RHESSI has been successfully operating in space for over 16 years. Starting on April 12, 2018 we will be rejuvenating the detectors once again through an "anneal" procedure. This is RHESSI's sixth anneal (the most recent was in 2016). RHESSI’s germanium detectors degrade steadily over time due to radiation damage from charged particles, and heating up the germanium restores lost sensitivity and resolution. The detectors will be heated up from their operating temperature of ~155 K to ~85 deg C (~358 K), held at that temperature for two weeks, and then cooled back down. The entire procedure is estimated to take about two months. During this time period, RHESSI will be unable to make X-ray or gamma-ray observations.
RHESSI continues to be the only active observatory that can provide imaging spectroscopy of the energetic electrons that carry such a predominant part of the energy released in a flare.
RHESSI has now covered more than a complete 11-year solar cycle and is continuing to make observations as activity decreases to solar minimum (expected in ~2019). During that time it has recorded over 114,000 X-ray events, 42 with gamma-ray emission above 300 keV, and 27 with gamma-ray line emission.
The RHESSI spacecraft and instrument continue to operate well. The slowly rising detector temperature resulting from decreasing cryocooler efficiency is of concern, and only two of the nine detectors are turned on during periods of low solar activity to minimize the temperature increase. When there is enhanced activity and during short observing campaigns with other space- and ground-based observatories, we will turn on additional detectors for optimum coverage. In this way, we expect to be able to maintain RHESSI's unique hard X-ray imaging spectroscopy capability for the foreseeable future.
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is a NASA Small Explorer Mission, launched on February 5, 2002.
RHESSI's primary mission is to explore the basic physics of particle acceleration and explosive energy release in solar flares. This is achieved through imaging spectroscopy in X-rays and gamma-rays with fine angular and energy resolution to reveal the locations and spectra of the accelerated electrons and ions and of the hottest plasma.
Solar flares and their associated coronal mass ejections are of great scientific interest since they are so little understood. They present severe challenges to explain how the energy equivalent of billions of megatons of TNT is released in the solar atmosphere on time scales of minutes, and how so many electrons, protons and heavier ions are accelerated to such high energies. These super-energetic solar eruptive events are the most extreme drivers of space weather and present significant dangers in space and on Earth.