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Register a new userToward quantitative model for simulation and forecast of solar energetic particle production during gradual events -- II: kinetic description of SEP
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Solar Energetic Particles (SEPs) possess a high destructive potential as they pose multiple radiation hazards on Earth and onboard spacecrafts. The present work continues a series started with the paper by Borovikov et al.(2018) describing a computational tool to simulate and, potentially, predict the SEP threat based on the observations of the Sun. Here we present the kinetic model coupled with the globalMHD model for the Solar Corona (SC) and Inner Heliosphere (IH), which was described in the first paper in the series. At the heart of the coupled model is a self-consistent treatment of the Alfven wave turbulence. The turbulence not only heats corona, powers and accelerates the solar wind, but also serves as the main agent to scatter the SEPs and thus controls their acceleration and transport. The universal character of the turbulence in the coupled model provides a realistic description of the SEP transport by using the level of turbulence as validated with the solar wind and coronal plasma observations. At the same time, the SEP observations at 1 AU can be used to validate the model for turbulence in the IH, since the observed SEPs have witnessed this turbulence on their way through the IH.
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Solar Energetic Particles (SEPs) are an important aspect of space weather. SEP events posses a high destructive potential, since they may cause disruptions of communication systems on Earth and be fatal to crew members onboard spacecrafts and, in extreme cases, harmful to people onboard high altitude flights. However, currently the research community lacks efficient tools to predict such hazardous threat and its potential impacts. Such a tool is a first step for mankind to improve its preparedness for SEP events and ultimately to be able to mitigate their effects. The main goal of the presented research effort is to develop a computational tool that will have the forecasting capability and can be serve in operational system that will provide live information on the current potential threats posed by SEP based on the observations of the Sun. In the present paper the fundamentals of magneto-hydrodynamical (MHD) simulations are discussed to be employed as a critical part of the desired forecasting system.
We fit the $sim$0.1-500 MeV/nucleon H-Fe spectra in 46 large SEP events surveyed by Desai et al. (2016) with the double power-law Band function to obtain a normalization constant, low- and high-energy parameters $gamma_a$ and $gamma_b$; and break energy $E_B$. We also calculate the low-energy power-law spectral slope $gamma_1$. We find that: 1) $gamma_a$, $gamma_1$, and $gamma_b$ are species-independent within a given SEP event, and the spectra steepen with increasing energy; 2) $E_B$s are well ordered by Q/M ratio, and decrease systematically with decreasing Q/M, scaling as (Q/M)$^alpha$ with $alpha$ varying between $sim$0.2-3; 3) $alpha$ is well correlated with Fe/O at $sim$0.16-0.23 MeV/nucleon and CME speed; 4) In most events: $alpha<$1.4, the spectra steepen significantly at higher energy with $gamma_b$-$gamma_a >$3; and 5) Seven out of 9 extreme SEP events (associated with faster CMEs and GLEs) are Fe-rich, have $alpha >$1.4, have flatter spectra at low and high energies with $gamma_b$-$gamma_a <$3. The species-independence of $gamma_a$, $gamma_1$, and $gamma_b$ and the systematic Q/M dependence of $E_B$ within an event, as well as the range of values for $alpha$ suggest that the formation of double power-laws in SEP events occurs primarily due to diffusive acceleration at near-Sun CME shocks and not due to scattering in the interplanetary turbulence. In most events, the Q/M-dependence of $E_B$ is consistent with the equal diffusion coefficient condition while the event-to-event variations in $alpha$ are probably driven by differences in the near-shock wave intensity spectra, which are flatter than the Kolmogorov turbulence spectrum but still weaker compared to that inferred for the extreme events.
An interval of exceptional solar activity was registered in early September 2017, late in the decay phase of solar cycle 24, involving the complex Active Region 12673 as it rotated across the western hemisphere with respect to Earth. A large number of eruptions occurred between 4-10 September, including four associated with X-class flares. The X9.3 flare on 6 September and the X8.2 flare on 10 September are currently the two largest during cycle 24. Both were accompanied by fast coronal mass ejections and gave rise to solar energetic particle (SEP) events measured by near-Earth spacecraft. In particular, the partially-occulted solar event on 10 September triggered a ground level enhancement (GLE), the second GLE of cycle 24. A further, much less energetic SEP event was recorded on 4 September. In this work we analyze observations by the Advanced Composition Explorer (ACE) and the Geostationary Operational Environmental Satellites (GOES), estimating the SEP event-integrated spectra above 300 keV and carrying out a detailed study of the spectral shape temporal evolution. Derived spectra are characterized by a low-energy break at few/tens of MeV; the 10 September event spectrum, extending up to ~1 GeV, exhibits an additional rollover at several hundred MeV. We discuss the spectral interpretation in the scenario of shock acceleration and in terms of other important external influences related to interplanetary transport and magnetic connectivity, taking advantage of multi-point observations from the Solar Terrestrial Relations Observatory (STEREO). Spectral results are also compared with those obtained for the 17 May 2012 GLE event.
Heavy ion ratio abundances in Solar Energetic Particle (SEP) events, e.g.~Fe/O, often exhibit decreases over time. Using particle instruments on the ACE, SOHO and STEREO spacecraft, we analysed heavy ion data from 4 SEP events taking place between December 2006 and December 2014. We constructed 36 different ionic pairs and studied their time evolution in each event. We quantified the temporal behaviour of abundant SEP ratios by fitting the data to derive a decay time constant $B$. We also considered the ratio of ionic mass--to--charge for each pair, the $S$ value given e.g.~for Fe/O by $S_{rm Fe/O} = (M/Q)_{rm Fe}big/(M/Q)_{rm O}$. We found that the temporal behaviour of SEP ratios is ordered by the value of $S$: ratios with $S>1$ showed decreases over time (i.e.~$B<0$) and those with $S<1$ showed increases ($B>0$). We plotted $B$ as a function of $S$ and observed a clear monotonic dependence: ratios with a large $S$ decayed at a higher rate. A prominent discontinuity at $S=2.0$ (corresponding to He/H) was found in 3 of the 4 events, suggesting anomalous behaviour of protons. The X/H ratios often show an initial increase followed by a decrease, and decay at a slower rate. We discuss possible causes of the observed $B$ versus $S$ trends within current understanding of SEP propagation.
The PAMELA satellite experiment is providing first direct measurements of Solar Energetic Particles (SEPs) with energies from about 80 MeV to several GeV in near-Earth space, bridging the low energy data by other space-based instruments and the Ground Level Enhancement (GLE) data by the worldwide network of neutron monitors. Its unique observational capabilities include the possibility of measuring the flux angular distribution and thus investigating possible anisotropies. This work reports the analysis methods developed to estimate the SEP energy spectra as a function of the particle pitch-angle with respect to the Interplanetary Magnetic Field (IMF) direction. The crucial ingredient is provided by an accurate simulation of the asymptotic exposition of the PAMELA apparatus, based on a realistic reconstruction of particle trajectories in the Earths magnetosphere. As case study, the results for the May 17, 2012 event are presented.
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