Acceleration Of Solar Energetic Particles Research Articles

Solar Fast‐Wind Regions as Sources of Shock Energetic Particle Production

If the production of solar energetic particles (SEPs) near the Sun is due to shocks driven by coronal mass ejections (CMEs), then two factors might favor SEP acceleration in slow rather than fast solar wind streams. The first is that both the MHD fast-mode and solar wind flow speeds are higher in the fast-wind streams. The second is that shock seed populations in the fast-wind streams consist of weak suprathermal ion tails with soft spectra. An earlier study used observed fast west-limb or halo CMEs and solar wind O+7/O+6 ratios to compare associated SEP events and fast CMEs in fast wind with those in slow wind. Properties of the few fast CMEs in fast-wind regions suggested a bias in terms of associated SEP event size and/or CME speed against shock production of SEPs in fast-wind regions. Here the previous 1998-2000 study is expanded through 2002. The results of the much larger event sample now show no significant bias against SEP production in fast-wind regions. We discuss the implications of this result for shock seed populations and for shock propagation in coronal fast-wind regions.

Telemachus: a mission for a polar view of solar activity

Telemachus is a polar solar-heliospheric mission described in the current NASA Sun–Earth Connections Roadmap (2003–2028). To continue the quest begun by Ulysses, Telemachus will be injected into a 0.2 AU × 2.5 AU solar polar orbit after gravitational encounters with Venus, Earth (twice) and Jupiter, followed by a maneuver at first perihelion. With an orbital period of 1.5 years, it will pass over the solar poles at a radial distance of 0.37 AU. The behavior of solar polar flows (heretofore unobservable) will provide critical boundary conditions on the transport of solar magnetic fields. Manifestations of the global flows and fields are the solar dynamo, formation of active regions, emission of the solar wind and the heliospheric magnetic field, launching of coronal mass ejections, eruption of solar flares, acceleration of solar energetic particles, and ultimately the dynamics of the entire heliosphere.

Solar Energetic Particle Acceleration in Refracting Coronal Shock Waves

Gradual solar energetic particle (SEP) events are known to be correlated with coronal mass ejections (CMEs) and soft X-ray flares. The current paradigm of particle acceleration in these events attributes it to CME-driven shock waves in the solar corona and in interplanetary space. Even in small gradual SEP events related to CMEs with speeds in the (possibly submagnetosonic) range of 300-800 km s-1, shock waves at global coronal scales, as evidenced by associated metric type II radio bursts, are important. Recent observational evidence from soft X-ray imaging data supports models of coronal shock wave propagation in the solar atmosphere as freely propagating blast waves that refract toward the solar surface as they propagate away from the flare site. Based on these observations, we study a model of test-particle acceleration in such a global refracting coronal shock wave. Such shocks may also be generated by solar eruptions other than flares, e.g., slow CMEs. The geometry of the shock wave results in the observer in the interplanetary medium being magnetically connected with the downstream region of the shock wave. Thus, steady-state diffusive shock acceleration predicts that the energy spectrum of the escaping ions is a power law, as typically observed—a result that is not obtained naturally if the observer is connected to the upstream region of the shock wave. Using parameters of upstream turbulence obtained from models of a cyclotron-heated solar corona, we calculate typical timescales of diffusive proton acceleration and show them to be consistent with the maximum proton energies typically observed in small, gradual SEP events. Acceleration in refracting coronal shock waves may also provide a preacceleration mechanism for further acceleration in CME-driven shocks in large gradual SEP events.

Properties of high heliolatitude solar energetic particle events and constraints on models of acceleration and propagation

We analyse 9 large solar energetic particle (SEP) events detected by the Ulysses spacecraft at high heliolatitudes during the recent solar maximum polar passes. Properties of time intensity profiles from the Ulysses/COSPIN instrument are compared with those measured by SOHO/COSTEP and Wind/3DP near Earth. We find that onset times and times to maximum at high latitude are delayed compared to in‐ecliptic values. We show that the parameter which best orders these characteristics of time profiles is the difference in latitude between the associated flare and the spacecraft. We find that the presence of a shock is not necessary for the establishing of near equal intensities at Ulysses and in the ecliptic during the decay phase. The model of SEP acceleration by coronal mass ejection driven shocks does not appear to account for our observations, which would more easily be explained by particle diffusion across the interplanetary magnetic field.

Shock surfing acceleration

Analytical and numerical analysis identify shock surfing acceleration as an ideal pre-energization mechanism for the slow pick-up ions at quasiperpendicular shocks. After gaining sufficient energy by shock surfing, pick-up ions undergo diffusive acceleration to reach their observed energies. Energetic ions upstream of the cometary bow shock, acceleration of solar energetic particles by magnetosonic waves in corona, ion enhancement in interplanetary shocks, generation of anomalous cosmic rays from interstellar pick-up ions at the termination shock are some of the cases where shock surfing acceleration apply. Inclusion of the lower-hybrid wave turbulence into the laminar model of shock surfing can explain the preferential acceleration of heavier particles as observed by Voyager at the termination shock. At relativistic energies, unlimited acceleration of ions is theoretically possible; because for sufficiently strong shocks main limitation of the mechanism, caused by the escape of accelerated particles downstream of the shock during acceleration no longer exists.

Are CME “interactions” really important for accelerating major solar energetic particle events?

Recent studies have proposed that the presence or absence of an “interaction” with a preceding coronal mass ejection (CME) or other coronal structure within ∼50 Rs of the Sun discriminates large, fast CMEs associated with major solar energetic particle (SEP) events from those that are not. We conclude that there is no compelling evidence that, if such interactions take place, they play an important role in SEP acceleration. Reasons include: The reported statistical results are consistent with a chance association between interacting CMEs and SEP events; Energetic SEPs are detected at Earth typically before or around the time when the “primary” CME enters the LASCO C2 field of view ‐ interactions higher in the corona cannot play a role in acceleration of these particles; For ∼60% of major SEP events in 1997–2001, the preceding CME fades into the background corona or is relatively narrow (<40°), suggesting any interaction will be weak; Radio signatures attributed to CME interaction occur after SEP acceleration has commenced.

Long‐duration hectometric type III radio bursts and their association with solar energetic particle (SEP) events

It has recently been suggested by Cane et al. [2002] that a class of type III solar radio bursts, called type III‐l, is reliably associated with intense solar energetic particle (SEP) events. They proposed that the causative electrons for these bursts are accelerated in regions of reconnecting magnetic field in the wakes of coronal mass ejections (CMEs). In this paper, we examine the durations, intensities, and other characteristics of such radio bursts in the hectometric frequency range and compare them to several groups of control events. We conclude that simple criteria, based on hectometric data alone, can identify the majority (∼80%) of type III‐l radio bursts, which are associated with >20 MeV SEP proton events, while excluding almost 100% of the control events. Detailed study of these type III‐l bursts may play a significant role in a better understanding of the acceleration of SEPs and of the magnetic field evolution in the vicinity of CMEs.

Solar Energetic Particle Production by Coronal Mass Ejection–driven Shocks in Solar Fast‐Wind Regions

Gradual solar energetic particle (SEP) events at 1 AU are produced by coronal/interplanetary shocks driven by coronal mass ejections (CMEs). Fast (vCME 900 km s-1) CMEs might produce stronger shocks in solar slow-wind regions, where the flow and fast-mode MHD wave speeds are low, than in fast-wind regions, where those speeds are much higher. At 1 AU the O+7/O+6 ratios distinguish between those two kinds of wind streams. We use the 20 MeV proton event intensities from the EPACT instrument on Wind, the associated CMEs observed with the LASCO coronagraph on SOHO, and the ACE SWICS solar wind values of O+7/O+6 to look for variations of peak SEP intensities as a function of O+7/O+6. No significant dependence of the SEP intensities on O+7/O+6 is found for either poorly connected or well-connected CME source regions or for different CME speed ranges. However, in the 3 yr study period we find only five cases of SEP events in fast wind, defined by regions of O+7/O+6 < 0.15. We suggest that in coronal holes SEP acceleration may take place only in the plume regions, where the flow and Alfv?n speeds are low. A broad range of angular widths are associated with fast (vCME ? 900 km s-1) CMEs, but we find that no fast CMEs with widths less than 60? are associated with SEP events. On the other hand, nearly all fast halo CMEs are associated with SEP events. Thus, the CME widths are more important in SEP production than previously thought, but the speed of the solar wind source regions in which SEPs are produced may not be a factor.

Acceleration of solar energetic particles: the case of solar flares

Solar energetic particles are believed to originate from two different sources, solar flares and coronal mass ejections. These two sources are the most energetic particle accelerators in the heliosphere, as they can accelerate electrons from 10 keV to a few MeV and protons from a few MeV to a few GeV. In this contribution, we restrict our presentation to the case of solar flares, by reviewing the key observations of solar energetic particles, as well as the theoretical acceleration models, such as wave–particle acceleration, DC electric fields, and shock acceleration. Finally, we present a new theoretical approach connecting the acceleration with the energy release during solar flares, which might lead to a global modeling of solar flare energetics.

The correlation between solar energetic particle peak intensities and speeds of coronal mass ejections: Effects of ambient particle intensities and energy spectra

The correlation of peak intensities of solar energetic particle (SEP) events with the speeds of the associated coronal mass ejections (CMEs) is understood to be a result of SEP acceleration at shocks driven by the CMEs. However, the peak SEP intensities associated with CMEs of a given speed vary over ∼ 4 orders of magnitude. We examine a database of 71 E &gt; 10 MeV SEP events observed with the GOES satellite to determine whether enhanced ambient SEP intensities at the times of the CMEs and/or variations among SEP event spectra contribute to the large range of peak SEP event intensities. A statistical analysis shows that enhanced ambient SEP intensities may be a contributing factor to the range of SEP events of higher peak intensities, probably by providing sources of energetic seed particles for the shock acceleration process. Another factor is the variation of energy spectra among the SEP events, which generally have harder spectra with increasing peak intensities. The observed increase of peak SEP intensities and hardening of peak SEP spectra with increasingly westward solar source regions is only a minor factor in the range of SEP peak intensities in the CME speed correlation.

Solar Particle Events from a risk management perspective.

Solar Particle Events pose a health risk to astronauts in space. Today, SPE forecasts are inadequate to provide advance warning with sufficient credibility to lead operators to initiate protective measures. However, research on SPEs and Coronal Mass Ejections suggests that the space weather community is on the verge of substantial improvements in understanding solar energetic particle acceleration and propagation. This paper describes the impact of SPEs, reviews the physics of SPEs, discusses current SPE forecast tools, and describes an approach to provide the comprehensive space weather data necessary to implement physics-based SPE risk management.

Milagro as a solar observatory

The Milagro Gamma Ray Observatory, a ground-based water Čherenkov detector designed primarily for observing very high energy gamma ray sources, can also be used to study the Sun. Measurements of high energy emission from solar flares can lead to an understanding of the solar energetic particle acceleration mechanism(s), and Milagro, with enhanced data acquisition electronics, will measure solar cosmic rays in the poorly studied energy range from 10 to 300 GeV. Monte Carlo simulations show that proton and neutron solar cosmic rays at these energies will be detectable at the ground level through the single muons and/or mini showers they produce. Milagro can detect these events in a scaler counting mode similar to that of a neutron monitor. Additionally, it can operate in a telescope mode that reduces background by pointing at the source region. We also report a preliminary detection, using the Milagro prototype, Milagrito, of a ground level event (GLE) on 6 November 1997 associated with an X-class solar flare.

Post-Impulsive-Phase Acceleration in a Wide Range of Solar Longitudes

We have analyzed five solar energetic particle (SEP) events observed aboard the SOHO spacecraft during 1996–1997. All events were associated with impulsive soft X-ray flares, Type II radio bursts and coronal mass ejections (CMEs). Most attention is concentrated on the SEP acceleration during the first 100 minutes after the flare impulsive phase, post-impulsive-phase acceleration, being observed in eruptions centered at different solar longitudes. As a representative pattern of a (nearly) well-connected event, we consider the west flare and CME of 9 July 1996 (S10 W30). Similarities and dissimilarities of the post-impulsive-phase acceleration at large heliocentric-angle distance from the eruption center are illustrated with the 24 September 1997 event (S31 E19). We conclude that the proton acceleration at intermediate scales, between flare acceleration and interplanetary CMEdriven shock acceleration, significantly contributes to the production of ≳ 10 MeV protons. This post-impulsive-phase acceleration seems to be caused by the CME lift-off.

Energy Spectra of Accelerated Solar Protons From Different Sources – I. Reconstruction and Properties of the Source Spectrum

It is widely accepted now that a significant fraction of the solar energetic particles (SEPs) observed at 1 AU after major solar flares are actually accelerated at a CME-driven shock. In addition, in the emerging new paradigm for SEP acceleration in different sources at or near the Sun, the existence of two types of flares – impulsive and gradual – is recognized. Within this concept, it is tempting also to separate SEPs into two groups – interacting and escaping – and to derive their 'source spectra' from observational data on various flare emissions (protons, gamma rays, neutrons, etc.). By different techniques, those spectra have been reconstructed for 80 solar proton events (SPE) in 1949–1991. In this paper, all available data on the source spectra of solar protons are summarized and revised. We discuss in detail existing uncertainties in the derived spectral indexes, consider other methodological problems involved in this study, and suggest several possible lines for the future investigations of solar flares and SCRs using the source spectrum data. It is noted that some peculiarities of the spectra, for instance, spectral steepening for high energies, may be characteristic of large events of the 23 February 1956 type.

High coronal flares and impulsive acceleration of solar energetic particles

view Abstract Citations (31) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High Coronal Flares and Impulsive Acceleration of Solar Energetic Particles Cliver, E. ; Kahler, S. Abstract Evidence is reviewed which suggests that the source regions of high coronal flares (HCFs) are hot. A scenario is considered in which the source region of He-3 enhancement in solar energetic particle events from impulse flares is distinct from the source region of the enhanced abundances of high-Z ions. It is proposed that HCFs occur due to reconnection in the neutral current sheets of coronal streamers. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/185916 Bibcode: 1991ApJ...366L..91C Keywords: Energetic Particles; Particle Acceleration; Particle Emission; Solar Corona; Solar Flares; Coronal Loops; Current Sheets; Helium Ions; Solar Electrons; Solar Physics; SUN: FLARES; SUN: PARTICLE EMISSION full text sources ADS |

Elemental composition of solar energetic particles

The Low Energy Telescopes on the Voyager spacecraft have been used to measure the elemental composition (2< or =Z< or =28) and energy spectra (5--15 MeV per nucleon) on solar energetic particles (SEPs) in seven large flare events. Four flare events were selected which have SEP abundance ratios approximately independent of energy per nucleon. For these selected flare events, SEP composition results may be described by an average composition plus a systematic flare-to-flare deviation about the average. The four-flare average composition plus a systematically different from the solar composition determined by photospheric spectroscopy. These systematic composition differences are apparently not due to SEP propagation or acceleration effects. In contrast, the four-flare average SEP composition is in agreement with measured solar wind abundances and with a number of recent spectroscopic coronal abundance measurements. These findings suggest that SEPs originate in the corona, and that both SEPs and the solar wind sample a coronal composition which is significantly and persistently different from that measured for the photosphere. Our observations thus provide a measure of the coronal abundances of 15 elements.