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Reuven Ramaty High Energy Solar Spectroscopic Imager

Reuven Ramaty High Energy Solar Spectroscopic Imager

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Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI, or more rarely Explorer 81) is the sixth mission in the line of NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 Small Explorer missions (also known as SMEX). Launched on 5 February 2002, its primary mission is to explore the basic physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 of particle acceleration
Particle acceleration
In a compressible sound transmission medium - mainly air - air particles get an accelerated motion: the particle acceleration or sound acceleration with the symbol a in metre/second². In acoustics or physics, acceleration is defined as the rate of change of velocity. It is thus a vector...

 and explosive energy release in solar flares.

HESSI was renamed to RHESSI on March 29, 2002 in honor of Reuven Ramaty
Reuven Ramaty
Reuven Ramaty was a pioneer in the fields of solar physics, gamma-ray astronomy, nuclear astrophysics, and cosmic rays. He was a HESSI Co-Investigator and one of the founding members of the HESSI team. His active involvement and enthusiastic support were critical for HESSI’s selection by NASA as...

, a pioneer in the area of high energy solar physics - RHESSI is the first space mission named after a NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 scientist. RHESSI was designed and is operated at the Space Sciences Laboratory
Space Sciences Laboratory
The Space Sciences Laboratory is an Organized Research Unit of the University of California, Berkeley. It is located in the Berkeley Hills above the university campus...

 in Berkeley
Berkeley, California
Berkeley is a city on the east shore of the San Francisco Bay in Northern California, United States. Its neighbors to the south are the cities of Oakland and Emeryville. To the north is the city of Albany and the unincorporated community of Kensington...

 California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

.

Mission concept


RHESSI is 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 up to gamma-ray energies of ~20 MeV. Furthermore, it has 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) and protons and other ions (emitting primarily gamma rays). The new approach of the RHESSI mission is 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 will enable 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 is 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 magnetospheres to active galaxies. Consequently, the importance of understanding these processes transcends the field of solar physics; it is one of the major goals of space physics and astrophysics.

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.

The acceleration of electrons is revealed by hard X-ray and gamma-ray bremsstrahlung 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 is based on a Fourier-transform technique using a set of 9 Rotational Modulation Collimator
Rotational Modulation Collimator
Rotational Modulation Collimators are a specialization of the modulation collimator, an imaging device invented by Minoru Oda. Devices of this type create images of high energy X-rays . Since high energy X-rays are not easily focused, such optics have found applications in various instruments...

s (RMCs) as opposed to mirrors and lenses. Each RMC consist of two sets of widely-spaced, fine-scale linear grids. As the spacecraft rotates, these grids block and unblock any X-rays which may be coming from the Sun modulating the photon signal in time. The modulation can be measured with a detector having no spatial resolution placed behind the RMC since the spatial information is now stored in the time domain. The modulation pattern over half a rotation for a single RMC provides 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, provide coverage over a full range of flare source sizes. Images are then reconstructed from the set of measured Fourier components in exact mathematical analogy to multi-baseline radio interferometry.

RHESSI provides spatial resolution of 2 arcseconds 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 can also see gamma rays coming from off-solar directions. The more energetic gamma rays pass through the spacecraft structure, and impact the detectors from any angle. This mode is used to observe gamma ray burst
Gamma ray burst
Gamma-ray bursts are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical...

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 shield 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
Multilateration
Multilateration is a navigation technique based on the measurement of the difference in distance to two or more stations at known locations that broadcast signals at known times. Unlike measurements of absolute distance or angle, measuring the difference in distance results in an infinite number of...

 on the ground with other spacecraft in the IPN (Interplanetary Network) to provide a fine solution. The large area and high sensitivities of the Ge crystal assembly make 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 occurs near the Sun, in the collimated field of view. The grids then provide full information, and RHESSI can provide a fine GRB location even without IPN correlation.

Spacecraft and instrument


The entire spacecraft rotates to provide the necessary signal modulation. The four, fixed solar panels are designed to provide enough gyroscopic moment to stabilize rotation about the solar vector. This largely eliminates the need for attitude control.

The instrument detectors are nine high-purity germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....

 crystals. Each is cooled to cryogenic temperatures by a mechanical cryocooler. Germanium provides not only detections by the photoelectric effect
Photoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...

, 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 forms the bulk of the spacecraft. Its purpose is to hold the collimators above the Ge crystals at known, fixed positions.

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. As of September 2008, RHESSI is mentioned in 970 publications, books, and presentations (as listed on NASA ADS). Between February 2006 to 2008, 200 publications have been published about RHESSI observations.

Some significant RHESSI results follow:
  • RHESSI was the first satellite
    Satellite
    In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

     to image solar gamma rays from a solar flare
    Solar flare
    A solar flare is a sudden brightening observed over the Sun surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy . The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day...

    .

  • RHESSI was the first satellite
    Satellite
    In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

     to accurately measure terrestrial gamma-ray flashes
    Terrestrial gamma-ray flash
    Terrestrial gamma-ray flashes are bursts of gamma rays in the Earth's atmosphere. TGFs have been recorded to last 0.2 to 3.5 milliseconds, and have energies of up to 20 MeV. They are probably caused by electric fields produced above thunderstorms...

     that come from thunder storms, and RHESSI found that such flashes occur more often than thought and the gamma rays have a higher frequency
    Frequency
    Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

     on average than the average for cosmic
    COSMIC
    Constellation Observing System for Meteorology, Ionosphere, and Climate is a program designed to provide advances in meteorology, ionospheric research, climatology, and space weather by using GPS satellites in conjunction with low Earth orbiting satellites...

     sources.

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