# Presentation: 804310

## SP21A-02Transition Region Emission and the Energy Input to Thermal Plasma in Solar Flares

- Gordon D. Holman (*Gordon.D.Holman@nasa.gov*),& Brian R. Dennis (*Brian.R.Dennis@nasa.gov*)(NASA/GSFC),Leah Haga (*lhaga@lasp680.gsfc.nasa.gov*)(CUA, NASA/GSFC),John C. Raymond (*jraymond@cfa.harvard.edu*),& Alexander Panasyuk (*apanasyuk@cfa.harvard.edu*)(CfA)

## Motivation

- Understanding the energetics of solar flares depends on obtaining reliable determinations of the energy input to flare plasma.  X-ray observations of the thermal bremsstrahlung from hot flare plasma provide temperatures and emission measures which, along with estimates of the plasma volume, allow the energy content of this hot plasma to be computed.  However, if thermal conduction energy losses are significant or if significant energy goes directly into cooler plasma, this is only a lower limit on the total energy injected into thermal plasma during the flare.

## Summary

- We use SOHO Ultraviolet Coronagraph Spectrometer (UVCS) observations of flare O VI line emission, scattered by coronal O VI ions, to deduce the flare emission at transition region temperatures between 100,000 K and 1 MK for three X-class flares.
- We find that the radiated energy at these temperatures does not significantly increase the deduced energy input to the thermal plasma.
- We compare these radiative energy losses with estimates of thermal conduction losses from the X-ray-emitting plasma observed by GOES and RHESSI.
- We find that in most cases the radiative losses exceed conduction losses computed using the half length of the flare loop.

## The Observations

- The 42 arcmin long UVCS slit was placed parallel to a tangent to the solar limb at 1.64 R๏ for each flare.    UVCS obtained a sequence of 120 s exposures of O VI lines at 1032 Å and 1037 Å separated by about 10 s intervals.
- The GOES satellites obtained flare X-ray fluxes every 3 s in two bands, 1.6 – 12 keV and 3 – 25 keV.
- The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) obtained flare spectra and images every 20 s in 1 keV energy bands above 6 keV.

## Obtaining the Flare Transition Region Energy Fluxes from UVCS Data

- Determine the O VI column density in the corona before the flare, assuming a spherically symmetric corona with density decreasing as r****4.
- Use this column density and the scattering cross section to determine the illuminating flux from the flare.
- Divide the O VI luminosity by the emissivity to determine the emission measure (*EM*) of the O VI emitting gas.
- Assuming that *EM(log T) **** (log T)**0.5* for *5.0 **≤ log T ≤ 6.0*, multiply the emission measures times the theoretical cooling rates to obtain the energy flux.

## Obtaining the ThermalEnergy in the Flare Plasma

- The thermal energy content at time *t* is 		*E(t) = 3 kT(t) ( EM(t) f V(t) )**1/2**,*where *V* is the “observed” plasma volume and *f* is the plasma filling factor, taken to be 1.
- The plasma temperature *T(t)* and emission measure *EM(t)* are obtained from GOES fluxes and their ratios, and from isothermal fits to RHESSI spectra.
- The volumes* V(t)* are obtained from RHESSI images.  They are computed from measured 12-25 keV RHESSI source areas: 	*V(t) = [A**12-25**(t)]**3/2*.

## ResultsThe plots show:

*dotted curve* – the energy content of the X-ray-emitting plasma as a function of time, *E(t)*
*solid **blue** curve *– the accumulated energy injected into the X-ray-emitting plasma as a function of time, accounting for the transition region (TR) energy losses,     *E(t)* + (*computed* *TR radiative energy loss rate*)*t*
*blue** bars* – the average radiative energy loss rate from the transition region during each UVCS observing period
*yellow** bars* – the average rate of change of the energy in the thermal plasma during each UVCS observing period,     *E* / (120 s)
*red** bars* – the average minimum power in nonthermal electrons during each UVCS observing period

## Conclusions

- Radiative energy losses from transition region plasma in the 105 **** 106 K temperature range have a negligible impact on the deduced energy input to the thermal flare plasma.
- The radiative energy losses from the transition region in most cases exceed the conduction losses from the X-ray-emitting plasma computed using the half length of the flare loop.  *Either* the transition region plasma is directly heated *or* the conduction scale length is significantly less than the loop half length.

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## Thermal Conduction

- Energy losses due to thermal conduction can be estimated as *P**cond** = **κ**T**7/2**A/L*, where *κ** *is a constant and *A* is the cross sectional area of the flare loop.  The temperature gradient scale length, *L*, is often taken to be the half length of the loop.
- The following table compares the transition region energy losses obtained from the UVCS data with conduction energy losses from the X-ray-emitting plasma, computed using loop areas, *A,* and half lengths, *L**1/2*, estimated from RHESSI images.  An effective temperature gradient scale length, *L**eff*, that gives conduction losses equal to the transition region losses, is also computed. *L**eff* is in most cases smaller than *L**1/2*.

## Comparison of Computed Transition Region and Conduction Energy Fluxes