Finding the Critical Decay Index in Solar Prominence Eruptions

Abstract

The background fi eld is assumed to play a prime role in erupting structures like prominences. In the fl ux rope models, the critical decay index ( n c ) is a measure of the rate at which background fi eld intensity decreases with height over the fl ux rope or erupting structure. In the real observations, the critical height of the background fi eld is unknown, so a typical value of n c

=

1.5 is adopted from numerical studies. In this study, we determined the n c of 10 prominence eruptions ( PEs ) . The prominence height in 3D is derived from two-perspective observations of the Solar Dynamics Observatory and the Solar TErrestrial RElations Observatory . Synoptic maps of photospheric radial magnetic fi eld are used to construct the background fi eld in the corona. During the eruption, the height – time curve of the sample events exhibits the slow- and fast-rise phases and is fi tted with the linear-cum-exponential model. From this model, the onset height of fast-rise motion is determined and is considered as the critical height for the onset of the torus instability because the erupting structure is allowed to expand exponentially provided there is no strapping background fi eld. Corresponding to the critical height, the n c values of our sample events are varied to be in the range of 0.8 – 1.3. Additionally, the kinematic analysis suggests that the acceleration of PEs associated with fl ares are signi fi cantly enhanced compared to fl areless PEs. We found that the fl are magnetic reconnection is a more dominant contributor than the torus instability to the acceleration process during the fast- rise phase of fl are-associated PEs in low corona ( < 1.3 R e ) .