Magnetic and spectroscopic properties of supergranular-scale coronal jets and erupting loops in a polar coronal hole^*

¹ Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau 37191, Germany e-mail: jshept@gmail.com
² School of Earth and Space Sciences, Peking University, Beijing 100871, PR China
³ School of Space Science and Physics, Shandong University, Weihai 264209, PR China

Received: 1 April 2010
Accepted: 25 June 2010

Abstract

Context. Coronal jets and mass ejections associated with erupting loops are two distinct and frequently observed types of transient upflows of plasma in coronal holes (CHs). But the magnetic and spectroscopic properties of these events at the supergranular scale are not well known.

Aims. Here we aim at studying in a polar hole the plasma and field characteristics of coronal jets and erupting loops of a supergranular size, for which we use observations from XRT, EIS and SOT on Hinode as well as EUVI on STEREO.

Methods. The open magnetic field structures related to the coronal jets are obtained by magnetic field extrapolation into the corona from SOT magnetograms. Furthermore, we use the EIS observations to analyze ultraviolet line intensities and Doppler shifts in association with the erupting loops.

Results. We find that the coronal jet plasma is indeed ejected along open field lines, thus confirming the conjecture of jet formation in an open magnetic environment. The magnetic evolution at the jet base is investigated, and the results indicate that the interaction between two flux tubes of opposite magnetic polarities as well as the squeezing of several tubes with identical polarities might be responsible for the jet initiation. We reveal for the first time the spectroscopic signatures of a supergranular-size erupting loop at its early stage, which consists of three steps. The first step is the onset, which is featured by a sudden brightening of one footpoint, as well as by the occurrence of blueshifts along almost its entire path. The second step is the initial expansion of the closed loop, which is estimated to move upward at a speed of about 20 km s^-1, as derived from the line-of-sight (LOS) blueshift and the loop enlargement projected onto the plane of the sky. In the third step, the loop's bright footpoint is apparently diminishing its intensity and enhancing its blueshift, which indicates that plasma upflow from the leg is filling the expanding loop volume.

Conclusions. From our results we conclude that in polar CHs, where the steady fast solar wind is known to emanate, there are also at least two possible ways of causing transient plasma outflows at supergranular scale. One is related to coronal jets guided by open field lines, the other to the eruption of closed loops, which is triggered by magnetic reconnection at their footpoints.