magnetosphere contributed to the better development of auroral undulations and SAPS‐WS.

Occurrence characteristics and lowest speed limit of subauroral polarization stream (SAPS) observed by the SuperDARN Hokkaido East radar

We investigate the characteristics of the subauroral polarization stream (SAPS), with focus on the relationship between geomagnetic parameters and occurrence characteristics of SAPS. This study’s observations were made using the Super Dual Auroral Radar Network (SuperDARN) Hokkaido East radar, which can observe the Far East region of Russia and has been in operation since 2006. In particular, we identify the lowest limit of SAPS speed, which has not been discussed in previous literature, in order to examine the lowest threshold of electric field able to generate SAPS as a result of magnetosphere-ionosphere (M-I) coupling. In order to conduct a comprehensive investigation of SAPS occurrence characteristics, we analyzed events with wider ranges of velocity and magnetic latitude (MLAT) than those in previous studies. As a result of quantitative estimation, we found two categories of westward flows that were reasonably separated using a speed threshold of 150–200 m/s. For the faster flows above the speed threshold, there is a clear correlation between MLAT and SYM-H geomagnetic index, whereas for the slower flows, there is no such correlation. The faster flows are considered to be SAPS, whereas the slower flows are probably associated with mid-latitude F-region ionospheric irregularities not directly related to storms or substorms. This slowest limit of SAPS gives us a minimum electric field of 7.5–10 mV/m that generates SAPS. However, this field strength is not strong enough to cause frictional heating, which is generally considered to be a crucial mechanism for generating SAPS. This result suggests that frictional heating is not always necessary to generate SAPS.

Formation of polar ionospheric tongue of ionization during minor geomagnetic disturbed conditions

AbstractPrevious investigations of ionospheric storm‐enhanced density (SED) and tongue of ionization (TOI) focused mostly on the behavior of TOI during intense geomagnetic storms. Little attention has been paid to the spatial and temporal variations of TOI during weak to moderate geomagnetic disturbed conditions. In this paper we investigate the source and development of TOI during a moderate geomagnetic storm on 14 October 2012. Multi‐instrumental observations including GPS total electron content (TEC), Defense Meteorological Satellite Program (DMSP) in situ measured total ion concentration and ion drift velocity, SuperDARN measured polar ion convection patterns, and electron density profiles from the Poker Flat Incoherent Scatter Radar (PFISR) have been utilized in the current analysis. GPS TEC maps show salient TOI structures persisting for about 5 h over high latitudes of North America on 14 October 2012 in the later recovery phase of the storm when the magnitudes of IMF By and Bz were less than 5 nT. The PFISR electron density profiles indicate that the extra ionization for TEC enhancements mainly occurred in the topside ionosphere with no obvious changes in the bottomside ionosphere and vertical plasma drifts. Additionally, there were no signatures of penetration electric fields in the equatorial electrojet data and upward ion drifts at high latitudes. At the same time, strong subauroral polarization streams with ion drift speeds exceeding 2.5 km/s carried sunward fluxes and migrated toward lower latitudes for about 5° based on the DMSP cross‐track drift measurements. Based on those measurements, we postulate that the combined effects of initial build‐up of ionization at midlatitudes through daytime production of ionization and equatorward (or less poleward than normal daytime) neutral wind reducing downward diffusion along the inclined filed lines, and an expanded polar ion convection pattern and its associated horizontal plasma transport are important in the formation of the TOI.

Thermospheric poleward wind surge at midlatitudes during great storm intervals

AbstractWe report a significant poleward surge in thermospheric winds at subauroral and midlatitudes following the 17–18 March 2015 great geomagnetic storm. This premidnight surge is preceded by strong westward winds. These disturbances were observed over three sites with geodetic latitudes 35–42°N in the American sector by Fabry‐Perot interferometers at 630 nm wavelength. Prior to the wind disturbances, subauroral polarization streams (SAPS) were measured by the Millstone Hill incoherent scatter radar between 20 and 02 UT. We identify the observed neutral wind variations as driven by SAPS, through a scenario where strong ion flows cause a westward neutral wind, subsequently establishing a poleward wind surge due to the poleward Coriolis force on that westward wind. These regional disturbances appear to have prevented the well‐known storm time equatorward wind surge from propagating into low latitudes, with the consequence that the classic disturbance dynamo mechanism failed to occur.

Azimuthal flow bursts in the inner plasma sheet and possible connection with SAPS and plasma sheet earthward flow bursts

AbstractWe have combined radar observations and auroral images obtained during the Poker Flat Incoherent Scatter Radar Ion Neutral Observations in the Thermosphere campaign to show the common occurrence of westward moving, localized auroral brightenings near the auroral equatorward boundary and to show their association with azimuthally moving flow bursts near or within the subauroral polarization stream (SAPS) region. These results indicate that the SAPS region, rather than consisting of relatively stable proton precipitation and westward flows, can have rapidly varying flows, with speeds varying from ~100 m/s to ~1 km/s in just a few minutes. The auroral brightenings are associated with bursts of weak electron precipitation that move westward with the westward flow bursts and extend into the SAPS region. Additionally, our observations show evidence that the azimuthally moving flow bursts often connect to earthward (equatorward in the ionosphere) plasma sheet flow bursts. This indicates that rather than stopping or bouncing, some flow bursts turn azimuthally after reaching the inner plasma sheet and lead to the bursts of strong azimuthal flow. Evidence is also seen for a general guiding of the flow bursts by the large‐scale convection pattern, flow bursts within the duskside convection being azimuthally turned to the west, and those within the dawn cell being turned toward the east. The possibility that the SAPS region flow structures considered here may be connected to localized flow enhancements from the polar cap that cross the nightside auroral poleward boundary and lead to flow bursts within the plasma sheet warrants further consideration.

Global features of the disturbance winds during storm time deduced from CHAMP observations

AbstractA wind‐driven disturbance dynamo has been postulated many decades ago. But due to the sparseness of thermospheric wind measurements, details of the phenomena could not be investigated. In this study we use the CHAMP zonal wind observations from 2001 to 2005 to investigate the global features of the disturbance winds during magnetically disturbed periods. The disturbance zonal wind is mainly westward, which increases with magnetic activity and latitude. At subauroral region, the westward zonal wind is strongly enhanced in the magnetic local time (MLT) sector from afternoon to midnight, which we relate to the plasma drift within the subauroral polarization streams. At middle and low latitudes, the disturbance zonal wind is largely independent of season. Peak values of the disturbance zonal wind occur at different MLTs for different latitudes. That is around 1800 MLT at subauroal region, with average values of about 200 m/s; around 2300 MLT at middle latitudes, with average values of about 80 m/s; and around 0300 MLT at low latitudes, with average values up to 50 m/s. The shift of the peak values of the westward disturbance zonal wind in local time at different latitudes could be considered as a response of the disturbance wind when it propagates from high to low latitudes. Further by applying for the first time a superposed epoch analysis, we show that the disturbance zonal wind responds with a delay to the sudden changes of solar wind input, which is different for the various latitudinal ranges. The propagation time of disturbance wind from the auroral region to the equator is about 3–4 h. This is consistent with the speed of traveling atmospheric disturbances. Based on CHAMP observations, we try to illustrate the whole chain of processes from the solar wind driving to the ionospheric effects at lower latitudes.

Modeling subauroral polarization streams during the 17 March 2013 storm

AbstractThe subauroral polarization streams (SAPS) are one of the most important features in representing magnetosphere‐ionosphere coupling processes. In this study, we use a state‐of‐the‐art modeling framework that couples an inner magnetospheric ring current model RAM‐SCB with a global MHD model Block‐Adaptive Tree Solar‐wind Roe Upwind Scheme (BATS‐R‐US) and an ionospheric potential solver to study the SAPS that occurred during the 17 March 2013 storm event as well as to assess the modeling capability. Both ionospheric and magnetospheric signatures associated with SAPS are analyzed to understand the spatial and temporal evolution of the electrodynamics in the midlatitude regions. Results show that the model captures the SAPS at subauroral latitudes, where Region 2 field‐aligned currents (FACs) flow down to the ionosphere and the conductance is lower than in the higher‐latitude auroral zone. Comparisons to observations such as FACs observed by Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), cross‐track ion drift from Defense Meteorological Satellite Program (DMSP), and in situ electric field observations from the Van Allen Probes indicate that the model generally reproduces the global dynamics of the Region 2 FACs, the position of SAPS along the DMSP, and the location of the SAPS electric field around L of 3.0 in the inner magnetosphere near the equator. The model also demonstrates double westward flow channels in the dusk sector (the higher‐latitude auroral convection and the subauroral SAPS) and captures the mechanism of the SAPS. However, the comparison with ion drifts along DMSP trajectories shows an underestimate of the magnitude of the SAPS and the sensitivity to the specific location and time. The comparison of the SAPS electric field with that measured from the Van Allen Probes shows that the simulated SAPS electric field penetrates deeper than in reality, implying that the shielding from the Region 2 FACs in the model is not well represented. Possible solutions in future studies to improve the modeling capability include implementing a self‐consistent ionospheric conductivity module from inner magnetosphere particle precipitation, coupling with the thermosphere‐ionosphere chemical processes, and connecting the ionosphere with the inner magnetosphere by the stronger Region 2 FACs calculated in the inner magnetosphere model.

Investigating storm‐enhanced density and polar tongue of ionization development during the 22 October 1999 great storm

AbstractWe investigate large‐scale plasma density increases occurring during the 22 October 1999 great storm and focus on storm‐enhanced density (SED) and polar tongue of ionization (TOI) development. Observations include two‐hourly Global Ionosphere Map series coupled with multi‐instrument in situ, space‐based, and ground‐based data plots, with Super Dual Auroral Radar Network two‐cell convection maps and with model‐generated neutral wind vector maps. Results demonstrate the equatorial electrojet events occurring in the Australian and American sectors, the high‐density plasma features, and their underlying plasma transportation processes. During the main phase, a series of four prompt penetration electric field (PPEF) events occurred with subauroral polarization stream E field development forming a plasmaspheric drainage plume. These E field events caused the repeated development of equatorial ionization anomaly (EIA), SED bulge, and SED plume during the local dusk‐midnight hours in those sectors that covered these local times. Showing a westward movement in accordance with their local time dependence, the EIA‐SED structure developed first in the American sector, later on over the Pacific, and finally in the Australian sector. The SED plume plasma found its way into the polar cap through the dayside cusp region, created in the north some large polar cap enhancements reaching up to seven times of the quiet time levels, and evolved in each hemisphere into a polar TOI. We speculate that the enhanced growth of EIA, transporting high‐density solar‐produced plasma to the SED bulge via strong net eastward E field effects, and the mechanical effects of equatorward neutral winds contributed to the polar TOI's increasingly better development.

Counter equatorial electrojet and overshielding after substorm onset: Global MHD simulation study

AbstractBy performing a global magnetohydrodynamic (MHD) simulation, we have demonstrated for the first time that an electrojet at the dayside magnetic equator can be reversed and an overshielding condition can be established in the inner magnetosphere after substorm onset without northward turning of the interplanetary magnetic field. Near the substorm onset, the plasma pressure is highly enhanced in the inner magnetosphere on the nightside. The Region 2 field‐aligned current diverges from the diamagnetic current on the surface of the dayside extension of the high‐pressure region, which is connected to the ionosphere in the relatively low‐conductivity region a few degrees equatorward of the main auroral oval that is formed as the projection of the plasma sheet. The separation of the equatorward boundary of the auroral region and the equatorward boundary of the Region 2 current results in dusk‐dawn electric fields that generate a counter electrojet (CEJ) at the dayside magnetic equator. Poleward electric fields in a narrow latitudinal width, which may be regarded as subauroral ion drift and subauroral polarization stream, are simultaneously intensified. The dusk‐dawn electric fields may propagate to the inner magnetosphere along a field line as shear Alfvén waves. Then, the inner magnetosphere is completely constrained by the overshielding condition. The intensity and polarity of the CEJ depend largely on at least the ionospheric conductivity that is related to the plasma pressure (probably associated with diffuse aurora). This may explain the observational fact that overshielding does not always occur after onset.

Large plasma density enhancements occurring in the northern polar region during the 6 April 2000 superstorm

AbstractWe focus on the ionospheric response of northern high‐latitude region to the 6 April 2000 superstorm and aim to investigate how the storm‐enhanced density (SED) plume plasma became distributed in the regions of auroral zone and polar cap plus to study the resultant ionospheric features and their development. Multi‐instrument observational results combined with model‐generated, two‐cell convection maps permitted identifying the high‐density plasma's origin and the underlying plasma transportation processes. Results show the plasma density feature of polar cap enhancement (PCE; ~600 × 103 i+/cm3) appearing for 7 h during the main phase and characterized by increases reaching up to 6 times of the quiet time values. Meanwhile, strong westward convections (~17,500 m/s) created low plasma densities in a wider region of the dusk cell. Oppositely, small (~750 m/s) but rigorous westward drifts drove the SED plume plasma through the auroral zone, wherein plasma densities doubled. As the SED plume plasma traveled along the convection streamlines and entered the polar cap, a continuous enhancement of the tongue of ionization (TOI) developed under steady convection conditions. However, convection changes caused slow convections and flow stagnations and thus segmented the TOI feature by locally depleting the plasma in the affected regions of the auroral zone and polar cap. From the strong correspondence of polar cap potential drop and subauroral polarization stream (SAPS), we conclude that the SAPS E‐field strength remained strong, and under its prolonged influence, the SED plume provided a continuous supply of downward flowing high‐density plasma for the development and maintenance of PCEs.

The spatial distribution of region 2 field-aligned currents relative to subauroral polarization stream

Abstract. To test the current-generation model of subauroral polarization stream (SAPS), we have investigated the relative positions of field-aligned currents (FACs) with respect to SAPS in a statistical way by using CHAMP (CHAllenging Minisatellite Payload) and DMSP (Defense Meteorological Satellite Program) satellite observations as well as model simulations. Comparative studies have been performed for consecutive CHAMP observations in different magnetic local time (MLT) sectors with respect to SAPS. The latitude of the peak westward zonal wind deduced from CHAMP measurements has been used to represent the location of the SAPS peak. Both the density and the sheet current strength of R2 (region 2) FACs are enhanced when SAPS occur. Subsequently R2 FACs decay in intensity and correspondingly the centers retreat poleward. The latitudes of the center of the R2 FAC, small- and medium-scale FACs, and SAPS shift equatorward with increasing MLT. The SAPS peaks are located between R2 and R1 (region 1) FAC peaks in all MLT bins under study. The SAPS peaks are closer to R2 centers in the later MLT sectors. The peaks of small- and medium-scale FACs are located poleward of SAPS, mainly in the upward R1 FACs region. The upward R1 FACs are partly closed by the downward R1 FACs in the dawn–morning sector. Based on model simulation, when R2 shifts equatorward to the subauroral region, the plasma flow also shifts equatorward with its peak located poleward of that of R2 FACs. Both the model and observations provide evidence that SAPS behave as caused by a magnetospheric current source.

Fine structure of subauroral electric field and electron content

AbstractSmall‐scale structure of the plasma convection and electron content within the subauroral polarization stream (SAPS) is investigated. We present ionospheric observations during the main phase of the geomagnetic storm on 17 March 2013, during which a sequence of intense, highly localized, and fast‐moving electric field (EF) structures within SAPS was observed by the Super Dual Auroral Radar Network Christmas Valley West (CVW) radar. The CVW EF measurements at 60 s resolution are analyzed in context of coincident GPS measurements of the total electron content (TEC) at 30 s resolution. The strong and narrow feature of the subauroral ion drift (SAID) was observed poleward of the TEC trough, with a TEC enhancement (peak) seen in the SAPS (SAID) region. The SAPS wave activity commenced ~2 h (15 min) after first appearance of SAPS (SAID). The SAPS structures appeared near the poleward edge of the trough, propagated westward, and merged with SAID near TEC peak. The propagation velocity was comparable with convection velocity within each EF structure. The SAPS TEC exhibited a general decrease toward the end of the period. On a smaller time scale, TEC exhibited a small but appreciable decrease within EF structures. The wavelet spectra of EF and TEC showed similar variations, with wave period of ~5 min period near onset and increasing to 8–10 min toward the end of the period with significant wave activity. A scenario is discussed, in which the SAPS wave activity may modify the ionospheric conductance and TEC at small scales, with large‐scale magnetosphere‐ionosphere feedback acting to continuously deplete TEC where/when such activity does not occur.

First Palmer and Millstone Hill midlatitude conjugate observation of thermospheric winds

AbstractThe first midlatitude conjugate thermospheric wind observations in the American sector showed various degrees of conjugacy between Palmer (64°S, 64°W, magnetic latitude (MLAT) 50°S) and Millstone Hill (42.82°N, 71.5°W, MLAT 53°N) under three different geomagnetic conditions (recovery after a substorm, moderately active, and quiet). The agreement with the National Center for Atmospheric Research's Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations also varies with the geomagnetic activity level. During substorm recovery, the observations at Palmer (PA) and Millstone Hill (MH) both showed strong westward zonal winds, which the standard TIEGCM greatly underestimated. Inadequate ion convection pattern size and lack of effect from Subauroral Polarization Streams (SAPS) may be the cause of the large discrepancy. The TIEGCM with a SAPS model produced stronger westward zonal winds near PA but did not change the zonal wind near MH. The empirical SAPS model needs further refinements. In general, there is better conjugacy with moderate geomagnetic activity levels. The TIEGCM also agrees better with the observations. Under geomagnetically quiet conditions, the meridional winds appear to be less conjugate. The agreement between the observations and model is reasonable. Optical conjugate observations are severely limited by the seasons and weather conditions in the two hemispheres. Yet they are necessary to understanding the thermospheric dynamics in the subauroral region and its relationship with geomagnetic activity levels. The comparisons with TIEGCM are necessary for future model improvements.

Storm time observations of plasmasphere erosion flux in the magnetosphere and ionosphere

AbstractPlasmasphere erosion carries cold dense plasma of ionospheric origin in a storm‐enhanced density plume extending from dusk toward and through the noontime cusp and dayside magnetopause and back across polar latitudes in a polar tongue of ionization. We examine dusk sector (20 MLT) plasmasphere erosion during the 17 March 2013 storm (Dst ~ −130 nT) using simultaneous, magnetically aligned direct sunward ion flux observations at high altitude by Van Allen Probes RBSP‐A (at ~3.0 Re) and at ionospheric heights (~840 km) by DMSP F‐18. Plasma erosion occurs at both high and low altitudes where the subauroral polarization stream flow overlaps the outer plasmasphere. At ~20 UT, RBSP‐A observed ~1.2E12 m−2 s−1 erosion flux, while DMSP F‐18 observed ~2E13 m−2 s−1 sunward flux. We find close similarities at high and low altitudes between the erosion plume in both invariant latitude spatial extent and plasma characteristics.

Mesoscale and large‐scale variability in high‐latitude ionospheric convection: Dominant modes and spatial/temporal coherence

Empirical orthogonal function (EOF) analysis, a variant of principle component analysis, is applied to 20 months of plasma drift data from the Super Dual Auroral Radar Network radars in the high‐latitude region of the Northern Hemisphere. Dominant modes of ionospheric electric field variability are identified and the spatial and temporal coherence of this variability is quantified. The first three modes of variability, which, together with the mean, account for ∼50% of the observed squared electric field (E2), are characterized by global spatial scales and long time scales (∼1 h). The first and second modes of variability represent the strengthening/weakening of the global convection pattern and the shaping of the convection pattern into asymmetrical round‐ and crescent‐shaped cells. These two modes are correlated with the Bz and By components of the interplanetary magnetic field. The third mode represents the expansion/contraction of the convection pattern and is weakly correlated with the solar wind velocity. For EOFs beyond EOF 3, the power contained in the modes falls off rapidly, the characteristic spatial and temporal scales decrease, and weak correlations with external driving parameters are observed. These higher‐order EOFs likely capture more random behavior of the electric field variability. The notable exception to this trend is EOF 11, which captures midlatitude variations on the duskside and is enhanced during subauroral polarization stream events. The EOF technique described in this paper is applied in a companion paper to characterize the covariance of ionospheric electric fields for use in an assimilative mapping procedure.

Seasonal variation of the ion upflow in the topside ionosphere during SAPS (subauroral polarization stream) periods

Abstract. A statistical study has been performed by using two years of DMSP (Defense Meteorological Satellite Program) plasma observations to investigate the seasonal effect of SAPS (subauroral polarization stream) on the ion upflow in the duskside ionosphere of the Northern Hemisphere. There are obvious upflows occurring in the topside ionosphere around the SAPS region, exceeding 200 m s−1 at winter solstice, indicating an important relationship between SAPS and the local plasma upward motion. Both SAPS and ion upward velocities show similar seasonal variations, largest in winter and smallest in summer, irrespective of geomagnetic activity. A good correlation is found and a linear relationship is derived between SAPS and the ion upflow velocities. During December solstice the average upflow flux can reach about 2 × 108 cm−2 s−1 for more disturbed periods, which is comparable to the typical upflow flux in the dayside cusp region. The depression of the ion temperatures around the peak SAPS region can be understood in terms of the adiabatic cooling. The hot ion cools down when expanding into the low ion concentration region. The electron temperature elevates around the SAPS region because of the reduced Coulomb cooling in the low ion density region. Both the changes of ion and electron temperatures are larger in winter than in summer, however, for Kp < 4 the electron temperatures are almost seasonably independent. The present work highlights the important role of the SAPS-related frictional heating at mid-latitudes on the local formation of the strong upward flow, which might provide a direct ionospheric ion source for the ring current and plasmasphere in the duskside sector.