Rotational Flows in Solar Coronal Flux Rope Cavities

Rotational flows in the prominence cavity Author: [Valeriia Liakh]

In this paper publsihed in The Astrophysical Journal Letters, we present a 2.5D magnetohydrodynamic simulation of a systematically rotating prominence inside its coronal cavity using the open-source MPI-AMRVAC code. Our simulation starts from a nonadiabatic, gravitationally stratified corona, permeated with a sheared arcade magnetic structure. The flux rope (FR) is formed through converging and shearing footpoints driving, simultaneously applying randomized heating at the bottom. The latter induces a left-right asymmetry of temperature and density distributions with respect to the polarity inversion line. This asymmetry drives flows along the loops before the FR formation, which gets converted to net rotational motions upon reconnection of the field lines. As the thermal instability within the FR develops, angular momentum conservation about its axis leads to a systematic rotation of both hot coronal and cold condensed plasma. The initial rotational velocity exceeds 60 km s-1. The synthesized images confirm the simultaneous rotations of the coronal plasma seen in 211 and 193 Å and condensations seen in 304 Å. Furthermore, the formation of the dark cavity is evident in 211 and 193 Å images. Our numerical experiment is inspired by observations of so-called giant solar prominence tornadoes and reveals that asymmetric FR formation can be crucial in triggering rotational motions. We reproduce observed spinning motions inside the coronal cavity, augmenting our understanding of the complex dynamics of rotating prominences.

In the accompanying animation, we show the synthesized images in AIA channels: 304, 211, 193, and 171 Å of the evolution during the FR formation and the rotational evolution (16–141 minutes). From 40.0 to 58.2 minutes, the condensation forms and grows gradually. Due to the low temperature of the condensation, it appears as a dark structure in all four channels. In channel 304 Å, only the prominence–corona transition region(PCTR) appears as a bright structure. It should be noted that this is a result of the approximate, optically thin treatment of 304 Å in the synthetic images, as explained by [1]. Nevertheless, the prominence appearance in the different channels clearly manifests rotation along with the bright coronal plasma (see panels (a)–(c) of the animation associated with Figure 4). Using SDO/AIA observations, [2] noticed an extremely similar appearance of their tornado in the 304 and 171 Å channels and also suggested that the cavity contained both hot and cold plasma. At 58.2 minutes, the formation of a dark cavity can be seen in the 211 and 193 Å channels. Initially, the darkening is caused by the strong cooling of this region due to the developing thermal instability. Later, the FR recovers its temperature but still appears dark in the 211 and 193 Å channels because it is strongly depleted of plasma due to the formation of condensations. At this time, channel 304 Å displays the ongoing rotation of the prominence mostly seen through the bright PCTR inside the dark cavity. Similarly, [2] showed the cavity as a dark structure best visible in the 211 and 193 Å channels from SDO/AIA observations. When the tornado developed, the authors observed bright plasma moving along the dark cavity, highlighting the magnetic field structure that was otherwise invisible. From 70.8 minutes, all four channels show the dimming region in the loops surrounding the FR. As seen in the animation, this region is also depleted of plasma. The border of the dimming region appears more luminous due to the compression of plasma in the overlying loops. At 102.5 minutes, the compressed material at the height of 50–60 Mm also condenses and drops down along the field lines.

[1] Xia, C., Teunissen, J., Mellah, I. E., Chané, E., & Keppens, R. 2018, ApJS, 234, 30

[2] Li, X., Morgan, H., Leonard, D., & Jeska, L. 2012, ApJL, 752, L22