Simulation of transient energy distributions in sub-ns streamer formation

Hodosán

et al. 2014

ApJ

Mineral clouds in substellar atmospheres play a special role as a catalyst for a variety of charge processes. If clouds are charged, the surrounding environment becomes electrically activated, and ensembles of charged grains are electrically discharging (e.g. by lightning), which significantly influences the local chemistry creating conditions similar to those thought responsible for life in early planetary atmospheres. We note that such lightning discharges contribute also to the ionisation state of the atmosphere. We apply scaling laws for electrical discharge processes from laboratory measurements and numerical experiments to Drift-Phoenix model atmosphere results to model the discharge's propagation downwards (as lightning) and upwards (as sprites) through the atmospheric clouds. We evaluate the spatial extent and energetics of lightning discharges. The atmospheric volume affected (e.g. by increase of temperature or electron number) is larger in a brown dwarf atmosphere (10 8 − 10 10 m 3 ) than in a giant gas planet's (10 4 − 10 6 m 3 ). Our results suggest that the total dissipated energy in one event is < 10 12 J for all models of initial solar metallicity. First attempts to show the influence of lightning on the local gas phase indicate an increase of small carbohydrate molecules like CH and CH 2 at the expense of CO and CH 4 . Dust forming molecules are destroyed and the cloud particle properties are frozen-in unless enough time is available for complete evaporation. We summarise instruments potentially suitable to observe lightning on extrasolar objects.

Section: Segment Lengthmentioning

confidence: 99%

Ionization in Atmospheres of Brown Dwarfs and Extrasolar Planets Vi: Properties of Large-Scale Discharge Events

Bailey

Hodosán

et al. 2014

ApJ

“…The resulting self-electric field (i.e. the self-repulsion of the electrons) accelerates the local electrons to an equivalent energy, ionizing the incoming neutral atoms that have an electron-impact ionization threshold, eφ I , that is less than 1 2 m gas v 2 0 (MacLachlan et al 2009(MacLachlan et al , 2013. Note that it is only the relative speed between the magnetized plasma and gas that is important; a moving magnetized plasma can encounter a stationary neutral component: the critical requirement is that the plasma is localized by the ambient magnetic field.…”

Section: Alfvén Ionizationmentioning

confidence: 99%

“…In the model atmosphere results considered here, excitation to metastable states would need to occur via an alternative process such as gas discharges; cosmic-ray interactions; or even as part of the Alfvén ionization mechanism where bombardment from neutral flow drives the excitation. For example, simulations of electron avalanches (the precursor to a full gas discharge) in 1 mm gap of atmospheric Nitrogen have shown that the metastable density can reach ≈ 10 12 cm −3 (MacLachlan et al 2013). Furthermore, if in a localized volume of a BD, [M/H]= 0.0 model atmosphere (for example at an atmospheric pressure of ≈ 10 −10 bar), Alfvén ionization excited all the Helium from its ground state to its first metastable state, the metastable density would be ≈ 10 10 cm −3 (see Fig.…”

Section: Metastable States Of Gas-phase Speciesmentioning

confidence: 99%

Ionization in Atmospheres of Brown Dwarfs and Extrasolar Planets. V. Alfvén Ionization

Stark

Diver

et al. 2013

ApJ

Observations of continuous radio and sporadic X-ray emission from low-mass objects suggest they harbour localized plasmas in their atmospheric environments. For low-mass objects, the degree of thermal ionization is insufficient to qualify the ionized component as a plasma, posing the question: what ionization processes can efficiently produce the required plasma that is the source of the radiation? We propose Alfvén ionization as a mechanism for producing localized pockets of ionized gas in the atmosphere, having sufficient degrees of ionization (≥ 10 −7 ) that they constitute plasmas. We outline the criteria required for Alfvén ionization and demonstrate it's applicability in the atmospheres of low-mass objects such as giant gas planets, brown dwarfs and Mdwarfs for both solar and sub-solar metallicities. We find that Alfvén ionization is most efficient at mid to low atmospheric pressures where a seed plasma is easier to magnetize and the pressure gradients needed to drive the required neutral flows are the smallest. For the model atmospheres considered, our results show that degrees of ionization of 10 −6 − 1 can be obtained as a result of Alfvén ionization. Observable consequences include continuum Bremsstrahlung emission, superimposed with spectral lines from the plasma ion species (e.g. He, Mg, H 2 or CO lines). Forbidden lines are also expected from the metastable population. The presence of an atmospheric plasma opens the door to a multitude of plasma and chemical processes not yet considered in current atmospheric models. The occurrence of Alfvén ionization may also be applicable to other astrophysical environments such as protoplanetary disks.

“…A major challenge is to model cloud formation and its consequent effect on the surrounding environment, involving plasma processes such as non-spherical dust growth (Stark et al 2006) and electrical discharge events (MacLachlan et al 2013). Opik (1956) investigated the electrostatic disruption (or Coulomb explosion) of dust grains, where the electrostatic stress of a body holding a net charge exceeds its mechanical tensile strength (the maximum stress or pressure that a material can withstand while being pulled), resulting in it fracturing.…”

Section: Introductionmentioning

confidence: 99%

Inhomogeneous cloud coverage through the Coulomb explosion of dust in substellar atmospheres

Stark

Diver

2015

A&A

Context. Recent observations of brown dwarf spectroscopic variability in the infrared infer the presence of patchy cloud cover. Aims. This paper proposes a mechanism for producing inhomogeneous cloud coverage due to the depletion of cloud particles through the Coulomb explosion of dust in atmospheric plasma regions. Charged dust grains Coulomb-explode when the electrostatic stress of the grain exceeds its mechanical tensile stress, which results in grains below a critical radius a < a Coul crit being broken up. Methods. This work outlines the criteria required for the Coulomb explosion of dust clouds in substellar atmospheres, the effect on the dust particle size distribution function, and the resulting radiative properties of the atmospheric regions. Results. Our results show that for an atmospheric plasma region with an electron temperature of T e = 10 eV (≈10 5 K), the critical grain radius varies from 10 −7 to 10 −4 cm, depending on the grains' tensile strength. Higher critical radii up to 10 −3 cm are attainable for higher electron temperatures. We find that the process produces a bimodal particle size distribution composed of stable nanoscale seed particles and dust particles with a ≥ a Coul crit , with the intervening particle sizes defining a region devoid of dust. As a result, the dust population is depleted, and the clouds become optically thin in the wavelength range 0.1-10 μm, with a characteristic peak that shifts to higher wavelengths as more sub-micrometer particles are destroyed. Conclusions. In an atmosphere populated with a distribution of plasma volumes, this will yield regions of contrasting radiative properties, thereby giving a source of inhomogeneous cloud coverage. The results presented here may also be relevant for dust in supernova remnants and protoplanetary disks.