SpreadFast Project

CMEs are some of the most energetic events in the solar system, and one of the main manifestations of short-term solar activity. They can influence the interplanetary environment over vast longitudinal and radial ranges by injecting fast solar wind plasma, increased magnetic flux, and highly energetic ions and electrons (solar energetic particles, or SEP). SEPs present a significant radiation damage risk to astronauts and spacecraft situated beyond low-earth orbit. The period between onset of a solar eruption and arrival of SEP fluxes of harmful energies to 1 astronomical unit (AU) is generally on the order of several hours. This is often very short time to warn astronauts and spacecraft operators. Thus, it is important to develop a capability for predicting interplanetary SEP fluxes as early as possible after the CME onset.

Most CMEs exhibit a typical, three-stage kinematic profile from their onset until they reach interplanetary space (Zhang & Dere 2006)¹. The first stage corresponds to the initial release of the erupting material via flux rope destabilization. The second stage corresponds to rapid acceleration driven by impulsive conversion of the stored magnetic energy to kinetic energy and, depending on the background plasma environment before the eruption, a possible CME overexpansion and shock driving. This stage typically peaks around 1.5 R_ʘ and ends below 3 R_ʘ (Bein et al. 2011², Temmer 2017³). The third stage corresponds to a gradual equilibration of the CME speed to the ambient solar wind surrounding it, as it continues to interplanetary space. It is the second stage, which occurs in the low and middle corona, when CMEs exhibit the most dynamic behaviour, and are expected to be most active in SEP production.

Shocks may be driven if CMEs exhibit significant over-expansion, or if they enter regions of decreased fast magnetosonic (i.e., Alfvén) speed. Such regions are expected above many active regions (ARs), as well as in quiet sun (QS) regions (Zucca et al. 2014⁴; Evans et al. 2008⁵). In fact, multiple observational and modeling studies have shown that shocks can be driven at heights as low as 1.1 R_ʘ (Gopalswamy & Yashiro 2011⁶; Kozarev et al. 2015⁷). Recent modeling work has also suggested that driven shocks and blast waves caused by CMEs and flares can energize ions and electrons out of the supra-thermal coronal populations up to solar energetic particles (SEP) energies of tens of MeV, even below 5 R_ʘ (Kozarev et al. 2013⁸; Battarbee et al. 2013⁹). The combination of large densities of supra-thermal particles, variable acceleration and velocity profiles, and relatively low Alfvén speeds make this region a likely location for early-stage shock acceleration of particles. This will be explored by the proposed SEP prediction prototype.