[1] The interaction of the solar wind and Earth's magnetosphere is complex, and the phenomenology of the interaction is very different for interplanetary coronal mass ejections (ICMEs) compared to their sheath regions. In this paper, a total of 71 intense (Dst ≤ −100 nT) geomagnetic storm events in 1996-2006, of which 51 are driven by ICMEs and 20 by sheath regions, are examined to demonstrate similarities and differences in the energy transfer. Using superposed epoch analysis, the evolution of solar wind energy input and dissipation is investigated. The solar wind-magnetosphere coupling functions and geomagnetic indices show a more gradual increase and recovery during the ICME-driven storms than they do during the sheath-driven storms. However, the sheath-driven storms have larger peak values. In general, solar wind energy input (the epsilon parameter) and dissipation show similar trends as the coupling functions. The trends of ion precipitation and the ratio of ion precipitation to the total (ion and electron) are quite different for both classes of events. There are more precipitating ions during the peak of sheath-driven storms. However, a quantitative assessment of the relative importance of the different energy dissipation branches shows that the means of input energy and auroral precipitation are significantly different for both classes of events, whereas Joule heating, ring current, and total output energy display no distinguishable differences. The means of electron precipitation are significantly different for both classes of events. However, ion precipitation exhibits no distinguishable differences. The energy efficiency bears no distinguishable difference between these two classes of events. Ionospheric processes account for the vast majority of the energy, with the ring current only being 12%-14% of the total. Moreover, the energy partitioning for both classes of events is similar.Citation: Guo, J., X. Feng, B. A. Emery, J. Zhang, C. Xiang, F. Shen, and W. Song (2011), Energy transfer during intense geomagnetic storms driven by interplanetary coronal mass ejections and their sheath regions,

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[1] A three-dimensional time-dependent, numerical magnetohydrodynamic (MHD) model is used to investigate the propagation of coronal mass ejections (CMEs) in the nonhomogenous background solar wind flow. On the basis of the observations of the solar magnetic field and K-coronal brightness, the self-consistent structure on the source surface of 2.5 Rs is established with the help of MHD equations. Using the self-consistent source surface structures as initial-boundary conditions, we develop a three-dimensional MHD regional combination numerical model code to obtain the background solar wind from the source surface of 2.5 Rs to the Earth's orbit (215 Rs) and beyond. This model considers solar rotation and volumetric heating. Time-dependent variations of the pressure and velocity configured from a CME model at the inner boundary are applied to generate transient structures. The dynamical interaction of a CME with the background solar wind flow between 2.5 and 215 Rs (1 AU) is then investigated. We have chosen the well-defined halo-CME event of 6-12 January 1997 as a test case. Because detailed observations of this disturbance at 1 AU (by WIND spacecraft) are available, this event gives us an excellent opportunity to verify our MHD methodology and to learn about the physical processes of the Sun-Earth connection. In this study, we find that this three-dimensional MHD model, with the self-consistent structures on the source surface as input, provides a relatively satisfactory comparison with the WIND spacecraft observations. Citation: Shen, F., X. Feng, S. T. Wu, and C. Xiang (2007), Three-dimensional MHD simulation of CMEs in three-dimensional background solar wind with the self-consistent structure on the source surface as input: Numerical simulation

[1] In this paper, we develop a time-dependent MHD model driven by the daily-updated synoptic magnetograms (MHD-DUSM) to study the dynamic evolution of the global corona with the help of the 3D Solar-Interplanetary (SIP) adaptive mesh refinement (AMR) space-time conservation element and solution element (CESE) MHD model (SIP-AMR-CESE MHD Model). To accommodate the observations, the tangential component of the electric field at the lower boundary is specified to allow the flux evolution to match the observed changes of magnetic field. Meanwhile, the time-dependent solar surface boundary conditions derived from the method of characteristics and the mass flux limit are incorporated to couple the observation and the 3D MHD model. The simulated evolution of the global coronal structure during 2007 is compared with solar observations and solar wind measurements from both Ulysses and spacecrafts near the Earth. The MHD-DUSM model is also validated by comparisons with the standard potential field source surface (PFSS) model, the newly improved Wang-Sheeley-Arge (WSA) empirical formula, and the MHD simulation with a monthly synoptic magnetogram (MHD-MSM). Comparisons show that the MHD-DUSM results have good overall agreement with coronal and interplanetary structures, including the sizes and distributions of coronal holes, the positions and shapes of the streamer belts, and the transitions of the solar wind speeds and magnetic field polarities. The MHD-DUSM results also display many features different from those of the PFSS, the WSA, and the MHD-MSM models.

Though coronal mass ejections (CMEs) are magnetized fully ionized gases, a recent observational study of a CME collision event in 2008 November has suggested that their behavior in the heliosphere is like elastic balls, and their collision is probably superelastic [C. Shen et al., 2012]. If this is true, this finding has an obvious impact on the space weather forecasting because the direction and velocity of CMEs may change. To verify it, we numerically study the event through three‒dimensional MHD simulations. The nature of CMEs' collision is examined by comparing two cases. In one case, the two CMEs collide as observed, but in the other, they do not. Results show that the collision leads to extra kinetic energy gain by 3–4% of the initial kinetic energy of the two CMEs. It firmly proves that the collision of CMEs could be superelastic.

[1] A three-dimensional (3-D) time-dependent, numerical magnetohydrodynamic (MHD) model with asynchronous and parallel time-marching method is used to investigate the propagation of coronal mass ejections (CMEs) in the nonhomogenous background solar wind flow. The background solar wind is constructed based on the self-consistent source surface with observed line-of-sight of magnetic field and density from the source surface of 2.5 R s to the Earth's orbit (215 R s ) and beyond. The CMEs are simulated by means of a very simple flux rope model: a high-density, high-velocity, and hightemperature magnetized plasma blob is superimposed on a steady state background solar wind with an initial launch direction. The dynamical interaction of a CME with the background solar wind flow between 2.5 and 220 R s is investigated. The evolution of the physical parameters at the cobpoint, which is located at the shock front region magnetically connected to ACE spacecraft, is also investigated. We have chosen the well-defined halo-CME event of 4-6 April 2000 as a test case. In this validation study we find that this 3-D MHD model, with the asynchronous and parallel time-marching method, the self-consistent source surface as initial boundary conditions, and the simple flux rope as CME model, provide a relatively satisfactory comparison with the ACE spacecraft observations at the L1 point.

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We present here a time‐dependent three‐dimensional magnetohydrodynamic (MHD) solar wind simulation from the solar surface to the Earth's orbit driven by time‐varying line‐of‐sight solar magnetic field data. The simulation is based on the three‐dimensional (3‐D) solar‐interplanetary (SIP) adaptive mesh refinement (AMR) space‐time conservation element and solution element (CESE) MHD (SIP‐AMR‐CESE MHD) model. In this simulation, we first achieve the initial solar wind background with the time‐relaxation method by inputting a potential field obtained from the synoptic photospheric magnetic field and then generate the time‐evolving solar wind by advancing the initial 3‐D solar wind background with continuously varying photospheric magnetic field. The model updates the inner boundary conditions by using the projected normal characteristic method, inputting the high‐cadence photospheric magnetic field data corrected by solar differential rotation, and limiting the mass flux escaping from the solar photosphere. We investigate the solar wind evolution from 1 July to 11 August 2008 with the model driven by the consecutive synoptic maps from the Global Oscillation Network Group. We compare the numerical results with the previous studies on the solar wind, the solar coronal observations from the Extreme ultraviolet Imaging Telescope board on Solar and Heliospheric Observatory, and the measurements from OMNI at 1 astronomical unit (AU). Comparisons show that the present data‐driven MHD model's results have overall good agreement with the large‐scale dynamical coronal and interplanetary structures, including the sizes and distributions of the coronal holes, the positions and shapes of the streamer belts, the heliocentric distances of the Alfvénic surface, and the transitions of the solar wind speeds. However, the model fails to capture the small‐sized equatorial holes, and the modeled solar wind near 1 AU has a somewhat higher density and weaker magnetic field strength than observed. Perhaps better preprocessing of high‐cadence observed photospheric magnetic field (particularly 3‐D global measurements), combined with plasma measurements and higher resolution grids, will enable the data‐driven model to more accurately capture the time‐dependent changes of the ambient solar wind for further improvements. In addition, other measures may also be needed when the model is employed in the period of high solar activity.