Solar flares are violent eruptions suddenly releasing
magnetically stored free energy in the form of plasma heating, mass motions,
energetic particles, and electromagnetic radiation from radio waves to
gamma rays.
X-ray and extreme-ultraviolet observations and flare models
seem to confirm magnetic reconnection as flare mechanism.
For the first time, we found a spectral fine structure of solar meter wave
radio burst emission which can be due to sawtooth oscillations
in the hot flare plasma.
The sawteeth occur during the impulsive flare phase hard X-ray emission and
consist of a sequence of almost identical narrow band (~ 1%) drift bursts.
Similar oscillations are familiar in tokamak plasmas
and understood as signature of the kink instability of the toroidal current.
We believe that the radio sawteeth are nonthermal plasma emission due to 2-4%
density fluctuations of the flare plasma. The fluctuations can be excited
by a current instability in a coronal flare loop or in a vertical flaring
current sheet e.g. occuring behind a rising magnetic flux rope. This is
in analogy to kink instability effects observed in laboratory plasmas.
Top panel: dynamic radio spectrum of the solar flare on 3 November 1997.
The radio intensity is color coded where dark means bright emission.
The spectrum displays the evolution of radio emission in the corona at
heights between about 10 000 and 570 000 km (800-40 MHz).
The white and black time-parallel stripes are due to terrestrial interferences.
The box contains the sawtooth pattern and is enlarged below.
The bottom panel (enlargement): the arrow points to the sawtooth
pattern. The period of the sawtooth oscillations is about 5 seconds.
The transition jumps between
single teeth appear sometimes synchronized with sharp type III-like emission
revealing simultaneous electron beam generation.
In 1995-2000 we found 13 more or less well expressed sawtooth events indicating
that the effect is rare but not unique.
Our finding newly underlines an important analogy between coronal and
laboratory plasma processes.
( Paper by Klassen, Aurass and Mann, A&A 2001, v.370, L41-L44)
