Properties of a Coronal Shock Wave as a Driver of Early Sep Acceleration

Abstract

Coronal mass ejections (CMEs) are thought to drive collisionless shocks in the solar corona, which in turn have been shown capable of accelerating solar energetic particles (SEPs) in minutes. It has been notoriously difficult to extract information about energetic particle spectra in the corona, due to lack of in-situ measurements. It is possible, however, to combine remote observations with datadriven models in order to deduce coronal shock properties relevant to the local acceleration of SEPs and their heliospheric connectivity to near-Earth space. We present such novel analysis applied to the May 11, 2011 CME event on the western solar limb, focusing on the evolution of the eruption-driven, dome-like shock wave observed by the Atmospheric Imaging Assembly (AIA) EUV telescopes on board the Solar Dynamics Observatory spacecraft. We analyze the shock evolution and estimate its strength using emission measure modeling. We apply a new method combining a geometric model of the shock front with a potential field source surface model to estimate time-dependent field-to-shock angles and heliospheric connectivity during shock passage in the low corona. We find that the shock was weak, with an initial speed of ∼450 km/s. It was initially mostly quasiparallel, but significant portion of it turned quasi-perpendicular later in the event. There was good magnetic connectivity to near-Earth space towards the end of the event as observed by the AIA instrument. The methods used in this analysis hold a significant potential for early characterization of coronal shock waves and forecasting of SEP spectra based on remote observations.

The Coronal Analysis of SHocks and Waves (CASHeW) framework

“…Results from previous studies of the density ratios in the low corona (e.g. Veronig et al, 2011;Kozarev et al, 2011;Vanninathan et al, 2015;Kozarev et al, 2015) suggest similar values from DEM and spectrometric calculations À ∼3-20% increase in the density, consistent with the results from this study.…”

“…Here, there is a significant difference with the previous OCBF, in that a large patch of high-u BN values close to the flanks of the OCBF develops over time, including many open field line crossings. Similar patches, or 'valleys' of quasi-perpendicularity were previously discussed by Kozarev et al (2015) and Rouillard et al (2016), and could mean that efficient particle acceleration may take place along a long linear structure on the shock surface. The nominal connectivity to Earth throughout the event is very good, and increases, as well as the spread of connected field lines (panels G and H).…”

“…cfa.harvard.edu/cashew). The reader can find further examples of CASHeW products in Kozarev et al (2015) and Kozarev & Schwadron (2016).…”

“…We have further improved the method by using pixel-specific values for the radial locations and the average coronal temperature from our DEM model results, rather than just a single value for the coronal temperature (as described in Zucca et al (2014)). Within the CASHeW framework, the DEM model can be run for every pixel in the field of view, as well as for individual groups of pixels specified by the user (Kozarev et al, 2015), or small groups of pixels (∼25) surrounding the time-dependent locations where PFSS field lines cross the CSGS surface (Kozarev & Schwadron, 2016). As the observations are of the twodimensional plane of the sky, it is implicitly assumed here that the emission measure is the same on both the near and far side of the OCBF, along any given line of sight.…”

Solar Energetic Particle Event Associated With the 2012 July 23 Extreme Solar Storm

“…A broad coronal or IP shock is one of these (e.g., Heras et al 1995;Lario et al 1998;Reames & Lal 2010;Dresing et al 2012). Particles observed at 1 au in large SEP events are accelerated by coronal mass ejection (CME)-driven shocks and injected onto the magnetic field lines connecting the observers and the coronal shock (e.g., Wild et al 1963;Cliver et al 2004Cliver et al , 2005Zank et al 2007;Battarbee et al 2011;Kozarev et al 2015), while the angular size of a wide coronal shock can extend up to 300° (Cliver et al 1995) and an IP shock at 1 au can provide a large acceleration region with its longitudinal extent as large as 180°(e.g., Cane 1996;Liu et al 2008). On the other hand, EUV wave observed in the lower corona has been used as a proxy for the longitudinal extent of the CME during the initial expansion phase (e.g., Torsti et al 1999;Rouillard et al 2012;Park et al 2013).…”