| Solar flares are associated with a strong enhancement of the emission of electromagnetic
waves over a broad spectrum from the radio over the visible up to the hard X- and γ-ray
range, indicating the generation of energetic electrons during flares. In addition, the
occurrence of the γ-ray line emission during flares shows that protons are also accelerated at
the flare. RHESSI imaging observations reveal that the enhanced hard X- and γ-ray line
emission occur at the same time but their sources are spatially separated of about 20”. That
indicates that the electrons and protons are simultaneously accelerated but at different places
in the corona. It is intended to explain that the energetic electrons and protons are generated
at different location in the corona during flares. In the framework of the reconnection scenario
of solar flares, jets of hot plasma shoot away from the reconnection region due to the
relaxation of the new magnetic field configuration. If the jet velocity exceeds the local
Alfvén speed, a standing shock wave, so-called termination shock (TS), can be
established in the flare region. Such TS is able to accelerate both electrons and protons
via the shock-drift acceleration (SDA). A fully relativistic study of SDA at the TS
confirms that this mechanism is able to accelerate protons up to few tens of GeV under
flaring conditions in the solar corona. This model implies different locations for
electron and proton acceleration at the TS. That can explain the separation of the hard
X- and γ-ray sources as really seen by RHESSI imaging observations. Such an
agreement between the model and the observations supports the idea that the TS is the
source of energetic particles as originally proposed by Tsuneta & Naito (1998). |