Solar Energetic Particle Spectra Research Articles

The First Ground‐Level Enhancement of Solar Cycle 25 as Seen by the High‐Energy Particle Detector (HEPD‐01) on Board the CSES‐01 Satellite

AbstractIn this work we present the High‐Energy Particle Detector (HEPD‐01) observations of proton fluxes from space during the 28 October 2021 solar energetic particle event, which produced a ground‐level enhancement on Earth. The event was associated with the major, long‐duration X1‐class flare and the concomitant coronal mass ejection (CME) that erupted from the Active Region 12887. This is the first direct measurement from space of solar particles emitted during the current solar cycle, recorded by a single instrument in the energy range from ∼50 MeV/n up to ∼250 MeV/n. We have performed a Weibull‐modeled spectral analysis of the energy spectrum in the wide energy range 300 keV–250 MeV, obtained from combination of HEPD‐01 proton measurements with the ones from ACE/ULEIS, SOHO/EPHIN, and SOHO/ERNE. The good agreement between data and model, also corroborated by a comparison with other spectral shapes commonly used in these studies, suggests that particles could have possibly been accelerated out from the ambient corona through the contribution of stochastic acceleration at the CME‐driven shock, even if the presence of seed populations influencing spectral shape could not be excluded. Finally, a Solar Proton Release time of 16:01 UTC ± 13 min and a magnetic path‐length of L = 1.32 ± 0.24 AU have been obtained, in agreement with previous results for this event. We remark that new and precise data on protons in the tens/hundreds MeV energy range—like the one provided by HEPD‐01—could shed more light on particle acceleration as well as provide a reliable parametrization of solar energetic particle spectra for Space Weather purposes.

Radio Astronomical Tools for the Study of Solar Energetic Particles II.Time-Extended Acceleration at Subrelativistic and Relativistic Energies

Solar energetic particle (SEP) events are commonly separated in two categories: numerous “impulsive” events of relatively short duration, and a few “gradual” events, where SEP-intensities may stay enhanced over several days at energies up to several tens of MeV. In some gradual events the SEP spectrum extends to relativistic energies (>1 GeV), over shorter durations. The two categories are strongly related to an idea developed in the 1960s based on radio observations: Type III bursts, which were addressed in a companion chapter, outline impulsive acceleration of electrons to subrelativistic energies, while the large and the relativistic SEP events were ascribed to a second acceleration process. At radio wavelengths, typical counterparts were bursts emitted by electrons accelerated at coronal shock waves (type II bursts) and by electron populations in large-scale closed coronal structures (type IV bursts). Both burst types are related to coronal mass ejections (CMEs). Type II bursts from metric to kilometric wavelengths tend to accompany large SEP events, which is widely considered as a confirmation that CME-driven shocks accelerate the SEPs. But type II bursts, especially those related to SEP events, are most often accompanied by type IV bursts, where the electrons are rather accelerated in the wake of the CME. Individual event studies suggest that although the CME shock is the most plausible accelerator of SEPs up to some yet unknown limiting energy, the relativistic SEP events show time structure that rather points to coronal acceleration related to type IV bursts. This chapter addresses the question what type II bursts tell us about coronal shock waves and how type II and type IV radio bursts are related with relativistic proton signatures as seen by particle detectors on the Earth and by their gamma-ray emission in the solar atmosphere, focusing on two relativistic SEP events, on 2005 Jan 20 and 2017 Sep 10. The importance of radio emissions as a complement to the upcoming SEP observations from close to the Sun is underlined.

Ionization effect in the Earth’s atmosphere during the sequence of October–November 2003 Halloween GLE events

The effect of precipitating high-energy particles on atmospheric physics and chemistry is extensively studied over the last decade. In majority of the existing models, the precipitating particles induced ionization plays an essential role. For such effects, it is necessary to possess enhanced increase in ion production, specifically during the winter period. In this study, we focus on highly penetrating particles — cosmic rays. The galactic cosmic rays are the main source of ionization in the Earth’s stratosphere and troposphere. On the other hand, the atmospheric ionization may be significantly enhanced during strong solar energetic particle events, mainly over the polar caps. A specific interest is paid to the most energetic solar proton events leading to counting rate enhancement of ground-based detectors, namely the so-called ground level enhancements (GLEs). During solar cycle 23, several strong ground level enhancements were observed. A sequence of three GLEs was observed in October–November 2003, the Halloween events. Here, on the basis of 3-D Monte Carlo model, we computed the energetic particles induced atmospheric ionization, explicitly considering the contribution of cosmic rays with galactic and solar origin. The ion production rates were computed as a function of the altitude above sea level using reconstructed solar energetic particles spectra. The 24 h and event averaged ionization effects relative to the average due to galactic cosmic rays were also computed.

The physics of space weather/solar-terrestrial physics (STP): what we know now and what the current and future challenges are

Abstract. Major geomagnetic storms are caused by unusually intense solar wind southward magnetic fields that impinge upon the Earth's magnetosphere (Dungey, 1961). How can we predict the occurrence of future interplanetary events? Do we currently know enough of the underlying physics and do we have sufficient observations of solar wind phenomena that will impinge upon the Earth's magnetosphere? We view this as the most important challenge in space weather. We discuss the case for magnetic clouds (MCs), interplanetary sheaths upstream of interplanetary coronal mass ejections (ICMEs), corotating interaction regions (CIRs) and solar wind high-speed streams (HSSs). The sheath- and CIR-related magnetic storms will be difficult to predict and will require better knowledge of the slow solar wind and modeling to solve. For interplanetary space weather, there are challenges for understanding the fluences and spectra of solar energetic particles (SEPs). This will require better knowledge of interplanetary shock properties as they propagate and evolve going from the Sun to 1 AU (and beyond), the upstream slow solar wind and energetic “seed” particles. Dayside aurora, triggering of nightside substorms, and formation of new radiation belts can all be caused by shock and interplanetary ram pressure impingements onto the Earth's magnetosphere. The acceleration and loss of relativistic magnetospheric “killer” electrons and prompt penetrating electric fields in terms of causing positive and negative ionospheric storms are reasonably well understood, but refinements are still needed. The forecasting of extreme events (extreme shocks, extreme solar energetic particle events, and extreme geomagnetic storms (Carrington events or greater)) are also discussed. Energetic particle precipitation into the atmosphere and ozone destruction are briefly discussed. For many of the studies, the Parker Solar Probe, Solar Orbiter, Magnetospheric Multiscale Mission (MMS), Arase, and SWARM data will be useful.

A novel synthetic diamond Cherenkov radiator for measuring space radiation

The measurement of cosmic rays and Solar energetic particles in space is basic to our understanding of the Galaxy, the Sun, phenomena in the Heliosphere and the emerging field of space weather . For these reasons, cosmic ray instruments are common on both scientific spacecraft and operational spacecraft such as weather satellites. Cosmic rays (CRs) and Solar energetic particles (SEPs) include ions over the full range of elements found in the Solar System. High-resolution measurements of the energy spectra of space radiation are key to understanding both acceleration and propagation processes. An inherent challenge is the large range of energies of such spectra. Cosmic ray energies range up to over 1 0 21 eV , while SEPs can reach a few GeV. Multi-instrument measurements are currently required to cover the full range of particle energies. Indeed, the highest energy particles, due to the rarity, can only be measured with ground-based instruments using the atmosphere as a calorimeter . Over limited energy ranges, SEP spectra are often approximated by a power law; however, over the full energy range, SEP events exhibit changing spectral shapes (e.g. “knees”, “roll-overs” and “cut-offs”). These features give information about the acceleration processes, such as size of the acceleration region, time of acceleration, morphology of the magnetic field during acceleration, and others, all of which can vary from event to event. Measurements of such features are often compromised by the need to combine measurements from more than one instrument, each with its own limited energy range. Even if there are no gaps between the energy intervals of the instruments, differing systematics can severely impact data analysis. A single instrument capable of measurements over a continuous and extended energy range would offer vastly more reliable measurement of the energy spectra of SEP events as well as replacing multiple instruments on resource-limited spacecraft. The most common method to measure GCRs and SEPs from a few to ∼ 100 MeV for protons, is Si Solid-State Detector (SSD) stacks. Above these energies, Cherenkov detectors are typically used together with SSDs. Ideally, to provide full energy coverage with no gaps, this requires a Cherenkov radiator with a threshold of ∼ 100 MeV. No suitable Cherenkov detector with such a low threshold has been developed. We are in the process of developing a synthetic diamond Cherenkov detector for this purpose. Diamond’s high index of refraction (2.42) results in a theoretical threshold of 92 MeV for protons. Even with a practical threshold of ∼ 110 MeV, this is ideal for extending the energy range from that of SSDs alone to that of sapphire Cherenkov detectors (202 MeV threshold) with higher energies attainable using plastic Cherenkov detectors. Both Sapphire and plastic Cherenkov radiators have spaceflight heritage.

Solar Energetic Particle Events Observed by the PAMELA Mission

Abstract Despite the significant progress achieved in recent years, the physical mechanisms underlying the origin of solar energetic particles (SEPs) are still a matter of debate. The complex nature of both particle acceleration and transport poses challenges to developing a universal picture of SEP events that encompasses both the low-energy (from tens of keV to a few hundreds of MeV) observations made by space-based instruments and the GeV particles detected by the worldwide network of neutron monitors in ground-level enhancements (GLEs). The high-precision data collected by the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) satellite experiment offer a unique opportunity to study the SEP fluxes between ∼80 MeV and a few GeV, significantly improving the characterization of the most energetic events. In particular, PAMELA can measure for the first time with good accuracy the spectral features at moderate and high energies, providing important constraints for current SEP models. In addition, the PAMELA observations allow the relationship between low- and high-energy particles to be investigated, enabling a clearer view of the SEP origin. No qualitative distinction between the spectral shapes of GLE, sub-GLE and non-GLE events is observed, suggesting that GLEs are not a separate class, but are the subset of a continuous distribution of SEP events that are more intense at high energies. While the spectral forms found are to be consistent with diffusive shock acceleration theory, which predicts spectral rollovers at high energies that are attributed to particles escaping the shock region during acceleration, further work is required to explore the relative influences of acceleration and transport processes on SEP spectra.

Upgrade of GLE database: Assessment of effective dose rate at flight altitude

A new database for assessment of radiation doses at cruise flight altitude in the Earth atmosphere, related to ground level enhancement (GLE) events is created under VarSiTi/SCOSTEP support and incorporated to the International ground level enhancement (GLE) database (gle.oulu.fi). The upgraded database provides, for each GLE event, where possible, information on the estimated energy/rigidity spectra of solar energetic particles and the corresponding computed effective doses at cruise flight altitude of 35 kft (10,668 m above sea level). The computations are performed for various reconstructions of solar energetic particles spectra, available in literature, thus for some events there are several results. Computations were performed using a recent model for assessment of effective dose due to cosmic ray particles, applied specifically in the polar region, where the exposure is maximal. This upgrade allows one to estimate the radiation effects at cruise flight altitude caused by major GLE events over several decades.

Neutron monitor count rate increase as a proxy for dose rate assessment at aviation altitudes during GLEs

Radiation exposure due to cosmic rays, specifically at cruising aviation altitudes, is an important topic in the field of space weather. While the effect of galactic cosmic rays can be easily assessed on the basis of recent models, estimate of the dose rate during strong solar particle events is rather complicated and time consuming. Here we compute the maximum effective dose rates at a typical commercial flight altitude of 35 kft (≈11 000 m above sea level) during ground level enhancement events, where the necessary information, namely derived energy/rigidity spectra of solar energetic particles, is available. The computations are carried out using different reconstructions of the solar proton spectra, available in bibliographic sources, leading to multiple results for some events. The computations were performed employing a recent model for effective dose and/or ambient dose equivalent due to cosmic ray particles. A conservative approach for the computation was assumed. A highly significant correlation between the maximum effective dose rate and peak NM count rate increase during ground level enhancement events is derived. Hence, we propose to use the peak NM count rate increase as a proxy in order to assess the peak effective dose rate at flight altitude during strong solar particle events using the real time records of the worldwide global neutron monitor network.

A Generalized Approach to Model the Spectra and Radiation Dose Rate of Solar Particle Events on the Surface of Mars

Abstract For future human missions to Mars, it is important to study the surface radiation environment during extreme and elevated conditions. In the long term, it is mainly galactic cosmic rays (GCRs) modulated by solar activity that contribute to the radiation on the surface of Mars, but intense solar energetic particle (SEP) events may induce acute health effects. Such events may enhance the radiation level significantly and should be detected as immediately as possible to prevent severe damage to humans and equipment. However, the energetic particle environment on the Martian surface is significantly different from that in deep space due to the influence of the Martian atmosphere. Depending on the intensity and shape of the original solar particle spectra, as well as particle types, the surface spectra may induce entirely different radiation effects. In order to give immediate and accurate alerts while avoiding unnecessary ones, it is important to model and well understand the atmospheric effect on the incoming SEPs, including both protons and helium ions. In this paper, we have developed a generalized approach to quickly model the surface response of any given incoming proton/helium ion spectra and have applied it to a set of historical large solar events, thus providing insights into the possible variety of surface radiation environments that may be induced during SEP events. Based on the statistical study of more than 30 significant solar events, we have obtained an empirical model for estimating the surface dose rate directly from the intensities of a power-law SEP spectra.

The Effects of Interplanetary Transport in the Event-intergrated Solar Energetic Particle Spectra

Abstract Previous investigations on the energy spectra of solar energetic particle (SEP) events revealed that the energy spectra observed at 1 au often show double power laws with break energies from one to tens of MeV/nuc. In order to determine whether the double power-law features result from the SEP source or the interplanetary transport process from the Sun to 1 au, we separately analyze the SEP spectra in the decay phase, during which the transport effect is minimum. In this paper, we reported three events observed by the Interplanetary Monitory Platform 8 spacecraft, which occurred on 1977 September 19, November 22, and 1979 March 1. For the first two events, the event-integrated spectra of protons possess double power-law profiles with break energies in a range of several MeV to tens of MeV, while the spectra integrated in the decay (reservoir) phase yield single power laws. Moreover, a general trend from a double power law at the rising phase to a single power law at the decay phase is observed. For the third event, both the event-integrated and the reservoir spectra show double power-law features. However, the difference between the low- and high-energy power-law indices is smaller for the reservoir spectrum than the event-integrated spectrum. These features were reproduced by solving the 1D diffusion equation analytically and we suggest that the transport process, especially the diffusion process, plays an important role in breaking the energy spectra.

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.

SPECTRAL PROPERTIES OF LARGE GRADUAL SOLAR ENERGETIC PARTICLE EVENTS. II. SYSTEMATIC Q/M DEPENDENCE OF HEAVY ION SPECTRAL BREAKS

ABSTRACT We fit ∼0.1–500 MeV nucleon−1 H–Fe spectra in 46 large solar energetic particle (SEP) events with the double power-law Band function to obtain a normalization constant, low- and high-energy parameters γ a and γ b, and break energy E B, and derive the low-energy spectral slope γ 1. We find that: (1) γ a, γ 1, and γ b are species-independent and the spectra steepen with increasing energy; (2) E B decreases systematically with decreasing Q/M scaling as (Q/M) α ; (3) α varies between ∼0.2–3 and is well correlated with the ∼0.16–0.23 MeV nucleon−1 Fe/O; (4) in most events, α < 1.4, γ b–γ a > 3, and O E B increases with γ b–γ a; and (5) in many extreme events (associated with faster coronal mass ejections (CMEs) and GLEs), Fe/O and 3He/4He ratios are enriched, α ≥ 1.4, γ b–γ a < 3, and E B decreases with γ b–γ a. The species-independence of γ a, γ 1, and γ b and the Q/M dependence of E B within an event and the α values suggest that double power-law SEP spectra occur due to diffusive acceleration by near-Sun CME shocks rather than scattering in interplanetary turbulence. Using γ 1, we infer that the average compression ratio for 33 near-Sun CME shocks is 2.49 ± 0.08. In most events, the Q/M dependence of E B is consistent with the equal diffusion coefficient condition and the variability in α is driven by differences in the near-shock wave intensity spectra, which are flatter than the Kolmogorov turbulence spectrum but weaker than the spectra for extreme events. In contrast, in extreme events, enhanced wave power enables faster CME shocks to accelerate impulsive suprathermal ions more efficiently than ambient coronal ions.

DOUBLE POWER LAWS IN THE EVENT-INTEGRATED SOLAR ENERGETIC PARTICLE SPECTRUM

ABSTRACT A double power law or a power law with exponential rollover at a few to tens of MeV nucleon−1 of the event-integrated differential spectra has been reported in many solar energetic particle (SEP) events. The rollover energies per nucleon of different elements correlate with a particle's charge-to-mass ratio (Q/A). The probable causes are suggested as residing in shock finite lifetimes, shock finite sizes, shock geometry, and an adiabatic cooling effect. In this work, we conduct a numerical simulation to investigate a particle's transport process in the inner heliosphere. We solve the focused transport equation using a time-backward Markov stochastic approach. The convection, magnetic focusing, adiabatic cooling effect, and pitch-angle scattering are included. The effects that the interplanetary turbulence imposes on the shape of the resulting SEP spectra are examined. By assuming a pure power-law differential spectrum at the Sun, a perfect double-power-law feature with a break energy ranging from 10 to 120 MeV nucleon−1 is obtained at 1 au. We found that the double power law of the differential energy spectrum is a robust result of SEP interplanetary propagation. It works for many assumptions of interplanetary turbulence spectra that give various forms of momentum dependence of a particle's mean free path. The different spectral shapes in low-energy and high-energy ends are not just a transition from the convection-dominated propagation to diffusion-dominated propagation.

SIMULATIONS OF THE SPATIAL AND TEMPORAL INVARIANCE IN THE SPECTRA OF GRADUAL SOLAR ENERGETIC PARTICLE EVENTS

The spatial and temporal invariance in the spectra of energetic particles in the gradual solar events is reproduced in the simulations. Based on a numerical solution of the focused transport equation, we obtain the intensity time profiles of solar energetic particles (SEPs) accelerated by an interplanetary shock in the three-dimensional interplanetary space. The shock is treated as a moving source of energetic particles with a distribution function. The time profiles of particle flux with different energies are calculated in the ecliptic at $1$ AU. According to our model, we find that shock acceleration strength, parallel diffusion and adiabatic cooling are the main factors in forming the spatial invariance in SEP spectra, and perpendicular diffusion is a secondary factor. In addition, the temporal invariance in SEP spectra is mainly due to the effect of adiabatic cooling. Furthermore, a spectra invariant region, which agrees with observations but is different than the one suggested by Reames and co-workers, is proposed based on our simulations.

PROPERTIES OF A CORONAL SHOCK WAVE AS A DRIVER OF EARLY SEP ACCELERATION

Coronal mass ejections (CMEs) are thought to drive collisionless shocks in the solar corona, which in turn have been shown capable of accelerating solar energetic particles (SEPs) in minutes. It has been notoriously difficult to extract information about energetic particle spectra in the corona, due to lack of in-situ measurements. It is possible, however, to combine remote observations with data-driven models in order to deduce coronal shock properties relevant to the local acceleration of SEPs and their heliospheric connectivity to near-Earth space. We present such novel analysis applied to the May 11, 2011 CME event on the western solar limb, focusing on the evolution of the eruption-driven, dome-like shock wave observed by the Atmospheric Imaging Assembly (AIA) EUV telescopes on board the Solar Dynamics Observatory spacecraft. We analyze the shock evolution and estimate its strength using emission measure modeling. We apply a new method combining a geometric model of the shock front with a potential field source surface model to estimate time-dependent field-to-shock angles and heliospheric connectivity during shock passage in the low corona. We find that the shock was weak, with an initial speed of ~450 km/s. It was initially mostly quasi-parallel, but significant portion of it turned quasi-perpendicular later in the event. There was good magnetic connectivity to near-Earth space towards the end of the event as observed by the AIA instrument. The methods used in this analysis hold a significant potential for early characterization of coronal shock waves and forecasting of SEP spectra based on remote observations.

THE GROUND-LEVEL ENHANCEMENT OF 2012 MAY 17: DERIVATION OF SOLAR PROTON EVENT PROPERTIES THROUGH THE APPLICATION OF THE NMBANGLE PPOLA MODEL

In this work, we apply an updated version the Neutron Monitor (NM) Based Anisotropic GLE Pure Power Law (NMBANGLE PPOLA) model, in order to derive the characteristics the ground-level enhancement (GLE) on 2012 May 17 (GLE71), the spectral properties the related solar energetic particle (SEP) event, the spatial distributions the high-energy solar cosmic ray fluxes at the top the atmosphere, and the time evolution the locationoftheGLEsource.Ourmodeling,baseduniquelyontheuseofground-levelNMdata,leadstothefollowing mainresults.TheSEPspectrumrelatedtoGLE71wasrathersoftduringthewholedurationoftheevent,manifesting some weak acceleration episodes only during the initial phase (at 01:55‐02:00UT) and at 02:30‐02:35UT and 02:55‐03:00UT. The spectral index the modeled SEP spectrum supports the coronal mass ejection‐shock driven particle acceleration scenario, in agreement with past results based on the analysis satellite measurements. During the initial phase GLE71, the solar proton source at the top the atmosphere was located above the northernhemisphere,implyingthattheasymptoticdirectionsofviewingofthenorthernhemisphereNMsweremore favorably located for registering the event than the southern ones. The spatial distribution the solar proton fluxes at the top the atmosphere during the main phase manifested a large variation along longitude and latitude. At the rigidity 1GV, the maximum primary solar protonflux resulted on the order of 3# 10 4 part. m $ 2 s $ 1 sr $ 1 GV $ 1 .

Charge states of heavy ions, as determined from the parameters of solar energetic particle spectra

The charge states of He, C, O, and Fe ions are determined for 51 gradual solar energetic particle (SEP) events of solar cycle 23 using the parameters of the particle energy spectra consisting of two power-law regions separated by the so-called knee. Experimental data from the GOES satellites (protons) and ULEIS (all particles) and SIS (H, C, O, Fe ions) instruments are employed. The charge states of the heavy ions are found to be independent of the SEP event magnitude and particle energy (in the interval of 0.3 to 30 MeV/nucleon).

NON-STANDARD ENERGY SPECTRA OF SHOCK-ACCELERATED SOLAR PARTICLES

We consider a numerical model for the shock acceleration of energetic ions in the magnetic environment of the solar corona. The model is motivated by observations of the deka-to-hecto-MeV proton energy spectra, ion and electron timing, and abundances in the beginning of major solar energetic particle (SEP) events, prior to the event's main phase associated with coronal mass ejection (CME) driven shock in the solar wind. Inasmuch as the obliquity of the CME-liftoff-associated shocks in solar corona and hence the seed-particle supply for the shock acceleration are essentially time dependent, a steady state energy spectrum of accelerated protons near the shock could not be attained. Energy spectrum of the SEP emission depends on the spatial and energy distribution of seed particles for the coronal shock acceleration, on the shock wave history, and on the location and scenario of the energetic particle escape into the interplanetary medium. We use a numerical model of the shock acceleration on a semicircular magnetic field line to learn a significance of different effects. If the shock geometry in a particular magnetic tube changes from nearly parallel to perpendicular, the resulting SEP spectrum in most distant sections of the tube, e.g., at the top of a transequatorial loop, resembles a wide beam, which is very different from the standard power-law spectrum that would be expected in a steady state. Possible escape of the shock-accelerated particles from more than one coronal location, stochastic re-acceleration, and the magnetic tube expansion can make the SEP spectra even more complicated.

OBSERVATIONS OF THE 2006 DECEMBER 13 AND 14 SOLAR PARTICLE EVENTS IN THE 80 MeV n–1-3 GeV n–1RANGE FROM SPACE WITH THE PAMELA DETECTOR

We present the space spectrometer PAMELA observations of proton and helium fluxes during the December 13 and 14, 2006 solar particle events. This is the first direct measurement of the solar energetic particles in space with a single instrument in the energy range from $\sim$ 80 MeV/n up to $\sim$ 3 GeV/n. In the event of December 13 measured energy spectra of solar protons and helium were compared with results obtained by neutron monitors and other detectors. Our measurements show a spectral behaviour different from those derived from the neutron monitor network. No satisfactory analytical fitting was found for the energy spectra. During the first hours of the December 13 event solar energetic particles spectra were close to the exponential form demonstrating rather significant temporal evolution. Solar He with energy up to ~1 GeV/n was recorded on December 13. In the event of December 14 energy of solar protons reached ~600 MeV whereas maximum energy of He was below 100 MeV/n. The spectra were slightly bended in the lower energy range and preserved their form during the second event. Difference in the particle flux appearance and temporal evolution in these two events may argue for a special conditions leading to acceleration of solar particles up to relativistic energies.

Solar Energetic Particle Spectrum on 2006 December 13 Determined by IceTop

On 13 December 2006 the IceTop air shower array at the South Pole detected a major solar particle event. By numerically simulating the response of the IceTop tanks, which are thick Cherenkov detectors with multiple thresholds deployed at high altitude with no geomagnetic cut-off, we determined the particle energy spectrum in the energy range 0.6 to 7.6 GeV. This is the first such spectral measurement using a single instrument with a well defined viewing direction. We compare the IceTop spectrum and its time evolution with previously published results and outline plans for improved resolution of future solar particle spectra.