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At Earth’s bow shock, the supersonic solar wind is slowed down and deflected around the magnetosphere. To many this is "just a bow shock", a simple and quite passive element of solar-terrestrial physics. However, it has recently been realized that the bow shock plays a significantly more important role with currents on the bow shock connecting through the magnetosheath to the magnetospheric current systems. The bow shock current cannot close locally, since the magnetic field compression in the magnetosheath cannot be maintained globally. The bow shock current is inevitably a generator current extracting mechanical energy from the supersonic solar wind, and feeding it to other processes such as acceleration of the magnetosheath flow, local particle acceleration at the bow shock and dissipation in the distant ionosphere. Here we use data from the first dayside season of the Magnetospheric Multiscale (MMS) mission to investigate the generator properties of the terrestrial bow shock. Typically, the main shock ramp shows clear generator properties, but for some of the more turbulent bow shocks, generator properties may also be observed slightly downstream the ramp. This may be due to effects from shock motions and shock nonstationaity and reformation. Moreover, sometimes a weaker load can be seen in the upstream foot region due to local particle acceleration. We also find that the generator capacity of the bow shock decreases with decreasing bow shock angle as well as with increasing upstream plasma beta and solar Mach number. A better understanding of the energy conversion properties of the terrestrial bow shock will be useful also for the understanding of other astrophysical shock currents. The currents must close somewhere and deposit energy somewhere.

The magnetopause is the boundary established by pressure balance between the solar wind flow in the magnetosheath and the magnetosphere. Generally this pressure balance is represented to be between the solar wind the dynamic pressure and the magnetic pressure of Earth's dipole field. The plasma actually in contact with the magnetosphere is the slowed, compressed, and heated solar wind downstream of the shock. The force exerted on the magnetosheath plasma is the JxB force produced by the Chapman-Ferraro current that flows on the magnetopause. Under typical solar wind conditions of relatively high magnetosonic Mach number flow (>6), this simple picture is a reasonable description of the situation. However, under conditions of low solar wind magnetosonic Mach number flow (~2) the force on the solar wind plasma is not exerted at the magnetopause and must be exerted at the bow shock by currents that connect to the Region 1 currents. In this paper we present observations from two magnetopause crossings observed by the THEMIS spacecraft to compare and contrast the force balance with the solar wind for two situations with two very different solar wind magnetosonic Mach numbers.

The data for X‐ray images studied in this paper can be downloaded from Zenodo (https://doi.org/10.5281/zenodo.1415285).

The magnetopause location is generally believed to be determined by the solar wind dynamic pressure and by the sign and value of the interplanetary magnetic field vertical ( B Z ) component. The contribution of other parameters is usually considered to be minor or negligible near the equatorial plane. Recent papers have shown a magnetopause expansion during intervals of a nearly radial IMF but our ability to predict the magnetopause location under steady or slowly changing upstream conditions remains rather weak even if the effect of radial magnetic field is considered. We present a statistical study based on more than 10,000 magnetopause crossings identified in the THEMIS data in the course of the last half of the solar cycle. The observed magnetopause locations are compared with an empirical magnetopause model of Shue et al. (1997) and the sources of differences between observations and model predictions are analyzed. This analysis reveals that the magnetopause location depends on the solar activity being more compressed during the solar maximum. Furthermore, we have found that, beside the solar wind dynamic pressure and vertical magnetic field component, the solar wind speed and ionospheric conductivity ( F 10.7 used as a proxy) are important physical quantities controlling this compression.

Using global magnetohydrodynamic(MHD) simulation, we investigate the effect of the interplanetary magnetic field (IMF) on the location of the open-closed field line boundary(OCB), in particular the duskside and dawnside OCB and their asymmetry. We first model the typical OCB-crossing events on 22 October 2001 and 24 October 2002 observed by DMSP. The MHD model presents a good estimate of OCB location under quasi-steady magnetospheric conditions. We then systemically study the location of the OCB under different IMF conditions. The model results show that the dawnside and duskside OCBs respond differently to IMF conditions when B Y is present. An empirical expression describing the relationship between the OCB latitudes and IMF conditions has been obtained. It is found that the IMF conditions play an important role in determining the dawn-dusk OCB asymmetry, which is due to the magnetic reconnection at the dayside magnetopause. The differences between the dawn and dusk OCB latitudes from MHD predictions are in good agreement with the observations.

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.

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.

In order to improve the production of physics teachers, and high school science teachers in general, at The University of Texas at Arlington, the authors obtained grant funding to offer National Science Foundation Robert Noyce Teacher Scholarships and to support a replication of the successful UTeach science and mathematics teacher preparation program. The Noyce grant was obtained first, and a modest increase in science teacher production was seen. The UTeach replication has been implemented on a four-year schedule, culminating in the establishment of a new student teaching program in January 2014. The combination of a UTeach replication and availability of Noyce Scholarships has UT Arlington poised to improve its science teacher production by an order of magnitude.

The general view is that the response of the magnetosphere-ionosphere system to a complex, variable solar wind input will be a complicated and nonlinear function of that input. We investigate this question using a global MHD code to simulate the interaction and determine the responses of the system to isolated aspects of the solar wind input. We present evidence from the simulations that the solar wind-geospace interaction can be linearly decomposed into component merging interactions for interplanetary magnetic field (IMF) By separate from Bz and a separate viscous interaction. One can run the global simulation (in our case, the Lyon-Fedder-Mobarry code) using just the solar wind plasma and Bz time series, do a second simulation using just the solar wind plasma and By time series, add the results of the two simulation outputs, then subtract the output from a simulation done with only the plasma input and no magnetic field, since the sum of the By and Bz runs has two viscous interactions (one for each run), and get an output that is very close to the result of a single run using the entire IMF merging field (By and Bz) along with the plasma time series. This demonstrates that the components of merging and viscous interactions between the solar wind and geospace are linearly separable to a very large degree.

[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).

[1] Simulations with the Global Ionosphere-Thermosphere Model (GITM) show that both Poynting flux and soft electron precipitation are important in producing neutral density enhancements near 400 km altitude in the cusp that have been observed by the Challenging Minisatellite Payload (CHAMP) satellite. Imposing a Poynting flux of 75 mW/m2 in the cusp within the model increases the neutral density by 34%. The direct heating from 100 eV, 2 mW/m2 soft electron precipitation produces only a 5% neutral density enhancement at 400 km. However, the associated enhanced ionization in the F-region from the electron precipitation leads to a neutral density enhancement of 24% through increased Joule heating. Thus, the net effect of the soft electron is close to 29%, and the combined influence of Poynting flux and soft particle precipitation causes a more than 50% increase in neutral density at 400 km, which is consistent with CHAMP observations in extreme cases. The effect of electron precipitation on the neutral density at 400 km decreases sharply with increasing characteristic energy such that 900 eV electrons have little effect on neutral density. Finally, the impact of 2 keV, 0.3 mW/m2 proton precipitation on the neutral density is negligible due to a lowering of the altitude of Joule heating.

The Whole Heliosphere Interval (WHI), comprising March 20–April 16, 2008 (DOY 80–107), is a single Carrington Rotation (2068) designated for intense study through observations and simulations. We used solar wind data from the WHI to run the Coupled Magnetosphere–Ionosphere–Thermosphere (CMIT) and stand-alone Lyon–Fedder–Mobarry (LFM) models. The LFM model was also run with the WHI solar wind plasma parameters but with zero interplanetary magnetic field (IMF). With no IMF, we expect that the cross-polar cap potential (CPCP) is due entirely to the viscous interaction. Comparing the LFM runs with and without the IMF, we found that during strong driving with southward IMF B z , the viscous potential could be a significant fraction of the total CPCP. During times of northward IMF B z , the CPCP was generally lower than the CPCP value from the IMF=0 run. LFM tends to produce high polar cap potentials, but by using the Bruntz et al. (2012) viscous potential formula ( Φ V = μn 0.439 V 1.33 , where μ =0.00431) and the IMF=0 LFM run, we calculated a scaling factor γ =1.54, which can be used to scale the LFM CPCP during the WHI down to realistic values. The Newell et al. (2008) viscous merging term can similarly be used to predict the viscous potential using the formula: Φ V = νn 1/2 V 2 , where the value ν =6.39×10 −5 was also found using the zero IMF run. Both formulas were found to perform better when V (solar wind)= V x , rather than V total , yielding similar, accurate predictions of the LFM viscous potential, with R 2 >0.91 for both formulas. The γ factor was also used to scale down the LFM CPCP from the full solar wind run, with most of the resultant values matching the CPCP from the Weimer05 model well, even though γ was derived independent of the Weimer05 model or the full LFM data. We interpret this to be an indication that the conductivity model in LFM is producing values that are too low, thus elevating the CPCP values.

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.

Space science is perhaps the science topic that elicits the greatest interest in students in the United States and also worldwide. NASA has always endeavored to leverage that widespread interest to advance STEM education, not least because NASA requires a thriving STEM workforce for the future.

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.

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.

Treating the Teukolsky perturbation equation numerically as a 2+1 PDE and smearing the singularities in the particle source term by the use of narrow Gaussian distributions, we have been able to reproduce earlier results for equatorial circular orbits that were computed using the frequency domain formalism. A time domain prescription for a more general evolution of nearly geodesic orbits under the effects of radiation reaction is presented. This approach can be useful when tackling the more realistic problem of a stellar-mass black hole moving on a generic orbit around a supermassive black hole under the influence of radiation reaction forces., 8 pages, 5 figures, problems with references and double-printing fixed

Global MHD models have become more sophisticated, and computing power has increased, so that it is now possible to model actual events using solar wind data as a boundary condition. Simulations during very strong driving, such as during a magnetic storm, suggest that quasi-steady reconnection regions form relatively close to the Earth, at about 30 Earth radii, and remain there during the period of strong driving. Such a reconnection region would be consistent with quasi-steady convection during a period of sustained, steady southward solar wind magnetic field. Using the simulation as a guide, we have investigated magnetotail magnetic field data during the main phase of magnetic storms. We find that the observations are consistent with our interpretation based on the simulation that during periods of strong, steady driving from the solar wind, a reconnection region forms in the magnetotail at about 30 Earth radii, and that it sits there, reconnecting newly merged flux that is convected into the tail from the dayside.

In this paper we examine the motion of ions in the magnetotail current sheet, and the effect on that motion of both an average normal component to the current sheet and the presence of magnetic fluctuations. We find that these two quantities have competing effects on the ion motion, with the normal component causing the ions to be ejected from the current sheet and the fluctuations (under certain circumstances) causing the ions to have an increased residence time in the current sheet. We also consider the implications for particle energization and the resulting ion distributions in the lobes.

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.

3-D MHD simulations were used to investigate the behavior of the high-latitude convection and the polar cap variations during two events characterized by sudden southward IMF turnings. In agreement with recent observations the simulation results indicate that the convection pattern across the entire polar cap begins to change a few minutes after the arrival of the southward IMF. In contrast, the onset of the equatorward motion of the open closed field-line boundary depends on the local time, with equatorward motion of the midnight boundary delayed by about 20 minutes relative to the the onset of the boundary motion at noon. We interpret this delay as the time required to convect newly merged flux from the dayside to the nightside. We belive that these two different responses can reconcile apparent contradictions in studies of ionospheric reconfigurations in response to changes in the IMF.

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.

Space physics is primarily focused on the study of plasmas and electromagnetic fields in space, principally the plasma physics processes that give rise to the solar wind and the interaction of that wind with planetary magnetospheres. And the key to understanding space physics interactions is the transfer of energy from one part of the system to another, such as the sudden transfer of energy from stressed magnetic fields to the plasma during magnetic reconnection. Over the years, I have observed that graduate and undergraduate students are conceptually unprepared to create a coherent conceptual framework for understanding space physics because they do not have a firm grasp of the meaning of energy and the role of energy in interactions, or the role of fields in energy transfer. These difficulties are rooted in their precollege education, and are not well addressed in university education. In this paper I will outline some aspects of how the concept of energy is used in space plasma physics. I will also discuss some difficulties that students have with applying the concept of energy in two specific cases. In this context, I will discuss ideas around the teaching of energy that have applicability to the problem at all academic levels.

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.

In this study we examine observations made by AMPTE/CCE of energetic ion bursts during seven substorm periods when the satellite was located near the neutral sheet, and CCE observed the disruption cross-tail current in situ. We compare ion observations to analytic calculations of particle acceleration. We find that the acceleration region size, which we assume to be essentially the current disruption region, to be on the order of 1 R(sub E). Events exhibiting weak acceleration had either relatively small acceleration regions (apparently associated with pseudobreakup activity on the ground) or relatively small changes in the local magnetic field (suggesting that the magnitude of the local current disruption region was limited). These results add additional support for the view that the particle bursts observed during turbulent current sheet disruptions are due to inductive acceleration of ions.

[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

A 9 day periodic oscillation in solar wind properties, geomagnetic activity, andupper atmosphere has been reported for the year 2005. To understand the energytransfer processes from the high‐speed solar wind streams into the upper atmosphere, weexamined Joule heating and hemispheric power (HP) from the assimilative mapping ofionospheric electrodynamics (AMIE) outputs for 2005. There are clear 9 day periodvariations in all AMIE outputs, and the 9 day periodic oscillation in the global integratedJoule heating is presented for the first time. The band‐pass filter centered at 9 dayperiod shows that both Joule heating and HP variations are correlated very well to theneutral density variation. It indicates that the energy transfer process into the upperatmosphere associated with high‐speed solar wind streams is a combination of Jouleheating and particle precipitation, while Joule heating plays a dominant role. Thesensitivities of Joule heating and HP to the solar wind speed are close to 0.40 and0.15 GW/(km/s), respectively.

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.

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.