Manifestations of CMEs in the source region are best visible in EUV and in soft X-rays. There is a wide concensus that both CMEs and flares are interlinked processes of the same magnetic instability in coronal eruptive events. The triggering mechanism of the magnetic instability could be (i) a tether-cutting or flux cancellation mechanism, (ii) shearing motions, (iii) the magnetic breakout model, (iv) an emerging flux triggering mechanism, (v) a flux injection triggering mechanism, or (vi) a kink instability or torus instability (Chen, 2011; Cheng et al., 2010). In all cases, a part of the unstable coronal volume starts to expand and to raise upward, which diminishes the density inside the expanding volume according to,EUV dimming. A statistical study that investigated the relationship between EUV dimming (detected with SOHO/CDS) and CME events (detected with SOHO/LASCO) for the time span of 1998-2005 found that 55% of identified dimming regions are associated with CMEs, while 84% of the detected CMEs could be tracked back to dimming regions (Bewsher et al., 2008). The mutual correlation might be even higher if dimming regions on the backside of the Sun could be observed. Of course, there are also exceptions, so-called stealth CMEs, which apparently start higher up in the corona and expel so little mass that they cannot be detected in EUV, but are visible in polarized brightness in white-light (Robbrecht et al., 2009). One out of three CMEs during the solar minimum were found to be a stealth CME without coronal signatures, such as dimming, waves, filament eruptions, flares, or post-eruptive arcade on the disk (Ma et al., 2010).
An example of a CME with a bubble-like expansion, observed with STEREO/EUVI/A and B on 2008 Mar 25 is shown in Figure 47. The EUV light curve (diamonds in Figure 47 top panel) shows a massive drop of the total EUV brightness about at the same time when the hard X-ray emission of the associated flare peaks (solid curve in Figure 47 top panel), which illustrates the simultaneity of the magnetic instability that causes the launch of the CME, and the coupled magnetic reconnection process that drives the flare with particle acceleration, precipitation (hard X-ray bremsstrahlung), and chromospheric evaporation (free-free emission of upflowing heated plasma seen in soft X-rays). A geometric model of a spherical CME bubble expansion and the related EUV dimming calculated from the line-of-sight integral of the emission measure through the solar corona across the CME bubble is shown in Figure 48. The relative dimming in EUV is strongest in the lowest density scale height of the solar corona where most of the expelled mass resides. The 4D modeling of the CME expansion and related EUV dimming of this CME event of 2008 Mar 25 is visualized with a numerical simulation in Figure 49 and is described in more detail in several studies (Aschwanden, 2009b; Aschwanden et al., 2009b; Patsourakos et al., 2010).
The amount of EUV dimming can quantitatively be used to estimate the mass of a CME. If we characterize the footprint area of a CME with a spherical area with radius and CME angle (latitude or longitude extend in units of radian), and the vertical mass extent with a hydrostatic density scale height , the original CME source volume is,et al., 2010), e.g., for the three STEREO/EUVI filters in 171, 195, and 284 Å. Applying Eq. (48) to the 2008 Mar 25 flare, where the EUV dimming extends over an opening angle of , we estimate a CME mass of , which includes the CME plasma in the temperature range of and, thus, represents a lower limit for the entire CME mass. More detailed geometric modeling for arbitrary locations in the solar corona and combining the emission measures from different temperature filters in 6 CME events yielded CME masses in the range of , which agreed between the two STEREO/A and B spacecraft () as well as with white-light measurements by COR2 () (Aschwanden et al., 2009a). Previous CME masses were determined by assuming the propagation direction to be aligned with the plane-of-sky (Vourlidas et al., 2010), but improved values that are corrected for the (stereoscopically triangulated) “true” 3D propagation direction have been derived in the latter comparison (Colaninno and Vourlidas, 2009), based on the assumption that both STEREO spacecraft measure equal CME masses.
A numerical simulation of a coronal wave – CME – dimming event (2009 Feb 13) was carried out when the STEREO spacecraft were in quadrature, which provided a top-down as well as a side-view of the expanding CME (Cohen et al., 2009). STEREO quadrature observations also revealed that the coronal dimming occurs before the chromospheric eruption, indicating that the removal of the overlying coronal magnetic field is the trigger in miniature CMEs (Innes et al., 2010). Sequential (homologous) CME events may cause “double dimmings” (Li et al., 2010b). EUV dimming may occur at two footpoint locations of an eruptive loop, especially in cases with emerging flux trigger mechanisms (Zheng et al., 2011). The detection and measurement of coronal EUV dimming regions can now be conducted with automated algorithms (Attrill and Wills-Davey, 2010). It would be interesting to compare the automatically detected dimming areas from the two STEREO spacecraft and to stereoscopically triangulate the altitude of the centroids of the dimming areas, which is expected to correspond to a half density scale height.
Living Rev. Solar Phys. 8, (2011), 5
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