Double Plasma Resonance at Ion Cyclotron Harmonics in the Jovian Magnetosphere

Abstract

The ion cyclotron waves instability near the frequencies of ionic cyclotron harmonics is studied under conditions typical for the Jovian kilometer radio emission source. This instability is caused by the presence of the ions with a nonequilibrium loss cone-type distribution function in the source. Expressions for the growth rate of the ion cyclotron wave's instability are obtained, and it is shown that the amplification of these waves significantly increases when the frequency of the lower-hybrid resonance coincides with one of the ion cyclotron harmonics. Possible mechanisms for conversion of the ion cyclotron waves excited in the Jovian kilometer radio emission source into electromagnetic radiation are discussed. It is shown that the conversion of ion cyclotron waves, due to their coalescence with high-frequency plasma waves, is possible only into ordinary electromagnetic waves, while the process capable of providing conversion of ion cyclotron waves into fast extraordinary electromagnetic waves is scattering by suprathermal electron fluxes. In the latter, despite the thermal spread of electron velocities, the radiation that leaves the source region is concentrated in a narrow frequency range Δ ≪ near the local gyrofrequency ≃ Be , and the polarization of this radiation corresponds to the extraordinary wave. The estimates of the electron energy, which is necessary for such conversion, have shown the possibility of the process realizing under conditions characteristic for the Jovian kilometer radio emission source.

Origin of the zebra structure in the Jovian decameter radio emission

“…The DPR effect at ion cyclotron harmonics consists in the efficient excitation of the ion cyclotron waves propagating practically across the magnetic field due to the cyclotron instability (the instability growth rate increases about one to two orders of magnitude). The growth rate increasing occurs in a narrow frequency range of ∆ f f , located above the value of the cyclotron harmonic frequency s f Bi ≈ f pi (Shaposhnikov et al 2018). It should be noted that a small difference between the frequency of the excited wave and the frequency of the ion cyclotron harmonic is insignificant for our study, thus for simplicity, we assume f = s f Bi = f pi .…”

“…The theory of the DPR effect at the ion cyclotron harmonics is presented in Shaposhnikov et al (2018). Here, we limit our discussion to the information that allows us to understand the proposed interpretation of the origin of the ZS in the decameter emission of Jupiter.…”

“…where f pi = e 2 N/πm i and f Bi = eB/2πm i are the plasma and cyclotron ion frequencies, accordingly, m i is the mass of ion, s is the ion cyclotron harmonic number. In the case when f 2 Le f 2 Be and f 2 pi f 2 Bi , the condition of double plasma resonance at ion cyclotron harmonics takes a simpler form (Zlotnik et al 2016;Shaposhnikov et al 2018):…”

“…According to Zlotnik et al (2016), radiation sources, each of which generates radiation corresponding to one of the so-called zebra stripes, are located along the magnetic field lines at heights where the lower hybrid resonance frequency coincides with one of the local ion cyclotron harmonics. Excited ion cyclotron waves are converted due to the scattering by suprathermal magnetized electrons into extraordinary electromagnetic waves with frequencies close to the local electron cyclotron frequency f ≈ f Be + f 2 Le / f Be (Shaposhnikov et al 2018). As a result, a zebra-like structure is formed in the dynamic spectrum of the Jovian radiation.…”

Interpretation of Harmonic Structure in Jovian Decametric Radiation

“…Эффект DPR заключается в эффективном возбуждении за счет циклотронной неустойчивости (инкремент возрастает примерно на один-два порядка) ионных циклотронных волн, распространяющихся почти поперек магнитного поля. Возрастание инкремента происходит в узком интервале частот ∆ ≪ , расположенным над частотой циклотронной гармоники[5]. Конверсия возбужденных ионных циклотронных волн в быстрые необыкновенные волны (декаметровое радиоизлучение Юпитера соответствует необыкновенной моде) происходит при рассеянии на потоках надтепловых электронных с энергиями ≳ 20~кэВ.…”