EIT waves appear as propagating diffuse bright fronts in the difference EUV images. It is believed that they are strongly related to coronal mass ejections (CMEs). There have been numerous observations to this kind of phenomenon during last decade. However, its physical nature is still under hot debate. One popular interpretation is that EIT waves are fast-mode MHD waves, i.e., the coronal counterpart of H? Moreton waves; while some scientists believe that they are non-wave perturbations, for example, the counterpart of the CME in the lower corona.
Recent observations tend to illustrate that there are two different EUV waves associated with a CME (Chen & Wu 2011; Warmuth & Mann 2011; Asai et al. 2012, Cheng et al. 2012), i.e., a faster EUV wave which should be a fast-mode MHD wave or shock wave, and a slower EUV wave which may correspond to the CME. The two-wave scenario was proposed in the field-line stretching model of Chen et al (2002, 2005).
Spectroscopic observations are thought to be very valuable to constrain the physical model of EIT waves. Since the pioneering work of Harra & Sterling (2003), the spectroscopic observations that can catch EIT waves well are still very rare. Here we report one the of the few works that reveal the spectroscopic features of EIT waves.
The event we studied occurred on 12 June 2010 and was associated with an M2.0 class flare. The EIS slit was placed to the south-east of the active region where the wave was generated. The wave front passed through the EIS field-of-view (FOV) is not the strongest part, but it's still clear enough, as illustrated in Figure 1
The EIS observations have 12 consecutive data sets with a FOV of 240"×5". We binned up the 5 columns in the horizontal direction and made a time series with the 12 rasters, which is shown in Figure 2
. Meanwhile, we could measure the positions of the intersection of the wave front and the EIS slit. These positions are denoted by asteroids in each panel in Figure 2. The dotted lines connecting them illustrate the propagation of the EIT wave front.
Figure 2 - The line intensity, Doppler velocity, and line width for the Fe XII and Fe XIII lines as a function of time. The time is related to 00:30:00 UT.
The most interesting finding is in the Doppler velocity and non-thermal velocity maps. There is a clear coronal upflow region from y=250" to y=280" emanating an upflow with a velocity of ~20 km/s. The non-thermal velocity deduced from the line width in this upflow region is significantly higher than the ambient region, especially the quiet sun. Surprisingly, the upward velocity in this region suddenly and significantly drops right after the transit of the wave front. At the same time, the non-thermal velocity in this region also dramatically decreases to a quiet Sun level. These are clearly shown by the contrast of upflow velocity and non-thermal velocity at the both sides of the dotted line.
Why would this happen to a stable upflow region within 5 minutes? Since the upflow exists in a low intensity region, we conjecture that this region is a small coronal hole, where the magnetic field line is very likely to be open. If the EIT wave would be a fast-mode wave, its impinging to the small coronal hole would have produced oscillation of the field lines in the hole and led to the oscillation of the upflow velocity rather than sudden diminishing. Our explanation to the sudden diminishing of the upflow velocity is that the open field in this region is not strong enough to stop the wave propagation, instead, the direction of these open field lines are pushed away from the LOS by the expanding/stretched field lines associated with the EIT wave, as proposed in the model of Chen et al (2002, 2005). Considering a well accepted idea that non-thermal velocity seen in upflow region may come from spatially unresolved multi-components of the Doppler velocity, the amplitudes of these multiple components will also decrease as the real flows are re-directed away from the LOS. Therefore, it is natural that the non-thermal velocity decreases with the Doppler velocity. Our results strongly support that the EIT wave is associated with the field-line stretching, rather than being a fast-mode wave.
This nugget is the summary of a paper published on The Astrophysical Journal 740 (2011), 116 by F. Chen, M. D. Ding, P. F. Chen and L. K. Harra. (See: Chen et al 2011 here.