Data from the VLF receivers aboard the Alouette and Isis spacecraft contain a number of examples of the ‘saucer’ radio phenomenon, so named because of its hyperbolic shape in the amplitude‐frequency‐time displays. These have been interpreted in terms of a model involving a small source of emission. Frequency‐time signatures have been calculated by using a ray‐tracing program assuming that the appropriate dispersion curves are those of the whistler mode on its resonance cone surface. Spatial dimensions of the sources have been deduced: typically, these are 0.5 km horizontally by no more than 10 km vertically. Total power radiated is about 10 mW. In the model, suprathermal electron beams flow through the source region and engage in an instability with background electrons. The deduced source dimensions are consistent with quantitative predictions of beam‐plasma theory.

VLF/ELF electric field wave data acquired on the ISIS 1, ISIS 2, and ISEE 1 satellites demonstrate the existence of a new phenomenon in which initially narrowband (∼1 Hz) upgoing signals from ground‐based VLF transmitters undergo a significant spectral broadening as they propagate through the ionosphere and protonosphere up to altitudes in the range 600–3800 km. For transmitter signals in the range 10–20 kHz, the spectral broadening can be as high as 10% of the nominal frequency of the input signal. Spectral broadening occurs only in the presence of impulsive VLF hiss and/or a lower hybrid resonance (LHR) noise band with an irregular lower cutoff frequency, and only for signals whose frequency exceeds the LHR frequency at the satellite location. It is often observed in association with a band of impulsive ELF hiss below 700 Hz. In many cases, the bandwidth of the spectrally broadened signals is a strong function of the electric dipole antenna orientation with respect to the local direction of the earth's magnetic field. Unusual dispersion in the components of the spectrally broadened pulses suggests that the spectral broadening may be due to a doppler shift effect in which the initial signals scatter from irregularities in the F region and couple into quasi‐electrostatic modes of short wave length. The large doppler shift associated with these short wavelength modes produces a significant increase in the bandwidth of the signal, as observed on a moving satellite. Since impulsive VLF hiss and irregular LHR noise bands have been linked to energetic (<1 keV) electron precipitation in the past, it is conjectured that the spectral broadening effect may be driven by precipitating electrons.

The Enhanced Polar Outflow Probe (e-POP) on the polar-orbiting CASSIOPE small satellite (325 × 1500 km, 80°inclination) is a suite of 8 plasma instruments, including imaging plasma and neutral particle sensors, magnetometers, dual-frequency Global Positioning System (GPS) receivers, charge coupled-device (CCD) cameras, a radio wave receiver and a beacon transmitter. The scientific objective of e-POP is to make observations of mesoscale and microscale plasma processes in the topside high-latitude ionosphere at the highest-possible resolution, specifically to study the microscale characteristics of plasma outflow and related acceleration processes, the occurrence morphology of neutral escape, and the effects of auroral currents on plasma outflow and those of plasma microstructures on radio propagation: the strategy is to use the large data storage and high-speed telemetry downlink capacity of a companion, experimental communications payload on board CAS-SIOPE to support the high-resolution observations of particle distributions, waves and magnetic fields to 10-ms time scale (∼ 100 m spatial scale) and the imaging of the aurora on 100-ms time scale, as well as imaging studies of the ionosphere in conjunction with groundbased transmitters and ground receiving stations on comparable (10-100 ms) time scales.

Heavy (O+) ion energization and field‐aligned motion in and near the ionosphere are still not well understood. Based on observations from the CAScade, Smallsat and IOnospheric Polar Explorer (CASSIOPE) Enhanced Polar Outflow Probe at altitudes between 325 km and 730 km over 1 year, we present a statistical study (24 events) of ion heating and its relation to field‐aligned ion bulk flow velocity, low‐frequency waves, and field‐aligned currents. The ion temperature and field‐aligned bulk flow velocity are derived from 2‐D ion velocity distribution functions measured by the suprathermal electron imager (SEI) instrument. Consistent ion heating and flow velocity characteristics are observed from both the SEI and the rapid‐scanning ion mass spectrometer instruments. We find that transverse O+ ion heating in the ionosphere can be intense (up to 4.5 eV), confined to very narrow regions (∼2 km across B), is more likely to occur in the downward current region and is associated with broadband extremely low frequency (BBELF) waves. These waves are interpreted as linearly polarized perpendicular to the magnetic field. The amount of ion heating cannot be explained by frictional heating, and the correlation of ion heating with BBELF waves suggests that significant wave‐ion heating is occurring and even dominating at altitudes as low as 350 km, a boundary that is lower than previously reported. Surprisingly, the majority of these heating events (17 out 24) are associated with core ion downflows rather than upflows. This may be explained by a downward pointing electric field in the low‐altitude return current region.

The Radio Receiver Instrument (RRI) is a four-channel digital receiver fed by four 3-metre monopoles, arranged in a crossed configuration, each connected to a high input impedance preamplifier. The RRI bandwidth extends from 10 Hz to 18 MHz. The receiver measures the electric fields of either spontaneous radio emissions or waves created by ground transmitters, such as ionosondes, high-frequency radars and ionospheric heaters. In order to measure accurately the intensity, frequency, direction of propagation and signal delay of such fields over the broad frequency range, modern digital receiver technology is employed. The amplified signals from the monopoles are digitized at a rate of 40 megasamples per second, and from there on, the signal is down-converted, filtered, time-stamped and communicated in digital form. The characterization results of the RRI flight model are reported. Formats for data commands for configuring the digital receiver and for data output are described.

Radio waves at frequencies between 525 and 5850 Hz were observed simultaneously on the ground and in the VLF receiver of the ISIS 1 spacecraft during a flight over the Max‐Planck‐Institut für Aeronomie ionospheric heater near Tromsø, Norway, on December 9, 1981. The heater carrier frequency, 4.04 MHz, was amplitude modulated with a set of four frequencies: 525, 1725, 2925, and 4125 Hz. The satellite detected all these fundamental frequencies plus harmonics of the 525 Hz and 2925 Hz components, caused by the nonsinusoidal modulation of the carrier. Characteristics of the signal received along the satellite track are in approximate agreement with the results of three‐dimensional ray tracing applied to a model of the ionosphere around Tromsø. The time dispersion of the signals is best fitted with an ionospheric density model based on real time data with relatively low peak values. Observed signal levels have been transformed to power flux by using a plasma dipole theory and wave polarization information obtained from the three‐dimensional ray solutions. Through the use of a magnetoplasma dipole theory for the induced D/E region current, the observed levels are found to correspond to radiation created by effective dipole currents between about 1.0 and 100 A. The geometrical optics theory also predicts two solutions for rays reaching a given satellite point near the polar limit of the reception zone, and a northern limit corresponding to a caustic surface. The frequencies of signal fades observed in this region are also predicted by the geometrical optics theory, but the fade depths are not. Simultaneous ground observations correspond to source region currents of about 1 A or less. Also, the ELF waves observed on the ground have different harmonic signal ratios and much smaller fluctuations than those observed on ISIS.

Abstract. An HF transmitter was operated at one end of the tethered sounding rocket payload OEDIPUS C, and a synchronized receiver was operated at the other end. Both the transmitter and the receiver were connected to dipoles. On the flight downleg after the tether had been cut, direct bistatic propagation experiments were carried out with the transmitter-receiver pair. During the flight, sharp minima which can be attributed to interference fringes were detected in the directly transmitted signal. Fringe frequencies observed on OEDIPUS C ionograms for four different interference schemes have been examined using the cold-plasma theory. First, fringes were observed which can be attributed to the Faraday rotation of the plane of linear electric field polarization for the

The soft‐particle spectrometers aboard the spacecrafts ISIS I and ISIS II detect sounder‐accelerated particles, i.e., fluxes of electrons and ions energized by the 100‐μs transmitter pulse (nominal peak power: 400 W). Fluxes of up to 108 st−1 eV−1 cm−2 s−1 are observed. Typical highest electron and ion energies are several hundred electronvolts and 100–200 eV, respectively. Sounder‐accelerated electron fluxes detected on ISIS II are energized near the major electron resonant frequencies: ƒpe (plasma frequency), ƒce (gyrofrequency), 2ƒce and the oblique resonance frequency domains. Ion fluxes are present from the lowest sounder frequency (0.1 MHz) up to the greater of ƒpe and ƒce. Electrons are observed at pitch angles near 90° while ions are present at all pitch angles. The observations can be interpreted using a model of particle motion and spacecraft dc potential both induced by the intense rf field (∼100 V/m). The ion results indicate that at ƒ < ƒpe, ƒce, a negative potential of about 100 V is on the spacecraft, whereas at ƒ > ƒpe, ƒce, the potential is much smaller. ISIS I data from equatorial perigee conditions show that electrons remain energized for a few milliseconds after the end of the rf pulse.