Summary of quantitative interpretation of IMAGE far ultraviolet auroral data

Direct imaging of the magnetosphere by instruments on the IMAGE spacecraft is supplemented by simultaneous observations of the global aurora in three far ultraviolet (FUV) wavelength bands. The purpose of the multi-wavelength imaging is to study the global auroral particle and energy input from the magnetosphere into the atmosphere. This paper describes the method for quantitative interpretation of FUV measurements. The Wide-Band Imaging Camera (WIC) provides broad band ultraviolet images of the aurora with maximum spatial resolution by imaging the nitrogen lines and bands between 140 and 180 nm wavelength. The Spectrographic Imager (SI), a dual wavelength monochromatic instrument, images both Doppler-shifted Lyman-α emissions produced by precipitating protons, in the SI-12 channel and OI 135.6 nm emissions in the SI-13 channel. From the SI-12 Doppler shifted Lyman-α images it is possible to obtain the precipitating proton flux provided assumptions are made regarding the mean energy of the protons. Knowledge of the proton (flux and energy) component allows the calculation of the contribution produced by protons in the WIC and SI-13 instruments. Comparison of the corrected WIC and SI-13 signals provides a measure of the electron mean energy, which can then be used to determine the electron energy flux. To accomplish this, reliable emission modeling and instrument calibrations are required. In-flight calibration using early-type stars was used to validate the pre-flight laboratory calibrations and determine long-term trends in sensitivity. In general, very reasonable agreement is found between in-situ measurements and remote quantitative determinations.     

Far ultraviolet imaging from the IMAGE spacecraft. 3. Spectral imaging of Lyman-α and OI 135.6 nm

Two FUV Spectral imaging instruments, the Spectrographic Imager (SI) and the Geocorona Photometer (GEO) provide IMAGE with simultaneous global maps of the hydrogen (121.8 nm) and oxygen 135.6 nm components of the terrestrial aurora and with observations of the three dimensional distribution of neutral hydrogen in the magnetosphere (121.6 nm). The SI is a novel instrument type, in which spectral separation and imaging functions are independent of each other. In this instrument, two-dimensional images are produced on two detectors, and the images are spectrally filtered by a spectrograph part of the instrument. One of the two detectors images the Doppler-shifted Lyman- while rejecting the geocoronal `cold Ly-, and another detector images the OI 135.6 nm emission. The spectrograph is an all-reflective Wadsworth configuration in which a grill arrangement is used to block most of the cold, un-Doppler-shifted geocoronal emission at 121.567 nm. The SI calibration established that the upper limit of transmission at cold geocoronal Ly- is less than 2%. The measured light collecting efficiency was 0.01 and 0.008 cm² at 121.8 and at 135.6 nm, respectively. This is consistent with the size of the input aperture, the optical transmission, and the photocathode efficiency. The expected sensitivity is 1.8×10^–2 and 1.3×10^–2 counts per Rayleigh per pixel for each 5 s viewing exposure per satellite revolution (120 s). The measured spatial resolution is better than the 128×128 pixel matrix over the 15°×15° field of view in both wavelength channels. The SI detectors are photon counting devices using the cross delay line principle. In each detector a triple stack microchannel plate (MCP) amplifies the photo-electronic charge which is then deposited on a specially configured anode array. The position of the photon event is measured by digitizing the time delay between the pulses detected at each end of the anode structures. This scheme is intrinsically faster than systems that use charge division and it has a further advantage that it saturates more gradually at high count rates. The geocoronal Ly- is measured by a three-channel photometer system (GEO) which is a separate instrument. Each photometer has a built in MgF₂ lens to restrict the field of view to one degree and a ceramic electron multiplier with a KBr photocathode. One of the tubes is pointing radially outward perpendicular to the axis of satellite rotation. The optic of the other two subtend 60° with the rotation axis. These instruments take data continuously at 3 samples per second and rely on the combination of satellite rotation and orbital motion to scan the hydrogen cloud surrounding the earth. The detective efficiencies (effective quantum efficiency including windows) of the three tubes at Ly- are between 6 and 10%.     

Scientific requirements for future spatially resolved white-light and broad-band high-cadence observations of the Sun

Several important issues are open in the field of solar variability and they wait their solution which up to now was attempted using critical ground-based instrumentations. However, accurate photometric data are attainable only from space. New observational material should be collected with high enough spatial and spectral resolution, covering the whole visible range of the electromagnetic spectrum as well infrared and ultraviolet to reconstruct the total solar irradiance: (1) the absolute contributions of different small-scale structural entities of the solar atmosphere from the white light flares and from micro-flares are still poorly known; (2) we do not know the absolute contributions of different structural elements of the solar atmosphere to the long-term and to the cyclic variations of the solar irradiance, including features of the polar regions of the Sun; (3) the variations of the chromospheric magnetic network are still poorly evaluated; (4) only scarce information is available about the spectral variations of different small-scale features in the high photosphere. Variability of the Sun in white light can be studied with higher spectral, spatial and time resolution using space-born telescopes, which are more appropriate for this purpose than ground based observatories because of better seeing conditions, no interference of the terrestrial atmosphere and a more precise calibration procedure. Scientific requirements for such observations and the possible experimental tools proposed for their solution. Suggested solar studies have broader astrophysical importance.     

Electrostatic accelerometer with bias rejection for gravitation and Solar System physics

Radio tracking of interplanetary probes is an important tool for navigation purposes as well as for testing the laws of physics or exploring planetary environments. The addition of an accelerometer on board a spacecraft provides orbit determination specialists and physicists with an additional observable of great interest: it measures the value of the non-gravitational acceleration acting on the spacecraft, i.e. the departure of the probe from geodesic motion.     

Space-borne coronagraphy

We consider the main aspects of a rather delicate discipline, space-borne coronagraphy, which is essentially performed with white-light, externally-occulted coronagraphs. Methods of observation and recent results are presented in a scientific section; a short account of historical rocket-borne and balloon-borne experiments is also given. The review concentrates on both the NRL-type coronagraphs and the HAO-type coronagraphs, stressing the essential features and merits of numerous experiments which were flown. A small section is devoted to other designs. A large number of figures, including many drawings, are used to illustrate the growing complexity of experiments.Dedicated to: B. Lyot, who built the first ground-based coronagraph; G. Newkirk, who built the first flying coronagraph; G. Nikolsky, who built the largest, each of whom prematurely passed away.On leave from Paris Institut d'Astrophysique CNRS, 98 Bis Bd Arago, F-75014, Paris, France.     

Unbiased acceleration measurements with an electrostatic accelerometer on a rotating platform

The Gravity Advanced Package is an instrument composed of an electrostatic accelerometer called MicroSTAR and a rotating platform called Bias Rejection System. It aims at measuring with no bias the non-gravitational acceleration of a spacecraft. It is envisioned to be embarked on an interplanetary spacecraft as a tool to test the laws of gravitation.     

Simulation of Ambiguity Effects in Doppler Tracking of Pioneer Probes

Various models have been proposed to interpret the anomalous Pioneer Doppler data. We present in this paper a simulation tool aiming at determining signatures that could be searched in the data reduction process in order to discriminate between these different explanations. We discuss preliminary results on the seasonally modulated anomalies and compare two interpretations corresponding to a constant anomalous acceleration on one hand, to an anomalous curvature in the second sector of the gravity law on the other hand. Though the second sector interpretation could naturally induce large modulated anomalies, the adjustment of the initial conditions partly compensate these modulations and thus produces a signature resembling that of a constant acceleration. The difference between the predictions of the two interpretations is in fact close to the rms level of the residuals after the data analysis.     

A search of short-period coronal waves

In 1973, during the total solar eclipse, we flew an experiment aboard the Concorde supersonic airliner in order to investigate the possible presence of white-light coronal waves. Our experiment failed to detect any significant effects, so it became clear that such waves should be searched for by use of finer, i.e., spectroscopic methods.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

Space solar arrays and concentrators

This paper presents some research activities conducted at the Centre Spatial de Liege (CSL) in the field of space solar arrays and concentration.With the new generation of high efficiency solar cells, solar concentration brings new insights for future high power spacecrafts. A trade-off study is presented in this paper. Two different trough concentrators, and a linear Fresnel lens concentrator are compared to rigid arrays. Thermal and optical behaviors are included in the analysis.Several technical aspects are discussed:

• Off-pointing with concentrators induces collection loss and illumination non uniformity, reducing the PV efficiency.

• Concentrator deployment increases the mission risk.

• Reflective trough concentrators are attractive and already proven. Coating is made of VDA (Aluminum). A comprehensive analysis of PV conversion increase with protected silver is presented.

• Solar concentration increases the heat load on solar cells, while the conversion efficiency is significantly decreasing at warm temperatures.

To conclude, this paper will point out the new trends and the key factors to be addressed for the next generation of solar generators.