Ultraviolett observations of the solar corona, recently obtained in several coronal emission lines by the Extreme ultraviolet Imaging Telescope onboard SOHO (EIT henceforth, Delaboudiniére et al. ) lead to the discovery of coronal wave transients (Moses et al. ; Thompson et al. ). Difference images of EIT snapshots reveal a bright rim sometimes nicely circularly expanding around the flaring active region (Fig. 1), sometimes only propagating in a sector of the active region surroundings but clearly influenced by magnetic structures on the propagation path.
Thompson et al. () invoked that these waves may be related with the flare waves seen in the chromosphere. These Moreton waves (Moreton & Ramsey ; Svestka  and Ref. therein) were discovered in the early 1960s. They appear as a bright front in the center of H or as a dark front in the H wings propagating away from the flare. Moreton waves have velocities in the range 440-1125 kms-1with a mean value of 650kms-1. Some flare waves are invisible but their existence can be inferred indirectly by the observation of filament oscillations outside the flare region. According to Smith & Harvey (1971) these "invisible'' waves have a speed of 880kms-1 (mean value) scattering between 410 and 2000kms-1.
Several authors (e.g. Svestka ) have discussed the possibility that magnetic and thermal
energy released during solar flares can escape in the form of MHD waves or shocks.
A further part of the energy
drives pistons (e.g. flare sprays, jets, evaporation fronts). On a larger spatial scale and a longer
time scale magnetic field energy can be released as disappearing filaments and coronal mass
ejections (CMEs, e.g. Kahler ; Hundhausen )
|Figure 1: The event on 12 May 1997. The top panel presents EIT 195 Å running-difference images of the wave transient (arrow). North is top, east is to the left. The bottom panel shows the dynamic spectrogram in the range 40-800 MHz. Start and end of the type II burst are indicated by arrows. Bright parallel stripes are due to terrestrial disturbances|
Propagating waves and pistons are candidates for exciting coronal and interplanetary shock waves. From timing arguments (e.g. Aurass ), and after the discovery of flare-related radio precursors of coronal type II bursts (Klassen et al. ) it is evident that in the majority of cases coronal shocks have flare-related drivers. This does not exclude that a preexisting CME yields a disturbed coronal background in which a flare-related disturbance grows to a shock (Wagner & McQueen ). Interplanetary shocks may well be driven by CMEs (e.g. Gopalswamy et al. ).
A fast mode MHD-like coronal shock wave can accelerate electrons leading to nonthermal radio emission in terms of type II bursts (Mann  for a review). Type II bursts appear as slowly drifting lanes in dynamic radio spectra (the type II burst backbone component) which are superposed by a special kind of drift bursts emanating from the lanes (the herringbones). The mean coronal type II burst speed is in the range 765-930 km s-1 (Robinson ). The radio spectra show in many cases bandsplitted lanes, often multiple lanes at the fundamental, the second, sometimes also at the third harmonic of the local plasma frequency (see Zlotnik et al. ). Repeated lanes in time may be due to repeated shock formation in different coronal magnetoplasma structures (Aurass et al. ; Klassen et al. ).
Moreton waves are reported to be accompanied by type II bursts (Kai ;
Svestka  and Ref. therein).
It is generally accepted that both phenomena (Moreton waves and the type II bursts) are signatures
same driving agent (Uchida ). In Uchida's interpretation the hydromagnetic fast
mode wave propagates in
the corona, and a skirt of this wave front sweeps over the chromosphere with a velocity exeeding the
fast mode velocity in the chromosphere itself. Both Moreton waves and EIT transient wave events
move away from the flare site and have a reminiscent front structure. Wave fronts appear
semicircular for Moreton waves and sometimes circular for EIT waves. Moreton waves represent a wave
motion seen in a 104 K plasma. EIT waves are visible in EUV spectral lines
(for example FeXII,
The speed of Moreton waves (> 400 km s-1) exceeds the speed of EIT waves (170-350 km
s-1). There is not enough information to judge about speed changes of the wave fronts
|Figure 2: EIT wave onset related with the start of associatedtype III bursts (indicating the start of the impulsive flare phase). The start time of the type III bursts is zero on the time axis|
|Figure 4: Velocity histogram of type II bursts (top) and EIT waves (bottom). The mean speed of type II bursts is about three times larger than the velocity of EIT waves|
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