Flare Energetics

A solar flare/CME event is basically a process in which stored magnetic energy 
is converted into various forms that propagate in the solar atmosphere and 
through interplanetary space. Ultimately, all of the energy released appears 
either as kinetic energy of ejected particles and fields or as enhanced 
radiative output. By this time, however, much of the information on the energy 
release process has been lost.  Information about the fundamental energy release 
process can be acquired only through a more detailed determination of the 
partition of energy amongst various components (e.g., energetic particles, 
thermal plasma) present as the flare proceeds.
 
Emslie et al. (2004) studied the partitioning of energy in two specific solar 
flare/CME events (April 21, 2002 and July 23, 2002), using RHESSI data in 
conjunction with data from a variety of other instruments, such as ACE, SOHO, 
and GOES. The RHESSI data was crucial to this endeavor: hard X-ray and gamma-ray 
observations were used to deduce the energy transported by energetic electrons 
and ions, respectively, and RHESSI images were used to estimate the volume of 
the hot thermal source in order to convert from emission measure to thermal 
energy content.  They reached the "cautious" conclusion that the coronal mass 
ejection was the dominant component of the released energy, with the energy 
content in electrons, ions, and thermal plasma being approximately a factor of 3 
smaller, and of comparable magnitude to each other.
 
It should also be noted that not all these energy components are independent 
(for example, the energy in nonthermal electrons is converted through Coulomb 
collisions into energy in the thermal plasma).  Emslie et al. (2005) therefore 
provided a subsequent breakdown of the energy values into "primary" components 
(i.e., the magnetic field), "intermediate" components (i.e., those that are 
produced during the energy release process and subsequently transport energy 
throughout the flare plasma, such as nonthermal particles), and "final" 
components (i.e., those that leave the system, such as interplanetary 
high-energy particles, bulk mass motion in the CME, and radiation).
 
The energy present in energetic particles is, of course, far greater than that 
in the diagnostic radiation fields (hard X-rays and gamma rays, respectively) 
used to obtain the energy estimates; the remaining energy is deposited in the 
solar atmosphere and ultimately radiated as optical and UV emission. The RHESSI 
observations showed that the accelerated particles carry a surprisingly large 
fraction of the released energy during the impulsive phase of these two flares 
but relatively little if any during the gradual phase; this is consistent with 
the SORCE observations (Woods et al. 2004) of the total flare irradiance, which 
suggest that the total-irradiance light curve peaked prior to the GOES soft 
X-ray peak.