Plasma mechanism of exoplanetary radio emissions

Abstract

As an alternative to the traditionally considered electron cyclotron maser (ECM) mechanism of exoplanetary radio emission (RE), we study plasma maser mechanism. The latter, contrary to ECM operates in dense and weakly magnetized plasmas, where electron cyclotron frequency fc is less than Langmuir frequency fL [1]. Similar mechanism is known to contribute the generation of RE in solar corona, as well as in magnetospheres of the Solar System planets [2,3]. It is a two-step process. At first, the plasma waves are excited due to Cherenkov instability in a weakly anisotropic background plasma by a small admixture of hot electrons with a loss-cone type non-equilibrium distribution function. Then, the electromagnetic radiation at fRE arises due to, e.g., plasma wave scattering on the background ions (Rayleigh scattering, fRE = fL), or nonlinear coupling of two plasma waves (Raman scattering, fRE = 2fL). In the first case, the maser effect at plasma frequency fL takes place under certain conditions, leading to an exponential grow of the RE intensity with the growing energy of plasma waves. In the case of Raman scattering of two plasma waves, resulting in generation of the RE at doubled plasma frequency, the maser effect is absent, but the collisional dissipation of RE is significantly reduced at the same time. This improves the requirements regarding the brightness temperature of the RE source, to provide a detectable on Earth radiation flux. In both cases the frequency band of the exoplanetary RE is defined not by magnetic field, but by the structure of planetary plasmasphere and density distribution there.We evaluate the efficiency of plasma mechanism of the RE generation and its detectability on Earth for the case of hot Jupiter HD189733b, for which the 3D structure of plasmasphere is simulated with the global multi-fluid self-consistent numerical model [4], taking into account the realistic stellar wind and radiation conditions. It is shown that the RE flux at doubled plasma frequency sharply increases from several mJy at 20MHz to several tens of Jy at 4 MHz. This means that the most favorable frequency range for detection of the RE from HD189733b falls into the decameter band in vicinity of the ionospheric cut-off.1. Zaitsev, V.V., Shaposhnikov, V.E., MNRAS, 2022, 513, 4082 (DOI:10.1093/mnras/stac1140)2. Zaitsev V. V., et al., A&A, 1986, 169, 345-354 (ISSN 0004-6361)3. Zlotnik E. Y., et al., JGR Space Physics, 2016, 121, 5307-5318 (DOI: 10.1002/2016JA02265)4. Rumenskikh, M. S., et al., ApJ, 2022, 927(2):238 (DOI: 10.3847/1538-4357/ac441d)