Empirical modelling of solar energetic particles

Solar energetic particles (SEPs) are an important component of space weather. They pose a serious radiation hazard to electronic equipment and biological organisms in space. They are produced in explosive events such as solar flares and coronal mass ejections, where particles can be accelerated up to several orders of magnitude above their thermal energy. During large SEP events, observed particle fluxes may increase by many orders of magnitude in timescales of minutes, and the increases may last several days. The occurrence of SEP events is approximately correlated with the overall activity of the Sun. The solar activity follows a nearly periodic cycle with the average period of approximately 11 years, and the amplitude of the cycle is modulated by longer variations. During the most recent solar cycle, activity has stayed at a very low level compared to previous cycles. The low activity has been interpreted as a result of long-term variability, and it is expected to continue at least for the recently started cycle 25. This thesis presents research into two closely related topics, the first of which is the comparison of the properties of SEPs during the two recent solar cycles, namely cycles 23 and 24. We studied the heavy ion intensities and abundances, and found that the mean abundances of heavy ions were lower during cycle 24, and the overall number of SEP events with heavy ion enhancements was lower. These results reflect the reduced efficiency of SEP acceleration processes. We used shock acceleration theory and simulation results to show that the reduced efficiency can be explained by reduced average densities of coronal plasma and suprathermal seed particles. Designers of space missions require accurate knowledge about the radiation environment their equipment or astronauts will be exposed to. As the processes behind SEPs are extremely complex, long-term forecasting is not possible according to current knowledge. Therefore, statistical models of previous SEPs events are the only option in estimating the particle radiation environment. The second topic of this thesis is the development of such models, with special emphasis on the high energy part of the SEP spectrum. We developed models for solar high energy proton fluences and peak fluxes. The models were based on space-borne and ground-based measurements over several solar cycles, and they provide improved estimations of the high energy radiation environment

ERNEn suurienergiaisten hiukkasten tunnistus ja analysointi

Siirretty Doriast

Why is solar cycle 24 an inefficient producer of high-energy particle events?

The aim of the study is to investigate the reason for the low productivity of high-energy SEPs in the present solar cycle. We employ scaling laws derived from diffusive shock acceleration theory and simulation studies including proton-generated upstream Alfv\'en waves to find out how the changes observed in the long-term average properties of the erupting and ambient coronal and/or solar wind plasma would affect the ability of shocks to accelerate particles to the highest energies. Provided that self-generated turbulence dominates particle transport around coronal shocks, it is found that the most crucial factors controlling the diffusive shock acceleration process are the number density of seed particles and the plasma density of the ambient medium. Assuming that suprathermal populations provide a fraction of the particles injected to shock acceleration in the corona, we show that the lack of most energetic particle events as well as the lack of low charge-to-mass ratio ion species in the present cycle can be understood as a result of the reduction of average coronal plasma and suprathermal densities in the present cycle over the previous one

Updated Model of the Solar Energetic Proton Environment in Space

The Solar Accumulated and Peak Proton and Heavy Ion Radiation Environment (SAPPHIRE) model provides environment specification outputs for all aspects of the Solar Energetic Particle (SEP) environment. The model is based upon a thoroughly cleaned and carefully processed data set. Herein the evolution of the solar proton model is discussed with comparisons to other models and data. This paper discusses the construction of the underlying data set, the modelling methodology, optimisation of fitted flux distributions and extrapolation of model outputs to cover a range of proton energies from 0.1 MeV to 1 GeV. The model provides outputs in terms of mission cumulative fluence, maximum event fluence and peak flux for both solar maximum and solar minimum periods. A new method for describing maximum event fluence and peak flux outputs in terms of 1-in-x-year SPEs is also described. SAPPHIRE proton model outputs are compared with previous models including CREME96, ESP-PSYCHIC and the JPL model. Low energy outputs are compared to SEP data from ACE/EPAM whilst high energy outputs are compared to a new model based on GLEs detected by Neutron Monitors (NMs).Comment: 37 pages, 17 figure

New reconstruction of event-integrated spectra (spectral fluences) for major solar energetic particle events

Fluences of solar energetic particles (SEPs) are not easy to evaluate, especially for high-energy events (i.e. ground-level enhancements, GLEs). Earlier estimates of event-integrated SEP fluences for GLEs were based on partly outdated assumptions and data, and they required revisions. Here, we present the results of a full revision of the spectral fluences for most major SEP events (GLEs) for the period from 1956 -- 2017 using updated low-energy flux estimates along with greatly revisited high-energy flux data and applying the newly invented reconstruction method including an improved neutron-monitor yield function. Low- and high-energy parts of the SEP fluence were estimated using a revised space-borne/ionospheric data and ground-based neutron monitors, respectively. The measured data were fitted by the modified Band function spectral shape. The best-fit parameters and their uncertainties were assessed using a direct Monte Carlo method. As a result, a full reconstruction of the event-integrated spectral fluences was performed in the energy range above 30 MeV, parametrised, and tabulated for easy use along with estimates of the 68% confidence intervals. This forms a solid basis for more precise studies of the physics of solar eruptive events and the transport of energetic particles in the interplanetary medium, as well as the related applications.Comment: 19 pages, 3 figures, to be published in Astronomy and Astrophysic

Annual integral solar proton fluences for 1984-2019

Aims. Long-term fluxes or integral fluences of solar energetic particles (SEPs), and their variability within and beyond the 11-year solar cycle, make an important contribution to space physics. However, large uncertainties exist in the evaluation of average SEP fluxes or fluences over the last few decades, as they have been assessed by different methods and from different datasets. Here we revisit the derivation of annual integral SEP fluences from available data based on in situ measurements since 1984.Methods. We reconstructed a full time series of integral SEP fluxes above 10, 30, 60, 100, and 200 MeV for the period from 1984 to 2019 using observations performed by the GOES satellites. Intercalibration of the fluxes was performed via a linear relation between overlapping pairs of observations in order to obtain a uniform dataset. Galactic cosmic ray (GCR) background subtraction and identification of SEP event periods were carefully performed, allowing for a precise calculation of annual SEP fluences.Results. Annual integral fluences of SEPs with energies above 10, 30, 60, 100, and 200 MeV were calculated for the period from 1984 to 2019 (solar cycles 22-24), along with their uncertainties. It is shown that solar cycle 24 was significantly (by a factor of 5-8) weaker in the SEP fluence than the preceding cycles 22 and 23. The cumulative occurrence probability of years with the fluence above a given value is found to be perfectly described by the Weibull distribution. This can be used as a projection for the occurrence of solar extreme eruptive events on the secular timescales.</p

37th International Cosmic Ray Conference (ICRC2021)

Here we report a new reconstruction of the event-integrated spectra of solar energetic particles (SEP) detected by neutron monitor (NM) network and space-borne detectors (mainly GOES data) during ground-level enhancement (GLE) events. The reconstruction of SEP fluences is based on the “bow-tie” method employing the latest advances in the NM data analysis (a time-dependent GCR background and the use of the NM yield function directly verified with the AMS-02 experiment data), usage of different space-borne detectors data and a detailed study of different uncertainties. As a result of this work, we obtained integral fluences of SEPs in the energy range from 30 MeV to a few GeV for 58 moderate and strong GLE events since 1956. The results were fitted with the modified Band-function (a double power-law function with two exponential cutoffs). An easy-to-use presentation of SEP fluences in the form of an analytical expression forms a solid basis for new studies in different fields, such as the influence of SEPs on the atmosphere and a statistical study of extreme solar activity.</p

New reconstruction of event-integrated spectra (spectral fluences) for major solar energetic particle events

Aims. Fluences of solar energetic particles (SEPs) are not easy to evaluate, especially for high-energy events (i.e. ground-level enhancements, GLEs). Earlier estimates of event-integrated SEP fluences for GLEs were based on partly outdated assumptions and data, and they required revisions. Here, we present the results of a full revision of the spectral fluences for most major SEP events (GLEs) for the period from 1956 to 2017 using updated low-energy flux estimates along with greatly revisited high-energy flux data and applying the newly invented reconstruction method including an improved neutron-monitor yield function.Methods. Low- and high-energy parts of the SEP fluence were estimated using a revised space-borne/ionospheric data and ground-based neutron monitors, respectively. The measured data were fitted by the modified Band function spectral shape. The best-fit parameters and their uncertainties were assessed using a direct Monte Carlo method.Results. A full reconstruction of the event-integrated spectral fluences was performed in the energy range above 30 MeV, parametrised and tabulated for easy use along with estimates of the 68% confidence intervals.Conclusions. This forms a solid basis for more precise studies of the physics of solar eruptive events and the transport of energetic particles in the interplanetary medium, as well as the related applications.</p

Catalogue of 55-80 MeV solar proton events extending through solar cycles 23 and 24

We present a new catalogue of solar energetic particle events near the Earth, covering solar cycle 23 and the majority of solar cycle 24 (1996-2016), based on the 55-80 MeV proton intensity data gathered by the SOHO/ERNE experiment. In addition to ERNE proton and heavy ion observations, data from the ACE/EPAM (near-relativistic electrons), SOHO/EPHIN (relativistic electrons), SOHO/LASCO (coronal mass ejections, CMEs), and GOES soft X-ray experiments are also considered and the associations between the particle and CME/X-ray events deduced to obtain a better understanding of each event. A total of 176 SEP events have been identified as having occurred during the time period of interest; their onset and solar release times have been estimated using both velocity dispersion analysis (VDA) and time-shifting analysis (TSA) for protons, as well as TSA for near-relativistic electrons. Additionally, a brief statistical analysis has been performed on the VDA and TSA results, as well as the X-rays and CMEs associated with the proton/electron events, both to test the viability of the VDA and to investigate possible differences between the two solar cycles. We find, in confirmation of a number of previous studies, that VDA results for protons that yield an apparent path length of 1 AU < s <~ 3 AU seem to be useful, but those outside this range are probably unreliable, as evidenced by the anticorrelation between apparent path length and release time estimated from the X-ray activity. It also appears that even the first-arriving energetic protons apparently undergo significant pitch angle scattering in the interplanetary medium, with the resulting apparent path length being on average about twice the length of the spiral magnetic field. The analysis indicates an increase in high-energy SEP events originating from the far eastern solar hemisphere; e.g., such an event...Comment: 33 pages, 12 figures (2 with multiple image files), 1 appendix as an external PDF file. Article is in the accepted manuscript/referee (single column) forma

Consistency of the average flux of solar energetic particles over timescales of years to megayears

Aims. Solar energetic particles (SEPs) have been measured directly in space over the past decades. Rare extreme SEP events are studied based on terrestrial cosmogenic proxy data for the past ten millennia. Lunar rocks record the average SEP fluxes on the megayear timescale. The question of whether the SEP fluxes averaged over different timescales are mutually consistent is still open. Here we analyze these different datasets for mutual consistency.Methods. Using the data from directly measured SEPs over the past decades and reconstructions of extreme SEP events in the past, we built a distribution function of the occurrence of annual SEP fluences for SEPs with energies above 30, 60, 100, and 200 MeV. The distribution function was fit with the Weibull and other types of distributions, and the long-term average SEP flux was computed and compared with the megayear SEP flux estimated from lunar data.Results. In contrast to the current paradigm, the direct space-era data are not representative of the long-term averaged SEP flux because they are only 20-55% of it, while the major fraction was formed by rare extreme SEP events in the past. The combined statistics of direct and proxy data are fully consistent with megayear lunar data, implying that our knowledge of the whole range of the SEP fluxes, from frequent weak to rare extreme events, is now consistent.</p