Main Objective:
Investigating the role of plasma processes on the stability of solar filaments
Scientific Justification:
As elongated tubes of cool, dense plasma suspended in the hot solar corona, understanding the formation and evolution of solar filaments requires knowledge of the surrounding magnetic structure which provides the magnetic support for the plasma. Magnetic flux ropes are formed by the continuous emergence and cancellation of small-scale magnetic features along polarity inversion lines (PILs) in the photosphere (Martin et al. 1985; van Ballegooijen & Martens 1989). These small-scale features are interpreted as representing the footpoints of two magnetic flux systems sheared across the PIL. The observed cancellation of magnetic flux leads to the development of these into two (new) distinct magnetic flux systems with a different connectivity to the pre-reconnection pair; a small loop which is pulled below the photosphere by magnetic tension (the observational manifestation of flux cancellation) and a longer, highly sheared loop. The highly sheared nature of this long loop provides a number of dips which can support cool, dense plasma, leading to the development of a tube of filamentary plasma within the flux rope above the PIL as the process of flux emergence and cancellation continues.
In order for this filamentary feature to erupt it must first become unstable, leading either to a coronal mass ejection (CME), or the material collapsing back onto the solar surface as a failed eruption. The loss of stability is thought to be driven by an imbalance between the different magnetohydrodynamic (MHD) forces influencing the filament, namely the upward magnetic pressure and downward magnetic tension. This deviation from equilibrium is explored in each of the breakout reconnection, tether-cutting, and torus instability models. For example, the torus instability (Bateman 1978; Kliem & T ̈or ̈ok 2006) is known to be sensitive to the variation with height of the magnetic field overlying the flux rope (i.e., the decay index). However, the effects of plasma flow within the filament and injection (draining) into (out of) the footpoints of the filament are largely neglected by these models. Despite this, it is known that the effects of the filament plasma can play a key role in deforming the field lines comprising the magnetic configuration on the order of hours (cf. D ́emoulin 1998). Increased plasma flows driven by the change in force balance as the filament rises may also provide an early indication of an imminent eruption. While the results of the eruption of overdense filaments have previously been studied (e.g., Carlyle et al. 2014), the pre-eruption effects of the plasma on the filament are still uncertain, while the sudden loss of stability leading to the eruption is also not very well understood. |
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