Details

Autor(en) / Beteiligte

• Kowalski, Adam F.
• Hawley, S. L.
• Carlsson, M.
• Allred, J. C.
• Uitenbroek, H.
• Osten, R. A.
• Holman, G.

Titel

New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation

Ist Teil von

• Solar physics, 2015-12, Vol.290 (12), p.3487-3523

Ort / Verlag

Dordrecht: Springer Netherlands

Erscheinungsjahr

2015

Link zum Volltext

Quelle

SpringerLink

Beschreibungen/Notizen

• The heating mechanism at high densities during M-dwarf flares is poorly understood. Spectra of M-dwarf flares in the optical and near-ultraviolet wavelength regimes have revealed three continuum components during the impulsive phase: 1) an energetically dominant blackbody component with a color temperature of T ≈ 10 4 K in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at λ ≤ 3 646  Å, and 3) an apparent pseudo-continuum of blended high-order Balmer lines between λ = 3 646  Å and λ ≈ 3 900  Å. These properties are not reproduced by models that employ a typical “solar-type” flare heating level of ≤ 10 11 erg cm − 2 s − 1 in nonthermal electrons, and therefore our understanding of these spectra is limited to a phenomenological three-component interpretation. We present a new 1D radiative-hydrodynamic model of an M-dwarf flare from precipitating nonthermal electrons with a high energy flux of 10 13 erg cm − 2 s − 1 . The simulation produces bright near-ultraviolet and optical continuum emission from a dense ( n > 10 15 cm − 3 ), hot ( T ≈ 12 000 – 13 500 K ) chromospheric condensation. For the first time, the observed color temperature and Balmer jump ratio are produced self-consistently in a radiative-hydrodynamic flare model. We find that a T ≈ 10 4 K blackbody-like continuum component and a low Balmer jump ratio result from optically thick Balmer ( ∞ → n = 2 ) and Paschen recombination ( ∞ → n = 3 ) radiation, and thus the properties of the flux spectrum are caused by blue ( λ ≈ 4 300  Å) light escaping over a larger physical depth range than by red ( λ ≈ 6 700  Å) and near-ultraviolet ( λ ≈ 3 500  Å) light. To model the near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer lines, we include the extra Balmer continuum opacity from Landau–Zener transitions that result from merged, high-order energy levels of hydrogen in a dense, partially ionized atmosphere. This reveals a new diagnostic of ambient charge density in the densest regions of the atmosphere that are heated during dMe and solar flares.