Multiwavelength observations Multiwavelength observations of a partially occulted solar of a partially occulted solar

flareflare

Laura Bone, John C.Brown, Lyndsay Fletcher.

OutlineOutline

BackgroundObservationsInterpretationConclusions

Coronal HXR sourcesCoronal HXR sources First observed in occulted events in the 1970’s using data

from OSO 5 and OSO 7Earliest imaging observations found coronal emission at 3.5-

16keV extending to 30000km.Observations with Hinotori extended the energy range to

25keV. Several further sources observed with yohkoh/HXT;

– Masuda (1994).– Kosugi et al. (1995).– Feldman et al. (1994).

Since the launch of RHESSI in 2002 a number of coronal sources have been observed.

Theoretical interpretationsTheoretical interpretationsLocation of a fast mode shock, occuring

where the outflow jet from a coronal reconnection region impacts on a dense and static loop system below.

Signature of the current sheet itselfParticles trapped and possibly accelerated in

the field below a reconnecting coronal structure

Thick target bremsstrahlung from non thermal particles in a dense part of the corona.

20 July 2002 X3.3 flare20 July 2002 X3.3 flare

RHESSI image reconstructionRHESSI image reconstruction

PIXON image reconstruction algorithm used. High quality, excellent noise suppression, photometrically

accurate reconstruction. BUT! Very time consuming.

RHESSI imagesRHESSI images

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

RHESSI spectraRHESSI spectra

T=26.4MK

EM=6.7e49cm-3

=3.9

I()=A

=1e6-3.6

Where the photon spectrum can be approximated by a power law IAthe instantaneous number of electrons is given by the formula;

integrating over energy we can get N(>10keV)=7.0e35 electrons.

21241 ]

2

1,

2

1[)1)(/1067.5()(

AEBnEN i

OVSAOVSA(Owens Valley Solar Array)(Owens Valley Solar Array)

2 x 27m and 5 x 2m dishesTunable to any harmonic of 200MHz from

1-18GHz.Records left, right and circular polarisation.

OVSA dataOVSA dataDynamic spectrum shows impulsive nature of flare

From spectrum we can derive different parameters.

Derivation of N and B from Radio Derivation of N and B from Radio datadata

Can fit radio spectrum using a function of the form (Stähli et al.,1989)

))exp(1( BAS

Where and are respectively, the low and high frequency slopes. For the optically thin part of the spectrum shown , Using;

9.022.1 (Dulk and Marsh,1982) we can obtain a value for the electron spectral index of the radio emission to be =3.13

Assuming a line of sight angle we can use the polarisation measurements to determine magnetic field strength. rc =0.15 => using the expression given in Dulk (1985);

cos545.0782.0

cos071.0035.0 1010 26.1

bc

vr

We find at 10.6GHz, b~25 at the flare peak (21:30).

Thus from;

We estimate the magnetic field strength to be ~150G.

BfB6108.2

085.050.0

06.036.031.09 )(sin10102.2

Beff vT

We can determine the effective temperature from:

and optical depth/electron line of sight density.

since

S is measured from the radio emission and estimated from the radio emission, thus we calculate NV=1.5e36 electrons.

BNdr b

1)(sin10104.1

98.030.1

72.009.022.09

Ndr

T

c

fk

Ndr

S

NdrT

Ndr

T

bb

effb

2

22

Density estimatesDensity estimatesCan estimate the plasma density in the loop from the

emission measure Two separate measurements, RHESSI and GOES.RHESSI EM=6.7x1049 cm–3 GOES EM=28.0x1049cm-

3

Gives density estimate of between 1.0x1011cm-3 and 2.2x1011cm-3. Column density 1-2.2x1020cm-2

This implies that electrons of energies 28-41keV being fully stopped in the corona.

VnEM 2

4

19

2

8.83

eK

NKNEloop

Beam driven evaporationcBeam driven evaporationc

Power >25keV

Therefore;

-12725 s 108.7 ergP

219108.3 cm

Conductive evaporationConductive evaporation

Thus for this event;

hard for this event to differentiate between beam andconductive evaporation!

2201056.6 cmNcond

Cooling timescalesCooling timescales Conductive cooling time assuming constant density and no flows.

For values derive gives cooling time of <100sec. Radiative cooling time is given by

Which is ~2000-6000 sec, either conductive cooling is being inhibited or constant heating is occuring.

25

210104

T

nLtc

190

10

1058.1

3

Tn

ktr

ConclusionsConclusions

Contrary to typical observations, very high energy electrons observed in a coronal source.

~1036electrons instantaneously in the flaring system, V=6.98x1027cm3.

Magnetic field ~150G. T~30MKDensity >1011cm-3,leading to electrons <40keV

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.