The Earth as a radio source: Terrestrial kilometric radiation

Abstract

Radio wave experiments on the Imp 6 and 8 satellites have shown that the earth emits very intense electromagnetic radiation in the frequency range of about 50–500 kHz. At peak intensity the total power emitted in this frequency range is about 109 W. The earth is therefore a very intense planetary radio source, with a total power output comparable to the decametric radio emission from Jupiter. We refer to this radio emission from the earth as terrestrial kilometric radiation. Terrestrial kilometric radiation appears to originate from low altitudes (less than 3.0 RE) in the auroral region. The intensity of the noise has a pronounced dependence on both the local time and the magnetic latitude of the observing point. At large radial distances the radiation is primarily observed on the poleward side of two cone‐shaped surfaces that are centered on the earth and symmetrically located with respect to the northern and southern auroral zones. The magnetic latitude of the cone‐shaped boundaries varies from greater than 50° in the local morning to near the magnetic equator in the local evening. Poleward of these boundaries the noise occurs in sporadic ‘storms’ lasting from ½ hour to several hours. Comparisons with auroral photographs obtained from the low‐altitude polar‐orbiting Dapp satellite show that the terrestrial kilometric radiation is closely correlated with the occurrence of discrete auroral arcs, which occur in the local evening region of the auroral zone. This association indicates that the kilometric radiation is probably generated by intense ‘inverted V’ electron precipitation bands, which cause the discrete auroral arcs. Possible mechanisms that can explain the generation and propagation of the terrestrial kilometric radiation are discussed.

Numerical simulations of unbounded cyclotron-maser emissions

“…A DC offset is present in the measurement due to lowfrequency electromagnetic field components associated with the electron beam propagation. After 180 ns, a peak rf output power of 3 kW was measured corresponding to an rf conversion efficiency of 1.1% -comparable with the estimate of 1% for the astrophysical phenomena (Gurnett 1974).…”

“…This trend is reflected in the results of unbounded PiC simulations of cyclotron-wave coupling, where radiation propagation has been observed with a distinctive backward-wave character to it, typically directed in a slightly backward direction, only a few degrees away from the perpendicular to the magnetostatic field. The corresponding rf conversion efficiencies are comparable with estimates for the astrophysical phenomena (Gurnett 1974;Treumann 2006;Bingham et al 2013). …”

Control of Jupiter's radio emission and aurorae by the solar wind

“…The processes involved appear to be very similar to those occurring in Earth's magnetosphere. The comparable type of radio emission at Earth is auroral kilometric radiation, which is known to be closely associated with the terrestrial aurora 19 . Just as with the jovian hectometric radiation, the terrestrial kilometric radiation is highly variable and is closely correlated with magnetic storms 20 , including those triggered by interplanetary shocks.…”

Universal time control of AKR: Earth is a spin‐modulated variable radio source

“…Early solar system exploration led to the discovery of hectometric radiation from Jupiter [Brown, 1974;Desch and Carr, 1974] and kilometric radiation from Saturn [Warwick et al, 1981], Uranus [Warwick et al, 1986], and Neptune [Warwick et al, 1989]. Auroral kilometric radiation (AKR) from the terrestrial polar region has been observed by Earth-orbiting satellites [Dunckel et al, 1970;Gurnett, 1974]. These planetary radio waves, whose emission frequency is very close to the local electron gyrofrequency, are generated by accelerated auroral electrons through the cyclotron maser instability [Wu and Lee, 1977;Pritchett, 1984;Pritchett and Strangeway, 1985].…”