A. Bahnsen

VLF Electromagnetic Waves Observed Onboard GEOS-1

This paper is concerned mainly with the information which can be extracted from frequency-time spectra in the VLF range. The instrument used is the correlator which has a good frequency resolution (50 Hz) and time resolution (30 ms) in one magnetic and one electric component simultaneously. By suitable computer analysis, it is possible for instance to distinguish between the two dominant electromagnetic emissions, hiss and chorus, as well as to display the complete spectra. This treatment is applied to the Survey periods, which are a fixed sequence of modes, repeated every hour on the hour in order to have reference data from GEOS analogous to many ground-based observatories. One result of this treatment obtained already is that hiss and chorus normally appear together, although one or the other may be dominating in intensity. The occurrence rate of these emissions in local time is also given.For continuous surveillance the filterbank data are used. There are 16 frequency filters supplying magnetic and electric amplitude at few different frequencies. Using these data, a storm sudden commencement can be followed with good time resolution (1 s), and an interesting correlation has been found in a few cases between the VLF signal amplitude and the cold plasma density (as measured by the active part of the S-300 experiment).

Auroral kilometric radiation sources: In situ and remote observations from Viking

The present paper is a summary of studies carried out from Viking measurements on the propagation and the generation of the auroral kilometric radiation (AKR hereafter). Advantage is taken of the spin modulation of the AKR observed as Viking was rotating in the cartwheel mode. This, together with the study of the cutoff of the various spectral components, confirms that low-amplitude Z and O modes are generated at the same time as a larger-amplitude X mode. Hence Z, O and X mode AKR is all generated by the same sources. The spectrum of the dominant polarization, the X mode, usually contains several spectral peaks. An AKR source crossing is characterized by a minimum in the frequency of the lowest-frequency peak (fpeak) and by a maximum of its amplitude. About 50 AKR source crossings are used to demonstrate that fpeak approaches fce, the electron gyrofrequency: (fpeak - fce)/fce ≈ 0.025 in AKR sources. Similarly, the low-frequency cutoff of the AKR is found, on average, to coincide with fce. The density inside AKR sources is determined from four sets of independent measurements, namely (1) the upper frequency cutoff of the hiss, (2) the relaxation sounder, (3) the Langmuir probe, and (4) particle measurements. It is shown that an AKR source coincides with a strong depletion in the density of the cold/cool electrons that becomes comparable to or less than the density of energetic electrons (E ≥ 1 keV). The total density inside AKR sources is of the order of 1 cm−3, typically a factor 5 to 10 below that of the surrounding regions. AKR sources are found to coincide also with an acceleration region characterized by a potential drop of ≥1 kV, both below and above the spacecraft. Evidence for this comes from the observation of electrons accelerated above the spacecraft and ions accelerated below it. In addition to a strong depletion in the density of the cool electrons, particle measurements on Viking give evidence of several possible free energy sources that could drive unstable the AKR, namely (1) a loss cone, (2) a hole for parallel velocities smaller than that of the observed downgoing electron beam, and (3) a trapped electron component for a pitch angle a ≈ 90°. The trapped electron component, bounded at low perpendicular energies (a few keVs) by an enhanced loss cone, is observed inside, and only inside, AKR sources. It is therefore concluded that the corresponding ∂f/∂v⊥ > 0, for small parallel velocities, is the free energy source that drives unstable the cyclotron maser.

Some early results of the keV plasma experiment on GEOS-1

Some preliminary new results are presented of the keV plasma experiment on GEOS-1. Electrons and ions have been observed to stream along the magnetic field lines in the dayside magnetosphere from the ionospheric side of the satellite towards the equatorial plane during magnetic storms, with streaming velocities corresponding to a kinetic energy of the order of a keV. The opposite streaming velocity has also been seen, but primarily in the ions only and with a smaller flux ratio for the two opposite directions along the field lines. The transition between the two opposite streaming directions, as seen by the satellite, has been found to occur even in a fraction of a second.

Microstrip proportional counters for X-ray astronomy

Abstract Four position sensitive proportional counters will be delivered by the Danish Space Research Institute as focal plane instruments for the Soviet Danish Rontgen Telescope (SODART). These detectors will incorporate the novel microstrip electrode design. This design has been tested with xenon gas and an 55Fe X-ray source. The energy resolution is better than 14% FWHM up to gas gains of 104, the rise time of pulses from X-ray events is fast, 100 ns, allowing for efficient background rejection by rise time analysis. A position resolution of 1 mm FWHM is easily obtained by resistive charge division of the signals from the cathodes and from a wire grid placed in front of the microstrip plate. The detector performs without significant degradation of gain or energy resolution at fluxes exceeding 106/s cm2 and a total dose of 1011 photons.

Quiet-time convection electric field properties derived from keV electron measurements at the inner edge of the plasma sheet by means of GEOS-2

Abstract From an analysis of the local time distribution of the electron upper energy limit reached by the geostationary satellite GEOS-2 in cutting through the innermost part of the electron plasma sheet during fairly quiet conditions the following results have been obtained, among others. An electric field model given by E = −▽{AR 4 sin (φ+ π 4 )} , with the dusk singular point of the forbidden region boundary at 1500, instead of at 1800 M.L.T., is in quite good agreement with the observations. This means that effects due to the shielding by the hot plasma of the inner magnetosphere from the convection electric field are quite strong in situations of low disturbance level. The quiet-time convection electric field strength at 2100 M.L.T. in the geostationary orbit obtained from this analysis varies in the range 0.15–0.3 kV/ R e . Six hours earlier or later in the satellite orbit the convection field is four times stronger. Also when the convection field varies, some information about its magnitude can be obtained from the keV electron measurements.

First VIKING results: high frequency waves

The high frequency wave experiment onboard the Viking satellite measures electric and magnetic fields and the plasma frequency in the range 4 to 700 kHz. The magnetic field sensor is a loop antenna with its axis parallel to the satellite spin axis, the electric field sensor is one of the radial boom dipoles also used for DC and low frequencies. The sensitivity is 10?14 Tesla/?Hz for magnetic fields, 5 ? 10?8V/m?Hz for electric fields. The plasma frequency is measured by wave activation and by impedance measurements of the plasma. The frequency range available corresponds to plasma densities from 1 to 3000 per cubic cm. Data from the initial operating period show frequent wave activity. In approximately half of the passes hiss and auroral kilometric radiation is observed in the auroral region in the evening side and sometimes also in the morning side. The radiation shows strong variations within a few seconds in spectral shape, and is often clearly limited by cutoffs in frequency. Very low plasma frequencies are often registered, corresponding to a few electrons per cubic cm at 1 to 2 Earth radii above the polar region.

Rocket-borne and groundbased observations of coincident field-aligned currents, electron beams, and plasma density enhancements in the afternoon auroral oval

Abstract Results are reported from a rocket experiment conducted at Sondre Stromfjord, Greenland, on 22 August 1976, at 16.00 M.L.T. A series of plasma, particles, and fields and wave experiments were carried on board the payload, and the venture was supported by data from the AE - C satellite and by groundbased ionosondes and magnetometers at the launch site and at Godhavn. Two regions of field-aligned electron precipitation, electron density and temperature enhancements, and field-aligned upflowing current sheets were intercepted by the rocket. The density enhancements were also observed by groundbased ionosondes. Significant discrepancies were found between the currents carried by the streaming electrons in the 0.15–10 keV range and the upflowing currents seen by the on board magnetometer, suggesting that the upflowing current could not be the primary driver of the electron acceleration mechanism. The E -region was unstable to the combined Gradient-Drift and Farley-Buneman instability, and plasma turbulence was observed in situ , but the absolute density fluctuations were too small to return detectable HF-radar power to the ground.

Observation on GEOS-1 of 10.2 to 13.6 kHz ground based transmitter signals

The Omega transmitter signals on 10.2, 11.33 and 13.6 kHz are frequently observed on GEOS-1 near the end of the pass. The signals show large fluctuations in amplitude and in E/B ratio on a time scale of 0.1 s. The time delay of the signals vary considerably over a period of a few minutes, and often the duration of the received pulses on the dipole antenna is longer than the duration of the transmitted pulses. The propagation aspects of these observations will be discussed on the basis of raytracing calculations from the ground to the vicinity of GEOS-1.

Application of microstrip proportional counters at the Danish Space Research Institute

Abstract The low energy proportional counter LEPC (0.2–8 keV) and the high energy proportional counter HEPC (2–25 keV) for the Danish-Russian X-ray telescopes XSPECT/SODART are presently being tested at DSRI. The sensor principle of these detectors is based on the novel microstrip proportional counter (MSPC) where the strip electrodes are deposited by photolithography onto a rigid substrate. The MSPC offers many advantages: a uniform gas gain, excellent energy (∼ 13% at 6 keV) and position-resolutions (⪡ 1 mm), a fast charge collection and a low operating voltage. The energy response, imaging and background rejection performance of LEPC (82 × 82 mm) and HEPC ( O = 160 mm ) will be discussed. The presentation will also include first results obtained with a high energy (∼ 150 keV) MSPC which is planned as a candidate for the X-ray monitor for the International Gamma-Ray Astrophysical Laboratory (INTEGRAL).

Polar Magnetopause Distance

The distance from Earth to the magnetopause at very high latitudes has been measured on the HEOS-2 satellite. The wave experiment on this satellite measures magnetic and electric signals at five frequencies between 20 and 236 Hz. The transition from the magnetosphere into the sheath region is clearly indicated in these signals. Measurements over one half year show periodic variations in polar magnetopause distance around an average of 14.3 R E. The measurements are compared to calculations of the magnetopause distance that take into account the solar wind pressure and the angle between the solar wind velocity and the Earths magnetic dipole. The periodic variations are predicted reasonably well by these calculations, but an average difference in the distance of approximately 20% exists between calculations and measurements.