OpenGGCM Simulations for the THEMIS MissionAbstractIntroductionThe OpenGGCM ModelOuter MagnetosphereIonosphere and MI CouplingOpenGGCM Products and the 23 March 2007 Substorm ExampleSatellite Time SeriesIonosphere and AuroraMagnetosphere Flow and Field EvolutionSummary and ConclusionsAcknowledgementsReferencesSpace Sci RevDOI 10.1007/s11214-008-9421-5OpenGGCM Simulations for the THEMIS MissionJoachim Raeder ·Douglas Larson ·Wenhui Li ·Emil L. Kepko ·Timothy Fuller-RowellReceived: 17 March 2008 / Accepted: 18 July 2008© Springer Science+Business Media B.V. 2008Abstract The THEMIS mission provides unprecedented multi-point observations of themagnetosphere in conjunction with an equally unprecedented dense network of groundmeasurements. However, coverage of the magnetosphere is still sparse. In order to tie to-gether the THEMIS observations and to understand the data better, we will use the OpenGeospace General Circulation Model (OpenGGCM), a global model of the magnetosphere-ionosphere system. OpenGGCM solves the magnetohydrodynamic (MHD) equations inthe outer magnetosphere and couples via field aligned current (FAC), electric potential,and electron precipitation to a ionosphere potential solver and the Coupled ThermosphereIonosphere Model (CTIM). The OpenGGCM thus provides a global comprehensive view ofthe magnetosphere-ionosphere system. An OpenGGCM simulation of one of the first sub-storms observed by THEMIS on 23 March 2007 shows that the OpenGGCM reproduces theobserved substorm signatures very well, thus laying the groundwork for future use of theOpenGGCM to aid in understanding THEMIS data and ultimately contributing to a com-prehensive model of the substorm process.Keywords THEMIS · OpenGGCM · Magnetosphere · MHD · Simulation · SubstormJ. Raeder () · D. Larson · W. Li ·E.L. KepkoSpace Science Center, University of New Hampshire, Durham, NH, USAe-mail: [email protected]. Larsone-mail: [email protected]. Lie-mail: [email protected]. Kepkoe-mail: [email protected]. RaederPhysics Department, University of New Hampshire, Durham, NH, USAT. Fuller-RowellCIRES, Colorado University, Boulder, CO, USAe-mail: [email protected]. Raeder et al.1 IntroductionThe substorm debate has been a central part of space physics for over four decades andcenters on the question of what physical process(es) precipitate the sudden energy release inthe magnetotail and the sudden auroral brightening and expansion (Akasofu 1977;Lui1991;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 poweredby magnetic reconnection. Reconnection signatures are often observed in the tail duringthe course of substorms. However, the location of the associated x-lines is typically ob-served ∼20REfrom Earth or further down the tail. Conversely, the initial brightening ofthe aurora maps much closer to Earth. Thus, the question is commonly posed as to whetherreconnection causes the process that brightens the aurora or whether the process that bright-ens the aurora causes reconnection. The THEMIS mission (Sibeck and Angelopoulos 2008;Angelopoulos 2008) is designed to answer this question by providing simultaneous mea-surements 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 becauseprocesses such as dipolarization of the field or earthward flows may not necessarily occurstrictly radially but sweep azimuthally over the spacecraft, creating an apparent radial mo-tion that does not correspond to the real one. Furthermore, substorms come in different sizesand 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 changeswhile others occur spontaneously. Furthermore, there are other forms of geomagnetic ac-tivity such as pseudo-breakups and Steady Magnetospheric Convection (SMC) events, thathave some traits of substorms but differ in certain aspects. THEMIS will undoubtedly clar-ify the phenomena and the relationships between different forms of activity and substormtriggers. However, the physical processes will not be understood fully until we are able tomodel them.We will thus complement the THEMIS mission with global simulations of the mag-netosphere. While it is possible to use local models to study isolated processes such asreconnection in detail, it is not possible to apply local models to substorms. Substormsare inherently global and encompass physical processes ranging from the dayside magne-topause, the lobes, the plasma sheet, and the inner magnetosphere to the ionosphere andto the ground. There have been a few attempts in the past to model substorms with globalmodels, 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 thesesimulations has been able to reproduce a substorm in its entirety. Some substorm-relatedphenomena such as particle injections are beyond the MHD description of the models.However, even the phenomena that global MHD based models should be able reproducedo often not come out well. For example, all models have a tendency to enter an SMC-likestate, where nightside reconnection closely balances dayside reconnection and no loading-unloading cycle occurs. Models then often require tweaking of parameters for a substorm tooccur (Raeder et al. 2001b). The necessity for such tweaking reflects the multi-scale natureof substorms, i.e., the effects of small-scale processes, such as anomalous resistivity, kineticinstabilities, or other processes that break the frozen flux condition. Such processes are notincluded 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 inglobal models is not possible, one hopes that these parameterizations are good enough tocapture the substorm physics correctly. In essence, these parameterizations constitute hy-potheses concerning the underlying physical processes, and, by comparison of the modelOpenGGCM Simulations for the THEMIS Missionresults with in situ data, we test them. We present one example of such a comparison laterin this paper which shows a quite reasonable agreement with the data. However, many