SM13B-1310
Nightside Region-2 electric field evolution during substorm growth and expansion phases
Observations of the strong coupling of Harang reversal region dynamics to substorm evolutions suggest that nightside Region-2 physics is closely related with substorm dynamics. We use SuperDARN HF radar and DMSP observations to evaluate the Region-2 electric field evolution for nine substorm events. Auroral images from the IMAGE spacecraft, when available, are superposed with these ionospheric convection patterns to better illustrate the relation between the Region-2 electric fields near the Harang reversal region and the substorm dynamics. Based on these nine events, we found that the Harang reversal forms and becomes well defined during the growth phase of all the nine substorms. Azimuthal flows equatorward of the Harang reversal increase and reach at least ~500 m/s. These observational evidences indicate that shielding, subauroral ion drifts (SAIDS), and the Harang reversal evolve together as part of the growth phase development of the Region-2 system and they become well developed during the substorm growth phase. In addition, we find that substorm onset seems to occur slightly poleward of the center of the Harang flow shear and in the weak equatorward flows near the western edge of the dawn convection cell.
SM13B-1311
Ionospheric signature of flow bursts in the magnetotail: Geotail-SuperDARN conjunction study
Ionospheric convection signatures associated with flow bursts in the magnetotail are examined statistically on the basis of the simultaneous observations made by the Geotail spacecraft and the SuperDARN radars covering the footprint of Geotail. Our statistical study shows that most (~ 82 %) of the flow bursts in the magnetotail are accompanied by a significant enhancement of the ionospheric convection flow at the footprint of Geotail mapped along the field line. Generally the magnetotail flows enhance and decay rapidly, while the corresponding ionospheric flows develop as quickly but tend to fade away gradually. However, the start time of most of the magnetotail tail flows coincides with that of the ionospheric flow enhancements within a few minutes. Both the spatial and temporal correspondence suggests that those flow bursts take place as a M-I-coupled process. While the ionospheric convection often shows an overall enhancement of the nightside part of the dawn or dusk convection cell, the most significant enhancement tends to take place at or near the foot point of the corresponding magnetotail flow bursts. There the ionospheric flow enhancement appears as an uniform convection channel or a local shear/vortex-like convection cell. Both types of convection enhancement are found with roughly the same probability, regardless of the magnetotail flow characteristics. How a flow burst in the magnetotail can have those different counterparts on the ionospheric side will be discussed by considering substorm effects as well as the spatial characteristics of the flow bursts and the ionosphere.
SM13B-1312
Effects of plasma sheet condition on the evolution of shielding and the Harang reversal under weak convection: RCM simulations
Shielding of the convection electric field and the Harang reversal associated with the region 2 field-aligned current system are fundamental manifestations of the large-scale plasma transport within the tail plasma sheet and the electrodynamic coupling of this transport to the ionosphere. However, how their formation and evolution is affected by the plasma sheet density and temperature is not well understood. We have used the RCM with Tsyganenko 96 magnetic field model to investigate this effect of plasma sheet conditions under weak convection. We have found that the existence of an overlap in local time of Region 2 upward and downward field aligned currents is necessary for the formation of the Harang reversal. The downward field aligned current in this overlap region is associated with electrons and low energy ions, while the upward field aligned current is associated with high-energy ions. Higher plasma sheet pressure can cause quicker shielding of the penetration electric field. If the time scale of the shielding is much shorter than the drift time scale of the low energy particles, these low energy particles cannot penetrate earthward enough to cause the overlap, thus preventing formation of the Harang reversal. Under the same plasma sheet pressure, higher auroral conductance can result in slower shielding. Higher plasma temperature enhances the upward field-aligned currents in the overlap region, therefore leading to a stronger Harang reversal. Currently we are running the simulation under different strengths of convection and with more realistic MLT dependent boundary conditions based on the Geotail observations.
SM13B-1313
Three-Dimensional Distributions of Ionospheric Electric Potentials Determined by a Global MHD Simulation
The ionospheric electric potential is one of the most important parameters for the study of the magnetosphere- ionosphere system since it shows not only the electric field but the convection pattern in geospace system. In recent global MHD simulations and several models of the ionospheric electric potential (e.g. KRM, AMIE, Weimer's model), the ionosphere is treated as a thin layer although the real ionosphere has a three-dimensional structure. To examine 3-dimensional distributions of the ionospheric potential and the current system in the ionosphere, therefore, a solver of the 3-dimensional distribution of the ionospheric electric potential is adopted in M-I coupling process of the global MHD simulation code developed by Tanaka [1995, JGR]. In determining distributions of the electric potentials, field-aligned currents (FACs) on the upper boundary of the ionosphere and 3-dimensional distribution of the ionospheric conductivity are used. As done in most of global MHD simulations, FACs are mapped from the inner boundary of the magnetosphere. As for the ionospheric conductivity, the horizontal distribution of the ionospheric conductivity is determined by the global MHD simulation, including the effects of the solar EUV, diffuse precipitation driven by plasma pressure and temperature, and discreet precipitation modeled by field-aligned current, as described by Tanaka [2000, JGR]. In addition, the height profile of the ionospheric conductivity is proportional to the model distribution determined by IRI 2001. The calculation area covers the polar region which expands 30 degrees of colatitudes. In the present study, a model substorm, which is produced by southward turning of northward IMF, is simulated. Resultant potential patterns in lower ionosphere, where the parallel conductivity corresponds to the Pedersen and Hall conductivities, are similar to the potential patterns determined by the original scheme. The parallel currents mainly connect with the Pedersen currents. On the other hand, the parallel currents also connect with the Hall current in the cusp region and the nightside of the ionosphere, where the distribution of the conductivity is complicated.
SM13B-1314
The Current-Driven Magnetosphere
Convection in the ionosphere and magnetosphere is driven by solar wind energy, which powers the generation of a large-scale electric field that produces plasma motion. It is generally believed that there are two mechanisms by which solar wind energy generates convection: reconnection and a viscous interaction at the magnetopause. In this paper we propose that a third, fundamentally distinct mode of imposing convection on the magnetosphere and ionosphere exists. Current must flow across the bow shock as determined by the jump conditions, and the bow shock current represents a dynamo. The bow shock current closes in part through the ionosphere, which is a load. Thus it is possible to generate an ionospheric potential that is neither a product of reconnection nor of mechanical transport of energy across the magnetopause. The current-driven potential is imposed directly on the ionosphere by field-aligned currents that connect to the bow shock and which are driven by the work done on the solar wind as it crosses the bow shock.
SM13B-1315
Determining the Viscous Potential from MHD Simulations and Comparing it to Observations
The viscous potential is produced by a mechanical interaction between the magnetosphere and the solar wind and is generally thought to have a value of about 20 kV. Preliminary investigations using the Lyon-Fedder-Mobarry global MHD simulation indicate that the viscous potential increases with increasing solar wind density. To determine if this is in fact the case, we have selected solar wind intervals where the ionospheric potential due to merging with the solar wind should be extremely small. During those periods, we use the DMSP satellites to determine the value of the transpolar potential, which we assume to be driven primarily by the viscous interaction. In this study we will compare those observations to the MHD results.
SM13B-1316
Responses of the Magnetosphere-Ionosphere System to A Sudden Pressure Commencement
The magnetospheric response to a sudden step increase of the solar wind dynamic pressure when interplanetary magnetic field is northward is studied using the University of Michigan MHD code. Two responses in the ionosphere are observed in terms of cross polar cap potential (CPCP) as well as travelling convection vortices (TCVs). The first response right after the high pressure commencement hits the dayside magnetopause is associated with a pair of NBZ-like high-latitude cells, set up by a dusk-to-dawn electric field in the dayside magnetopause carried by the fast mode compressional wave, mapped to the ionosphere along the field lines. A subsequent pair of TCVs appears at relatively low latitude on the dayside, with the opposite sense than the previous set, resulting from field-aligned currents driven by the magnetospheric vortices in the equatorial plane that are mainly driven by pressure gradients. These vortices are located inside the magnetopause, implying the positions of this Region-1-like field-aligned current is not from the magnetopause. The final CPCP for the new compressed configuration of the system is found to be higher than the prefront value, which appears to be a result of the higher pressure around the Earth.
SM13B-1317
MHD Simulations of the Magnetosphere's Response to Solar Wind Tangential Discontinuities With Dynamic Pressure Changes
Global MHD simulations of the coupled solar wind-magnetosphere-ionosphere system are used to examine responses of the magnetosphere to tangential discontinuities (TD) with associated dynamic pressure changes in the solar wind. When a TD carrying an increase in density crosses the earth's bow shock, the transmitted TD convects through the magnetosheath sandwiched between an earthward moving bow shock, and a fast compressional wave. The fast compression wave carries a fraction of the original density change with the remainder at the TD. The arrival of the fast compression wave at the magnetopause initiates enhanced merging, earthward movement of the magnetopause, and a reflected fast wave that propagates upstream through the magnetosheath that brings the bow shock to rest at its new equilibrium position. The bow shock-fast wave interaction generates a second TD that convects through the magnetosheath establishing the final magnetosheath equilibrium state, and returning the merging rate to near its initial value. The transpolar potential increases in response to enhanced merging but then decays slowly towards original values after merging subsides. The reconnection rate in the magnetotail also varies during this process but does not contribute to the strength of the transpolar potential. The magnetosphere's response to a solar wind TD with an associated decrease in density will also be discussed. In this case, the transpolar potential decreases in response to reduced merging and then recovers slowly toward its original value.
SM13B-1318
Effect of solar wind dynamic pressure enhancements on convection and the aurora under different IMF conditions: modeling and observations
Recent studies have clearly demonstrated the significant effect solar wind dynamic pressure enhancements have on auroral precipitation, ionospheric and magnetospheric currents, and convection. Sudden pressure enhancements can cause significant and rapid closure of the polar cap and thus reduction in the open flux, widening and strengthening of the auroral oval at all local times, as well as an increase of the cross polar cap potential (CPCP) and the efficiency of coupling of the solar wind to the magnetosphere. The effects are most dramatic during periods of strongly southward IMF Bz but are evident at various degrees under most IMF conditions. MHD model simulations of two events, using the Open GGCM model, have also shown increase in the CPCP, but the reduction of open flux was only reproduced in one of the two cases. What seems to be the deciding factor is whether dayside or nightside reconnection dominates. We now simulate a larger number of events that occur under various IMF conditions and compare the results with observations. We use observations from the DMSP and FAST low-altitude spacecraft and auroral images from the Polar UVI instrument. We focus specifically on the CPCP, the area of open flux, the dynamic evolution of the separatrix boundary, and convection. With the larger number of events simulated we focus on the question of why these properties (i.e., the CPCP, open flux, etc) respond differently for different IMF conditions, and how well does the model reproduce such differences. Does the model reproduce the global closure of the polar cap during strong southward IMF Bz as opposed to closure only on the nightside for near-zero IMF Bz, and how are the associated CPCP changes reproduced? Does the dayside or nightside reconnection rate dominate in the changes observed? Can the model reproduce other significant features, like the propagation of the auroral precipitation enhancement from dayside to nightside under zero or positive IMF Bz, or the added effects of dynamic pressure aurora and a simultaneous substorm at nightside. Finally, we also use the model to predict various properties (like the precipitating flux) along virtual spacecraft (DMSP and FAST) passes of the oval and compare them directly with the actual observations of DMSP and FAST.
SM13B-1319
The Dynamic Reconnection-Driven Polar Cap as seen by HF Radar (PolarDARN)
The SuperDARN HF radar pair (called PolarDARN) at Rankin Inlet (73.2° AACGM LAT) and Inuvik (71.2° AACGM LAT) is now fully operational (Rankin, May 2006 and Inuvik, Nov. 2007). For the first time, detailed two-dimensional radar convection maps of the polar cap region and poleward portion of the auroral oval are available with a one-minute time resolution. These radar images show clearly the signature of reconnection in the proximity of the ionospheric mapping of the OCFLB (open-closed field line boundary). Near magnetic noon (midnight), high-speed flows into (out of) the polar cap, away from (toward) the radars, are seen regularly, presumably mapping the fast outflows from the dayside and nightside reconnection regions. The convection maps reveal a very dynamic polar cap, with substantial changes in the convection pattern from one minute to the next. Examples of the reconnection-driven convection are presented, for By-dominant conditions and for both Bz+ and Bz- IMF conditions. The noon region for Bz+ conditions appears to be the counterpart of the auroral proton spot seen clearly with the IMAGE FUV instrument (Frey et al., JGR, 107, A7, 2002; Fusilier et al., JGR, 107, A7, 2002; Frey et al., Nature, 426, 533, 2003; Phan et al., GRL, 30, 10, 2003).
SM13B-1320
Open flux estimation using SuperDARN
The open flux is estimated from SuperDARN observations of the convection reversal boundary (CRB). The CRB as a proxy for the OCB was established by Sotirelis et al. [2005]. Since that time the CRB detection algorithm has been improved and a procedure for estimating the open flux from a collection of CRBs devised. The correlation of open flux with solar wind/IMF driving is tested. Since these flux estimates are made every few minutes, the expanding and contracting polar cap conditions may possibly be distinguished. The practicality of using this procedure for this purpose will be tested.
SM13B-1321
Inonospheric Convecton During Very Weak IMF and Solar Wind Driving
A series of high latitude ionospheric convection patterns determined using the combined data from the Greenland incoherent scatter radar, SuperDARN radars and DMSP driftmeter are analyzed during the interval May 12, 2007 – May 17, 2007. These days are particularly unique as characterized by the magnetospheric driving parameters in the solar wind. The IMF is unusually weak with B total between 1 and 3 nT. The solar wind is also unusually slow with V ranging between 300 and 350 km/s and the density near 1/cc and sometimes less than 1/cc. The May 12 – 17, 2007 interval has a much weaker IMF and slower solar wind than the previously investigated "Day the solar wind went away", May 11, 1999. The present investigation examines the driving conditions for extremely weak coupling by reconnection and by the so-called viscous driving. Preliminary plots show polar cap potentials during the quietest driving conditions of around 20 – 25 kV.
SM13B-1322
Ionospheric Response to the Changes in the Interplanetary Magnetic Field (IMF) Bz Under Strong Positive IMF By Conditions, as Seen using SuperDARN and PolarDARN Radars.
The interplanetary magnetic field (IMF) is known to have a profound influence on the ionospheric convection pattern at high latitudes. Thus, when the IMF has a dominant southward (Bz) component, the convection over the central polar cap is anti-sunward and closes through the dawn and dusk sectors to produce a familiar 2-cell convection pattern. For strong northward IMF conditions, the pattern is thought to break into four cells, with sunward convection taking place over the noon sector of the polar cap. For strong IMF By conditions the standard 2-cell convection pattern is believed to become asymmetric and/or to rotate from its basic alignment with the noon-to- midnight meridian to a more dusk-to-dawn or dawn-to-dusk alignment. With the help of the new PolarDARN radar in combination with other SuperDARN radar data, we have been able to observe changes occurring deep into the polar cap during a period of strong sustained positive IMF By, when the Bz component was always less than 20% of the By component in magnitude. The basic shape of the convection pattern was highly reminiscent of the Heppner-Maynard `BC' convection pattern predicted for strong positive By conditions ( Heppner and Maynard, JGR, 92, 4467, 1987). We note that we observed very clear changes in the convection pattern as Bz changed signs, with a distinct sub-cell on the evening side being present while the IMF Bz component was greater than zero. The sub-cell completely vanished when the Bz component changed sign. These variations were unexpected in view of the relatively small IMF Bz magnitudes involved. Interestingly enough, this feature would have been missed, had the PolarDARN radar data not been added to the data obtained with the rest of the SuperDARN radars. We will discuss the implications of our findings for our understanding of ionospheric/magnetospheric convection in response to solar wind inputs.
SM13B-1323
Reverse Convection Potential Behavior During Northward IMF
We report the results of an investigation of the reverse convection potentials in the ionosphere during periods of steady northward interplanetary magnetic field (IMF). While it has been shown that the polar cap potential in the ionosphere exhibits non-linear saturation behavior when the IMF is southward, it has yet to be shown whether the high latitude reverse convection cells in response to northward IMF exhibit similar behavior. We use solar wind data from the ACE satellite from 1998 to 2005 to search for events in the solar wind when the IMF is northward and the interplanetary electric field is stable within a given bin for more than 40 minutes. We then use bin- averaged SuperDARN convection data and apply a spherical harmonic fit to calculate the average potential pattern for each bin. Preliminary results suggest that the reverse convection cells do, in fact, exhibit non-linear saturation behavior.
SM13B-1324
The Relationship Between Convection Velocities in Simultaneous High-Latitude Ionospheres
Depending on the dipole tilt angle of the Earth's magnetosphere, the northern and southern hemispheres can have dramatically different average conductances. Because the ionosphere provides a path through which magnetospheric field-aligned currents coupling the two regions close, it is expected that the electric field and current density would also exhibit asymmetric behavior between hemispheres that depend on the dipole tilt angle. Some magnetohydrodynamic models of the magnetosphere show that the ratio of the global hemispheric electric potential can be a factor of 2-4 for large dipole tilt angles. Statistical studies of ionospheric electric fields using ground-based radars and spacecraft also show that there is a seasonal dependence of the global ionospheric potential but that the difference is at most ~20%. Here we use merged line-of-sight velocity vectors obtained simultaneously in both hemispheres from the SuperDARN network of HF radars to investigate the relationship of the convection velocity for conditions of varying hemispheric conductance. We find a marked difference in the hemispheric relationship of the simultaneous convection velocities between the dayside and the nightside ionospheres. Using a dataset consisting of seven years of SuperDARN merged velocities centered roughly on the most recent solar maximum, we show statistically that the ratio of velocities restricted to the dayside in the Summer hemisphere to those in the dayside Winter hemisphere depends only marginally on the dipole tilt angle and is at most a factor of ~1.2 for extreme tilt angles. The ratio of merged velocities restricted to the nightside, however, show a strong dependence on dipole tilt angle and can be as high as 2 for extreme tilt angles. The results are consistent with the notion that the dayside magnetosphere-ionosphere circuit behaves as if driven by a constant voltage source, whereas the nightside acts more like a circuit with a constant current source.
SM13B-1325
Comparison of Simultaneously Measured Cross Polar Cap Potentials
One of the outstanding questions in polar ionospheric studies is whether the cross polar cap potentials in both hemispheres are identical or different. Theoretically the two should be the same, but there have been observational results from both satellites and ground-based radars than indicated that differences can occur sometimes. Unfortunately such results can not be proven definitively because of either lack of coverage (radars) or the observations were separated in time (satellites), thus the question remained unresolved. But since the November 2006 launch of DMSP-F17 there are now two DMSP spacecraft (F13 and F17) operating simultaneously in the dawn-dusk polar orbit orientation, the ideal orbit for measuring the cross polar cap potential. The two spacecraft cycle in and out of phase with each other over a period of about 60 days. The early results of a comparison between the polar cap potentials measured during the periods when the two satellites are simultaneously in opposite polar regions will be presented. Differences between the two measured potentials will be examined to determine if they are demonstrably real or not.
SM13B-1326
The Distribution of the Cross-polar Potential Drop Based on Particle Precipitation Characteristics
There are several proposed physical processes which may contribute to the cross-polar potential drop and thus drive ionospheric convection over the poles. It is generally believed that magnetic reconnection is the dominant process, however low latitude closed field line dynamos such as viscous interaction are other possible contributors. A comprehensive statistical study of DMSP F13 satellite data has been conducted for passages along the dawn- dusk meridian, with focus on typical two cell convection patterns during steady IMF conditions. An average potential distribution pattern for regions of different particle precipitation characteristics has been established, and the contribution from open and closed field line dynamos is estimated for different solar wind conditions.
SM13B-1327
Variations of Ionospheric Convection Electric Field: Under Steady and Non-steady Conditions
It is well known that magnetic reconnections at the dayside magnetopause and in the distant magnetotail drive the polar cap convection that has a two-cell pattern in potential. This pattern is twisted westward due to southward turning of electric field in midnight sector during substorm expansion phases. To quantitatively describe the variations of nightside ionospheric electric fields, the natural orthogonal components (NOC) algorithm is applied to separate convection E field and a southward E field that drives the substorm electrojet. It is found that the convection E field is mainly westward in the midnight sector and is stable when there is a lack of substorm expansion phases. A non-steady convection may be enhanced by ~30% of the steady E field magnitude when substorms occur. In addition, the southward E field is found to vary more dramatically than the convection E field from ~5 to 40 mV/m when substorm expansion phase takes place. We will also discuss the interplanetary drivers of the polar cap convection, such as magnetic clouds and other sudden changes in the solar wind.
SM13B-1328
Synthesis of Various Ionospheric Convection Patterns for IMF BY-Dominated Periods: Split Crescent Cells, Exchange Cells, and Theta Aurora Formation
When the dawn-to-dusk component of the interplanetary magnetic field (IMF BY) is dominant, ionospheric convection usually exhibits a distorted two-cell pattern with a round cell on the dawnside (or duskside) and a crescent-shaped cell on the duskside (dawnside). However, this convection pattern is an average picture, and actual convection sometimes deviates from the basic round/crescent cell pattern. Such nonstandard convection patterns include the dayside-enhanced crescent cell pattern, nightside-enhanced crescent cell pattern, split crescent cell pattern, and exchange cell pattern. Although at times the nonstandard convection patterns appear to be quite different from the basic round/crescent cell pattern, the underlying physical processes (i.e., reconnection) and field line topologies are basically the same. We show that, by considering the topologies of magnetic field lines involved in the eight types of reconnection that occur in IMF BY- dominated periods, these various convection patterns can be understood with one unified picture. We also discuss the formation mechanism of theta auroras in terms of magnetic topology and ionospheric convection. The theta aurora results from transient reconnection-driven convection when the IMF BY polarity switches. During the transition, the magnetospheric topology is expressed by a system with four magnetic nulls located on the separator circle encircling the Earth. The nightside portion of the separator circle (which corresponds to the plasma sheet in the magnetotail) is kinked at the two nulls on the nightside. The bifurcation of the polar cap occurs as a consequence of the accumulated closed magnetic flux around the kinked portion of the plasma sheet and the newly added open magnetic flux in the adjacent open field line region.
SM13B-1329
Global Topology, Motion, and Convection of a Northward IMF Reconnection Event
In this study we explore the global structure of a northward IMF reconnection event observed by the Cluster and IMAGE spacecraft on March 18, 2002. In a previous paper we analyzed magnetic field and electron and ion moments to infer the instantaneous structure and size or a reconnection site local to the null region. Using simulation studies, DMSP data, and Iridium data, we examine the same event not at Cluster spacecraft separation and magnetopause thickness length scales, but at global separator length scales. The simulations posit a separator at the position of the Cluster spacecraft and 3-D nulls near the position of Cluster. We also find that the low-altitude satellite data support sunward ionospheric convection in a region that maps along field lines to both the position of Cluster as well as the magnetopause location of the large-scale separator seen in resistive MHD simulations. Having derived a reconnection electric field from the Cluster data, we can compare its value to the DMSP polar cap potential drop to derive a length for the reconnection line perpendicular to the earth- sun line. This length can be compared to the length observed both in the simulations and in the mapping from the DMSP sunward flows. We will then compare the simulated separator structure to previous simulations and observations of global anti-parallel and component reconnection. Cluster data and simulations indicate a possible tearing mode structure, whose motion we derive from a four spacecraft wavevector analysis.