Solar Ultraviolet Bursts in the Joint Footpoints of Multiple Transition Region Loops

Zhenyong Hou,Hui Fu,Zhenghua Huang,Dayang Liu,Ning Tang,Youqian Qi,Lidong Xia

doi:10.3390/sym13081390

Abstract

Solar Ultraviolet bursts (UBs) associated with flux emergence are expected to help understand the physical processes of the flux emergence itself. In the present study, we analyse imaging and spectroscopic observations of a special group of UBs (including twelve of them) occurring in the joint footpoint regions of multiple transition region loops above the flux emerging regions. Consistent with previous studies of common UBs, we found that the spectral characteristics of this group of UBs are varied. Our results show that the responses of UBs in Ni ii, NUV continuum, Mg ii h and O i are originated from locations differ from that emits Si iv. The imaging data show that UBs have connections with the dynamics in the transition region loops. Brightenings starting from UB-regions and propagating along loops can be seen in SJ 1400/1330 Å and AIA 304 Å images and the corresponding time-space images. The apparent velocities are tens of kilometers per second in AIA 304 Å. For symmetry, the brightenings can propagate from the UB-regions towards opposite directions with similar apparent velocities in some cases. Given that these UBs are magnetic reconnection phenomena, we suggest that the propagating brightenings are the signals of the plasma flows resulted from heatings in the UB-regions.

Highlights

Flux emergence is a key process leading to the formation of active region, which hosts the most intense magnetic activities on the Sun
All these Ultraviolet bursts (UBs) are rooted in the joint footpoints of multiple transition region loops as seen in the IRIS SJ images
In order to display the UBs and their associated transition region loops in the same image, the SJ images shown in Figure 1 are the artificial images generated by the the technique described in Huang et al [34] that takes for each pixel the highest value obtained during the observing periods

Summary

Introduction

Flux emergence is a key process leading to the formation of active region, which hosts the most intense magnetic activities on the Sun. The superimposition of absorption lines in the transition region emission lines is the main distinction between this class and the other types of UBs (such as the narrow line-width UBs [4]) This class of UBs was first discovered by Peter et al [5] with observations of the Interface Region Imaging Spectrograph (IRIS [6]). UBs were suggested to be observational signatures of magnetic reconnection associated with U-loops forming in the process of magnetic flux emergence in the lower solar atmosphere, i.e., photosphere [5]. They might have strong connection to the so-called “Ellerman bombs” (EBs [7–11]) that are believed to be magnetic reconnection in the photosphere [8,12], Kim et al [13] and Vissers et al [14] found that EBs can produce UB-type spectra.

Observations and Data Analysis

20 April 2015 07:59–08:35

Results

Activities of the Transition Region Loops Associated with UBs

Discussion and Conclusions