Propagation of solar cosmic rays: Diffusion versus focused diffusion

This paper is designed to bring to the attention the fact that the effect of focusing of solar energetic particles is always essential as compared with scattering, no matter how small the value of the mean free path may be. That is why, an ordinary (focusing-free) diffusion approach can not be applied to the solar cosmic ray transport. In the case of high-energy solar particles, the focused diffusion is demonstrated to lead to a power law decay of energetic particle intensity much like an ordinary diffusion. However, the power law index of the decay is renormalized by the focusing.     

Sources of SEP Acceleration during a Flare – CME Event

A high-speed, halo-type coronal mass ejection (CME), associated with a GOES M4.6 soft X-ray flare in NOAA AR 0180 at S12W29 and an EIT wave and dimming, occurred on 9 November 2002. A complex radio event was observed during the same period. It included narrow-band fluctuations and frequency-drifting features in the metric wavelength range, type III burst groups at metric – hectometric wavelengths, and an interplanetary type II radio burst, which was visible in the dynamic radio spectrum below 14 MHz. To study the association of the recorded solar energetic particle (SEP) populations with the propagating CME and flaring, we perform a multi-wavelength analysis using radio spectral and imaging observations combined with white-light, EUV, hard X-ray, and magnetogram data. Velocity dispersion analysis of the particle distributions (SOHO and Wind in situ observations) provides estimates for the release times of electrons and protons. Our analysis indicates that proton acceleration was delayed compared to the electrons. The dynamics of the interplanetary type II burst identify the burst source as a bow shock created by the fast CME. The type III burst groups, with start times close to the estimated electron-release times, trace electron beams travelling along open field lines into the interplanetary space. The type III bursts seem to encounter a steep density gradient as they overtake the type II shock front, resulting in an abrupt change in the frequency drift rate of the type III burst emission. Our study presents evidence in support of a scenario in which electrons are accelerated low in the corona behind the CME shock front, while protons are accelerated later, possibly at the CME bow shock high in the corona.     

Interpreting Solar EUV Wave Observations from Different Viewing Angles Using an MHD Model

We study the effect of projection and line-of-sight integration on the interpretation of the morphology and kinematics of EUV waves. We have performed a three-dimensional magnetohydrodynamic simulation of a coronal mass ejection (CME) erupting in an environment that mimics the low solar corona and calculated the resulting emission measure of the event from five different viewing angles. Our study provides more quantitative information about the impact of the viewing angle and projection effect on the properties of EUV waves than previous studies on the subject. Analyzing the emission measure of the lower corona reveals wave-like increases that move away from the eruption site, which we interpret as EUV waves. Behind the EUV wave front we can recognize coronal dimming regions. A comparison of the emission measure and calculated density supports the view that EUV waves are true waves. Our results show that the origin of the observed EUV wave is height-dependent, which means that the measured speed and the morphology depend on the viewing direction. Consequently, care should be taken when EUV observations are used to infer the true propagation speeds of EUV wave fronts.     

The 1990 May 24 solar cosmic-ray event

This paper presents an integrated analysis of GOES 6, 7 and neutron monitor observations of solar cosmic-ray event following the 1990 May 24 solar flare. We have used a model which includes particle injection at the Sun and at the interplanetary shock front and particle propagation through the interplanetary medium. The model does not attempt to simulate the physical processes of coronal transport and shock acceleration, therefore the injections at the Sun and at the shock are represented by source functions in the particle transport equation. By fitting anisotropy and angle-average intensity profiles of high-energy (>30 MeV) protons as derived from the model to the ones observed by neutron monitors and at GOES 6 and 7, we have determined the parameters of particle transport, the injection rate and spectrum at the source. We have made a direct fit of uncorrected GOES data with both primary and secondary proton channels taken into account.The 1990 May 24–26 energetic proton event had a double-peaked temporal structure at energies 100 MeV. The Moreton (shock) wave nearby the flare core was seen clearly before the first injection of accelerated particles into the interplanetary medium. Some (correlated with this shock) acceleration mechanism which operates in the solar corona at a height up to one solar radius is regarded as a source of the first (prompt) increase in GOES and neutron monitor counting rates. The proton injection spectrum during this increase is found to be hard (spectral index 1.6) at lower energies ( 30 MeV) with a rapid steepening above 300 MeV. Large values of the mean free path ( 1.8 AU for 1 GV protons in the vicinity of the Earth) led to a high anisotropy of arriving protons. The second (delayed) proton increase was presumably produced by acceleration/injection of particles by an interplanetary shock wave at height of 10 solar radii. Our analysis of the 1990 May 24–26 event is in favour of the general idea that a number of components of energetic particles may be produced while the flare process develops towards larger spatial/temporal scales.Visiting Associate from St. Petersburg State Technical University, St. Petersburg 195251, Russia.     

Adiabatic Deceleration of Solar Energetic Particles as Deduced from Monte Carlo Simulations of Interplanetary Transport

Monte Carlo simulations of interplanetary transport are employed to study adiabatic energy losses of solar protons during propagation in the interplanetary medium. We consider four models. The first model is based on the diffusion-convection equation. Three other models employ the focused transport approach. In the focused transport models, we simulate elastic scattering in the local solar wind frame and magnetic focusing. We adopt three methods to treat scattering. In two models, we simulate a pitch-angle diffusion as successive isotropic or anisotropic small-angle scatterings. The third model treats large-angle scatterings as numerous small-chance isotropizations. The deduced intensity–time profiles are compared with each other, with Monte Carlo solutions to the diffusion-convection equation, and with results of the finite-difference scheme by Ruffolo (1995). A numerical agreement of our Monte Carlo simulations with results of the finite-difference scheme is good. For the period shortly after the maximum intensity time, including deceleration can increase the decay rate of the near-Earth intensity essentially more than would be expected based on advection from higher momenta. We, however, find that the excess in the exponential-decay rate is time dependent. Being averaged over a reasonably long period, the decay rate of the near-Earth intensity turns out to be close to that expected based on diffusion, convection, and advection from higher momenta. We highlight a variance of the near-Earth energy which is not small in comparison with the energy lost. It leads to blurring of any fine details in the accelerated particle spectra. We study the impact of realistic spatial dependencies of the mean free path on adiabatic deceleration and on the near-Earth intensity magnitude. We find that this impact is essential whenever adiabatic deceleration itself is important. It is also found that the initial angular distribution of particles near the Sun can markedly affect MeV-proton energy losses and intensities observed at 1 AU. Computations invoked during the study are described in detail.     

Anomalous Cosmic-Ray Helium,Nitrogen and Oxygen in 1996 – Measurements of the ERNE Instrument on-board SOHO

The energy spectra of the anomalous components of helium, nitrogen and oxygen have been measured by the ERNE experiment on board the SOHO spacecraft. During February 28–April 30, 1996, the maximum intensity of anomalous helium was found to be 3.8 × 10^-5 cm^-2 sr^-1 s^-1 (MeV nucl^-1)^-1 in the energy range 10–15 MeV nucl^-1. During the period January 26–April 30, 1996, the maximum oxygen intensity was 1.2 × 10^-5 cm^-2 sr^-1 s^-1 (MeV nucl^-1)^-1 at 4–7 MeV nucl^-1, and the maximum nitrogen intensity 1.7 × 10^-6 cm^-2 sr^-1 s^-1 (MeV nucl^-1)^-1 at 4–9 MeV nucl^-1. These peak intensities are at the same level as two solar cycles ago in 1977, but significantly higher than in 1986. This gives observational evidence for a 22-year solar modulation cycle. A noteworthy point is that the spectra of anomalous nitrogen and oxygen appear to be somewhat broader than in 1977.     

A joint analysis of high-energy neutrons and neutron-decay protons from a flare

A joint analysis of neutron monitor and GOES data is performed to study the production of high-energy neutrons at the Sun. The main objects of the research are the spectrum of >50 MeV neutrons and a possible spectrum of primary (interacting) protons which produced those neutrons during the major 1990 May 24 solar flare. Different possible scenarios of the neutron production are presented. The high magnitude of the 1990 May 24 neutron event provided an opportunity to detect neutron decay protons of higher energies than ever before. We compare predictions of the proposed models of neutron production with the observations of protons on board GOES 6 and 7. It is shown that the precursor in high-energy GOES channels observed during 20:55–21:09 UT can be naturally explained as originating from decay of neutrons in the interplanetary medium. The ratio of counting rates observed in different GOES channels can ensure the selection of the model parameters.The set of experimental data can be explained in the framework of a scenario which assumes the existence of two components of interacting protons in the flare. A hard spectrum component (the first component) generates neutrons during a short time while the interaction of the second (soft spectrum) component lasts longer. Alternative scenarios are found to be of lesser likelihood. The intensity-time profile of neutron - decay protons as predicted in the framework of the two-component exponential model of neutron production (Kocharov et al., 1994a) is in an agreement with the proton profiles observed on board GOES. We compare the deduced characteristics of interacting high-energy protons with the characteristics of protons escaping into the interplanetary medium. It is shown that, in the 100–1000 MeV range, the spectrum of the second component of interacting protons was close to the spectrum of the prompt component of interplanetary protons. However, it is most likely that, at 300 MeV, the interacting proton spectrum was slightly softer than the spectrum of interplanetary protons. An analysis of gamma-ray emission is required to deduce the spectrum of interacting protons below 100 MeV and above 1 GeV.     

Multi-spacecraft Study of the 8 November 2000 SEP Event: Electron Injection Histories 100° Apart

We present the analysis of a large solar near-relativistic electron event observed by the Ulysses and the ACE spacecraft on 8 November 2000, when Ulysses was located at a heliocentric distance of 2.4 AU and at a heliographic latitude of ??80° S. We use a particle propagation model to infer the local interplanetary transport conditions and the injection histories of the near-relativistic electrons observed by both spacecraft. We find different local transport conditions for each set of observations. The inferred injection profiles for both spacecraft extend for several hours; but the injection at Ulysses was smaller and started later. The association with type II radio emission suggests that the heliospheric electrons were provided by coronal shock acceleration. An analysis of the in situ magnetic field and plasma measurements indicates that the global configuration of the heliosphere (disturbed by transient structures) could play a role in shaping the characteristics of solar energetic particle events observed from different locations.     

Scientific Analysis within SEPServer – New Perspectives in Solar Energetic Particle Research: The Case Study of the 13 July 2005 Event

Solar energetic particle (SEP) events are a key ingredient of solar?Cterrestrial physics both for fundamental research and space weather applications. Multi-satellite observations are an important and incompletely exploited tool for studying the acceleration and the coronal and interplanetary propagation of the particles. While STEREO uses for this diagnostic two identical sets of instrumentation, there are many earlier observations carried out with different spacecraft. It is the aim of the SEPServer project to make these data and analysis tools available to a broad user community. The consortium will carry out data-driven analysis and simulation-based data analysis capable of deconvolving the effects of interplanetary transport and solar injection from SEP observations, and will compare the results with the electromagnetic signatures. The tools and results will be provided on the web server of the project in order to facilitate further analysis by the research community. This paper describes the data products and analysis strategies with one specific event, the case study of 13 July 2005. The release time of protons and electrons are derived using data-driven and simulation-based analyses, and compared with hard X-ray and radio signatures. The interconnection of the experimental and the simulation-based results are discussed in detail.