H.F. van Beek

Hard X-ray imaging of two flares in active region 2372

We discuss hard X-ray images of two flares observed by the Hard X-Ray Imaging Spectrometer (HXIS) aboard SMM on 1980 April 7 and 10. A comparison with H..cap alpha.. images and the photospheric magnetic field maps shows that the emission originated in (arcades of) loops which differ greatly in the hardness of the X-ray spectra. On April 7 the hardest X-ray emission coincided with the brightest H..cap alpha.. patch. On April 10 the most intense X-ray emission appeared to be concentrated in a looplike structure with a softer spectrum at the top and a harder spectrum in the legs. Temperature estimates from flux ratios in different energy bands tend to confirm that small, hot components are embedded in more extensive, cooler flaring regions. Temperatures in excess of 8 x 10/sup 7/ K have been found in the impulsive phase, but alternately a power-law spectrum with ..gamma..roughly-equal5.4 might fit the observed counts better.

A long-range capacitive displacement sensor having micrometre resolution

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.

AN ABSOLUTE CAPACITIVE POSITION TRANSDUCER FOR LITHOGRAPHY.

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.

Time Profiles and Photon Spectra of Solar Hard X-Rays

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.

A Position Sensitive Detector System Consisting of an Array of Mini-Proportional Counters

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.

Coronal interconnection of two active regions observed in 3.5–5.5 keV X-rays

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.

A MWPC Read-Out System with Novel Photon Identification Capabilities

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.

Structural development of the X-ray limb flare of 30 April 1980

We describe the development of the limb flare of 30 April 1980, 20:20 UT, as observed by the Hard X-ray Imaging Spectrometer (HXIS) aboard the Solar Maximum Mission (SMM). It consisted of a short-lived bright nucleus (FWHM < 10,000 km), just inside the Suns limb; a longer lasting tongue, extending to a height of ∼ 30,000 km, and a more complicated feature, approximately situated at the Suns limb. The tongue was a pre-existing magnetic structure that started emitting X-rays only a few seconds after the bright nucleus, and which had a slightly higher temperature than the nucleus; its X-ray emission may be caused by electrons escaped from the nucleus.

Development and manufacture of collimators for gamma ray imaging

Abstract 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.

Solar maximum mission experiment: Early results of the hard X-ray imaging experiment

Abstract 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 instruments 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.