Extreme Solar Energetic Particle Research Articles

Revisiting empirical solar energetic particle scaling relations. II. Coronal mass ejections

The space radiation environment conditions and the maximum expected coronal mass ejection (CME) speed are assessed by investigating scaling laws between the peak proton flux and fluence of solar energetic particle (SEP) events with the speed of the CMEs. We used a complete catalog of SEP events, covering the last sim 25 years of CME observations (i.e., 1997 to 2017). We calculated the peak proton fluxes and integrated event fluences for events that reached an integral energy of up to E$>$ 100 MeV. For a sample of 38 strong SEP events, we first investigated the statistical relations between the recorded peak proton fluxes ($I_ P $) and fluences ($F_ P $) at a set of integral energies of E $>$10 MeV, E$>$30 MeV, E$>$60 MeV, and E$>$100 MeV versus the projected CME speed near the Sun ($V_ CME $) obtained by the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph (SOHO/LASCO). Based on the inferred relations, we further calculated the integrated energy dependence of both $I_ P $ and $F_ P $, assuming that they follow an inverse power law with respect to energy. By making use of simple physical assumptions, we combined our derived scaling laws to estimate the upper limits for $V_ CME $, $I_ P $, and $F_ P $ by focusing on two cases of known extreme SEP events that occurred on 23 February 1956, (GLE05) and in AD774/775, respectively. Based on the physical constraints and assumptions, several options for the upper limit $V_ CME $ associated with these events were investigated. A scaling law relating $I_ P $ and $F_ P $ to the CME speed as $V_ CME ^5$ for CMEs ranging between sim 3400-5400 km/s is consistent with values of $F_ P $ inferred for the cosmogenic nuclide event of AD774/775. At the same time, the upper CME speed that the current Sun can provide possibly falls within an upper limit of $V_ CME 5500 km/s.

Effects of Coronal Magnetic Field Configuration on Particle Acceleration and Release during the Ground Level Enhancement Events in Solar Cycle 24

Ground level enhancements (GLEs) are extreme solar energetic particle (SEP) events that are of particular importance in space weather. In solar cycle 24, two GLEs were recorded on 2012 May 17 (GLE 71) and 2017 September 10 (GLE 72), respectively, using a range of advanced modern instruments. Here we conduct a comparative analysis of the two events by focusing on the effects of large-scale magnetic field configuration near active regions on particle acceleration and release. Although the active regions are both located near the western limb, temporal variations of SEP intensities and energy spectra measured in situ display different behaviors at early stages. By combining a potential field model, we find the coronal mass ejection (CME) in GLE 71 originated below the streamer belt, while in GLE 72 it originated near the edge of the streamer belt. We reconstruct the CME shock fronts with an ellipsoid model based on nearly simultaneous coronagraph images from multiple viewpoints and further derive the 3D shock geometry at the GLE onset. The highest-energy particles are primarily accelerated in the shock–streamer interaction regions, i.e., likely at the nose of the shock in GLE 71 and the eastern flank in GLE 72, due to quasi-perpendicular shock geometry and confinement of closed fields. Subsequently, they are released to the field lines connecting to near-Earth spacecraft when the shocks move through the streamer cusp region. This suggests that magnetic structures in the corona, especially shock–streamer interactions, may have played an important role in the acceleration and release of the highest-energy particles in the two events.

The First Ground Level Enhancement Seen on Three Planetary Surfaces: Earth, Moon, and Mars

AbstractOn 28 October 2021, solar eruptions caused intense and long‐lasting solar energetic particle (SEP) flux enhancements observed by spacecraft located over a wide longitudinal range in the heliosphere. SEPs arriving at Earth caused the 73rd ground level enhancement (GLE) event recorded by ground‐based neutron monitors. In particular, this is also the first GLE event seen on the surface of three planetary bodies, Earth, Moon, and Mars, by particle and radiation detectors as shown in this study. We derive the event‐integrated proton spectrum from measurements by near‐Earth spacecraft and predict the lunar and martian surface radiation levels using particle transport models. Event doses at the lunar and martian surfaces of previous GLE events are also modeled and compared with the current event. This statistical and comparative study advances our understanding of potential radiation risks induced by extreme SEP events for future human explorations of the Moon and Mars.

What Is the Radiation Impact of Extreme Solar Energetic Particle Events on Mars?

AbstractSolar Energetic Particles (SEP) are one of the major sources of the Martian radiation environment. It is important to understand the SEP‐induced Martian radiation environment for future human habitats on Mars. Due to the lack of a global intrinsic magnetic field, Solar Energetic Particles (SEPs) can directly propagate through and interact with its atmosphere before reaching the surface and subsurface of Mars. Mars has many high mountains and low‐altitude craters where the atmospheric thickness can be more than 10 times different than one another. The SEP‐induced surface radiation level may therefore be very different from one location to another. We thus consider the influence of the atmospheric depths on the Martian radiation levels including the absorbed dose, dose equivalent, and (human‐)body effective dose induced by SEPs at varying heights above and below the Martian surface. The state‐of‐the‐art Atmospheric Radiation Interaction Simulator based on GEometry And Tracking Monte‐Carlo method has been employed for simulating particle interactions with the Martian atmosphere and terrain. We find that even the thinnest Martian atmosphere reduces radiation dose from that in deep space by at least 65%, and the shielding effect increases for denser atmosphere. Furthermore, we present a method to quickly forecast the SEP‐induced radiation in different regions of Mars with different surface pressures.

No signature of extreme solar energetic particle events in high-precision 14C data from the Alaskan tree for 1844–1876 CE

Cosmogenic nuclides – 14C from tree rings and 10Be and 36Cl from ice cores serve as an effective proxy for past extreme solar energetic particle (SEP) events. After identifying the first signature of an extreme SEP event in 774 CE, several candidates have been found in these proxy archives, such as 993 CE, 660 BCE, and 7176 BCE. Their magnitudes have been estimated to be tens of times larger than that of the largest SEP event ever observed since the 1950s. Although a detailed survey of such extreme SEP events is ongoing, the detection of intermediate-sized SEP events that bridge the gap between modern observations and extreme events detected in cosmogenic nuclides has not progressed sufficiently, primarily because of the uncertainties in cosmogenic nuclide data. In this study, we measured 14C concentrations in tree rings in the 19th century (1844–1876 CE) to search for any increases in 14C concentrations corresponding to intermediate-size extreme SEP events. We utilized Alaskan tree-ring samples cut into early and latewoods to suppress the potential seasonal variations in intra-annual 14C data. Notably, no significant 14C variations were observed between early and latewoods (0.0 ± 0.3‰), and the annual resolution 14C data series displayed an error of ~ 0.8‰. Over the entire study period, no significant increase in 14C concentrations characterized by other candidates of extreme SEP events such as the 774 CE event was detected in the annual 14C data. The present result imposes a constraint on the SEP fluence when the largest class of recorded solar storms occurred (especially those in 1859 CE and 1872 CE).

Solar Energetic-Particle Ground-Level Enhancements and the Solar Cycle

Severe geomagnetic storms appear to be ordered by the solar cycle in a number of ways. They occur more frequently close to solar maximum and the declining phase, are more common in larger solar cycles, and show different patterns of occurrence in odd- and even-numbered solar cycles. Our knowledge of the most extreme space-weather events, however, comes from spikes in cosmogenic-isotope (14C, 10Be, and 36Cl) records that are attributed to significantly larger solar energetic-particle (SEP) events than have been observed during the space age. Despite both storms and SEPs being driven by solar-eruptive phenomena, the event-by-event correspondence between extreme storms and extreme SEPs is low. Thus, it should not be assumed a priori that the solar-cycle patterns found for storms also hold for SEPs and the cosmogenic-isotope events. In this study, we investigate the solar-cycle trends in the timing and magnitude of the 67 SEP ground-level enhancements (GLEs) recorded by neutron monitors since the mid-1950s. Using a number of models of GLE-occurrence probability, we show that GLEs are around a factor of four more likely around solar maximum than around solar minimum, and that they preferentially occur earlier in even-numbered solar cycles than in odd-numbered cycles. There are insufficient data to conclusively determine whether larger solar cycles produce more GLEs. Implications for putative space-weather events in the cosmogenic-isotope records are discussed. We find that GLEs tend to cluster within a few tens of days, likely due to particularly productive individual active regions, and with approximately 11-year separations, owing to the solar-cycle ordering. However, these timescales would not explain any cosmogenic-isotope spikes requiring multiple extreme SEP events over consecutive years.

Regional Differences in Carbon-14 Data of the 993 CE Cosmic Ray Event

Cosmogenic nuclides such as 14C from tree rings and 10Be and 36Cl from ice cores are excellent proxies for the past extremely large solar energetic particle (SEP) events, which are dozens of times larger than the largest SEP event in the history of observation. So far, several rapid 14C increases have been discovered, which are considered to have originated from extreme SEP events (or set of successive SEP events) from verifications using multiple cosmogenic nuclide analyses in natural archives. Although these events are characterized by a rapid increase in cosmogenic nuclide concentrations, 14C data recorded worldwide do not always show similar variations, especially during the 993 CE event, where a rapid increase was recorded in either 992–993 CE or 993–994 CE in several records. We present new 14C data of the Japanese cedar sample for the 993 CE event. Although the latest data show no significant increase in 1 year, an overall increase pattern is consistent with the previously reported 14C data of the Japanese cedar, which supports that a significant 14C increase occurred from 993 to 994 CE in the Japanese sample. Given the dominant 14C production in high latitudes by SEPs, the difference in timing of increase may be a transport effect in the atmosphere. Moreover, the difference in the timing of the 14C increase can cause a 1-year age-determination error using the 993 CE radiocarbon spike. Compared with the 14C data between tree samples from high latitude and midlatitude, including Japan, high-latitude data can capture 14C changes originating from SEP events more quickly and clearly and may be more suitable for a SEP event exploration in the past.

Tree Rings Show Record of Newly Identified Extreme Solar Activity Event

Mass spectroscopy of tree ring material indicates a sharp, single-year rise in carbon-14 concentrations consistent with an extreme solar energetic particle event that occurred around 5410 BCE.

A Single-Year Cosmic Ray Event at 5410 BCE Registered in 14C of Tree Rings.

The annual 14C data in tree rings is an outstanding proxy for uncovering extreme solar energetic particle (SEP) events in the past. Signatures of extreme SEP events have been reported in 774/775 CE, 992/993 CE, and ∼660 BCE. Here, we report another rapid increase of 14C concentration in tree rings from California, Switzerland, and Finland around 5410 BCE. These 14C data series show a significant increase of ∼6‰ in 5411–5410 BCE. The signature of 14C variation is very similar to the confirmed three SEP events and points to an extreme short‐term flux of cosmic ray radiation into the atmosphere. The rapid 14C increase in 5411/5410 BCE rings occurred during a period of high solar activity and 60 years after a grand 14C excursion during 5481–5471 BCE. The similarity of our 14C data to previous events suggests that the origin of the 5410 BCE event is an extreme SEP event.

Study of the Impact of Past Extreme Solar Events on the Modern Air Traffic

AbstractThe ancient solar energetic particle (SEP) events of 774/775 CE and 993/994 CE were characterized thanks to radionuclide productions stored in environmental archives as ice cores or tree rings. Primary cosmic ray spectra deduced from these cosmogenic isotope data indicate that the impact of these extreme SEP events would have been much more significant than any of the ones observed during the modern era. However, the impact of these should be studied more accurately in the framework of the ambient dose equivalent impacting aircrew and passengers in the air traffic context by considering physical parameters such as time profile or anisotropy properties. In this study, the impact that 774/775 CE and 993/994 CE past extreme SEP events could have had on modern air traffic is discussed. Possible event spectra for these ancient events are derived from the spectra ground‐level enhancement (GLE) 5 and GLE 69, which have been observed during the modern era and have been widely studied/characterized using measurements. The investigations include the impact of the SEP activity on ambient dose equivalent, including detailed analyses considering route, airplane characteristics (departure, arrival, continent, airplane type), and the time occurrence of the SEP event. Statistical analyses show that additional dose levels can reach values on the order of 70 mSv, which is absolutely significant considering the current air traffic recommendations. The orders of magnitude of the ambient dose equivalent induced during past extreme SEP events raises a number of issues, both for aircrews and for avionics hardware. This study demonstrates that simulations can be useful for the evaluation of risks in case of extreme SEP events.

The Ground‐Level Enhancement Event of September 2017 and Other Large Solar Energetic Particle Events of Cycle 24

AbstractThe 10 September 2017 solar energetic particle (SEP) event was the largest since June 2015 and one of only two ground‐level enhancement (GLE) events of solar cycle 24. GLE events are subset of large SEP events (~15% of events identified by Space Weather Prediction Center) with particularly hard spectra, making them a substantial space weather hazard to space‐based instrumentation and exposed astronauts. We present analysis of the 10 September 2017 event and compare it to the other cycle 24 GLE events, to those of cycle 23, and also to two extreme SEP events observed by Solar Terrestrial Relations Observatory (STEREO). We find the 10 September 2017 event had a broken power law spectrum typical of GLE events but was softer than average at high energies. However, it was hard at low energies with a relatively high break energy, which led to 100‐MeV proton fluences within a factor of 4.5 of the largest cycle 23 GLE events. The composition was nominal, except for a low Fe/O ratio, which has also been seen in large SEP events this cycle, but is somewhat atypical of the cycle 23 GLE events. The extreme events seen by STEREO exhibited very hard high‐energy spectra, with one event producing ~80‐MeV proton fluences larger than the largest cycle 23 GLE event. However, even including STEREO events, the top 10 largest cycle 24 events are, on average, 2.4 times smaller than the top 10 of cycle 23 based on >10‐MeV proton fluences.

Spatial Organization of Seven Extreme Solar Energetic Particle Events

Abstract Emission of relativistic protons and helium responsible for extreme solar particle events (ground level enhancements (GLEs)) is often structured. We investigate its organization depending on the eruption stage characterized by the heliocentric height of associated coronal mass ejections (CMEs). Seven GLEs are considered: events on 1997 November 6, 1998 May 2, 2000 July 14, 2001 December 26, 2003 November 2, 2006 December 13, and 2012 May 17, which are half of the SOlar and Heliospheric Observatory (SOHO )-era GLEs, excluding very weak events. Count-rate profiles of the GLEs plotted as a function of the CME height reveal two types (or two components) of the high-energy particle emission. The first component rises in a step-like manner during the CME transit from 2 R ⊙ to 3 R ⊙, when the CME exits from predominantly closed coronal magnetic structures, irrespective of the CME speed (type H). This component is of coronal origin. The second component of the GLE-producing particles starts to rise when CME is at about 4 R ⊙, achieves its maximum at 6–10 R ⊙, and declines shortly after that (type J). The type J particle injection into the interplanetary space coincides with the decametric–hectometric radio burst complex that includes enhanced emission of type II and concurrent low-frequency type III bursts, indicative of the CME interaction with a streamer-like structure at a few solar radii from the Sun. Those could be delayed particles from the flare region. A possible additional contribution of the CME-bow-shock acceleration in unstructured solar wind is not large in the two considered types of events.

Analysis of Ground Level Enhancements (GLE): Extreme solar energetic particle events have hard spectra

Nearly 70 Ground Level Enhancements (GLEs) of cosmic rays have been recorded by the worldwide neutron monitor network since the 1950s depicting a big variety of energy spectra of solar energetic particles (SEP). Here we studied a statistical relation between the event-integrated intensity of GLEs (calculated as count-rate relative excess, averaged over all available polar neutron monitors, and expressed in percent-hours) and the hardness of the solar particle energy spectra. For each event the integral omnidirectional event-integrated fluences of particles with energy above 30MeV (F30) and above 200MeV (F200) were computed using the reconstructed spectra, and the ratio between the two fluences was considered as a simple index of the event’s hardness. We also provided a justification of the spectrum estimate in the form of the Band-function, using direct PAMELA data for GLE 71 (17-May-2012). We found that, while there is no clear relation between the intensity and the hardness for weak events, all strong events with the intensity greater 100%∗h are characterized by a very hard spectrum. This implies that a hard spectrum can be securely assumed for all extreme GLE events, e.g., those studied using cosmogenic isotope data in the past.

Particle acceleration at a pair of parallel shocks near the Sun

AbstractA “twin‐coronal mass ejection (CME)” scenario has been proposed recently to account for extreme solar energetic particle (SEP) events. In this scenario, two CMEs erupt from the same or nearby active regions within a short period of time. They both drive shocks, and the presence of the first CME‐driven shock leads to a more efficient particle acceleration process at the second shock. All Ground Level Enhancement events in solar cycle 23 and many large SEP events support the twin‐CME scenario. In a subset of these events, the two CMEs are separated by only a few minutes and particle acceleration is further enhanced due to particles reflecting between the two shocks. Here we examine particle acceleration at a parallel shock pair using a test particle approach. We follow single‐particle trajectories using two methods: close to the shock we follow particle's motion by solving the Lorentz equation directly; beyond certain distances, we ignore the gyration degree of freedom of a particle and approximate its motion by its guiding center motion and pitch angle scattering. We do not consider wave generation due to the streaming protons at both shocks. Instead, we assume the background turbulence upstream of the first shock is given and follow its transmission to the downstream of the second shock. Our results show that the intensity of accelerated particles in the shock pair case is significantly enhanced over a single shock case, and the maximum energy obtained in a shock pair system can be a few to 10 times that of a single shock case.

The carbon-14 spike in the 8th century was not caused by a cometary impact on Earth

A mysterious increase of radiocarbon 14C ca. 775 AD in the Earth’s atmosphere has been recently found by Miyake et al. (Miyake, F., Nagaya, K., Masuda, K., Nakamura, T. [2012]. Nature, 486, 240). A possible source of this event has been discussed widely, the most likely being an extreme solar energetic particle event. A new exotic hypothesis has been presented recently by Liu et al. (Liu, Y. [2014]. Sci. Rep., 4, 3728) who proposed that the event was caused by a cometary impact on Earth bringing additional 14C to the atmosphere. Here we calculated a realistic mass and size of such a comet to show that it would have been huge (≈100km across and 1017–1020g of mass) and would have produced a disastrous geological/biological impact on Earth. The absence of an evidence for such a dramatic event makes this hypothesis invalid.

On the identification of time interval threshold in the twin‐CME scenario

Recently it has been suggested that the “twin‐CME” scenario may be a very effective mechanism in causing extreme solar energetic particle (SEP) events and, in particular, ground level enhancement (GLE) events. Ding et al. (2013) performed a statistical examination of the twin‐CME scenario with a total of 126 fast and wide western coronal mass ejections (CMEs). They found that CMEs having a preceding CME with a speed > 300 km/s within 9 h from the same active region have larger probability of leading to large SEP events than CMEs that do not have preceding CMEs. The choice of 9 h being the time lag τ between the preceding CME and the main CME was based on some crude estimates of the decay time of the turbulence downstream of the shock driven by the preceding CME. In this work, we examine this choice. For the 126 fast wide CMEs examined in, we vary the time lag τ from 1 h to 24 h with an increment of 1 h. By considering three quantities whose values depend on the choice of this time lag τ, we show that the choice of 13 h for τ is more appropriate. Our study confirms our earlier result that twin CMEs are more likely to lead to large SEP events than single fast CMEs. The results shown here are of great relevance to space weather studies.

Extreme fluxes in solar energetic particle events: Methodological and physical limitations

In this study, all available data on the largest solar proton events (SPEs), or extreme solar energetic particle (SEP) events, for the period from 1561 up to now are analyzed. Under consideration are the observational, methodological and physical problems of energy-spectrum presentation for SEP fluxes (fluences) near the Earth's orbit. Special attention is paid to the study of the distribution function for extreme fluences of SEPs by their sizes. The authors present advances in at least three aspects: 1) a form of the distribution function that was previously obtained from the data for three cycles of solar activity has been completely confirmed by the data for 41 solar cycles; 2) early estimates of extremely large fluences in the past have been critically revised, and their values were found to be overestimated; and 3) extremely large SEP fluxes are shown to obey a probabilistic distribution, so the concept of an “upper limit flux” does not carry any strict physical sense although it serves as an important empirical restriction. SEP fluxes may only be characterized by the relative probabilities of their appearance, and there is a sharp break in the spectrum in the range of large fluences (or low probabilities). It is emphasized that modern observational data and methods of investigation do not allow, for the present, the precise resolution of the problem of the spectrum break or the estimation of the maximum potentialities of solar accelerator(s). This limitation considerably restricts the extrapolation of the obtained results to the past and future for application to the epochs with different levels of solar activity.

Possible effect of extreme solar energetic particle events of September–October 1989 on polar stratospheric aerosols: a case study

Abstract. The main ionization source of the middle and low Earth's atmosphere is related to energetic particles coming from outer space. Usually it is ionization from cosmic rays that is always present in the atmosphere. But in a case of a very strong solar eruption, some solar energetic particles (SEPs) can reach middle/low atmosphere increasing the ionization rate up to some orders of magnitude at polar latitudes. We continue investigating such a special class of solar events and their possible applications for natural variations of the aerosol content. After the case study of the extreme SEP event of January 2005 and its possible effect upon polar stratospheric aerosols, here we analyze atmospheric applications of the sequence of several events that took place over autumn 1989. Using aerosol data obtained over polar regions from two satellites with space-borne optical instruments SAGE II and SAM II that were operating during September–October 1989, we found that an extreme major SEP event might have led to formation of new particles and/or growth of preexisting ultrafine particles in the polar stratospheric region. However, the effect of the additional ambient air ionization on the aerosol formation is minor, in comparison with temperature effect, and can take place only in the cold polar atmospheric conditions. The extra aerosol mass formed under the temperature effect allows attributing most of the changes to the "ion–aerosol clear sky mechanism".

A FOCUSED TRANSPORT APPROACH TO THE TIME-DEPENDENT SHOCK ACCELERATION OF SOLAR ENERGETIC PARTICLES AT A FAST TRAVELING SHOCK

Some of the most sophisticated models for solar energetic particle (SEP) acceleration at coronal mass ejection driven shocks are based on standard diffusive shock acceleration theory. However, this theory, which only applies when SEP pitch-angle anisotropies are small, might have difficulty in describing first-order Fermi acceleration or the shock pre-heating and injection of SEPs into first-order Fermi acceleration accurately at lower SEP speeds where SEP pitch-angle anisotropies upstream near the shock can be large. To avoid this problem, we use a time-dependent focused transport model to reinvestigate first-order Fermi acceleration at planar parallel and quasi-parallel spherical traveling shocks between the Sun and Earth with high shock speeds associated with rare extreme gradual SEP events. The focused transport model is also used to investigate and compare three different shock pre-heating mechanisms associated with different aspects of the nonuniform cross-shock solar wind flow, namely, the convergence of the flow (adiabatic compression), the shear tensor of the flow, and the acceleration of the flow, and a fourth shock pre-heating mechanism associated with the cross-shock electric field, to determine which pre-heating mechanism contributes the most to injecting shock pre-heated source particles into the first-order Fermi acceleration process. The effects of variations in traveling shock conditions, such as increasing shock obliquity and shock slowdown, and variations in the SEP source with increasing shock distance from the Sun on the coupled processes of shock pre-heating, injection, and first-order Fermi acceleration are analyzed. Besides the finding that the cross-shock acceleration of the solar wind flow yields the dominant shock pre-heating mechanism at high shock speeds, we find that first-order Fermi acceleration at fast traveling shocks differs in a number of respects from the predictions and assumptions of standard steady-state diffusive shock acceleration theory as is discussed below.

A Twin-CME Scenario for Ground Level Enhancement Events

Ground Level Enhancement (GLEs) events are extreme Solar Energetic Particle (SEP) events. Protons in these events often reach ∼GeV/nucleon. Understanding the underlying particle acceleration mechanism in these events is a major goal for Space Weather studies. In Solar Cycle 23, a total of 16 GLEs have been identified. Most of them have preceding CMEs and in-situ energetic particle observations show some of them are enhanced in ICME or flare-like material. Motivated by this observation, we discuss here a scenario in which two CMEs erupt in sequence during a short period of time from the same Active Region (AR) with a pseudo-streamer-like pre-eruption magnetic field configuration. The first CME is narrower and slower and the second CME is wider and faster. We show that the magnetic field configuration in our proposed scenario can lead to magnetic reconnection between the open and closed field lines that drape and enclose the first CME and its driven shock. The combined effect of the presence of the first shock and the existence of the open close reconnection is that when the second CME erupts and drives a second shock, one finds both an excess of seed population and an enhanced turbulence level at the front of the second shock than the case of a single CME-driven shock. Therefore, a more efficient particle acceleration will occur. The implications of our proposed scenario are discussed.