LESIA-Observatoire de Paris & CNRS, 92190 Meudon, France
Solar energetic particles (SEPs) are accelerated in relationship with eruptive events in the solar corona, flares and coronal mass ejections (CMEs). Depending on the energy of the SEPs, the acceleration occurs in the corona or during the interplanetary propagation of a CME. Enhanced proton fluxes at energies above 10 MeV, the reference of SEP events categorised by NOAA, can be produced both by acceleration processes in the corona and by CME-driven shock waves as they approach and intercept the Earth. The CME-driven shocks often, but not always, produce very high fluxes at energies up to a few tens of MeV. The higher the particle energy, the closer the acceleration occurs to the Sun. At energies above 500 MeV, which are relevant for nuclear cascades in the Earth‘s atmosphere and for radiation doses at aircraft altitudes, the interplanetary and near-Earth shock acceleration plays no role.
The forecasting of SEP events relies presently on early observations of eruptive signatures in the solar corona. A number of features visible at radio frequencies is physically related to, and can be used in short-term forecasting of SEP events:
- Radio emission at metric and longer wavelengths, coming from the corona at heights above about 0.2 solar radii above the photosphere, seems to be a necessary condition for SEPs to be released to interplanetary space.
Even strong soft X-ray bursts located in the western solar hemisphere may not be followed by SEPs when flare-accelerated particles remain confined in the corona and no CME occurs. The absence of radio emission is a tracer of this situation.
- Within the HESPERIA project (European-Union H 2020 programme), we (Zucca et al. 2017 JSWSC 7, A13) examined the possibility to use microwave whole Sun observations within the UMASEP forecasting tool developed at the University of Malaga. Microwaves turn out to be a useful ingredient of this forecasting scheme. The usefulness might still be enhanced if the forecasting could be combined with in situ information from energetic electrons, as in the RELEASE forecasting scheme.
- Within the HESPERIA project we also examined the idea (Chertok et al. 2009 Astron. Rep. 53, 1059) of an empirical relationship between the hardness of the SEP proton spectrum between 10 and 100 MeV and the ratio of the peak flux densities of the microwave burst at frequencies of 9-15 GHz. This relationship could be exploited in the forecasting of the hardness of the SEP spectrum. While we found similar trends in the data as Chertok et al., we found no convincing correlation in solar cycle 24.
Besides being a direct forecasting tool for SEP events, microwave emission can also serve as an early estimate of CME speed. The importance of this aspect in sophisticated models of CME propagation and CME shock-related particle acceleration in the interplanetary space will be briefly discussed.
While societal needs for forecasting are to be formulated by the relevant organisms, the usefulness of radio
observations, which can be conducted from ground, should be considered. Dedicated patrol observations would be useful for SEP forecasting and other space weather applications.