Radiative Hydrodynamic Models of the Optical and Ultraviolet Emission from Solar Flares

Abstract

We report on radiative hydrodynamic simulations of moderate and strong solar flares. The flares were simulated by calculating the atmospheric response to a beam of non-thermal electrons injected at the apex of a one-dimensional closed coronal loop, and include heating from thermal soft Xray, extreme ultraviolet and ultraviolet (XEUV) emission. The equations of radiative transfer and statistical equilibrium were treated in non-LTE and solved for numerous transitions of hydrogen, helium, and Ca ii allowing the calculation of detailed line profiles and continuum emission. This work improves upon previous simulations by incorporating more realistic non-thermal electron beam models and includes a more rigorous model of thermal XEUV heating. We find XEUV backwarming contributes less than 10% of the heating, even in strong flares. The simulations show elevated coronal and transition region densities resulting in dramatic increases in line and continuum emission in both the UV and optical regions. The optical continuum reaches a peak increase of several percent which is consistent with enhancements observed in solar white light flares. For a moderate flare (∼M-class), the dynamics are characterized by a long gentle phase of near balance between flare heating and radiative cooling, followed by an explosive phase with beam heating dominating over cooling and characterized by strong hydrodynamic waves. For a strong flare (∼X-class), the gentle phase is much shorter, and we speculate that for even stronger flares the gentle phase may be essentially non-existent. During the explosive phase, synthetic profiles for lines formed in the upper chromosphere and transition region show blue shifts corresponding to a plasma velocity of ∼120 km s −1 , and lines formed in the lower chromosphere show red shifts of ∼40 km s −1 .

IRIS observations of the Mg ii h and k lines during a solar flare

“…Modelling of this line in a flare atmosphere should indicate if a direct mapping of centroid velocity to upper atmospheric velocity exists. A downward motion is expected in models of explosive chromospheric evaporation (Fisher et al 1985;Allred et al 2005), and has been observed previously in solar flares, including the optically thick He ii line which showed a similar magnitude of centroid shift as we observe in Mg ii (see Milligan & Dennis 2009 and references therein). Svestka et al (1980) and Ding et al (1995) discuss finding similarly shortlived downflows that were only associated with the outer edge of the ribbon.…”

The Origin of Superflares on G-Type Dwarf Stars of Various Ages

“…This analysis allowed them to trace the behaviour of the Balmer continuum at λ < 3646Å and features of the line profiles both in weak and large flares. Observations in the blue spectral range agree well with both the two-temperature gas-dynamical modeling of an impulsive flare on the red dwarf (Katsova, Boiko, and Livshits, 1997) and the new single-temperature radiative hydrodynamic flare models by Allred et al (2005). The most important result of the analysis of these observations by Kowalski et al (2013) is the detection of two components of the flare optical continuum: blue continuum at the flare maximum that linearly decreases with wavelength from λ = 4000 − 4800Å, indicates the hot, black-body emission with typical temperatures of T BB ∼ 9000 − 14 000 K; the red continuum gives evidence for a T BB ∼ 8000 K component at the beginning of the gradual decay phase during eight flares, most of which are impulsive events.…”

Hαspectroscopy and multiwavelength imaging of a solar flare caused by filament eruption

“…A two-dimensional magnetic reconnection model called CSHKP (Carmichael 1964;Sturrock 1966;Hirayama 1974;Kopp & Pneuman 1976), suggests that the plasma surrounding a null point in the corona is heated such that high coronal pressure, thermal conduction, and non-thermal particles (mostly electrons) can efficiently carry energy from the magnetic reconnection site in the corona to the lower solar atmosphere along the magnetic field lines (Magara et al 1996). Thermal radiation from soft X-rays, EUV, and UV can also contribute to this process, but this contribution was found to be very small (Allred et al 2005). Other more recent works have raised questions about the viability of this mechanism in the light of recent observations (Fletcher & Hudson 2008) and suggested Alfvén wave propagation as an alternate energy transport mechanism from the corona to chromosphere during flares (Russell & Fletcher 2013).…”