Flash ionization 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 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 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 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.

Insight into atmospheres of extrasolar planets through plasma processes

“…The cyclotron radiation emerges nearly perpendicular to the magnetic field lines, but it will be refracted by the atmospheric material. The refracted radio beam may then cross atmospheric regions where lightning occurs (orange arrows in Figure 3), and the lightning will affect the beam's electromagnetic field such that it leaves a time-signature on it [Vorgul and Helling, 2016]. The geometry of this is shown in Figure 3.…”

“…The geometry of this is shown in Figure 3. Examples for the different emission cone cases are discussed in Vorgul and Helling [2016] Cyclotron maser emission (CME) can occur at relatively low plasma density only, when the electron plasma frequency is significantly smaller than the electron cyclotron frequency. Terrestrial CME originates from elongated plasma cavities along magnetic field lines [Pritchett et al, 2002;Burinskaya and Shevelev, 2017].…”

“…Schellart et al demonstrated that terrestrial lightning leaves a fingerprint on the radio emission from cosmic ray induced air showers. Vorgul and Helling [2016] developed the mathematical framework for the effect of extraterrestrial lightning on coherent radio emission on brown dwarfs: They developed a general model to describe Vorgul and Helling, 2016]. the influence of flash ionization events (like lightning or expositions of similar intensity) on pre-existing radiation that passes through the region where the transient events happen.…”

“…the influence of flash ionization events (like lightning or expositions of similar intensity) on pre-existing radiation that passes through the region where the transient events happen. A (direct) problem of electromagnetic field transformation was solved as a result of the flash transient events and have provided a recipe for observers in Section 5.5 of Vorgul and Helling [2016]. The main findings are:…”

“…This can be done by deriving the dependence of the field responses' maximum values on the maximum conductivity change. The absence of a noticeable plateau in this dependence [see Figure 5 in Vorgul and Helling, 2016] suggests that the runaway breakdown discharge is likely to be a source of the ionization, which requires about 10 times lower voltage to enable the discharge in comparison to a conventional capacitor-like discharge. If the plateau is noticeable, then a capacitor-like discharge is more probable.…”