4.12 Global coronal waves

Global coronal waves (originally discovered with SOHO/EIT) propagate concentrically outward of a CME launch site, globally over the solar surface, with typical speeds of ≈ 170– 350 km s−1. Interpretations of these CME-triggered global waves include: (i) fast-mode MHD magnetoacoustic waves, (ii) successive stretching of magnetic field lines, (iii) successive magnetic reconnection (Attrill et al., 2007), (iv) current shell model, or (v) slow-mode wave or soliton wave model (see review by Chen, 2011, and references therein). Let us review what stereoscopic observations contributed to a better understanding of these global coronal waves.
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Figure 50: Sequence of median-filtered running difference images recorded in the EUVI 195 Å channel with a cadence of 10 minutes. The coronal wave (outlined by arrows) is observed on-disk in STEREO/B (top) and on the limb in STEREO/A (bottom) (from Kienreich et al., 2009Jump To The Next Citation Point).

The kinematics of global coronal waves were measured with STEREO/EUVI in all four passbands (304, 171, 195, 284 Å) and it was found that the difference in speeds are partly due to different cadences, but are essentially consistent with an impulsively generated fast-mode magnetoacoustic wave (Long et al., 2008). High-cadence EUVI observations of the 2007 May 19 event reveal a deceleration that is indicative of a freely propagating large-amplitude MHD wave (Veronig et al., 2008). The reflection of a propagating EUV disturbance off a coronal hole boundary was considered as a confirmation of its true wave nature (Gopalswamy et al., 2009), and has been numerically simulated with an MHD code (Schmidt and Ofman, 2010). Hinode/XRT and STEREO observations of a diffuse coronal wave observed near the limb show that the core coronal dimmings map to the core of the CME, secondary coronal dimmings map to the CME cavity, and the diffuse coronal wave maps to the outermost edge of the expanding CME shell (Attrill et al., 2009). The numerical simulations of a CME dimming and global wave event observed with STEREO in quadrature confirmed that the global wave front is made of a mass density (rather than a temperature) enhancement and propagates at a height of ≈ 1.1R ⊙ (Cohen et al., 2009). Ideal STEREO quadrature observations of a wave’s initiation at disk center for STEREO/B and exactly at the limb for STEREO-A show the 3D structure of global wave most clearly (Figure 50View Image), with a constant propagation velocity of ≈ 236 ± 16 km s− 1 (close to the fast magnetosonic speed in the quite corona) at a height of ≈ 80 –100 Mm (≈ 1.11 – 1.14 R ⊙) (Kienreich et al., 2009). Confusion in tracking and identifying a global coronal wave arises also from cospatial projection effects (of the column depth) of the expanding CME bubble, which may expand at a different speed in the upper corona than the global wave in the lower corona, especially for halo CMEs (Ma et al., 2009). Therefore, STEREO quadrature observations are the most unambiguous method to separate the CME structure from the EUV wave and confirms the fast-mode MHD wave interpretation (Patsourakos and Vourlidas, 2009; Patsourakos et al., 2009). However, the global EUV wave is not always restricted to a low coronal altitude. In some cases a clear dome-shaped 3D surface can be seen (Veronig et al., 2010Jump To The Next Citation Point), which probably coincides with the leading edge of the CME (Figure 51View Image). Arguments against the fast-mode MHD wave interpretation was mounted by STEREO observations of strongly variable propagation speeds (Zhukov et al., 2009), while a unified scenario was proposed that includes both a wave-like component moving at the fast magnetosonic speed and a coherent driven compression front related to the eruptive CME bubble (Downs et al., 2011).

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Figure 51: Running difference images (in 5-min intervals) of the dome-shaped EUV wave front as observed with the EUVI/B channels in the 171, 195, 284, and 304 Å wavelengths. Arrows outline the wave dome, crosses indicate the erupting CME loops inside the dome (from Veronig et al., 2010).

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