4.11 CME source regions and EUV dimming

In this review we limit our scope to stereoscopy and tomography in the solar corona, which includes the manifestations of Coronal Mass Ejections (CMEs) in the source regions, such as EUV dimming and global (EIT) waves, while stereoscopy and tomography of CMEs in the heliosphere and interplanetary space should be reviewed separately, given the massive amount of literature that emerged over the first four years of the STEREO mission. A review on models and observations of CMEs appeared recently in Living Reviews in Solar Physics (Chen, 2011Jump To The Next Citation Point), however, without treatment of stereoscopic and tomographic 3D reconstruction methods. Other recent reviews on CME theoretical models can be found, e.g., in Forbes et al. (2006), Schrijver (2009), or Démoulin and Aulanier (2010).

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, 2011Jump To The Next Citation Point; 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,

( ) r(t) −3 ne (t) = n0 ---- , (44 ) r0
in the approximation of a spherical CME bubble with initial radius r0 and density n0. The expansion of the CME bubble may be driven by a Lorentz force, which can cause a near-constant acceleration a during the initial phase, so that the kinematics can be described as,
1 r(t) = r0 + v0(t − t0) +-a(t − t0). (45 ) 2
The density decrease inside the expanding CME bubble causes a deficit in the emission measure in the corresponding part of the solar corona,
( )− 6 ∫ 2 ∫ 2 r(t) EM (x, y,t) = ne(x,y,z, t)dz = n0 r dz , (46 ) 0
which is called 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 47View Image. The EUV light curve (diamonds in Figure 47View Image 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 47View Image 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 48View Image. 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 49View Image and is described in more detail in several studies (Aschwanden, 2009b; Aschwanden et al., 2009bJump To The Next Citation Point; Patsourakos et al., 2010).

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Figure 47: Soft X-ray (GOES/Lo 0.5-4 Å and 1-8 Å, thin curves) and EUV (EUVI/A, diamonds) light curves, time derivative, dI (t)∕dt), of the harder soft X-ray light curve (thick solid line) are shown for the flare/CME event of 2008 Mar 25, 18:30 UT (top panel). Four EUVI/A images (second row) and running difference images (bottom row). Note the strong dimming in the EUV light curve. The diamond symbols mark the times of the EUV images, while the selected images shown below are marked with vertical lines. The peak EUV flux is F = 5.6 × 106 DN s− 1 (or 7.8% of the total flux). The FOV of the images is 512 EUVI pixels (≈ 600 Mm) (from Aschwanden et al., 2009bJump To The Next Citation Point).
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Figure 48: A numerical simulation of adiabatic CME expansion and resulting EUV dimming is shown in the x-z plane for three different times, with x the direction of the CME trajectory and z the line-of-sight direction of the observer. The relative EUV dimming qdimm(x, t) (normalized to the preflare value) resulting from the LOS-integrated emission measure is shown in the lower panels (from Aschwanden et al., 2009bJump To The Next Citation Point).
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Figure 49: Comparison of observed and simulated EUVI base-difference images at 5 times for the observations of STEREO/A 171 Å (left two columns) and STEREO/B 171 Å (right two columns). The pre-CME image at 18:36 UT was subtracted in these base-difference images (from Aschwanden et al., 2009b).

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 rCME = R ⊙ 𝜃 and CME angle 𝜃 (latitude or longitude extend in units of radian), and the vertical mass extent with a hydrostatic density scale height λT, the original CME source volume is,

2 VCME = (R⊙ 𝜃) λT , (47 )
which leads to the following CME mass estimate,
( 𝜃 )2 ( T ) ( n ) mCME = VCMEnemp = 1.16 × 1015 ---- ------ ----base-- [g]. (48 ) 10 ∘ 1 MK 109 cm −3
This yields for a typical base density of n = 109 cm −3 base, a coronal temperature of T = 1.0 MK, and an opening angle range of ∘ ∘ 𝜃 ≈ 3 –30, CME masses in the range of 14 16 mCME ≈ 10 – 10 g. More accurate estimates could easily be obtained by combining the coronal EUV filters in order to have more comprehensive temperature coverage (Robbrecht and Wang, 2010; Landi et al., 2010), e.g., T ≈ 0.7 –2.7 MK for the three STEREO/EUVI filters in 171, 195, and 284 Å. Applying Eq. (48View Equation) to the 2008 Mar 25 flare, where the EUV dimming extends over an opening angle of ∘ 𝜃 ≈ 13, we estimate a CME mass of 15 mCME ≈ 2 × 10 g, which includes the CME plasma in the temperature range of T ≈ 0.7 –1.3 MK 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 15 mCME = (2 –7) × 10 g, which agreed between the two STEREO/A and B spacecraft (mA ∕mB = 1.3 ± 0.6) as well as with white-light measurements by COR2 (m ∕m = 1.1 ± 0.3 EUVI 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., 2009Jump To The Next Citation Point). 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.

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