Solar Energetic Particle Spectra Research Articles

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.

Solar energetic particle spectra from the SOHO-EPHIN sensor by application of regularization methods

Context. The Electron Proton Helium Instrument (EPHIN) on ESA’s Solar and Heliospheric Observatory (SOHO) measures solar energetic electrons, protons, and alpha particles with a stack of six solid-state detectors forming a telescope. The energy deposit in these detectors must be inverted to derive the original energy of the incident particles, thus leading to the original energy spectrum of solar energetic particles. Normal inversion techniques, such as least-squares methods, rely on fitting a known functional behavior of the spectral dependence (normally a power law) to the measured data with some account taken for the instrument response. Such procedures can fail to retrieve accurate particle spectra, e.g., when count rates are low and unphysical negative counts result from the fitting procedure. Aims. We show how regularization methods can be applied to energetic particle measurements to unambiguously derive the original particle spectrum without any assumptions about its functional behavior, while also satisfying constraints such as non-negative counts. Methods. Such inversion techniques still require knowledge of the instrument response function, however, it is an improvement upon normal least-squares or maximum-likelihood fitting procedures because it does not require any a-priori knowledge of the underlying particle spectra. Given the instrument response function in matrix form (here derived using Monte Carlo techniques), the original Fredholm integral equations reduce to a discrete system of linear algebraic equations that can be solved by ordinary regularization methods such as singular value decomposition (SVD) or the Tikhonov method. This procedure alone may lead to unphysical negative results, requiring the further constraint of non-negative count rates. This technique avoids full deconvolution because it involves the solution of ill-conditioned or singular linear systems. Results. We analyze data from SOHO/EPHIN by full deconvolution of the measured data with the instrument response function. We apply the SVD and Thikonov methods with and without constraints to measured data from SOHO/EPHIN. Conclusions. The derived results agree well with those of other methods that rely on a-priori knowledge of the spectral shape of the particle distribution function, demonstrating the power of the regularization method for more general cases.

Solar Energetic Particle Composition, Energy Spectra, and Space Weather

Recent progress in measuring the composition and energy spectra of solar energetic particles (SEPs) accelerated by CME-driven shocks is reviewed, including a comparison of the observed charge-to-mass dependence of breaks in SEP spectra with model predictions. Also discussed is a comparison of SEP and CME kinetic energies in seventeen large SEP events, and estimates of the SEP radiation dose that astronauts would be subject to once they venture outside the protective cover of Earth’s magnetosphere.

Forecasting of radiation hazard: 1. Alerts on great FEP events beginning; probabilities of false and missed alerts; on-line determination of solar energetic particle spectrum by using spectrographic method

We show that exact forecast can be made by using high-energy particles (few GeV/nucleon and higher) which transportation from the Sun is characterized by much bigger diffusion coefficient than for small and middle energy particles. Therefore, high-energy particles came from the Sun much more early (8–20 min after acceleration and escaping into solar wind) than main part of smaller energy particles caused dangerous situation for electronics (more than 30–60 min later). We describe here principles and experience of automatically working of program “FEP-Search”. The positive result which shows the exact beginning of FEP event on the Emilio Segre’ Observatory (2025 m above sea level, R c = 10.8 GV), is determined now automatically by simultaneously increasing on 2.5 St. Dev. in two sections of neutron supermonitor. The next 1-min data the program “FEP-Search” uses for checking that the observed increase reflects the beginning of real great FEP or not. If yes, automatically starts to work on line the programs “FEP-Research”. We determine also the probabilities of false and missed alerts. The first of programs “FEP-Research” is the program “FEP-Research/Spectrum”. We consider two variants: (1) quiet period (no change in cut-off rigidity), (2) disturbed period (characterized with possible changing of cut-off rigidity). We describe the method of determining of the spectrum of FEP in the 1-st variant (for this we need data for at least two components with different coupling functions). For the 2-nd variant we need data for at least three components with different coupling functions. We show that for these purposes can be used data of total intensity and some different multiplicities, but better to use data from two or three NM with different cut-off rigidities. We describe in details the algorithms of the program “FEP-Research/Spectrum”. We show how worked this program on examples of some historical great FEP events.

Forecasting of radiation hazard: 2. On-line determination of diffusion coefficient in the interplanetary space, time of ejection and energy spectrum at the source; on-line using of neutron monitor and satellite data

In Paper 1 [Dorman, L.I., Pustil’nik, L.A., Sternlieb, A., Zukerman, I.G. Forecasting of Radiation Hazard: 1. Alerts on Great FEP Events Beginning; Probabilities of False and Missed Alerts; on-Line Determination of Solar Energetic Particle Spectrum by using Spectrographic Method, Paper tCOSPAR tPSW1-0022-04, This Issue, 2005] it was described the behavior of programs “FEP-Search” and “FEP-Research/Spectrum” estimating, on the basis of on-line one-minute NM data, the beginning of event and FEP spectrum out of the Earth’s magnetosphere. We show that after these two steps it is possible to determine the time of ejection, diffusion coefficient in the interplanetary space, and energy spectrum at the source of FEP. We consider the following possibilities: (1) one of the above parameters is unknown; (2) two parameters are unknown; (3) all three parameters are unknown. We show that in the first case it is necessary to determine the energy spectrum of FEP on the Earth in two different times and automatically, from two equations, the unknown parameter can be determined (energy spectrum at the source or diffusion coefficient, or time of ejection; the determination is done by one equation, and the other is used for control of the model). In the second case it is necessary to determine the energy spectrum of FEP on the Earth in three different times and from three equations two parameters can be determined automatically (for example, the energy spectrum at the source and diffusion coefficient in the interplanetary space). In the third case, by using data for four different times all three unknown parameters can be determined (time of ejection, diffusion coefficient in the interplanetary space and energy spectrum at the source of FEP), and one equation can be used for control of the model. We describe in detail the algorithms of the programs “FEP-Research/Time of Ejection”, “FEP-Research/Source” and “FEP-Research/Diffusion”. We show the behavior of these programs on some historical great FEP events. On the basis of these on-line programs the time of ejection, diffusion coefficient in the interplanetary space and energy spectrum at the source of FEP can be determined. To extend the obtained information to the region of very small energies, we use, together with NM data, also the available satellite one-minute data. On the basis of these results it is shown the forecasting feasibility of expected radiation hazard for computers, electronics, solar batteries, and technology in space at different distances from the Sun and at different helio-latitudes. We show that the same procedure can be done for satellites on different orbits in the magnetosphere, by taking into account the change of cut-off rigidities along the orbits (for human health, solar batteries, computers, electronics, technology). By the method of coupling functions for different altitudes in the atmosphere we describe some principles of on-line radiation hazard forecasting for air-planes on regular and non-regular lines in dependence of altitude and cut-off rigidity, and estimation of shielding will be made The on-line on-ground forecasting of radiation hazard for people health and technology will be done in dependence of cut-off rigidity and atmospheric pressure. If for some cases the expected radiation hazard will be higher than some definite dangerous level, on-line special Alerts will be send. These programs are tested on some examples of historical great FEP events.

Errors in the particle flux measurement data relevant to solar energetic particle spectra

Systematical errors of the spacecraft measured high-energy particle fluxes are analyzed. The errors are shown to be inherent to most of the measurements made to be the monitoring of the high-energy radiation in the space. The level of the systematic errors of the measurements varies with energy, thus resulting in distortions of the solar energetic particle spectra based on the measurement data. The erroneous experimental data have resulted in spurious estimates of space radiation environment and give rise to erroneous physical conclusions.

Spectral Properties of He and Heavy Ions in3He‐rich Solar Flares

Using advanced instrumentation on the ACE spacecraft, we have conducted a survey of solar energetic particle spectra in 3He-rich events over a broad energy range ~80 keV nucleon-1 to 15 MeV nucleon-1 during the period 1997 September-2001 March. The spectra of 4He and heavy ions (C, N, O, Ne, Mg, Si, S, Ca, Fe) were generally similar over this range but often hardened below ~1 MeV nucleon-1. In most of the events there was even stronger hardening of the 3He spectrum below ~1 MeV nucleon-1, leading to an energy-dependent 3He : 4He ratio. These observations point to unique and distinct properties of 3He in these events and place new constraints on models that seek to explain enhancements of 3He and heavy ions using the same mechanisms. In addition to the events with spectra in the form of power laws or double power laws, there is a second class of event in which the low-energy 3He and Fe spectra are rounded, while the 4He remains a power law. In these cases 3He and Fe spectra can be fitted at low energies by a stochastic acceleration model, but this model does not explain the higher energy portions of these spectra, nor the power-law spectral forms of the 4He. These observations appear to require an additional mechanism, such as acceleration by cascading MHD turbulence. The 3He enrichment pattern that we observe suggests that all these different spectral features might be due to processes with a common origin but then followed by different acceleration histories.

The correlation between solar energetic particle peak intensities and speeds of coronal mass ejections: Effects of ambient particle intensities and energy spectra

The correlation of peak intensities of solar energetic particle (SEP) events with the speeds of the associated coronal mass ejections (CMEs) is understood to be a result of SEP acceleration at shocks driven by the CMEs. However, the peak SEP intensities associated with CMEs of a given speed vary over ∼ 4 orders of magnitude. We examine a database of 71 E > 10 MeV SEP events observed with the GOES satellite to determine whether enhanced ambient SEP intensities at the times of the CMEs and/or variations among SEP event spectra contribute to the large range of peak SEP event intensities. A statistical analysis shows that enhanced ambient SEP intensities may be a contributing factor to the range of SEP events of higher peak intensities, probably by providing sources of energetic seed particles for the shock acceleration process. Another factor is the variation of energy spectra among the SEP events, which generally have harder spectra with increasing peak intensities. The observed increase of peak SEP intensities and hardening of peak SEP spectra with increasingly westward solar source regions is only a minor factor in the range of SEP peak intensities in the CME speed correlation.

Gamma-Ray Line Observations of the 2000 July 14 Flare and SEP Impact on the Earth

The HXS and GRS detectors on Yohkoh observed the 14 July 2000, X5.7 flare, beginning at ∼ 10:20 UT, ∼ 4 min before the peak in soft X-rays. The hard X-rays and γ-rays peaked ∼ 3 min later at ∼ 10:27 UT. Solar γ-ray emission lasted until ∼ 10:40 UT. Impact of high-energy ions at the Sun is revealed by the γ-ray lines from neutron capture, annihilation radiation and de-excitation that are visible above the bremsstrahlung continuum. From measurement of these lines we find that the flare-averaged spectrum of accelerated protons is consistent with a power law ge10 MeV with index 3.14±0.15 and flux 1.1×1032 protons MeV−1 at 10 MeV. We estimate that there were ∼1.5×1030 erg in accelerated ions if the power law extended without a break down to 1 MeV; this is about 1% of the energy in electrons > 20 keV from measurements of the hard X-rays. We find no evidence for spectral hardening in the hard X-rays that has been suggested as a predictor for the occurrence of solar energetic particle (SEP) events. This was the third largest proton event above 10 MeV since 1976. The GRS and HXS also observed γ-ray lines and continuum produced by the impact of SEP on the Earth's atmosphere beginning about 13 UT on 14 July. These measurements show that the SEP spectrum softened considerably over the next 24 hours. We compare these measurements with proton measurements in space.

Observational studies of gamma-rays and neutrons from solar flares

Gamma-ray observations from HINOTORI satellite and possible neutron observations from the Tokyo neutron monitor are reviewed. Time histories of gamma-ray and X-ray emissions for both typical impulsive and gradual flares are discussed in connection with the particle acceleration time. The gamma-ray spectral hardening observed around 400 keV is explained from superimposition of two different electron bremsstrahlung spectra. Proton-energy spectra derived from the gamma-ray observations are compared with the solar energetic particle spectra in interplanetary space. The weak correlation between the gamma-ray fluence and the proton flux is discussed in connection with the particle trapping and escaping in the flare region. The limb darkening of the 2.22 MeV line resulting from neutron-proton capture is interpreted in terms of the attenuation by the Compton scattering in the photosphere. Possible solar neutron events recorded by the Tokyo neutron monitor are presented and the correlation between the gamma-ray fluence and the neutron fluence are described.