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Solar radio physics |
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Particle acceleration during flares -
the part played by an outflow termination shock
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During solar
flares energetic nonthermal electromagnetic radiation up to the gamma-ray
range is emitted from the corona implying the generation of accelerated
electrons. Particle acceleration in cosmic plasma is of fundamental physical
interest. The solar corona offers detailed remote sounding of these processes.
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An interesting
example is the X17-flare of 28 October 2003, one of the strongest events
of the last activity cycle. Fig. 1 (left) shows the sun as seen by
SOHO-EIT about 10 min after the onset of the impulsive flare phase
still overexposed at active region 10486. Fig. 1 (right) is a TRACE-image
in the same spectral range, isolines overplotted are the Hard X-ray sources
imaged by the RHESSI spacecraft.
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Figure 1:
The sun at 195 Angstr. with the flaring active region NOAA 10486 at about
11:12 UT. Left: SOHO EIT. Right: TRACE with superposed RHESSI hard
X-ray sources (read: 12-25 keV, blue: >= 100-300 keV).
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For this event
an enhanced flux of hard X- and gamma-rays up to an energy of 10 MeV has
been observed by the INTEGRAL spacecraft (flux curve on top of right panel
in Fig. 2) indicating strong impulsive generation of relativistic electrons.
The radio signature of a standing shock wave occured in the AIP radio data
simultaneously with the enhanced energetic radiation (bright backbone and
herringbone fine structure emission in the dynamic spectrum, bottom of
right panel in Fig. 2).
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In the framework
of the magnetic reconnection scenario of eruptive flares (Fig. 2, left
panel), jets appear in the outflow region (bounded by slow mode shocks,
SMSS) above and underneath the diffusion region (DR). If the jets
penetrate with a super-Alfvenic speed into the surrounding plasma
then standing fast-mode shocks (FMSS) can be established near the postflare
loop top (PFL), and – less probably – behind the erupting prominence (EP).
These shocks are a source of energetic electrons and become visible
in meter wave radio emission under certain conditions (as was recognized
for the first time in AIP dynamic radio spectra, see previous work in
A&A 2002, 384, 273 and ApJ 2004, 615, 526).
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Figure 2: Left the
scheme of the plasma-magnetic field configuration
evolving in the corona during an eruptive flare. Right top: 7.5-10 MeV
flux measured by INTEGRAL (after Gros et al. 2004, ESA-SP 552, p. 669).
Right bottom: AIP dynamic radio spectrum,, range 200-400 MHz, same time
scale. The red arrow points from the fast mode shock in the scheme to the
radio signature of this shock in the AIP dynamic spectrum. For more explanation
see text.
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We quantitatively
checked the question how powerful an electron accelerator can contribute
to the observed flare energy release given the termination shock magnetic
field configuration. In contrast with earlier work, the theoretical considerations
are founded on realistic coronal parameters well observed for the huge
X17-flare of 28 October 2003 (see the comprehensive data analysis by
Aurass, Mann, Rausche & Warmuth,
A&A 2006, 457, 681-692).
As shown in
the diagram Fig. 3 the electrons are energized by shock drift acceleration
which was necessarily treated here in a fully relativistic manner. After
acceleration, the electrons travel along the postflare loop magnetic field
towards the denser chromosphere where they emit hard X- and gamma-ray radiation
via bremsstrahlung.
Using the derived
theoretical results, the observed photon fluxes in the range 7.5-10 MeV
are well explained. Our work confirms the prediction by Tsuneta and Naito
(ApJ 1998, 495, L67) in a quantitative manner. For more information see
Mann, Aurass & Warmuth (A&A 2006, 454, 969).
The presented results are a serious step toward a better understanding
of energy release in solar flares.
Gottfried Mann, Henry Aurass and Alexander Warmuth
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Illustration of the effect of shock drift acceleration in the velocity
space. For a shock wave with a normalized speed of 0.8 and
a magnetic field strength jump of 2, all particles in the grey area are
transformed into the dark area. In the same case, all particles right of
the dash-dotted line will be transmitted into the downstream region at
the second shock encounter.
Contact
Prof. Gottfried Mann
Astrophysikalisches Institut Potsdam
An der Sternwarte 16
D-14482 Potsdam
(0331) 7499 292
Courtesy: SOHO-EIT-, TRACE-, INTEGRAL-, RHESSI teams for the image data sources.
[Solar radio physics]
[AIP home page]
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