Properties of Energetic Ions in the Solar Atmosphere from γ-Ray and Neutron Observations

Abstract

Gamma-rays and neutrons are the only sources of information on energetic ions present during solar flares and on properties of these ions when they interact in the solar atmosphere. The production of γ-rays and neutrons results from convolution of the nuclear cross-sections with the ion distribution functions in the atmosphere. The observed γ-ray and neutron fluxes thus provide useful diagnostics for the properties of energetic ions, yielding strong constraints on acceleration mechanisms as well as properties of the interaction sites. The problem of ion transport between the accelerating and interaction sites must also be addressed to infer as much information as possible on the properties of the primary ion accelerator. In the last couple of decades, both theoretical and observational developments have led to substantial progress in understanding the origin of solar γ-rays and neutrons. This chapter reviews recent developments in the study of solar γ-rays and of solar neutrons at the time of the RHESSI era. The unprecedented quality of the RHESSI data reveals γray line shapes for the first time and provides γ-ray images. Our previous understanding of the properties of energetic ions based on measurements from the former solar cycles is also summarized. The new results -obtained owing both to the gain in spectral resolution (both with RHESSI and with the non solar-dedicated INTEGRAL/SPI instrument) and to the pioneering imaging technique in the γ-ray domain -are presented in the context of this previous knowledge. Still open questions are emphasized in the last section of the chapter and future perspectives on this field are briefly discussed.

Solar flares and energetic particles

“…A noticeable enhancement has been observed in the numbers of a-particles (up to 0.5 in some events), as well as of accelerated 3 He isotopes, compared with the standard coronal abundances. Furthermore, accelerated heavy ions, such as Ne, Mg and Fe, are also found to be over-abundant with respect to normal coronal compositions as reviewed in Vilmer et al [13]. This poses other constraints on models for ion acceleration.…”

“…This continuum (curves 3 and 5) is dominant below 1 MeV and again in the 10-50 MeV range. Energetic ions with energies in the more than 1 MeV per nuc to 100 MeV per nuc range produce, through interactions in the solar atmosphere, a complete g-ray line spectrum (curve 4), which consists of several nuclear de-excitation lines, neutron capture and positron annihilation lines (for reviews, see [11][12][13]). When ions over a few hundred MeV per nuc are produced in the flare, nuclear interactions with the ambient medium produce secondary pions whose decay produces a broadband continuum at photon energies above 10 MeV (with a broad peak around 70 MeV from neutral pion radiation; curve 6) and also secondary neutrons which, if energetic enough, may escape from the Sun and be directly detected in interplanetary space or at ground level (respectively, more than 10 MeV or 200 MeV neutrons; for a review, see [14]).…”

“…Statistically significant displacements between HXR and g-ray line sources were observed in three of the five events. Given the limited number of observations, it is still difficult to draw firm conclusions on the relative locations of electron and ion interaction sites in many flares (for a review, see [13]). Improving the imaging in the g-ray line domain is clearly one of the goals of future instruments.…”

“…In most powerful flares, the photon spectra may reveal double power laws below 300 keV, with the spectral index from below 30 to 60 keV being smaller (flatter) than that above 60 keV by two to four units [48]. For the most energetic flares, emission from relativistic and ultrarelativistic electrons can be observed up to a few hundred MeV (for reviews, see [13,14]). At photon energies more than 500 keV, arising from electrons of higher energies, the spectrum tends to flatten again.…”

Variability of Low Energy Cosmic Rays Near Earth

“…Production of  -rays and neutrons results from convolution of the nuclear cross-sections with the ion distribution functions in the atmosphere. Recently Vilmer et al (2011) reviewed the -ray and neutron observations with the emphasis on the very detailed RHESSI measurements, namely the high spectral resolution revealing line shapes and fluences, and gamma-ray imaging technique. The authors point out also still open question for the study of high energy neutral emissions from the Sun.…”