SA44A-01 INVITED
Sources of Electrodynamic Variability at Mid and Low Latitudes: Quiet and Disturbed
Quantifying the storm-time electrodynamic response requires a baseline from which to measure the changes. Day-to-day changes in quiet-time electric fields at mid and low latitudes are a consequence of leakage of magnetospheric fields from high latitudes, variability in the thermospheric wind system, and solar-induced conductivity changes. On the dayside, E-region winds tends to dominate the wind-driven electrodynamic variability due to the high conductivities; on the nightside, F-region winds become significant as E-region plasma decays. The quiet-time day-to-day changes in equatorial vertical plasma drift on the dayside are of order of 50% and exhibit multi-day periodicities, indicative of a lower atmosphere source. On the nightside, fluctuating F-region winds from geomagnetic activity are a likely source of variability. During extreme events the storm changes are dramatic, so deviation from quiet conditions is more easily identified. In the storm case, electrodynamic changes arise from penetrating magnetospheric fields and the disturbance dynamo. The penetration electric fields are immediate but tend to recover quickly. The disturbance dynamo is slower to develop but also longer-lived. The fastest disturbance dynamo effect in numerical simulation is apparent within an hour or two of onset of geomagnetic activity, which could become confused with apparent long-lived penetration on the nightside. Understanding the physics of the dynamo, during both quiet and disturbed times, is necessary to determine the source of the storm-time changes, and to correctly partition the variability to the different sources.
SA44A-02 INVITED
Penetration of Solar Wind and Magnetospheric Electric Fields to the Inner Magnetosphere
Almost 40 years ago Nishida showed that magnetic field fluctuations measured in the solar wind were sometimes highly correlated with magnetic fields measured near the magnetic equator. With the development of electric field measurements, tracking the chain of events from the solar wind to the inner magnetosphere is now possible. Here we present several case studies illustrating this chain. We often find almost identical waveforms for the dawn-to-dusk component of the interplanetary electric field (IEF) and the zonal component of the ionospheric field at latitudes from the auroral zone to the equator. The response is symmetric in that the inner magnetosphere responds to both IEF increases and decreases, implying that an energy storage element in the system can be traced to inductance of the ring current. Thus, when the IEF abruptly turns toward the dusk-to-dawn direction, the inner magnetospheric current system (region 2 currents) continues to flow, with a portion closing in the ionosphere. Likewise, when the IEF dawn-to-dusk component increases, region 1 currents intensify but region 2 currents cannot change immediately. Again, this imbalance is associated with currents in the inner ionosphere. To study the temporal response of this system, four years of electric field data were compared to the IEF using ACE data. The ratio of these parameters is like a transfer function (TF) between the interplanetary and ionospheric systems, which is a function of frequency. Using the average of many such TFs revealed a distinct logarithmic dependence on Kp with a 16 db difference between low and high values. The average TF is significant for periods from 30 min. to 6 hours and displays a small peak near a 1 hour period, suggesting that some capacitance exists in the system and possibly a weak resonance. The average TF indicates that about 3% of the IEF appears in the equatorial ionosphere whereas case studies indicate values as high as 10%. Case studies also show a higher frequency response than the average TF since sharp transitions in the IEF are also seen in equatorial fields, suggesting that wavelet analysis might be more accurate than Fourier methods. We have studied penetration events from widely different longitudes and are building a better understanding of this world-wide effect.
SA44A-03 INVITED
A Survey of Subauroral Electric Fields and Plasma Flow Observed With the Wallops SuperDARN Radar During Quiet and Disturbed Periods
The Wallops SuperDARN radar has now been in operation for more than one year and has obtained a virtually continuous record of the state of the subauroral ionosphere in the vicinity of the northeastern U.S. and maritime Canada. For several of the storm intervals that have occurred, there have been expanded oval configurations during which large electric fields and plasma flows have expanded as far south as the Middle Atlantic States. There have also been numerous examples of Sub-Auroral Ionization Drifts (SAIDs) that have been recorded and compared with measurements from the DMSP, NOAA POES and TIMED satellites. One of the interesting features of SAID measurements with the Wallops radar is the ability of the radar to obtain multi-hour records of the latitude and maximum plasma drift speed within the SAID flow channel. While the ionospheric flow velocity is often at low values (less than 50 m/s) equatorward of the SAID, there are periods when these values can become much larger. Some of these periods are transient in nature and appear to be related to substorms, but others are more enduring. It is not entirely clear what process or processes lead to these enduring increases in the electric field. In this presentation, examples from each of these types of subauroral electric fields will be reviewed and discussed.
SA44A-04 INVITED
Recent Rice Convection Model Calculations of Subauroral Electric Fields
We present results from recent Rice Convection Model simulations of inner magnetospheric and subauroral ionospheric electric fields, with emphasis on prompt penetration electric fields and the formation of Subauroral Polarization Streams (SAPS). During a magnetic storm prompt penetration electric fields are most apparent when the plasma sheet is hot and the ionospheric conductance is high. In an idealized major storm simulation, while prompt penetration was substantial, shielding was still significant. Much stronger penetration fields were recorded in another simulation run in which the region-2 Birkeland currents were artificially set to zero. We present results from computer experiments designed to investigate to what extent electric-field penetration in a major storm is due to the change in cross-polar-cap potential drop, and to what extent it is due to reconfiguration of the magnetospheric magnetic field. We report on computer experiments that explore the dependence of SAPS electric fields on ionospheric conductance and other factors. Downward region-2 currents and a conductance drop at the equatorward edge of the diffuse electron aurora seem to be necessary conditions for the formation of a SAPS. A localized reduction in Pedersen conductance in the SAPS region results in an electric field enhancement, roughly in proportion to the inverse square root of the conductance.
SA44A-05
Prompt Penetration of Magnetospheric Convection to Low Latitudes: What is the Physical Mechanism?
At very low magnetic latitudes, plasma flows in the ionosphere and disturbances of the geomagnetic field are sometimes observed to undergo temporal variations closely correlated with changes in solar wind parameters and associated variations of magnetospheric convection at high latitudes. Conventionally, such events are interpreted as the result of an electric field imposed by the solar wind, penetrating from the polar cap down toward the equator; time scales and other properties have been calculated often from purely electromagnetic considerations, assuming that electric fields drive conduction currents in the ionosphere and produce plasma bulk flows by ${\bf E} \times {\bf B}$ drifts. It has been amply demonstrated, however, that in large-scale plasmas the electric field is produced by the flow rather than the other way around, and that ionospheric currents arise directly from deformation of the magnetic field as the result of plasma-neutral collisional drag. The penetration of magnetospheric convection to low latitudes must therefore be described directly in terms of flows and stresses; the conventional formulation in terms of electric fields, although it may offer some mathematical advantages, is not a viable description of the physical process. At low altitudes, where any change of the magnetic field is necessarily very small in comparison to the Earth's dipole field, the two-dimensional (height-averaged) flow is effectively incompressible, irrespective of any vertical flows or compressibility of the plasma, because the magnetic field cannot be appreciably compressed. We show that, as a consequence, any changes in magnetospheric convection flow over the polar cap are immediately propagated to all latitudes at the fast-mode speed. At any radial distance $R$, the time scale for penetration is $\sim R/V_A$, shortest just above the ionosphere (combination of enhanced $V_A$ because of the low density and minimal $R$). This time scale differs from those given (under various approximations) by the purely electromagnetic approach; thus, although largely equivalent in the steady state, the two approaches do differ when time variations are involved.
SA44A-06
Relating the Interplanetary Electric Fields (IEFs) With the Low Latitude Electric Fields (LLEFs) Under Both Geomagnetically Quiet and Disturbed Conditions
We present first results that illustrate how dependent the ionospheric, daytime low latitude electric field (LLEF) is on the Interplanetary Electric Field (IEFy) under both geomagnetically quiet and disturbed conditions. The IEFy values are calculated from the ACE L1 satellite observations of the Bz and Vx components in the GSM coordinate system (IEFy = -Vx x Bz) and time shifted to the magnetopause position using the solar wind speed, Vx. Daytime, eastward ionospheric F region electric fields are inferred from ground-based magnetometer observations using a recently-developed neural network algorithm that realistically calculates daytime, vertical ExB drift velocities at the magnetic equator incorporating the difference in the horizontal (H) components between a magnetometer on the magnetic equator and one located 6 to 9 degrees away in dip latitude. Magnetometer observations on and off the magnetic equator for all of the days in 2001 have been obtained for the Peruvian, Philippine and Indian longitude sectors. We analyze the IEFy observations as a function of Universal Time (UT) for each of the days chosen using 1.) Fourier transform, 2.) Lomb-Scargle transform and 3.) Morlet wavelet transform to obtain the dominant periods between 10 minutes and 6 hours. We compare these periodicities with the periodicities obtained for the LLEF observations using the same techniques at each of the 3 longitude sectors as a function of UT for each of the days chosen. We present these comparisons and illustrate how, for quiet days and portions of the UT day that are quiet, the IEFy periodicities match the LLEF periodicities (long periods) while for the disturbed part of the UT day, the short periodicities in IEFy match the LLEF periodicities in the appropriate longitude sectors. The implications of these findings are discussed with respect to prompt penetration and overshielding electric fields and how the evolution of quiet-to-disturbed LLEF signatures can be related to IEFy conditions as they evolve from low frequency fluctuations to higher and higher frequency fluctuations.
SA44A-07
Variations of disturbance dynamo and ionospheric electron densities during geomagnetic storms
The new version of the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model includes the National Center for Atmospheric Research (NCAR)-Thermosphere- Ionosphere-Electrodynamic Global Circulation Model (TIEGCM). This enables us to study the global ionospheric disturbance dynamo during geomagnetic storms. The CMIT model has been run to simulate the geospace response to the April 2004 geomagnetic storm. We investigate global neutral wind variations during the storm, changes and evolutions of the disturbance dynamo field induced by these variations, and their effects on global ionospheric F region electron densities. Several issues that will be addressed in this presentation are: how long does it take for the changes to propagate to the geomagnetic equator; and how long does it take for the dynamo to recovery after the main phase of the storm.