On the Efficiency of Radio Emissions at the Double Plasma Frequency in the Magnetosphere of Exoplanet HD189733b

Abstract

On exoplanets with a weak magnetic field, the so-called plasma maser can be effectively implemented instead of an electron cyclotron maser. This maser involves the generation of plasma waves by energetic electrons and their conversion into radio emissions at the plasma frequency or at the double frequency. Under specific conditions, a maser effect occurs at the plasma frequency, which manifests itself in an exponential increase in radio emissions intensity with an increase in the energy of plasma waves. In this paper, we study the Raman scattering of excited plasma waves with the formation of an electromagnetic wave at the double plasma frequency in the plasmasphere of the exoplanet HD189733b, for which the three-dimensional structure of the plasma envelope has been studied. Although the maser effect is absent in the case of Raman scattering, the collisional absorption of radiation is significantly reduced at the second harmonic and the requirement for the brightness temperature in the source is reduced as well. It has been shown that the radio flux at the second harmonic increases sharply for this exoplanet from a few millijanskys at a frequency of 20 MHz to tens of janskys at a frequency of ≈4 MHz. This means that the decameter range near the cutoff frequency of the Earth’s ionosphere is the most promising range for the detection of second harmonic radio emissions by modern radio telescopes. In this case, the radio emissions of the second harmonic can provide information about the properties of plasmaspheres around exoplanets at considerable distances that are inaccessible during observations at the main plasma frequencies.