Magnetic Fields in Coronal Mass Ejection Source Regions III
Presiding: O Abramenko, Moscow State University; V Yurchyshyn, Big Bear Solar Observatory
SH54B-01 INVITED 15:30h
Shape and Reconnection of the Exploding Magnetic Field in the Onset of CMEs
From chromospheric and coronal images and line-of-sight and vector magnetograms of magnetic regions that produce CMEs, and from chromospheric and coronal movies of the onsets of CME eruptions, it appears that the magnetic field that explodes to drive the CME is initially the strongly sheared core of a magnetic arcade encasing a polarity dividing line in the magnetic flux. Before or during the onset of the explosion, the sheared core field becomes a flux rope, often carrying chromospheric material within it. For the erupting flux rope to drive the explosion, that is, for its magnetic energy content to decrease in the explosion, the flux rope's cross-sectional area must increase faster than its length. For instance, for isotropic expansion, the area increases as the square of the length, and the magnetic energy content of the flux rope decreases as the inverse of the length. The instability that initiates the eruption of the flux rope might be an ideal MHD kink instability, or might involve runaway tether-cutting reconnection. The reconnection begins below the flux rope (internal to the arcade) when the overall field configuration of the region is effectively that of a single bipole. When the flux rope resides in a multi-bipolar configuration having a magnetic null above the flux rope, the runaway tether-cutting reconnection might begin either below the flux rope or at the null above (external to) the arcade. We present examples of observed CME onsets that illustrate the above alternatives. In each example, reconnection below the flux rope begins early in the eruption. This indicates that internal tether cutting reconnection (classic tether-cutting reconnection) is important in unleashing the CME explosion in all cases, including those in which the explosion may be triggered by MHD kinking or by external reconnection (classic breakout reconnection).
SH54B-02 INVITED 15:45h
Observational Evidence of the Kink Instability In Solar Eruptions
Two approaches to studying the role of source region magnetic fields for CMEs are described. First, observational evidence is used to infer that the MHD helical kink instability is associated with at least some filament eruptions and CMEs. In seven cases, the sense of twist and writhe in the source regions were determined from movies of erupting filaments. In every case the sense of twist was the same as the sense of writhe, as required for a kink. Next, we consider six different cases in which the chirality and axis orientation of interplanetary flux ropes could be compared with the corresponding signatures in the source regions at the Sun. In four cases, the chirality and orientation inferred from these pre-eruption flux rope signatures agreed well with the interplanetary flux rope signatures. In two cases, the flux rope axis orientations differed by about 150 degrees. These results suggest that the flux ropes existed prior to eruption, that they become flux ropes in interplanetary space, and, at least in some cases, that the kink instability is the likely cause of eruption.
SH54B-03 16:00h
Magnetic Twist and Writhe of Delta Active Regions
Active regions with a Δ magnetic configuration from 1996 to 2002 were selected to study how important a role the kink instability plays in such active regions. We employ the systematic tilt angle of each active region as a proxy for the writhe of a fluxtube and the force-free parameter, αbest, as a proxy for the magnetic field twist in the fluxtube. It is found that 65-67% of the 104 active regions have the same sign of twist and writhe, which violate the Hale-Nicholson and Joy's Laws (HNJL) or the hemispheric helicity rule (HHR). 68% (46/68) of these active regions produced more than five large flares. Active regions violating HNJL, but following HHR, have a much stronger tendency to produce X-class flares and/or strong proton events. Continuously clockwise rotation of magnetic configuration of a long-lived active region (AR 9604-9632-9672-9704-9738) which produced major flares, fast CMEs and many strong proton events shows that a kink instability would play very important role in such active regions. These results support the prediction for the presence of a kink instability, that the twist and writhe of the magnetic fields exhibit the same sign for Δ active regions (Linton et al, 1998, 1999, and Fan et al., 1999). Finally, we analyze possible origins of the twist and writhe of the magnetic fields for the active regions with different relations between the twist and writhe.
SH54B-04 16:15h
In situ observation of filament plasma and their magnetic structure
Many CMEs, when observed during their expansion in the corona, are associated with a three-part structure: A bright core associated with prominence plasma, a dark area or cavity, that is bounded by a bright rim, presumably associated with the dynamic interactions of CMEs with their plasma environment. Many Interplanetary CMEs (ICMEs) only have two key parts: A magnetically dominated inner part, generally associated with the low density volume appearing dark in the corona, and the interaction sheath, presumably associated with the interaction of magnetic clouds in the corona. The vast majority of ICME do not have recognizable filament plasma. There are a few observations of ICMEs with singly charged He contributions that are generally associated with filaments. However, there are two model scenarios for producing such low ionic charge states. First, they can be achieved through filament-like material that never heats up during the eruption. Second, they can result from a rapid freeze-in process at high density in the corona. We discuss in situ observations of ICMEs that are characterized by unusually cool material and the ionic composition of all observable constituents that allow us to distinguish between these models. We discuss these data and interpret them in the context of CME models and the evolution of CMEs in the inner corona. We will also discuss the relation of the filament material to the magnetically dominated structures, such as flux ropes.
SH54B-05 16:30h
Modeling Active Region Coronal Mass Ejections*
The fastest coronal mass ejections (CMEs) typically originate from active regions on the Sun. From a theoretical standpoint, fast CMEs are the most difficult to understand and model, because they require that large amounts of magnetic energy (1032 ergs) be released rapidly (~1000 seconds). We describe MHD computations of eruptive behavior in an active region arising from the flux cancellation mechanism. The active region is modeled as a localized bipole within a global dipolar configuration, and is similar to the solar magnetic flux for the May 12, 1997 CME. We find that a localized 3D configuration erupts as a consequence of flux cancellation, in the same way that 2D axisymmetric and large-scale 3D fields erupt. We discuss the role of the interaction of the local active region magnetic field with the global magnetic fields due to the surrounding magnetic flux, as well the implications of our work for the initiation of fast CMEs. *Work supported by NASA and the Center for Integrated Space Weather Modeling (an NSF Science and Technology Center).
SH54B-06 16:45h
Structural Analysis of the Pre-Eruptive Magnetic Field for the May 12, 1997 CME Event*
A simple analytical model of the coronal magnetic field prior to the CME eruption on May 12, 1997 is developed in the current-free approximation. The magnetic field is constructed by superimposing a large-scale background field and a localized bipole field to model the active region. The background field is determined from the normal component of the observed photospheric magnetic field averaged over the longitude of the Sun. The influence of the solar wind is taken into account by imposing a source-surface boundary condition that makes the field radial at a specified radius. The field of the active region is modeled with the help of a subphotospheric dipole whose strength, location, and orientation are optimized to fit the magnetic field obtained from an MDI magnetogram. A corresponding force-free magnetic field is developed then by shearing and twisting the potential configuration. The structure of the potential and force-free configurations is analyzed and related to the characteristics of the observed eruption. *Research supported by NASA and the Center for Integrated Space Weather Modeling (an NSF Science and Technology Center).