High-resolution Observations of a Flux Rope with the Interface Region Imaging Spectrograph

TL;DR

This study presents high-resolution IRIS observations of a small flux rope at transition region temperatures, revealing its twisted structure, unwinding motion, and magnetic properties, illustrating flux ropes of various scales on the Sun.

Contribution

First detailed IRIS observation of a small flux rope showing its twisted structure, unwinding motion, and magnetic characteristics at transition region temperatures.

Findings

Flux rope exhibited a total twist of about 4π.

Unwinding motion observed with Doppler redshifts of 6-24 km/s.

Similarity between small flux rope and large CME core morphology.

Abstract

We report the observations of a flux rope at transition region temperatures with the \emph{Interface Region Imaging Spectrograph} (IRIS) on 30 August 2014. Initially, magnetic flux cancellation constantly took place and a filament was activated. Then the bright material from the filament moved southward and tracked out several fine structures. These fine structures were twisted and tangled with each other, and appeared as a small flux rope at 1330 {\AA}, with a total twist of about 4$\pi$. Afterwards, the flux rope underwent a counter-clockwise (viewed top-down) unwinding motion around its axis. Spectral observations of C {\sc ii} 1335.71 {\AA} at the southern leg of the flux rope showed that Doppler redshifts of 6$-$24 km s$^{-1}$ appeared at the western side of the axis, which is consistent with the counter-clockwise rotation motion. We suggest that the magnetic flux cancellation initiates reconnection and some activation of the flux rope. The stored twist and magnetic helicity of the flux rope are transported into the upper atmosphere by the unwinding motion in the late stage. The small-scale flux rope (width of 8.3$^{\prime\prime}$) had a cylindrical shape with helical field lines, similar to the morphology of the large-scale CME core (width of 1.54 $R_{\odot}$) on 2 June 1998. This similarity shows the presence of flux ropes of different scales on the Sun.