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Space Sci Rev DOI 10 1007 s11214 008 9421 5 OpenGGCM Simulations for the THEMIS Mission Joachim Raeder Douglas Larson Wenhui Li Emil L Kepko Timothy Fuller Rowell Received 17 March 2008 Accepted 18 July 2008 Springer Science Business Media B V 2008 Abstract The THEMIS mission provides unprecedented multi point observations of the magnetosphere in conjunction with an equally unprecedented dense network of ground measurements However coverage of the magnetosphere is still sparse In order to tie together the THEMIS observations and to understand the data better we will use the Open Geospace General Circulation Model OpenGGCM a global model of the magnetosphereionosphere system OpenGGCM solves the magnetohydrodynamic MHD equations in the outer magnetosphere and couples via field aligned current FAC electric potential and electron precipitation to a ionosphere potential solver and the Coupled Thermosphere Ionosphere Model CTIM The OpenGGCM thus provides a global comprehensive view of the magnetosphere ionosphere system An OpenGGCM simulation of one of the first substorms observed by THEMIS on 23 March 2007 shows that the OpenGGCM reproduces the observed substorm signatures very well thus laying the groundwork for future use of the OpenGGCM to aid in understanding THEMIS data and ultimately contributing to a comprehensive model of the substorm process Keywords THEMIS OpenGGCM Magnetosphere MHD Simulation Substorm J Raeder D Larson W Li E L Kepko Space Science Center University of New Hampshire Durham NH USA e mail J Raeder unh edu D Larson e mail Douglas Larson unh edu W Li e mail wenhuil cisunix unh edu E L Kepko e mail Larry Kepko unh edu J Raeder Physics Department University of New Hampshire Durham NH USA T Fuller Rowell CIRES Colorado University Boulder CO USA e mail Tim Fuller Rowell noaa gov J Raeder et al 1 Introduction The substorm debate has been a central part of space physics for over four decades and centers on the question of what physical process es precipitate the sudden energy release in the magnetotail and the sudden auroral brightening and expansion Akasofu 1977 Lui 1991 Fairfield 1992 Kennel 1992 McPherron 1991 Baker et al 1999 It is probably fair to say that it is widely accepted that substorms are ultimately powered by magnetic reconnection Reconnection signatures are often observed in the tail during the course of substorms However the location of the associated x lines is typically observed 20RE from Earth or further down the tail Conversely the initial brightening of the aurora maps much closer to Earth Thus the question is commonly posed as to whether reconnection causes the process that brightens the aurora or whether the process that brightens the aurora causes reconnection The THEMIS mission Sibeck and Angelopoulos 2008 Angelopoulos 2008 is designed to answer this question by providing simultaneous measurements at five locations in order to establish how events proceed in time and space However in spite of the unprecedented coverage ambiguities will likely remain because processes such as dipolarization of the field or earthward flows may not necessarily occur strictly radially but sweep azimuthally over the spacecraft creating an apparent radial motion that does not correspond to the real one Furthermore substorms come in different sizes and shapes and at this point it is only a hypothesis that they all follow the same scheme It is well known that some substorms are triggered by various solar wind or IMF changes while others occur spontaneously Furthermore there are other forms of geomagnetic activity such as pseudo breakups and Steady Magnetospheric Convection SMC events that have some traits of substorms but differ in certain aspects THEMIS will undoubtedly clarify the phenomena and the relationships between different forms of activity and substorm triggers However the physical processes will not be understood fully until we are able to model them We will thus complement the THEMIS mission with global simulations of the magnetosphere While it is possible to use local models to study isolated processes such as reconnection in detail it is not possible to apply local models to substorms Substorms are inherently global and encompass physical processes ranging from the dayside magnetopause the lobes the plasma sheet and the inner magnetosphere to the ionosphere and to the ground There have been a few attempts in the past to model substorms with global models such as the GEM substorm challenge Slinker et al 1995 Fedder et al 1995 Wiltberger et al 2000 Raeder and Maynard 2001 Raeder et al 2001b None of these simulations has been able to reproduce a substorm in its entirety Some substorm related phenomena such as particle injections are beyond the MHD description of the models However even the phenomena that global MHD based models should be able reproduce do often not come out well For example all models have a tendency to enter an SMC like state where nightside reconnection closely balances dayside reconnection and no loadingunloading cycle occurs Models then often require tweaking of parameters for a substorm to occur Raeder et al 2001b The necessity for such tweaking reflects the multi scale nature of substorms i e the effects of small scale processes such as anomalous resistivity kinetic instabilities or other processes that break the frozen flux condition Such processes are not included self consistently in the model but they are represented at least to some extent by parameterizations As long as self consistent treatment of such small scale processes in global models is not possible one hopes that these parameterizations are good enough to capture the substorm physics correctly In essence these parameterizations constitute hypotheses concerning the underlying physical processes and by comparison of the model OpenGGCM Simulations for the THEMIS Mission results with in situ data we test them We present one example of such a comparison later in this paper which shows a quite reasonable agreement with the data However many more such studies are needed to firmly establish the validity of the model We will thus use the OpenGGCM in at least three different ways to support THEMIS and to better understand substorms 1 We will test and constrain the model by simulating a number of substorm events with THEMIS and other observations These simulations will be driven with observed solar wind and IMF data which are usually available from solar wind monitors such as


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