Multiwavelength observations of a partially occulted solar flare

Multiwavelength observations of a partially occulted solar flare Laura Bone, John C. Brown, Lyndsay Fletcher.

Outline l Background l Observations l Interpretation l Conclusions

Coronal HXR sources First observed in occulted events in the 1970’s using data from OSO 5 and OSO 7 l Earliest imaging observations found coronal emission at 3. 5 -16 ke. V extending to 30000 km. l Observations with Hinotori extended the energy range to 25 ke. V. l Several further sources observed with yohkoh/HXT; l – Masuda (1994). – Kosugi et al. (1995). – Feldman et al. (1994). l Since the launch of RHESSI in 2002 a number of coronal sources have been observed.

Theoretical interpretations l Location of a fast mode shock, occuring where the outflow jet from a coronal reconnection region impacts on a dense and static loop system below. l Signature of the current sheet itself l Particles trapped and possibly accelerated in the field below a reconnecting coronal structure l Thick target bremsstrahlung from non thermal particles in a dense part of the corona.

20 July 2002 X 3. 3 flare

RHESSI image reconstruction PIXON image reconstruction algorithm used. l High quality, excellent noise suppression, photometrically accurate reconstruction. l BUT! Very time consuming. l

RHESSI images As energy increases, emission concentrated more in looptop, contrary to traditional thick target model.

RHESSI spectra T=26. 4 MK EM=6. 7 e 49 cm-3 g=3. 9 I(e)=Ae-g =1 e 6 e-3. 6

Where the photon spectrum can be approximated by a power law I(e)=Ae-g the instantaneous number of electrons is given by the formula; integrating over energy we can get N(>10 ke. V)=7. 0 e 35 electrons.

OVSA (Owens Valley Solar Array) l 2 x 27 m and 5 x 2 m dishes l Tunable to any harmonic of 200 MHz from 1 -18 GHz. l Records left, right and circular polarisation.

OVSA data Dynamic spectrum shows impulsive nature of flare From spectrum we can derive different parameters.

Derivation of N and B from Radio data Can fit radio spectrum using a function of the form (Stähli et al. , 1989) Where a and a-b are respectively, the low and high frequency slopes. For the optically thin part of the spectrum shown , a-b =-1. 6. Using; (Dulk and Marsh, 1982) we can obtain a value for the electron spectral index of the radio emission to be d=3. 13

l Assuming a line of sight angle q we can use the polarisation measurements to determine magnetic field strength. rc =0. 15 => using the expression given in Dulk (1985); We find at 10. 6 GHz, n/nb~25 at the flare peak (21: 30). Thus from; We estimate the magnetic field strength to be ~150 G.

We can determine the effective temperature from: and optical depth/electron line of sight density. since S is measured from the radio emission and DW estimated from the radio emission, thus we calculate NV=1. 5 e 36 electrons.

Density estimates Can estimate the plasma density in the loop from the emission measure l Two separate measurements, RHESSI and GOES. l RHESSI EM=6. 7 x 1049 cm– 3 GOES EM=28. 0 x 1049 cm -3 l l Gives density estimate of between 1. 0 x 1011 cm-3 and 2. 2 x 1011 cm-3. Column density 1 -2. 2 x 1020 cm-2 l This implies that electrons of energies 28 -41 ke. V being fully stopped in the corona.

Beam driven evaporationc l Power >25 ke. V l Therefore;

Conductive evaporation Thus for this event; hard for this event to differentiate between beam and conductive evaporation!

Cooling timescales l Conductive cooling time assuming constant density and no flows. For values derive gives cooling time of <100 sec. l Radiative cooling time is given by l l Which is ~2000 -6000 sec, either conductive cooling is being inhibited or constant heating is occuring.

Conclusions l l l Contrary to typical observations, very high energy electrons observed in a coronal source. ~1036 electrons instantaneously in the flaring system, V=6. 98 x 1027 cm 3. Magnetic field ~150 G. T~30 MK Density >1011 cm-3, leading to electrons <40 ke. V being stopped in a coronal thick target scenario. Long duration event, mass must be continuously evaporated into the flaring system through both conductive and beam driven evaporation.