SM31B-1223 0800h
What are Sawtooth Events?
Sawtooth events are quasi-periodic, large-amplitude flux oscillations with a periodicity of 2-4 hours often seen during storms when the ring current is enhanced. They are driven by moderate ($|-B_z| < 10$nT) and steadily southward IMF conditions. The oscillations have been termed `sawtooth events' because their shape -- a series of slow flux decreases followed by rapid increases -- resemble the teeth of a saw blade. The `sawtooth' shape is particularly well-defined in the high energy proton channels. At geosynchronous orbit, each tooth is associated with strong stretching and dipolarization of the B-field and, while this activity is typically strongest on the night-side, it can also extend well into the day-side for larger sawtooth events. An abrupt recovery in Sym-H is typically observed in association with each dipolarization and each tooth is associated with dispersed particle injections and pre-midnight sector auroral substorm onsets Nevertheless, the auroral activity rapidly engages a wider than usual azimuthal range and the injections can also be dispersionless over a wider than usual range of MLTs. Thus, despite the many similarities to substorms, the more distributed (in local time) nature of sawtooth events has led to an on-going debate as to whether or not they really are substorms. Here, we compare and contrast the phenomenology associated with a number of individual sawtooth and substorm events. In particular, we present a re-analysis of a well-studied interval of substorm activity (the CDAW-9C interval) and demonstrate that it was also a sawtooth event. We will also present the results of a statistical study showing how substorms and sawtooth events relate to one-another on average.
SM31B-1224 0800h
Ionospheric Convection Pattern for Sawtooth Events from AMIE Simulation
Sawtooth events are quasi-periodic, large-amplitude oscillations of energetic particle flux enhancement with a periodicity of 2-4 hours, which are observed globally and simultaneously at the geosynchronous orbit. Statistical studies of sawtooth events imply an enhanced solar wind - magnetosphere - ionosphere coupling. Herein the ionosphere convection potentials and currents systems are examined using the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) simulation results. Comparison of convection pattens for sawtooth events with isolated substorms, steady magnetosphere convection (SMC) events are also made.
SM31B-1225 0800h
Ring Current Composition During Sawtooth Storms
Recent IMAGE observations have shown that the gradual development of the hydrogen ring current during sawtooth storms is accompanied by a bursty occurrence of oxygen. The observed oxygen energetic neutral atom (ENA) fluxes are as high, if not sometimes higher, than oxygen fluxes associated with non-sawtooth storms. They appear predominantly during individual sawtooth injections. Once a sawtooth injection has occurred, oxygen ENA fluxes quickly abate due to a combination of precipitation, drift loss and charge exchange. We use ENA inversion techniques to estimate the amount of hydrogen and oxygen plasma in the inner magnetosphere during sawtooth events. Due to the orbit of IMAGE we can monitor the plasma content for 70-80 percent of the time, split into uninterrupted stretches of up to 10 hours apiece. This coverage allows us not only to study the time development of the ring current and the oxygen content during a storm, but to also study several consecutive "teeth" without interruption. We present results from several sawtooth storms that occurred since the fall of 2001. We show the time development of the ring current during those storms, and contrast it with the time history of observed oxygen. For comparison, we will also show data from non-sawtooth storms since 2001.
SM31B-1226 0800h
Reoccurrence Substorms and Their Responses to IMF Variance
The critical problem in magnetospheric substorm study is the cause of substorm onset. We examined the reoccurrent substorms which well defined with energetic particle data and magnetic data in geosynchronous ground geomagnetic data. 5 events are analyzed. During these events, nearly simultaneous particle flux enhancements and magnetic field variances occurred at geosynchronous, coincided well with the geomagnetic field H-component dramatically decreasing in high latitude nightside regions, which are defined as substorm signatures. With examining IMF data, 4 events occurred when the IMF stayed sustaining southward, but disturbed, One event occurred during relative stable southward IMF. All these events occurred during magnetic storm main phase, and ended at early stage of recovery phase. We find that the reoccurrence substorm onsets are associated with relative change in IMF, especially IMF Bz northward turning. The periods of substorm reoccurrence are 2-3 hour, which dependent on the variances in IMF. Based on present study, we suggest that the necessary condition for substorm onset is enough energy accumulated in magnetotail, leading to an instable magnetotail, external variances in IMF trigger the substorm onset.
SM31B-1227 0800h
Interpretation of Automated Forward Modeling Parameters for Sawtooth Events and Steady Magnetospheric Convection
Automated Forward Modeling (AFM) is an inversion technique based on magnetic data alone, which can indicate physical parameters associated with electrojets. From perturbations along a meridian, the total electric current crossing the meridian may be determined, as well as the latitudes between which it flowed. The technique is based on nonlinear optimization of the parameters of a forward model. It is possible to compare model output to the original input to ensure that the routine has functioned well and that output parameters are reliable and presumably have physical meaning. Characteristic behaviors of substorms are readily seen in modeling output: the current strengthens rapidly and considerably at an expansive phase onset, following a growth phase during which the electrojet borders move equatorward, usually with some strengthening of current. At onset the poleward border is often seen to move poleward rapidly. Poleward border activity may be noted then and also at other times. After an onset, the recovery phase is often marked by a retreat of the equatorward border, indicating the well-known shrinkage of the auroral oval. These complete cycles of activity are absent in steady magnetospheric convection (SMC) and sawtooth events. Our output parameters can be diagnostic of onsets and useful in determining their location and role in SMC. Sawtooth events have many of the characteristics of expansive phase onsets, but maximum poleward expansion of the poleward border is followed by equatorward movement reminiscent of a growth phase. Since this is correlated with the interplanetary magnetic field remaining southward, the difference from common expansive phase phenomenology may simply be the lack of a recovery phase. For SMC, onset-like activity appears to be concentrated at the poleward border and of small magnitude compared to the overall convection electrojets. The activity is reminiscent of poleward border intensifications.
SM31B-1228 0800h
Characteristic Time Scales of Sawtooth Phenomenon in the Magnetosphere
The sawtooth phenomenon in the magnetosphere reflect the coupling between the inner magnetosphere and magnetotail, and thus have important implications on storm-substorm relationship. In this study the data of sawtooth events from the LANL geosynchronous satellites during July 2000 - December 2001 (Skoug et al., 2002 Spring AGU Meeting) are used to obtain their characteristic features. The data set consists of 71 events for which the electron and proton fluxes in many energy channels are measured by the geosynchronous satellites. For the electrons there are 10 energy channels ranging from 50 keV to >1.5 MeV, and for the protons there are 12 channels with energies ranging from 50 keV to 50 MeV. The data for each channel in electron or proton energy are combined together by taking the peaks in each of them as the common reference point. The fluxes are then averaged over the events to get the mean shape of the sawtooth oscillations. The rise and decay times computed from these provide the characteristics time scales quantifying the sawtooth nature of the oscillations. For example, the rise time is found to be typically half of the decay time. These time scales for electrons and protons, and their variability are studied in order to characterize of the sawtooth oscillations quantitatively.
SM31B-1229 0800h
The specific entropy of the plasma sheet as a controlling factor for the injection of a storm-time ring current
Simulation results are presented which explore the importance of the inner plasma sheet specific entropy ($pV^{5/3}$, where p is pressure and $V = \int ds/B$ is the flux tube volume) for controlling the strength and depth of ring current particle injections during magnetic storms. The simulation model is the RCM--E (Rice Convection Model -- Equilibrium), which calculates the energy-dependent magnetic and electric drifts of particles within self-consistently computed magnetic and electric fields. The results demonstrate that inner plasma sheet reductions in the specific entropy can drive region 1 Birkeland currents that enhance the westward electric field in the depletion region; the strong electric field then propels plasma earthward, into the ring current. In contrast, earthward convection of heavily loaded (insufficiently depleted) plasma sheet flux tubes is impeded by the immense pressure gradient that would result from squeezing that much plasma into the much smaller volume of a ring current flux tube. In other words, insufficient depletion disallows the flux tubes from taking on the quasi-dioplar shape necessary to penetrate into the ring current. Flux tube content reductions are observed during the substorm expansion phase, suggesting one potential process (though perhaps not the only one) that could lead to ring current injection. The ability of the plasma sheet specific entropy to influence the injection of ring current particles, and the implications for understanding the differences between SMC and Sawtooth events will be examined.
SM31B-1230 0800h
Magnetospheric Convection During an Extended Interval with Southward Interplanetary Magnetic Field
Recently we reported on a new convection state of the magnetosphere in which the late growth phase and expansion phase of a large magnetospheric substorm on December 22, 1996 were characterized by large twin vortical flows in the near-Earth magnetotail [Ashour-Abdalla et al. 2003]. We also found a state in which flux tubes formed by reconnection at a near-Earth neutral line do not convect to the dayside magnetopause. Instead these flux tubes convect around one of the vortices back to the tail neutral line where they reconnect with IMF field lines formed by the original neutral line and return the flux to the tail lobes. A second interval on February 13 and 14, 2001 was very similar to December 22, 1996 in that the interplanetary magnetic field remained southward for many hours. In this talk we will present the results from a study of convection during this event. February 13 and 14, 2001 were characterized by a number of substorms. The largest of which started at about 2200 on February 13 with significant intensifications at about 0030UT and 0500UT on February 14. We have used a global magnetohydrodynamic simulation to model the interval starting at 2000UT and including this substorm. A persistent near-Earth neutral line formed during this interval between x=-30RE and x=-50RE. A large-scale vortex formed earthward of the neutral line in the pre-midnight quadrant. This vortex intensified during the event eventually encompassing the entire magnetosphere earthward of the neutral line with earthward flows in the dawn quadrant and tailward flows in the dusk quadrant. The earthward flow reverses direction in the high-pressure region of the inner tail. Flux tubes in this large vortex reconnect with IMF field lines to form new closed field lines.
SM31B-1231 0800h
Continuous Lobe Reconnection in the Mid-Tail: Observational Signatures and Relation to Substorm onset
Magnetospheric substorms represent a global interaction between the solar wind, the magnetosphere, and the ionosphere. Energy extracted from the solar wind is mainly stored in the form of excess magnetic flux in the magnetotail lobes. There is little doubt that reconnection occurs in the magnetotail at some point during substorms. However, whether or not explosive release of this energy is required to cause the substorm and whether reconnection precedes or succeeds expansion onset are still subjects of big debate and controversy. In past three years (2001-2003) Cluster constellation passed through the plasma sheet more than one hundred times. Base on survey of the three year four spacecraft data, we have selected 39 continues lobe reconnection (CLR) events. A careful study of these events indicates that the CLR and plasma sheet closed field line reconnection manifest quite differently. The CLR occurs when the IMF is persistently southward (say, for more than a few tens of minutes) and maintains for more than about 20 minutes. It creates a low-density and low-temperature structure with high-speed plasma flows near the central plasma sheet. Quite often the CLRs appear quasi-periodically and in association with the presence of a magnetic storm. Comprehensive investigations have been made in this paper on the relationship between the occurrence of CLRs in the mid-tail and the substorm onsets in the near-Earth region. The 39 CLRs are all found to be corresponding to the appearance of intense substorms. In 37 events the CLRs precede substorms expansion onsets, while other two are opposite. These suggest that tail lobe unloading via CLR is a critical issue for the expansion onset of substorms occurring in persistently southward IMF periods. Nevertheless, this study does not exclude that substorms of other types may have different causes and that dynamical processes in the near-Earth tail are important in triggering the substorm expansion onset.
SM31B-1232 0800h
Convection and Auroral Electrojet Behavior deduced from a Simplified Magnetosphere: Is a Steady State possible when Curvature and Gradient Drifts are considered?
A simplified model of the magnetosphere is used to study convection in the presence of reconnection and energetic particles. The latitude-limited band of magnetic flux which contains azimuthal convection from the nightside to the dayside is sandwiched between a high-latitude region, in which magnetic flux tubes are too tail-like to convect to the dayside, and a low-latitude region in which the plasma does not strongly affect convection. In order to include the effects of curvature and gradient drifts, the band of azimuthally-convecting flux is treated as two regions: the dawn convection channel (westward electrojet) and the dusk channel (eastward electrojet). The author has not been able to find a simple steady-state solution to the convection if the Harang discontinuity is a true discontinuity (variations only in the perpendicular direction). However, a time-dependent solution exists in which: (1) there is no flux transfer from the highest-latitude region into the convection channels, (2) the Harang discontinuity drifts westward with the eastern edge of energetic particles (partial ring current), (3) the convection channels become thinner with time, (4) the westward electrojet overlaps the eastward electrojet. If flux transfer from the highest latitude region into the convection channels occurs as sporadic events, each event would be expected to produce a new pair of electrojets similar to the above, and the pair would be added to the poleward edge of the pre-existing system. This would produce multiple current sheets and electrojets near midnight. The author has previously suggested that azimuthal convection from the nightside near-earth plasma sheet might play a major role in controlling dipolarization and reconnection events. If this is true, then the onset of the events will be related to the above behavior of the Harang discontinuity.
SM31B-1233 0800h
Convection electric field in the near-Earth tail during the super magnetic storm on November 20-21, 2003
We have examined the electric field observations made by the Geotail spacecraft in the near-Earth magnetotail during the intense magnetic storm that occurred on November 20-21, 2003. The solar wind observation obtained by the ACE spacecraft shows the encounter of a shock-like change in the solar wind/IMF parameters at about 07:30 UT on November 20, 2003, which is followed by an intense southward dip of IMF (-50 nT $<$ IMF-Bz $<$ -20 nT) lasting until the end of the day. On the arrival of this solar wind structure, ground magnetograms indicate that a very intense magnetic storm started to develop at about 8 UT and reached its minimum Dst ($<$ -450 nT) around 18 UT, then Dst recovered gradually until early November 21. During this storm, Geotail traversed the magnetotail at R ~ 9-12 Re from dusk to dawn through the midnight sector at ~ 17 UT on November 20 and observed the convection electric field in the near-Earth plasma sheet for the storm main phase and early recovery phase. The electric field in the plasma sheet during the main phase shows large fluctuations with amplitudes of several mV/m, implying the highly turbulent characteristics of the tail electric field in a wide range of local time. Aside from its large deviation, the average intensity of the duskward electric field is ~ 0.6 mV/m, which is not so different in magnitude from those observed during other moderate level storms (~ 0.3-0.6 mV/m). The observed tail electric field is compared with those calculated with some empirical ionospheric electric field models as well as that estimated with the polarcap potential drop actually observed simultaneously by the DMSP satellites. It is shown that the model electric fields calculated with the Boyle model and Weimer model both tend to overestimate the average storm-time intensity of the tail electric field by a factor of 1.5-3 during the main phase. On the other hand, the polarcap potential obtained by DMSP gives a better estimate for the tail electric field during this intense storm.
SM31B-1234 0800h
The Transpolar Potential During the November 2003 Magnetic Storm
The November 2003 magnetic storm was particularly severe, with the minimum value of Dst reaching - 472 nT at 2000h UT on November 20. Four hours earlier (1600h) IMFBz reached its minimum value of ~ -50 nT (clock angle = 176o), with Psw = 18 nPa, Vz ~600 km/s, H+ density = 30 ions/cm-3, Esw = 29 mV/m. Inputting these parameters into the Hill-Siscoe (Siscoe et al. [2002], Ober et al. [2003]) transpolar potential model, a maximum polar transpotential of 300 kV for an estimated polar cap conductance of 10.1 mhos (F10.7 ~ 170). According to the Hill-Siscoe model this requires a Region I current of ~9 MA. The DMSP (Defense Meteorological Satellite Program) satellites F13 and F15 trajectories passed through various sections of the two-cell polar cap potential configuration, so that a direct measurement of the full transpolar cap potential is not probable. However, a theoretical two-cell potential configuration can be obtained from the Nopper-Carovillano (N-C) Model [Nopper and Carovillano, 1978] which then allows the experimental data to be normalized to a known transpolar cap potential. In this light, we find general agreement between the time-history predicted by the Hill-Siscoe model and the DMSP data, using 104 transpolar passes. We also find the maximum experimental value for the transpolar cap potential of 220 50 kV which is to be compared to the value predicted by the Hill-Siscoe Model (300 kV). R.W.Nopper and R.L. Carovillano, Geophys. Res. Ltrs.,5,699,1978. Siscoe et al., J. Geophys. Res., 107, A6, SMP 8, 2002. Ober et al., J. Geophys. Res., 108, A12, SMP 27, 2003.
SM31B-1235 0800h
The Saturation of Dayside Magnetosphere Erosion
We carry out a statistical investigation relating the depression of the geostationary magnetic field near noon to the strength of the southward component of the IMF ($B_z$). We use measurements made by GOES, WIND, ACE and ground-based magnetograms. The period studied is 1996--2003. Events have been selected during intervals with (1) a steady dynamic pressure, $P_{dyn}$; (2) a steady and negative IMF $B_z$; (3) when one of the GOES spacecraft was within 2 hours of local noon; and (4) during substorm growth phase (as ascertained by readings from nightside ground magnetometers). The investigation starts with a correction of raw values of the total geostationary field, $B_{tot}$, for the compression due to $P_{dyn}$. To do this, we chose a number of ``reference" days encompassing a wide range of $P_{dyn}$ values when the IMF was strongly northward (clock angle $<$ 45$^{\circ}$), and fitted $B_{tot}$ to the functional form $B_{tot} = a + b P_{dyn}^{1/2}$. We then fit the corrected $\Delta B_{tot}$ values as a function of $B_z$. This work extends the investigation of {\sl M\"uhlbachler et al., 2004} by a set of events with extremly large and negative $B_z$ in the range of [0,-52]nT. Thus, we find that saturation of erosion occurs for $B_z$ in the range -12 to -16 nT. We fit the data in several ways and find $\Delta B_{tot} = -27.76 + 26.1 \exp(0.012 B_{z})$ to lie closest to our set of data.
SM31B-1236 0800h
Solar wind and IMF control of substorm onset
Few studies of solar wind control of substorm onset exist and only two have good statistics using the IMF. Those studies with solar wind statistics employ only a few 10's of events, yet it has been proposed that solar wind dynamic pressure changes may trigger substorms. While a small percentage of substorms are believed to be triggered by solar wind dynamic pressure variations and hence, solar wind control of the substorms is not surprising, a significant number of substorm events exist that appear to have either IMF triggers or no triggers at all. We will show the results of a statistical examination of solar wind control of substorm onset using propagated solar wind data. Furthermore, we will demonstrate the difference in the solar wind control of the substorm onset for triggered and non-triggered substorms. This type of study is important for both space weather predictions of substorm onset and for use in substorm modeling. Moreover, the results of this study will be of interest to the upcoming THEMIS mission.
http://www.igpp.ucla.edu/jweygand
SM31B-1237 0800h
Geosynchronous particle flux disturbances caused by the solar wind dynamic pressure enhancement: Dependepnce on convection strength
When a variation in the solar wind dynamic pressure hits the magnetosphere, various types of disturbances are created. The geosynchronous particle flux disturbance is closely related to the issues of magnetic storms, substorms, and sawtooth oscillations. In this work, using the LANL particle flux data, we have done an intensive examination on how the geosynchronous particle flux in the energy range of tens to hundreds of keV responds to a sudden increase in the solar wind dynamic pressure. We find significant differences in the flux response between different conditions of the accompanied IMF (interplanetary magnetic field). When the IMF remains northward at and prior to the time of the pressure increase, the most common type of the flux response we find is a simple dispersionless increase simultaneously at all availbale MLT positions. However we also find a significant number of events where the flux response is a decrease or virtually no notable change at all. Also, for this northward IMF condition, we find no notable differences in the flux response between protons and electrons. When the accompanied IMF is weakly southward, the flux response to the pressure increase is very similar to that in the case of the northward IMF. However, as the IMF becomes further southward, the flux response becomes more complex. When the IMF remains strongly southward (around -10 nT or even less) at and prior to the pressure increase, we find that the flux change is strikingly different between different species. In a number of events we have examined, the electron flux response is characterized by dispersionless increases near midnight and by well-defined dispersions at MLT regions away from midnight. This feature is very similar to the flux change pattern during a typical substorm injection. In contrast, the proton flux often exhibits global near-simultaneous increases with little or even no energy dispersion at (dayside) MLT locations where one would normally expect a significant dispersion signature from a typical substorm injection. We will discuss how our result is associated with magnetic storms, substorms, and sawtooth phenomena.
SM31B-1238 0800h
Strongly Southward IMF Substorms, Dynamic Pressure Disturbances, and Null Events
Solar wind discontinuities can lead to important large-scale disturbances that significantly affect the space environment, including energetic particle fluxes, the aurora, and magnetospheric and ionospheric current systems. Understanding what discontinuity characteristics lead to what kind of disturbance is thus critical for disturbance prediction and understanding. Global auroral images from the wideband imaging camera (WIC) on the IMAGE spacecraft show striking new information on this relationship. Two well-studied types of discontinuity driven disturbance are: substorms resulting from northward interplanetary magnetic field (IMF) turnings and dynamic pressure (P) disturbances that result from enhancements of solar wind dynamic pressure. During typical substorms, auroral activity initiates near the equatorward boundary of the auroral oval within a ~1-2 hr MLT sector within the Harang electric-field reversal region and then expands to cover a few hours in MLT. Typical P disturbances show rapid global enhancement of auroral emissions as well as a significant poleward motion of the poleward boundary of the aurora, but enhancement related to the Harang reversal is not evident. The WIC images show that, during periods of strongly southward IMF, substorms expand to a significantly broader MLT range than do typical substorms, and that, in addition to a global auroral enhancement, P disturbances exhibit a substorm-like auroral enhancement within the Harang reversal that extends over a broad MLT range. These observations show that, for strongly southward IMF, both IMF and P changes cause Harang region activation. Because of this, it is reasonable to expect that IMF and P interplay effects may be important for solar wind discontinuities having both a significant IMF change and a significant P change. The WIC images show that such interplay effects can indeed be important. In particularly, discontinuities having a significant IMF northward turning and a significant decrease in P or having a significant increase in P and a significant southward turning of the IMF are found to not lead to a substorm-like aurora disturbance within the Harang reversal region. We refer to such events as "null events," since the IMF northward turning or P increase for each would, by themselves, be expected to cause a large substorm disturbance, but the effects of these appear to be nullified by the simultaneous change in the other quantity.
SM31B-1239 0800h
Firehose Turbulence as the Source of Pi2 Precursors to Dipolarization Events
The nonlinear dynamics of the firehose instability provides a possible explanation for the onset of the magnetic fluctuations associated with bursty bulk flows and substorms. The Pi2 geomagnetic pulsations are observed as precursors to the dipolarization events associated with substorms and sawtooth injections [Kepko and Kivelson, JGR 2001 and Sigsbee et al., JGR 2002]. A scenario of rapid Earthward flux tube motion producing a parallel ion pressure anisotropy triggering firehose driven Alfven wave turbulence is presented. Kinetically corrected MHD theory is required to describe the nonlinear magnetic fluctuations. Strong magnetic fluctuations are typically associated with substorms and start a few minutes before the arrival of the dipolarization pulse. Secondary current driven instabilities are considered and may also be seen in the data at ion cyclotron and lower hybrid frequencies. Nonlinear saturation is caused by the nonlinear weakening at large magnetic field line distortions of the destabilizing magnetic curvature force driven by the pressure anisotropy. Horton, W., B.-Y. Xu, and H. Vernon Wong, Firehose driven magnetic fluctuations in the magnetosphere, GRL 31, 2004. This work was supported by National Science Foundation Grant ATM-0229863.
SM31B-1240 0800h
Characteristics of Dayside Pi 2 Events During Magnetospheric Substorms
Thirty-six isolated substorms were identified using the energetic electron flux from the LANL 1994-089 satellite. Ground-based magnetometer data was collected from the MM210 and MEASURE arrays to compare Pi 2 wave power and Pi 2 peak frequencies between the night and the daysides. Each substorm event was classified as combinations of Pi 2, Pc3, and Pc 4. Strong dayside Pi 2's were present in 25% of the events increasing to 83% for all categories of Pi 2. The amplitude of the night and day Pi 2 increases with L shell along the similar trend. The Night Pi 2 occurred between 2:00 and 3:00 a.m. putting them in the category of a flank Pi 2. The frequency of the night Pi 2 decreased with an increasing L shell and the night Pi 2, between an L = 1.0 and L = 2.5 showed a tendency for equality. The day Pi 2frequency occurred within 1.5 hours of local noon. The day Pi 2 frequencies also showed a tendency for equality between an L = 1.0 and L = 2.5. The median frequency for the low L shell day Pi 2, showed a similarity to the mid to high L shell night Pi 2. The majority of day Pi 2 appears to occur before the night Pi 2 approximately 20 minutes before the energetic particle injection of the substorm. A hint of a linear trend between solar wind dynamic pressure and the day Pi 2 suggests the day Pi 2 is a directly driven field line oscillation coupling with a cavity resonance mode minutes before a substorm.
SM31B-1241 0800h
Analysis of the GEM Storms with the WINDMI Models
An important problem in magnetospheric physics is to develop integrated dynamical systems capable of modeling storm and substorm databases with the long term aim of developing space weather forecasting tools. The 20 parameter WINDMI model is used to analyze the global dynamical behaviour of Geomagnetic storms and substorms. The model is a system of non-linear ordinary differential equations that describe the solar wind-magnetosphere-ionosphere coupling and the interaction of basic energy components in the nightside magnetosphere. An electromotive force derived from Solar Wind parameters is input into the model as a driver. These parameters are measured by the ACE satellite and the data provided by the NSSDC. The output of the model is the westward Auroral Electrojet current and the energy stored in the Earth's ring current which is used to predict AL and Dst. The model has been tested on the seven GEM storms selected for community wide study. These geomagnetic storms cover periods of days and typically contain many substorms. The prediction of the model is compared to the AL and Dst indices provided by the WDC Kyoto. A globally optimum best fit to the data has been performed using the Genetic Algorithm to find the physical parameters of the model. For the GEM storms we find strong correlation between the model output and the data. We compare the performance of this model for the GEM storms with the Temerin-Li Dst model. This work is supported by the National Science Foundation Grant ATM-0229863.
SM31B-1242 0800h
Substorm Injections Produce Sufficient Electron Energization to Account for MeV Flux Enhancements Following Some Storms
One of the main questions concerning radiation belt research is the origin of very high energy ($>1 MeV$) electrons following many space storms. Under the hypothesis that the plasma sheet electron population is the source of these electrons which are convected to the outer radiation belt region during substorms, we estimate the flux of particles generated at geosynchronous orbit. We use the test particle method of following guiding center electrons as they drift in the electromagnetic fields during substorm dipolarization. The dipolarization pulse model electromagnetic fields are taken from the {\it Li et al.}~[1998] substorm particle injection model. We find that a substorm dipolarization can produce enough electrons within geosynchronous orbit to account for the electrons seen following storms. To do this we compute transport ratios of plasma sheet electrons, that is the relative ratio of plasma sheet electrons that are transported and trapped in the inner magnetosphere during substorms, as well as the change in energy of the electrons. Since high fluxes of $MeV$ electrons are only seen following storms and not isolated substorms, it is likely that these electrons may serve as a source population for other energization mechanisms which accelerate the electrons to MeV energies. Furthermore we do parametric studies of the dipolarization model to understand physically what conditions enable the generation of this source population. This work supported in part by NSF Grant No. ATM-0229863.
SM31B-1243 0800h
Plasmatrough SAPS Flow and its Implications for Global Patterns of Convection and Field-aligned Current
In the northern hemisphere, the sub-auroral polarization stream (SAPS)is characterized by strong westward convection and the accompanying poleward electric field in the plasmatrough during disturbed times. The westward SAPS convection is immediately equatorward of the eastward auroral electojet flows in the midnight sector. Since the convection contours must be closed, the SAPS flow contours lead to an interesting modification to the normal two-cell "directly driven" convection pattern. That modification is mainly due to the effects of disturbance-related "unloading" convection cells. When the resulting overall convection pattern (including directly-driven and unloading effects) is examined, it is remarkably consistent with the predictions of Iijima and Potemra (1976) about the global field-aligned current (FAC) pattern. Recent results are presented to support this overall picture of the convection and FAC patterns during disturbed periods.