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

RHESSI (the Reuven Ramaty High-Energy Solar Spectroscopic Imager) had a long and tortured gestation period from the initial concept perhaps as long ago as the 1970's to its final selection as a Small Explorer (SMEX) mission in 1997 and its subsequent delayed launch in 2002. We take the opportunity of RHESSI Science Nugget number 100 to summarize the many steps along the way.

Hard X-ray Imaging

The original observational goals that ultimately lead to RHESSI were high angular resolution hard X-ray imaging and fine energy resolution X-ray and gamma-ray spectroscopy of solar flares. The early instruments tackled one or the other of these objectives. The Solar Maximum Mission, launched in 1980, carried the Hard X-ray Imaging Spectrometer (HXIS), the first instrument capable of imaging in X-rays up to 30 keV with about 10 arcsecond resolution. Hinotori followed soon thereafter using rotating modulation collimators to achieve similar angular resolution but greater sensitive area. And then of course, the Hard X-Ray Telescope (HXT) on Yohkoh in the 1990's improved on the angular resolution and extended the energy range to close to 100 keV. The ambitious Pinhole Occulter Facility (P/OF - pronounced "poff") was proposed in the 1980s with a boom that could be extended to 50 m to achieve 0.1 arcsecond resolution up to 100 keV. It was initially proposed for the Space Shuttle (see figure) and later for the Space Station but was never selected for funding. The instrument that was used to test out the rotating modulation collimator concept and the solar aspect system used on RHESSI was the balloon-borne High Energy Imaging Device (HEIDI) built at Goddard and flown once from Texas in 1993.

Gamma-Ray Spectroscopy

The first gamma-ray spectroscopy of flares was achieved on OSO-3 using sodium iodide NaI(Tl) scintillation detectors, and similar observations were made with the Gamma-Ray Spectrometer (GRS) on SMM in the 1980s. The first high resolution gamma-ray measurements using germanium detectors were made on HEAO-1 but the instrument that evolved into RHESSI's high spectral resolution capability was the balloon-borne X-ray spectrometer with cooled germanium detectors. This was flown several times from the Antarctic and made seminal observations of the "super-hot component" and micro-flares and galactic center X-ray sources. It was supported and reflown as part of the NASA funded Max'91 program that also supported HEIDI.

Combined Imaging and Spectroscopy After the idea of putting P/OF on an in-orbit refurbishment of the SMM spacecraft was rejected, the Solar High-Energy Astrophysical Plasmas Explorer (SHAPE) proposal was submitted in 1986. This carried both a Gamma-Ray Imaging Device (GRID) with 34 RMCs and grids separated by 6.7 m for arcsecond imaging and a High-resolution Gamma-Ray and Neutron Spectrometer (HIGRANS) with 12 dual-segment high-purity germanium detectors and a 5-cm thick BGO shield. Unfortunately, the Challenger accident occurred about this time and all new missions were put on hold. By the time the review panel made the selection it was deem to be too late to make the solar maximum of 1990. However, the HEIDI and

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.

Figure 1: Two of the RHESSI predecessors - P/OF and HESP.

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. The Berkeley and Goddard groups persevered, and the objectives of imaging and spectroscopy together finally met in the form of RHESSI. In the meanwhile there were memorable milestones on the way:

The HEIDI payload had two RMCs with 25 arcsecond and 11 arcsecond angular resolution. It could detect photons up to 700 keV. It flew for the first time on June 22, 1993.

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. In the middle, behind Bob, one can catch a glimpse of the 11-m telemetry dish of the Berkeley Ground Station in the hills behind the Space Sciences Lab.

Figure 2: Three key RHESSI people. From left to right: Brian Dennis, Bob Lin, Arnold Benz.

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.

References

Imaging Solar Flare Gamma-Rays and Hard X-rays with The Gamma Ray Imaging Device (GRID) on a Balloon

Results from the First Flight of the High Energy Imaging Device (HEIDI) Balloon Payload

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