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During a large geomagnetic storm, the electric field from the polar ionosphere can expand far enough to affect the mid-latitude and equatorial electric fields. These changes in the equatorial zonal electric field, called the penetration field, will cause changes in the meridional ion flows that can be observed by radars and spacecraft. In general this E × B ion flow near the equator caused by the penetration field during undershielding conditions will be upward on the dayside and downward on the nightside of the Earth. Previous analysis of the equatorial meridional flows observed by CINDI instrument on the C/NOFS spacecraft during the 26 September 2011 storm showed that all of the response flows on the dayside were excess downward flows instead of the expected upward flows. These observed storm-time responses are compared to a prediction from a physics-based coupled model of thermosphere–ionosphere–inner-magnetosphere in an effort to explain these observations. The model results suggest that the equatorial downward flow could be attributed to a combined effect of the overshielding and disturbance dynamo processes. However, some discrepancy between the model and observation indicates a need for improving our understanding of how sensitive the equatorial electric field is to various model input parameters that describe the magnetosphere–ionosphere coupling processes.

The Ion Velocity Meter (IVM), a part of the Coupled Ion Neutral Dynamic Investigation (CINDI) instrument package on the Communication/Navigation Outage Forecast System (C/NOFS) spacecraft, has made over 5 yr of in situ measurements of plasma temperatures, composition, densities, and velocities in the 400–850 km altitude range of the equatorial ionosphere. These measured ion velocities are then transformed into a coordinate system with components parallel and perpendicular to the geomagnetic field allowing us to examine the zonal (horizontal and perpendicular to the geomagnetic field) component of plasma motion over the 2009–2012 interval. The general pattern of local time variation of the equatorial zonal ion velocity is well established as westward during the day and eastward during the night, with the larger nighttime velocities leading to a net ionospheric superrotation. Since the C/NOFS launch in April 2008, F10.7 cm radio fluxes have gradually increased from around 70 sfu to levels in the 130–150 sfu range. The comprehensive coverage of C/NOFS over the low-latitude ionosphere allows us to examine variations of the topside zonal ion velocity over a wide level of solar activity as well as the dependence of the zonal velocity on apex altitude (magnetic latitude), longitude, and solar local time. It was found that the zonal ion drifts show longitude dependence with the largest net eastward values in the American sector. The pre-midnight zonal drifts show definite solar activity (F10.7) dependence. The daytime drifts have a lower dependence on F10.7. The apex altitude (magnetic latitude) variations indicate a more westerly flow at higher altitudes. There is often a net topside subrotation at low F10.7 levels, perhaps indicative of a suppressed F region dynamo due to low field line-integrated conductivity and a low F region altitude at solar minimum.

The newly installed SuperDARN Hokkaido HF radar monitors ionospheric plasma flow between magnetic latitudes of 45° and 65° and thus has a great potential for studies of subauroral polarization streams (SAPS) in combination with another SuperDARN radar located at King Salmon, Alaska as well as the DMSP satellites and ground-based instruments in the Alaskan sector of the Arctic. Preliminary survey shows that although SAPS are often detected with the Hokkaido radar, their velocities are rather low, to the order of 150 m/s in its most suitable central beams. In this study, observations of unusually fast Hokkaido flows of up to 800 m/s are presented. The event of 1 April 2007 is investigated in detail. It is shown that high-velocity echoes appear after substorm onsets over North America with a delay of ~30 min. In terms of latitude, the velocity peaks just outside the auroral oval; signatures of a detached polarization jet are occasional and not pronounced. The King Salmon radar operating concurrently detects SAPS signatures as well but at different times and locations. Simulation with the Comprehensive Ring Current Model for the 1 April event reasonably identifies the period of fast flow occurrence but the velocity is underestimated. The event studied suggests that substorm-injected particle populations may intensify the pre-existing SAPS flow and lead to a mismatch of the predictions and observations.

The two-cell aurora is characterized by azimuthally elongated regions of enhanced auroral brightness over extended local times in the dawn and dusk sectors. Its association with the convection, particle precipitation, and field-aligned currents under various phases of substorms has not been fully understood. With Polar Ultraviolet Imager auroral images in conjunction with Defense Meteorological Satellite Program (DMSP) F12 spacecraft on the dusk-side branch of the two-cell aurora, we are able to investigate an association of the auroral emissions with the electric fields, field-aligned currents, and energy flux of electrons. Results show that the substorm expansion onset does not significantly change the orientation of the dusk-side branch of the two-cell aurora. Also, the orientation of the magnetic deflection vector produced by the region1 field-aligned current changed from 73±1° to the DMSP trajectory during the substorm growth phase, to 44±6° to the DMSP trajectory during the substorm expansion phase. With a comparison between the orientation of the dusk-side branch of the two-cell aurora and the orientation of the magnetic deflection vector, it is found that the angular difference between the two orientations is 28±5° during the substorm growth phase, and 13±6° during the substorm expansion phase.

Cross-track ion drifts measured by the DMSP satellites are compared with line-of-sight SuperDARN HF velocities in approximately the same directions. Good overall agreement is found for a data set comprising of 209 satellite passes over the field of view of nine SuperDARN radars in both the Northern and Southern Hemispheres. The slope of the best linear fit line relating the SuperDARN and DMSP velocities is of the order of 0.7 with a tendency for SuperDARN velocities to be smaller. The agreement implies that the satellite and radar data can be merged into a common set provided that spatial and temporal variations of the velocity as measured by both instruments are smooth. Keywords. Ionosphere (Ionospheric irregularities; Plasma convection; Auroral ionosphere)

During the long-duration steady convection activity on 11 December 1998, the development of a few dozen auroral streamers was monitored by Polar UVI instrument in the dark northern nightside ionosphere. On many occasions the DMSP spacecraft crossed the streamer-conjugate regions over the sunlit southern auroral oval, permitting the investigation of the characteristics of ion and electron precipitation, ionospheric convection and field-aligned currents associated with the streamers. We confirm the conjugacy of streamer-associated precipitation, as well as their association with ionospheric plasma streams having a substantial equatorward convection component. The observations display two basic types of streamer-associated precipitation. In its polewardmost half, the streamer-associated (field-aligned) accelerated electron precipitation coincides with the strong (≥2–7μA/m2) upward field-aligned currents on the westward flank of the convection stream, sometimes accompanied by enhanced proton precipitation in the adjacent region. In the equatorward portion of the streamer, the enhanced precipitation includes both electrons and protons, often without indication of field-aligned acceleration. Most of these characteristics are consistent with the model describing the generation of the streamer by the narrow plasma bubbles (bursty bulk flows) which are contained on dipolarized field lines in the plasma sheet, although the mapping is strongly distorted which makes it difficult to quantitatively interprete the ionospheric image. The convective streams in the ionosphere, when well-resolved, had the maximal convection speeds ∼0.5–1km/s, total field-aligned currents of a few tenths of MA, thicknesses of a few hundreds km and a potential drop of a few kV across the stream. However, this might represent only a small part of the associated flux transport in the equatorial plasma sheet.Key words. Ionosphere (electric fiels and currents). Magnetospheric physics (aurroal phenomena; energetic particles, precipitating)

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)

During northward interplanetary magnetic field (IMF), it is generally believed that ionospheric convection appears as a four-cell structure for |Bz/By| > 1 and as a distorted two-cell structure for |Bz/By| 1 on November 11, 1998. We show in detail the evolution of the convection patterns as |Bz/By| changes. Nearly symmetric four-cell convection, with two reverse cells in the polar cap and two normal cells at lower latitudes, occurs for |Bz/By| ≈ 7. The ionospheric flow associated with the reverse cells is closed almost completely on the dayside. A shifted four-cell convection pattern, with the reverse cells shifted toward earlier magnetic local time (MLT) for negative By and toward later MLT for positive By, is observed for |Bz/By| ≈ 2.3. When |Bz/By| decreases to ∼1.7, the convection appears as a three-cell pattern, with a single reverse cell focused near noon and two normal cells. The normal morning and afternoon cells are focused at quite high magnetic latitudes (between 76° and 80°); the spatial extent of the normal cells is 10°–15° or 1000–1500 km in the latitudinal direction. We also present Defense Meteorological Satellite Program (DMSP) satellite data which show sunward convection over the polar cap in the Southern Hemisphere at the same time as the Northern Hemisphere radar observations. We propose a new model of convection patterns during northward IMF for |Bz/By| > 1 and By < 0 on the basis of the combined observations. In the model the convection appears as a symmetric four-cell structure for |Bz/By| ≥ 3, a shifted four-cell structure for |Bz/By| = 2–3, and a three-cell structure for |Bz/By| = 1–2.

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) on NASA's Polar spacecraft provides the first global images of the auroral oval in X-rays and allows very accurate measurements of the timing of geomagnetic disturbances to a degree of temporal resolution not available from previous imagers due to its photon counting characteristics. On October 19, 1998, a magnetic cloud associated with a CME encountered the Earth's magnetopause near 0500 UT, generating a magnetic storm that reached a minimum value in Dst of -139 nT. The z component of the interplanetary magnetic field (IMF) (B z ) remained remarkably steady for the first 10 hours of the storm as did the solar wind particle pressure. The PIXIE and UVI instruments on the Polar spacecraft were both imaging the auroral oval from 0800 to 1800 UT; six distinct impulsive auroral enhancements were observed by the imagers during this time period. Global imaging combined with geosynchronous particle observations allowed classification of the geomagnetic disturbances associated with the events. Only two of the events were classified as substorms; one was classified as a poleward boundary intensification, one was a convection bay, and one was a pseudobreakup. A sixth event occurred after a dramatic northward turning of the IMF at the end of the 10-hour B z south period but was very weak and transient. The effects of the northward turning were counteracted by a simultaneous increase in the B y component of the IMF. The first sign of significant substorm activity occurred over 8 hours after the cloud encountered the Earth and was not associated with any change in the solar wind magnetic field or particle pressure. The cross polar cap potential remained large (> 100 kV), and most of the X-ray emissions observed were associated with enhanced earthward convection caused by large cross-tail electric fields; 50% were collected from the 0000 - 0600 magnetic local time (MLT) sector.

Discrete F region electron density enhancements of a factor of 2 or more have been observed in the high-latitude ionosphere. These enhancements have been termed patches if they occur within the polar cap and blobs if they occur outside of the polar cap. It is important to understand the formation and evolution of these structures because they are associated with large phase and amplitude scintillation in transionospheric radio signals. Blobs are generally thought to result from the breakup of patches as they exit the polar cap; however, this process has not previously been observed. Detailed study of high-latitude ionospheric plasma transport is generally difficult because of the sparseness (spatial and temporal) of electron density and velocity observations. In this paper, we present electron density enhancements measured from the Qaanaaq Digisonde, the Millstone Hill incoherent scatter radar, and the DMSP F8 satellite during a 5-hour interval of the March 21, 1990, storm period and show definitively how a patch is transformed into a blob. We present a new trajectory analysis package that is capable of using ionospheric convection patterns to determine the motion of ionospheric plasma over a period of several hours. The new package uses convection patterns from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique to track the motion of observed patches from one site to another and thus determines where the measured electron density enhancements originated and where they went after being observed. The trajectory analysis also establishes that there is a direct connection between the enhancements observed by the different instruments at different locations. In this case, within ∼4 hours, plasma observed by a Digisonde near the pole is convected through 35° of latitude to the northeastern United States, where it is observed by the Millstone Hill radar, then roughly equal portions are transported westward to Alaska and eastward to Scandinavia where they are observed by the DMSP satellite. This study demonstrates that the changing convection pattern can significantly distort the patch shape and trajectory, and illustrates the high degree of mixing of ionospheric plasma by convection. The changing convection pattern leads to the simultaneous existence of a boundary blob and a subauroral blob which are both observed by the Millstone Hill radar. This work is very relevant to our future ability to specify and forecast ionospheric conditions at high latitudes. It represents a critical step from a merely qualitative ability to model the evolution of patches and blobs to a quantitative ability.

Precipitating ions on the poleward edge of the nightside auroral oval sometimes exhibit sharp low-energy cutoffs in their energy spectra. These truncated spectra are interpreted as signatures of magnetic reconnection in the magnetotail. The energy cutoff is frequently smoothly dispersed in latitude, allowing an interpretation in terms of quasi-steady reconnection. These events are designated velocity-dispersed ion structures (VDIS) type 2. Roughly one third of type 2 VDIS are accompanied by a sharp transition in the polar rain near the open-closed boundary that aids in their analysis. From 886 nightside open-closed boundary crossings by DMSP spacecraft, 148 type 2 VDIS were identified. They were found most frequently within 2–3 hours of midnight and for 40% of the open-closed boundary crossings between 2200 and 0100 magnetic local time. Minimum variance fits to the cutoff energies and polar rain transition are performed on 49 of these events. For four of them the information from the minimum variance fit and observed convection velocities are used to infer distances to the reconnection site that varied from 30 to 80 RE. In three of these four cases a sharp transition in the convection velocity is observed, coincident with the arrival of ions from the reconnection site. If the reconnection region is viewed as a voltage source, lobe field lines can be insufficiently populated to carry the current necessary to impose the required voltage on the ionosphere. This explains the coincidence between the arrival of ions and a discontinuity in convection, that is, that an electric field from the reconnection site is imposed on the ionosphere but only after sufficient density populates the field lines that connect the regions.

We study the response of midtail plasma and field parameters to enhanced solar wind electric field input for two substorm intervals on November 22, 1995. The solar wind input signatures were quite different for these two substorms, which had major Pi2 onsets at 1108 and 1502 UT. The solar wind input for the 1108 UT substorm had a short timescale (∼0.5 hour) electric field enhancement up to 1.5 mV/m, whereas the solar wind electric field for the 1502 UT substorm continuously exceeded 1.5 mV/m for ∼2 hours. In association with the expansion phase onsets of both substorms, the electric field fluctuation in the midtail plasma sheet commenced. The electric field disturbances lasted for a time interval close to the length of time, in which the solar wind electric field was enhanced. The midtail plasma sheet electric field responded well to the enhanced solar wind input with a time delay of 45–80 min. The pressure decrease, which started at the onset of both substorms, ended quite differently for these two substorms: the pressure decreased until it approached the quiet time level in the course of the 1108 UT substorm, whereas the pressure dropped below the quiet time level during the expansion phase of the 1502 UT substorm and stayed at this low level until late in the recovery phase. Using polar cap potential drop and the pressure profile in the midtail, we estimate the relationship between the dayside and nightside potential drop. We suggest that the difference in the nightside flux transport rate could control the configuration of the midtail and could explain why the tail pressure responded differently during the expansion and recovery phase of the two substorms.

In this study we applied a linear regression analysis technique to the interplanetary magnetic field (IMF) data and the ionospheric electrostatic potentials obtained from thermal ion drift measurements made by the Defense Meteorological Satellite Program (DMSP) satellites F8 and F10–F13 in 1993–1996. The ionospheric potentials are binned by every 1° of the corrected geomagnetic latitude and 0.5 hour of magnetic local time over both the northern and southern polar regions. The regression analyses are applied to the DMSP data in each bin in a similar way to what was done with the ground magnetometer data in the IZMIRAN Electrodynamic Model (IZMEM). This allowed us to parameterize the DMSP ionospheric potentials by the IMF strength and direction, as well as to recalibrate the IZMEM model against experimental satellite data. From the analysis it is calculated that the “background” cross-polar potential drop estimated from DMSP data equals ∼33 kV if IMF ≈ 0. The calculated ionospheric potential responses to changes in the southward and northward IMF components are 11.5 kV/nT and −5.0 kV/nT, respectively.

During the National Space Weather Program (NSWP) event study period of November 2–11, 1993 there were three operational DMSP meteorological satellites (F8, F10, and F11) in orbit, each carrying the Special Sensor for Ions, Electrons, and Scintillation (SSIES) plasma instrument package. Ion flow data from these instruments are used to determine the electrostatic potential drop across both the northern and southern polar caps. The magnitude and distribution of the potential are used to characterize the convection patterns present in the polar ionospheres. The results from all three satellites ate presented to show an overall observational history of the potential drop and the convection pattern during the study period. These observational parameters provide crucial inputs and checks to several ionospheric and magnetospheric models being used in this study. Evidence is presented of an unambiguous difference in the cross polar cap potential drop between the two hemispheres for an extended period of time.

The response of the dayside ionospheric flow to a sharp change in the direction of the interplanetary magnetic field (IMF) measured by the WIND spacecraft from negative Bz and positive By, to positive Bz and small By, has been studied using SuperDARN radar, DMSP satellite, and ground magnetometer data. In response to the IMF change, the flow underwent a transition from a distorted twin-cell flow involving antisunward flow over the polar cap, to a multi-cell flow involving a region of sunward flow at high latitudes near noon. The radar data have been studied at the highest time resolution available (~2 min) to determine how this transition took place. It is found that the dayside flow responded promptly to the change in the IMF, with changes in radar and magnetic data starting within a few minutes of the estimated time at which the effects could first have reached the dayside ionosphere. The data also indicate that sunward flows appeared promptly at the start of the flow change (within ~2 min), localised initially in a small region near noon at the equatorward edge of the radar backscatter band. Subsequently the region occupied by these flows expanded rapidly east-west and poleward, over intervals of ~7 and ~14 min respectively, to cover a region at least 2 h wide in local time and 5° in latitude, before rapid evolution ceased in the noon sector. In the lower latitude dusk sector the evolution extended for a further ~6 min before quasi-steady conditions again prevailed within the field-of-view. Overall, these observations are shown to be in close conformity with expectations based on prior theoretical discussion, except for the very prompt appearance of sunward flows after the onset of the flow change.Key words. Ionosphere (Auroral ionosphere) · Magnetospheric physics (Magnetopause · cusp · and boundary layers; Magnetosphere · ionosphere interaction)

The DE 1 imaging instrumentation provides a full view of the entire auroral oval every 12 min for several hours during each orbit. We examined five examples of global evolution of the aurora that occurred during the northern hemisphere winter of 1981-1982 when the z component of the interplanetary magnetic field was positive and the y component was changing sign. Evolution of an expanded auroral emission region into a theta aurora appears to require a change in the sign of By during northward interplanetary magnetic field (IMF). Theta aurora are formed both from expanded duskside emission regions (By changes from positive to negative) and dawnside emission regions (By changes from negative to positive), however the dawnside-originating and duskside-originating evolutions are not mirror images. The persistence of a theta aurora after its formation suggests that there may be no clear relationship between the theta aurora pattern and the instantaneous configuration of the IMF.

DMSP satellite plasma flow data from 1987–1990 are used to derive empirical models of the polar cap potential for quasi-steady interplanetary magnetic field (IMF) conditions. The large data set, due to the high duty cycle and nearly Sun synchronous DMSP orbits, allowed very stringent data selection criteria. The analysis indicates that a good description of the unskewed (Heppner Maynard pattern A) steady state polar cap potential is ΦA = 10−4v2+11.7B sin3 (θ/2) kV, where v is the solar wind velocity in kilometers per second, B is the magnitude of the interplanetary magnetic field in nanoteslas, and θ = arccos (Bz/|B|)GSM. The IMF-dependent contribution to the cross polar cap potential does not depend significantly on solar wind pressure. Functional forms for the potential do benefit from inclusion of an IMF independent term proportional to the solar wind flow energy. Best fits to IMF-independent contributions to the steady state polar cap potential yield ∼16 kV for vsw = 400 kilometers per second. During steady IMF the total unskewed polar cap potential drop is shown to be approximately ΦA = 16.5 + 15.5 Kp kV. The distribution of potential around the polar cap is examined as a function of magnetic local time. A sinusoidal distribution is an excellent description of the distribution, and more complex forms are not justified by this data set. Analysis of this data set shows no evidence of saturation of the polar cap potential for large |MF|. A simple unified description of the polar cap potential at all magnetic local times (MLT) and IMF, Φ(IMF, MLT) = −4.1 + 0.5 sin ((2π/24) MLT + 0.056 + 0.015 Byeff) (1.1 × 10–4 v2 + 11.1 B sin3 (θ/2)) kV, is generated, where Byeff is By (−BY) in the northern (southern) hemisphere. If IMF data is unavailable, the polar cap potential is well described by ΦA(Kp, MLT) = − 4.1 + 1/2 sin ((2π/24) MLT + ϕHM)(16.4 + 15.2 Kp) kV, where OϕHM is a small phase correction of (−0.054, −0.031, 0.040) for Heppner-Maynard convection patterns (BC, A, DE), respectively.

[1] We present the observations of equatorial plasma bubbles in the evening sector by the Communication/Navigation Outage Forecasting System (C/NOFS) satellite during 2011. We illustrate with a few examples the overall properties of the equatorial ionosphere as the solar activity approached maximum. C/NOFS was often below the F peak and this allowed us to examine the early phases of irregularity formation. We show examples when C/NOFS detected a continuous generation of plasma bubbles near the sunset terminator over eight successive orbits (∼12 h). A clear prereversal enhancement of upward plasma drift occurred between 18:00 and 19:00 LT when plasma bubbles were detected by C/NOFS, and the peak value of the upward ion drift at or near the magnetic equator was 40–70 m s−1. In some cases, C/NOFS was well below the Fpeak and detected wide regions with very low plasma density over ∼3000 km in longitude in the evening sector, and plasma bubbles were generated within the low-density region. C/NOFS also detected simultaneous existence of plasma bubbles between 19:00 and 03:00 LT, corresponding to a longitudinal coverage of ∼12,000 km. Significant differences in the characteristics of plasma bubbles between periods of low and high solar activity are identified. Large plasma bubbles occur in the midnight-dawn sector at low solar activity but in the evening sector at high solar activity. The lifetime of plasma bubbles is long (7 h or longer) at low solar activity but is short (∼3 h) at high solar activity. Broad plasma depletions occur near dawn at low solar activity, but wide low-density regions with multiple plasma bubbles occur in the evening sector at high solar activity.

[1] When the interplanetary magnetic field (IMF) is dawnward or duskward, magnetic merging between the IMF and the geomagnetic field occurs near the cusp on the dayside flanks of the magnetosphere. While these periods are usually considered “quiet,” they can lead to intense localized energy deposition into the dayside ionosphere. We analyze two intervals during the geomagnetic storm on 24 August 2005: one with steady duskward IMF and one with steady dawnward IMF. Using outputs from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) and data from the Defense Meteorological Satellite Program, we show that intense Joule heating exists on fast ionospheric flow channels which lie on open field lines. In addition, the flow channels on open field lines have large components in the sunward direction and therefore resist the bulk solar wind and magnetosheath flow. We compare observed velocities with predicted reconnection jet speeds using magnetosheath and cusp parameters from an MHD simulation. Results suggest that the fast ionospheric flow corresponds to portions of the reconnection jet populated by low-density plasma. The importance of ionospheric conductance in determining the ionospheric flow is also discussed.

[1] We report the results of an investigation into the effect of solar wind properties on the saturation of the polar cap potential (CPCP) during periods of strongly southward IMF. We use propagated solar wind data to search for periods between 1998 and 2007 when the interplanetary electric field is stable for more than 50 min and placed further conditions on the availability of SuperDARN and DMSP velocity data. CPCP values are calculated from these data sets and various fits of the polar cap potential to the interplanetary electric field (IEF) are compared. It is found that the trend is nonlinear, with a square root function fitting better than a straight line, and that the CPCP does not appear to exhibit asymptotic behavior. The nonlinearity of the CPCP is then correlated with various interplanetary parameters to test the various models of polar cap potential saturation. It is also found that the deviation of the CPCP from a linear fit has statistically significant correlation with solar wind Alfvenic Mach number and no significant correlation with solar wind dynamic pressure.

We analyze ionospheric convection patterns over the polar regions during the passage of an interplanetary magnetic cloud on January 14, 1988, when the interplanetary magnetic field (IMF) rotated slowly in direction and had a large amplitude. Using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure, we combine simultaneous observations of ionospheric drifts and magnetic perturbations from many different instruments into consistent patterns of high-latitude electrodynamics, focusing on the period of northward IMF. By combining satellite data with ground-based observations, we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres. We present evidence that a lobe convection cell was embedded within normal merging convection during a period when the IMF By and Bz components were large and positive. As the IMF became predominantly northward, a strong reversed convection pattern (afternoon-to-morning potential drop of around 100 kV) appeared in the southern (summer) polar cap, while convection in the northern (winter) hemisphere became weak and disordered with a dawn-to-dusk potential drop of the order of 30 kV. These patterns persisted for about 3 hours, until the IMF rotated significantly toward the west. We interpret this behavior in terms of a recently proposed merging model for northward IMF under solstice conditions, for which lobe field lines from the hemisphere tilted toward the Sun (summer hemisphere) drape over the dayside magnetosphere, producing reverse convection in the summer hemisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap. The positive IMF Bx component present at this time could have contributed to the observed hemispheric asymmetry. Reverse convection in the summer hemisphere broke down rapidly after the ratio |By/Bz| exceeded unity, while convection in the winter hemisphere strengthened. A dominant dawn-to-dusk potential drop was established in both hemispheres when the magnitude of By exceeded that of Bz, with potential drops of the order of 100 kV, even while Bz remained northward. The later transition to southward Bz produced a gradual intensification of the convection, but a greater qualitative change occurred at the transition through |By/Bz| = 1 than at the transition through Bz = 0. The various convection patterns we derive under northward IMF conditions illustrate all possibilities previously discussed in the literature: nearly single-cell and multicell, distorted and symmetric, ordered and unordered, and sunward and antisunward.

Observations are presented of the ionospheric convection in cross sections of the polar cap and auroral zone as part of the study of the interaction of the Earth's magnetosphere with the magnetic cloud of January 13-15, 1988. For strongly northward IMF, the convection in the Southern Hemisphere is characterized by a two-cell convection pattern comfined to high latitudes with sunward flow over the pole. The strength of the flows is comparable to that later seen under southward IMF. Superimposed on this convection pattern there are clear dawn-dusk asymmetries associated with a one-cell convection component whose sense depends on the polarity of the magnetic cloud's large east-west magnetic field component. When the cloud's magnetic field turns southward, the convection is characterized by a two-cell pattern extending to lower latitude with antisunward flow over the pole. There is no evident interhemispheric difference in the structure and strength of the convection. Superimposed dawn-dusk asymmetries in the flow patterns are observed which are only in part attributable to the east-west component of the magnetic field.

[1] During a magnetic storm on 14–16 December 2006, a polar cap tongue of ionization (TOI) was detected by an all-sky imager (ASI) at Resolute Bay, Canada (74.73°N, 265.07°E). We investigate the temporal evolution and spatial structure of the TOI in detail by combining the optical data with other observations (e.g., solar wind, GPS total electron content, SuperDARN, and DMSP and NOAA POES satellites). The TOI was observed as a bright and elongated 630 nm airglow plume for 4 h during the main phase of the storm. This interval corresponded to a period of prolonged stable large-amplitude southward IMF during a coronal mass ejection (CME). One to one and a half hours before the appearance of TOI, the polar cap boundary expanded rapidly far equatorward, and a positive ionospheric storm occurred. This implies that both the “expansion of the high-latitude plasma convection” and “build up of the source plasma in the midlatitudes” are necessary conditions for the formation of a TOI. Because both of them were triggered by a major southward turning of the IMF, the prolonged large-amplitude southward IMF orientation in the trailing part of the CME was primarily responsible for the generation of TOI. After its appearance, the TOI exhibited dynamic motion in the dawn to dusk direction. Simultaneous SuperDARN data suggest that a longitudinal progression of subauroral polarization stream controlled this dynamic motion. The optical TOI was found to be a continuous stream elongated in the noon-midnight direction although it contained some mesoscale patterns. Absence of large-scale temporal changes in the cusp plasma flow during the stable IMF period allowed the TOI to remain continuous without being broken into polar cap patches. The mesoscale structures within the TOI were probably produced by small-scale velocity fluctuations in the cusp plasma flow. The TOI as visualized with the all-sky airglow imager was found to be much more dynamic and much more complicated than we ever thought. The current study indicates that such a behavior of the TOI was presumably caused by a combination of temporal variations in the global-scale plasma circulation system, expansion and contraction of the polar cap area, and plasma density changes in the dayside low to midlatitudes.

[1] The purpose of this paper is to show a “proof of the existence” of the ionospheric situation that is expected for the interchange cycle, during periods of favorable interplanetary magnetic field (IMF) and dipole tilt conditions. To do so, we present three case studies of dayside high-latitude ionospheric convection that is observed around the equinoxes (near-zero dipole tilt) and at small IMF clock angles (one θc ∼ −30° event and two θc ∼ 30° events, where θc ≡ Arg(BZ + iBY)), using Super Dual Auroral Radar Network (SuperDARN)/Defense Meteorological Satellite Program (DMSP)/National Oceanic and Atmospheric Administration (NOAA) data in the Northern Hemisphere and, when available, DMSP data in the Southern Hemisphere. The convection pattern exhibits twin reverse cells in both hemispheres, but the constituents of each cell are different. In the Northern Hemisphere, for θc ∼ 30° (θc ∼ −30°), the center of the dawnside (duskside) cell is located poleward of the polar cap boundary, while the center of the duskside (dawnside) cell is located equatorward of the polar cap boundary. For θc ∼ 30°, we confirmed that the above-mentioned dawn-dusk relation reverses in the Southern Hemisphere. The north-south asymmetric behavior of the conjugate reverse cells, on the dawnside and duskside each, is consistent with two independent interchange cycles that result from the coupling of IMF-lobe reconnection in one hemisphere with lobe-closed reconnection in the opposite hemisphere.

[1] A fortuitous multisatellite observation of a dayside merging event driven by a sharp southward interplanetary magnetic field (IMF) turning illuminates several ionospheric particle and auroral reconnection signatures. Shortly after the southward IMF turning, as propagated from ACE, DMSP F13 observed a region equatorward of the cusp emptied of magnetospheric ions and bounded by two sharp intensifications of electron energy flux. The equatorward electron burst occurred at 74.4° magnetic latitude. That is also both the boundary to which magnetospheric ions were depleted and to which magnetosheath ions appeared in a DMSP F12 satellite pass 4 min later. These latter ions evinced a low-energy cutoff, implying that about 4-5 min elapsed since merging. The equatorward electron burst observed by F13 reached an intensity of 8.6 ergs/cm 2 s, and thus must have produced a strong auroral signature. Indeed, Polar UVI observed a dayside auroral transient at the appropriate latitude and local time. This burst was likely Alfvenic (it was not monoenergetic (not associated with a quasi-static electric field)). The poleward electron burst observed by F13 (up to 4.6 ergs/cm 2 s), was at the equatorward edge of the old cusp and also had a broadband (Alfvenic) signature. Drift meter data on F13 indicates that the merging burst produced flow speeds up to 2 km/s and a 47 kV cusp contribution to the polar cap potential, of which 25 kV was in a narrow high-speed spike. Thus, this event probably constituted a significant fraction of the cross-polar cap potential.

[1] Contemporary magnetosphere models now include species-dependent dynamics. Energetic O+ has significant consequences for the energy stored in the ring current, the rate of reconnection, and perhaps the timing of substorm injections. The mechanism by which thermal O+ escapes from the top of the ionosphere and into the magnetosphere is not fully understood. Previous studies have used dynamic auroral boundary coordinates to describe the outflowing energetic O+ ions above the ionosphere. In this study we focus on the vertical flow of O+ ions at lower altitudes before they are accelerated to escape velocity. An algorithm has been devised to identify auroral zone boundaries using precipitating electron observations from the Defense Meteorological Satellite Program (DMSP) spacecraft. Vertical ion flows measured by the DMSP special sensor for ions electrons and scintillation ion drift meter and the retarding potential analyzer instruments aboard the F12 (noon-midnight) and F13 (dawn-dusk) spacecraft from 1997 to 1998 were projected into dynamic auroral boundary coordinates and used to investigate the dependence of Southern Hemisphere bulk flows on interplanetary magnetic field (IMF) and geomagnetic conditions. Initial results show that (1) net upward flows occur primarily in the auroral zone and net downward flows occur primarily in the polar cap, (2) there exists a strong upward flow at 9 magnetic local time (MLT) near the polar cap boundary, 3) the downward ion flow orientation is strongly dependent on IMF By, and 4) the auroral boundary does not coincide exactly with the upward/downward boundary for bulk flows.

[1] The CINDI plasma instrument package on board the C/NOFS spacecraft performs in situ measurements of the velocity, temperature, composition, and density of ionospheric ions. In a 402 by 849 km orbit with a 13° inclination its apogee reaches the topside ionosphere at the same altitude as the polar-orbiting DMSP spacecraft that also carry similar plasma measurement packages. Over the course of about 66 days the apogee of C/NOFS' orbit cycles once through all solar local times. This paper presents the apogee observations from the CINDI instruments between 21 August and 5 September 2008 when the C/NOFS' apogee passed through the orbital planes of the duskside equatorial crossings of the four operational DMSP spacecraft. Combining the data from all five spacecraft reveals the spatial distribution of the total ion density, the ion temperature, and the ion composition at these altitudes. Comparisons of these observed distributions from CINDI and DMSP are used to check the agreement between the new CINDI instruments and the long-operating DMSP instruments. The results of the density and temperature measurements show good agreement, however there are two regions where the composition and temperature measurements show discrepancies that result from the limitation of the plasma instruments in an environment of extremely low density plasma composed of over 85% light ions (H+ and He+).

[1] In order to understand temporal variations and spatial distributions of plasma density enhancements in the polar magnetosphere during geomagnetic storms, nearly simultaneous observations of storm time electron densities in the polar magnetosphere by the Akebono satellite and ion upflows in the polar ionosphere by the DMSP satellites were investigated. Akebono observations show that the electron densities were highest (>100 cm−3 at ∼9000 km altitude) from the main to early recovery phases of geomagnetic storms. The regions of enhanced electron density were not localized but widely spread in the polar magnetosphere. Coordinated observations by the DMSP satellites detected ion upflows with large fluxes (∼1010/cm2/s) in and near the cusp, when the electron density enhancements were observed by the Akebono satellite. This result indicates that the storm time electron density enhancements are caused by cleft ion fountain mechanisms from the polar ionosphere. Very low-energy component (

[1] The Defense Meteorological Satellite Program (DMSP) has flown spacecraft with ionospheric payloads since 1987. Over this period, DMSP has collected over two hundred million electron density measurements. This paper presents a statistical analysis of the DMSP topside electron density measurements from 1995 to 2005 with additional data from 1992. Nearly 140 million quality-checked data points are binned by solar activity, magnetic latitude, season, and solar zenith angle or magnetic local time. When binned, the data usually obey a three-parameter lognormal distribution. These parameters represent the density floor (the minimum possible density), the median density, and the variability in the densities. In general, the density floor, the median density, and the variability increase with solar activity and peak near the magnetic equator. Away from the equator, these parameters are higher during local summer and for lower solar zenith angles. However, the median density and the variability near the magnetic equator are greater in the dusk sector where the prereversal enhancement pushes F region plasma to higher altitudes. In addition, all three parameters show a marked asymmetry between the northern and southern hemispheres. A probabilistic climatology model is created by modeling these parameters and using the lognormal probability distribution function. This new model provides an occurrence frequency for different densities given the F10.7 solar radio flux, magnetic latitude, day of year, and either solar zenith angle or magnetic local time.

[1] The Ion Velocity Meter (IVM), a part of the CINDI instrument package on board the C/NOFS spacecraft, makes in situ measurements of plasma temperature, composition, density, and velocity. The 16 April 2008 launch of C/NOFS coincided with the deepest solar minimum since the space age began with F10.7 cm radio fluxes in the 60–70 solar flux unit range. Because of the 13° inclination of the orbit the location of the perigee advances through all local times in about 66 days. This allows seasonal sampling of ionospheric temperature, density, and composition as a function of local time, magnetic latitude, and altitude. Measurements taken near the spacecraft's 402 km perigee altitude indicate an unusually cold low-density ionosphere with nighttime ion temperatures at the magnetic equator reaching as low as 600 K with an [O+]/[H+] ratio of 4 and maximum daytime temperatures of 1300 K. The O+ to H+ transition height is very low and at the highest altitudes measured H+ comprises over 75% of the ionospheric plasma at all local times. We compare average values of the measured parameters with those from the International Reference Ionosphere and with incoherent scatter radar measurements from Jicamarca.

[1] A study of the behavior of the ionospheric signature of the low-latitude boundary layer is conducted from signatures of the precipitating electrons and the ionospheric convection velocity. Observations are made under southward interplanetary magnetic field (IMF) conditions in a highly dynamic environment where temporal changes produce large excursions in the location of both the convection reversal boundary and the poleward edge of the particle precipitation associated with the outer edge of the low-latitude boundary layer. We describe the latitudinal width of the boundary layer as the displacement between these boundaries and the bulk plasma flow as the potential difference across the region. On either side of local noon the boundary layer has a latitudinal width of about one degree, widening to about two degrees near dawn and dusk. The potential drop across the boundary layer increases toward local noon and accounts for no more than 30% of the total potential across the polar cap. While the boundary layer is physically wider on the duskside than on the dawnside, the average potential across the dawnside boundary layer is larger than that across the duskside layer. The resulting electric field across the dawnside layer is approximately twice that found in the duskside layer. We also find a higher level of variability in the boundary layer width and potential on the dawnside than on the duskside that may be related to higher levels of turbulence and wave activity associated with the orientation of the IMF with respect to the bow shock.

[1] During major geomagnetic storms anomalous enhancements of the ionospheric density are seen at high and middle latitudes. A number of physical mechanisms have been invoked to explain these storm time density anomalies including an expansion of high-latitude electric plasma convection to midlatitudes, thermospheric neutral winds, and changes in the ionospheric composition. However, it remains unclear which mechanism plays the dominant role in the formation of storm time density anomalies, partly because of insufficient coverage of the measurements of global electric convection and thermospheric winds at midlatitudes. This paper describes a novel technique for extracting the storm time E × B convection boundary from in situ measurements of plasma bulk motion obtained by LEO DMSP satellites. The convection boundary estimated from the DMSP data during major magnetic storm of 20 November 2003 has been compared with the global distributions of the ionospheric plasma deduced from characteristics of GPS signals acquired by a ground-based network of GPS receivers. The tomographic inversion of GPS data using a three-dimensional time-dependent inversion technique reveals the spatial and temporal evolution of the storm time density anomaly. Comparison between the tomographic reconstructions of the ionospheric plasma distributions and in situ DMSP measurements of plasma bulk motion suggests that the convective flow expanded low enough in latitude to encompass, in part, the formation of the midlatitude TEC anomaly. Some features of the TEC dynamics observed during the 20 November 2003 storm, however, suggest that mechanisms other than the expanded ionospheric convection (such as thermospheric neutral winds) are also involved in the formation of the midlatitude anomaly.

[1] The high-latitude response of sunward E × B flow and Birkeland field-aligned currents (FAC) is analyzed for the 15 May 2005 magnetic cloud that generated a great magnetic storm (SYM-H = −305 nT at 0820 UT). The interplanetary magnetic field (IMF) clock angle, θ = arctan(By/Bz), gradually rotated from 65° to −80° during the 10-h long northward IMF period and the recovery of this storm. DMSP observations confirm a dawnward migration of a Northern Hemisphere sunward E × B flow channel (FC) between a downward and upward FAC pair. This FAC system developed during southward IMF (θ = 109°) at the poleward edge of the duskside auroral oval as part of a four-sheet FAC system 23 min before the IMF became northward. TIMED/GUVI observations show that the dawnward migration of the upward FAC coincides with a drifting transpolar auroral arc (TPA). IMAGE/WIC did not observe a TPA in the southern (winter) hemisphere. DMSP and Iridium observations are in good agreement with MHD simulation predictions of a northward IMF reorientation of high-latitude FACs. The northern FC migration was likely due to summer hemisphere conductances, a strong average IMF Bx = −35 nT and the sunward dipole tilt angle that favor a northern high-latitude reconnection mechanism for a well-organized sunward FC and FAC system migration. The storm recovery rate appeared to be related with the region 2 FAC. A fast 11.4 nT/h rate was observed for a weak or nonexistent region 2 system during the high-latitude FAC redistribution. The SYM-H recovery slowed significantly to 0.9 nT/h following the 1800 UT region 2 system recovery.

[1] During geomagnetic storms the ability of the Super Dual Auroral Radar Network (SuperDARN) to measure ionospheric convection becomes limited when the radars suffer from absorption and the auroral disturbance expands equatorward of the radar sites. To overcome these shortcomings, it was decided to construct a SuperDARN radar at middle latitudes on the grounds of the NASA Wallops Flight Facility. This paper presents the first comprehensive analysis of Doppler measurements from the Wallops radar, which commenced operations in May 2005. Wallops measurements are compared with the Goose Bay radar during the onset of a geomagnetic storm on 31 August 2005: Goose Bay measured the onset of geomagnetic activity at high latitude while Wallops monitored the expansion of convection to middle latitudes. Average convection patterns binned by the Kp geomagnetic index are also presented. During weak-moderate geomagnetic activity (Kp ≤ 3) the Wallops radar observes ionospheric irregularities between 50° and 60° magnetic latitude drifting westward across much of the nightside. When these measurements are incorporated into the calculation of an average SuperDARN convection pattern, the streamlines of polar cap outflow on the nightside become kinked in a manner reminiscent of the Harang discontinuity. This morphology arises quite naturally when the two-cell convection at high latitudes merges with the prevailing westward convection at middle latitudes. During increased geomagnetic activity (Kp ≥ 3), Wallops is able to measure the expansion of auroral electric fields to middle latitudes and the average SuperDARN cross-polar cap potential is increased by 25%.

[1] We have examined characteristics of the vertical O+ flux in the topside high-latitude ionosphere from measurements of the vertical ion drift and ion number density made by the Defense Meteorological Satellite Program (DMSP) F13 spacecraft from June 1996 to January 1997, June 1998 to January 1999, and June 2001 to January 2002. In the polar cap the vertical ion flux is, on average, downward at all locations. However, in the auroral zone the ion flux is highly structured, and a net upward flux is produced primarily by spatially and temporally confined events containing upward fluxes in excess of 109 cm−2s−1. These dominant high-flux events tend to be produced more commonly by high densities rather than by high velocities. The range of vertical velocity and number density changes with season and solar cycle with greater variability in the vertical velocity occurring during winter and at lower levels of solar activity. There is also evidence that the vertical fluxes are a function of the interplanetary magnetic field, with upward fluxes in the auroral zones for negative Bz and upward fluxes in the polar cap for positive Bz.

[1] The ion drift velocities and electric field derived from the Assimilative Mapping of Ionospheric Electrodynamics technique (AMIE) are compared with the same parameters observed by the Defense Meteorological Satellite Program's (DMSP) Special Sensor-Ions, Electrons, and Scintillation (SSIES) instrument. In addition the along-track cross polar cap potential, the correlation between the curves, and other metrics are compared and those results are binned by various criteria. The criteria include spacecraft, year, season, and magnetic activity level. We observed a reasonable correlation between the AMIE results and DMSP data with respect to the along-track potential, though the AMIE results tend to show a lower potential by some 30-50 percent, depending on spacecraft and orbit. Spacecraft in the same general orbits show similar relations with AMIE, implying that there is good intersatellite calibration for the DMSP array. There is a great deal of variability in the correlation, depending on season and activity level, and correlations with some of the spacecraft show a solar cycle dependance. It is further found that when we set a multifaceted criteria on each orbit for a DMSP-AMIE match, including along-track potential, V y correlation, and peak potential locations, the results correspond between 35% and 55% for the dawn-dusk spacecraft, while they only correspond 7%-12% of the time for noon-midnight satellites.

[1] Magnetic reconnection that involves overdraped lobe field lines is called internal reconnection since it occurs inside the magnetopause. When the interplanetary magnetic field (IMF) is due northward and the Earth's dipole is tilted significantly, internal reconnection occurs in the winter hemisphere, not only between a summer lobe field line and a winter lobe field line but also between a summer lobe field line and a closed field line. The latter internal reconnection drives “reciprocal cells” in the winter ionosphere that circulate exclusively in the closed field line region. The reciprocal cells are intimately related to the lobe cells in the summer ionosphere in that in the steady state, the reconnection voltage associated with merging of IMF and open field lines is equal to the sum of the lobe cell potential and the reciprocal cell potential. In this paper we present observations of convection patterns consistent with those expected for reciprocal cells, using ionospheric radar and low-altitude satellite data. We also show the concurrence of lobe cells and reciprocal cells. The observations of reciprocal cells provide support for the internal reconnection between a summer lobe field line and a closed field line. In addition, we show that equatorward of the polar cap boundary, magnetosheath-like ions are drifting from noon toward the flankside in both hemispheres. We suggest that these ions are of magnetosheath origin and that they entered the closed region of the magnetosphere through the rotational discontinuity associated with internal reconnection. These magnetosheath-like ion observations strongly support the occurrence of internal reconnection.

[1] We investigated the impact on the terrestrial ring current of a coronal mass ejection (CME) and the associated magnetic cloud that severely disturbed the Earth's magnetosphere on 20 November 2003. This CME decreased the Dst index to −472 nT, which makes it the second largest storm, based on the minimum Dst index values, observed between 1957 and 2004. Data from the DMSP, NOAA, and LANL satellites showed the unique characteristics of this storm; a polar cap potential that increased to at least 200 kV, a polar cap boundary that moved as low as about 60° MLAT, a plasma sheet density that increased to 5 cm−3 at L = 6.6 when the Dst index was near its minimum, and the inner edge of the plasma sheet ion population that penetrated into a region for which L ≤ 1.5. In order to study the dynamics of the ring current and the associated magnetosphere-ionosphere coupling, we performed a ring current simulation that computed the evolution of the phase space density of the ring current ions and the closure of the electric current between the magnetosphere and the ionosphere. Major results were as follows: (1) The ring current, in terms of the Dst index and the inner edge of the plasma sheet, can result from the enhancement of the convection electric field, given the polar cap potentials used in the model; (2) The solar wind particles probably penetrated quickly into the geosynchronous altitude on the nightside with a lag of about 80 min, resulting in further enhancement of the ring current; (3) Dense geocoronal neutral hydrogen or a large coefficient of pitch angle diffusion (>10−4 s−1) is probably needed to account for the rapid motion of the inner edge of the plasma sheet (or the ring current) population to a higher L value; (4) Both the simulated and observed field-aligned current (FAC) distributions show multiple current sheets, rather than the normally expected two current sheets. Fluctuations in the polar cap potential and the plasma sheet density are believed to cause the multiple sheets of field-aligned currents; (5) The equatorward edge of the Region 2 type field-aligned currents was observed to expand as low as 40° MLAT, which is consistent with the simulation; and (6) The convection pattern can be much more complicated than an average one due to a strong Region 2 FAC. A noticeable feature was the reversal of the zonal ionospheric plasma flow that emerged on the dawnside. In particular, a westward flow was observed in the equatorial region of the eastward plasma flow at dawn. Its speed had a local maximum of about 5° equatorward of the flow reversal. The flow reversal is thought to have resulted from the relatively strong shielding electric field.

[1] The question of whether the cross polar cap potential drop in the Earth's ionosphere saturates under conditions of extreme electric field in the solar wind has been tested observationally during the past several years. The challenge to proving the existence of this phenomenon is that periods of such extreme electric fields in the solar wind are relatively rare. The three superstorms of October and November 2003 provided ideal cases for testing this idea. We first review the earlier evidence of the saturation seen by the DMSP-F13 spacecraft during the 31 March 2001 superstorm and other storm events during the 1998–2002 time period. Then we present observations from the DMSP-F13 spacecraft during the October and November 2003 superstorms that show definite evidence of this saturation. In addition, some of the electric fields during these superstorms were almost twice as large as the largest fields previously studied, thus increasing the range of our sample set and further increasing our confidence in the existence of the saturation phenomenon. The data are compared with the saturated potentials predicted by the Hill-Siscoe model to test its validity. The DMSP measurements indicate that the saturation limit of the cross polar cap is about 260 kV.

[1] This paper reports results from a detailed analysis of theta auroras that occurred during the passage of a magnetic cloud on 8 November 2000. The interplanetary magnetic field (IMF) was generally northward for more than 12 hours, while the y-component of the IMF changed signs several times. Auroral images of the Northern Hemisphere acquired from the Ultraviolet Imager (UVI) on board the Polar satellite showed clear development of theta auroras during the prolonged northward IMF period. It is found that the theta aurora can be enhanced by compression of the magnetosphere associated solar wind pressure pulses. We also examine large-scale ionospheric plasma convection flow data from the SuperDARN radar network to study the ionospheric plasma flow pattern associated with the theta aurora. The radar data, when available, showed that two different plasma flows can coexist within the day-night part of the theta aurora. The nightside extension of the theta aurora is always on a region of antisunward convecting flows, whereas there is no consistent flow pattern found in the dayside portion of the theta aurora studied. In general, plasma flow outside the theta aurora cannot be determined because of lack of scattered radar signals, but from a few instances it suggested that the nightside theta aurora is flanked by antisunward flows. In contrast to the prevailing view that theta auroras reside in regions of closed magnetic flux that convect sunward, this study suggests one more aspect of the theta aurora and provides constraints for models and theories.

[1] We present an observational study of the global dynamics of the plasmasphere, aurora, ring current, and subauroral ionosphere on 17 April 2002 during a substorm. Global observations by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) and in situ observations by DMSP F13 provide a comprehensive view of the coupled response of the inner magnetosphere and ionosphere. At 1900 UT a substorm onset initiated a sunward convective impulse, which caused a ring current injection. The motion of this impulse past the plasmasphere caused ripples to propagate along the plasmapause, eastward and westward from premidnight magnetic local time (MLT). The motion of the ripples agrees exceptionally well with the motion of the aurora and the ring current, implying strong coupling. The westward moving ripple (on the duskside) participated in a two-phase plasmapause undulation effect. In the first phase (1915 UT to 1936 UT), a mild 0.4–0.5 RE bulge formed near 2000 MLT, probably caused by an E-field induced by local reduction of the magnetic field by the ring current pressure increase. In the second phase (1936 UT to 2037 UT) this mild bulge was removed by a subauroral polarization stream (SAPS) westward flow that stripped away the outer 1 RE of the duskside plasmasphere. The SAPS effect was observed in the ionosphere by DMSP between about 1930 UT and 2000 UT and is evident in vector E-fields inferred from plasmapause motion. All the observations of this event suggest strong coupling among the plasma populations of the magnetosphere-ionosphere system. This event represents the first identification of the directly observed global plasmaspheric effects of a substorm-driven impulse, the SAPS flow channel, and the ring-current magnetic field reduction.

[1] The evolution of the plasmasphere during the moderate 22–23 April 2001 storm is studied by comparing observed plasmasphere behavior to the predictions of a simulation. The plasmasphere observations include global images from IMAGE EUV and density and flow measurements from LANL MPA. The subjective uncertainty in the EUV plasmapause was found to be 0.2L. Inward plasmapause motion was correlated with southward interplanetary magnetic field (IMF), with a 27 min delay. The electric (E) field at the plasmapause was approximately 13% of the solar wind E-field. The observations support the idea that plasmaspheric erosion begins with a partial plasmapause indentation which then gradually widens in magnetic local time (MLT) to encompass the entire nightside. In situ measurements confirm a dayside plume of sunward flowing plasma, and images show that the plume subsequently experienced phases of MLT narrowing and rotation/wrapping. To simulate the event, we employed a test particle representation of the plasmapause using a parametric E-field model that includes corotation and convection from two sources: dayside magnetopause reconnection (DMR) and the subauroral polarization stream (SAPS). The model captures the phases of plume evolution, and it reproduces the observed plasmapause with maximum (mean) error

[1] The major magnetic storm of 4–5 June 1991 was well observed with the Combined Release and Radiation Experiment (CRRES) satellite in the duskside inner magnetosphere and with three Defense Meteorological Satellite Program (DMSP) spacecraft in the polar ionosphere. These observations are compared to results from the Rice Convection Model (RCM), which calculates the inner magnetospheric electric field and particle distribution self-consistently. This case study, which uses the most complete RCM runs to date, demonstrates two significant features of magnetospheric storms, the development of subauroral polarization streams (SAPS) and plasma-sheet particle injection deep into the inner magnetosphere. In particular, the RCM predicts the electric field peak near L = 4 that is observed by the CRRES satellite during the second injection. The RCM calculations and DMSP data both show SAPS events with similar general characteristics, though there is no detailed point-by-point agreement. In the simulation, SAPS are generated by the deep penetration of plasma sheet protons to L < 4 and Earthward of the plasma sheet electrons. Similarly, the vast majority of the ions that make up the storm-time ring current came from the plasma sheet; most of the particles that made up the prestorm quiet-time ring current escaped through the dayside magnetopause during ring current injection. The RCM demonstrates the capability of plasma sheet ions to reach all ring current orbits and predicts the location of the injected particles (both ions and electrons) reasonably well. However, it overpredicts the ion flux in the inner magnetosphere.

This study presents, for the first time, detailed spatiotemporal measurements of the reconnection electric field in the Northern Hemisphere ionosphere during an extended interval of northward interplanetary magnetic field. Global convection mapping using the SuperDARN HF radar network provides global estimates of the convection electric field in the northern polar ionosphere. These are combined with measurements of the ionospheric footprint of the reconnection X-line to determine the spatiotemporal variation of the reconnection electric field along the whole X-line. The shape of the spatial variation is stable throughout the interval, although its magnitude does change with time. Consequently, the total reconnection potential along the X-line is temporally variable but its typical magnitude is consistent with the cross-polar cap potential measured by low-altitude satellite overpasses. The reconnection measurements are mapped out from the ionosphere along Tsyganenko model magnetic field lines to determine the most likely reconnection location on the lobe magnetopause. The X-line length on the lobe magnetopause is estimated to be ~6–11 RE in extent, depending on the assumptions made when determining the length of the ionospheric X-line. The reconnection electric field on the lobe magnetopause is estimated to be ~0.2mV/m in the peak reconnection region. Key words. Space plasma physics (Magnetic reconnection) – Magnetospheric physics (Magnetopause, cusp and boundary layers) – Ionosphere (Plasma convection)

[1] In this paper, for steady-state northward interplanetary magnetic field (IMF) with ∣BY∣ ∼ BZ, we describe a new merging sequence that results in polar cap bifurcation and accompanying paired “exchange cells” in the ionospheric convection pattern. Although the IMF is northward, it reconnects with the closed geomagnetic field on the dayside high-latitude magnetopause, creating two types of open geomagnetic field lines. For the first type the neutral point and the foot point are in the same hemisphere; for the second type the neutral point and the foot point are in opposite hemispheres. The latter type of field lines slips on the magnetopause in the azimuthal direction opposite to the normal BY- associated flux transport and forms an overdraped tail lobe. The ionospheric signature of this overdraped lobe is the appearance of an open magnetic flux island inside the dawn/dusk plasma sheet (i.e., polar cap bifurcation). For BY > 0 the island emerges in the duskside (dawnside) plasma sheet in the northern (southern) ionosphere and conversely for BY 0 the pair is located in the noon-dusk and midnight-dawn (dawn-noon and dusk-midnight) quadrants of the northern (southern) ionosphere; for BY < 0 a mirror image with respect to the noon-midnight meridian applies to the convection pattern. We demonstrate observational evidence that supports this model.

Using ionospheric data from the SuperDARN radar network and a DMSP satellite we obtain a comprehensive description of the spatial and temporal pattern of day-side reconnection. During a period of southward interplanetary magnetic field (IMF), the data are used to determine the location of the ionospheric projection of the dayside magnetopause reconnection X-line. From the flow of plasma across the projected X-line, we derive the reconnection rate across 7 h of longitude and estimate it for the total length of the X-line footprint, which was found to be 10 h of longitude. Using the Tsyganenko 96 magnetic field model, the ionospheric data are mapped to the magnetopause, in order to provide an estimate of the extent of the reconnection X-line. This is found to be ~ 38 RE in extent, spanning the whole dayside magnetopause from dawn to dusk flank. Our results are compared with previously reported encounters by the Equator-S and Geotail spacecraft with a reconnecting magnetopause, near the dawn flank, for the same period. The SuperDARN observations allow the satellite data to be set in the context of the whole magnetopause reconnection X-line. The total potential associated with dayside reconnection was ~ 150 kV. The reconnection signatures detected by the Equator-S satellite mapped to a region in the ionosphere showing continuous flow across the polar cap boundary, but the reconnection rate was variable and showed a clear spatial variation, with a distinct minimum at 14:00 magnetic local time which was present throughout the 30-min study period.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; magnetosphere-ionoshere interactions) – Space plasma physics (magnetic reconnection)

[1] The ionospheric response to the magnetic storm of 15 July 2000 is investigated using the global total electron content (TEC) maps provided by global positioning system and the measurements of ion density, composition, and drift velocity from the Defense Meteorological Satellite Program (DMSP) F13 and F15 spacecraft. The global TEC maps showed clear seasonal effects that can be characterized by a dominance of a negative ionospheric storm (decrease in plasma density) in the summer (northern) hemisphere and the pronounced positive ionospheric storm (increase in plasma density) in the winter (southern) hemisphere. The northern negative storm phase rapidly expanded to the equator at midnight and even penetrated to the opposite hemisphere during the storm main phase. In the southern hemisphere, the negative storm phase began in the morning sector but was confined to narrow latitude and local time sectors owing to strong poleward winds and ion drag on the dayside. The negative storm phases in the opposite hemispheres kept out of phase, lasted for a day, and corotated with Earth. These characteristics show good qualitative similarity with the predictions of global model simulations. The positive storm phase prevailed in the southern low-middle latitudes and was most pronounced during nighttime. In that region, the quiet time DMSP measurements at 1800 LT, 2100 LT, 0600 LT, and 0900 LT showed low ion density, low O+ proportion, and large downward ion drift velocity compared with those in the northern hemisphere. During storm time the O+ proportion and ion concentration increased to the levels seen in the northern hemisphere while the downward ion drift velocity was much decreased. The excellent temporal and spatial correspondence of the increase in ion concentration with the decrease in downward ion drift velocity indicates that the maintenance of the F layer at high altitudes by the storm-induced equatorward neutral winds was the main driver of the positive ionospheric storm. The quiet time hemispheric asymmetry was most significant at nighttime, and therefore the positive storm effect appeared most pronounced at nighttime in the winter hemisphere.