Coronal Mass Ejections II
Presiding: D A Biesecker, NOAA Space Environment Center; B Welsch, Space Sciences Laboratory, University of California, Berkeley
SP43C-01 INVITED 13:30h
Harvey Prize Lecture: The calm before the storm: the link between quiescent cavities and CMEs
Coronal mass ejections (CMEs) are thought to be driven by magnetic energy, stored in twisted or sheared magnetic fields. Magnetic clouds, which are interplanetary manifestations of CMEs, are commonly modeled as flux ropes of twisted magnetic field. It has also become quite standard to model the erupting CME as a flux rope. However, the question of whether the flux rope is formed during the eruption, or whether the flux rope existed prior to the eruption, remains controversial. CMEs often possess a three-part morphology in white light observations of a bright front, followed by a relatively dark cavity, and lastly a bright core associated with an erupting prominence. The three-part structure of CMEs has been shown in a variety of models to be a consequence of a magnetic flux rope topology. The same physical reasons for the presence of the cavity system in eruption hold true in quiescence, and so it is significant that the three-part structure, in the form of helmet-streamer/cavity/prominence-core often exists quiescently in the corona. I will present an analysis of a few case studies of white light quiescent cavities as observed by the HAO Mauna Loa Solar Observatory Mk4 coronagraph. In particular I will consider the 3D structure and evolution of these cavities, and how these are related to CMEs. Finally, I will discuss the implications that these observations may have for the state of the corona just prior to a CME, and more generally for the nature of coronal MHD equilibria.
SP43C-02 14:00h
3D Numerical Simulations of the Breakout Model
We present the continuing progress of the numerical simulations of the breakout model for coronal mass ejection initiation. To validate the 3D spherical ARMS code we have run the 2.5D breakout problem and compare the eruption to the published 2D results. The ARMS 2.5D CME also forms a large magnetic island ahead of the erupting plasmoid due to the code's excellent maintenance of equatorial symmetry. Progress on the fully 3D breakout problem is also discussed. To build up enough magnetic free energy for an eruption the active region field must be strong with a steep gradient near the polarity inversion line and the shear must be highly concentrated there. This requires adaptive griding techniques. In the current simulation, the active region to background field ratio is 20-to-1 and the neutral line is long compared to the active region width. We present the evolution of this topology under Br-conserving shearing flow and discuss implications for a 3D eruption. This work is supported by NASA and ONR. BJL is supported by NASA GSRP grant NGT5-50453.
SP43C-03 14:15h
Maximizing the Energies of Force-Free Coronal Magnetic Flux Ropes
Coronal mass ejections (CMEs) and other solar eruptive events are believed to be powered largely by the release of magnetic energy stored in non-potential magnetic fields in the solar corona. Whatever the CME energy source, there must be sufficient energy available to (1) open the coronal magnetic field, (2) lift the ejecta against solar gravity, and (3) accelerate the ejecta to speeds of typically several hundred km/s. However, the magnetic energy of force-free fields grounded in the solar photosphere is incapable of all three tasks, since the energy of the fully open field is an upper limit to the energies of such fields. Magnetic configurations containing detached flux ropes do not share this limitation. The work presented here describes the maximum energies possible in such flux-rope fields. We explore a two-dimensional parameter space that includes variations in the photospheric magnetic flux distribution as well as a high-latitude cutoff of the region containing non-potential fields. The energy we seek to maximize is the free energy above that of the open-field state, since this is the energy available for tasks (2) and (3) above. The maximum such energy appears to be in the range from 15 to 20 percent of the energy of a fully potential field, and is determined by a tradeoff between the hold-down effect of an overlying potential field and a decrease in the open-field energy as magnetic flux moves poleward.
SP43C-04 14:30h
Resistive MHD Modeling of CME Acceleration and Impulsive Magnetic Reconnection
CME (or Flux rope) acceleration and impulsive magnetic reconnection are modeled by resistive MHD simulations with anomalous resistivity. We discuss the role of nonuniform anomalous resistivity on the time scale of flux rope acceleration and reconnection rate. Our simulation results show that the reconnection electric field reaches a peak value of O(1 keV) for X-class flares during the GOES X-ray flare rise phase, which is consistent with estimates obtained from several observations of two ribbon expansion in flare-CME events. Our simulations provide quantitative agreement with observations of CME acceleration during the flare rise phase and predict an enhanced magnetic reconnection rate during this period. We will discuss the physical scenario of how the phenomenological anomalous resistivity can be created in collisionless turbulent plasmas in the current sheet, how the current sheet can be significantly wider than the ion skin depth, and how particles are accelerated in the globally evolving EM fields with turbulence in the current sheet.
SP43C-05 14:45h
Formation and Expulsion of Large Scale Flux Ropes in Major Solar Eruptions by Reconnection Among Smaller Scale Flux Tubes
Observations of solar eruptions often show a sudden appearance and expulsion of a large scale loop structure following a brightening of smaller scale loops. Also, the distance between the legs of a CME loop is generally much larger than the separation distance between Hα flare ribbons or between hard X-ray emission features. To explain this, Kopp-Pneuman type standard solar eruption models would require a footpoint shearing over a huge distance, which, however, is not observationally confirmed. To address this problem, we perform a 3D MHD simulation study on evolution of an idealized active region composed of several magnetic flux tubes whose field connectivities are individually discrete. Footpoint twisting motions are applied within individual flux tubes so that the longitudinal magnetogram does not change at all whereas magnetic shear near the polarity inversion line rapidly increases as could be seen in a vector magnetogram. Magnetic reconnection between the flux tubes results in a new field line connection, in which the distance between the footpoints is larger than the original one. After a certain amount of magnetic flux is reconnected, a helical flux rope of a visibly larger scale is created and it is expelled out of the active region leaving the underneath field lines open. An observational example corresponding to this simulation will also be presented.