Hard X-rays and Sympathy

From RHESSI Wiki

Introduction

This Nugget returns to the "gee whiz" style in which a very recent and interesting phenomenon gets some quick scrutiny. In this case the phenomenon is a sympathetic flare, ie one induced by another event. The occurrence of such flares has long been known, but the mechanisms remain somewhat murky. The general opinion is that an active region that is about to flare finds itself in a state of marginal stability, so that an energetically minor trigger of some sort can just push it over the edge. The problem is in the physical nature of the trigger, if that is indeed the right way to describe the interaction. Very often powerful flare simply are ignored elsewhere on the Sun, as discussed in an early [1] Nugget.

In the case reported here, the flare SOL2011-07-11T10:56 occurred in NOAA active region [11249] but was rather obviously triggered by a slightly earlier flare in NOAA active region [11247]. This was noticed in the Max Millennium [milligan.pdf Message of the Day], written by Ryan Milligan of Queen's University Belfast, who was the duty scientist.

The Web view

Armed with the alert message, one can rather quickly understand the basic phenomenon by use of the many Web-based resources now available; these lead to wonderful data from a wide variety of spacecraft, most of which eagerly put their images on line as quickly as possible. Three of them, in Figure 1, show some of the trail of evidence for our sympathetic event.

Figure 1: Three images from the on-line archive for the sympathetic flares of 11 July 2011. Left, the flare locator image from Sam Freeland's events service. MIddle, the RHESSI locator image, which identifies the event in 1247 as the second event of the pair. Right, the hard X-ray time series from the [Fermi] [Gamma-Ray Burst Monitor]. This shows the sympathy in action, with the two participating active regions labeled 1247 and 1249. These latter two items are part of the RHESSI [Browser] material.

This survey led to a little discovery: the very rapid spike in the blue line (25-50 keV) of the hard X-ray data in the right-hand panel of Figure 1. By a leap of imagination, this could well be the time of the impulse that led to the second flare. Different mechanisms are feasible. One could have an Alfvenic disturbance propagating above the photosphere, moving rapidly through the corona. There are several possibilities here. Or, one could have a sunquake propagating beneath the photosphere, in which case the disturbance would be more like a simple sound wave.

One can study various movies of the flare development, for example by opening iSolSearch and querying by time and phenomenon ("flare" here). This rapidly leads to movies in all of the [AIA] wavelengths, reduced by the "cutout" service to manageable file sizes.