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Aims. The Spectrometer Telescope for Imaging X-rays (STIX) on Solar Orbiter is a hard X-ray imaging spectrometer, which covers the energy range from 4 to 150 keV. STIX observes hard X-ray bremsstrahlung emissions from solar flares and therefore provides diagnostics of the hottest (⪆10 MK) flare plasma while quantifying the location, spectrum, and energy content of flare-accelerated nonthermal electrons. Methods. To accomplish this, STIX applies an indirect bigrid Fourier imaging technique using a set of tungsten grids (at pitches from 0.038 to 1 mm) in front of 32 coarsely pixelated CdTe detectors to provide information on angular scales from 7 to 180 arcsec with 1 keV energy resolution (at 6 keV). The imaging concept of STIX has intrinsically low telemetry and it is therefore well-suited to the limited resources available to the Solar Orbiter payload. To further reduce the downlinked data volume, STIX data are binned on board into 32 selectable energy bins and dynamically-adjusted time bins with a typical duration of 1 s during flares. Results. Through hard X-ray diagnostics, STIX provides critical information for understanding the acceleration of electrons at the Sun and their transport into interplanetary space and for determining the magnetic connection of Solar Orbiter back to the Sun. In this way, STIX serves to link Solar Orbiter’s remote and in-situ measurements.

RHESSI is the sixth in the NASA line of Small Explorer (SMEX) missions and the first managed in the Principal Investigator mode, where the PI is responsible for all aspects of the mission except the launch vehicle. RHESSI is designed to investigate particle acceleration and energy release in solar flares, through imaging and spectroscopy of hard X-ray/gamma-ray continua emitted by energetic electrons, and of gamma-ray lines produced by energetic ions. The single instrument consists of an imager, made up of nine bi-grid rotating modulation collimators (RMCs), in front of a spectrometer with nine cryogenically-cooled germanium detectors (GeDs), one behind each RMC. It provides the first high-resolution hard X-ray imaging spectroscopy, the first high-resolution gamma-ray line spectroscopy, and the first imaging above 100 keV including the first imaging of gamma-ray lines. The spatial resolution is as fine as ~ 2.3 arc sec with a full-Sun (≳ 1°) field of view, and the spectral resolution is ~ 1–10 keV FWHM over the energy range from soft X-rays (3 keV) to gamma-rays (17 MeV). An automated shutter system allows a wide dynamic range (> 107) of flare intensities to be handled without instrument saturation. Data for every photon is stored in a solid-state memory and telemetered to the ground, thus allowing for versatile data analysis keyed to specific science objectives. The spin-stabilized (~ 15 rpm) spacecraft is Sun-pointing to within ~ 0.2° and operates autonomously. RHESSI was launched on 5 February 2002, into a nearly circular, 38° inclination, 600-km altitude orbit and began observations a week later. The mission is operated from Berkeley using a dedicated 11-m antenna for telemetry reception and command uplinks. All data and analysis software are made freely and immediately available to the scientific community.

Solar Orbiter is a Sun-observing mission led by the European Space Agency, addressing the interaction between the Sun and the heliosphere. It will carry ten instruments, among them the X-ray imaging spectrometer STIX. STIX will determine the intensity, spectrum, timing, and location of thermal and accelerated electrons near the Sun through their bremsstrahlung X-ray emission. This report gives a brief overview of the STIX scientific goals and covers in more detail the instrument design and challenges.

The Spectrometer Telescope for Imaging X-rays (STIX) is one of 10 instruments on board Solar Orbiter, a confirmed Mclass mission of the European Space Agency (ESA) within the Cosmic Vision program scheduled to be launched in 2017. STIX applies a Fourier-imaging technique using a set of tungsten grids (at pitches from 0.038 to 1 mm) in front of 32 pixelized CdTe detectors to provide imaging spectroscopy of solar thermal and non-thermal hard X-ray emissions from 4 to 150 keV. The status of the instrument reviewed in this paper is based on the design that passed the Preliminary Design Review (PDR) in early 2012. Particular emphasis is given to the first light of the detector system called Caliste-SO.

We report the development of very fine collimator slit patterns for gamma ray imaging. Astronomical objects emitting gamma rays cannot be imaged in a classical way. However, it is possible to measure Fourier components of the images by observing gamma ray sources through pairs of patterns of fine slits placed at a mutual distance of several meters. The finest pattern should have slit 50 μm wide at a pitch of 100 μm. To achieve high contrast images, the slit geometry must meet high precision goals. The absolute slit position accuracy must be on the order of a few micrometers within an area of 130 mm × 130 mm. The slit patterns have been realized by stacking logn thin blades separated by small spacers. The application of flexible spacers and precision reference frames results in a high pitch accuracy. The spacer design was optimized using finite-element analysis. The spacers were made via a multistep precision etching technique. Measurements on the full-size slit patterns show low slit position errors.

Describes a capacitive displacement sensor using the spatial sampling principle. A first experimental prototype has been manufactured. Measurements show that this prototype achieves a resolution of 1 mu m and an accuracy of 3 mu m over a displacement of 150 mm. The linear displacement range can be as large as several metres. The prototype confirms low sensitivity to both alignment inaccuracies and external influences like dust and electromagnetic disturbances.

This paper describes a new capacitive position transducer with tens of nanometer accuracy and a long dynamic range (5 × 106). Contrary to a laser interferometer, its position determination is absolute and instead of referring to the wavelength of light, which is temperature, pressure and humidity dependent, it refers to a form standard that can be made out of quartz or zerodur. The principle of this transducer enables the extension of the system so that true two-dimensional performance is obtained, where the momentary position of a movable sensor head is referred to a mechanically stable two-dimensional standard. A one-dimensional laboratory model has been investigated. This model exhibits a resolution of 40 nm and an absolute accuracy of 100 nm (3σ). The lay-out of the transducer and the method of position determination are described.

During spaceflights, it is important to measure an astronaut's body mass ('weight'), both for investigating the influence of the space environment on the human body and for monitoring the physical (health) condition of the astronaut. This paper reports the development of a mass measuring device that is compact, user friendly, and has an absolute measuring accuracy better than 60 gram. The measurement accuracy turns out to be restricted by the way a human body is configured by nature and not by the instrument itself, that has an accuracy much better than required.

Using HXIS data, we have studied the further development of the coronal arch extending towards the SE above active region No. 17255 in November 1980. The arch, studied originally by Svestka (1984) disappeared on 10 November (after pronounced revival), but since 9 November HXIS revealed another arch-like structure towards the SW. We have studied the development of this new structure which appeared to be most likely an arch interconnecting AR 17255 with AR 17251, located nearly 30° to the west. This interconnection revived many times during the following days with intensity varying with the activity in both active regions. We have estimated the physical characteristics in this coronal structure and compared them with observations of interconnecting loops made at lower energies by Skylab in 1973. The temperature (maximum values 7.5–14 × 106 K) and the density (1.1–5.0 × 109 cm−3) are found to be higher than in the Skylab loops (a result that could be expected because the HXIS energy range was harder than that of Skylab) and similar to the values deduced for the earlier arch system extending to the SE. However, much shorter decay times of the brightness variations indicate the presence of conduction in contrast to the SE arch in which conduction was clearly inhibited. This supports the assumption that the SE and SW coronal structures were two different phenomena.

Using HXIS data, we have studied the further development of the coronal arch extending towards the SE above active region No. 17255 in November 1980. The arch, studied originally by Svestka (1984) disappeared on 10 November (after pronounced revival), but since 9 November HXIS revealed another arch-like structure towards the SW. We have studied the development of this new structure which appeared to be most likely an arch interconnecting AR 17255 with AR 17251, located nearly 30° to the west. This interconnection revived many times during the following days with intensity varying with the activity in both active regions. We have estimated the physical characteristics in this coronal structure and compared them with observations of interconnecting loops made at lower energies by Skylab in 1973. The temperature (maximum values 7.5–14 × 106 K) and the density (1.1–5.0 × 109 cm−3) are found to be higher than in the Skylab loops (a result that could be expected because the HXIS energy range was harder than that of Skylab) and similar to the values deduced for the earlier arch system extending to the SE. However, much shorter decay times of the brightness variations indicate the presence of conduction in contrast to the SE arch in which conduction was clearly inhibited. This supports the assumption that the SE and SW coronal structures were two different phenomena.

On July 5, 1980 the Hard X-Ray Imaging Spectrometer on board the Solar Maximum Mission observed a complex flare event starting at 22 : 32 UT from AR 2559 (Hale 16955), then at N 28 W 29, which developed finally into a 2-ribbon flare. In this paper we compare the X-ray images with Hα photographs taken at the Big Bear Solar Observatory and identify the site of the most energetic flare phenomena. During the early phases of the event the hard X-rays (>16 keV) came from a compact source located near one of the two bright Hα kernels; we believe the latter are at the footpoints of a compact magnetic loop. The kernel identified with the X-ray source is immediately adjacent to one of the principal sunspots and in fact appears to ‘rotate’ around the sunspot over 90° in the early phase of the flare. Two intense X-ray bursts occur at the site of the rotating kernel, and following each burst the loop fills with hot, X-ray emitting plasma. If the first burst is interpreted as bremsstrahlung from a beam of electrons impinging on a collisionally dominated medium, the energy in such electrons, >16 keV, is ∼ 5 × 1030 erg. The altitude of the looptop is 7–10 × 103 km. The temperature structure of the flare is extremely non-homogeneous, and the highest temperatures are found in the top of the loop.

We have selected four widely different flares from the early period of operations of the Hard X-Ray Imaging Spectrometer (HXIS) on SMM to illustrate the characteristic imaging properties of this experiment. For the small flare of April 4, 1980, we demonstrate the instrument's capability for locating a compact source. In the weak, but extensive, flare of April 6 we show how well the instrument can display spatial structure, and also the low level of the instrument background. In the 1B flare of April 7 we are able to locate positions of the X-ray emission in the soft and hard channels, and estimate the positional variations of the emission patches. Finally, in the IN flare of April 10, which produced the strongest hard X-ray burst we have seen so far, we repeat some of the studies made for the April 7 event, and also demonstrate the capability of the HXIS instrument to study the development, with high time resolution, of individual 8″ × 8″ elements of the flare.

A new position sensitive detector system has been developed for the Hard X-ray Imaging Spectrometer launched aboard the NASA Solar Maximum Mission. The instrument operates in the energy range 3.5-30 keV. The Xe-gas filled detector system consists of a photon absorption compartment and an array of 900 mini-proportional counters. The detector lay-out and the system performance are described.

Particle fluxes have been measured by means of shielded Geiger-Müller telescopes mounted m a rocket, which was launched from ESRANGE(Kiruna) into a diffuse aurora. The analysis of the dependence of the counting rates on altitude indicates that a weak flux of energetic electrons, 1–10 MeV, has been detected.

The Utrecht Hard Solar X-Ray Spectrometer on board the ESRO TD-1A satellite (launch March 1972) is permanently Sun-pointed and measures the solar radiation between 30 keV and 1000 keV, in 12 logarithmically spaced energy channels, with a continuous fine time resolution, viz. 1.2 s for the four lowest energy channels and 4.8 s for the rest. The detector has a 5 cm2 Cs I (Na) crystal; counts due to particles are rejected and even during the largest solar flares saturation effects (e.g. pulse pile-up) are absent. For further details see Van Beek (1973), and Van Beek and De Feiter (1973).

A MWPC read-out system is described that determines the position of a photon interaction by calculating the center of gravity of the charge induced in sense wires. The width of the charge induction is determined to distinguish photon interactions from disturbing events that occur in the system. The position resolution is 0.3 mm FWHM at 6 keV. The energy range is 2 - 30 keV.

SummaryThe importance and difficulties of determining the height of hard X-ray sources in the solar atmosphere, in order to distinguish source models, have been discussed by Brown and McClymont (1974) and also in this Symposium (Brown, 1975; Datlowe, 1975). Theoretical predictions of this height, h, range between and 105 km above the photosphere for different models (Brown and McClymont, 1974; McClymont and Brown, 1974). Equally diverse values have been inferred from observations of synchronous chromospheric EUV bursts (Kane and Donnelly, 1971) on the one hand and from apparently behind-the-limb events (e.g. Datlowe, 1975) on the other.

The result of a design study on a Focal Plane Instrument for the 6.5 to 175 nm wavelength range is described. The instrument is meant to be used in combination with a grazing incidence telescope consisting of a sector out of a full revolution configuration mirror system, type Wolter II. The presented design is based on the principles of grazing incidence reflection and conical diffraction. The absolute efficiency of a grating in a conical mount is high. The combination of a plane grating with some mirrors for imaging therefore results in an arrangement with a satisfactory instrumental transmission. The outcome of efficiency calculations is presented. The spectral image can be made stigmatic over the entire wavelength range in a field of view with sufficiently large dimensions. Simultaneously, radiation from 240 spatial pixels of 1 arcsec x 1 arcsec is analyzed in four wavelength bands.With a change of the angle of incidence on the grating it is possible to choose these bands, with some restrictions, anywhere in the mentioned wavelength range. The spectral resolution varies from 0.003 nm to 0.0075 nm. For detection micro-channel arrays are chosen.

With a spectrometer aboard the ESRO TD-1A satellite, time profiles and photon spectra of solar X-rays have been observed in the energy range above 24 keV. Most of the X-ray bursts studied thus far appear to consist of numerous short-lived spikes with rise and decay times of a few seconds. Such spikes, called ‘Elementary Flare Bursts’ (EFB’s) cluster together in large numbers and constitute the conventional high energy flare. The form of the photon spectra could be determined very accurately and was in all cases of the power-law type. For the August 4, 1972 flare, the photon spectra appear to be combinations of two power-law spectra; for the lower energies the spectral coefficient is smaller than for the higher energies. The transition energy is about 60 keV.

The Utrecht Hard Solar X-Ray Spectrometer on board the ESRO TD-1A satellite (launch March 1972) is permanently Sun-pointed and measures the solar radiation between 30 keV and 1000 keV, in 12 logarithmically spaced energy channels, with a continuous fine time resolution, viz. 1.2 s for the four lowest energy channels and 4.8 s for the rest. The detector has a 5 cm2 Cs I (Na) crystal; counts due to particles are rejected and even during the largest solar flares saturation effects (e.g. pulse pile-up) are absent. For further details see Van Beek (1973), and Van Beek and De Feiter (1973).

During the Polar Cap Absorption event occurring after the flares of 24th and 25th February 1969, three rockets have been launched successfully at Andenes (Norway).

In order to observe celestial X-ray sources an instrument is under development to be launched in the Astronomical Netherlands Satellite (ANS) in 1974. The aim of the experiment is to measure the spectral distribution of the sources. If there is some evidence that a source should be a pulsar the instrument can be switched from the normal mode to the pulsar mode. In this latter mode the detected photons are labeled in time using an on board clock. The ANS will have a pointing mode and a slow scanning mode. The maximum observing time per orbit for one object will be about 2000 s.

The hard x-ray imaging spectrometer (HXIS) is one of the instruments aboard the NASA Solar Maximum Mission satellite. For the imaging of solar flares in hard x rays HXIS employs a grid collimator with an 8-sec of arc FWHM angular resolution. The collimator consists of ten grid plates in a sequence. The position accuracy of the grids in their planes must be better than a few microns. This paper describes a simple optical alignment method developed to perform measurements of the grid positions with a precision of 1 microm.

X-ray imaging of the limb event of 1980 April 30 shows that the flaring involved two distinct components: a pointlike component, which was the source of the initial hard X-ray burst and an extensive tongue reaching some 30,000 km above the limb. The tongue had a higher temperature than the other parts of the structure and seemed to be enhanced by energetic electrons that derived their energy from the initial source.

We report observations on the two-ribbon flare of 1980 May 21 by the Hard X-Ray Imaging Spectrometer, the Hard X-Ray Burst Spectrometer, and the UV Spectrometer and Polarimeter aboard the Solar Maximum Mission, as well as H..cap alpha.. and magnetic field data. Our conslusions are (1) the impulsive 16--30 keV X-ray spike bursts originated from separate locations of approx.8'' width, coinciding in position with H..cap alpha.. flare kernels. This hard X-ray emission was caused by beams of accelerated electrons. (2) The impulsive spike bursts came from the footprints of loops arched over a 30'' region where new magnetic flux emerged, which caused eruption of a filament immediately before the flare. (3) Temperatures in excess off 7 x 10/sup 7/ K were observed during the thermal phase. The hot plasma component cooled by anomalous heat conduction, requiring a continuous energy release of approx.2 x 10/sup 29/ ergs s/sup -1/ initially, to approx.4 x 10/sup 28/ ergs s/sup -1/ later on.