Observations of Photospheric Vortical Motions During the Early Stage of Filament Eruption

Abstract

Solar filaments/prominences exhibit rotational motion during different phases of their evolution from their formation to eruption. We have observed the rotational/vortical motion in the photosphere near the ends of ten filaments during their initial phase of eruption, at the onset of the fast rise phase. All the filaments were associated with active regions. The photospheric vortical motions we observed lasted for 4 – 20 minutes. In the vicinity of the conjugate ends of the filament the direction of rotation was opposite, except for two cases, where rotational motion was observed at only one end point. The sudden onset of a large photospheric vortex motion could have played a role in destabilizing the filament by transporting axial flux into the activated filament thereby increasing the outward magnetic pressure in it. The outward magnetic pressure may have pushed the filament/flux rope to the height where the torus instability criterion was satisfied, and hence it could have caused the filament instability and eruption.

Appendix

The filament locations in He II 304 Å SDO/AIA channel for events 5 – 10 are displayed in Figures 7–12(a), respectively. The position of the filament on an averaged dopplergram (greyscale) and averaged horizontal flow map (arrows) is shown by overlaying the contours of the filament extracted from 304 Å images. These are shown in Figures 7–12(b), respectively. The observed rotational motions at the ends of the filament footpoints for all the events are also shown in bottom-left(c) and bottom-right(d) rows in Figures 7–12. The date and time of the observations, the filament location on the Sun, associated active region number, the filament activation time, the importance of the associated flare, and the velocities of the erupting filaments are given in Table 1. The starting and ending time and the type of the rotational motions seen at both ends of the filaments for all events are also given in the same table.

Figure 7

(a) The filament observed in SDO/AIA 304 Å channel for event 5 is indicated by a white arrow. Magnetic field isocontours from SHO/HMI overlaid on the 304 Å image. The red and green contours represent the positive and negative polarities with magnetic field strength values of ±150, 400, 600, and 900 G, respectively. (b) Averaged horizontal velocity vectors shown in arrows are overlaid upon an averaged dopplergram. The duration of the averaging time is given above the panel. The contour of the filament, extracted from the 304 Å image is also shown. The boxed regions 1 and 2 show the location of the eastern and western ends of the filament, respectively. (c, d) Rotational velocity pattern observed for this event. (c) Image corresponds to location 1 and (d) to location 2 in (b).

Figure 8

Same as Figure 7, but for event 6.

Figure 9

Same as Figure 7, but for event 7.

Figure 10

Same as Figure 7, but for event 8.

Figure 11

Same as Figure 7, but for event 9.

Figure 12

Same as Figure 7, but for event 10. (c) Rotational velocity pattern observed in the location 1 for this event.

The filament in event 5 is a long one, associated with two active regions. One of its ends is located in AR 11451 and the other end is in AR 11450. Before the filament eruption, bidirectional flows were observed in the filament. There were three filament eruptions observed in AR 11936 (events 6 – 8). Event 6 was observed on 31 December 2013. The other two events were observed on 01 January 2014. On 31 December 2013 only one filament existed in the active region and before the eruption this filament bifurcated into two halves. The right-side half erupted and the left-side one did not erupt. Later, again a filament formed in the same region and the bifurcation disappeared. Event 7 also occurred in the same active region, but the filament was located North of the 31 December 2013 event. This was a failed filament eruption. Event 8 also occurred on the same day, a few hours after event 7. The erupting filament was the same as the one which erupted on 31 December 2013. Unlike the 31st December event, however, here the whole filament was activated with no bifurcation. But it was a failed eruption. In event 9, AR 12027 was surrounded by filaments in all directions. It was not a single filament, in fact it was a group of 3 – 4 segmented filaments. On 04 April 2014 the North–East portion of the filament erupted (event 9). Before the eruption a large scale mass flow was observed inside the filament. Event 10 occurred in AR 12035. The filament was hard to see before the eruption in the active region. But during the filament activation a cusp shaped filament was observed in the active region. This filament erupted completely during a C7.3 class flare.