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

By J. Allred, S. Hawley, W. Abbett, M. Carlsson. 2005

331 citations recorded by the source corpus.

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 nonthermal electrons injected at the apex of a one-dimensional closed coronal loop and include heating from thermal soft X-ray, 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 on previous simulations by incorporating more realistic nonthermal electron beam models and includes a more rigorous model of thermal XEUV heating. We find that XEUV back-warming 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 nonexistent. During the explosive phase, synthetic profiles for lines formed in the upper chromosphere and transition region show blueshifts corresponding to a plasma velocity of ~120 km s-1, and lines formed in the lower chromosphere show redshifts of ~40 km s-1.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/dcfca10b42e4fc070aae88b48e27d164602b1b49

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. James R. A. Davenport
2. M. Temmer
3. J. Leenaarts
4. M. Kretzschmar
5. Wei Liu
6. M. Carlsson
7. H. Hudson
8. E. Hilton

Citations in the corpus

• Approximations for radiative cooling and heating in the solar chromosphere (2012, background): “…et al. 1976, 1978; Carlsson & Stein 1992, 1994, 1995, 1997, 2002; Rammacher & Ulmschneider 2003; Fossum & Carlsson 2005, 2006), sunspots (Bard & Carlsson 2010), solar flares (Abbett & Hawley 1999; Allred et al. 2005; Kašparová et al. 2009; Cheng et al. 2010) and stellar flares (Allred et al. 2006).”
• VELOCITY CHARACTERISTICS OF EVAPORATED PLASMA USING HINODE/EUV IMAGING SPECTROMETER (2009, background): “Indeed, most evaporation models predict this to be the case (Fisher et al. 1985a,b,c; Mariska et al. 1989; Allred et al. 2005).”
• TIME-RESOLVED PROPERTIES AND GLOBAL TRENDS IN dMe FLARES FROM SIMULTANEOUS PHOTOMETRY AND SPECTRA (2013, methodology): “The sophisticated one-dimensional RHD models of Abbett & Hawley (1999) and Allred et al. (2005, 2006) used the RADYN code (Carlsson & Stein 1994, 1995, 1997) to simulate flares on an M dwarf and on the Sun using moderate (1010 ergs cm−2 s−1, F10) and large (1011 ergs cm−2 s−1, F11) fluxes of mildly…”
• Global Forces in Eruptive Solar Flares: The Lorentz Force Acting on the Solar Atmosphere and the Solar Interior (2010, background): “from Allred et al. (2005). Note that in all cases, the results from the gasdynamic simulations are either close to the approximated maximum pressures from the estimates derived above, or else are bracketed by our estimates.”
• A UNIFIED COMPUTATIONAL MODEL FOR SOLAR AND STELLAR FLARES (2015, background): “Many previous studies (e.g., Abbett & Hawley 1999; Allred et al. 2005, 2006; Cheng et al. 2010) implemented the analytic expression for beam penetration depth derived by E78.”
• The Sun as a star: observations of white-light flares (2011, methodology): “WL continuum emission is thought to occur in the minimum temperature region or below, where heating is provided by radiative backwarming from the chromosphere as has been found in simulations (Allred et al. 2005; Allred et al. 2006; Cheng et al. 2010).”
• Impulsive Phase Flare Energy Transport by Large-Scale Alfvén Waves and the Electron Acceleration Problem (2007, background): “In the radiative hydrodynamic models of Allred et al. (2005) the density and temperature gradients between chromosphere and corona are indeed at first on average smoothed out by atmospheric heating in the impulsive phase, but then steeper temperature gradients occur as the corona heats.”
• An Observational Overview of Solar Flares (2011, background): “In beam-driven radiative hydrodynamic simulations, to achieve such temperatures at the appropriate heights in the atmosphere requires beam energy fluxes on the order of 1011 erg cm−2 s−1, or alternatively a lower electron flux for hundreds of seconds (Allred et al. 2005).”