摘要:

This review describes the energetic particle populations observed in the heliosphere including gradual and impulsive solar energetic particle (SEP) events, interplanetary energetic particle enhancements, the anomalous cosmic ray (ACR) component, and galactic cosmic rays (GCRs). Also described briefly are the interstellar pickup ions and the solar wind itself, which are not energetic in the usual sense but are substantially heated and/or accelerated and provide the seed particles for the heliospheric populations. Firstly several overarching themes are presented which are important for all the energetic particles: acceleration mechanisms, particle transport theory, species-dependent transport and acceleration, injection mechanisms, and energetic particles as probes of heliospheric structure and dynamics. Secondly each population of energetic particles is reviewed including our current understanding of the source of the particles, the acceleration mechanism, and the transport, and any puzzles or challenges for theory. Shock acceleration is the dominant acceleration mechanism. Gradual SEP events are accelerated at coronal/interplanetary shocks, the corotating ion events are accelerated at the shocks bounding corotating interaction regions (CIRs) in the solar wind, diffuse ions are accelerated at planetary bow shocks, the ACRs are accelerated at the solar wind termination shock, and GCRs are accelerated at supernova remnant shocks. Only impulsive SEP events do not originate at shocks; they are accelerated by direct electric fields or stochastic acceleration at sites of magnetic reconnection in solar flares. For GCRs in particular, transport in the heliosphere and interstellar space affects their observed energy spectra and composition. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324276

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

I review the evolution of research on solar energetic particle events, beginning with Forbush’s report of the ground level event of 1946, through the most recent observations of the Advanced Composition Explorer (ACE). The emphasis is on research that attempted to link solar flare electromagnetic emissions with the solar energetic particles (SEPs) observed in space following flares. The evolution of thought on this topic is traced from the initial paradigm in which SEPs were accelerated at the flare site (a &dgr;-function in space and time) to the current two-class picture accommodating both impulsive acceleration at the flare site (small3He-rich events) and prolonged acceleration at extended shocks driven by coronal mass ejections (large proton events). I conclude with some open questions; the most prominent of these concerns the relative contributions of the flare and shock acceleration processes to “mixed” or hybrid SEP events in which the distinguishing characteristics of the impulsive and gradual classes are blended. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324277

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

New measurements of energetic solar electrons from the WIND spacecraft are reviewed, and the implications for particle acceleration mechanisms discussed. In non-relativisticelectron/3He-rich (so-called impulsive) events the electron energy spectrum is often found to extend below ∼1 keV, indicating that acceleration occurs high in the corona. Comparison of the escaping electrons with the electrons producing the associated hard X-ray burst suggests that acceleration is occurring over a wide range of altitudes. For Large Solar Energetic Particle (LSEP, or so-called gradual) events, WIND observations show the low energy ∼1–10 keV electron component is sometimes missing. In many LSEP events the electrons are released from the Sun up to ∼0.5 hour later than the onset of the solar type III radio burst, and coronal transient waves are detected traveling across the Sun by the SOHO EIT instrument. Onset timing analyses show two types of LSEPs; in some events the first arriving ∼0.1–6 MeV protons are released ∼0.5–2 hours after the electrons and travel a path length of ∼1.2 AU (essentially scatter-free), while in other events the protons are released at the same time as the electrons but appear to travel ∼2 AU. If we assume the observed energetic particles are accelerated by a shock in front of an outward propagating fast CME, the electrons are accelerated earlier and lower in the corona(∼0.5&hthinsp;RSun)and the protons later and higher,∼4&hthinsp;RSunfor the first type of event, and from∼4&hthinsp;to >∼10&hthinsp;RSun,with the more energetic protons accelerated lower for the second type. In mid-2000 the High Energy Solar Spectroscopic Imager (HESSI) mission will be launched to provide detailed X-ray and gamma-ray imaging and spectroscopy observations to study particle acceleration and energy release processes at the Sun. Comparisons between HESSI and ACE/WIND should provide new insights into the origins of energetic solar particles. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324278

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Electric fields induced by the changing magnetic field at sites of magnetic reconnection can efficiently accelerate charged particles in the solar corona. This review begins with estimates for the electric field magnitude in flare models and presents some of the theoretical results for the electron and proton acceleration in reconnecting current sheets in solar flares. Particular emphasis is placed on models for collisionless acceleration in a large-scale reconnecting current sheet with a nonzero magnetic field and a highly super-Dreicer electric field of order a few V cm−1. Particle orbits in model current sheets are discussed using an approximate analytical approach that allows one to identify the effects of both the electric and magnetic field components on the particle motion. Formulas for the particle energy gains and acceleration times are presented. Given a super-Dreicer electric field in the sheet, it is the magnetic field structure in the sheet that determines both the electron to proton ratio for the accelerated particles and their typical energies and spectra. The analytical results form the basis for the electric field acceleration models in solar flares. In particular, physical conditions can be identified that lead to either flares in which electrons primarily generate hard X-rays in the energy range of tens of keV or flares with unusually large electron fluxes at gamma-ray energies extending up to a few tens of MeV. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324279

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Using instrumentation on board theACEspacecraft we describe short-time scale (∼3 hour) variations observed in the arrival profiles of ∼20 keV nucleon−1to ∼2 MeV nucleon−1ions from impulsive solar flares. These variations occurred simultaneously across all energies and were generally not in coincidence with any local magnetic field or plasma signature. These features appear to be caused by the convection of magnetic flux tubes past the observer that are alternately filled and devoid of flare ions even though they had a common flare source at the Sun. In these particle events we therefore have a means to observe and measure the mixing of the interplanetary magnetic field due to random walk. In a survey of 25 impulsive flares observed at ACE between 1997 November and 1999 July these features had an average time scale of 3.2 hours, corresponding to a length of ∼0.03 AU. The changing magnetic connection to the flare site sometimes lead to an incomplete observation of a flare at 1 AU; thus the field-line mixing is an important effect in studies of impulsive flare energy spectra. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324280

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Since the launch of ACE in August 1997, the Solar Isotope Spectrometer (SIS) has observed 11 large solar particle events in which elemental and isotopic composition was determined over a large energy range. The composition of these events has raised many issues and challenged generally accepted characterizations of solar energetic particle (SEP) events. In particular,3He/4Heenhancements have been observed in several large events as well as enhancements of heavy ions typically associated with smaller impulsive events. The isotopic composition varies substantially from event to event (a factor of 3 for22Ne/20Ne) with enhancements and depletions that are generally correlated with elemental composition. This correlation suggests that the isotopic enhancements may be related to the Q/M fractionation typically evident in the elemental composition of SEP events. However, there are also significant deviations from this pattern, which may imply that wave-particle resonances or other mass fractionation processes may be involved. We review the recent isotopic observations made with ACE and discuss their implications for particle acceleration and transport. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324281

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

With the launch of the Advanced Composition Explorer, it has become possible through the SEPICA instrument to make direct ionic charge state measurements for individual Solar Energetic Particle events. In large events, the charge state may even be measured as a function of time, revealing changes that may be created by phenomena such as injections from different acceleration mechanisms, or confinement by magnetic field structures. The charge state can be a sensitive indicator of separate SEP populations. Several examples of SEP events will be presented. One of these, the November, 1997 event, displayed a trend in which the mean charge state for several ions increased with energy. These measurements may be the result of several processes, including a mixture of plasma with different source and acceleration histories, and abundance formation and possibly additional charge state modification by collisional or other means in the corona. A wide range of iron charge states have been measured for a variety of SEP events, ranging from⟨Q⟩=10+&hthinsp;to&hthinsp;20+.The mean charge states of C, O, Ne, Mg and Si all increased as the iron charge state increased. In events with the highest iron charge states, there were abundance enhancements in Ne with respect to oxygen in those cases, even though the mass/charge of the O and Ne were similar. In events with the lowest iron charge states, all these ions except Mg showed mean charge states generally consistent with coronal material of an equilibrium temperature of 1.3–1.6 million degrees K. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324282

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Recent measurements of the mean ionic charge states of solar energetic iron and silicon by SAMPEX and ACE during the large solar events of 1992 November 1 and 1997 November 6 show a mean ionic charge that increases with energy. This feature has implications for the use of the observed charge state as a probe of the coronal electron temperature and density, as well as for models of ion acceleration and transport in the coronal plasma. In this paper, we show results of a nonequilibrium model for the mean ionic charge that includes shock-induced acceleration in addition to charge-changing processes. The model is able to reproduce the general features observed without, however, specifying uniquely the acceleration time and the plasma electron density. Based on our simulations for iron and silicon for the 1992 and 1997 events, and assuming a characteristic shock-acceleration time of ∼10 sec, our model suggests an equilibration-acceleration site at heights ∼1 solar radius above the solar surface, a density∼109&hthinsp;cm−3,and an electron temperature∼1−1.33&hthinsp;MK.For ions with kinetic energy ≳30 MeV/nucleon we estimate the amount of coronal material the ions traverse to be ∼100 &mgr;g/cm2. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324283

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

In the largest solar energetic particle (SEP) events, acceleration takes place at shock waves driven out from the Sun by fast coronal mass ejections. Protons streaming away from strong shocks generate Alfve´n waves that trap particles in the acceleration region, limiting outflowing intensities but increasing the efficiency of acceleration to higher energies. Early in the events, with the shock still near the Sun, intensities at 1 AU are bounded and spectra are flattened at low energies. Elements with different charge-to-mass ratios,Q/A,differentially probe the wave spectra near shocks, producing abundance ratios that vary in space and time. An initial rise in He/H, while Fe/O declines, is a typical symptom of the non-Kolmogorov wave spectra in the largest events. Strong wave generation can cause cross-field scattering near the shock and unusually rapid reduction in anisotropies even far from the shock. At the highest energies, shock spectra steepen to form a “knee.” For protons, this spectral knee can vary from ∼10 MeV to ∼1 GeV depending on shock conditions for wave growth. In one case, the location of the knee scales approximately asQ/Ain the energy/nucleon spectra of other species. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324284

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The 3-D Plasma and Energetic Particles experiment on the WIND spacecraft was designed to provide high sensitivity measurements of both suprathermal ions and electrons down to solar wind energies. A statistical survey of 26 solar proton events has been investigated. For all these proton events, a temporally related electron event is observed. The presented results focus on the properties of protons released near the Sun which show a velocity dispersion when detected at 1 AU. The particle flux onset times observed at 1 AU in the energy range between 30 keV and 6 MeV suggest that there are two classes of proton events: (1) For one class (70&percent; of the events), the first arriving protons are traveling almost scatterfree as indicated by the derived path lengths between 1.1 and 1.3 AU, (2) whereas the events of the second class show significantly larger path lengths of around 2 AU. Relative to the electron release time at the Sun, the almost scatterfree traveling protons of the first class of events are release delayed by 0.5 to 2 hours. For the events of the second class, protons and electrons seemed to be released simultaneously within the accuracy of 20 minutes. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324285

出版商:AIP    

年代:1900

数据来源: AIP