RHESSI - Concept to Fruition
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| Nugget | |
|---|---|
| Number: | 100 |
| 1st Author: | Hugh Hudson |
| 2nd Author: | |
| Published: | 27 April 2009 |
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Contents |
Introduction
Our RHESSI (the Reuven Ramaty High-Energy Solar Spectroscopic Imager) has its own interesting history, and we take the opportunity to devote RHESSI Science Nugget number 100 to a discussion of how this solar observatory in space came into existence. In addition to the technical aspects of creating and operating the instruments, a great deal of effort went into the writing of proposals prior to final acceptance. Below we trace about twenty years' worth of this preparatory work prior to obtaining the first observations.
About three years after launch we began to write Science Nuggets, such as this one. Most are contributed by volunteer authors, and most deal with RHESSI discoveries in one way or another. The RHESSI Science Nuggets are roughly biweekly, and the first RHESSI Science Nugget was put on line on 21 March 2005 - a few years after RHESSI's launch date, and three years after the discoveries began. The Nuggets are an informal channel for publicizing RHESSI (and solar) discoveries, and the dozens of volunteer authors work towards making them readable by all. This seems to work - the previous Nugget 99 has had 1,301 Web accesses in its first two weeks. This level of citation of a scientific paper might attract the attention of the Nobel Prize committee!
Hard X-ray imaging
At their core, solar flares are highly nonthermal in nature. That is, they involve highly intense energy release in a manner characterized by non-equilibrium particle distributions evolving on short time scales (below one sec) and small spatial scales (below a few arc sec). On larger scales they are also involved in the beautiful eruptions termed CMEs. To study the core nonthermal physics of the flare, hard X-ray bremsstrahlung and radio gyrosynchrotron radiation have proven to be the most interesting, although there are many other direct and indirect manifestations. RHESSI has provided the best view to date of the hard X-ray bremsstrahlung, while at the same timing doing remarkably new things such as gamma-ray imaging and spectroscopy. These Nuggets (both the old series and our current Wiki series) provide many glimpses into these discoveries.
Where does RHESSI fit in the history of hard X-ray solar science? There were three previous hard X-ray imagers: a rotating modulation collimator on the Hinotori spacecraft; a multi-grid collimator on the Solar Maximum Mission, an array of fixed bigrid collimators on Yohkoh, and now our multiple rotating collimators on RHESSI. These three predecessor instruments were all quite distinct, even though they all relied upon modulation imaging - only now are we beginning to get the technology needed for hard X-ray imaging with mirrors. RHESSI provided tremendous improvements over its predecessors in most observing properties.
RHESSI's proposal history
To get RHESSI into orbit (February 2002, in time to see the previous solar maximum and possibly the next one as well) required a great deal of preparation. Two U.S. laboratories needed to combine their efforts (the Space Sciences Lab at Berkeley, and NASA's Goddard Space Flight Center) with the aid of major Swiss contributions (the Paul Scherrer Institute and ETH.
Several U.S. groups each participated in earlier design studies and/or proposals over the years, each aiming to go beyond the simple scintillation-counter spectrometers towards imaging and spectroscopy. These objectives finally met in RHESSI, but in the meanwhile we could list several items:
The predecessors included most spectacularly the Pinhole/Occulter Facility, an ambitious plan to deploy a 50-meter boom from the cargo bay of the Space Shuttle. The long boom would give high angular resolution and large area, but it would have to be stabilized dynamically with sensors, actuators, and feedback. Such a boom did fly on STS-99, but not with solar X-ray instrumentation. The right-hand panel of Figure 1 shows a more conservative design that looks a lot more like RHESSI, except that the proposals aimed at a medium-sized satellite rather than the SMEX that RHESSI became.
The key people
The two U.S. groups involved in building RHESSI, operating it, and learning from its data are headed by Bob Lin (Berkeley) and Brian Dennis (Goddard); see Figure 2 for pictures of them and of Arnold Benz, who inspired the major Swiss contributions to the program.
Conclusions
Much more could be said about RHESSI's history, but a Nugget really doesn't have the scope necessary. Perhaps one of the principals - maybe somebody pictured in Figure 2 - will write a fuller account. In the meanwhile RHESSI is still going strong; the new solar cycle has not really begun but that too is very interesting. New studies involving RHESSI data continue to appear and amaze us, so the Science Nuggets will continue to appear.
| RHESSI Nugget Date | 27 April 2009 + |
| RHESSI Nugget First Author | Hugh Hudson + |
| RHESSI Nugget Index | 100 + |
