3.6 Shock waves and SEPs

If the speed of a CME exceeds the local Alfvén speed in the corona and interplanetary medium it can drive a forward shock. Type II radio bursts, caused by Langmuir waves forming as a result of plasma motion ahead of a shock, are associated with CMEs. Type IV bursts, especially “moving” bursts associated with CMEs, imply magnetic plasma ejections, possibly associated with EPs, and nonthermal particles from field-line reconnection. Studies performed using SOHO data seem to confirm that metric type II bursts arise from shock waves driven by CMEs (Cliver et al., 1999) although the association between metric type II bursts and solar activity has been established since their discovery in the 1950s. Type II bursts in various wavelength domains appear to be organized by the kinetic energy of the CMEs: metric type II bursts (< 2R ⊙) are associated with CMEs with above-average kinetic energy; those extending into decameter-hectometric (DH) wavelengths (> 2 R ⊙) have moderate CME kinetic energy; and type II bursts seen in both the metric and DH domains and extending to kilometric (km) wavelengths (covering the entire Sun-Earth distance) are associated with CMEs of the largest energy. This hierarchical relationship implies that all type II bursts are associated with CMEs, i.e., mass ejecta (e.g., Gopalswamy et al., 2005). More details about solar radio events will appear in an upcoming Living Review; also see Schwenn (2006Jump To The Next Citation Point).

Historically, identifying shocks in white light coronagraph images has been very difficult. Kinks in streamers deflected by CMEs and changes in type II dynamic spectra have been used to infer the existence of shocks on the flanks of CMEs. Sharp, bright rims ahead of fast CMEs occasionally observed by LASCO are now considered by some to be evidence of shocks (e.g., Vourlidas et al., 2003; Ontiveros and Vourlidas, 2009; Vourlidas and Ontiveros, 2009). The kinematics of a spherical shock ahead of a bubble CME has been determined in EUV data from SDO-AIA (Ma et al., 2011).

Ultraviolet spectroscopy provides an unambiguous means to observe coronal shocks and determine their properties (Kohl et al., 2006). Shock compression causes an immediate increase in the emissivity of dominant ions, and the bulk motion of the shocked plasma causes Doppler dimming of H I Lyα and O VI lines. Electron heating causes a more gradual change in the ionization state, and heating of the ions can be measured through line width increases. However, since the shocked gas passes quickly through the UVCS slit, the signatures of only a few shocks have been reported. In all these cases broad O VI profiles were detected and the O temperatures were > 108 K.

Using SOHO and radio observations of a fast CME, Bemporad and Mancuso (2010Jump To The Next Citation Point) were able to provide a complete characterization of pre- and post-shock plasma physical parameters in the corona. The UVCS slit was centered at 4.1R ⊙ in the flank of the expanding CME, the highest UV detection of a shock obtained so far with UVCS. The white-light and EUV data were combined to estimate the shock compression ratio, plasma temperature, and the strength of the magnetic fields. For the compression ratio of 2.06, the coronal plasma was heated across the shock from an initial temperature of 2.3 × 105 K up to 1.9 × 106 K, while the magnetic field was compressed such that its strength increased from ∼ 0.02 G to ∼ 0.04 G. Magnetic and kinetic energy density increases at the shock were comparable and more than two times larger than the thermal energy density increase.

CME-driven shocks can accelerate electrons and ions producing solar energetic particle (SEP) events. The close association between SEP events and fast CMEs implies that SEPs are accelerated by CME-driven shocks (Reames, 1999). Early work with solar energetic particles in the 1960s suggested that a two-stage acceleration process must take place to achieve the energies observed in these particles (Wild et al., 1963), a process later confirmed using in-situ data in the 1980s and 1990s (e.g., Gloeckler et al., 1994). The first stage, up to around 100 keV for electrons, is provided by the flare, and the rest provided by a fast magnetohydrodynamic (MHD) shock, now believed to be produced by the CME. A few hundred large SEP events have been recorded during the SOHO period, most of them occurring around the solar maximum (e.g., Gopalswamy et al., 2008Jump To The Next Citation Point). The associated CMEs were fast (average speed ∼ 1500 km s–1), apparently wide (mostly full halos) and decelerating (possibly due to coronal drag). Large SEP events with the most energetic particles, ground level enhancements (GLEs – e.g., Forbush, 1946), are associated with the fastest CMEs (> 2000 km s–1; Gopalswamy et al., 2008). The fastest particles can arrive at Earth only minutes after the impulsive flare and associated shock. A comparison of the LASCO fast (v > 1000 km s–1) CMEs between the CDAW (manual) and CACTus (automatic) catalogs shows that the CDAW CME widths are considerably wider (Yashiro et al., 2008b), but nearly all of the CMEs associated with GLEs are halos (W > 180°) in both catalogs. The source regions of the SEP-associated CMEs are generally located in the Sun’s western hemisphere, because the particles travel along the Parker spiral interplanetary field lines. The SEP-CME distribution is different from that of the CMEs producing geomagnetic storms (Figure 25View Image). Thus, all frontsided fast and wide CMEs are potentially important for Earth’s space weather. An important SOHO result is that the high-intensity SEPs are associated with active regions that are associated with repeated CMEs, suggesting that CME interactions may be important in accelerating the particles in large SEPs (e.g., Gopalswamy et al., 2004b).

Emslie et al. (2004) have shown that the CME kinetic energy is by far the largest component in the energy budget of an eruption. As much as 10% of the CME kinetic energy might go into SEPs, suggesting that CME-driven shocks are very efficient particle accelerators (Mewaldt, 2006). More details about Solar Energetic Particles will appear in an upcoming Living Review; also see Schwenn (2006Jump To The Next Citation Point).

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Figure 25: Locations of associated solar surface activity related to CMEs that produce major (Dst ≤ − 100 nT) geomagnetic storms (left) and large SEP events (right). The circle sizes represent the significance of the resultant event (Gopalswamy, 2010b).

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