RHESSI Resumes Observations

From RHESSI Wiki

Introduction

As announced in an earlier Nugget, RHESSI entered its fourth annealing in late June. This process is periodically necessary to minimize the radiation damage of the detectors that accumulates gradually in orbit, and a still earlier Nugget explains the process in further detail.

We are happy now that the process is complete, and that RHESSI is functioning again in a satisfactory, although (as expected) not quite as well as in 2002!

Detectors

Of RHESSI's nine detectors, each of which can make images, we now have five (1,3,...) that are segmented, meaning that they can operate simultaneously as hard X-ray and soft X-ray detectors. Without segmentation they are only useful for imaging as hard X-ray detectors above about 20 keV. The anneal has thus resulted in some functionality lost, but also some functionality gained (see below), and at this stage in RHESSI's precious life - anticipating a few more powerful flares in this cycle - we are happy to have the higher spectral resolution that the annealing allowed.

Imaging

In Figure 1 we show an image of SOL2014-07-24T12, an almost-X-class event that had interesting structures. The Figure shows that RHESSI can image, at high resolution, the hard X-ray footpoint sources, as well as capturing the soft X-ray coronal loop structure at high temperatures.

Fig. 1: Images of SOL2014-08-24T12 in hard and soft X-rays (RHESSI) superposed on an EUV context image from AIA.

Note the interesting difference between RHESSI's soft X-ray contours and the AIA structures: RHESSI is showing us a better representation of the energetically important loop structures, which differ substantially from the EUV appearance.

Spectroscopy

How well can RHESSI now do imaging spectroscopy? There is essentially no change as a result of the anneal above 20 keV, as Figure 2 shows. This is an example (one example for each RHESSI detector) of the spectrogram representation of the RHESSI data.

Fig. 1: Spectrogram representation of each RHESSI detector's response to the flare SOL2014-08-24T12, showing that each of the nine detectors properly see >20 keV photons.