J. Torsti

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.

Injection of 10 MeV Protons in Association with a Coronal Moreton Wave

We report extreme-UV observations of the coronal Moreton wave and concurrent observations of ~10-100 MeV protons. Observations are carried out with the Extreme-UV Imaging Telescope and the Energetic and Relativistic Nuclei and Electron instrument on board the SOHO spacecraft. We study the proton events associated with coronal mass ejections (CMEs) centered near the central meridian. Observations reveal the initial injection of 10 MeV protons during the period when the coronal Moreton wave was traversing the western hemisphere of the Sun, this being an early signature of the CME launch. Acceleration of the CME-associated protons starts during the CME liftoff, while the main proton production occurs several hours later, when the CME expands in the interplanetary medium. Between the first proton production and the maximum intensity time, a spectral softening is observed. We analyze in detail the 1997 September 24 event. Development of the event indicates that the spectral softening may be due to a change in the acceleration regime, so the proton production starts with the less intensive but hard-spectrum injection and then moves to the more intensive but soft-spectrum injection farther from the Sun.

Energetic (∼ 1 to 50 MeV) protons associated with Earth‐directed coronal mass ejections

During the period from January through mid-May, 1997, four large Earth-directed CMEs were observed by the Large Angle Spectroscopic Coronograph (LASCO). These CMEs were associated with long-lasting fluxes of >1.6 MeV protons detected by the Energetic and Relativistic Nuclei and Electron instrument (ERNE). However, the magnitudes of energetic proton events differed dramatically on different occasions. In strong proton events, production of 10-50 MeV protons started during expansion of the coronal Moreton wave in the western hemisphere of the Sun. The new SOHO observations suggest that potentialities of CMEs to produce energetic particles in the interplanetary medium crucially depend on the previous evolution of the explosion below ∼2R⊙. Forecasting of the near-Earth >10 MeV proton intensity requires multiwavelength observations of the early phase of an event particularly the Extreme-ultraviolet Imaging Telescope (EIT) observations.

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 γ ≈ 1.6) at lower energies (∼ 30 MeV) with a rapid steepening above 300 MeV. Large values of the mean free path (λ ≈ 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.

Response of BGO and CsI(Tl) scintillators to heavy ions

Abstract Response of BGO and CSI(Tl) scintillators to ions heavier than helium has been studied at energies below 30 MeV/n. An argon beam was scattered from gold target. The reaction products, covering ions from lithium to argon, were identified by using the conventional ΔE - E method. A thin silicon surface barrier detector was used as an energy loss detector and a scintillator as a residual energy detector. The energies of incident nuclei were determined by using the signals from the calibrated ΔE detector, and the light yield of the scintillators was obtained from the observed pulse heights. Telescope consisting of a silicon transmission detector and either BGO of CsI(Tl) scintillators as a residual energy detector were found to have similar identification capabilities of heavy ions. The light output of BGO was found to be linear at all energies observed, except for the lightest nuclei, where some nonlinearity was apparent below about 6–7 MeV/n. The response of CsI(Tl) was linear for all ions above 6 MeV/n.

Modeling the Shock Aftermath Source of Energetic Particles in the Solar Corona

Recent observations on board the Solar and Heliospheric Observatory (SOHO) indicate that acceleration of solar energetic particles (SEPs) at intermediate scales in the solar corona, between flare acceleration and interplanetary CME-driven shock acceleration, significantly contributes to the production of >10 MeV protons. Coronal shocks seem to be the most plausible candidate for the post-impulsive phase acceleration, which emits ~1-100 MeV protons into the interplanetary medium for about 1 hr after the flare. We have employed a Monte Carlo technique to model the diffusive shock acceleration of protons in a turbulent layer at the base of the solar wind. We find that a power-law spectrum of energetic protons can be emitted from the trailing turbulent layer left behind the shock into the solar wind for a few tens of minutes after the CME liftoff. In contrast to an earlier expectation, the propagation direction of the shock wave is found not to be crucial. Both outward-propagating and refracting shocks can load the corona with energetic protons. Those protons escape into the interplanetary medium well after the passage of the shock. We have studied successive transformations of the particle spectra during shock acceleration, coronal transport, and possible reacceleration, for different populations of seed particles. The simulated production time profiles and energy spectra are found to be consistent with observations of the 1996 July 9 event by the Energetic and Relativistic Nuclei and Electron (ERNE) instrument on board SOHO. The new model can be easily combined with our previous interplanetary transport models, forming a basis on which to interpret SEP observations made at 1 AU.

Energetic (∼10–65 MeV) protons observed by ERNE on August 13–14, 1996: Eruption on the solar back side as a possible source of the event

The onset of the >10-MeV proton event of August 13-14, 1996, revealed a velocity dispersion, which is a signature of its solar origin, but no associated soft X ray flare was observed. The LASCO CME observations, the presence of AR 7981 beyond the west limb, and type II and IV radio burst timing with respect to the proton event onset indicate that the parent solar eruption may be centered on the back side of the Sun, at ∼150°W. In such a case, expanding CME-associated wave can reach the Earth-connected interplanetary magnetic field line in ∼1 hour and so give rise to the >10-MeV proton event observed with the Energetic and Relativistic Nuclei and Electron (ERNE) instrument onboard SOHO. We verify this hypothesis against observational data and conclude that a solar back side eruption is the most plausible explanation of the August 13, 1996, event. We compare the August 13, 1996, event with events associated with Earth directed CMEs and show that the August 13, 1996, event reveals many properties common to >10-MeV proton events originating from solar eruptions centered ∼90° away from the root of the Earth-connected interplanetary magnetic field line. In such events, the first detected protons are released ∼1 hour after the start time of type II and IV radio bursts. The first injection spectrum is essentially harder than the spectrum at the intensity maximum; that is, the hard but less intensive proton production is followed by the major soft-spectrum production when CME expands farther from the Sun.

DYNAMICAL CYCLES IN CHARGE AND ENERGY FOR IRON IONS ACCELERATED IN A HOT PLASMA

We consider a unified model of Fe ion acceleration in the solar corona. The model comprises charge-changing processes, Coulomb energy losses, and both regular and stochastic acceleration. At a given acceleration scenario, the type of acceleration is found to have a minor effect on the mean charge states, but the shapes of the charge-state distributions produced by regular acceleration and by stochastic acceleration are different. During a continual acceleration at coronal temperatures, iron ions typically follow rising trajectories on the charge-energy plane. These trajectories are situated below the mean equilibrium charge curve defined from the balance of ionization and recombination at fixed energy. During stopping, the iron ions cross the equilibrium charge curve and run through a series of charge states above the mean equilibrium charge at current energy, because the Coulomb deceleration rate significantly exceeds the rate of the ion recombination in a hot plasma. As a result, the variety of possible trajectories on the ion charge-energy plane turns out to be much wider than would be expected based on the equilibrium charge-state approximation. In particular, we find dynamical cycles in charge and energy, so that accelerated and highly stripped ions may reappear at low energies. We also find that the equilibrium charge curve cannot be reproduced without strong reduction in the total number of accelerated particles. This implies that the observed iron charge-state distributions essentially depend on the scenario of their acceleration and transport.

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.

The Origin of High-Energy 3He-rich Solar Particle Events

Observed isotope abundances in the energy range >15 MeV nucleon-1 by the ERNE instrument on board Solar and Heliospheric Observatory show a group of high-energy 3He-rich solar particle events that apparently has not been identified in earlier observations. Those events are observed to start well after the flare, when the associated coronal mass ejection (CME) already extends to ~0.3 AU from the Sun. We present the first model of interplanetary reacceleration of 3He that explains main properties of the new group of solar particle events. Our numerical simulations suggest that appearance of 3He in the unusually high-energy range can be caused by a reacceleration of ~1 MeV solar ions in oblique shocks or compressions driven by not very fast CMEs associated with those events.