Search

In this paper we examine the response of the magnetosphere-ionopshere (M-I) system to a transient northward excursion in the interplanetary magnetic field (IMF) using the Lyon-Fedder-Mobarry (LFM) global MHD simulation. The simulated IMF transitions hold from a steady southward IMF to a steady northward IMF before suddenly transitioning back to southward IMF after 20 min. Once the IMF returns southward, the M-I system is in a state of reduced energy dissipation for approximately an hour as it reconfigures back into a standard southward IMF configuration. We find that the northward IMF excursion affects both the viscous and reconnection interactions with the solar wind. The flow of plasma in the magnetosphere is significantly disrupted by the reconnection cycle under northward IMF. This reduce the transfer of mechanical energy from the solar wind due to the viscous interaction, and the magnetosphere-ionosphere system is in a mixed topological configuration containing elements produced by both of southward IMF reconnection and the Dungey cycle, as well as northward IMF reconnection and the presence of reverse cell convection at high latitudes. The effects of the transient northward IMF must be completely cleared out before the system can return to an optimal state of energy transfer characteristic of steady southward IMF. As a result, a simple 20 min excursion of northward IMF can put the magnetosphere-ionosphere system into a reduced state of coupling to the solar wind for some time following the return to steady southward IMF; for LFM we saw a reduced state lasting an hour

An argument is presented to support the view that the rate of merging between the geomagnetic field and the interplanetary magnetic field across the dayside magnetosphere is controlled primarily by the solar wind parameters. The controlling parameters are the solar wind electric field in the Earth's frame of reference and the solar wind magnetosonic fast mode Mach number. These factors control to first order the total amount of magnetic flux that is carried by the magnetosheath flow to the dayside merging region. We argue that the global dayside merging rate is governed by the amount of flux that is delivered to the dayside merging line by the magnetosheath flow. The ionospheric conductance also plays an important role by modulating the shape of the magnetospheric obstacle around which the magnetosheath flow is deflected. The local conditions at the magnetopause, especially changes in magnetospheric plasma density will affect the local reconnection rate, but not the global dayside merging rate because to change the global merging rate the entire pattern of magnetosheath flow must be changed. The conceptual model presented here can explain how dayside merging depends on solar wind values, including both linear and nonlinear dependencies, through the application of a single, unifying perspective, without the need for ad hoc mechanisms that limit the dayside merging rate.

The polar cap boundary (PCB) is a fundamental indicator of magnetospheric activities especially during a substorm cycle. Taking a period on 8 March 2008 as an example, we investigate the location of PCB and its dynamics during a substorm event. The PCB location is determined from the Piecewise Parabolic Method with a Lagrangian Remap (PPMLR) -Magnetohydrodynamic (MHD) simulation data and Defense Meteorological Satellite Program (DMSP) observations, respectively. Model-observation comparison indicates that the PPMLR-MHD model gives a reliable estimate of PCB location during a complex substorm sequence. We further analyze the evolution of PCB in that period. The polar cap expands under southward interplanetary magnetic field (IMF), since the low-latitude dayside reconnection produces new open magnetic flux. Meanwhile, more solar wind energy enters and stores in the magnetosphere with the decreasing SML (SuperMAG Auroral Lower) index. After the substorm expansion onset, the polar cap contracts for a while due to the explosive increase of nightside reconnection. When the IMF direction turns northward, the polar cap contracts continuously, since the dayside reconnection ceases and no more open magnetic flux are supplied, and the storage energy in the magnetosphere releases with the increasing SML index. The model results are in good accord with the features from observations.

Changes in the Dst index, or the similarly constructed high-resolution SYM-H index, are thought to indicate changes in the total energy content of the ring current. However, this is not always the case. In this paper we examine an intense (SYM-H ∼ −435 nT) magnetic storm that occurred on March 31, 2001. The arrival at Earth of strongly southward IMF produced an immediate negative response in the SYM-H index. While energetic particle and magnetometer data from geosynchronous orbit and inner magnetosphere energetic neutral atom imaging indicate that two substorm injections took place during the main phase, there was about one hour when the SYM-H decreased more than 200 nT with no evidence in the data for ring current enhancement. Instead the near-Earth magnetotail exhibited a growth phase indicative of a strong, growing cross-tail current, with the large substorm expansion phase and the associated injection of energetic particles coming significantly later. Data from the DMSP spacecraft demonstrate that the polar cap flux grew rapidly in response to the strongly southward IMF. We present observations showing that the decrease in SYM-H occurred when polar cap flux was increasing and there was no evidence of injection into the ring current. Our findings strongly support the relationship between Dst and the polar cap flux proposed by theoretical studies that determined that the tail current system could be a significant contributor to Dst.

In this study we examine the recovery of relativistic radiation belt electrons on November 15-16, 2014, after a previous reduction in the electron flux resulting from the passage of a Corotating Interaction Region (CIR). Following the CIR, there was a period of high-speed streams characterized by large, nonlinear fluctuations in the interplanetary magnetic field (IMF) components. However, the outer radiation belt electron flux remained at a low level for several days before it increased in two major steps. The first increase is associated with the IMF background field turning from slightly northward on average, to slightly southward on average. The second major increase is associated with an increase in the solar wind velocity during a period of southward average IMF background field. We present evidence that when the IMF Bz is negative on average, the whistler mode chorus wave power is enhanced in the outer radiation belt, and the amplification of magnetic integrated power spectral density in the ULF frequency range, in the nightside magnetosphere, is more efficient as compared to cases in which the mean IMF Bz is positive. Preliminary analysis of the time evolution of phase space density radial profiles did not provide conclusive evidence on which electron acceleration mechanism is the dominant. We argue that the acceleration of radiation belt electrons requires (i) a seed population of keV electrons injected into the inner magnetosphere by substorms, and both (ii) enhanced whistler mode chorus waves activity as well as (iii) large-amplitude MHD waves.

[1] The two primary methods responsible for solar wind magnetosphere coupling are magnetic reconnection and the viscous interaction. The viscous interaction is generated due to the antisunward dragging of plasma inside the magnetopause by the plasma flowing in the magnetosheath, creating a return flow deeper inside the magnetosphere and producing a circulation pattern. This viscous circulation pattern is mapped into the ionosphere via magnetic field lines, which results in ionospheric electric field in the nonrotating Earth's frame. We measure this interaction in terms of an electric potential, the viscous potential. In this paper, we use the results obtained from the Lyon-Fedder-Mobarry (LFM) simulation model during periods of purely northward interplanetary magnetic field (IMF) for different solar wind velocity and ionospheric conductivity, showing a reduction of the viscous potential with increasing magnitude of northward IMF. The viscous potential is found to settle around 5–10 kV for large +Bz values. The decrease in viscous potential was found to be associated with a weak or nonexistent sunward plasma flow in the nightside plasmasheet. Instead, the return flow to the dayside occurs at high latitudes and is associated with the reconnection topology and dynamics that occur during northward IMF periods. We also show that the magnetosphere remains closed during purely northward IMF, except for two small regions—one on each hemisphere, where the magnetic reconnection occur. We argue that the reduction of the viscous potential is due to a reduction of the velocity shear across the magnetopause and the lack of sunward convection in the equatorial tail.

[1] Ring current formation is mainly attributed to enhanced global magnetospheric convection and particle injection. One of the indicators of enhanced global magnetospheric convection is the transpolar potential. The transpolar potential has been shown to respond to dawn-dusk oriented interplanetary magnetic field (IMF), enhancing as the IMF magnitude grows. This suggests that the ring current should respond to dawn-dusk oriented IMF. This work examines the ring current response during periods of dawn-dusk oriented IMF using the Lyon-Fedder-Mobarry and Comprehensive Ring Current Model simulations. Exploring three hypotheses, this work shows through simulation results that the ring current does respond during periods of dawn-dusk oriented IMF, that the inner magnetospheric response is different for periods of dawn-dusk oriented IMF than for periods of southward IMF, and that these differences are attributed to both lower magnetospheric convection on closed field lines and the location of the reconnection region on the nightside. Specifically, the simulation results show that as the magnitude of the dawn-dusk oriented IMF increases, producing a corresponding increase in the transpolar potential, the ring current response increases. The response is always much less than a comparable southward IMF would produce. This lower response is due to both magnetospheric convection on closed field lines, which builds the ring current but at a slower rate, and flank reconnection, which allows energy to flow through the inner magnetosphere without building the ring current plasma population (as a conduit).

In this paper we examine the role of dayside merging between the interplanetary magnetic field (IMF) and the geomagnetic field in the generation of the polar cap potential in the ionosphere during the Whole Heliospheric Interval using the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) and Lyon–Fedder–Mobarry (LFM) global simulations of the geospace system from the Center for Integrated Space Weather Modeling (CISM). We isolate the portion of the total ionospheric potential due to the viscous interaction by simulating the interval with a zero IMF, but with the same solar wind plasma conditions. For southward IMF, the cross polar cap potential is the sum of the merging potential and the viscous potential, so we can determine the merging potential by subtracting the viscous potential from the total potential. From the dependence of the merging potential on southward IMF we calculate a geoeffective length of 5 R E . For northward IMF the situation is more complicated since the cross polar cap potential, defined as the peak to peak potential, will be almost always either the value of the viscous potential or of the merging potential, whichever is larger. We find that during periods of northward IMF the cross polar cap potential can be less than what the viscous interaction would produce with no IMF present. This means that the viscous interaction is weakened by the cycle of merging and reconnection for northward IMF. Our results also indicate that current representations of merging rates or electric fields are flawed in the manner in which they describe northward IMF. Typical representations simply produce a weak reconnection rate when the IMF is northward that adds to the viscous potential to create a cross polar cap potential that is larger than the viscous potential, whereas the effect of merging for northward IMF reduces the viscous interaction so that the cross polar cap potential for moderate northward IMF values is smaller than the value that would be expected from solar wind plasma conditions of the viscous potential in isolation.

While it is widely known that coronal mass ejections and their related solar wind features are significant drivers of activity with geospace it is less known that corotating interaction regions (CIRs) and the high speed stream (HSS) periods that precede them are also drivers of activity within geospace. The most recent extended and weak solar minimum interval has brought renewed attention to the space weather impacts of CIR+HSS periods since the highly structured and relatively stable coronal hole features on the Sun resulted in numerous CIR+HSS periods. In this paper we examine two Carrington Rotations (CRs) using the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) model. CR2060 lasted from August 14, 2007 to September 11, 2007 and contained three CIR+HSS periods. CR2068, also known as the Whole Heliosphere Interval (WHI), began on March 20, 2008 and lasted until April 16, 2008 and contained two CIR+HSS periods. For each CR simulations driven by both L1 solar wind observations from the OMNI data set and L1 conditions extracted from CORHEL heliospheric simulations were conducted. The heliospheric simulation results capture the velocity and density structures seen in the solar wind well for CR2060 and only get one of the CIR+HSS periods in CR2068. In each CR the heliospheric simulations produce a much weaker IMF and have less temporal variability in all parameters. We compare the results of the CMIT simulations for each CR to observations of the cross polar cap potential (CPCP), hemispheric power (HP), and SYM H index including the computation of RMS and cross correlation error metrics. We examine the response of the thermospheric density during these intervals by utilizing data from the CHAMP satellite. In the magnetosphere we use magnetic field data from the GOES spacecraft to asses the different simulations ability to describe the distribution and intensity the ULF wave power. Our results show that the L1 driven simulations under-estimates the SYM H index and HP and over-estimates the CPCP. We believe that over estimation of the CPCP is directly linked to the low HP highlighting the need for an improved precipitation model within CMIT. The ULF power in the L1 simulations compares well with the observations, especially for the compressional component important in radiation belt energization processes. In all cases, the CMIT simulations driven by the heliospheric simulation results produce dramatically inferior predictions highlighting the importance of having good IMF predictions in heliospheric model results and possibly indicating the importance of having fluctuations in the solar wind.

To gain an improved quantitative understanding of the solar wind/magnetosphere/ionosphere coupling processes, the capabilities of different models and observations to represent the variations of ionospheric forcing associated with high-speed solar wind streams are compared for year 2005. We investigate the high-latitude forcing from the measurements of DMSP satellite, empirical model Weimer05 and Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure, with ground magnetometer data only (AMIE 1) or with both ground magnetometer and DMSP convection data (AMIE 2). The yearly averages of the northern hemisphere integrated Joule heating (IJH) calculated from AMIE 1 and 2 are about 41% and 85% larger than that from Weimer05, respectively. A consistent 9-day periodicity is shown in both the cross polar cap potential and IJH, which corresponds to the 9-day oscillation in the high-speed solar wind streams. A band-pass filter centered at 9 day is used to examine the sensitivity of the IJH calculated from different sources to the solar wind condition. The comparison shows that the sensitivity of IJH from AMIE 1 and 2 are about 2.2 and 2.5 times larger than that from Weimer05, respectively.

In this paper we examine the role of the bow shock in coupling solar wind energy to the magnetosphere using global magnetohydrodynamic simulations of the solar wind-magnetosphere interaction with southward IMF. During typical solar wind conditions, there are two significant dynamo currents in the magnetospheric system, one in the high-latitude mantle region tailward of the cusp and the other in the bow shock. As the magnitude of the (southward) IMF increases and the solar wind becomes a low Mach number flow, there is a significant change in solar wind-magnetosphere coupling. The high-latitude magnetopause dynamo becomes insignificant compared to the bow shock and a large load appears right outside the magnetopause. This leaves the bow shock current as the only substantial dynamo current in the system, and the only place where a significant amount of mechanical energy is extracted from the solar wind. That energy appears primarily as electromagnetic energy, and the Poynting flux generated at the bow shock feeds energy back into the plasma, reaccelerating it to solar wind speeds. Some small fraction of that Poynting flux is directed into the magnetosphere, supplying the energy needed for magnetospheric dynamics. Thus during periods when the solar wind flow has a low Mach number, the main dynamo in the solar wind-magnetosphere system is the bow shock.

During periods of very strong southward magnetic field in the solar wind the polar cap potential becomes saturated. Recent investigations of the saturation of the polar cap potential suggest that the Region 1 current system plays a major role in balancing the solar wind pressure, leading to the saturation effect. To do this, the Region 1 current must flow on open field lines, closing outside Earth magnetosphere, where it can exert a J × B force on the solar wind. In this paper we present observations from DMSP F13 of the low-altitude distribution of field-aligned currents with Region 1 polarity relative to the boundary between open and closed field lines. The observations show that during a period of strongly southward solar wind magnetic field, a substantial amount of field-aligned current was flowing on field lines that had merged with the interplanetary magnetic field. On the other hand, during a period of nominal southward solar wind magnetic field, much less field-aligned current with Region 1 polarity was found in the open field line region. These observations support the view that Region 1 field-aligned currents can play a role in the force balance between the magnetosphere and the solar wind during periods of strongly southward IMF.

A signature feature of magnetic storms is the development of a ring current. Recent ideas on ring current formation suggest that during the main phase of storms the ring current is primarily asymmetric, becoming symmetric only during the recovery phase. In this paper, we examine ring current formation during a magnetic storm using global magnetohydrodynamic simulations of the interaction between the solar wind and the magnetosphere. The magnetohydrodynamic simulation uses solar wind data as a boundary condition, allowing the modeling of actual events. The event modeled here is the January 1997 storm. Although the code does not develop a true ring current, the simulated electromagnetic potentials predict the trajectory of drifting energetic particles in a non-self-consistent manner. Comparing drift paths of ring current-energy particles during the main phase and the recovery phase, we find that the MHD simulation predicts an asymmetric main phase ring current that evolves into a symmetric ring current during the recovery phase.

In this paper we will examine ionospheric Joule heating during magnetic storms using global MHD simulations of the interaction of the solar wind with the magnetosphere. The MHD simulations use solar wind data as a boundary condition, so that we are able to model actual events. Among the events that we have modeled are the November 1998 and January 1997 storms. We will begin by comparing the MHD dissipation rates with those estimated from AE in order to benchmark the simulation. We will then discuss the relationship between the energy dissipated in the ionosphere through Joule heating and the input from the solar wind. We will also investigate the time scale for energy dissipation in order to specify the direct-driving time scales for these events.

Exploration of plasma–plasma interactions at the high-latitude magnetopause versus a simulated sheared current sheet with strong fluctuations of realistic spectral shape, revealed a new type of dynamic equilibrium, in which nonlinear disturbances serve as an effective obstacle for 80% of the incident magnetosheath ions, providing also the exchange by ∼10% of plasma particles with the stagnant high-beta boundary layer in the minimum field region over the polar cusps. The measured waves, reflected upstream by the boundary, interact in the 3-wave manner with the magnetosonic (MS) fluctuations of the incident flow, resulting in their amplification and then decay into accelerated MS-jets and Alfven waves, driving decelerated flows at the Alfven speed. This impulsive momentum loss via the MS-jets contributes in the average flow bend around the magnetosphere. The leading jet appearance is suggested to be phase-synchronized with both the initial MS fluctuations and nonlinear cascades upstream at the magnetopause, which constitutes the wavy obstacle with multiple decays into the smaller MS-jets and Alfvenic flows. High dynamic pressure in the MS-jets does not fit their acceleration by a reconnection; instead the jets are able to initiate the driven reconnection in the process of interaction with a downstream magnetopause. The acceleration of the MS-jets is consistent with a Fermi-type mechanism, in which electric wave-trains play the role of a moving non-continuous ‘wall’. Estimations of the jet scales from the approach of a nonlinear Cherenkov resonance conforms 2–3 reflections of the jet from the ‘wall’ before overcoming the ‘wall’ potential barrier. We demonstrate quantitative agreement of the acceleration of the leading MS-jet in the process of inertial ion drift in variable electric fields. Current sheets, generated due to opposite sign of the ion and electron inertial drift, can account for the intermittency of the TBL fluctuations.

The Lyon–Fedder–Mobarry global magnetospheric model couples a magnetohydrodynamic simulation of the solar wind magnetosphere interaction to a 2D simulation of the ionosphere. Event studies driven by solar wind observations made by WIND and ACE have proved the utility and accuracy of the model. Recently we have begun conducting a series of Gedanken experiments using the LFM as our testbed. By creating idealized solar wind conditions we are able to explore a portion of solar wind parameter space and determine the response of the coupled magnetosphere–ionosphere system. This paper will summarize results from two of these thought experiments. In the first experiment we used an abrupt transition from northward to southward interplanetary magnetic field to examine the development of polar cap convection in ionosphere and how the dayside magnetopause erodes under south-ward IMF. Next we use solar wind conditions which contained ultra low frequency (ULF) waves in the plasma density and show that it is possible to drive ULF waves in the magnetosphere with this direct driving from the solar wind. Using these computer assisted thought experiments we are able to move beyond the typical cartoon picture of the magnetosphere and probe its fundamental physics.

In this work, we examine convection during very strong driving during magnetic storms using global MHD simulations of the interaction of the solar wind with the terrestrial magnetosphere. Specifically, we examine the variation of the flux tube volume along the midnight meridian using the simulation of the January 10, 1997, magnetic storm. We find that during substorm-like periods in the simulation, the flux tube volume varies significantly, rising during the growth phase and shrinking during the expansion phase. During main phase of the storm, the flux tube volume grows and remains at elevated values. We suggest that this is an important feature of convection during storms.

In this paper, we will examine the variation of the polar cap potential during magnetic storms using global magnetohydrodynamic (MHD) simulations of the interaction of the solar wind with the magnetosphere. We present the time evolution of the polar cap potential and correlate changes in the potential with changes in the solar wind input during three events. We show that the simulation potential behaves very similar to predictions based on the solar wind electric field, but that there is also a noticeable dependence on solar wind density, with higher values of polar cap potential for larger values of solar wind density.

In this paper, we report the results of a study of eight participants, all physics majors with some exposure to space physics, who were provided with 2-D and 3-D depictions of the substorm current wedge. Student understanding of the substorm current wedge was probed by asking them to determine the magnetic perturbations produced by the current wedge system. None of the students were able to determine the complete correct set of north–south and east–west perturbations from the 2-D drawing, but all of them were able to do so from the 3-D rendering. Moreover, three students had interpreted the 2-D drawing incorrectly and were able to correct the misconception on seeing the 3-D rendering. Our analysis strongly suggests that the cognitive processing of mental images is a key factor in the students' ability to correctly determine the magnetic perturbation due to the current wedge, and that determining the direction of a magnetic field from an arbitrary current is not an impediment. We also discuss the broader implications of this result for teaching about magnetism and the relationship to electric current.

We examine the simulation of the January 10, 1997, magnetic storm using the Lyon-Fedder-Mobarry three-dimensional magnetohydrodynamic simulation of the interaction of the solar wind with the magnetosphere. We explore the response of the energy coupling between the solar wind and the magnetosphere during the extreme solar wind conditions produced by magnetic cloud that hit the Earth's magnetosphere during the first half of January 10. During the early part of the event, the simulation produced substorm-like behavior marked by a loading and unloading of stored magnetic flux. However, during the period of strong, sustained solar wind energy input, the loading-unloading behavior essentially goes away and the simulation entered a directly driven phase. During this direct-driving phase the solar wind dynamic pressure controlled the ionospheric Joule heating in the simulation.

Global MHD models hold the promise of providing a physics-based understanding of magnetospheric structure and dynamics. As such models have become more sophisticated, and computing power has increased, it is now possible to model actual events using solar wind data as a boundary condition. In this paper we present some results MHD simulations of actual storm and substorm events. We will demonstrate that not only can the simulations reproduce details of events, they also are reproducing fundamental aspects of energy coupling between the solar wind and the magnetosphere in such a manner that we can distinguish storms and substorms.

In this paper, we present results derived from global magnetohydrodynamic simulations of the response of the magnetosphere to magnetic clouds. The simulations were driven with solar wind data from 10 January 1997 and 4 May 1998. Both periods were major magnetic storms. We find that during moderate periods of magnetic merging between the interplanetary magnetic field (IMF) and the magnetospheric magnetic field, the simulated magnetosphere responds with a loading–unloading behavior characteristic of substorms. However, during periods of strong, sustained, southward IMF, the simulation evolves to a directly driven state, without the loading–unloading component. This behavior is the same as that of the real magnetosphere (Bargatze, L.F., Baker, D.N., McPherron, R.L., Hones, E.W., Jr., 1985. Magnetospheric response for many levels of geomagnetic activity. J. Geophys. Res., 90, 6387–6394), thus providing compelling evidence that the simulation is accurately representing the fundamental features of solar wind–magnetosphere coupling.

The onset and evolution of substorms is viewed by many researchers to be a central question in magnetospheric physics. The most popular model for substorms is the Near-Earth Neutral Line model. This model asserts that the formation of a new, near-Earth reconnection region is the central feature of the onset of the expansion phase and that all other phenomena flow from this event. In recent years the Current Sheet Disruption model has been proposed as an alternative. This model asserts that the initiation of the substorm expansion phase is triggered by the disruption of the thin near-Earth current sheet that forms at and just beyond geosynchronous orbit in the late growth phase, and that the formation of a reconnection region proceeds from this event. In this paper we will review the history of and evidence for the Current Sheet Disruption model, especially with regard to recent evidence in favor of the NENL model. We will also review recent global MHD simulation results that provide an interesting perspective on the relationship between these two models.

Spacecraft and ground data combined with multiscale computer models developed by the ISTP program are providing a new and coherent understanding of the magnetospheric substorms and storms. Global MHD simulations that include ionospheric response are dynamically driven by upstream satellite data and allow for direct comparison with the field and flow quantities measured by magnetospheric satellites, ground data and images from the POLAR satellite. Through the combined analysis of the simulations and observations, the first unified picture of a substorm from the magnetospheric and ionospheric viewpoint is currently emerging. Here we use MHD simulations of two particularly well observed and analyzed events to explore the factors that trigger and organize the substorm elements into a coherent entity. The first event — March 9, 1995 — produces clear evidence that impulsive penetration of a large electric field in the vicinity of -8 to -10 RE, possibly associated with magnetosonic energy focusing, acts as a trigger for substorm initiation. It is the element that connects the ionospheric to magnetospheric substorm. Particularly impressive is the timing of the chain of events and indices observed on the ground and their proxies computed in the simulation. This simulation is complemented by a “theoretician's” simulation, a step function transition of the IMF from northward to southward, which clarifies the physics of energy penetration into the magnetosphere and demonstrates Poynting flux focusing in the near earth tail. The second event — January 10–11, 1997 — was driven by the impact of a magnetic cloud in the magnetosphere. It induced major disturbances in the magnetosphere and the groundand resulted in the loss of a geosynchronous ATT satellite. It is a simulation “tour de force” and used continuous upstream data over 36 hours as input. The results provide a graphical and fascinating view of the global magnetospheric and tail response to a magnetic cloud impinging upstream, illustrate the importance of dynamics and indicate that pressure impulses play a key role in providing the coherence required for substorms.

A global MHD simulation of an isolated substorm that occurred on March 9, 1995 is presented. The simulation, driven with solar wind data provided by the Wind satellite, reproduced to a surprising degree the evolution of substorm activity. The onset of the expansion phase was coincident with the penetration of an electric field spike into the near- Earth region. This impulse launched a tailward propagating signal (rarefaction wave) that enhanced reconnection in the mid tail. Substorm intensification was correlated with th'e enhancement of the reconnection rate at the preexisting reconnection region located at 30 Re. The importance of the electric field spike in correlating ionospheric and magnetospheric aspects of the substorm is emphasized.

The results of a 3D MHD simulation of the January 10-11, 1997 geomagnetic storm are presented. The simulation results agree well with ground-based and geosynchronous observations. The 28 hours modeled by the simulation include the magnetic cloud responsible for the storm, the shock preceding the cloud, and the dense plasma filament following it. The simulation shows that during the period of southward (MF ionospheric activity was strongly correlated to the solar wind density. The arrival of the plasma filament during northward IMF pushed the dayside magnetopause well within geosynchronous orbit, but generated little ionospheric activity. It appears that n,w as well as the orientation of Bsw plays a role in controlling the intensity of ionospheric and magnetospheric activity.

The global conditions during a moderate geomagnetic disturbance event on May 15, 1996, are examined by comparing data from several ground-based instruments and inner tail satellites with global MHD simulations of the same event. The ground-based data show two substorm intensifications about 40 min apart, the first one being small and localized (a pseudobreakup) and the second leading to a major rearrangement of both the ionospheric auroral distribution and the magnetotail configuration. The simulation shows that during the pseudobreakup, open field lines were reconnecting in the midtail, but the flows were mainly tailward and very few effects were observable in the inner magnetosphere. The result that pseudobreakups can be associated with activity producing topological changes in the tail is an important new aspect that has not been discussed in earlier studies. Both the observations and the simulation show two distinct regions of activity: a thin current sheet in the inner tail magnetically connected with the auroral bulge and a reconnection region in the midtail associated with the most intense electrojet currents.

In this report we present the results of a study using data from AMPTE/CCE, ground stations, and DMSP F7 during substorms. We have examined 12 events during which CCE was located near the neutral sheet and observed the disruption of the cross-tail current. During these events there were simultaneous ground magnetic field data near to the CCE local time sector. DMSP F7 crossed the nightside auroral oval within 40 minutes of substorm onset at CCE during five of the events. We find that the data are consisent with a near-Earth substorm initiation. We also find that the Tsyganenko (1987) magnetic field model requires an additional tail-like stress during substorms, which corresponds to an equatorward displacement of auroral zone field lines. Assuming that the center of the electrojet is drawn from the near-Earth region, the magnitude of this displacement is about 1 deg to 2 deg. Magnetosphere, Magnetotail, Current.

In this study we examine a period of substorm activity that occurred on March 9, 1995. Using solar wind data as input, the event has been simulated using a 3-D MHD code. To determine how well the simulation fared, we have compared the simulation results to data. This comparison has taken two forms. The first is a comparison to individual spacecraft in the magnetotail. The second is a comparison of global energy storage and release, where we have compared the auroral heating to data-based estimates of auroral energy dissipation, as well as evaluating the variation of the open flux in the simulation. There is a generally good level of agreement between the simulation results and data. Thus our results show that MHD simulations can be used to model at least one magnetospheric substorm.

[1] In this paper we examine the response of the ionospheric cross-polar cap potential to steady, purely northward interplanetary magnetic field (IMF) using the Lyon-Fedder-Mobarry global magnetohydrodynamic simulation of the Earth's magnetosphere. The simulation produces the typical, high-latitude “reversed cell” convection that is associated with northward IMF, along with a two cell convection pattern at lower latitude that we interpret as being driven by the viscous interaction. The behavior of the potential can be divided into two basic regions: the viscous dominated region and the reconnection dominated region. The viscous dominated region is characterized by decreasing viscous potential with increasing northward IMF. The reconnection dominated region may be further subdivided into a linear region, where reconnection potential increases with increasing magnitude of northward IMF, and the saturation region, where the value of the reconnection potential is relatively insensitive to the magnitude of the northward IMF. The saturation of the cross-polar cap potential for northward IMF has recently been documented using observations and is here established as a feature of a global MHD simulation as well. The region at which the response of the potential transitions from the linear region to the saturation region is also the region in parameter space at which the magnetosheath transitions from being dominated by the plasma pressure to being dominated by the magnetic energy density. This result is supportive of the recent magnetosheath force balance model for the modulation of the reconnection potential. Within that framework, and including our current understanding of the viscous potential, we present a conceptual model for understanding the full variation of the polar cap potential for northward IMF, including the simulated dependencies of the potential on solar wind speed and ionospheric conductivity.

[1] The viscous interaction between the solar wind and Earth's magnetosphere is extremely difficult to study through direct observations. The viscous contribution to the polar cap potential, the viscous potential, is typically swamped by the much larger reconnection potential or obscured by rapidly changing solar wind conditions. We used the Lyon-Fedder-Mobarry (LFM) magnetohydrodynamic simulation to study the response of the viscous potential to a variety of ideal conditions both in the solar wind and the ionosphere. We found that the viscous potential in LFM increases with either increasing solar wind density or velocity, with a relation that is similar to some previous empirical results in form but different in detail. The density dependence scales as n0.439 (in cm−3) and velocity scales as Vx1.33 (in km s−1). Combining these results with a reference value, the viscous potential in LFM can be predicted using the formula ΦV = (0.00431)n0.439Vx1.33 kV. We also found that the viscous potential changes inversely in relation to constant Pedersen conductivity in an idealized ionosphere, a result that was previously predicted for LFM but not explored until now.

Magnetospheric substorms represent the episodic dissipation of energy stored in the geomagnetic tail that was previously extracted from the solar wind. This energy release produces activity throughout the entire magnetosphere-ionosphere system, and it results in a wide variety of phenomena such as auroral intensifications and the generation of new current systems. All of these phenomena involve the acceleration of particles, sometimes up to several MeV. In this paper we present a brief overview of substorm phenomenology. We then review some of the evidence for particle acceleration in Earth’s magnetosphere during substorms. Such in situ observations in this most accessible of all cosmic plasma domains may hold important clues to understanding acceleration processes in more distant astrophysical systems.Subject headings: acceleration of particles — Earth — solar wind

This paper reports the multisatellite and ground observations of two pseudo-substorm onset events that occurred successively at 0747 UT and 0811 UT, May 30, 1985, with more attention to the 0747 UT onset. The distinguishing features of the 0747 UT event are as follows. (1) The substorm-associated tail reconfiguration started in a very localized region in the near-Earth magnetotail. (2) The magnitude of the current disruption decreased markedly as the disruption region expanded tailward. (3) On the ground the onset of a very small negative bay (approx. 40 nT) was observed simultaneously with the onset of the current disruption, but over a much wider local time sector than the near-Earth tail reconfiguration. Positive bay onsets at mid-latitudes also had a longitudinally wide distribution. From these features we infer than in the present event the current disruption took place filamentarily near AMPTE/CCE at approx. 8.8 R(sub E). It is also inferred that pseudo-substorm onsets are distinguished from standard substorm onsets by the absence of a global expansion of the current disruption, and that the spatial scales of the onset region in the magnetosphere is not a major difference between the two. The present study suggests that the spatial distribution of the magnetic distortion before onsets is an important factor to determine the expansion scale of the current disruption. It is also suggested that the current disruption is basically an internal process of the magnetosphere.

We present an analytical approximation of the behavior of the velocity dispersion structure of the energetic particle bursts generated during magnetic field topology reconstruction within the Earth's magnetotail. We show that some critical distance S exists, such that if the distance S between the source and the spacecraft S >Sc, the burst exhibits only normal velocity dispersion (more energetic particles appear before low energy particles). For S < Sc, both inverse (for higher energies) and normal (for lower energies) parts coexist in the structure. These calculations are compared with the observational data.

Observations made during the growth phase and the onset of a substorm on August 31, 1986 are presented. About 20 min after the epsilon parameter at the magnetopause had exceeded 10 exp 11 W, magnetic field dipolarization with an increase of energetic particle fluxes was observed by the AMPTE Charge Composition Explorer (CCE) spacecraft at the geocentric distance of 8.7 R(E) close to magnetic midnight. The event exhibited local signatures of a substorm onset at AMPTE CCE and a weak wedgelike current system in the midnight sector ionosphere, but did not lead to a full-scale substorm expansion; neither did it produce large particle injections at GEO. Only after another 20 min of continued growth phase could the entire magnetosphere-ionosphere system allow the onset of a regular substorm expansion. The initial activation is interpreted as a 'pseudobreakup'. We examine the physical conditions in the near-Earth plasma sheet and analyze the development in the ionosphere using ground-based magnetometers and electric field observations from the STARE radar.

An intense geomagnetic substorm event on May 3, 1986, occurring toward the end of a strong storm period, is studied. The auroral electrojet indices and global imaging data from both the Northern and Southern Hemispheres clearly revealed the growth phase and expansion phase development for a substorm with an onset at 0111 UT. An ideally located constellation of four spacecraft allowed detailed observation of the substorm growth phase in the near-tail region. A realistic time-evolving magnetic field model provided a global representation of the field configuration throughout the growth and early expansion phase of the substorm. Evidence of a narrowly localized substorm onset region in the near-earth tail is found. This region spread rapidly eastward and poleward after the 0111 UT onset. The results are consistent with a model of late growth phase formation of a magnetic neutral line. This reconnection region caused plasma sheet current diversion before the substorm onset and eventually led to cross-tail current disruption at the time of the substorm onset.

The relationship between the geosynchronous magnetic field variations during substorms measured by GOES 5 and the auroral electroject as measured by AE and Poste de la Baleine is examined. It is found that the more taillike the field prior to the local onset, the greater the dipolarization of the field during the substorm. The greater the deviation of the field from a dipolar configuration, the larger the change in AE during the event. It is inferred that stronger cross-tail currents prior to the substorm are associated with larger substorm-associated westward electrojets and thus more intense substorms. Since the westward electroject is the ionospheric leg of the substorm current wedge, it is inferred that the substorm-associated westward electrojet is drawn from the near-earth region. Most of the current diversion is found to occur in the near-earth magnetotail.

A one-dimensional, hybrid simulation code is used to study current sheets with a nonzero normal magnetic field B(sub z) and a dawn-to-dusk electric field E(sub y). Such configurations are dependent upon only two parameters: we use the normalized normal magnetic field B-normalized (sub z) = B(sub z)/(4(pi)(n(sub b)) (v(exp 2 sub T))(exp 1/2) and normalized electric field V-normalized (sub D) = (1/V(sub T)(cE(sub y)/B(sub z)), where V(sub T) is the thermal velocity of ions prior to their interaction with the current sheet and n(sub b) is the number density outside the current sheet (at the simulation boundary). A third parameter that is relevant to the motion of particles in current sheets is kappa(sub A), the value of kappa = (R(sub min)/rho(sub max))(exp 1/2) for particles of average energy. We find that if either B-normalized (sub z) is close to or greater than 1, or if kappa(sub A) is close to 1, a rotational mode develops in which the z = 0 current rotates with the ion sense about the normal magnetic field, while for small values of both B-normalized (sub z) or kappa(sub A), the configuration is quasi-steady. To achieve values of kappa(sub A) of the order of or larger than 1, we decrease the value of V-normalized (sub D) uniformly. We find that the magnetic field fluctuations and particle distribution functions are similar in many respects to what was observed in the day 240, 1986, Active Magnetospheric Particle Tracer Explorer (AMPTE)/CCE current disruption event, an event that appears to be located at the site of initiation of current disruption and related particle energization.

In this paper we present evidence from the low-altitude DMSP F7 satellite that the poleward edge of auroral luminosity in the nightside auroral zone does not necessarily correspond to the boundary between plasma-filled flux tubes and flux tubes devoid of plasma. Assuming that that low-altitude boundary corresponds to the boundary between the lobe and the plasma sheet, this implies that the boundary between open and closed field lines may lie poleward of the most poleward auroral luminosity. Thus the assumption that the poleward boundary of auroral luminosity is a good indicator of the open-closed boundary may not always be correct. Furthermore, we show clear evidence that an auroral surge may also be located equatorward of the open-closed boundary. Therefore, tailward of the region of the plasma sheet to which the surge is connected there may exist undisturbed plasma sheet that has not yet been disconnected from the ionosphere. This means that substorm-associated reconnection does not necessarily begin to reconnect lobe field lines at the onset of a substorm. Moreover, available evidence strongly suggests that the arc that brightens at the onset of a substorm and that develops into a surge maps to the inner magnetotail, to that region at the inner edge of the plasma sheet where the magnetic field changes from a dipolar to a taillike configuration. This would be consistent with recent studies that connect auroral breakup to the near-Earth (

[1] For low values of the solar wind electric field, the response of the polar cap potential is essentially linear, but at high values of VBs, the polar cap potential saturates and does not increase further with increasing VBs. On the other hand, the ring current injection rate does increase linearly with VBs and shows no evidence of saturating. If enhanced convection is the origin of the ring current, this poses a paradox. How can the polar cap potential, and thus convection, saturate when the ring current does not? We examine a possible explanation based on the reexamination of the Burton equation by Vasyliunas (2006). We show that this explanation is not a viable solution to the paradox since it would require a changing polar cap flux, and we demonstrate that the polar cap flux saturates (at around 1 GWb) as the polar cap potential saturates. Instead, we argue that during storms a quasi-steady reconnection region forms in the tail near the Earth. This reconnection region moves closer to the Earth for higher values of solar wind Bs, although the polar cap potential, the dayside merging and nightside reconnection rates, and the amount of open flux do not change much as a function of Bs once the polar cap potential has become saturated. As the neutral line moves closer, the volume per unit magnetic flux in the closed field line region is less. Flux tubes leaving the reconnection region in general have lower PVγ as Bs increases, and lower PVγ flux tubes can penetrate deeper into the inner magnetosphere, leading to a corresponding greater injection of particles into the inner magnetosphere. Thus a reconnection region that is closer to Earth is more effective in creating a strong ring current. This leads to a continued dependence of the ring current injection rate on VBs, although the polar cap potential has saturated.

The CDAW-9 Event C focused upon the early part of 3 May 1986 when a large substorm onset occurred at 0111 UT. By modifying the Tsyganenko 1989 magnetic field model, a model is constructed in which the near-earth current systems are enhanced with time to describe the observed development of the tail magnetic field during the growth phase. The cross-tail current intensity and the thickness of the current sheet are determined by comparison with three spacecraft in the near-earth tail. The location of the auroral bulge as recorded by the Viking imager is mapped to the equatorial current sheet. The degree of chaotization of the thermal electrons is estimated, and the consequences to the tail stability towards on tearing are discussed. It is concluded that the mapping of the brightening region in the auroral oval corresponds to the regions in the tail where the current sheet may be unstable towards ion tearing.

This study involves a multi-satellite investigation of magnetic field perturbations in the magnetosphere due to solar wind dynamic pressure variations. The case study uses data acquired on 3 August 1986 from IMP-8, GOES-5, GOES-6, VIKING, and ground magnetograms. As expected, dramatic solar wind dynamic pressure variations, recorded at IMP-8, produced compressions and rarefactions of the magnetosphere as seen in GOES-5 and GOES-6 magnetic field observations at geostationary orbit. These same compressions and rarefactions were also recorded by the VIKING magnetic field experiment over the polar cap at mid-altitudes. The magnetic field perturbations are interpreted in terms of fast mode waves which were generated near the magnetopause and propagate anti-sunward into the inner magnetosphere. In addition, we compare the amplitude of the magnetic field perturbations recorded at geostationary orbit and VIKING. The wave amplitudes recorded by GOES and VIKING were nearly identical, suggesting a low damping rate of the waves in the magnetosphere.

The position of the near-earth neutral sheet throughout the nightside region between R = 5 R(E) and R = 8.8 R(E) is examined statistically using magnetic field data from AMPTE/CCE. As has been demonstrated previously, the latitudinal position of the neutral sheet is primarily controlled by the dipole tilt angle. The position also depends on local time, radial distance, and magnetic activity. These dependencis may be represented by the empirical formula MLAT = - (0.14 Kp + 0.69) /cos(Phi)/exp 1/3 (0.065 R exp 0.8 - 0.16) DTA, where MLAT is the position in degrees of the neutral sheet relative to the magnetic equator, Kp is the 3-hourly magnetic activity index, Phi is the magnetic local time in degrees (midnight = 0 deg), R is the radius in R(E), and DTA is the dipole tilt angle in degrees.

Simultaneous magnetic field and particle measurements by AMPTE/CCE with magnetic field measurements by GOES 5 and GOES 6 clarify the structure of the field-aligned Birkeland current system in the post-midnight region of the near-Earth magnetotail. The current system, consisting of current sheets flowing in opposite directions, exists near the boundary region of the plasma sheet; its magnetic effects are confined mainly to the current region because of its structure. During the expansion phase of substorms, a single-sheet current system is formed in the boundary region of the plasma sheet and net current is probably present in one meridian plane. This type of current system produces the substorm-related transient variations in the azimuthal (D) component normally observed at synchronous orbit.

It is generally thought that the local effects associated with substorm onset in the near-earth magnetotail (R less than about 10 earth radii) have a component of propagation which is radially earthward. This study presents a case during which it is possible that the local effects of substorm onset propagated radially antisunward, examining energetic-ion and magnetic-field data from AMPTE/CCE and energetic-particle data from the GEO satellite 1982-019 during a period of weak substorm activity on June 11, 1985. Although the effects of this event on the ground were very weak, the substorm had a dramatic effect on at least one portion of the near-earth magnetotail. During the period the spacecraft were in an unusually close radial alignment. The data show that phenomena associated with the local effects of substorm onset began at or near GEO before they were observed at CCE. This event is best explained by a near-earth disruption of the cross-tail current sheet and the consequent formation of a substorm current wedge, with a component of expansion antisunward down the tail.

The data base from the 1985-86 Balloon Campaign at South Pole Station, Antarctica, has been searched for impulsive magnetic and electric field events that occurred when IMP 8 was in the solar wind and data were acquired. Two events are shown that appear to be similar to published examples of high latitude twin vortex current systems. Neither event was preceded by a dynamic pressure pulse in the solar wind. These examples disprove the hypothesis that all such high-latitude impulses arise solely from dynamic pressure pulses produced by the solar wind. Further, on a statistical basis, a total of 42 events were found for which IMP 8 data are available. Of these events, 12 occurred when the Z{sub GSM} component (B{sub Z}) of the interplanetary magnetic field (IMF) was northward and 30 occurred when B{sub Z} was southward. Only 3 of the B{sub Z} north cases and only 5 of the B{sub Z} south cases were preceded by pressure pulses greater than 0.4 nPa in amplitude.

[1] In late October and early November of 2003, the Sun unleashed a powerful series of events known as the Halloween storms. The coronal mass ejections launched by the Sun produced several severe compressions of the magnetosphere that moved the magnetopause inside of geosynchronous orbit. Such events are of interest to satellite operators, and the ability to predict magnetopause crossings along a given orbit is an important space weather capability. In this paper we compare geosynchronous observations of magnetopause crossings during the Halloween storms to crossings determined from the Lyon-Fedder-Mobarry global magnetohydrodynamic simulation of the magnetosphere as well to predictions of several empirical models of the magnetopause position. We calculate basic statistical information about the predictions as well as several standard skill scores. We find that the current Lyon-Fedder-Mobarry simulation of the storm provides a slightly better prediction of the magnetopause position than the empirical models we examined for the extreme conditions present in this study. While this is not surprising, given that conditions during the Halloween storms were well outside the parameter space of the empirical models, it does point out the need for physics-based models that can predict the effects of the most extreme events that are of significant interest to users of space weather forecasts.

[1] It is generally believed that the coupling of energy between the solar wind and the magnetosphere depends almost exclusively on the solar wind speed and magnetic field, with density and temperature playing little or no role. However, recent studies have indicated that under certain conditions, such as the main phase of a storm, density can have a significant role in modulating the transfer of energy to the magnetosphere. In this paper we demonstrate the effect using global MHD simulations of the solar wind-magnetosphere interaction. We also identify the physical mechanism that leads to the density control, namely the modification of the compression ratio of the bow shock, and explain why it is only apparent during periods of strong, southward IMF.

[1] In this paper we use a global magnetohydrodynamic simulation of the magnetosphere to examine the behavior of the magnetopause position when the interplanetary magnetic field suddenly changes from northward to southward. The inward motion of the magnetopause under the influence of a southward IMF is generally referred to as magnetopause “erosion.” Physical models to explain erosion have been proposed, notably the “onion” peeling model, a model based on the effects of fringe fields from the Region 1 Birkeland currents and the nightside cross-tail current. The simulation shows behavior consistent with aspects of these models, but it also shows behavior that is most consistent with the major driver of erosion being the growth of the nightside cross-tail current. In particular, the simulation exhibits a marked delay between the arrival of the southward IMF at the magnetopause and the inward motion of the magnetopause. We attribute this delay to the delay in the growth of the nightside current system in response to the southward turning of the IMF.

[1] A numerical study of magnetosheath ion motion in the magnetopause current sheet in the presence of magnetic fluctuations is reported. Test particles are injected at the magnetosheath side only. The magnetic field turbulence is modeled as a power law spectrum in phase space, which reaches maximum intensity in the center of the magnetopause current sheet and decreases toward the magnetosheath and magnetosphere boundaries, creating magnetic islands. The number of particles entering the magnetosphere, reflected from the magnetopause, and flowing away from the flanks is computed, as a function of the fluctuation level of the turbulence and of the magnetic field shear parameter. All those quantities appear to be strongly dependent on the fluctuation level, with the number of particles entering the magnetopause increasing with the fluctuation level. The density profile for a localized source in the magnetosheath is not Gaussian but rather has a long tail extending into the magnetopause. The statistical analysis of the ion motion along the direction normal to the magnetopause reveals that the ions perform a Levy random walk, so that the penetrating particles could reach the magnetospheric side of the simulation box with few long jumps in between the magnetic islands. This shows that plasma transport across the magnetopause in presence of magnetic turbulence is non-Gaussian and needs to be described in terms of enhanced, superdiffusive transport.