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

Abstract

The present paper is a summary of studies carried out from Viking measurements on the propagation and the generauon 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 lo•v-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 o•fio•v.L > 0, for small parallel velocities, is the free energy source that drives unstable the cyclotron maser. 11,657 11,658 Roux ET AL.: AURORAL KILOMETRIC RADIATION SOURCES

Observational study of generation conditions of substorm-associated low-frequency AKR emissions

“…Since the altitude dependence of the magnetic field is well known, the AKR generation altitude can be estimated from the AKR frequency. The usual AKR frequencies (100-400 kHz) correspond to altitudes of 3300-9000 km (1.5-2.5 R E radial distance), which is in accordance with the idea that AKR is produced within the acceleration region (Roux et al, 1993;Strangeway et al, 2001). AKR propagates upward from the generation region in a cone centred along the magnetic field direction.…”

Modeling the radio signature of the orbital parameters, rotation, and magnetic field of exoplanets

“…In the Earth's magnetosphere, interaction with the solar wind (so-called " Dungey-cycle" Dungey 1961), lead to the development of a steady state current circuit that leads to the formation of parallel electric fields and potential drops in the auroral regions. Those accelerate electrons (and ions) generating horseshoe/shell distributions, and at the same time carve density cavities in the auroral plasma, leading to the amplification of auroral radio waves in the shell mode (<ω ce ) (Roux et al 1993).…”

Backward Wave Cyclotron-Maser Emission in the Auroral Magnetosphere

“…It has been suggested that small scale magnetic mirrors or convergent flux tubes may be formed within the jet via hydrodynamic instabilities or shocks, providing the means of generating the required electron velocity distribution Bing2003, Begel2005. In Earth's auroral region, observations by the Viking spacecraft [11,16] and the Fast Auroral Snapshot (FAST) satellite [14] led to the suggestion that the free energy source for AKR is a population of downward accelerated electrons having a large perpendicular velocity produced by a combination of parallel accelerating electric fields and converging magnetic field lines. The electron distribution takes the form of a horseshoe or crescent shape due to the first adiabatic invariance as the electrons move into the stronger magnetic field.…”