Transient effects of solar energetic particle flux enhancements on the inner magnetosphere of the earth

In one type of space weather, the sun emits intermittent enhancements of solar energetic particle (SEP) fluxes. A fraction of these fluxes that reach the envelope of geospace can be injected into the magnetospheric particle confinement region after transiting the geomagnetic tail domain, the polar cleft/cusp region, or directly through the front side magnetopause. Common for these processes is that they provide inward diffusive “leakage” whenever the immediate external flux environment is more intense than in the outer trapping region. Conversely, following injection events outward leakage can also occur whereby the confinement region becomes a source of Magnetosheath particles. Numerical modeling has been carried out to investigate the effects on the ambient fluxes in the Earth's radiation belts from this effect.     

Temporal profiles of solar energetic particle events from SOHO/EPHIN data

Temporal profiles of energetic ions and electrons observed at 1 AU during solar energetic particle events are mainly determined by particle injection features, the observer location relative to the source region at the Sun, and the interplanetary space plasma and field conditions during particle transport. In this work, temporal profiles of 18 solar energetic particle events have been analyzed by fitting a pulse function to them in order to find a set of parameters which can be used to characterize the events.     

A model of solar energetic particles for use in calculating LET spectra developed from ONR-604 data.

A model of solar energetic particles (SEP) has been developed and is applied to solar flares during the 1990/1991 CRRES mission using data measured by the University of Chicago instrument, ONR-604. The model includes the time-dependent behavior, heavy-ion content, energy spectrum and fluence, and can accurately represent the observed SEP events in the energy range between 40 to 500 MeV/nucleon. Results are presented for the March and June, 1991 flare periods.     

Prediction of energetic particle behavior at high solar latitudes

Emphasis is placed on predictions of the galactic cosmic-ray flux at high heliographic latitudes. Recent work on gradient and curvature drifts in the large-scale heliospheric magnetic field have modified the traditional argument that the cosmic rays should be essentially unmodulated over the solar poles. In fact, drift effects during the next solar cycle, when the International Solar Polar Mission is to fly, are predicted to cause considerable modulation in the polar regions. However, it is pointed out that the use of conventional drift formulae in the solar wind, which contains magnetic-field fluctuations with large amplitudes and perhaps systematic properties, is highly suspect. Prediction of cosmic-ray behavior over the solar poles is thus currently uncertain. Also considered is the behavior of the anomalous cosmic-ray component at high heliographic latitudes.     

To the problem on the reliability of solar energetic proton flux models

This work analyses basic issues of conformity of the most well-known models of solar energetic particles (SEP) fluxes to the experimental data. It is shown, that the postulates on neglecting SEP fluxes in quiet Sun years and on invariability SEP fluxes in active Sun years, underlying some models, contradict the experimental data.     

The inner heliosphere from solar minimum to solar maximum: Short- and long-term variations in the energetic particle population

Between 1975 and 1983 HELIOS 1 scanned the interplanetary medium between 0.3 and 1 AU 31 times. The observed variations in the differential and integral flux of protons and helium nuclei in the energy range from 4 to >50 MeV/n are characterized by large temporal changes in the intensities, moderate changes in the energy spectrum and changes in the gradient below the detection level (60%). During solar minimum conditions recurrent disturbances are caused mainly by corotating interaction regions. The onset of solar activity near the end of 1977, characterized by a large number of solar events, is accompanied by a monotonous decrease of galactic cosmic radiation. The successive reduction of the cosmic ray intensity to the level of solar maximum is discussed in view of the role of large transient disturbances as compared to processes as diffusion, convection, adiabatic energy losses and drifts.     

Solar flares,CMEs and solar energetic particle events during solar cycle 24

We present here a study of Solar Energetic Particle Events (SEPs) associated with solar flares during 2010–2014 in solar cycle 24. We have selected the flare events (≥GOES M-class), which produced SEPs. The SEPs are classified into three categories i.e. weak (proton intensity?≤?1?pfu), minor (1?pfu?<?proton intensity?<?10?pfu) and major (proton intensity?≥?10?pfu). We used the GOES data for the SEP events which have intensity greater than one pfu and SOHO/ERNE data for the SEP event less than one pfu intensity. In addition to the flare and SEP properties, we have also discussed different properties of associated CMEs.     

Solar flare particle injection spectra and spectra of flux maxima at the point of observation

Numerical models of impulsive solar flare particle events usually assume the radial diffusion coefficient to be independent of energy per nucleon, T, although the observations indicate a T^0.5 dependence (constant mean free path). The assumption of a constant diffusion coefficient results in a preservation of a power law injection spectrum at all radial distances throughout the event. We investigate the effect of an energy dependent diffusion coefficient on the spectrum of flux maxima at a fixed point in interplanetary space. This spectrum is harder than that of initial differential number densities close to the sun. Furthermore, the spectrum hardens with increasing radial distance which seems to be at variance with observations.     

Some problems with developing a standard for determining solar energetic particle fluxes

Based on the author’s experience in ISO TC20/SC14 Working Group 4, this paper discusses the common problems encountered when developing a standard for solar energetic particle (SEP) fluxes. The problem involving the reliability of the distribution function describing the SEP events and the interpolation of this function into the region of not-yet-observed large events are discussed. The problems with describing the fluences of SEPs over a wide range of energy in the form of energetic spectra are analyzed. Requirements for SEP flux models are formulated. The reliability of some SEP flux models is determined by comparing their predictions with the experimental data.     

The atmospheric cosmic- and solar energetic particle radiation environment at aircraft altitudes

Galactic cosmic rays interact with the solar wind, the earth's magnetic field and its atmosphere to produce hadron, lepton and photon fields at aircraft altitudes. In addition to cosmic rays, energetic particles generated by solar activity bombard the earth from time to time. These particles, while less energetic than cosmic rays, also produce radiation fields at aircraft altitudes which have qualitatively the same properties as atmospheric cosmic rays. We have used a code based on transport theory to calculate atmospheric cosmic-ray quantities and compared them with experimental data. Agreement with these data is seen to be good. We have then used this code to calculate equivalent doses to aircraft crews. We have also used the code to calculate radiation doses from several large solar energetic particle events which took place in 1989, including the very large event that occurred on September 29^th and 30^th of that year. The spectra incident on the atmosphere were determined assuming diffusive shock theory.     

Power-law spectra of energetic electrons in solar flares from the maximum entropy and dimensional considerations

The maximum entropy formalism and dimensional analysis are used to derive a power-law spectrum of accelerated electrons in impulsive solar flares, where the particles can contain a significant fraction of the total flare energy. Entropy considerations are used to derive a power-law spectrum for a particle distribution characterised by its order of magnitude of energy. The derivation extends an earlier one-dimensional argument to the case of an isotropic three-dimensional particle distribution. Dimensional arguments employ the idea that the spectrum should reflect a balance between the processes of energy input into the corona and energy dissipation in solar flares. The governing parameters are suggested on theoretical grounds and shown to be consistent with solar flare observations. The flare electron flux, differential in the non-relativistic electron kinetic energy E, is predicted to scale as $E^{-3}$. This scaling is in agreement with RHESSI measurements of the hard X-ray flux that is generated by deka-keV electrons, accelerated in intense solar flares.     

Solar energetic particle events during the rise phases of solar cycles 23 and 24

We present a comparative study of the properties of coronal mass ejections (CMEs) and flares associated with the solar energetic particle (SEP) events in the rising phases of solar cycles (SC) 23 (1996–1998) (22 events) and 24 (2009–2011) (20 events), which are associated with type II radio bursts. Based on the SEP intensity, we divided the events into three categories, i.e. weak (intensity < 1 pfu), minor (1 pfu < intensity < 10 pfu) and major (intensity ? 10 pfu) events. We used the GOES data for the minor and major SEP events and SOHO/ERNE data for the weak SEP event. We examine the correlation of SEP intensity with flare size and CME properties. We find that most of the major SEP events are associated with halo or partial halo CMEs originating close to the sun center and western-hemisphere. The fraction of halo CMEs in SC 24 is larger than the SC 23. For the minor SEP events one event in SC23 and one event in SC24 have widths < 120° and all other events are associated with halo or partial halo CMEs as in the case of major SEP events. In case of weak SEP events, majority (more than 60%) of events are associated with CME width < 120°. For both the SC the average CMEs speeds are similar. For major SEP events, average CME speeds are higher in comparison to minor and weak events. The SEP event intensity and GOES X-ray flare size are poorly correlated. During the rise phase of solar cycle 23 and 24, we find north–south asymmetry in the SEP event source locations: in cycle 23 most sources are located in the south, whereas during cycle 24 most sources are located in the north. This result is consistent with the asymmetry found with sunspot area and intense flares.     

Composition of energetic particles from solar flares.

We present a model for composition of heavy ions in the solar energetic particles (SEP). The SEP composition in a typical large solar particle event reflects the composition of the Sun, with adjustments due to fractionation effects which depend on the first ionization potential (FIP) of the ion and on the ratio of ionic charge to mass (Q/M). Flare-to-flare variations in composition are represented by parameters describing these fractionation effects and the distributions of these parameters are presented.     

Geomagnetic transmission disturbances and heavy-ion fluences observed in low Earth orbit during the solar energetic particle events of October 1989.

The large solar energetic particle (SEP) events and simultaneous large geomagnetic disturbances observed during October 1989 posed a significant, rapidly evolving space radiation hazard. Using data from the GOES-7, NOAA-10, IMP-8 and LDEF satellites, we determined the geomagnetic transmission, heavy ion fluences, mean Fe ionic charge state, and effective radiation hazard observed in low Earth orbit (LEO) for these SEPs. We modeled the geomagnetic transmission by tracing particles through the combination of the internal International Geomagnetic Reference Field (IGRF) and the Tsyganenko (1989) magnetospheric field models, extending the modeling to large geomagnetic disturbances. We used our results to assess the radiation hazard such very large SEP events would pose in the anticipated 52 degrees inclination space station orbit.     

Propagation of energetic particles in the solar wind

The characteristics of solar energetic particle events as observed in interplanetary space depend on many physical processes acting at the source and during the transport from the source to the observer. These processes, such as acceleration at the Sun and the propagation near the Sun and in interplanetary space depend, in general, on both the particle velocity and rigidity. Thus, the evaluation of both the nuclear charge and/or atomic mass and the ionic charge of heavy ions turns out to be essential for the interpretation of the physical parameters observed, such as the energy spectra and the compositional variations during individual solar energetic particle events. In this paper recent results on the direct determination of the charge states of He, C, O, and Fe will be summarized. Using these results the compositional variations during individual solar particle events will be discussed. It will be shown that ratio changes by a factor of ～ 10 during the onset phase of solar particle events, as frequently observed, could be explained not only by rigidity dependent interplanetary propagation, but also by rigidity dependent diffusive propagation in the corona. However, there is now increasing experimental evidence that also other processes, such as compositional variations at the source and discontinuities of the interplanetary magnetic field, separating two different particle populations, may be important. Thus the picture emerges that these variations do not have a unique explanation but rather that each event has to be investigated individually.     

A program to measure new energetic particle nuclear interaction cross sections.

The Transport Collaboration, consisting of researchers from institutions in France, Germany, Italy and the USA, has established a program to make new measurements of nuclear interaction cross sections for heavy projectiles (Z > or = 2) in targets of liquid H2, He and heavier materials. Such cross sections directly affect calculations of galactic and solar cosmic ray transport through matter and are needed for accurate radiation hazard assessment. To date, the collaboration has obtained data using the LBL Bevalac HISS facility with 20 projectiles from 4He to 58Ni in the energy range 393-910 MeV/nucleon. Preliminary results from the analysis of these data are presented here and compared to other measurements and to cross section prediction formulae.     

Energization of charged particle in a time-dependent chaotic magnetic field with an implication of the production of seed particles in solar energetic particle events

We investigate the acceleration of charged particles in a time-dependent chaotic magnetic field in this work. In earlier works, it has been demonstrated that in an asymmetric wire-loop current systems (WLCSs), the magnetic field is of chaotic in nature. Furthermore, observations also showed that there exist time-varying current loops and current filaments in solar corona. It is therefore natural to conceive that the magnetic field on the solar surface is chaotic and time-dependent. Here, we develop a numerical model to study the acceleration process of charged particles in a time-varying chaotic magnetic field that is generated by an ensemble of 8 WLCSs. We found that the motion of energetic particles in the system is of diffusive in nature and a power law spectrum can quickly develop. The mechanism examined here may serve as an efficient pre-acceleration mechanism that generates the so-called seed particles for diffusive shock acceleration at a coronal mass ejection (CME) driven shock in large solar energetic particle (SEP) events.     

Research needed to improve solar flare and particle forecasting.

We briefly review the status of our physical understanding of energy buildup and release in transient active phenomena on the Sun. Such understanding is necessary in order to improve our capabilities to predict such events and their effects in interplanetary space and near-Earth environment. We then discuss the research that we consider is needed for such improvement.     

The influence of in situ pitch-angle cosine coverage on the derivation of solar energetic particle injection and interplanetary transport conditions

Modelization of solar energetic particle (SEP) events aims at revealing the general scenario of SEP injection and interplanetary propagation and relies on in situ measurements of SEP distributions. In this paper, we study to what extent the LEFS60 and LEMS30 electron telescopes of the Electron Proton Alpha Monitor (EPAM) on board the Advanced Composition Explorer are able to scan pitch-angle distributions during near-relativistic electron events. We estimate the percentage of the pitch-angle cosine range scanned by both telescopes for a given magnetic field configuration. We obtain that the pitch-angle coverage is always higher for LEFS60 than for LEMS30. Therefore, LEFS60 provides more information of the directional distribution of the observed particles. The aim of the paper is to study the relevance of the coverage when fitting LEFS60 particle measurements in order to infer the solar injection and the interplanetary transport conditions. By studying synthetic electron events, we obtain that at least 70% of the pitch-angle cosine range needs to be scanned by the telescope. Otherwise, multiple scenarios can explain the data.     

Relationship of solar flare accelerated particles to solar energetic particles (SEPs) observed in the interplanetary medium

Observations of hard X-ray (HXR)/γ-ray continuum and γ-ray lines produced by energetic electrons and ions, respectively, colliding with the solar atmosphere, have shown that large solar flares can accelerate ions up to many GeV and electrons up to hundreds of MeV. Solar energetic particles (SEPs) are observed by spacecraft near 1 AU and by ground-based instrumentation to extend up to similar energies as in large SEP events, but it appears that a different acceleration process, one associated with fast coronal mass ejections is responsible. Much weaker SEP events are observed that are generally rich in electrons, ³He, and heavy elements. The energetic particles in these events appear to be similar to those accelerated in flares. The Ramaty high energy solar spectroscopic imager (RHESSI) mission provides high-resolution spectroscopy and imaging of flare HXRs and γ-rays. Such observations can provide information on the location, energy spectra, and composition of the flare accelerated energetic particles at the Sun. Here, preliminary comparisons of the RHESSI observations with observations of both energetic electron and ion near 1 AU are reviewed, and the implications for the particle acceleration and escape processes are discussed.