The Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment

The Diviner Lunar Radiometer Experiment on NASA’s Lunar Reconnaissance Orbiter will be the first instrument to systematically map the global thermal state of the Moon and its diurnal and seasonal variability. Diviner will measure reflected solar and emitted infrared radiation in nine spectral channels with wavelengths ranging from 0.3 to 400 microns. The resulting measurements will enable characterization of the lunar thermal environment, mapping surface properties such as thermal inertia, rock abundance and silicate mineralogy, and determination of the locations and temperatures of volatile cold traps in the lunar polar regions.     

Physical mechanisms for turbulent dissipation in collisionless shock waves

Temporal and spectral characteristics of solar hard X-ray bursts are briefly reviewed. The merits of non-thermal and thermal flare models are discussed. The validity of these models may be checked by future measurements of X-ray polarization. Finally, some important results of recent satellite experiments are described providing information on the spatial distribution of hard X-ray sources: the multi-spacecraft observation of X-ray bursts and the imaging of X-ray sources by means of the HXIS instrument.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

The spacelab Lyman alpha and white light coronagraphs program

The Harvard-Smithsonian Center for Astrophysics and the High Altitude Observatory have defined a joint coronagraphs experiment for a future Spacelab mission. The instrumentation package would include an ultraviolet light coronagraph to measure the intensity and profiles of spectral lines formed between 1.2 and 8 solar radii from Sun center and a white light coronagraph to measure the intensity and polarization of visible light. The overall goals of the joint program are to use new coronal plasma diagnostic techniques to understand the physical processes and mechanisms operating in the solar corona, to understand the acceleration of high-speed and low-speed solar wind streams and to extrapolate this knowledge to other stars in order to help understand the physics of stellar coronae and stellar mass loss.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

The GRASP telescope

The GRASP mission Gamma-Ray Astronomy with Spectroscopy and Positioning addresses the scientific goals of fine spectroscopy with imaging and accurate positioning of gamma-ray sources, an unexplored area within gamma-ray astronomy. The assessment of GRASP as a future space astronomy mission in the mid-1990s has led to the design of the instrument outlined in this article. Thus GRASP is a third generation gamma-ray telescope and is designed to operate as a high quality spectral imager in the mid-1990s, when, following the GRO, SIGMA, and GAMMA-1 missions, there will be requirement for a more sophisticated instrument to maintain the momentum of advance in gamma-ray astronomy. The telescope will be capable of locating point sources with a precision of typically 1 arc min, whilst making a fine spectral analysis (E/E 1000) of any gamma-ray line features. The high sensitivity of this instrument and the long (> 2 year) lifetime of the mission will enable a large number ( 1000) of astronomical objects to be studied. The GRASP mission has the potential to move gamma-ray astronomy from an era of basic exploration to one in which detailed and novel measurements can be used to gain a better understanding of many astrophysical problems.     

Galileo Net Flux Radiometer experiment

The Galileo Net Flux Radiometer (NFR) is a Probe instrument designed to measure the vertical profile of upward and net radiation fluxes in five spectral bands spanning the range from solar to far infrared wavelengths. These unique measurements within Jupiter's atmosphere, from which radiative heating and cooling profiles will be derived, will contribute to our understanding of Jovian atmospheric dynamics, to the detection of cloud layers and determination of their opacities, and to the estimation of water vapor abundance. The NFR uses an array of pyroelectric detectors and individual bandpass filters in a sealed detector package. The detector package and optics rotate as a unit to provide chopping between views of upward and downward radiation fluxes. This arrangement makes possible the measurement of small net fluxes in the presence of large ambient fluxes. A microprocessor-controlled electronics package handles instrument operation.     

Solar microwave bursts — A review

We review the observational and theoretical results on the physics of microwave bursts that occur in the solar atmosphere. We particularly emphasize the advances made in burst physics over the last few years with the great improvement in spatial and time resolution especially with instruments like the NRAO three element interferometer, Westerbork Synthesis Radio Telescope and more recently the Very Large Array (VLA).We review the observations on pre-flare build-up of an active region at centimeter wavelengths. In particular we discuss the observations that in addition to the active region undergoing brightness and polarization changes on time scales of the order of an hour before a flare, there can be a change of the sense of polarization of a component of the relevant active region situated at the same location as the flare, implying the emergence of a flux of reverse polarity at coronal levels. The intensity distribution of cm- bursts is similar to that of soft X-ray and hard X-ray bursts. Indeed, it appears that the flaring behavior of the Sun at cm wavelengths is similar to that of some other cosmic transients such as flare stars and X-ray bursters.We discuss three distinct phases in the evolution of cm bursts, namely, impulsive phase, post-burst phase, and gradual rise and fall. The radiation mechanism for the impulsive phase of the microwave burst is gyrosynchrotron emission from mildly relativistic electrons that are accelerated near the energy release site and spiral in the strong magnetic field in the low corona. The details of the velocity distribution function of the energetic electrons and its time evolution are not known. We review the spectral characteristics for two kinds of velocity distribution, e.g., Maxwellian and Maxwellian with a power law tail for the energetic electrons. In the post-burst phase the energetic electrons are gradually thermalized. The thermal plasma released in the energy release region as well as the expanded parts of the overheated upper chromosphere may alter the emission mechanism. Thus, in the post-burst phase, depending on the average density and temperature of the thermal plasma, the emission mechanism may change from gyrosynchrotron to collisional bremsstrahlung from a thermal plasma. The gradual rise and fall (GFR) burst represents the heating of a flare plasma to temperatures of the order of 10⁶ K, in association with a flare or an X-ray transient following a filament disruption.We discuss the flux density spectra of centimeter bursts. The great majority of the bursts have a single spectral maximum, commonly around 6 cm- The U-shaped signature sometimes found in cm-dcm burst spectrum of large bursts is believed to a be a reflection of only the fact that there are two different sources of burst radiation, one for cm- and the other for dcm-, with different electron energy distributions and different magnetic fields.Observations of fine structures with temporal resolutionof 10–100 ms in the intensity profiles of cm- bursts are described. The existence of such fine time structures imply brightness temperatures in burst sources of order 10¹⁵ K; their interpretation in terms of gyrosynchrotron measuring or the coherent interaction of upper hybrid waves excited by percipitating electron beams in a flaring loop is discussed.High spatial resolution observations (a few seconds of arc to 1 arc) are discussed, with special reference to the one- and two-dimensional maps of cm burst sources. The dominance of one sense of circular polarization in some weak 6 cm bursts and its interpretation in terms of energetic electrons confined in an asymmetric magnetic loop is discussed. Two-dimensional snapshot maps obtained with the VLA show that multi-peak impulsive 6 cm burst phase radiation originates from several arcades of loops and that the burst source often occupies a substantial portion of the flaring loop, and is not confined strictly to the top of the loop. This phenomenon is interpreted in terms of the trapping of energetic electrons due to anomalous doppler resonance instability and the characteristic scale length of the magnetic field variation along the loop. The VLA observations also indicate that the onset of the impulsive phase of a 6 cm burst can be associated with the appearance of a new system of loops. The presence of two loop systems with opposite polarities or a quadrupole field configuration is reminiscent of flare models in which a current sheet develops in the interface between two closed loops.We provide an extensive review of the emission and absorption processes in thermal and non-thermal velocity distributions. Unlike the thermal plasma where absorption and emission are inter-related through Kirchoff's law, the radiation emitted from a small population of non-thermal electrons can be reabsorbed from the same electrons (self-absorption) or from the background (thermal) electrons through gyro-resonance absorption, and free-free absorption. We also suggest that the non-thermal electrons can be unstable and these instabilities can be the source of very high brightness temperature, fine structure ( 10 ms) pulsations.Finally in the last part of this review we present several microwave burst models-the magnetic trap model, the two-component model, thermal model and the flaring loop model and give a critical discussion of the strength and weakness of these models.     

Variations of Solar Ultraviolet Irradiance Observed by the UARS SOLSTICE — 1991 to 1999

The Solar Stellar Irradiance Comparison Experiment (SOLSTICE) is one of ten instruments on the Upper Atmosphere Research Satellite (UARS) — one of two instruments measuring the solar ultraviolet irradiance. The instrument is a three channel spectrometer covering the spectral range 120 to 420nm with a spectral resolution of approximately 0.2nm. It has been successfully operating since October 1991, and has now provided more than eight years of data, extending from near the peak of solar cycle 22, through solar minimum and into the new cycle. The data provide time series that display solar variations over time scales from a few days up to the 11-year solar cycle. Quantitative estimates of amplitudes of both rotational modulation and the solar cycle variation in the 1991–1999 epoch are given for the UV spectrum between 119 and 300nm.     

Probing the solar wind acceleration region using spectroscopic techniques

Measurements of the intensities and profiles of UV and EUV spectral lines can provide a powerful tool for probing the physical conditions in the solar corona out to 8 R and beyond. We discuss here how measurements of spectral line radiation in conjunction with measurements of the white light K-corona can provide information on electron, proton and ion temperatures and velocity distribution functions; densities; chemical abundances and mass flow velocities. Because of the fundamental importance of such information, we provide a comprehensive review of the formation of coronal resonance line radiation, with particular emphasis on the H i L line, and discuss observational considerations such as requirements for rejection of stray light and effects of emission from the geocorona and interplanetary dust. Finally, we summarize some results of coronal H i L and white light observations acquired on sounding rocket flights.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

Coupling of the coronal he abundance to the solar wind

Models of the transition region — corona — solar wind system are investigated in order to find the coronal helium abundance and to study the role played by coronal helium in controlling the the solar wind proton flux. The thermal force on -particles in the transition region sets the flow of helium into the corona. The frictional coupling between -particles and protons and/or the electric polarization field determines the proton flux in the solar wind as well as the fate of the coronal helium content.     

The Lightning And Radio Emission Detector (LRD) instrument

The Lightning and Radio Emission Detector (LRD) instrument will be carried by the Galileo Probe into Jupiter's atmosphere. The LRD will verify the existence of lightning in the atmosphere and will determine the details of many of its basic characteristics. The instrument, operated in its magnetospheric mode at distances of about 5, 4, 3, and 2 planetary radii from Jupiter's center, will also measure the radio frequency (RF) noise spectrum in Jupiter's magnetosphere. The LRD instrument is composed of a ferritecore radio frequency antenna ( 100 Hz to 100 kHz) and two photodiodes mounted behind individual fisheye lenses. The output of the RF antenna is analyzed both separately and in coincidence with the optical signals from the photodiodes. The RF antenna provides data both in the frequency domain (with three narrow-band channels, primarily for deducing the physical properties of distant lightning) and in the time domain with a priority scheme (primarily for determining from individual RF waveforms the physical properties of closeby-lightning).     

A high-resolution Ge spectrometer for Gamma-Ray burst astronomy

The Transient Gamma-Ray Spectrometer (TGRS) to be flown aboard the WIND spacecraft is primarily designed to perform high resolution spectroscopy of transient -ray events, such as cosmic -ray bursts and solar flares over the energy range 25 keV to 8.2 MeV with an expected spectroscopic resolution of 3 keV at 1 MeV. The detector itself consists of a 215 cm³ high purityn-type Ge crystal kept at cryogenic temperatures by a passive radiative cooler. The geometric field of view defined by the cooler is 1.8 steradian. To avoid continuous triggers by soft solar events, a thin BeCu Sun-shield around the sides of the cooler has been provided. A passive Mo/Pb occulter, which modulates signals from within ±5° of the ecliptic plane at the spacecraft spin frequency, is used to identify and study solar flares, as well as emission from the galactic plane and center. Thus, in addition to transient event measurements, the instrument will allow the search for possible diffuse background lines and monitor the 511 keV positron annihilation radiation from the galactic center. In order to handle the typically large burst count rates, which can be in excess of 100 kHz, burst data are stored directly in an onboard 2.75 Mbit burst memory with an absolute timing accuracy of ±1.5 ms after ground processing. The memory is capable of storing the entire spectral data set of all but the largest bursts. WIND is scheduled to be launched on a Delta II launch vehicle from Cape Canaveral on November 1, 1994. After injection into a phasing orbit, the spacecraft will execute a double lunar swing-by before being moved into a controlled halo orbit about theL1 Lagrangian point (250R _e towards the Sun). This will provide a 5 light-second light travel time with which to triangulate gamma-ray burst sources with Earth-orbiting systems, such as those on-board the Gamma-Ray Observatory (GRO). The response of instrument to transient -ray events such as GRB's and solar flares will be presented as well as the expected response to steady state point sources and galactic center line emission.     

X-Ray Spectroscopy of Galaxy Clusters: Beyond the CIE Modeling

X-ray spectra of galaxy clusters are dominated by the thermal emission from the hot intracluster medium. In some cases, besides the thermal component, spectral models require additional components associated, e.g., with resonant scattering and charge exchange. The latter produces mostly underluminous fine spectral features. Detection of the extra components therefore requires high spectral resolution. The upcoming X-ray missions will provide such high resolution, and will allow spectroscopic diagnostics of clusters beyond the current simple thermal modeling. A representative science case is resonant scattering, which produces spectral distortions of the emission lines from the dominant thermal component. Accounting for the resonant scattering is essential for accurate abundance and gas motion measurements of the ICM. The high resolution spectroscopy might also reveal/corroborate a number of new spectral components, including the excitation by non-thermal electrons, the deviation from ionization equilibrium, and charge exchange from surface of cold gas clouds in clusters. Apart from detecting new features, future high resolution spectroscopy will also enable a much better measurement of the thermal component. Accurate atomic database and appropriate modeling of the thermal spectrum are therefore needed for interpreting the data.     

Spartan 201 coronal spectroscopy during the polar passes of ulysses

Spartan 201 is a shuttle deployed spacecraft that is scheduled to perform ultraviolet spectroscopy and white light polarimetry of the extended solar corona during two 40 hour missions to occur in September 1994 and August 1995. The spectroscopy is done with an ultraviolet coronal spectrometer which measures the intensity and spectral line profile of HI Ly up to heliocentric heights of 3.5 solar radii. It also measures the intensities of the OVI doublet at 1032 and 1037 Å and of Fe XII at 1242 Å. The HI Ly line profile measurements are used to determine the random velocity distribution of coronal protons along the line-of-sight. The absolute HI Ly intensities can be used together with electron densities from the white light coronagraph to estimate electron temperatures from hydrogen ionization balance calculations, and bulk outflow velocities from models of Doppler dimmed resonant scattering. Intensities of minor ion lines are used to determine coronal abundances and outflow velocities of O⁵⁺. Ultraviolet spectroscopy of extended coronal regions from the 11 April 1993 mission of Spartan 201 are discussed.     

Background limited infrared and submillimeter instruments

The temperature and emissivity of infrared and submillimeter telescopes are basic parameters that drive the optical and thermal design of astronomical space projects. They determine also, among other parameters, the self-emission of the instrument and the photon noise produced by this radiation on the detectors. By comparing the telescope brightness with that of the sky in the 1m–1 cm wavelength range, general conditions for background limited photometry are derived. For <0.4 mm, temperature is the driving parameter, and for >0.4 mm, temperature and emissivity have equivalent importances. It can be shown on actual projects that these two regimes determine different optical and thermal concepts. Although based on a simplistic approach, this work intends to help designers to handle some basic system parameters of infrared and submillimeter instruments.     

The Descent Imager/Spectral Radiometer (DISR) Experiment on the Huygens Entry Probe of Titan

The payload of the Huygens Probe into the atmosphere of Titan includes the Descent Imager/Spectral Radiometer (DISR). This instrument includes an integrated package of several optical instruments built around a silicon charge coupled device (CCD) detector, a pair of linear InGaAs array detectors, and several individual silicon detectors. Fiber optics are used extensively to feed these detectors with light collected from three frame imagers, an upward and downward-looking visible spectrometer, an upward and downward looking near-infrared spectrometer, upward and downward looking violet phtotometers, a four-channel solar aerole camera, and a sun sensor that determines the azimuth and zenith angle of the sun and measures the flux in the direct solar beam at 940 nm. An onboard optical calibration system uses a small lamp and fiber optics to track the relative sensitivity of the different optical instruments relative to each other during the seven year cruise to Titan. A 20 watt lamp and collimator are used to provide spectrally continuous illumination of the surface during the last 100 m of the descent for measurements of the reflection spectrum of the surface. The instrument contains software and hardware data compressors to permit measurements of upward and downward direct and diffuse solar flux between 350 and 1700 nm in some 330 spectral bands at approximately 2 km vertical resolution from an alititude of 160 km to the surface. The solar aureole camera measures the brightness of a 6° wide strip of the sky from 25 to 75° zenith angle near and opposite the azimuth of the sun in two passbands near 500 and 935 nm using vertical and horizontal polarizers in each spectral channel at a similar vertical resolution. The downward-looking spectrometers provide the reflection spectrum of the surface at a total of some 600 locations between 850 and 1700 nm and at more than 3000 locations between 480 and 960 nm. Some 500 individual images of the surface are expected which can be assembled into about a dozen panoramic mosaics covering nadir angles from 6° to 96° at all azimuths. The spatial resolution of the images varies from 300 m at 160 km altitude to some 20 cm in the last frames. The scientific objectives of the experiment fall into four areas including (1) measurement of the solar heating profile for studies of the thermal balance of Titan; (2) imaging and spectral reflection measurements of the surface for studies of the composition, topography, and physical processes which form the surface as well as for direct measurements of the wind profile during the descent; (3) measurements of the brightness and degree of linear polarization of scattered sunlight including the solar aureole together with measurements of the extinction optical depth of the aerosols as a function of wavelength and altitude to study the size, shape, vertical distribution, optical properties, sources and sinks of aerosols in Titan's atmosphere; and (4) measurements of the spectrum of downward solar flux to study the composition of the atmosphere, especially the mixing ratio profile of methane throughout the descent. We briefly outline the methods by which the flight instrument was calibrated for absolute response, relative spectral response, and field of view over a very wide temperature range. We also give several examples of data collected in the Earth's atmosphere using a spare instrument including images obtained from a helicopter flight program, reflection spectra of various types of terrain, solar aureole measurements including the determination of aerosol size, and measurements of the downward flux of violet, visible, and near infrared sunlight. The extinction optical depths measured as a function of wavelength are compared to models of the Earth's atmosphere and are divided into contributions from molecular scattering, aerosol extinction, and molecular absorption. The test observations during simulated descents with mountain and rooftop venues in the Earth's atmosphere are very important for driving out problems in the calibration and interpretion of the observations to permit rapid analysis of the observations after Titan entry. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ultraviolet spectroscopy of the extended solar corona during the Spartan 201 mission

The instruments on the Spartan 201 spacecraft are an Ultraviolet Coronal Spectrometer and a White Light Coronagraph. Spartan 201 was deployed by the Space Shuttle on 11 April 1993 and observed the extended solar corona for about 40 hours. The Ultraviolet Coronal Spectrometer measured the intensity and spectral line profile of HI Ly and the intensities of OVI 103.2 and 103.7 nm. Observations were made at heliocentric heights between 1.39 and 3.5 R. Four coronal targets were observed, a helmet streamer at heliographic position angle 135°, the north and south polar coronal holes, and an active region above the west limb. Measurements of the HI Ly geocorona and the solar irradiance were also made. The instrument performed as expected. Straylight suppression, spectral focus, radiometric sensitivity and background levels all appear to be satisfactory. The uv observations are aimed at determining proton temperatures and outflow velocities of hydrogen, protons and oxygen ions. Preliminary results from the north polar coronal hole observations are discussed.     

Measurements of the properties of solar wind plasma relevant to studies of its coronal sources

Interplanetary measurements of the speeds, densities, abundances, and charge states of solar wind ions are diagnostic of conditions in the source region of the solar wind. The absolute values of the mass, momentum, and energy fluxes in the solar wind are not known to an accuracy of 20%. The principal limitations on the absolute accuracies of observations of solar wind protons and alpha particles arise from uncertain instrument calibrations, from the methods used to reduce the data, and from sampling biases. Sampling biases are very important in studies of alpha particles. Instrumental resolution and measurement ambiguities are additional major problems for the observation of ions heavier than helium. Progress in overcoming some of these measurement inadequacies is reviewed.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

The Coronal Helium Abundance Experiment on Spacelab 2

The Coronal Helium Abundance Spacelab Experiment, (CHASE), basically consists of a grazing incidence telescope and spectrometer sensitive over the range 150–1335 Å. Whilst aimed primarily at deriving the solar helium abundance from measurements of coronal resonance scattering, its specification has been extended in order to provide a more general purpose solar XUV facility. The instrument will be flown on the Spacelab 2 Mission, currently scheduled for launch in November 1984.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

Ultraviolet spectrometer experiment for the Voyager mission

The Voyager Ultraviolet Spectrometer (UVS) is an objective grating spectrometer covering the wavelength range of 500–1700 Å with 10 Å resolution. Its primary goal is the determination of the composition and structure of the atmospheres of Jupiter, Saturn, Uranus and several of their satellites. The capability for two very different observational modes have been combined in a single instrument. Observations in the airglow mode measure radiation from the atmosphere due to resonant scattering of the solar flux or energetic particle bombardment, and the occultation mode provides measurements of the atmospheric extinction of solar or stellar radiation as the spacecraft enters the shadow zone behind the target. In addition to the primary goal of the solar system atmospheric measurements, the UVS is expected to make valuable contributions to stellar astronomy at wavelengths below 1000 Å.     

The Radiation Assessment Detector (RAD) Investigation

The Radiation Assessment Detector (RAD) on the Mars Science Laboratory (MSL) is an energetic particle detector designed to measure a broad spectrum of energetic particle radiation. It will make the first-ever direct radiation measurements on the surface of Mars, detecting galactic cosmic rays, solar energetic particles, secondary neutrons, and other secondary particles created both in the atmosphere and in the Martian regolith. The radiation environment on Mars, both past and present, may have implications for habitability and the ability to sustain life. Radiation exposure is also a major concern for future human missions. The RAD instrument combines charged- and neutral-particle detection capability over a wide dynamic range in a compact, low-mass, low-power instrument. These capabilities are required in order to measure all the important components of the radiation environment. RAD consists of the RAD Sensor Head (RSH) and the RAD Electronics Box (REB) integrated together in a small, compact volume. The RSH contains a solid-state detector telescope with three silicon PIN diodes for charged particle detection, a thallium doped Cesium Iodide scintillator, plastic scintillators for neutron detection and anti-coincidence shielding, and the front-end electronics. The REB contains three circuit boards, one with a novel mixed-signal ASIC for processing analog signals and an associated control FPGA, another with a second FPGA to communicate with the rover and perform onboard analysis of science data, and a third board with power supplies and power cycling or “sleep”-control electronics. The latter enables autonomous operation, independent of commands from the rover. RAD is a highly capable and highly configurable instrument that paves the way for future compact energetic particle detectors in space.