Energetic electrons produced by lower hybrid waves in the cometary environment and soft X ray emission: Bremsstrahlung and K shell radiation

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Abstract
Electron acceleration by lower hybrid waves is a phenomenon well known in fusion studies (e.g., the so‐called lower hybrid current drive), and it is also responsible for electron energization in many space environments. In the latter case, lower hybrid wave turbulence is of a self‐generated type. It can be produced by the modified two‐stream instability (MTSI) of the counterstreaming ion populations resulting from the mass‐loading effect, for example, contamination of the solar wind flow or interstellar plasma with the newly born ions. In the cometary case analyzed in this paper these ions are produced by photoionization of the neutral gas outflow evaporated from the cometary nucleus. The interesting feature about lower hybrid oscillations is that they can be in simultaneous Cherenkov resonance with unmagnetized slow ions and fast but magnetized electrons, and owing to this, serve as a powerful tool for energy coupling leading to electron energization. In situ measurements at Comet Halley revealed the existence of intense lower hybrid turbulence inside the cometary bow shock. The quasi‐linear theory of the MTSI evolution constructed in this paper demonstrates a consistency of these wave measurements with the large flux of energetic electrons also measured during the encounter with Comet Halley. Energetic electrons penetrating inside the cometary atmosphere produce soft X ray emission by a combination of bremsstrahlung and line radiation of oxygen atoms. Theoretical results are found to be in good agreement with six independent observations of the X ray cometary emission by the Röntgen satellite and Beppo‐SAX telescopes.