Solar Events – read more

The Sun has long been known to be a source of energetic particles (mostly protons, electrons and light-nuclei). The problem concerning the mechanism and site of acceleration of such particles, called SEPs, remains an open question up until now. Particle populations emitted by the Sun possess energies ranging from a few tens of keV to a few GeV; these emissions are often associated with solar flares and Coronal Mass Ejection (CME) but this causality relation is not always observed.

The correct amount of particle species injected in the interplanetary space by such powerful events largely varies from event to event and it is heavily linked to the production mechanisms that take place. Whether the Sun accelerates particles at low altitudes through magnetic reconnection or higher in the outermost layers of its atmosphere (like the corona)
through coronal mass ejection-driven shocks, or perhaps an admixture of the two, is still unclear. This kind of uncertainty involves both low energy particles measured in situ and the higher energy populations which lead to particularly energetic phenomena called Ground Level Enhancements (GLEs). These are produced when solar protons in the ∼ GeV range start a nuclear cascade through the Earth’s atmosphere that can be observed by detectors at ground level, such as Neutron Monitors, as an increase above the background produced by ordinary galactic cosmic rays. GLEs are very rare (only 71 have been registered so far) but very important because provide a good opportunity to detect matter ejected from the Sun that reaches the Earth within tens of minutes. During its ten years of data-taking, PAMELA registered over 25 solar events, among which two GLEs, one on December 14, 2006 and the other on May 17, 2012 (see Figure 3).

No qualitative distinction between the spectral shapes of GLE and non-GLE events has been observed, suggesting that GLEs are not a separate class, but are the subset of a continuous distribution of SEP events that are more intense at high energies. Nevertheless, both GLEs showed peculiar characteristics: the GLE of 2006 showed a rapid increase of the Helium nuclei component with respect to the proton one, suggesting a possible phenomenon of Helium enrichment. Moreover, the 2012 GLE showed two different proton populations with different pitch angle distributions (a low-energy population that extends to pitch angle of 90° and a population that is beamed at energies above 1 GeV) ; this behavior has been interpreted as the first comprehensive measurements of the effects of SEPs transport in the Earth’s magnetosphere.

A long-lasting CME was produced after the GLE of 2006, causing a depletion of the flux of GCRs with respect to the normal condition (Forbush effect or decrease). Thanks to the high precision of PAMELA, it was possible to follow such depletion in time and rigidity uo until the complete GCRs recovery (almost 1 month later). The daily averaged GCR proton intensity was used to investigate the rigidity dependence of the amplitude and the recovery time of the Forbush decrease. Additionally, for the first time, the temporal variations in the helium and electron intensities during a Forbush decrease were studied. Interestingly, the temporal evolutions of the helium and proton intensities during the Forbush decrease were found in good agreement, while the low rigidity electrons (< 2 GV) displayed a faster recovery. This difference in the electron recovery is interpreted as a charge-sign dependence introduced by drift motions experienced by the GCRs during their propagation through the heliosphere. This is shown in the six panels of Figure 4, where the time profiles of such populations are compared