M. Jespersen

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.

Auroral kilometric radiation from transpolar arcs

Observations from the Swedish satellite Viking allow us to study the relationship between auroral kilometric radiation (AKR) and discrete auroral features. Previous work has shown that AKR generation is most often associated with nightside aurora. We present wave data which show that under certain circumstances the source regions may also occur on discrete features, identified as transpolar arcs. The wave spectrograms detected during crossings or closest approaches to such sources exhibit structures similar to those observed during nightside AKR source crossings. Also, the associated ion beams and trapped conical electron populations with enhanced upward directed loss cones peak at comparable energies (∼1 keV).

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.

Electron Density Observations During Auroral Absorption Related to Radio Wave Communication Problems

This report deals with results from propagation experiments for measurements of the density and collision frequency of the free electrons during conditions of auroral absorption. The results are discussed with emphasis on their importance for problems of radio wave communication at high latitudes.