Reuven Ramaty High Energy Solar Spectroscopic Imager

Reuven Ramaty High Energy Solar Spectroscopic Imager

RHESSI spacecraft observing the Sun
Names	Explorer 81 HESSI High Energy Solar Spectroscopic Imager RHESSI SMEX-6
Mission type	Solar observatory
Operator	NASA/Space Sciences Laboratory
COSPAR ID	2002-004A
SATCATno.	27370
Website	RHESSI
Mission duration	2 years (planned) 16 years, 6 months, 10 days (achieved)
Spacecraft properties
Spacecraft	Explorer LXXXI
Spacecraft type	Reuven Ramaty High Energy Solar Spectroscopic Imager
Bus	RHESSI
Manufacturer	Spectrum Astro Inc.
Launch mass	293 kg (646 lb)
Dimensions	2.16 × 5.76 m (7 ft 1 in × 18 ft 11 in)
Power	414watts
Start of mission
Launch date	5 February 2002, 20:58:12UTC
Rocket	Pegasus XL(F31)
Launch site	Cape Canaveral,Stargazer
Contractor	Orbital Sciences Corporation
Entered service	2002
End of mission
Deactivated	16 August 2018
Last contact	11 April 2018
Decay date	20 April 2023 (UTC)
Orbital parameters
Reference system	Geocentric orbit
Regime	Low Earth orbit
Perigee altitude	579 km (360 mi)
Apogee altitude	607 km (377 mi)
Inclination	38.04°
Period	96.50 minutes
Main telescope
Type	Coded aperture mask
Focal length	1.55 m (5 ft 1 in)
Collecting area	150 cm2(23 sq in)
Wavelengths	X-ray/gamma ray(γ-ray)
Resolution	2 arcseconds up to 100 keV 7 arcseconds up to 400 keV 36 arcseconds above 1 MeV
Instruments
Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)
HESSI mission patch Explorer program

Reuven Ramaty High Energy Solar Spectroscopic Imager(RHESSI,originallyHigh Energy Solar Spectroscopic ImagerorHESSIorExplorer 81) was aNASAsolar flareobservatory. It was the sixth mission in theSmall Explorer program(SMEX), selected in October 1997and launched on 5 February 2002, at 20:58:12UTC.Its primary mission was to explore the physics ofparticle accelerationand energy release in solar flares.

The spacecraft re-entered Earth's atmosphere at 00:21 UTC on 20 April 2023, 21 years after its launch.

Spacecraft

HESSI was renamed to RHESSI on 29 March 2002 in honor of Dr.Reuven Ramaty,a pioneer in the area of high energy solar physics. RHESSI was the first space mission named after a NASA scientist.RHESSI was built bySpectrum AstroforGoddard Space Flight Centerand was operated by theSpace Sciences LaboratoryinBerkeley, California.Theprincipal investigatorfrom 2002 to 2012 wasRobert Lin,who was succeeded by Säm Krucker.

The entire spacecraft rotated to provide the necessary signal modulation. The four, fixedsolar panelswere designed to provide enough gyroscopic moment to stabilize rotation about the solar vector. This largely eliminated the need forattitude control.The instrument detectors were nine high-puritygermaniumcrystals. Each was cooled to cryogenic temperatures by a mechanical cryocooler. Germanium provided not only detections by thephotoelectric effect,but inherent spectroscopy through the charge deposition of the incoming ray. The crystals were housed in a cryostat, and mounted with low-conductivity straps. A tubular telescope structure formed the bulk of the spacecraft. Its purpose was to hold the collimators above the Ge crystals at known, fixed positions.

Thesatellite busconsisted of the structure and mechanisms, the power system (including thebattery,solar panels,and control electronics), theattitude control system,thermal control system,command and data handling system(C&DH), andtelecommunicationssystem. The spacecraft structure provided support for the telescope and other components. It was manufactured out ofaluminumparts to be light weight but strong. The equipment platform had a honeycomb structure to further reduce the weight. The spacecraft was manufactured inGilbert, ArizonabySpectrum Astro, Inc.

The Imaging Telescope Assembly consisted of the telescope tube, grid trays, Solar aspect system (SAS), and Roll angle system (RAS). It was constructed, assembled, aligned, and tested at thePaul Scherrer InstituteinSwitzerland.The front and rear grid trays were attached to the telescope tube. It maintained the separation and alignment of the trays. Nine grids were mounted on a grid tray at each end of the telescope tube. The grid pairs modulated the transmission of solar flareX-rayandgamma rayemissions through to the detectors as the spacecraft spins around the axis of the telescope tube. The modulated count rates in the nine detectors were used in computers on the ground to construct images of solar flares in different energy bands. The five coarse grids (square) were constructed by Van Beek Consultancy inNetherlands.The four fine grids (round) were constructed by Thermo Electron Tecomet inMassachusetts.All grids were characterized both optically and with X-rays atGoddard Space Flight Centerbefore being shipped to the Paul Scherrer Institute for integration into the imaging telescope assembly.

Thespectrometercontained ninegermanium detectorsthat were positioned behind the nine grid pairs on the telescope. These artificially grown crystals, pure to over one part in a trillion, were manufactured by theORTEC division of Perkin Elmer Instruments.When they were cooled to cryogenic temperatures and a high voltage was put across them (up to 4000volts), they converted incoming X-rays and gamma-rays to pulses of electric current. The amount of current was proportional to the energy of the photon, and was measured by sensitive electronics designed at theLawrence Berkeley National Laboratoryand theSpace Sciences Laboratory,at Berkeley, California. The detectors were cooled with an electromechanicalStirling-cyclecryocooler built bySunPower Inc.,and flight qualified at Goddard Space Flight Center.It maintained them at the required operating temperature of −198 °C (−324.4 °F), or 75° aboveabsolute zero).

Mission concept

RHESSI was designed to image solar flares inenergetic photonsfrom soft X-rays (~3keV) to gamma rays (up to ~20 MeV) and to provide high resolutionspectroscopyup to gamma-ray energies of ~20 MeV. Furthermore, it had the capability to perform spatially resolved spectroscopy with high spectral resolution.

Scientific objectives

Researchers believe that much of the energy released during a flare is used to accelerate, to very high energies,electrons(emitting primarily X-rays) andprotonsand otherions(emitting primarily gamma rays). The new approach of the RHESSI mission was to combine, for the first time, high-resolution imaging in hard X-rays and gamma rays with high-resolution spectroscopy, so that a detailed energy spectrum could be obtained at each point of the image. This new approach enabled researchers to find out where these particles are accelerated and to what energies. Such information will advance understanding of the fundamental high-energy processes at the core of the solar flare phenomena.

The primary scientific objective of RHESSI was to understand the following processes that take place in the magnetized plasmas of the solar atmosphere during a flare:

• Impulsive energy release
• Particle acceleration
• Particle and energy transport

These high-energy processes play a major role at sites throughout the Universe ranging frommagnetospheresto activegalaxies.Consequently, the importance of understanding these processes transcends the field ofsolar physics;it is one of the major goals ofspace physicsandastrophysics.

The high energy processes of interest include the following:

• The rapid release of energy stored in unstable magnetic configurations
• The equally rapid conversion of this energy into the kinetic energy of hot plasma and accelerated particles (primarily electrons, protons and ions)
• The transport of these particles through the solar atmosphere and into interplanetary space
• The subsequent heating of the ambient solar atmosphere

These processes involve:

• Particle energies to many GeV
• Temperatures of tens or even hundreds of millions of degrees
• Densities as low as 100 million particles persquare cm
• Spatial scales of tens of thousands of kilometers, and
• Magnetic containment times of seconds to hours

It is impossible to duplicate these conditions in laboratories on theEarth.

The acceleration of electrons is revealed by hard X-ray and gamma-raybremsstrahlungwhile the acceleration of protons and ions is revealed by gamma-ray lines and continuum. The proximity of the Sun means not only that these high-energy emissions are orders of magnitude more intense than from any other cosmic source but also that they can be better resolved, both spatially and temporally.

Imaging

Since X-rays are not easily reflected or refracted, imaging in X-rays is difficult. One solution to this problem is to selectively block the X-rays. If the X-rays are blocked in a way that depends on the direction of the incoming photons, then it may be possible to reconstruct an image. The imaging capability of RHESSI was based on aFourier transformtechnique using a set of 9Rotational Modulation Collimators(RMCs) as opposed to mirrors and lenses. Each RMC consisted of two sets of widely spaced, fine-scale linear grids. As the spacecraft rotated, these grids blocked and unblocked any X-rays which may have come from theSunmodulating thephotonsignal in time. The modulation could be measured with a detector having no spatial resolution placed behind the RMC since the spatial information was now stored in the time domain. The modulation pattern over half a rotation for a single RMC provided the amplitude and phase of many spatial Fourier components over a full range of angular orientations but for a small range of spatial source dimensions. Multiple RMCs, each with different slit widths, provided coverage over a full range of flare source sizes. Images were then reconstructed from the set of measured Fourier components in exact mathematical analogy to multi-baseline radio interferometry. RHESSI provided spatial resolution of 2arcsecondsat X-ray energies from ~4 keV to ~100 keV, 7 arcseconds to ~400 keV, and 36 arcseconds for gamma-ray lines and continuum emission above 1 MeV.

RHESSI could also see gamma rays coming from off-solar directions. The more energetic gamma rays passed through the spacecraft structure, and impacted the detectors from any angle. This mode was used to observegamma-ray bursts(GRBs). The incoming gamma rays were not modulated by the grids, so positional and imaging information was not recorded. However, a crude position could still be derived by the fact that the detectors had front and rear pickups. Also, the detectors near the burst shielded the ones away from the burst. Comparing signal strengths around the nine crystals, and front-to-back, then gave a coarse, two-dimensional position in space.

When combined with high-resolution time stamps of the detector hits, the RHESSI solution could becross-referencedon the ground with other spacecraft in the IPN (Interplanetary Network) to provide a fine solution. The large area and high sensitivities of the germanium crystal assembly made RHESSI a formidable IPN component. Even when other spacecraft could provide burst locations, few could provide as high-quality spectra of the burst (in both time and energy) as RHESSI. Rarely, however, a GRB occurred near the Sun, in the collimated field of view. The grids then provided full information, and RHESSI was able to provide a fine GRB location even without IPN correlation.

Experiment

Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)

RHESSI was intended to image at high resolution solar flares in X-rays and gamma rays. The X-rays and gamma rays covered an energy range of 3 keV-20 MeV with an energy resolution of about 1 keV and a spatial resolution of just a few seconds of arc. The imaging was accomplished by a 45 × 170 cm (18 × 67 in) tube containing nine pairs (one behind the other, spaced 1.5 m (4 ft 11 in) apart) oftungstenormolybdenumwire grids of width 9 cm (3.5 in) mounted parallel to the rotation axis of the tube pointing at the Sun. The tube rotated about its axis as the spacecraft spun at a rate of 15rpm.During a rotation, a photon from any point on the Sun could either pass through a grid-pair or be blocked by one or other of the grids. This caused a modulation of the intensity of photons emanating from that point. The depth of modulation was zero for the photons arriving exactly along the spin axis and gradually increases to the off-axis photons. Behind each grid-pair was a cryogenic (75 K) germanium detector of 7.1 cm diameter and 8.5 cm (3.3 in) thickness. The output from each of the nine detectors, at any given energy, could be Fourier-analyzed to provide a full two-dimensional spatial spectrum of an extended source region on the Sun. The full spatial spectrum was possible because each wire grid pair had a different slit width, spacing and wire thickness. Data accumulation was about 16Gbduring a 10-minutes rotation. Thetelemetrydata was collected at Berkeley (California),Wallops Flight Facility(WFF),Virginia,Santiago,Chileand Weilheim,Germany.Science analysis of the data involved close collaboration with many dedictated ground based and satellite based solar observatories. A secondary goal of RHESSI was to observe astronomical sources such asCrab Nebula.

Results

RHESSI observations changed our perspective on solar flares, particularly on high-energy processes in flares. Its observations led to numerous publications in scientific journals and presentations at conferences. Through 2017, the satellite was referenced in 2,474 publications, books, and presentations.The collection "High-Energy Aspects of Solar Flares: A RHESSI Inspired Monograph"contains reviews of RHESSI solar flare research up to 2011.

• RHESSI was the first satellite to image gamma rays from a solar flare.
• It was the first satellite to accurately measureterrestrial gamma-ray flashesthat come fromthunderstorms,and it found that such flashes occur more often than thought and the gamma rays have a higherfrequencyon average than the average for cosmic sources.

Following communication difficulties, RHESSI ceased science operations on 11 April 2018 at 01:50UTC.It was decommissioned on 16 August 2018and remained in a stablelow Earth orbitwhich gradually decayed due to atmospheric drag.

See also

• Explorers program

External links

• RHESSI websiteArchived26 June 2019 at theWayback Machineby NASA's Goddard Space Flight Center
• HESSI archive websiteby theSpace Sciences Laboratory
• RHESSI data browserby the Space Sciences Laboratory
• RHESSI Science Nuggets,biweekly series explaining recent science results