Solar energetic particle observations and radio emission: progress and challenges

Karl-Ludwig Klein, Observatoire de Paris, LESIA, F-92190 Meudon

The origin of solar energetic particle (SEP) events is still uncertain. It is widely advocated that SEP events can be divided in two categories, based on the elemental abundances, ion charge states, durations and the parent activity, and that this distinction is a key to identify the acceleration mechanism. The numerous smaller "impulsive" SEP events are ascribed to flares and acceleration processes that are broadly related to magnetic reconnection and turbulence. Radio observations of electron beams, especially bidirectional ones, then give evidence on the acceleration region at thermal electron densities of 10^9 to 10^{11} cm^{-3}, depending on the events. This shows some consistency with density estimates derived from the observed charge states of Fe. Intense long-duration SEP events, so-called gradual SEP events, are usually ascribed to the acceleration at CME shocks. This conclusion has some observational support, such as the long durations of gradual SEP events at MeV-energies, and their association with fast coronal mass ejections (CMEs) and with type II radio bursts. However, it is generally overlooked that type II bursts are usually accompanied by type IV bursts, which indicate continued energy release related to the evolution of the driver of the shock wave traced by the type II burst, and which are independent signatures of energy release and particle acceleration in the corona, behind the rising CME. Another intriguing fact is that few, if any, of the distinctive criteria between impulsive and gradual events established in the 1990s for SEPs at MeV energies are confirmed by SEP-observations at higher energies. It will be argued in this talk that rather than an a priori conviction on the plausibility of different acceleration processes, a thorough observational analysis that compares SEP events with remote sensing signatures of energetic particles at hard X-ray, gamma-ray and radio wavelengths is needed. Such comparative studies will be furthered by the availability of SEP measurements from vantage points close to the Sun (Parker Solar Probe, Solar Orbiter), where we expect to be able to see acceleration signatures in the time profiles of SEP events that are wiped out by interplanetary particle transport when observed at 1 AU. Radio observations will be an important contribution to this endeavour, because they show acceleration signatures in dilute plasmas, give evidence on magnetic activity between coronal acceleration sites and interplanetary