Poleward Electric Field Research Articles

Investigating the impact of the latitudinal velocity profile on nonlinear gradient drift instability development in the subauroral ionosphere

Subauroral polarization streams (SAPS) are regions in the subauroral ionosphere with lower plasma density and large westward flow driven by a poleward electric field. Density irregularities have been observed in the SAPS region; these irregularities can negatively impact radio signals by causing phase and amplitude fluctuations. Previous work identified the gradient drift instability (GDI) as a generation mechanism of such ionospheric irregularities in SAPS and investigated the impact of the velocity profile on the GDI development. The turbulence spectra of the GDI are important to understand as they can provide insight into spatial scales associated with the turbulent evolution that may impact radio propagation. We hypothesize that the background electric field, through the neutral wind, and the nature of the velocity shear have an impact on the power laws from the turbulence spectra. A 2D fluid model is used to investigate the turbulence spectra of GDI in SAPS with a fixed background density profile and different latitudinal velocity profiles. Data from Super Dual Auroral Radar Network (SuperDARN) radar and Global Positioning System (GPS) provide the background ionospheric conditions. The neutral wind direction is chosen to be in the equatorward direction, which is in line with typical observations; however, this can be used to modify the electric field to consider new possibilities in the SAPS region. The angle of the electric field is manifested through the combination of the background velocity profile ( drift) and the neutral wind. The turbulence spectra of cases with no velocity profile and with different neutral wind directions are analyzed. The impact of velocity shear is studied by translating the velocity shear location relative to the density gradient. Numerical spectral analysis results are presented and compared to recent experimental observations. Results of spectral slopes of GDI simulations with no background velocity ( drift) were found to be and for the normalized density and the perturbed electric potential, respectively. Spectral slopes for simulations with different velocity shear locations were found to follow power laws of and for the normalized density and perturbed electric potential (inside SAPS region), respectively.

The Role of Diffuse Electron Precipitation in the Formation of Subauroral Polarization Streams

AbstractThe role of diffuse electron precipitation in the formation of subauroral polarization streams (SAPS) is investigated with the Multiscale Atmosphere‐Geospace Environment (MAGE) model. Diffuse precipitation is derived from the distribution of drifting electrons. SAPS manifest themselves as a separate mesoscale flow channel in the duskside ionosphere, which gradually merges with the primary auroral convection toward dayside as the equatorward auroral boundary approaches the poleward Region‐2 field‐aligned currents (FACs) boundary. SAPS expand to lower latitudes and toward the nightside during the main phase of a geomagnetic storm, associated with magnetotail earthward plasma flows building up the ring current and intensifying Region‐2 FACs and electron precipitation. SAPS shrink poleward and sunward as the interplanetary magnetic field turns northward. When diffuse precipitation is turned off in a controlled MAGE simulation, ring current and duskside Region‐2 FACs become weaker, but subauroral zonal ion drifts are still comparable to auroral convection. However, subauroral and auroral convection manifest as a single broad flow channel without showing any mesoscale structure. SAPS overlap with the downward Region‐2 FACs equatorward of diffuse precipitation, where poleward electric fields are strong due to a low conductance in the subauroral ionosphere. The Region‐2 FACs extend to latitudes lower than the diffuse precipitation because the ring current protons penetrate closer to the Earth than the electrons do. This study reproduces the key physics of SAPS formation and their evolution in the coupled magnetosphere‐ionosphere during a geomagnetic storm. Diffuse electron precipitation is demonstrated to play a critical role in determining SAPS location and structure.

Westward Plasma Drifts in the Nighttime Equatorial Ionosphere During Severe Magnetic Storms: A New Type of Penetration Electric Fields Caused by Subauroral Polarization Stream

AbstractPenetration electric fields during geomagnetic storms have important effects on the equatorial ionospheric electrodynamics. Previous studies focus on the vertical ion drifts, corresponding to electric fields in the zonal direction, in the equatorial ionosphere. In this study, we analyze the characteristics of the zonal ion drifts in the equatorial region measured by the Defense Meteorological Satellite Program (DMSP) satellites during 10 severe magnetic storms with a minimum Dst < −200 nT in the solar maximum years of 2000–2003. It is found that the net change of the equatorial zonal ion drifts caused by magnetic storms is westward and can reach 200–300 m s−1 in the evening sector and that the westward ion drifts remain in the same direction during both the storm main and recovery phases. The storm‐induced zonal ion drifts are highly correlated with the Dst index, and the correlation coefficient between the average zonal drift and the average Dst value during the 10 storms is 0.89. The magnitude of the storm‐induced westward ion drifts is approximately linearly proportional to the Dst index in the evening sector, and on average, a change of −100 nT in Dst causes a change of −52 m s−1 in the zonal drift. The westward ion drifts caused by magnetic storms become small when Dst recovers to the range of −55 to −123 nT. A new mechanism is proposed to explain the generation of the westward plasma drifts in the nighttime equatorial ionosphere. This new mechanism is that the poleward electric fields associated with the subauroral polarization stream (SAPS) penetrate or extend to low latitudes, producing the westward plasma drifts in the equatorial region. The results of this study provide new insight into equatorial ionospheric electrodynamics during geomagnetic storms.

Formation of red arc in the polarization jet band

Complex optical and ionospheric measurements in the region of the formation of red arcs accompanying the polarization jet development are analyzed. Optical measurements were done at subauroral station Maimaga, and ionospheric measurements were fulfilled at subauroral stations Yakutsk and Zhigansk. Spatial location and intensity of red-arcs were detected by the meridional scanning photometer. The temperature of the neutral atmosphere was recorded by Fabry-Perot interferometer. It is shown that the temperature of the neutral atmosphere increases in the red arc region and changes in accordance with variations in the emission. On 02 December 1989 the neutral temperature increased by 500 K in the red arc relative to the undisturbed level determined for quiet days during the month. This evidences on the strong heating process in the red arc region. According to ionospheric and satellite data on 18 February 1999 the red arc was observed simultaneously with polarization jet development – fast westward plasma drift in the poleward electric field. The magnitude of this electric field was 50–80 mV/m. Due to frictional interaction of ions and neutrals, ions were heated up to 5000–6000 K and neutrals gain energy of some hundreds degrees.

SAPS/SAID revisited: A causal relation to the substorm current wedge

AbstractWe present multispacecraft observations of enhanced flow/electric field channels in the inner magnetosphere and conjugate subauroral ionosphere, i.e., subauroral polarization streams (SAPS) near dusk and subauroral ion drifts (SAID) near midnight. The channels collocate with ring current (RC) injections lagging the onset of substorms by a few to ∼20 min, i.e., significantly shorter than the gradient‐curvature drift time of tens of keV ions. The time lag is of the order of the propagation time of reconnection‐injected hot plasma jets to the premidnight plasmasphere and the substorm current wedge (SCW) to dusk. The observations confirm and expand on the previous results on the SAID features that negate the paradigm of voltage and current generators. Fast‐time duskside SAPS/RC injections appear intimately related to a two‐loop circuit of the substorm current wedge (SCW2L). We suggest that the poleward electric field inherent in the SCW2L circuit, which demands closure of the Region 1 and Region 2 sense field‐aligned currents via meridional currents, is the ultimate cause of fast RC injections and SAPS on the duskside.

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.

An auroral westward flow channel (AWFC) and its relationship to field-aligned current, ring current, and plasmapause location determined using multiple spacecraft observations

Abstract. An auroral westward flow channel (AWFC) is a latitudinally narrow channel of unstable F-region plasma with intense westward drift in the dusk-to-midnight sector ionosphere. AWFCs tend to overlap the equatorward edge of the auroral oval, and their life cycle is often synchronised to that of substorms: they commence close to substorm expansion phase onset, intensify during the expansion phase, and then decay during the recovery phase. Here we define for the first time the relationship between an AWFC, large-scale field-aligned current (FAC), the ring current, and plasmapause location. The Tasman International Geospace Environment Radar (TIGER), a Southern Hemisphere HF SuperDARN radar, observed a jet-like AWFC during ~08:35 to 13:28 UT on 7 April 2001. The initiation of the AWFC was preceded by a band of equatorward expanding ionospheric scatter (BEES) which conveyed an intense poleward electric field through the inner plasma sheet. Unlike previous AWFCs, this event was not associated with a distinct substorm surge; rather it occurred during an interval of persistent, moderate magnetic activity characterised by AL~−200 nT. The four Cluster spacecraft had perigees within the dusk sector plasmasphere, and their trajectories were magnetically conjugate to the radar observations. The Waves of High frequency and Sounder for Probing Electron density by Relaxation (WHISPER) instruments on board Cluster were used to identify the plasmapause location. The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) EUV experiment also provided global-scale observations of the plasmapause. The Cluster fluxgate magnetometers (FGM) provided successive measurements specifying the relative location of the ring current and filamentary plasma sheet current. An analysis of Iridium spacecraft magnetometer measurements provided estimates of large-scale ionospheric FAC in relation to the AWFC evolution. Peak flows in the AWFC were located close to the peak of a Region 2 downward FAC, located just poleward of the plasmapause. DMSP satellite observations confirmed the AWFC was located equatorward of the nightside plasmasheet, sometimes associated with ~10 keV ion precipitation.

Modeling subauroral polarization streams equatorward of the plasmapause footprints

We report on a modeling study on the effects of subauroral polarization stream (SAPS) events equatorward of the plasmapause footprints, using the Sheffield Coupled Thermosphere‐Ionosphere‐Plasmasphere (CTIP) model. SAPS events are simulated by imposing a poleward electric field for 2 hours in the premidnight sector between 50° and 60° magnetic latitude. The presence of the SAPS results in a decrease in vertical total electron count and F region ion density, while the height of the F region can rise by up to 100 km. A study of the relative importance of the physical processes finds that the most significant contribution is the temperature increase due to ion‐neutral friction, resulting in increased reaction rates of O+ loss. The recovery time for the SAPS studied is up to 10 hours.

Interhemispheric asymmetries in the occurrence of magnetically conjugate sub-auroral polarisation streams

Abstract. Earthward injections of energetic ions and electrons mark the onset of magnetospheric substorms. In the inner magnetosphere (L4), the energetic ions drift westward and the electrons eastward, thereby enhancing the equatorial ring current. Wave-particle interactions can accelerate these particles to radiation belt energies. The ions are injected slightly closer to Earth in the pre-midnight sector, leading to the formation of a radial polarisation field in the inner magnetosphere. This maps to a poleward electric field just equatorward of the auroral oval in the ionosphere. The poleward electric field is subsequently amplified by ionospheric feedback, thereby producing auroral westward flow channels (AWFCs). In terms of electric field strength, AWFCs are the strongest manifestation of substorms in the ionosphere. Because geomagnetic flux tubes are essentially equi-potentials, similar AWFC signatures should be observed simultaneously in the Northern and Southern Hemispheres. Here we present magnetically conjugate SuperDARN radar observations of AWFC activity observed in the pre-midnight sector during two substorm intervals including multiple onsets during the evening of 30 November 2002. The Northern Hemisphere observations were made with the Japanese radar located at King Salmon, Alaska (57, and the Southern Hemisphere observations with the Tasman International Geospace Environment Radar (TIGER) located at Bruny Island, Tasmania (55. LANL geosynchronous satellite observations of energetic ion and electron fluxes monitored the effects of substorms in the inner magnetosphere (L6). The radar-observed AWFC activity was coincident with activity observed at geosynchronous orbit, as well as westward current surges in the ionosphere observed using ground-based magnetometers. The location of AWFCs with respect to the auroral oval was inferred from FUV auroral images recorded on board the IMAGE spacecraft. DMSP SSIES ion drift measurements confirmed the presence of AWFCs equatorward of the auroral oval. Systematic asymmetries in the interhemispheric signatures of the AWFCs probably arose because the magnetic flux tubes were distorted at L shells passing close to the substorm dipolarisation region. Transient asymmetries were attributed to the development of nearby field-aligned potential drops and currents.

Mid-latitude ionospheric fluctuation spectra due to secondary [formula omitted] instabilities

The most spectacular night of mid-latitude spread F ever recorded at Arecibo is analyzed and compared with barium cloud observations made in the 1970s and with equatorial spread F. Strong evidence is found for a secondary instability driven either by a poleward electric field and a westward density gradient (both presumably due to a primary Perkins instability), or by a zero-order eastward wind flowing across the westward gradient. Spectra computed from the incoherent scatter density profiles of this event are very similar to the spectra of both barium striations and equatorial spread F. Estimates of the strength of the radar wave scattering expected from the fluctuation spectra are in good agreement with the VHF observations from that night. Finally, a poleward-surging, mid-latitude plasma bubble is seen to bifurcate in a manner similar to equatorial spread- F simulations.

On the lifetime and extent of an auroral westward flow channel (AWFC) observed during a magnetospheric substorm

Abstract. A -190-nT negative bay in the geomagnetic X component measured at Macquarie Island ( -65° L) showed that an ionospheric substorm occurred during 09:58 to 11:10 UT on 27 February 2000. Signatures of an auroral westward flow channel (AWFC) were observed nearly simultaneously in the backscatter power, LOS Doppler velocity, and Doppler spectral width measured using the Tasman International Geospace Environment Radar (TIGER), a Southern Hemisphere HF SuperDARN radar. Many of the characteristics of the AWFC were similar to those occurring during a polarisation jet (PJ), or subauroral ion drift (SAID) event, and suggest that it may have been a pre-cursor to a fully developed, intense westward flow channel satisfying all of the criteria defining a PJ/SAID. A beam-swinging analysis showed that the westward drifts (poleward electric field) associated with the flow channel were very structured in time and space, but the smoothed velocities grew to ~ 800 ms-1 (47 mVm-1) during the 22-min substorm onset interval 09:56 to 10:18 UT. Maximum west-ward drifts of >1.3 km s-1 (>77 mVm-1) occurred during a ~ 5-min velocity spike, peaking at 10:40 UT during the expansion phase. The drifts decayed rapidly to ~ 300 ms-1 (18 mVm-1) during the 6-min recovery phase interval, 11:04 to 11:10 UT. Overall, the AWFC had a lifetime of 74 min, and was located near -65° L in the evening sector west of the Harang discontinuity. The large westward drifts were confined to a geographic zonal channel of longitudinal ex-tent >20° (>1.3 h magnetic local time), and latitudinal width ~2° L. Using a half-width of ~ 100 km in latitude, the peak electric potential was >7.7 kV. However, a transient velocity of >3.1 km s-1 with potential >18.4 kV was observed further poleward at the end of the recovery phase. Auroral oval boundaries determined using DMSP measurements suggest the main flow channel overlapped the equatorward boundary of the diffuse auroral oval. During the ~ 2-h interval following the flow channel, an ~ 3° L wide band of scatter was observed drifting slowly toward the west, with speeds gradually decaying to ~ 50 ms-1 (3 mVm -1). The scatter was observed extending past the Harang discontinuity, and had Doppler signatures characteristic of the main ionospheric trough, implicating the flow channel in the further depletion of F-region plasma. The character of this scatter was in contrast with the character of the scatter drifting toward the east at higher latitude.Key words. Ionosphere (auroral ionosphere; electric fields and currents; ionosphere-magnetospehere interactions) Magnetospheric physics (storms and substorms)

Simultaneous measurement of duskside subauroral irregularities from the CUTLASS Finland radar and EISCAT UHF system

Dusk scatter event (DUSE) (first reported by Ruohoniemi et al. [1988]) is one of the most reproducible features among the SuperDARN radar backscatter within the subauroral ionosphere. Hosokawa et al. [2001] analyzed the scattering occurrence percentage of the Northern Hemisphere SuperDARN radars in a statistical fashion and pointed out that the region where the DUSE appears has a close relationship with the duskside end of the midlatitude trough in longitudinal direction. They proposed a model explaining the generation of the DUSE which employs a Sunward density gradient at the duskside edge of the trough and an ambient poleward electric field. In order to confirm the model proposed by Hosokawa et al. [2001], we have investigated two DUSE events which had been observed by the CUTLASS Finland radar and the EISCAT UHF system simultaneously. Consequently, when the CUTLASS Finland radar observed the DUSE, a Sunward directed density gradient was observed by the EISCAT in the vicinity of the DUSE. After the passage of the DUSE the EISCAT observed an ion temperature enhancement which suggested that the EISCAT entered the trough through its duskside edge. These observational facts suggest that the geometry of the parameters around the DUSE is quite consistent with the model proposed by Hosokawa et al. [2001]. In addition, the EISCAT observed an enhancement of the poleward electric field around the DUSE, and a signature of the substorm was identified by the ground magnetometer on the nightside. We suggest generation mechanisms of the trough responsible for the DUSE during substorm conditions in terms of the role of the enhanced subauroral electric field and discuss a relationship between the DUSE and the other substorm‐related phenomena.

The convection electric field in auroral substorms

Dynamics Explorer 2 (DE 2) electric field and ion drift data are used in a statistical study of the ionospheric convection electric field in bulge‐type auroral substorms. Thirty‐one individual DE 2 substorm crossings were carefully selected and organized by the use of global auroral images obtained by DE 1. The selected passes, which occurred during substorm expansion phase, maximum, or early recovery phase, cover the entire nighttime substorm. The organization of the data used the method developed by Fujii et al. [1994], which divided the data into six local time sectors covering the nighttime substorm region. Following the procedures employed in the paper by Gjerloev and Hoffman [2000b], the latitudinal width and location of each auroral oval crossing was then adjusted to fit the sector average. In addition to the detailed study of the characteristics of the field within each sector this database enabled us to compile a model of the ionospheric convection electric field. The characteristics of the premidnight convection reversal show a pronounced local time dependency. Far west of the surge it is a fairly well defined point reversal or convection shear. Approaching the surge and within the surge it is a region of weak electric fields increasing in width toward midnight that separates regions of equatorward and poleward electric fields. Therefore we adopt the term Harang region rather than the Harang discontinuity for the premidnight convection reversal. A relatively narrow convection channel is coincident with the highest conductances located just poleward of the Harang region. This channel drives the substorm current wedge component of the westward electrojet in the surge and middle surge sectors. It is present in all premidnight passes and consequently is an integral part of the three‐dimensional substorm current wedge system.

Electrodynamic coupling of high and low latitudes: Simulations of shielding/overshielding effects

The penetration of the electric field and associated magnetic perturbations from high latitudes to low latitudes is studied with the Magnetosphere‐Thermosphere‐lonosphere‐Electrodynamics General Circulation Model (MTlEGCM) of Peymirat et al. [1998] in response to variations of the polar cap potential drop. For a sudden decrease of the polar cap potential of ∼ 40 kV, the model reproduces the well‐known overshielding phenomenon corresponding to a sudden reversal of the low‐latitude electric field. For quasi‐periodic oscillations of the polar cap potential drop of ∼ 40 min period, the model predicts that the poleward electric field and the eastward Hall current in the auroral zone lag slightly in phase (< 1 min), while the eastward electric field and current at the magnetic equator are advanced slightly in phase, with respect to the potential‐drop oscillations. These phase differences are interpreted as the consequence of the succession of shielding and overshielding episodes induced by the response of the region‐2 field‐aligned currents to the polar cap variations. The phase differences among the polar cap potential and the auroral and equatorial electric fields and currents increase with the plasma sheet pressure. The amplitude of the associated magnetic perturbations is very dependent on the distribution of the potential along the polar cap boundary. The model predictions are tested against the observations of Kobea et al. [this issue] for two events, one representing simple overshielding and the other associated with polar cap potential oscillations. The model underestimates the decay time of the magnetic perturbations by a factor of 2 during the overshielding event, and the model gives results compatible with the observations during the second event. The disagreements may be due to limitations of the model and uncertainties of the input parameters.

The double probe electric field experiment on Freja: Experiment description and first results

A description is given of the Freja double-probe electric field instrument. Its capability to perform high-resolution measurements of the aurora and its fine-structure as well as collect information on sub-auroral and low-latitude phenomena is illustrated by selected results from the first six months of operation. The instrument is highly flexible and possible to operate in a number of different modes. It is also equipped with a 4-Megabyte burst memory for high data sampling rate and temporary storage of data. It has been fully operational since October 1992, and delivers data from ≈22 hr day-1 including about 5–6 auroral crossings of the northern and southern auroral ionosphere. New and important information on the auroral fine structure and electrodynamics is obtained by means of burst resolution data (6144 samples s-1) and normal resolution data (768 samples s-l). Common burst data collection triggered by the electric field event detector has turned out to be very useful for the selection of scientifically interesting events. This is illustrated by high-resolution data of a pair of extremely intense and narrow electric field structures (1 V m-1) which are associated with a total absence of precipitating particles, depletions of the thermal plasma and with an intense wave activity. The low inclination of the Freja orbit provides a new perspective for studying large-scale phenomena associated with east-west gradients as is exemplified by electric field data from a satellite crossing over north-south oriented auroral structures presumably resulting from rotational distortions of east-west aligned auroral arcs. The different plasma regimes encountered by Freja are continuously monitored by means of current sweeps applied to the probes and by the satellite potential. In addition, overview data (8 samples s-1) are collected from full orbits and stored in the on-board memory and have proved to be extremely valuable, providing new information on global electric field phenomena at subauroral and lower latitudes, such as the intense poleward electric fields and Pc-1 observations that have been made near the plasmapause during substorm activity.

EXOS D observations of enhanced electric fields during the Giant Magnetic Storm in March 1989

We study spatial and temporal variations of the intense electric field that is observed in middle latitudes in the evening sector. The observations were made by the EXOS D (AKEBONO) satellite at altitudes of 2000‐6000 km during and after the great storm event of March 13‐14, 1989. During the main phase of the storm a sharp peak was observed in the poleward electric field. The strength of this field was ∼100 mV/m when projected to the ionosphere, and this corresponds to the plasma drift speed of 1.8 km/s. The position of the peak moved toward higher latitudes during the recovery phase. Comparison with the electron density profile deduced from the wave measurements (Oya et al., 1990) shows that the peak was located in the low‐density region outside the plasmapause. An intense electric field of the same kind was observed also on March 15‐17 when the geomagnetic field was only moderately disturbed.

Characterization of the IMF By‐dependent field‐aligned currents in the cleft region based on DE 2 observations

The interplanetary magnetic field (IMF) By‐dependent distribution of field‐aligned currents in the cleft region is studied, using the magnetic field and plasma data from 47 passes of Dynamics Explorer (DE) 2. These orbits were chosen on the conditions that cusp/cleft particles are detected and that at the same time the IMF By and Bz components satisfy the criteria |By|≥5 nT and |Bz|≤5 nT during the satellite's crossing of the relevant field‐aligned current region. When By is positive (negative) in addition to satisfying these conditions, there is a strong eastward (westward) magnetic perturbation caused by a pair of field‐aligned current sheets, consisting of an equatorward sheet with downward (upward) current and a poleward sheet having upward (downward) current. These By‐dependent field‐aligned currents in the equatorward and the poleward sheets are referred to as the low‐latitude cleft current (LCC) and the high‐latitude cleft current (HCC), respectively. The cusp/cleft electron precipitation region and the LCC region overlap with each other to a varying degree irrespective of the sign of By. For positive (negative) By, LCC has the same direction as the morning (afternoon) region 1 current or the afternoon (morning) region 2 current. Thus an interpretation has been given in the past that the LCC region is an extension of the region 1 or region 2 current system. However, in this paper we present an alternative view that the LCC region is not an extension of the region 1 or region 2 current system and that a pair of LCC and HCC constitutes the cleft field‐aligned current regime. The proposed pair of cleft field‐aligned currents is explained with a qualitative model in which this pair of currents is generated on the open field lines that have just been reconnected on the dayside magnetopause. The model assumes a quasi‐steady reconnection operating within certain longitudinal width extending to both sides of the stagnation point on the dayside magnetopause. The reconnected flux tubes move under the influences of the field tension and the magnetosheath flow. When the magnetosheath By is positive, the northern hemisphere field lines reconnected on the eastward side of the stagnation point are pulled toward higher latitudes, and the field lines reconnected on the westward side of the stagnation point are pulled along the dawnside magnetopause flank. The electric fields associated with these motions are present immediately inside the magnetopause (rotational discontinuity). This is the source region of LCC and HCC. The electric fields are transmitted along the field lines to the ionosphere, creating a poleward electric field and a pair of field‐aligned currents when By is positive; the pair of field‐aligned currents consists of a downward current at lower latitudes (LCC) and an upward current at higher latitudes (HCC). In the By negative case, the model explains the reversal of the field‐aligned current direction in the LCC and HCC regions.

Storm time electric field penetration observed at mid‐latitude

Under disturbed geomagnetic conditions the latitudinal profile of the westward ion convection (equivalent to poleward electric field) observed with the Millstone Hill incoherent scatter radar at dusk, often exhibits a double peak (dual maxima). During the height of the February 8–9, 1986, magnetic storm the Millstone Hill radar was in the evening local time sector (1600–2200 MLT). Radar observations indicate that high speed (>1000 m s−1) westward ion flow penetrated deeply below 50° invariant latitude (Λ) and persisted for 6 hours between 2100 UT on February 8 and 0300 UT on February 9. The double‐peaked ion convection feature was pronounced throughout the period, and the separation in the dual maxima ranged from 4° to 10°. The latitude positions of the high‐latitude ion drift peak and the convection reversal varied in unison. The low‐latitude ion drift peak (∼49°Λ or L = 2.3) did not show significant universal time/magnetic local time (UT/MLT) variation in its latitude location but showed a decrease in magnitude during the initial recovery phase of the storm. Using simultaneous particle (30 eV–30 keV) precipitation data from the DMSP F6 and F7 satellites, we find the high‐latitude ion drift peak to coincide with the boundary plasma sheet/central plasma sheet transition in the high ionospheric conductivity (>15 mho) region. The low‐latitude ion drift peak lay between the equatorward edges of the electron and soft (<1 keV) ion precipitation in the low conductivity region (∼1 mho). A comparison between the low‐altitude observations and simultaneous ring current observations from the high‐altitude AMPTE satellite further suggests that the low‐latitude ion drift peak is closely related to the maximum of the O+ dominated ring current energy density in magnetic latitude. The low‐latitude ion drift peak is the low‐altitude signature of the electric field shielding effect associated with ring current penetration into the outer layer of the storm time plasmasphere. Unlike the transient and localized subauroral ion drifts under moderately disturbed conditions, the intense westward ion drifts developed in response to heavy ion ring current shielding during a great magnetic storm can decouple from the high‐latitude electric field and penetrate to very low latitudes and persist for long durations in the dusk and early afternoon MLT sectors. These features confirm the active role of storm time ring current dynamics in generating the low‐latitude extension of the magnetospheric electric field.

A particle simulation of large‐amplitude undulations on the evening diffuse auroral boundary

The pattern of undulation on the equatorward boundary of the diffuse aurora, occasionally observed in the afternoon‐evening sector, is studied by two‐dimensional particle simulations for the motion of magnetospheric plasma perpendicular to the geomagnetic field. In the simulation model, the poleward electric field is assumed to be initially enhanced locally in latitude and independent of longitude, as suggested by the observations near the undulations of the diffuse auroral boundary. Such an electric field is assumed to be maintained by space charges carried by a cold plasma. The diffuse aurora is initially distributed over a longitudinally extended zone. Since this type of diffuse aurora is probably caused by precipitation of the energetic protons with energies in excess of a few keV, the temporal evolution of the diffuse auroral pattern, as displayed in photographs from the satellites, can be visualized by following the cluster of the energetic protons. Sheared plasma flow arising from the electric field distribution drives the Kelvin‐Helmholtz instability and causes undulations on the equatorward boundary of the diffuse aurora with wavelengths and amplitudes comparable to the typical observed values. Notably, the particle simulation can reproduce the outstanding indentations characteristic of large‐amplitude undulations which have occasionally been observed. The good agreement between the simulation results and the observed undulations suggests that the Kelvin‐Helmholtz shear‐driven process is responsible for undulations on the equatorward boundary of the diffuse aurora in the afternoon‐evening sector. It is also shown that the magnetic drift of the energetic protons producing the diffuse aurora is important in creating the characteristic pattern of the diffuse auroral boundary.

Ion composition changes during F-region density depletions in the presence of electric fields at auroral latitudes

F-region density depletions in the afternoon/evening sector of the auroral zone are studied with the EISCAT UHF radar. Four case studies are presented, in which data from three experiment modes are used. In each case the density depletion can be identified with the main ionospheric trough. For the two cases occurring in sunlit conditions the electron densities recovered significantly after the trough minimum. Tristatic ion velocity measurements show the development of poleward electric fields of typically 50–100 m Vm −1, which maximize exactly in the trough minimum. A special analysis technique for incoherent scatter measurements is introduced, based on the ion energy equation. By assuming that the ion temperature should obey this equation it is possible to fix this parameter in a second analysis and to allow the ion composition to be a free parameter. The results from two experiments with accurate velocity measurements indicate that the proportion of O + near the F-region peak decreased from 100% in the undisturbed ionosphere to only 10% and 30%, respectively, in the density minimum of the trough. The loss of O + is explained by the temperature dependence of recombination with nitrogen molecules. Temperatures derived from radar measurements are very sensitive to the assumed ion composition. For the above case of 10% O + the deduced electron temperature in the trough was transformed from a local minimum of < 2000 K to a local maximum of 4000 K.