Particle acceleration by coronal and interplanetary shock waves

Utilizing many years of observation from deep space and near-earth spacecraft a theoretical understanding has evolved on how ions and electrons are accelerated in interplanetary shock waves. This understanding is now being applied to solar flare-induced shock waves propagating through the solar atmosphere. Such solar flare phenomena as γ-ray line and neutron emissions, interplanetary energetic electron and ion events, and Type II and moving Type IV radio bursts appear understandable in terms of particle accleration in shock waves.     

Evolution of coronal mass ejection/shock system in interplanetary space

The evolution of coronal mass ejection/shock system is investigated by numerically solving the usual set of two-dimensional single-fluid polytropic magnetohydrodynamic equations from 1 R_s to 1 AU in the meridian plane. The simulation result reveals that the coronal mass ejection/shock system formed near the sun evolves into the magnetic cloud/shock system near the earth’s orbit through the following three phases: the initial formation, the dominant latitudinal expansion and the similar expansion.     

Energetic particle acceleration by coronal mass ejections

The current paradigm for the source of large, gradual solar energetic particle (SEP) events is that theyare accelerated in coronal/interplanetary shocks driven by coronal mass ejections (CMEs). Early studies established that there is a rough correlation between the logs of the CME speed and the logs of the SEP intensities. Here I review two topics challenging the basic paradigm, the recent discovery that CMEs are also associated with impulsive, high-Z rich SEP events and the search for gradual SEP sources other than CME-driven shocks. I then discuss three topics of recent interest dealing with the relationship between the shock or CME properties and the resulting SEP events. These are the roles that CME accelerations, interactions between fast and preceding slow CMEs, and widths of fast CMEs may play in SEP production.     

Multi-wavelength study of a coronal mass ejection: a flare event from AR#9393

The active region, AR#9393, produced a number of intense flares during March–April 2001. In this paper, we report the analysis of an X1.1 flare event of April 2, 2001 and its associated coronal mass ejection. The timing and location of the H_α eruption, radio burst activities, and the onset of mass ejection suggest an energy release that occurred close to the surface of the sun. At this region, as shown by the magnetogram, X-ray and EUV images, the field configuration was complex and the 3-D extrapolation revealed the presence of a magnetic null point. Results also suggest that the energy release is followed by the magnetic reconnection between the low-lying loops near the separator point and outlying loops. This study provides the support for the magnetic break-out process to trigger the energy release in eruptive flare event.     

Diffusive shock acceleration at interplanetary perpendicular shock waves: Influence of the large scale structure of turbulence on the maximum particle energy

We calculate the maximum energy that a particle can obtain at perpendicular interplanetary shock waves by the mechanism of diffusive shock acceleration. The influence of the energy range spectral index of the two-dimensional modes of the interplanetary turbulence is explored. We show that changes in this parameter lead to energies that differ in at least one order of magnitude. Therefore, the large scale structure of the turbulence is a key input if the maximum particle energy is calculated.     

Information on particle acceleration and transport derived from solar flare spectropolarimetry

The hydrogen Hα line has been found to be linearly polarized at some locations and times during a June 15th 2001 flare observed with THEMIS. This flare was accompanied by radio pulses and hard X-ray emission. Linear polarization is below the noise level in the flare kernels. However, it is present at the edges of these kernels, in the line center and near wings where the polarization degree exceeds 4%. The directions of polarization are not random but close within ±15° to the tangential and radial directions. This polarization can be due either to electron beams and their associated return currents or to electron and proton beams.     

Dynamical models of coronal transients and interplanetary disturbances

We review the status of the best “off-the-shelf” tool available for the study of dynamical behavior of coronal transients and traveling interplanetary disturbances. This tool involves numerical solution of the initial-boundary value problem of multi-dimensional time-dependent magnetohydrodynamics. While this tool cannot address questions of turbulence and kinetic behavior, we suggest that deeper understanding of large scale phenomena can be obtained by direct comparison of the MHD models with multi-disciplinary synoptic observations of specific events on the sun, and in the corona and interplanetary space. Conclusions reached after a recent critique (based on a limited set of observational and numerical data) of the MHD paradigm's application to coronal transients are examined and found to have limited validity. Substantial observational progress was achieved during SMY through ground- and space-based observations of solar and interplanetary events. Many of these observations can confidently be associated with one another for specific events. These associations can be combined into a reasonable scenario of geometrical extent and mass, energy and momentum transfer in the framework of the solar-terrestrial chain of cause and effect. Several of these events during STIP Interval VII in August 1979 are used to provide test cases for an MHD simulation that is described with some details. The bringing-together of diverse observations is necessary in order to outline a program for the testing of dynamical models and their more physically-restricted approximations.     

CME dynamics using coronagraph and interplanetary ejecta data

In this work, we present a study of the coronal mass ejection (CME) dynamics using LASCO coronagraph observations combined with in-situ ACE plasma and magnetic field data, covering a continuous period of time from January 1997 to April 2001, complemented by few extreme events observed in 2001 and 2003. We show, for the first time, that the CME expansion speed correlates very well with the travel time to 1 AU of the interplanetary ejecta (or ICMEs) associated with the CMEs, as well as with their preceding shocks. The events analyzed in this work are a subset of the events studied in Schwenn et al. (2005), from which only the CMEs associated with interplanetary ejecta (ICMEs) were selected. Three models to predict CME travel time to Earth, two proposed by Gopalswamy et al. (2001) and one by Schwenn et al. (2005), were used to characterize the dynamical behavior of this set of events. Extreme events occurred in 2001 and 2003 were used to test the prediction capability of the models regarding CMEs with very high LASCO C3 speeds.     

Synoptic observations of coronal transients and their interplanetary consequences

A small coronagraph has been placed in orbit to monitor the sun's outer corona from 2.5 to 10.0 solar radii, and five years of nearly continuous synoptic observations have now been completed. Rapid and sensitive image processing techniques have been developed to screen the data for transient phenomena, particularly coronal mass ejections (CMEs). About 50,000 coronal images have been examined, out of a five-year total of 68,000, and a standardized listing of more than 1,200 coronal transients for the period 1979–1982 has been prepared. These data have been analysed in the light of other available information, particularly on conditions in the interplanetary plasma. The dynamical characteristics of the active corona, as they are beginning to emerge from the data, are presented. We find that coronal mass ejections exercise significant influence on the interplanetary solar wind. They are the source of disturbances that are frequent and energetic, that tend to be somewhat focussed, that often reach shock intensity, and that propagate to large heliocentric distances, sometimes causing major geomagnetic storms.     

Relationships between energetic storm particle events and interplanetary shocks driven by full and partial halo coronal mass ejections

We have analysed energetic storm particle (ESP) events in 116 interplanetary (IP) shocks driven by front-side full and partial halo coronal mass ejections (CMEs) with speeds $>$400 km s^?1during the years 1996–2015. We investigated the occurrence and relationships of ESP events with several parameters describing the IP shocks, and the associated CMEs, type II radio bursts, and solar energetic particle (SEP) events. Most of the shocks (57 %) were associated with an ESP event at proton energies $>$1 MeV.The shock transit speeds from the Sun to 1 AU of the shocks associated with an ESP event were significantly greater than those of the shocks without an ESP event, and best distinguished these two groups of shocks from each other. The occurrence and maximum intensity of the ESP events also had the strongest dependence on the shock transit speed compared to the other parameters investigated. The correlation coefficient between ESP peak intensities and shock transit speeds was highest (0.73 $\pm$ 0.04) at 6.2 MeV. Weaker dependences were found on the shock speed at 1 AU, Alfvénic and magnetosonic Mach numbers, shock compression ratio, and CME speed. On average all these parameters were significantly different for shocks capable to accelerate ESPs compared to shocks not associated with ESPs, while the differences in the shock normal angle and in the width and longitude of the CMEs were insignificant.The CME-driven shocks producing energetic decametric–hectometric (DH) type II radio bursts and high-intensity SEP events proved to produce also more frequently ESP events with larger particle flux enhancements than other shocks. Together with the shock transit speed, the characteristics of solar DH type II radio bursts and SEP events play an important role in the occurrence and maximum intensity of ESP events at 1 AU.     

Interplanetary acceleration of low-energy protons as observed during the 25 september 1978 shock event

ISEE-3 observations of a long-lasting low-energy proton intensity increase during the 25 September 1978 shock event are presented as an example for interplanetary particle acceleration in association with shock waves. The observations are discussed in the light of current models for particle acceleration. The particular shape of the time intensity behaviour of the particle intensity increase, the existence of a shock spike and the observed particle distributions indicate that the particles are accelerated at the shock by the induced electric field $\text{E}\text{= ?}\text{1}\text{c}\text{V}\text{×}\text{B}$.     

Solar particle event organ doses and dose equivalents for interplanetary crews: variations due to body size.

Proper assessments of spacecraft shielding requirements and concomitant estimates of risk to critical body organs of spacecraft crews from energetic space radiation require accurate, quantitative methods of characterizing the compositional changes in these radiation fields as they pass through the spacecraft and overlying tissue. When estimating astronaut radiation organ doses and dose equivalents it is customary to use the Computerized Anatomical Man (CAM) model of human geometry to account for body self-shielding. Usually, the distribution for the 50th percentile man (175 cm height; 70 kg mass) is used. Most male members of the U.S. astronaut corps are taller and nearly all have heights that deviate from the 175 cm mean. In this work, estimates of critical organ doses and dose equivalents for interplanetary crews exposed to an event similar to the October 1989 solar particle event are presented for male body sizes that vary from the 5th to the 95th percentiles. Overall the results suggest that calculations of organ dose and dose equivalent may vary by as much as approximately 15% as body size is varied from the 5th to the 95th percentile in the population used to derive the CAM model data.     

Solar flare electron acceleration: Comparing theories and observations

A popular scenario for electron acceleration in solar flares is transit-time damping of low-frequency MHD waves excited by reconnection and its outflows. The scenario requires several processes in sequence to yield energetic electrons of the observed large number. Until now there was very little evidence for this scenario, as it is even not clear where the flare energy is released. RHESSI measurements of bremsstrahlung by non-thermal flare electrons yield energy estimates as well as the position where the energy is deposited. Thus quantitative measurements can be put into the frame of the global magnetic field configuration as seen in coronal EUV line observations. We present RHESSI observations combined with TRACE data that suggest primary energy inputs mostly into electron acceleration and to a minor fraction into coronal heating and primary motion. The more sensitive and lower energy X-ray observations by RHESSI have found also small events (C class) at the time of the acceleration of electron beams exciting meter wave Type III bursts. However, not all RHESSI flares involve Type III radio emissions. The association of other decimeter radio emissions, such as narrowband spikes and pulsations, with X-rays is summarized in view of electron acceleration.     

Long lasting energetic particle injection from a weak flare

A relatively weak solar cosmic ray event registered at the Earth orbit following the flare of December 17, 1976 is discussed. The main feature of the event is the existence of a prolonged unusually high proton and electron anisotropy; even at the end of the decay phase of the flare the motion of the particles were mainly directed away from the Sun. The durations of proton and electron anisotropies were different. If prolonged particle injection is neglected the value of the anisotropy considerable exceeds all diffusive estimates. Time-intensity and anisotropy profiles of electrons and protons are fitted by a diffusive model including prolonged particle injection at the Sun. The best agreement with the data is obtained if the duration of injection equals about 20 and 7 hours for protons and electrons, respectively.     

Diagnosis of coronal magnetic field and nonthermal electrons from Nobeyama observations of a simple flare

Coronal magnetic field and nonthermal electrons are very important parameters for understanding of the global heliophysical processes. A flare on November 1, 2004 is selected for self-consistent calculations of coronal magnetic field parallel and perpendicular to the line-of-sight, and density of nonthermal electrons from Nobeyama observations. Both of the diagnosis methods and results are discussed in this paper.     

Interplanetary acceleration of low energy protons in association with shock waves: The 11 February 1979 event

The problem of interplanetary acceleration of low energy protons in association with shock waves is examined in the context of the specific event observed on 11 February 1979 on board the ISEE-3 spacecraft. This event has been selected for special study as it apparently was not associated with a solar flare event. The low energy proton telescope system on ISEE-3 measures the proton distribution function with good spectral, directional and temporal resolution from E_p = 35 keV. The evolution of the anisotropies and of the energy spectrum during the event are consistent with particle acceleration taking place in the vicinity of the shock wave.     

Results from the study of an M7.6 flare and its associated CME

An M7.6 flare was well observed on October 24, 2003 in active region 10486 by a few instruments and satellites, including GOES, TRACE, SOHO, RHESSI and NoRH. Multi-wavelength study shows that this flare underwent two episodes. During the first episode, only a looptop source of <40 keV was observed in reconstructed RHESSI images, which showed shrinkage with a velocity of 12–14 km s⁻¹ in a period of about 12 min. During the second process, in addition to the looptop source, two footpoint sources were observed in energy channel of as high as ∼200 keV. One of them showed fast propagation along one of the two TRACE 1600 Å flare ribbons and the 195 Å loop footpoints, which could be explained by successive magnetic reconnection. The associated CME showed a mass pickup process with decreasing center-of-mass velocity. The decrease of the CME kinetic energy and the increase of its potential energy lead to an almost constant total energy during the CME propagation. Our results reveal that the flare and its associated CME have comparable energy content, and the flare is of non-thermal property.     

The temperature map of the loop and coronal sources of an occulted solar flare

An occulted solar flare occurred at about 06:07 UT on 2002, November 2. The RHESSI X-ray images show two separate parts. The lower part consists of a complete loop and the upper part a coronal source which well extends above the solar limb. The loop source shrank for about 3 min with a speed of ∼24 km s⁻¹ during the early impulsive phase and then expanded at ∼7 km s⁻¹, while the coronal source presented an upward motion at about 6 km s⁻¹. We obtained the temperature map of the loop source from RHESSI image spectrum. The temperature of the loop increases with altitude, indicating that the reconnection X-point of this flare is located above the loop source. However, the apparent coronal source is the top of another independent large-scale loop.     

Energetic particle signatures of geoeffective coronal mass ejections

We have studied statistically associations of moderate and intense geomagnetic storms with coronal mass ejections (CMEs) and energetic particle events. The goal was to identify specific energetic particle signatures, which could be used to improve the predictions of the geoeffectiveness of full and partial halo CMEs. Protons in the range 1–110 MeV from the ERNE experiment onboard SOHO are used in the analysis. The study covers the time period from August 1996 to July 2000. We demonstrate the feasibility of energetic particle observations as an additional source of information in evaluating the geoeffectiveness of full and partial halo CMEs. Based on the observed onset times of solar energetic particle (SEP) events and energetic storm particle (ESP) events, we derive a proxy for the transit times of shocks driven by the interplanetary counterparts of coronal mass ejections from the Sun to the Earth. For a limited number of geomagnetic storms which can be associated to both SEP and ESP signatures, we found that this transit time correlates with the strength of geomagnetic storms.     

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.