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.

The two U.S. groups each had participated in earlier proposals and design studies that we

The key people

Conclusions

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