Flash ionisation signature in coherent cyclotron emission from Brown Dwarfs

Abstract

Brown dwarfs form mineral clouds in their atmospheres, where charged particles can produce large-scale discharges in form of lightning resulting in a substantial sudden increase of local ionisation. Brown dwarfs are observed to emit cyclotron radio emission. We show that signatures of strong transient atmospheric ionisation events (flash ionisation) can be imprinted on a pre-existing radiation. Detection of such flash ionisation events will open investigations into the ionisation state and atmospheric dynamics. Such ionisation events can also result from explosion shock waves, bursts or 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 ionisation 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 disks. We show that the field responds with a characteristic flash-shaped pulse to a conductivity flash of intermediate intensity. More powerful ionisation 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 ionisation 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 ionisation. Our work suggests that cyclotron emission could be probe signals for electrification processes inside BD atmosphere.

References in corpus (13)

• Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs
• Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf
• Vertical Atmospheric Structure in a Variable Brown Dwarf: Pressure-dependent Phase Shifts in Simultaneous Hubble Space Telescope-Spitzer Light Curves
• Properties of Saturn Kilometric Radiation measured within its source region
• Atmospheric Circulation of Brown Dwarfs: Jets, Vortices, and Time Variability
• Probing Atmospheric Electric Fields in Thunderstorms through Radio Emission from Cosmic-Ray-Induced Air Showers
• Multiwaveband photometry of the irradiated brown dwarf WD0137-349B
• Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. IV. The Active, Young Binary NLTT 33370 AB (=2MASS J13142039+1320011)
• Wideband Dynamic Radio Spectra of Two Ultra-cool dwarfs
• Reference study to characterise plasma and magnetic properties of ultra-cool atmospheres
• The radio lighthouse CU Virginis: the spindown of a single main sequence star
• CU Virginis - The First Stellar Pulsar
• Relativistic runaway electron breakdown and Terrestrial Gamma ray Flashes: GEANT4 simulation

Cited by in corpus (7)

• Exoplanet Clouds
• 3D-modelling of the stellar auroral radio emission
• Lightning climatology of exoplanets and brown dwarfs guided by Solar System data
• Is lightning a possible source of the radio emission on HAT-P-11b?
• Triboelectrification of KCl and ZnS particles in approximated exoplanet environments
• Exploring terrestrial lightning parameterisations for exoplanets and brown dwarfs
• Insight into atmospheres of extrasolar planets through plasma processes