Slow-Mode Oscillations of Hot Loops Excited at Flaring Footpoints

TL;DR

This study presents observational evidence that slow-mode oscillations in hot coronal loops are excited by impulsive heating events at the loop footpoints, linking small flares to loop oscillations.

Contribution

It provides the first clear observational evidence that slow-mode standing waves are triggered by footpoint microflares in hot coronal loops.

Findings

Damped Doppler shift oscillations detected in Fe XIX line.

Oscillations triggered by small flare at loop footpoint.

Supports impulsive heating as excitation mechanism.

Abstract

The analysis of a hot loop oscillation event using SOHO/SUMER, GOES/SXI, and RHESSI observations is presented. Damped Doppler shift oscillations were detected in the Fe XIX line by SUMER, and interpreted as a fundamental standing slow mode. The evolution of soft X-ray emission from GOES/SXI and hard X-ray sources from RHESSI suggests that the oscillations of a large loop are triggered by a small flare, which may be produced by interaction (local reconnection) of this large loop with a small loop at its footpoint. This study provides clear evidence supporting our early conjecture that the slow-mode standing waves in hot coronal loops are excited by impulsive heating (small or microflares) at the loop's footpoint.

Full text

1 Introduction

A large number of strongly damped oscillations in hot ($>$6 MK) coronal loops have been observed by SOHO/SUMER in the past decade in Doppler shifts of flare lines (Fe XIX and Fe XXI) (Wang et al. 2002, 2003a). These oscillations with periods on the order of 10-30 min have been interpreted as fundamental standing slow modes (Ofman & Wang 2002; Wang et al. 2003b). They often manifest features such as recurrence and association with a flow (100-300 km/s) pulse preceding the oscillation, which suggests that they are most likely driven by microflares at the footpoints (Wang et al. 2005). These observations provide important constraints that can be used for improving theoretical models of magnetosonic wave excitation and for coronal seismology (Wang et al. 2007; Wang 2011, for recent review). Here we study an event on 2003 April 24 in AR 10339 (Fig. 1a) to explore its trigger using GOES/SXI and RHESSI data.

2 Observations

By examining 30 oscillation events observed by SUMER at solar limb during 2002-2003, hard X-ray sources are found in the initial phases of more than 10 events. However, due to the loss of Yohkoh, no SXT images can be used to identify the spatial relationship between the flare sources and the oscillating loop. Instead, in this work we analyze data from the Solar X-Ray Imager (SXI) onboard GOES-12. The coalignment between GOES/SXI and RHESSI images is based on SOHO/EIT 195 Å images.

3 Results and Discussion

Two oscillations are observed (Figs. 1b-d). One follows a GOES C6.7 flare at 04:56 UT showing strong damping, and the second follows a C4.5 flare at 05:40 UT showing weak damping. We measured the first oscillation with the period of 13.5 min, decay time of 6.2 min, and velocity amplitude of 189 km s*-1*, and the second one with the period of 14.2 min, decay time of 44.3 min, and amplitude of 8.2 km s*-1*. In the late phase, since more mass has been loaded in the loop by the chromospheric evaporation, it has the high density, and so oscillates with the smaller amplitude and weak damping. The RHESSI hard X-ray sources appear as a small loop structure, located at one footpoint of a large soft X-ray loop, suggesting that the flares and oscillations are triggered by their interaction (via local magnetic reconnection).

Acknowledgments

TW and LO are supported by NASA grants NNX08AE44G, NNX10AN10G, and NNX09AG10G, and WL is supported by AIA contract NNG04EA00C.