Flash ionization signature in coherent cyclotron emission from brown dwarfs

Iryna Vorgul, Christiane Helling

Research output: Contribution to journalArticlepeer-review

6 Citations (Scopus)


Brown dwarfs (BDs) form mineral clouds in their atmospheres, where charged particles can produce large-scale discharges in the form of lightning resulting in substantial sudden increase of local ionization. BDs are observed to emit cyclotron radio emission. We show that signatures of strong transient atmospheric ionization events (flash ionization) can be imprinted on a pre-existing radiation. Detection of such flash ionization events will open investigations into the ionization state and atmospheric dynamics. Such events can also result from explosion shock waves, material outbursts or (volcanic) eruptions. We present an analytical model that describes the modulation of a pre-existing electromagnetic radiation by a time-dependent (flash) conductivity that is characteristic for flash ionization events like lightning. Our conductivity model reproduces the conductivity function derived from observations of terrestrial gamma-ray flashes, and is applicable to astrophysical objects with strong temporal variations in the local ionization, as in planetary atmospheres and protoplanetary discs. We show that the field responds with a characteristic flash-shaped pulse to a conductivity flash of intermediate intensity. More powerful ionization events result in smaller variations of the initial radiation, or in its damping. We show that the characteristic damping of the response field for high-power initial radiation carries information about the ionization flash magnitude and duration. The duration of the pulse amplification or the damping is consistently shorter for larger conductivity variations and can be used to evaluate the intensity of the flash ionization. Our work suggests that cyclotron emission could be probe signals for electrification processes inside BD atmosphere.
Original languageEnglish
Pages (from-to)1041-1056
Number of pages16
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date29 Jan 2016
Publication statusPublished - 1 May 2016


  • Instabilities
  • Radiation mechanisms: non-thermal
  • Atmospheric effects
  • Methods: analytical
  • Stars: atmospheres
  • Brown dwarfs


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