Optical Emission During Solar and Stellar Flares
Understanding the nature of explosive and eruptive events on the Sun is one of the most active areas of research in solar physics; partly because this type of activity in the solar atmosphere can impact our terrestrial environment here on earth. The study of flares, for example, is intimately connected to the study of magnetic eruptions on the Sun (e.g., CMEs), since the two often occur hand in hand. It is widely believed that the energy release during a flare is the result of a sudden restructuring or reconnection of magnetic fields in the solar corona. The energy release is localized, but its effects on the atmospheric plasma can be widespread, since charged particles can be accelerated to high energies, and can spiral along the fieldlines of coronal loops. Among the observable effects of high energy, "non-thermal" (meaning the particle energy is not coupled to the local temperature of the plasma) particles propagating along fieldlines and interacting with the surrounding coronal plasma are hard X-ray bremsstrahlung and gyrosynchrotron microwave emission (radiation that is observable by e.g., the RHESSI satellite and earthbound arrays of radio telescopes respectively).
The components of the intensity contribution function (the integrand of the formal solution of the transfer equation for emergent intensity) for Ca II K after magnetically confined chromospheric plasma at the footpoints of a coronal loop is subjected to 50 seconds of strong flare heating. The lower right panel shows the line profile, and the fraction of the emergent intensity emanating from a given height in the atmosphere (grayscale). (from Abbett and Hawley 1999).
My particular interest lies in understanding the optical emission that results when these high energy particles impact the relatively high density plasma of the solar chromosphere and photosphere. This is a relatively complex task, since the line emission so produced generally originates from the chromosphere --- a region that is optically thick to strong spectral lines. This requires a dynamic numerical model capable of describing the non-local, non-equilibrium transport of radiation (non-LTE radiative hydrodynamics). The bottom line of Abbett and Hawley 1999 is to point out that during flares in particular, the optical emission one observes spectroscopically is highly dependent on the dynamics of the atmosphere --- emission can originate far from where one might expect, and the atmosphere is generally far from LTE in most of the strong lines (i.e. the Planck function is decoupled from the source function in regions where the plasma is in emission). For a nice tutorial on the theory of macroscopic radiation transport, check out R.J. Rutten's lecture notes on radiative transfer.
This type of flare modeling is not limited to the Sun --- in fact, with Suzanne Hawley and Joel Allred, we are applying this formalism to the atmosphere of the active M-dwarf star AD-Leo in an attempt to better understand the optical emission resulting from a series of flares observed during a coordinated campaign in March of 2000 (see Hawley et al. 2003).