E. Ungstrup

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).

Rocket‐borne wave, field, and plasma observations in unstable polar cap E‐region

This paper presents initial results from the first comprehensively instrumented rocket flown through a Farley-unstable polar cap E-region. Groundbased ionosondes and magnetometers at two locations and HF radar backscatter at the launch site were used to determine the presence of a geographically widespread ionospheric plasma instability. The observed wave direction, electric field, and current density fit the predictions of the linear theory of the Farley instability, whereas the gradient-drift instability seems to be excluded by the geometry of the observations.

ROCKET OBSERVATION OF VLF HISS IN AURORA.

Abstract Three rocket payloads with VLF receivers have been launched during auroral disturbances to study VLF hiss in the ionosphere. One payload was launched during relatively quiet conditions and penetrated a quiet auroral arc. Another payload was launched during the break up phase of the aurora at the start of a magnetic substorm. The third payload was launched in a situation that was similar to an after break up auroral glow situation. From the 7.35 kHz hiss observations on these three payloads it is concluded that in some circumstances auroral hiss is generated in the auroral arcs at altitudes of 105–110 km. The observations also show that auroral hiss often is present in the ionosphere even if it is not observed on the ground. The observed variation in intensity of auroral hiss is interpreted as being caused by absorption as the hiss propagates away from a generation region at 105–110 km altitude or propagates down from greater altitudes. This interpretation together with the occasional spin modulation of the observed signals indicates that the wave normal direction for auroral hiss often is within a few degrees of the resonance cone (89.7°).

Giotto's Mission to planet Earth

After its successful encounter with comet P/Halley and a four-years hibernation period ESAs Giotto spacecraft has been reactivated in February 1990 and performed the first-ever Earth gravity-assisted maneuver on July 2, 1990 to be retargeted for comet P/Grigg-Skjellerup. This swing-by is of unique scientific interest due to Giottos hyperbolic, high-inclination orbit. Here, we shall report on scientific results of the Giotto magnetic field experiment. Due to the high fly-by velocity and the relative quietness of the magnetosphere during the swingby period these measurements present a snapshot view of the Earth magnetosphere with clearly identified inbound and outbound bow shock and magnetopause crossings. The outbound crossings are of particular interest as surface waves at the polar magnetopause at a distance of 28 RE as well as a strong quasi-perpendicular bow shock at a distance of about 64 RE are observed

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.

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.

Electronic Waves in the Ionosphere

The study of electrostatic waves is important for the understanding of the fine structure of the ionosphere. Although a number of different instabilities occur in the ionosphere we shall limit ourselves to a discussion of two important instabilities that occur in the E-region of the ionosphere. These instabilities are known as the Farley-Buneman two-stream instability and the gradient drift instability. The instabilities will be discussed from in situ rocket observations in the ionosphere.