Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI, originally High Energy Solar Spectroscopic Imager or HESSI or Explorer 81) was a

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...

solar flare A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other sol ...

observatory. It was the sixth mission in the

Small Explorer program The Explorers program is a NASA exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was the first spacecraft of the United Stat ...

(SMEX), selected in October 1997 and launched on 5 February 2002, at 20:58:12 UTC. Its primary mission was to explore the physics of particle acceleration and energy release in solar flares.

Spacecraft

HESSI was renamed to RHESSI on 29 March 2002 in honor of Dr. 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 by

Spectrum Astro Spectrum Astro was a privately held American corporation that designed, manufactured, and sold satellites. Spectrum Astro's, at the time, unique satellite production method was to design and manufacture space satellites, in their entirety, at a ...

for

Goddard Space Flight Center The Goddard Space Flight Center (GSFC) is a major NASA space research laboratory located approximately northeast of Washington, D.C. in Greenbelt, Maryland, United States. Established on May 1, 1959 as NASA's first space flight center, GSFC empl ...

and was operated by the Space Sciences Laboratory in

Berkeley, California Berkeley ( ) is a city on the eastern shore of San Francisco Bay in northern Alameda County, California, United States. It is named after the 18th-century Irish bishop and philosopher George Berkeley. It borders the cities of Oakland and E ...

. The

principal investigator In many countries, the term principal investigator (PI) refers to the holder of an independent grant and the lead researcher for the grant project, usually in the sciences, such as a laboratory study or a clinical trial. The phrase is also often u ...

from 2002 to 2012 was

Robert Lin Robert Peichung Lin (; January 24, 1942 – November 17, 2012) was Chinese-born American astrophysicist. He was a professor and director of the Space Sciences Laboratory (1998–2008) at the University of California, Berkeley. As a pioneer of ...

, who was succeeded by Säm Krucker. The entire spacecraft rotated to provide the necessary signal modulation. The four, fixed

solar panels A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a photo ...

were designed to provide enough gyroscopic moment to stabilize rotation about the solar vector. This largely eliminated the need for

attitude control Attitude control is the process of controlling the orientation of an aerospace vehicle with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, etc. Controlling vehicle ...

. The instrument detectors were nine high-purity

germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors ...

crystals. Each was cooled to cryogenic temperatures by a mechanical cryocooler. Germanium provided not only detections by the

photoelectric effect The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...

, but inherent spectroscopy through the charge deposition of the incoming ray. The crystals are 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. The

satellite bus A satellite bus (or spacecraft bus) is the main body and structural component of a satellite or spacecraft, in which the payload and all scientific instruments are held. Bus-derived satellites are opposed to specially produced satellites. Bus- ...

consists of the structure and mechanisms, the power system (including the

battery Battery most often refers to: * Electric battery, a device that provides electrical power * Battery (crime), a crime involving unlawful physical contact Battery may also refer to: Energy source *Automotive battery, a device to provide power t ...

solar panel A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a photo ...

s, and control electronics), the

attitude control system Spacecraft attitude control is the process of controlling the orientation of a spacecraft (vehicle/satellite) with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, et ...

, thermal control, command and data handling (C&DH), and

telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that ...

. The spacecraft structure shown here provides support for the telescope and other components. It was manufactured out of

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

parts to be light weight but strong. The equipment platform has a honeycomb structure to further reduce the weight. The spacecraft was manufactured in

Gilbert, Arizona Gilbert is a town in Maricopa County, Arizona, United States, located southeast of Phoenix within the city's metropolitan area. Incorporated on July 6, 1920, Gilbert was once known as the "Hay Shipping Capital of the World". It is the fifth-lar ...

by Spectrum Astro, Inc. The Imaging Telescope Assembly consists of the telescope tube, grid trays, Solar aspect system (SAS), and Roll angle system (RAS). It was constructed, assembled, aligned, and tested at the

Paul Scherrer Institute The Paul Scherrer Institute (PSI) is a multi-disciplinary research institute for natural and engineering sciences in Switzerland. It is located in the Canton of Aargau in the municipalities Villigen and Würenlingen on either side of the River ...

Switzerland ). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...

. The front and rear grid trays are attached to the telescope tube. It maintains the separation and alignment of the trays. Nine grids are mounted on a grid tray at each end of the telescope tube. The grid pairs modulate the transmission of solar flare

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

and

gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

emissions through to the detectors as the spacecraft spins around the axis of the telescope tube. The modulated count rates in the nine detectors are 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 in

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. The four fine grids (round) were constructed by Thermo Electron Tecomet in

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. All grids were characterized both optically and with X-rays at

before being shipped to the Paul Scherrer Institute for integration into the imaging telescope assembly. The

spectrometer A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...

contains nine germanium detectors that are positioned behind the nine grid pairs on the telescope. These artificially grown crystals, pure to over one part in a trillion, were manufactured by the ORTEC division of Perkin Elmer Instruments. When they are cooled to cryogenic temperatures and a high voltage is put across them (up to 4000

volt The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defin ...

s), they convert incoming X-rays and gamma-rays to pulses of electric current. The amount of current is proportional to the energy of the photon, and is measured by sensitive electronics designed at the Lawrence

Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory (LBNL), commonly referred to as the Berkeley Lab, is a United States national laboratory that is owned by, and conducts scientific research on behalf of, the United States Department of Energy. Located in ...

and the Space Sciences Laboratory, at Berkeley, California. The detectors are cooled with an electromechanical Stirling-cycle cryocooler (built by SunPower Inc., and flight qualified at Goddard Space Flight Center). It maintains them at the required operating temperature of , or 75° above

absolute zero Absolute zero is the lowest limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reach their minimum value, taken as zero kelvin. The fundamental particles of nature have minimum vibra ...

Mission concept

RHESSI was designed to image solar flares in energetic photons from soft X-rays (~3 keV) to gamma rays (up to ~20 MeV) and to provide high resolution

spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter ...

up 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,

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...

s (emitting primarily X-rays) and

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

s and other ions (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 can 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 problem. 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 from

magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior d ...

s to active

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System ...

. Consequently, the importance of understanding these processes transcends the field of solar physics; it is one of the major goals of

space physics Space physics, also known as solar-terrestrial physics or space-plasma physics, is the study of plasmas as they occur naturally in the Earth's upper atmosphere ( aeronomy) and within the Solar System. As such, it encompasses a far-ranging number of ...

and

astrophysics Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline said, Astrophysics "seeks to ascertain the nature of the h ...

. 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 per square 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 the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

. The acceleration of electrons is revealed by hard X-ray and gamma-ray

bremsstrahlung ''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typical ...

while 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 a

Fourier transform A Fourier transform (FT) is a mathematical transform that decomposes functions into frequency components, which are represented by the output of the transform as a function of frequency. Most commonly functions of time or space are transformed ...

technique using a set of 9 Rotational 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 the Sun modulating the

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...

signal 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 2

arcseconds A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The ...

at 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 observe

gamma-ray burst In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant galaxies. They are the most energetic and luminous electromagnetic events since the Big Bang. Bursts can last from ten millise ...

s (GRBs). The incoming gamma rays are not modulated by the grids, so positional and imaging information is not recorded. However, a crude position can still be derived by the fact that the detectors have 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 gives a coarse, two-dimensional position in space. When combined with high-resolution time stamps of the detector hits, the RHESSI solution can be cross-referenced on 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 can provide burst locations, few can 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 is intended to image at high resolution solar flares in X-rays and gamma rays. The X-rays and gamma rays covers 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 is accomplished by a tube containing nine pairs (one behind the other, spaced apart) of

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...

molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ...

wire grids of width mounted parallel to the rotation axis of the tube pointing at the Sun. The tube rotates about its axis as the spacecraft spins at a rate of 15 rpm. During a rotation, a photon from any point on the Sun can either pass through a grid-pair or be blocked by one or other of the grids. This causes a modulation of the intensity of photons emanating from that point. The depth of modulation is zero for the photons arriving exactly along the spin axis and gradually increases to the off-axis photons. Behind each grid-pair is a cryogenic (75 K) germanium detector of 7.1 cm diameter and thickness. The output from each of the nine detectors, at any given energy, can be Fourier-analyzed to provide a full two-dimensional spatial spectrum of an extended source region on the Sun. The full spatial spectrum is possible because each wire grid pair has a different slit width, spacing and wire thickness. Data accumulaton is about 16 Gb during a 10-minutes rotation. The

telemetry Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', "remote", an ...

data was collected at Berkeley (California),

Wallops Flight Facility Wallops Flight Facility (WFF) is a rocket launch site on Wallops Island on the Eastern Shore of Virginia, United States, just east of the Delmarva Peninsula and approximately north-northeast of Norfolk. The facility is operated by the Godda ...

(WFF),

Virginia Virginia, officially the Commonwealth of Virginia, is a state in the Mid-Atlantic and Southeastern regions of the United States, between the Atlantic Coast and the Appalachian Mountains. The geography and climate of the Commonwealth are ...

Santiago Santiago (, ; ), also known as Santiago de Chile, is the capital and largest city of Chile as well as one of the largest cities in the Americas. It is the center of Chile's most densely populated region, the Santiago Metropolitan Region, whos ...

Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the eas ...

and Weilheim,

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...

. Science analysis of the data was involved close collaboration with many dedictated ground based and satellite based solar observatories. A secondary goal of RHESSI is to observe astronomical sources such as

Crab Nebula The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus. The common name comes from William Parsons, 3rd Earl of Rosse, who observed the object in 1842 u ...

Results

RHESSI observations have changed our perspective on solar flares, particularly on high-energy processes in flares. RHESSI observations has led to numerous publications in scientific journals and presentations at conferences. Through 2017, RHESSI is referenced in 2,474 publications, books, and presentations. * RHESSI was the first satellite to image gamma rays from a solar flare. * RHESSI was the first satellite to accurately measure terrestrial gamma-ray flashes that come from

thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are some ...

s, and RHESSI found that such flashes occur more often than thought and the gamma rays have a higher

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

on average than the average for cosmic sources. Following communication difficulties, RHESSI ceased science operations on 11 April 2018 at 01:50 UTC. RHESSI was decommissioned on 16 August 2018, and remains in a stable

low Earth orbit A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never m ...

. However, since it has no means of propulsion, atmospheric drag will eventually pull the spacecraft into

atmosphere of Earth The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...

, which may occur as early as 2022.