Abstract: | Solar type II radio bursts are interpreted as the radio signature of shock waves travelling through the solar corona. Some
of these shock waves are able to enter into the interplanetary medium and are observed as interplanetary type II bursts. The
nonthermal radio emission of these bursts indicates that electrons are accelerated up to superthermal and/or relativistic
velocities at the corresponding shocks. Plasma wave measurements at interplanetary shock waves support the assumption that
the fundamental type II radio emission is generated by wave-wave interactions of electron plasma waves and ion acoustic waves
and that the source region is located near the transition region of the shock. Therefore, the instantaneous bandwidth of type
II bursts should reflect the density jump across the shock. Comparing the theoretically predicted density jump of coronal
shock waves (Rankine-Hugoniot relations) and the measured instantaneous bandwidth of solar type II radio bursts it is appropriate
to assume that these bursts are generated by weak supercritical quasi-parallel shock waves. Two different mechanisms for the
accelaration of electrons at this kind of shock waves are investigated in the form of test particle calculations in given
magnetic and electric fields. These fields have been extracted from in-situ measurements at the quasi-parallel region at Earth’s
bow shock, which showed large amplitude magnetic field fluctuations (so-called SLAMS: Short Large Amplitude Magnetic Field
Structures) as constituent parts. The first mechanism treats these structures as strong magnetic mirrors, at which charged
particles are reflected and accelerated. Thus, thermal electrons gain energy due to multiple reflections between two approaching
SLAMS. The second mechanism shows that it is possible to accelerate electrons inside a single SLAMS due to a noncoplanar component
of the magnetic field in these structures. Both mechanism are described in the form of test particle calculations, which are
supplemented by calculations according to adiabatic theory. The results are discussed for circumstances in the solar corona
and in interplanetary space.
Astrophysikalisches Institut, Observatorium für solare Radioastronomie, Potsdam, Germany. Published from Izvestiya Vysshikh
Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 1, pp. 84–104, January, 1998. |