High-energy cosmic-ray electrons in the Galaxy

The intensity of cosmic-ray electrons is only ∼1% of the protons at 10 GeV, and decreases very rapidly with energy to be ∼0.1% of protons at 1 TeV. Nevertheless, electrons in cosmic-rays have unique features, complementary to all other cosmic-ray nucleonic components, because they enable us to find the origins of cosmic-rays and the properties of their propagation mechanisms in the Galaxy. High-energy electrons lose energy by synchrotron and inverse Compton processes during the propagation in the Galaxy. Since the energy loss rate by these processes is proportional to the square of energy, TeV electrons accelerated in the sources at distances larger than ∼1 kpc, or ages greater than a few 10⁵ yr, cannot reach the solar system. This suggests that some nearby sources leave unique signatures in the form of identifiable structures in the energy spectrum of TeV electrons, and show increases of the flux towards the sources. In this paper, I review the past observations of high-energy cosmic-ray electrons and discuss their astrophysical significance.     

Propagation of normal and faster CMEs in the interplanetary medium

We have analyzed 101 Coronal Mass Ejection (CME) events and their associated interplanetary CMEs (ICMEs) and interplanetary (IP) shocks observed during the period 1997–2005 from the list given by Mujiber Rahman et al. (2012). The aim of the present work is to correlate the interplanetary parameters such as, the speeds of IP shocks and ICMEs, CME transit time and their relation with CME parameters near the Sun. Mainly, a group of 10 faster CME events (V_INT > 2200 km/s) are compared with a list of 91 normal events of Manoharan et al. (2004). From the distribution diagrams of CME, ICME and IP shock speeds, we note that a large number of events tends to narrow towards the ambient (i.e., background) solar wind speed (∼500 km/s) in agreement with the literature. Also, we found that the IP shock speed and the average ICME speed measured at 1 AU are well correlated. In addition, the IP shock speed is found to be slightly higher than the ICME speed. While the normal events show CME travel time in the range of ∼40–80 h with a mean value of 65 h, the faster events have lower transit time with a mean value of 40 h. The effect of solar wind drag is studied using the correlation of CME acceleration with interplanetary (IP) acceleration and with other parameters of ICMEs. While the mean acceleration values of normal and faster CMEs in the LASCO FOV are 1 m/s², 18 m/s², they are −1.5 m/s² and −14 m/s² in the interplanetary medium, respectively. The relation between CME speed and IP acceleration for normal and faster events are found to agree with that of  and  except slight deviations for the faster events. It is also seen that the faster events with less travel time face higher negative acceleration (>−10 m/s²) in the interplanetary medium up to 1 AU.     

Monte Carlo calibration of the response of the University of Chicago’s Cosmic Ray Nuclei Experiment (CRNE) on IMP-8 to electrons above 0.5 MeV

Modern instrument-simulation techniques offer the possibility of increasing the scientific yield from archival space datasets. In this paper, we report on a simulation of the electron response of the University of Chicago’s Cosmic Ray Nuclei Experiment (CRNE) instrument on the IMP-8 satellite. IMP-8/CRNE returned data from 1973 to 2006. The CRNE particle telescope was designed to measure the isotopic composition of Galactic cosmic-ray (GCR) nuclei and has also been used in many studies of protons and ions above 10 MeV/nucleon from solar energetic particle (SEP) events. But CRNE also functions as a highly-capable detector for solar electrons above 0.5 MeV, an energy range that has not been extensively studied. Utilization of the CRNE electron data has heretofore been limited by the fact that CRNE was never calibrated for electrons. We have therefore used the GEANT4 Monte Carlo simulation package to model the CRNE response to electrons and (separately) protons for multiple energies and incident angles. The results were used to compute the energy- and angle-dependence of the effective area and the energy-dependence of the geometric factor. The response to protons, which was already well understood, was used to verify the mass model, the simulation settings, and the post-processing software. Our simulation of the IMP-8/CRNE electron response now allows analysis of hundreds of relativistic solar electron events observed by CRNE over the years, including studies of evolution of electron energy spectra with high time resolution. We show examples of these results and briefly discuss potential applications to future scientific investigations.     

Solar particle precipitation into the polar atmosphere and their dependence on hemisphere and local time

The precipitation of solar energetic particles, protons as well as electrons, at high latitudes is commonly assumed to be homogeneous across both polar caps. Using Low-Earth Orbit POES (Polar Orbiting Environmental Satellites) we determine particle penetration ratios into the polar atmosphere for protons ranging from about 0.1 MeV to 500 MeV and for electrons spanning about one order of magnitude in energy with a maximum of 0.3 MeV. Based on power law fits for the POES spectrum we show, that for energies interesting for middle and lower atmosphere chemistry, particle flux over the poles is comparable in magnitude to flux at the geostationary orbit or at L1 in interplanetary space. The time period under study are the solar energetic particle (SEP) event series of October/November 2003 and January 2005.     

Charging of dust grains by anisotropic solar wind multi-component plasma

In this paper we study the charging process of small grain particles by anisotropic multi-component solar wind plasmas (electrons, protons and heavy ions), versus two-component (electron/proton) plasmas. We are focusing attention on the important characteristics of the charging process, namely the charging time, floating potential and current content as functions of plasma parameters such as He⁺⁺/H⁺ (α/p) number density and T_α/T_p temperature ratios of alpha particles to protons, as well as plasma streaming velocity v₀. Measured statistical properties of solar wind plasma parameters at 1 AU show considerable variations in α/p-temperature ratios from 1 to 10, in α/p-number density ratio from 0.01 to 0.35, as well as in values of streaming velocity v₀ from 200 km/s to 1000 km/s and more. Periods of these variations could last for several days each, leading to significant variability in the charging process, according to newly derived general analytical expressions. Numerical calculations performed for protons/alphas plasmas showed large disparity in the charging characteristics. For example, in anisotropic plasma, grain charging time varies up to 90% depending on α/p-particles temperature and number density ratios, whereas changes in floating potential are up to 40%. In contrast, in isotropic plasma, charging characteristic for grains do not change very much for the same plasma parameters variations, with charging time varying about 12% and floating potential only varying about 4%. It is also shown that in highly anisotropic plasma, with all ballistic electrons and ions, dust grains could not hold their charges, and characteristic discharged time is calculated. We note that the analysis is equally applicable to any sized body immersed in solar wind plasma.     

Coronal mass ejections acceleration problem

A statistical study of acceleration and its error of coronal mass ejections (CMEs) observed by the Large Angle Spectrometric Coronagraph (LASCO) is performed. A total of 5594 CMEs events have been analyzed by using a least-square method and using the error in the height measures. We verify that slower CMEs (velocities in the interval from 200 to 500 km s⁻¹) tend to have a positive acceleration (about 1 m s⁻²) at heights above 5 solar radii, while less than 10% CMEs show an average negative acceleration (about −2.2 m s⁻²) as they propagate from 5 to 30 solar radii. For most individual CMEs one can not say if they are accelerated or decelerated, only for 8% of all observed CMEs events one can extract the sign of the acceleration in the 5–30 solar radii.     

Exploring the global shock scenario at multiple points between sun and earth: The solar transients launched on January 1 and September 23, 1978

We revisit the transient interplanetary events of January 1 and September 23, 1978. Using in-situ and remote sensing observations at locations widely separated in longitudes and distances from the Sun, we infer that in both cases the overall shock surface had a very fast “nose” region with speeds >900 and >1500 km⁻¹ in the January and September events, respectively, and much slower flank speeds (∼600 km⁻¹ or less), suggesting a shock surface with a strong speed gradient with heliospheric longitude. The shock-nose regions are thus likely efficient acceleration sites of MeV ions, even at 1 AU from the Sun. Our 3D magnetohydrodynamics modeling suggests that a 24° × 24° localized disturbance at 18 solar radii injecting momentum 100 times the background solar wind input over 1 h can produce a disturbance in semi-quantitative agreement with the observed shock arrival time, plasma density and velocity time series in the January 1978 event.     

Observations of solar radio bursts from meter to kilometer wavelengths

Using the Clark Lake Radioheliograph data we present direct evidence that type III electron streams propagate in dense coronal streamers. We also present imaging observations of meter-decameter microbursts, which appear to be similar to those observed in hard X-rays. At meter-decameter wavelengths, these microbursts appear to be due to plasma radiation. From observations made with ISSE-3, we discuss the characteristics of hectometer and kilometer wavelength radio bursts. In particular, we show that from studies of type III storms that the exciter electrons propagate along spiral structures, where the density is enhanced and that there is an acceleration of the solar wind. We discuss type II bursts at kilometer wavelengths, compare them with meter type II bursts and discuss their association with interplanetary shocks. We show that the interaction between type III electron streams and shocks at kilometer wavelengths can provide information on the interplanetary shock geometry. Finally, we discuss the possibility that some shock associated (SA) events may be emissions caused by electrons accelerated lower in the atmosphere rather than high in the corona in type II shocks.Recent advances in solar research have resulted from new work on plasma radiation theory, new observations of active regions and flares across the electromagnetic spectrum and the availability of spacecraft in situ measurements of solar ejecta. In this paper, we review some results obtained with the Clark Lake multifrequency radioheliograph at meter-decameter wavelengths and from satellite multifrequency directive observations at hectometer and kilometer wavelengths. We present evidence that type III electrons propagate in dense coronal streamers, and that frequently observed microbursts (presumably of type III) at meter-decameter wavelengths are due to plasma radiation. We discuss observations of hectometer and kilometer type III radio storms which reveal information about active region structures, interplanetary magnetic field configuration, and solar wind acceleration. We also discuss kilometer type II bursts, interactions between type III electrons and interplanetary shocks, and present some new results on shock associated (SA) events.     

Heating heavy ions in the polar corona by collisionless shocks: A one-dimensional simulation

Recently a new model for explaining the observations of preferential heating of heavy ions in the polar solar corona was proposed ( and ). In that model the ion energization mechanism is the ion reflection off supercritical quasi-perpendicular collisionless shocks in the corona and the subsequent acceleration by the motional electric field E = −V × B/c. The mechanism of heavy ion reflection is based on ion gyration in the magnetic overshoot of the shock. The acceleration due to the motional electric field is perpendicular to the magnetic field, giving rise to large temperature anisotropy with T_⊥ ? T_∥, in agreement with SoHO observations. Such a model is tested here by means of a one dimensional test particle simulation where ions are launched toward electric and magnetic profiles representing the shock transition. We study the dynamics of O⁵⁺, as representative of coronal heavy ions for Alfvénic Mach numbers of 2–4, as appropriate to solar corona. It is found that O⁵⁺ ions are easily reflected and gain more than mass proportional energy with respect to protons.     

Viewing radiation signatures of solar energetic particles in interplanetary space

A current serious limitation on the studies of solar energetic particle (SEP) events is that their properties in the inner heliosphere are studied only through in situ spacecraft observations. Our understanding of spatial distributions and temporal variations of SEP events has come through statistical studies of many such events over several solar cycles. In contrast, flare SEPs in the solar corona can be imaged through their radiative and collisional interactions with solar fields and particles. We suggest that the heliospheric SEPs may also interact with heliospheric particles and fields to produce signatures which can be remotely observed and imaged. A challenge with any such candidate signature is to separate it from that of flare SEPs. The optimum case for imaging high-energy (E > 100 MeV) heliospheric protons may be the emission of π⁰-decay γ-rays following proton collisions with solar wind (SW) ions. In the case of E > 1 MeV electrons, gyrosynchrotron radio emission may be the most readily detectible remote signal. In both cases we may already have observed one or two such events. Another radiative signature from nonthermal particles may be resonant transition radiation, which has likely already been observed from solar flare electrons. We discuss energetic neutrons as another possible remote signature, but we rule out γ-ray line and 0.511 MeV positron annihilation emission as observable signatures of heliospheric energetic ions. We are already acquiring global signatures of large inner-heliospheric SW density features and of heliosheath interactions between the SW and interstellar neutral ions. By finding an appropriate observable signature of remote heliospheric SEPs, we could supplement the in situ observations with global maps of energetic SEP events to provide a comprehensive view of SEP events.     

Cosmic Ray Energetics And Mass for the International Space Station (ISS-CREAM)

The Cosmic Ray Energetics And Mass (CREAM) instrument is configured with a suite of particle detectors to measure TeV cosmic-ray elemental spectra from protons to iron nuclei over a wide energy range. The goal is to extend direct measurements of cosmic-ray composition to the highest energies practical, and thereby have enough overlap with ground based indirect measurements to answer questions on cosmic-ray origin, acceleration and propagation. The balloon-borne CREAM was flown successfully for about 161 days in six flights over Antarctica to measure elemental spectra of Z = 1–26 nuclei over the energy range 10¹⁰ to >10¹⁴ eV. Transforming the balloon instrument into ISS-CREAM involves identification and replacement of components that would be at risk in the International Space Station (ISS) environment, in addition to assessing safety and mission assurance concerns. The transformation process includes rigorous testing of components to reduce risks and increase survivability on the launch vehicle and operations on the ISS without negatively impacting the heritage of the successful CREAM design. The project status, including results from the ongoing analysis of existing data and, particularly, plans to increase the exposure factor by another order of magnitude utilizing the International Space Station are presented.     

Forbush decreases and solar events seen in the 10–20 GeV energy range by the Karlsruhe Muon Telescope

Since 1993, a muon telescope located at Forschungszentrum Karlsruhe (Karlsruhe Muon Telescope) has been recording the flux of single muons mostly originating from primary cosmic-ray protons with dominant energies in the 10–20 GeV range. The data are used to investigate the influence of solar effects on the flux of cosmic rays measured at Earth. Non-periodic events like Forbush decreases and ground level enhancements are detected in the registered muon flux. A selection of recent events will be presented and compared to data from the Jungfraujoch neutron monitor. The data of the Karlsruhe Muon Telescope help to extend the knowledge about Forbush decreases and ground level enhancements to energies beyond the neutron monitor regime.     

A model of radiation conditions during spacecraft flights in the interplanetary space and in the Earth's magnetosphere.

Based on the available measurement data, simulations of radiation conditions during spacecraft flights in the interplanetary space and in the Earth's and Jupiter's radiation belts has been carried out. The > or = 10 MeV and > or = 30 MeV solar flare proton fluence forecast has been proposed for Cycle 22. Radiation conditions due to both magnetospheric electrons and protons and to solar flare protons, magnetic rigidity cutoff being taken into account, have been evaluated on spacecraft trajectories in the Earth's and Jupiter's magnetospheres.     

Escaping near-relativistic electron beams from the solar corona

Processes in the solar corona are prodigious accelerators of energetic ions, and electrons. The angular distribution, composition, and spectra of energetic particles observed near Earth gives information on the acceleration mechanisms. A class of energetic particle observations particularly useful in understanding the solar acceleration is the near-relativistic impulsive beam-like electron events. During five years of operation the Advanced Composition Explorer (ACE) has measured well over 400 electron events. Approximately 25% of these electron events are impulsive beam-like events that are released onto interplanetary field lines predominantly from western solar longitudes. We extend our initial 3 year study during the rise to solar maximum (Haggerty and Roelof, 2002; Simnett et al., 2002) to a five year statistical analysis of these beam-like energetic electron events in relationship to optical flares, microwave emission, soft X-ray emission, metric and decametric type-III radio bursts, and coronal mass ejections.     

On the early phase of relativistic solar particle events: Are there signatures of acceleration mechanism?

Many physical processes precede and accompany the solar energetic particles (SEP) occurrence on the Earth’s orbit. Explosive energy release on the Sun gives rise to a flare and a coronal mass ejection (CME). X-ray and gamma emissions are believed to be connected with flares. Radio emission is signature of disturbances traveling through the corona and interplanetary space. Particles can gain energy both in the flare and the accompanying wave processes. The beginning of the SEP events has the advantage of being the phase most close to the time of acceleration. Influence of interplanetary transport is minimal in the case of first arriving relativistic solar protons recorded by ground based neutron monitors in so called ground-level enhancements (GLE). The early phase of the SEP events attracts attention of many scientists searching for the understanding of particle acceleration. However, they come to the opposite conclusions. While some authors find arguments for coronal mass ejections as a sole accelerator of SEPs, others prove a flare to be the SEP origin. Here, the circumstances of SEP generation for several GLEs of the 23rd solar cycle are considered. Timing of X-ray, CME, and radio emissions shows a great variety from event to event. However, the time of particle ejection from the Sun is closer to maximum of X-ray emission than to any other phenomena considered. No correlation is found between the particle fluxes and the CME characteristics.     

Evaluation of solar proton spectra using balloon cosmic ray observations and Monte Carlo simulation results

We have developed a method to evaluate the spectrum of solar energetic protons at the top of the Earth’s atmosphere from the measurements of our balloon cosmic ray experiment. By using the Monte Carlo PLANETOCOSMICS code based on Geant4 we compute the interaction of solar protons [10 MeV–10 GeV] with the Earth’s atmosphere. We obtain the angular and energy distributions of secondary particles (p, e⁻, e⁺, photons, muons) at different atmospheric levels as a function of primary proton spectra. By comparing the calculated depth dependence of the particle flux with the data obtained by our balloon experiment we can deduce the parameters of the solar proton spectrum that best fit the observations. In this paper we discuss our solar proton spectrum estimation method, and present results of its application to selected solar proton events from 2001 to 2005.     

First-order Fermi shock acceleration in solar flares

First order Fermi shock acceleration of electrons, protons and alpha particles is compared to observations of energetic particle events. For each event, a unique shock compression ratio produces spectra in good agreement with observation. The simple model predicts that the acceleration time to a given energy will be approximately equal for electrons and protons and, for reasonable solar parameters, can be less than 1 second to ～ 100 MeV.     

Modeling and experimental study of the 27-day variation of galactic cosmic-ray intensity for a solar wind velocity depending on heliolongitude

We develop a three-dimensional (3-D) model of the 27-day variation of galactic cosmic-ray (GCR) intensity with a spatial variation of the solar wind velocity. A consistent, divergence-free interplanetary magnetic field is derived by solving the corresponding Maxwell equations with a variable solar wind speed, which reproduces in situ observed experimental data for the time interval to be analyzed (24 August 2007–28 February 2008). We perform model calculations for the GCR intensity using the variable solar wind and the corresponding magnetic field. Results are compatible with experimental data; the correlation coefficient between our model predictions and observed 27-day GCR variation is 0.80 ± 0.05.     

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.     

Cosmic-ray electron spectrum above 100 GeV from PPB-BETS experiment in Antarctica

Cosmic-ray electrons have been observed in the energy region from 10 GeV to 1 TeV with the PPB-BETS by a long duration balloon flight using a Polar Patrol Balloon (PPB) in Antarctica. The observation was carried out for 13 days at an average altitude of 35 km in January 2004. The PPB-BETS detector is an imaging calorimeter composed of scintillating-fiber belts and plastic scintillators inserted between lead plates. In the study of cosmic-ray electrons, there have been some suggestions that high-energy electrons above 100 GeV are a powerful probe to identify nearby cosmic-ray sources and search for particle dark matter. In this paper, we present the energy spectrum of cosmic-ray electrons in the energy range from 100 GeV to 1 TeV at the top of atmosphere, and compare our spectrum with the results from other experiments.