Diagnostics of solar flare energetic particles: neglected hard X-ray processes and neutron astronomy in the inner heliosphere

For work on my thesis dissertation, we have been studying some energetic processes in solar flares. On our work on hard X-ray (HXR) emission from flares, we have shown that non-thermal recombination emission can compare with the bremsstrahlung HXR flux for certain flare conditions. In this thesis, we show spectral features characteristic of non-thermal recombination HXR emission and suggest how it plays a signicant role in the flare HXR continuum, something that has been ignored in the past. It is important to note that these results could demand a reconsideration of the numbers of accelerated electrons since recombination can be much more efficient in producing HXR photons than bremsstrahlung. We go on to show that although nonthermal recombination is not likely to dominate the total HXR flux unless we consider extreme parameter regimes, it can still form a signicant proportion of the HXR flux for typical flare conditions, thereby remaining important for both spectral inversion and low energy electron cut-off diagnostic capabilities. In related work on diagnosing particle acceleration in flares, we also have an interest in studying solar neutrons. To this end, this thesis presents our work done with new-age neutron detectors developed by our colleagues at the University of New Hampshire. Using laboratory and simulated data from the detector to produce its response matrix, we then employ regularisation and deconvolution techniques to produce encouraging results for data inversion. As a corollary, we have been reconsidering the role of inverse Compton scattering (ICS) of photospheric photons. Gamma-ray observations clearly show the presence of 100 MeV electrons and positrons in the solar corona, by-products of GeV energy ions. We present results of ICS of solar flare photons taking proper account of radiation field geometry near the solar surface. If observed, such radiation would let us determine the number of secondary positrons produced in large flares, contributing to a full picture of ion acceleration and to predicting neutron fluxes to be encountered by future inner heliosphere space missions

Advanced characterization and simulation of SONNE: a fast neutron spectrometer for Solar Probe Plus

SONNE, the SOlar NeutroN Experiment proposed for Solar Probe Plus, is designed to measure solar neutrons from 1-20 MeV and solar gammas from 0.5-10 MeV. SONNE is a double scatter instrument that employs imaging to maximize its signal-to-noise ratio by rejecting neutral particles from non-solar directions. Under the assumption of quiescent or episodic small-flare activity, one can constrain the energy content and power dissipation by fast ions in the low corona. Although the spectrum of protons and ions produced by nanoflaring activity is unknown, we estimate the signal in neutrons and γ−rays that would be present within thirty solar radii, constrained by earlier measurements at 1 AU. Laboratory results and simulations will be presented illustrating the instrument sensitivity and resolving power

Design optimization and performance capabilities of the fast neutron imaging telescope (FNIT)

We describe the design optimization process and performance characterization of a next generation neutron telescope, with imaging and energy measurement capabilities, sensitive to neutrons in the 1-20 MeV energy range. The response of the Fast Neutron Imaging Telescope (FNIT), its efficiency in neutron detection, energy resolution and imaging capabilities were characterized through a combination of lab tests and Monte Carlo simulations. Monte Carlo simulations, together with experimental data, are also being used in the development and testing of the image reconstruction algorithm. FNIT was initially conceived to study solar neutrons as a candidate instrument for the Inner Heliosphere Sentinel (IHS) spacecraft. However, the design of this detector was eventually adapted to locate Special Nuclear Material (SNM) sources for homeland security purposes, by detecting fission neutrons. In either case, the detection principle is based on multiple elastic neutron-proton scatterings in organic scintillator. By reconstructing event locations and measuring the recoil proton energies, the direction and energy spectrum of the primary neutron flux can be determined and neutron sources identified. This paper presents the most recent results arising from our efforts and outlines the performance of the FNIT detector

Flare hard X-ray sources dominated by nonthermal recombination

It was recently shown that, in the hottest regions of flare plasma, nonthermal hard X-ray (HXR) emission in the few deka-keV range from nonthermal electrons by recombination (NTR) onto heavy ions (especially Fe) exceeds bremsstrahlung (NTB), contrary to earlier assumptions. Here we discuss what types of HXR events are so dominated. Though significant even at temperatures T down to 10<sup>6</sup>K, the dominance of such NTR radiation over NTB needs T > 10 MK in order for Fe22+ ions and above to be plentiful. Furthermore, even for an accelerated fraction of only 0.01, the total hot plasma thermal emission begins to exceed NTR only for T > 25 MK. The relative NTR contribution is greatest when the electron flux spectrum is steep and extends to low energies. Thus, in proper modeling of hot HXR sources, inclusion of NTR as well as NTB is essential and reduces the HXR electron number and power requirements by over an order of magnitude in some cases. This alleviates problems of electron acceleration efficiency, especially in coronal HXR sources. Even some chromospheric footpoint HXR sources may be NTR-dominated if the hot soft X-ray (SXR) footpoint plasma there contains fast electrons. Only a small fraction of the plasma emission measure observed in SXR footpoints need be in the form of nonthermals to provide the necessary HXR emission measure. Compared with the standard cold thick target (bremsstrahlung) model (CTTM), such a scenario would give fast electrons a lesser role in the flare energy budget and help solve various problems with the CTTM