Coronal mass ejections, type II radio bursts, and solar energetic particle events in the SOHO era

Abstract

Abstract.Using the extensive and uniform data on coronal mass ejections (CMEs), solar energetic particle (SEP) events, and type II radio bursts during the SOHO era, we discuss how the CME properties such as speed, width and solar-source longitude decide whether CMEs are associated with type II radio bursts and SEP events. We discuss why some radio-quiet CMEs are associated with small SEP events while some radio-loud CMEs are not associated with SEP events. We conclude that either some fast and wide CMEs do not drive shocks or they drive weak shocks that do not produce significant levels of particle acceleration. We also infer that the Alfvén speed in the corona and near-Sun interplanetary medium ranges from <200 km/s to ∼1600 km/s. Radio-quiet fast and wide CMEs are also poor SEP producers and the association rate of type II bursts and SEP events steadily increases with CME speed and width (i.e. energy). If we consider western hemispheric CMEs, the SEP association rate increases linearly from ∼30% for 800 km/s CMEs to 100% for ≥1800 km/s. Essentially all type II bursts in the decametre-hectometric (DH) wavelength range are associated with SEP events once the source location on the Sun is taken into account. This is a significant result for space weather applications, because if a CME originating from the western hemisphere is accompanied by a DH type II burst, there is a high probability that it will produce an SEP event.

Large gradual solar energetic particle events

“…of ESP events and the locally measured IP shock parameters (e.g., compression ratio) indicates that many factors can contribute to the local diffusive shock acceleration processes and cause the event-to-event variability. Emslie et al (2012) found that only 22 of the 38 largest solar eruptive events are associated with large SEP events, while Gopalswamy et al (2008) found that some of the most energetic CMEs are not associated either with type II radio bursts or large SEPs. More recently, Kahler (2013a) calculated three different SEP event timescales: (a) the time from inferred CME launch at 1 R S to the time of the 20 MeV SEP onset at Wind, (b) the time from SEP onset to the time the intensity reached half the peak value, and (c) the time during which the intensity remained above half the peak value.…”

Coronal Mass Ejections from Sunspot and Non-Sunspot Regions

“…(v) The sources of SEP-associated CMEs, on the other hand, are confined mostly to the western hemisphere with a large number of sources close to the limb. In fact there are also many sources behind the west limb, not plotted here (see Gopalswamy et al, 2008a for more details). This western bias is known to be due to the spiral structure of the IP magnetic field along which the SEPs have to propagate before being detected by an observer near Earth.…”

The density compression ratio of shock fronts associated with coronal mass ejections

“…They form large-scale spherical fronts seen as halos by white light observations or as EUV waves propagating against the solar coronal base (e.g., Kwon et al, 2013a, see also recent reviews by Vourlidas (2012), Warmuth (2015), and Long et al, (2017) for debates on the nature of EUV waves). Occasionally, type II radio bursts over a large spectral range accompany these shocks, particularly in IP space (e.g., Gopalswamy et al, 2008), while metric Type II emission seems to originate from the flanks of shock waves close to the Sun (e.g., Démoulin et al, 2012).…”