N. Vasantharaju1, P. Vemareddy2, B. Ravindra2, V. H. Doddamani1
1. Department of Physics, Bangalore University, Bengaluru-560 056, India
2. Indian Institute of Astrophysics, Koramangala, Bengaluru-560 034, India
Pre-eruptive structures like prominences/filaments, X-ray sigmoids are supported by magnetic flux rope topology. These structures are in equilibrium with the outward acting plasma and magnetic pressures being counterbalanced by inward acting gravitational and magnetic tension forces. Recent evidence suggests that the torus instability plays a prominent role in rupturing this equilibrium leading to eruption, manifested as a CME. According to torus instability, the flux rope becomes unstable when it reaches a critical height from where the overlying (background) field decreases at a critical rate indicated by a dimensionless parameter called critical decay index, nc. In observations, the critical height is unknown so a typical value of critical decay index, nc=1.5, is adopted from the numerical simulations. To know the role of the background field, therefore, requires determining the exact nc, which is at the heart of triggering mechanism of eruptions.
Figure 1| CME kinematics of the PE on March 7, 2011. Left panel: A slit is placed to track the ascending prominence apex in AIA 304Å. Right panels: Height–time stack image of the slit. Green asterisk symbols locate the data points for the ascending apex. Model fitting to the corrected height–time data (black asterisks, black solid line). Blue solid curve is derived velocity. GOES soft X-ray flux is plotted as a red solid curve. Red vertical dashed line (19:42 UT) marks the onset of the M3.7 flare associated with this eruption. Black vertical dashed line marks the time of onset (19:33 UT) of the fast-rise phase.
In this study, we computed the nc of 10 prominence eruptions (PEs) by computing the true heights of the prominences. Three vantage point observations from Solar Dynamics Observatory (SDO) and Solar Terrestrial Relations Observatory (STEREO) are used. The prominence kinematics are derived by placing a slit across the leading edge observed in 304Å images (Figure 1). Height correction is applied by tie-pointing method. Kinematics of erupting structures typically exhibit slow and rapid acceleration phases. For the critical height of onset of torus-instability, we adapted a novel idea that the onset time of fast-rise motion corresponds to critical height, from where the erupting structure is allowed to expand exponentially provided there is no strapping background field. The exact critical height is obtained by fitting the height-time curves with the linear-exponential model.
The decay-index is derived from the coronal background field computed from potential field source surface model. The Helioseismic Magnetic Imager’s daily updated synoptic maps are used as the boundary condition. We found that the nc is not a constant value and varies from event to event as the background field configuration depends on the field distribution in the source region. Corresponding to the critical height, the nc values of our sample events vary in the range of 0.8–1.3.
Figure 2| Variation of decay index with height above the photosphere. Height-time profile is plotted (blue curve) with time as y-axis scale on right. From the model fit, the critical height (vertical dashed line) is determined as the height at which erupting prominence commences the onset of the fast-rise motion. Corresponding to the critical height (Hc = 0.035 R ) and time (Tc = 19.55 hr), the nc = 1.02 ± 0.12.
Further, in our sample, we noticed that during rapid-acceleration phase, the flare associated events have average acceleration in the range of 400 m s−2 to 1550 m s−2 and flare-less events have average acceleration well below 200 m s−2, suggesting the acceleration of PEs associated with flares are significantly enhanced compared to flareless PEs. We found that the flare magnetic reconnection is a more dominant contributor than the torus instability to the acceleration process during the fast-rise phase of flare-associated PEs in low corona (<1.3Rsun).
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