SM43A-1133 1340h
Ionospheric Electric-Field Basis Functions Discontinuous at the Boundary Between Closed and Open Magnetic Field Lines
In some simple models of magnetospheric electrodynamics [e.g., Volland, Ann. Geophys., 31, 154-173, 1975], the normal component of the convection electric field is discontinuous across the boundary between closed and open magnetic field lines. The requisite discontinuity in {\bf E} is achieved by making the scalar potential proportional to a positive power (typically 1 or 2) of L on closed field lines and to a negative power (typically -1/2) of L on open (i.e., polar-cap) field lines. This suggests that it may be advantageous to construct more realistic (and thus more complicated) magnetospheric and ionospheric electric-field models from superpositions of mutually orthogonal (or not) vector basis functions having this same analytical property (i.e., discontinuity at L = L*, the boundary surface between closed and open magnetic field lines). The present work offers a few examples of such constructions.
SM43A-1134 1340h
A Solar Cycle Dependence of Nonlinearity in Magnetospheric Activity
The nonlinear dependencies inherent to the historical $\rm K_p$ data stream (1932-2003) are examined using mutual information and cumulant based cost as discriminating statistics. The discriminating statistics are compared with surrogate data streams that are constructed using the corrected amplitude adjustment Fourier transform (CAAFT) method and capture the linear properties of the original $\rm K_p$ data. Differences are regularly seen in the discriminating statistics a few years prior to solar minima, while no differences are apparent at the time of solar maximum. These results suggest that the dynamics of the magnetosphere tend to be more linear at solar maximum than at solar minimum. The strong nonlinear dependencies tend to peak on a timescale around 40-50 hours and are statistically significant up to one week. Because the solar wind driver variables, $\rm VB_s$ and dynamical pressure exhibit a much shorter decorrelation time for nonlinearities the results seem to indicate that the nonlinearity is related to internal magnetospheric dynamics. Because the timescales for the nonlinearity seem to be on the same order as that for storm/ring current relaxation, we suggest that the nonlinearity is likely associated with those processes.
http://w3.pppl.gov/~jrj/cumulant.html
SM43A-1135 1340h
Earth-ionosphere transmission line model for an impulsive geomagnetic disturbance at the dayside geomagnetic equator
The near instantaneous onset of a geomagnetic impulse such as the preliminary reverse impulse (PRI) of the geomagnetic sudden commencement at high latitude and at the dayside geomagnetic equator has been explained by means of the TM0 mode waves in the Earth-ionosphere waveguide (Kikuchi and Araki, J. Atmosph. Terrest. Phys., 41, 927-936, 1979). There is, on the other hand, a time lag of the order of 10 sec in the peak amplitude of the magnetic impulse at the dayside equator. To explain these two temporal aspects, we examine transmission of the TM0 mode in a finite-length Earth-ionosphere transmission line composed of a finitely conducting ionosphere and the perfectly conducting Earth, with a fixed electric potential at one end and null potential at the other end of the transmission line, corresponding to the foot of a field-aligned current on the dawn- or dusk-side in the polar cap and middle point on the noon-midnight meridian at low latitude, respectively. Successive transmission and reflection in the bounded transmission line lead to that the ionospheric currents start to grow instantaneously, but reach a steady state with a relaxation time proportional to the length of the transmission line and the ionospheric conductivity. The relaxation time is of the order of 10 sec when we give high conductivity applicable to the equatorial ionosphere, which matches the observed time lag in the peak amplitude of the equatorial geomagnetic impulse. Consequently, the TM0 mode in the finite-length Earth-ionosphere transmission line explains both the instantaneous onset and time lag in the peak amplitude of the geomagnetic impulse at the dayside geomagnetic equator.
SM43A-1136 1340h
Comparison of intense nightside shock induced aurora and substorms activity
Variations of the solar wind dynamic pressure induce perturbation of magnetospheric processes. These perturbations frequently induce dayside enhancements of activity with particular features such as low latitude proton flash, low latitude arcs and aurora propagating eastward and westward from noon to the night sector. In some cases, these shocks may also induce an enhancement of the nightside activity during which the power precipitated in the night sector may reach values as high as observed during substorms. Various studies have shown that high precipitated powers are more likely during period of negative values of the North - South IMF components. Liou et al (2003) have shown that substorm-like activity is not frequent after a shock and they concluded that a shock may not be considered as a substorm trigger. The question addressed in this study is to know up to what point the substorm like nightside activity triggered by a shock is comparable to a classical substorm. For this purpose, we analyze four events presenting nightside activity morphologically similar to substorms and occurring within a short time (less than 20 minutes) after the arrival of a pressure pulse on the front of the magnetosphere. Different features of these events such as the mean energy of precipitated electrons, the motion of boundaries before and after onset and the power precipitated in the nightside region are compared to typical substorms. Except for the absence of southward motion of activity before onset, shock induced substorms appear very similar to isolated substorms. We investigate the ability of a shock to trigger a substorm during periods characterized by particular conditions. We suggest that the sign of B$_z$ plays an important role as well as the history of solar wind and interplanetary magnetic field and the resulting state of the magnetosphere.
SM43A-1137 1340h
Bow Shock-Magnetosphere coupling under different IMF conditions
We perform global hybrid (kinetic ions, fluid electrons) simulations to study the solar wind interaction with a magnetic dipole under a variety of interplanetary magnetic field (IMF) conditions. The advantage of the hybrid code over fluid codes is that it treats ionscale microphysics in the context of the global interaction. We investigate the macrostructure of the bow shock, its dependence on foreshock upstream waves, its influence on the magnetosheath, and its coupling to the outer magnetosphere. The nature of the shock-magnetosheath-magnetosphere coupling is expected to be tied to the direction of IMF for a number of reasons. When the IMF is radial, the quasi-parallel shock and ion foreshock are upstream of the dayside magnetopause while during highly northward or southward IMF the quasi-parallel portion of the shock falls on the flanks and the dayside magnetopause is exposed to the quasi-perpendicular magnetosheath. The magnetosheath is permeated by a variety of waves that result from the convection of upstream waves and also from local generation. Wave characteristics are different at the quasi-parallel and quasi-perpendicular parts of the magnetosheath. We study the influence that magnetosheath waves have on the outer magnetopause, and on processes taking place there such as reconnection.
SM43A-1138 1340h
Impulse Response of the Polar Cap Index ({\it PC}) to Interplanetary Coupling Using a Revised Weimer Solar Wind Propagation Method
Currently, the methods used to model solar wind propagation delay from an upstream monitor to the subsolar bow shock are receiving a great deal of renewed interest. A new generation of delay correction techniques, such as that presented by {\it Weimer et al.} [2003], have been proposed to correct for timing differences between the arrivals of IMF features at the fleet of monitoring satellites situated in the interplanetary medium upstream of the Earth. The new techniques, which rely on the use of ``phase plane'' whose tilts vary continuously with time, can be used to match the IMF time series observed at different upstream monitors with a high degree of accuracy. This recent advance motivates a reassessment of our ability to model the solar wind impulse response of geomagnetic activity indices such as the {\it PC} index, which is based on horizontal magnetic field deviations from quiet levels as observed at a ground station located near either the northern or southern magnetic pole. With this goal in mind, results will be presented characterizing the effect of the choice of propagation delay technique on the empirical response of {\it PC} to solar wind-magnetosphere coupling. Preliminary results utilizing data from July 1999 indicate that the modeling efficiency, as measured using the ratio of the modeled variance to the original variance for the {\it PC} index time series, shows a modest increase from 47% to 48.5% when using the revised Weimer propagation delay technique in place of older techniques ({\it i.e.} the ballistic {\it X/V}, corotation, or "half-way" methods) that are based on fixed phase plane tilts. The small 1.5% increase in modeling efficiency obtained for the July 1999 data set is still at least an order of magnitude larger than the differences in modeling efficiencies obtained using the various older propagation delay techniques. Effects of the choice of propagation delay model on the temporal details of the {\it PC} index impulse response filter specifically related to causality and bimodal character will also be addressed.
SM43A-1139 1340h
Toward a Global Magnetospheric Equilibrium Model
A ``quasi-static" equilibrium between magnetic (Lorentz) forces and plasma pressure gradient forces exists at most times in the magnetosphere, except for periods of explosive activity such as substorms. The magnetospheric evolution is thus slow enough so that it can be portrayed as a time sequence of such equilibria. In order to model quasi-static equilibria, we have started the development of a comprehensive three-dimensional self-consistent model of the magnetosphere, by coupling a 3-D equilibrium code in flux coordinates [Cheng, 1995, Zaharia et al., 2004] covering the inner/middle magnetosphere with an asymptotic ``tail equilibrium" model [Birn, 1987]. The coupling project is facilitated by the similar magnetic field representation, in terms of Euler potentials, in both models. We will present initial results from this effort, focusing on global configurations (including both closed- and open-field regions) obtained by coupling the two models for particular cases, such as north-south symmetry and 2-D geometry. We will discuss the results obtained by using different coupling procedures, boundary conditions and input pressure functions. Presenting computed sequences of quasi-static equilibria, we will also discuss how the magnetosphere evolves under changing external (solar wind) conditions, modeled by changes in magnetic flux boundaries and pressure profiles. In particular, we will discuss the possibility for appearance of thin cross-tail current sheets under boundary deformations.
SM43A-1140 1340h
How Much do Numerics Affect the Results of Global MHD Magnetsopheric Calculations?
Global MHD simulations of the magnetosphere are increasingly used for interpretation of magnetospheric data and for providing a realistic picture of the state of the system. However, the major codes used often do not agree, sometimes at a level above details.In some limit, however, since the codes are all solving the same equations, the results should converge. The limits where this occurs (usually high spatial and temporal resolution) may be far beyond the capabilities of current computer systems. This paper is aimed at obtaining systematic insight into how numerical resolution, spatial order of differencing, and numerical shock capturing algorithms affect the ``accuracy'' of the resulting solution. We will do this by analyzing the results for a case with constant Northward IMF. Raeder (JGR, 104,17357, 1999) analyzed a similar case and concluded that numerical resistance was the determining factor in the length of the magnetotail. In our case, the LFM code was modified to examine a three dimensional parameter space in resolution, differe8663ncing order, and sharpness of shock capturing. We will present results for three quantities: general configuration, reconnection structure, and ionospheric field-aligned currents. Not surprisingly, the results can be quite different depending on the numerics, with the structure of the field-aligned currents perhaps being the most sensitive to calculation details.
SM43A-1141 1340h
Energy Deposition in the Polar Ionospheres: Simulation Studies of Geomagnetic Storm Events
Using the LFM magnetosphere simulation model, we study several geomagnetic disturbance events emphasizing the results for deposition of energy into the polar ionospheres. The major processes for energy deposition are ohmic dissipation of the ionospheric currents driven from the magnetosphere and precipitation of electrons into the auroral regions. The morphology and relative strengths of the Joule dissipation and precipitation during various phases of the events will be discussed. The total rate can be on the order of a terrawatt during intense disturbances. The energy drives winds and causes the atmosphere to swell increasing satellite drag. It also plays a role in the ionospheric storm at lower latitudes. The events presented include the geomagnetic storms of Jan. 10, 1997, May 15, 1997, Sep. 24, 1998, Oct. 18, 1998, and Nov. 20, 2003. Work supported by the NASA LWS program and the NAVO and ARL HPC centers.
SM43A-1142 1340h
Effect of the Anomalous Electron Heating on the Ionospheric Potential in the Global MHD Model.
Conventional approach to the modeling of the ionosphere in global MHD models is to solve a two-dimensional height-integrated electrostatic potential equation driven by the field aligned currents within the magnetosphere. The ionospheric simulation then supplies the inner boundary condition for the MHD solution in the magnetosphere. The ionospheric conductance entering the electrostatic potential equation is usually taken from empirical models including the ionization and deposition of energy due to the auroral precipitation. However, these models do not describe a direct dependence of the ionospheric conductance on the solar wind electric field. An objective of this paper is to modify the ionospheric module of the global LFM model by incorporating the anomalous electron heating which was computed based on realistic models of polar ionosphere. The anomalous electron heating due to the Farley-Buneman instability leads to an increase of plasma density through the reduction of the electron-ion recombination rate. It was shown that the enhancement of the ionospheric conductance due to the anomalous electron heating leads to the drop in the cross polar cap potential making the simulated potential close to realistic values corresponding to strong storms.
SM43A-1143 1340h
Use of substorm onset observations by IMAGE-FUV for THEMIS ground-based system analysis
Over the first 2.5 years of operation, the FUV instrument on the IMAGE spacecraft observed more than 2400 substorm onsets in the Northern Hemisphere. The observations confirm earlier results of statistical studies in terms of a median substorm onset location at 2300 hours MLT and 66.4 degrees magnetic latitude. The list is published as an electronic supplement to a publication in the Journal of Geophysical Research [Frey et al., 2004]. The list can easily be searched for onsets close to certain ground stations or at specific magnetic latitudes or local times. As one example of such use we demonstrate how the probability of onset observation was determined for the ground-based automatic observatories of the THEMIS (Time History of Events and Macroscale Interactions during Substorms) project.
SM43A-1144 1340h
Energy Deposition in Magnetic Cloud and High Speed Stream Driven Storms
The solar wind couples a large amount of energy into the magnetosphere-ionosphere system; this energy is released in the form of geomagnetic storms. While the precise mechanism for this coupling and release is yet unclear, it is well established that different solar wind conditions create different responses within the magnetosphere-ionosphere system. We are examining the impact of high speed stream-driven and magnetic cloud-driven storms on the global redistribution of energy throughout the magnetosphere-ionosphere system. Data are used from ACE, WIND, and ground magnetometers. We estimate the energy input and output for multiple geomagnetic storms spanning from1995 to 1998. The comparison of storms reveals high speed stream-driven storms deposit less energy per second, but over longer durations. The comparison further reveals magnetic cloud-driven storms have deeper Dst* depressions but with shorter durations. Our results suggest magnetic cloud-driven storms with similar input parameters as high speed stream-driven storms produce an overall lower energy deposition.
SM43A-1145 1340h
Monitoring Auroral Electrojet from Polar Cap Stations
The auroral electrojet AL and AE geomagnetic activity indices are important for monitoring geomagnetic substorms. In the northern hemisphere these indices are derived from measurements at a set of geomagnetic observatories located in the auroral zone. In the southern hemisphere the major portion of the auroral zone is located on the ocean; this does not allow us to derive the auroral electrojet indices in the same way. We showed that monitoring the auroral electrojet is possible from magnetic field measurements at polar cap stations. For this purpose we used hourly values of geomagnetic field variations at four polar cap stations, distributed along polar cap boundary and occupying a longitudinal sector of about 14 hours, and calculated mean values of the total magnetic field disturbance T = (X2 + Y2 + Z2)1/2 where X, Y, and Z are geomagnetic field components measured at these polar cap stations. The set of the obtained values were called the T index. This index has a clear physical mining: it is the summary of geomagnetic disturbance in all three components averaged over the polar cap boundary. We found that correlation coefficients for the dependence of the T index on both AL and AE indices are as high as ~0.9 and higher. The high correlation of the T index with the AL and AE indices takes place for any UT hour when the stations were located at the night side. The T index further shows good correlation with solar wind parameters: the correlation coefficient for the dependence of the T index on the solar wind-geomagnetic activity coupling function is ~0.8 and higher, which is close to the correlation coefficient for AL index. The T index may be especially important in the cases when ground-based measurements in the auroral zone are impossible as in the southern hemisphere.
SM43A-1146 1340h
Sensitivity Tests of the Temerin-Li Dst Model
The Dst historically has been used as a measure of the disturbance of the magnetosphere and an indicator of when a geomagnetic storm is occuring. The Temerin-Li [2002] (TL02) model is a semi-empirical Dst model which was trained on five years (1995 - 1999) of data. The TL02 model is the most successful Dst predictor in terms of prediction efficiency, with a reported prediction efficiency of greater then .90. The inputs into this model are the solar wind parameters, solar wind velocity, interplanetary magnetic field, and solar wind density. This model is based on the Burton et al. model, but has many parameters whose physical relavance are not well understood. To better understand the physical relevance of the empirical parameters, we use parameter sensitivity, impulse response, and driver sensitivity analysis. These techniques are used along with seasonal and diurnal analysis of the prediction error to extract a physical understanding of the most influential components of the model. By holding the input parameters constant and comparing this output and the output from sending pulses of delta t = 1 hour at regular intervals over a period of a year to the actual Dst, we can observe the diurnal and seasonal variations due to the individual parameters.
SM43A-1147 1340h
On the Characteristics and Source Regions of Dayside Proton Precipitation
The source regions of precipitating protons on the dayside and their dependence on solar wind conditions are studied using far-ultraviolet (FUV) spectral observations and imaging. The High-resolution Ionospheric and Thermospheric Spectrograph (HITS) aboard the Advanced Research and Global Observation Satellite (ARGOS) observes Doppler-shifted H Lyman-$\alpha$ emissions from precipitating protons with a spectral resolution of 1.5 Angstroms. The shapes of these Doppler spectra are indicative of the energy and pitch angle distributions of the proton precipitation. Global images of H Lyman-$\alpha$ emissions obtained by the SI-12 instrument on the IMAGE spacecraft are examined to relate the spectral observations to the dayside morphology of the proton aurora. During periods of sustained southward interplanetary magnetic field (IMF), the dayside proton aurora spectra exhibit broad Doppler shifts and are similar to those observed on the nightside with inferred mean energies typical of plasma sheet protons of magnetospheric origin. Global images of proton aurorae under these conditions show continuous regions of H Lyman-$\alpha$ emissions across the dayside extending from the nightside. In contrast, during periods of northward or variable IMF, proton aurora emissions on the dayside often appear in an isolated spot in the noon to late afternoon MLT sector. The Doppler-spectra of the proton emissions in these regions are narrow, indicating precipitation with low mean energies and from a different origin than that observed in the southward IMF cases. These spectra may be indicative of magnetosheath protons that have direct access to the ionosphere through high-latitude dayside reconnection. This study further quantifies the characteristics of dayside proton precipitation under various states of the magnetosphere and highlights the importance of IMF orientation on the coupling between the high-latitude, dayside ionosphere and its plasma sources at higher altitudes.
SM43A-1148 1340h
IMF By and seasonal dependence of the electric field in the inner magnetosphere
It is known that the electric field pattern at high-latitudes depends on the polarity of the Y component of the interplanetary magnetic field (IMF By) and season. In this study we investigate the above relationships in the inner magnetosphere by using the perigee data from Cluster passing through the magnetic equator. The data consist of both components of the electric field perpendicular to the magnetic field obtained by the electron drift instrument (EDI). These data are sorted by polarities of IMF By and Bz and by seasons or hemispheres. The possible causes of the dependence of the electric fields on IMF By and seasons are discussed.
SM43A-1149 1340h
On Short-Time Prediction of Geomagnetic Activity
Prediction of geomagnetic activity is one of important tasks related to Space Weather Program. Global geomagnetic activity is controlled by solar wind parameters, the conditions in the magnetosphere and ionosphere (which are dependent on previous geomagnetic activity), season, and universal time. We developed an approach, which allowed us to improve significantly the reliability of short-time geomagnetic activity forecast. We used a solar wind-geomagnetic activity coupling function that is some different from the Akasofu parameter and proportional to V*f(Bz) where V is solar wind speed and f(Bz) is a step-like function dependent on the IMF Bz component and season/UT conditions. The correlation coefficient for hourly values of this coupling function and the AL index is higher than 0.8. Moreover, it increases significantly if instead of the AL index we used a geomagnetic activity index including geomagnetic disturbances at both high and low latitudes. Such index shows the excellent correlation with the solar wind-geomagnetic activity coupling function; the correlation coefficient is about 0.9 and higher.
SM43A-1150 1340h
Modeling kinetic particle effects in the inner magnetosphere during a magnetic storm
The interaction of the solar wind with the Earth's geomagnetic field leads to dynamic current and pressure distributions in the inner magnetosphere. Modeling the inner magnetosphere during a storm is accomplished by a combination of three techniques: global MHD simulations of the magnetosphere and its interaction with the solar wind, particle tracing calculations and drift-kinetic ring current simulations. Each approach captures different aspects of the physical process. MHD simulations do not include complete particle drifts and underestimate pressure in the inner magnetosphere. The ring current simulations produce detailed phase space densities and showed the highest pressure within a simplified magnetospheric model. Particle tracing calculations do not yet have adequate statistics to obtained detailed distribution function. We will present analyses of events based on these approaches and will present results of our efforts to unify the three approaches. We will consider observed moderate storm events; comparing the plasma pressure distribution in the inner magnetosphere from the MHD result to observed storm intervals. This will allow us to quantify the contributions of particles in the inner magnetosphere.
SM43A-1151 1340h
Initial Results from An MHD Simulation of Ganymede's Magnetosphere
Three-dimensional resistive MHD simulations were conducted to study the interaction between Jupiter's corotating plasma and Ganymede's internal magnetic field. We ran the simulations using different initial states that represent the different plasma and magnetic field environments corresponding to the six Galileo encounters. Since Ganymede's internal field is nearly anti-parallel to Jupiter's magnetospheric field, a small reconnecting magnetosphere was produced in the simulation as expected. We've found a small closed field line region near the equator and a large polar cap region containing filed lines that link to Jupiter. The magnetic fields from the simulation were evaluated along each flyby trajectory and compared with Galileo observations. The results gave good agreement between the observations and the simulation. The locations of the separatrices between Jovian field lines and those field lines connected to Ganymede at one or both ends are consistent with the results inferred from a vacuum superposition model (Kivelson et al. [1998]) and the energetic particle observations. We also show that the dynamic pressure of the upstream flow and the nature of Ganymede's ionosphere can be important factors affecting the topology of the magnetosphere. High dynamic pressure can expand the polar cap region and cause the equatorial standoff point to move close to Ganymede. Different ionization rates and neutral distributions in Ganymede's ionosphere will be used in future work to investigate the effect of the ionospheric boundary on the magnetosphere.
SM43A-1152 1340h
The Spatial Distribution of the Io Plasma Torus
Ground-based coronographic images of the Io plasma torus in S$^+$~6371\AA\ emission have been pseudo-tomographically deconvolved from their inherent line-of-sight integration, yielding estimates of their three-dimensional (3D) S$^+$\ density distributions with spatial resolution of $\sim$0.05$R_{\mbox{\rm \scriptsize J}}$. By interpreting this derived time-dependent structure we can infer some of the characteristics of torus formation mechanisms. For example, the dawn-to-dusk electric field in the torus region of Jupiter's magnetosphere causes the warm outer torus (including the ``ribbon'' feature) and the outer edge of the cold torus (which is the innermost region of the torus) to move closer to Jupiter by $\sim$0.3$R_{\mbox{\rm \scriptsize J}}$\ as the torus plasma rotates from the dawn region to dusk. Although this electric field is often approximated as spatially uniform, the initial analyses of these and other observations (Dessler and Sandel, {\em GRL 19}:2099, 1992; Schneider and Trauger, {\em ApJ 450}:450, 1995) implied a variation of the field strength with subsolar magnetic longitude. Now, preliminary results of our analyses of these observations imply differential motion between the inner and outer edges of the cold torus, indicating that the field strength varies spatially as well, appearing at times to diminish at the inner edge of the cold torus. Such behavior can result from a process known as magnetospheric ``shielding'', which has been noted in the magnetospheres of Earth and possibly Uranus but this would be the first instance where it has been seen at Jupiter. This and other anomalies of torus structure, such as the difference in magnetic latitudes of the warm and cold tori and the large gap between them, will be discussed at the meeting. This work was supported by NASA under grants NAG5-12944 and 9079 (Geospace Sciences Program) and NAG5-8952 (Planetary Atmospheres Program).
SM43A-1153 1340h
Global MHD Simulation of Three-dimensional Time Dependent Magnetic Reconnection in the Magnetosphere
Results from investigating 3d reconnection of magnetic fields using the Geospace General Circulation Model (GGCM) [Raeder, JGR, 104, 1999] are presented. A single long run was done with IMF orientation varying from north to south for the purpose of the studying clock angle dependence of reconnection topology. At the start of the simulation the initial northward IMF conditions show evidence of dayside null-null separator reconnection [Lau and Finn, Astro. Journal, 350, 1990]. As the simulation progresses, a change in magnetic field topology occurs in which the two nulls migrate to the nightside (about 15 - 20 RE down the tail), where new nulls form. The nulls are tracked using Greene's algorithm [Greene, Comp. Phys., 98, 1992] and the time evolution of the magnetic field skeleton is visualized.
http://apollo.sr.unh.edu/
SM43A-1154 1340h
Effects of Ejecta-Ejecta Interactions on the Magnetosphere: The Double Great Storm on March 31, 2001
When ejecta interact with each other en route to Earth, their parameters are changed in significant ways, and with it their geoeffectiveness. Among the effects of such interactions are: heating of the plasma, acceleration of the leading ejecta and deceleration of the trailing ejecta, compressed field and plasma in the leading ejecta, disappearance of shocks originally driven by the trailing ejecta, and the strengthening of shocks driven by the accelerated ejecta. Here we model the ring current enhancement and radiation belt behavior and try to isolate the effects these changes had on the magnetosphere when 2 ejecta in the process of coalescing reached Earth on March 31, 2001. The magnetosphere senses the presence of the two ejecta and reacts with a re-activation of the ring current soon after it started to recover from the first ejection, giving rise to a double-dip great storm (Dst < -250 nT). The compression of the plasma in the leading ejecta by itself gives a contribution of > 100 nT to the Dst corrected for magnetopause currents. This is about three times that due (i) to the compression behind the shock and (ii) the compression in the trailing ejecta. The high, but monotonically decreasing, plasma sheet density is probably also a result of the compression of the plasma in the leading ejecta. The non-linear behavior of the magnetosphere is illustrated with the behavior of the cross-polar cap potential, as calculated using the AMIE technique. Acknowledgements. This work is supported by the Wind Grant NAG5-11803 and by NASA Living with a Star Grants NAG5-10883, NAG5-13512, NASW/02035, and NSF Space Weather grants ATM-0208414 and NATM-0309585.
SM43A-1155 1340h
On the Time of Arrival of the Storm of 31 March 2001
The storm of March 31, 2001 presented a unique opportunity to use 4 satellites to track solar wind features from far-upstream, through the earth's bow shock to the magnetopause. The ACE satellite was about 220 Re upstream but almost on the Sun-Earth line, WIND was at approximately Y=-250Re, and GEOTAIL and CLUSTER exited the magnetopause on the Earth's dayside owing to the storm compression. Using the location of these spacecraft, the arrival of the discontinuity at the Earth can be reliably calibrated at 38 minutes after passing over the ACE spacecraft. A closer look at the CLUSTER data during this event also shows several oscillations of the bowshock past the satellite which allows for calculation of the velocity of the shock relative to the spacecraft. Detailed comparison of CLUSTER and GEOTAIL data when the satellites are near the magnetopause gives indirect evidence of the draping of the magnetic field lines over the magnetopause. Results from the Rice Convection Model using solar wind data with a 38 minute time delay from the ACE spacecraft to the magnetopause will be presented and compared to previous runs of the same event.