Source Of Energetic Particles Research Articles

Ultraviolet spectroscopy of the extended solar corona

The first observations of ultraviolet spectral line profiles and intensities from the extended solar corona (i.e., more than 1.5 solar radii from Sun-center) were obtained on 13 April 1979 when a rocket-borne ultraviolet coronagraph spectrometer of the Harvard-Smithsonian Center for Astrophysics made direct measurements of proton kinetic temperatures, and obtained upper limits on outflow velocities in a quiet coronal region and a polar coronal hole. Following those observations, ultraviolet coronagraphic spectroscopy has expanded to include observations of over 60 spectral lines in coronal holes, streamers, coronal jets, and solar flare/coronal mass ejection (CME) events. Spectroscopic diagnostic techniques have been developed to determine proton, electron and ion kinetic temperatures and velocity distributions, proton and ion bulk flow speeds and chemical abundances. The observations have been made during three sounding rocket flights, four Shuttle deployed and retrieved Spartan 201 flights, and the Solar and Heliospheric Observatory (SOHO) mission. Ultraviolet spectroscopy of the extended solar corona has led to fundamentally new views of the acceleration regions of the solar wind and CMEs. Observations with the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO revealed surprisingly large temperatures, outflow speeds, and velocity distribution anisotropies in coronal holes, especially for minor ions. Those measurements have guided theorists to discard some candidate physical processes of solar wind acceleration and to increase and expand investigations of ion cyclotron resonance and related processes. Analyses of UVCS observations of CME plasma properties and the evolution of CMEs have provided the following: temperatures, inflow velocities and derived values of resistivity and reconnection rates in CME current sheets, compression ratios and extremely high ion temperatures behind CME shocks, and three dimensional flow velocities and magnetic field chirality in CMEs. Ultraviolet spectroscopy has been used to determine the thermal energy content of CMEs allowing the total energy budget to be known for the first time. Such spectroscopic observations are capable of providing detailed empirical descriptions of solar energetic particle (SEP) source regions that allow theoretical models of SEP acceleration to be tailored to specific events, thereby enabling in situ measurements of freshly emitted SEPs to be used for testing and guiding the evolution of SEP acceleration theory. Here we review the history of ultraviolet coronagraph spectroscopy, summarize the physics of spectral line formation in the extended corona, describe the spectroscopic diagnostic techniques, review the advances in our understanding of solar wind source regions and flare/CME events provided by ultraviolet spectroscopy and discuss the scientific potential of next generation ultraviolet coronagraph spectrometers.

Introduction

An overview is provided of the papers presented at the Discussion Meeting on ‘Laser-driven particle accelerators: new sources of energetic particles and radiation’ which was held at the Royal Society on 6 and 7 June 2005. A discussion of the future prospects of the field, held during the last session of the meeting, is summarized and the conclusions of this presented.

ENERGETIC PARTICLES IN THE HELIOSPHERE

The heliosphere is the region around the Sun that is filled by the solar wind and its embedded magnetic field. The interaction of the supersonic solar wind with the local interstellar medium leads to a transition from supersonic to subsonic speeds at the heliospheric termination shock. The latter is regarded to be the source of the anomalous component of cosmic rays. Within the heliosphere "local" energetic particle sources, like the Sun and interplanetary shock waves contribute to the cosmic ray flux, too. At energies below a few GeV the observed galactic and anomalous cosmic ray intensities are modulated by the heliospheric magnetic field. In my contribution, both the current knowledge and hypotheses about modulation and the transport of cosmic rays in the heliosphere are reviewed.

SOLAR ENERGETIC PARTICLES

Energetic particles recorded in the Earth environment and in interplanetary space have a multitude of origins, i.e. acceleration and propagation histories. At early days practically all sufficiently energetic particles were considered to have come either from solar flares or from interstellar space. Later on, co-rotating interplanetary shocks, the termination shock of the supersonic solar wind, planetary bow shocks and magnetospheres, and also coronal mass ejections (CME) were recognized as energetic particle sources. It was also recognized that less energetic (suprathermal) particles of solar origin and pick-up ions have also a vital role in giving rise to energetic particles in interplanetary disturbances. The meaning of the term "solar energetic particles" (SEP) is now somewhat vague, but essentially it refers to particles produced in disturbances fairly directly related to solar processes. Variation of intensity fluctuations with energy and with the phase of the solar cycle will be discussed. Particular attention will be given to extremes of time variation, i.e. to very quiet periods and to large events. While quiet-time fluxes are expected to shed light on some basic coronal processes, large events dominate the fluctuation characteristics of cumulated fluence, and the change of that fluctuation with energy and with the phase of the solar cycle may also provide important clues. Mainly ISEE-3 and long-term IMP-8 data will be invoked. Energetic and suprathermal particles that may never escape into interplanetary space may play an important part in heating the corona of the sun.

Energetic particle fluxes in the exterior cusp and the high-latitude dayside magnetosphere: statistical results from the Cluster/RAPID instrument

Abstract. In this paper we study the fluxes of energetic protons (30–4000 keV) and electrons (20–400 keV) in the exterior cusp and in the adjacent high-latitude dayside plasma sheet (HLPS) with the Cluster/RAPID instrument. Using two sample orbits we demonstrate that the Cluster observations at high latitudes can be dramatically different because the satellite orbit traverses different plasma regions for different external conditions. We make a statistical study of energetic particles in the exterior cusp and HLPS by analysing all outbound Cluster dayside passes in February and March, 2002 and 2003. The average particle fluxes in HLPS are roughly three (protons) or ten (electrons) times larger than in the exterior cusp. This is also true on those Cluster orbits where both regions are visited within a short time interval. Moreover, the total electron fluxes, as well as proton fluxes above some 100 keV, in these two regions correlate with each other. This is true even for fluxes in every energy channel when considered separately. The spectral indices of electron and proton fluxes are the same in the two regions. We also examine the possible dependence of particle fluxes at different energies on the external (solar wind and IMF) and internal (geomagnetic) conditions. The energetic proton fluxes (but not electron fluxes) in the cusp behave differently at low and high energies. At low energies (<70 keV), the fluxes increase strongly with the magnitude of IMF By. Instead, at higher energies the proton fluxes in the cusp depend on substorm/geomagnetic activity. In HLPS proton fluxes, irrespective of energy, depend strongly on the Kp and AE indices. The electron fluxes in HLPS depend both on the <Kp index and the solar wind speed. In the cusp the electron fluxes mainly depend on the solar wind speed, and are higher for northward than southward IMF. These results give strong evidence in favour of the idea that the high-latitude dayside plasma sheet is the main source of energetic particles in the exterior cusp. Energetic particles can reach HLPS from the near-Earth tail. The closed field lines of HLPS act as storage for these particles. Direct diffusion (for electrons and high-energy protons) and magnetic reconnection in the high-latitude magnetopause near HLPS (for low energy protons) control the number of particles released into the exterior cusp. Note that this explanation, in contrast to other suggested theories, works both for the energetic protons and electrons in the exterior cusp. Keywords. Magnetospheric physics (Magnetopause, cusp and boundary layers; Solar wind-magnetosphere interactions) – Space plasma physics (magnetic reconnection)

Absence of energetic particle effects associated with magnetic reconnection exhausts in the solar wind

We have examined energetic particle data (>38 keV for electrons and >47 keV for protons) obtained in intervals surrounding a set of 7 encounters with Petschek‐type reconnection exhausts in the solar wind by the Advanced Composition Explorer. We find no evidence for any substantial increases in energetic particle intensity associated with those encounters. This indicates that local reconnection is not a significant source of energetic particles in the solar wind and suggests the possibility that reconnection itself may not be a particularly effective process for populating other space and astrophysical environments with energetic particles.

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.

Theoretical interpretation of frequency sweeping observations in the Mega-Amp Spherical Tokamak

Frequency sweeping (chirping) of high frequency magnetohydrodynamic modes is widely observed in tokamak plasmas. In this paper observations of chirping in neutral-beam-heated plasmas in the Mega-Amp Spherical Tokamak (MAST) [A. Sykes, R. J. Akers, L. C. Appel et al., Nucl. Fusion 41, 1423 (2001)] are considered, and it is shown that these may be interpreted using the Berk–Breizman augmentation of the Vlasov–Maxwell equations. This model includes an energetic particle source: it leads not only to a single chirp but also to a series of bursting events. This repetitious behavior is characteristic of the chirping seen in experiments such as MAST. The similarity between features in velocity space and features in frequency space reinforces the theory that hole-clump pair formation is responsible for the observed frequency sweeping.

Relativistic laser–plasma bubbles: new sources of energetic particles and x-rays

Using direct three-dimensional particle-in-cell (PIC) simulations we study the acceleration of electrons and ions in laser–plasma. The PIC simulations allow us to understand the results of actual experiments better, and to look beyond the presently available technology and model for laser parameters that will be obtained in the nearest future. The code VLPL (Virtual Laser-Plasma Laboratory) has been further developed and now it can model the x-ray emission by relativistic electrons in the synchrotron regime. It appears that the strongly non-linear plasma wave excited by a short laser pulse works as a compact high-brightness source of x-ray radiation. We have also checked the possibility of proton trapping in the non-linear plasma wave and find that this is already possible at intensities around 1022 W cm−2.

3D modeling of shock-induced trapping of solar energetic particles in the Earth's magnetosphere

The prompt trapping of solar energetic particles (SEPs) in the inner magnetosphere around L = 2 –2.5, including protons and heavier ions, has been observed at both the Cycle 22 and 23 solar maxima, in association with high-speed interplanetary shocks and storm sudden commencements (SSCs). Recent observations include the Bastille Day 2000 CME-driven storm as well as two in November 2001, which produced a long-lived new proton belt, as well as trapping of heavy ions up to Fe in all three cases. A survey of such events around the most recent solar maximum, including high altitude measurements from Polar and HEO satellites along with low altitude measurements from `the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX)', indicates similarities to the well-studied March 24, 1991 SSC event. In this event, electrons and protons in drift resonance with a magnetosonic impulse were transported radially inward. A requirement for such shock-induced acceleration is a high-speed CME-shock at 1 AU, which launches a perturbation with comparable velocity inside the magnetosphere. Secondly, there must be a source population which is drift-resonant with the impulse. The CME-shock itself is a source of solar energetic particles, both protons and heavy ions, with higher fluxes and harder spectra associated with faster moving CMEs. Arrival of the interplanetary shock compresses and changes the magnetosphere topology, leading to a reduction of the geomagnetic cutoff, initially around L = 4 for SEP protons. This effect is modeled using a 3D Lorentz integration of SEP trajectories in electric and magnetic fields taken from the Lyon–Fedder–Mobarry (LFM) global MHD code, with solar wind input parameters taken from spacecraft measurements upstream from the bow shock, carried out for the November 24, 2001 SEP event. The results indicate that an enhancement in solar wind dynamic pressure for this event plays a role in the observed injection of ions to low L-values, to form a new proton belt which has lasted for more than 2 years.

Coronal Mass Ejections and Space Weather

Coronal mass ejections (CMEs) are a key aspect of coronal and interplanetary dynamics. They can inject large amounts of mass and magnetic fields into the heliosphere, causing major geomagnetic storms and interplanetary shocks, a key source of solar energetic particles. Studies over the past decade using the excellent data sets from the SOHO, TRACE, Wind, ACE and other spacecraft and ground-based instruments have improved our knowledge of the origins and early development of CMEs at the Sun and how they affect space weather at Earth. I review some key coronal properties of CMEs, their source regions, their manifestations in the solar wind, and their geoeffectiveness. Halo-like CMEs are of special interest for space weather because they suggest the launch of a geoeffective disturbance toward Earth. However, their correspondence to geomagnetic storms varies over the solar cycle. Although CMEs are involved with the largest storms at all phases of the cycle, recurrent features such as interaction regions and high speed wind streams can also be geoeffective. A new heliospheric experiment, the Solar Mass Ejection Imager, has completed its first year in orbit and I give some early results.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Observation of energy-time dispersed ion structures in the magnetosheath by CLUSTER: possible signatures of transient acceleration processes at shock

Abstract. We analyse energy-time dispersed ion signatures that have been observed by CLUSTER in the dayside magnetosheath. These events are characterized by sudden increases in the ion flux at energies larger than 10 keV. The high energy ions (30 keV) are first detected, with the transition to the low energy ions (5 keV) lasting about 100 s. These injections are often associated with transient plasma structures of a few minutes in duration, characterized by a hotter, less dense plasma and a diverted flow velocity, thus presenting similarities with "hot flow anomalies". They also involve modifications of the magnetic field direction, suggesting that the shock interacts with a solar wind discontinuity at the time of the event. The injections can originate from the magnetosphere or the shock region. Studying in detail a particular event, we discuss this last hypothesis. We show that the observed energy/time dispersion can be explained by combining a time-of-flight effect with a drift of the source of energetic particles along the shock. We propose that the acceleration results from a Fermi process linked to the interaction of the discontinuity with a quasi-perpendicular shock. This model explains the observed pitch-angle selection of the accelerated particles. The Fermi process acting on the beam of ions reflected from the shock appears to be sufficiently efficient to accelerate over short time scales (less than 30 s) particles at energies above 30 keV.Key words. Magnetospheric physics (solar-wind-magnetosphere interaction; magnetosheath) – Space plasma physics (shock waves)

Butterfly pitch-angle distributions observed by ISEE-1

The ISEE-1 satellite has observed butterfly pitch-angle distributions (PAD) in protons and electrons from 20 keV to 2 MeV in the low latitude outer magnetosphere, which showed a sharp dropout in the flux of 90° pitch angle particles. These distributions were present on the nightside near the magnetic equatorial plane and were far more prevalent in electrons than in protons. These distributions have been known for some time and have been associated with drift shell splitting in combination with either shadowing by the magnetopause or with a radial intensity gradient close to the Earth as equatorially mirroring particles drift around the Earth at radial distances from 6 to 12 R E. In both a study of individual three-dimensional distributions and in a statistical survey, the occurrence of the butterfly PADs was found to have the following consistent behavior: (1) The anisotropy in the distributions increased with an increase in particle energy for both the protons and the electrons. (2) The anisotropy was less pronounced in the protons than in the electrons and usually disappeared altogether for the higher energy protons. (3) The radial location of the transition from and to the butterfly distribution was found to have a dependence on particle energy. (3a) The transition position occurred closer to the Earth with increasing particle energy for the butterfly PAD to normal PAD transition whereas (3b) the transition from the butterfly PAD to isotropic PAD occurred closer to the Earth with decreasing particle energy. In other words, the PADs were isotropic near the apogee of ISEE-1, peaked at 90° pitch angle close to the Earth, and butterfly over a range of radial distances, the width of which expanded in both directions with increasing energy. Modeling results are presented that are interpreted to demonstrate that the magnetopause and its motion are responsible for producing the butterfly PADs. The differences between ion and electron butterfly PADs are interpreted to indicate that the magnetopause has a layer of energetic particles that becomes a source of energetic particles to the inner magnetosphere and preferentially provides ions as close as the geostationary orbit routinely. These ions have a softer spectrum than the ions present at the same location on stably trapped drift trajectories (i.e., for those particles mirroring well away from the equator).

Gravitational Interaction of Celestial Bodies and neutron flux bursts from Their Surfaces

The gravitational interaction between the Sun and the planetary solar system gives rise to the well-known tidal waves at the planets. The tidal wave originating in the Earth's crust perpetually transforms the microstructure of the Earth's crust leading to a variation of the concentration of natural radioactive gases in the terrestrial air and to changed conditions of their leakage to the Earth's atmosphere. These variations give rise to bursts of thermal and slow neutrons in the vicinity of the Earth's crust, because the radioactive gases are sources of energetic alpha particles that induce neutron production upon the interaction with the nuclei of elements of the Earth's crust and atmosphere. In this work, the idea of neutron production in the ground coat is extended to the other celestial bodies interacting with one another.

Multisatellite observations of MeV ion injections during storms

We report here on the formation of new ion radiation belts observed in connection with several solar energetic particle events and large geomagnetic storms in 1998 and 2000. We use observations from the Polar spacecraft, the highly elliptical orbit (HEO) 1997‐068, and the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), to study details of the inner zone radiation belt at high and low altitudes. We focus specifically on the four International Solar Terrestrial Physics events of August and September 1998 and April and July 2000. In several events we find new 2–15 MeV proton belts at various locations between L = 2.0 and L = 3.5. The low‐altitude SAMPEX observations revealed features not visible at high altitudes, such as radiation belts with multiple peaks in L shell. During the July 2000 event, energetic helium and iron were observed at L ∼ 2, suggesting a solar energetic particle source for these injected ions. We compare observations of these new belts and remark on the significant differences from event to event.

Energetic particle observations at the subsolar magnetopause

Abstract. The pitch-angle distributions (PAD) of energetic particles are examined as the ISEE-1 satellite crosses the Earth’s magnetopause near the subsolar point. The investigation focuses on the possible existence of a particular type of distribution that would be associated with a source of energetic particles in the high-latitude magnetosphere. PADs, demonstrating broad, persistent field-aligned fluxes filling a single hemisphere (upper/northern or lower/southern), were observed just sunward of the magnetopause current layer for an extended period of many minutes. These distributions are a direct prediction of a possible source of energetic particles located in the high altitude dayside cusp and we present five examples in detail of the three-dimensional particle distributions to demonstrate their existence. From these results, other possible causes of such PADs are examined.Key words. Magnetospheric physics (energetic particles, precipitating; magnetopause, cusp and boundary layers; magnetospheric configuration and dynamics)

Energetic particle data recorded onboard Giotto during the first encounter of an observing spacecraft coming from deep space with Planet Earth

The Giotto Spacecraft executed an Earth-Gravity-Assist manoeuvre on 2–3 July 1990 and two onboard scientific instruments, namely the energetic particle analyser (EPONA) and the magnetometer (MAG), were set into operation during the associated (33 h) encounter. Contemporaneous data show that the magnetosphere was relatively undisturbed during this Earth Swingby so that the signatures recorded by EPONA effectively provide a “snapshot” of the quiet magnetosphere in energetic particles along a unique, hyperbolic, high-inclination orbit. Protons in the range ∼30– 200 keV recorded in the region upstream of the bow shock are interpreted to have been accelerated, via the diffuse shock acceleration mechanism, at the (inbound) quasi-parallel bow shock. The temporal profiles of those proton and electron fluxes recorded near closest approach, where Giotto traversed part of the outer radiation belt, are compared with profiles generated using NASA's AP-8 MIN and AE-8 MIN programs to model particle fluxes along this segment of the trajectory. Characteristic features of the measured energetic proton and (MeV) electron fluxes are, thereby, shown to match the modelled signatures of trapped radiation in the (outer) Belt. The EPONA data provide no evidence of a midnight cusp source of energetic particles. Outbound, where magnetic records show that “flapping” of the magnetopause boundary was a feature, variations in the particle record reflect spacecraft traversals back and forth from the magnetosphere to the magnetosheath. It is inferred that proton fluxes in the range <30– >200 keV identified between both the inbound and the outbound magnetopause and the bow shock comprise a sheath of (Fermi) accelerated protons enveloping the magnetosphere. The transit time damping mechanism is suggested to have contributed to producing the rather high fluxes (relative to the inbound pass) of energetic particles recorded downstream of the bow shock.

A Source of Energetic Particles Associated with Solar Flares

Energetic particles produced at a solar flare event are an issue of great theoretical interest. A scenario involving magnetic field reconnection and newborn ions produced by ionization of neutral atoms in the lower corona is proposed by Wu. A series of two-dimensional hybrid simulations has been carried out to investigate this scenario further, and the results are presented in this paper. It is shown that under certain conditions the proposed process can produce protons with energies in the range of 10-100 MeV with a timescale of 10-4 to 10-3 s in a reconnection layer via the so-called ion pickup process. The acceleration process is sensitive to the ambient magnetic field. The maximum energy attainable is also discussed in terms of altitudes above the photosphere on the basis of a certain magnetic field model.

The Cusp as a Source of Magnetospheric Energetic Particles, Currents, and Electric Fields: A New Paradigm

Energetic particle instrumentation on the Polar satellite has discovered that significant fluxes of energetic particles are continuously present in the region of the dayside magnetosphere where they cannot be stably trapped. This region is associated with either open magnetic field lines or a magnetic topology associated with pseudo-trapping. Two distinct features [Time-Energy Dispersion (TED) signatures and Cusp Energetic Particle (CEP) events] are observed in these energetic particle fluxes that strongly suggest a local acceleration of mostly shocked solar wind particles. As the solar wind particles ram themselves into the cusp geometry, they form diamagnetic cavities with strong turbulence that are capable of accelerating particles to energies of 100s and 1000s of kiloelectronvolts. This process forms a layer of energetic particles on the magnetopause as well as permits such particles to enter via drift the equatorial nightside magnetosphere to distances as close as six Earth radii under the influence of gradient and curvature effects in the local magnetic field. The fluxes of these particles have all of the properties associated with the ring current and can supply the magnitude of the cross tail current required. ISEE-1 energetic particle data and their pitch angle distributions [PAD] are examined at the magnetic equatorial plane on the night side to investigate and possibly validate the insights gains from the Polar data and energetic particle trajectory tracing in a realistic magnetic field. The existence and properties of butterfly-type PADs strongly supports the concept of a dayside high latitude source of energetic particle fluxes. Because the CEP process is impulsive and time variable the charge separation produced by the drifting electrons (eastward) and ions (westward) on the magnetospheric nightside may be responsible for the cross tail electric field that has been ascribed to the reconnection/convection process.

SOHO/Energetic and Relativistic Nucleon and Electron Experiment Measurements of Energetic H, He, O, and Fe Fluxes during the 1997 November 6 Solar Event

A brilliant solar X-ray and Hα flare and a coronal mass ejection (CME) on 1997 November 6 were associated with high particle fluxes in interplanetary space at energies above MeV. The CME had an exceptionally high leading edge velocity (1560 km s-1) as observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO). The Energetic and Relativistic Nucleon and Electron experiment (ERNE), also on SOHO, measured high H, He, O, and Fe fluxes in several energy channels from 3 to 200 MeV nucleon-1. The oxygen energy spectrum in energy range 3-200 MeV nucleon-1 was of a broken-power-law form with a break around 50 MeV nucleon-1. In addition, ERNE observed abrupt changes in the intensity, elemental composition, and anisotropy of high-energy particles, which may indicate two energetic particle sources during several hours after the solar flare eruption. The observational results lead us to conclude that, most of the time, the O and Fe nuclei were injected by the interplanetary shock associated with the coronal mass ejection emitted around 12 UT on November 6. However, during two time periods the injection source might have been different or complementary. The first period was in the very beginning of the event, 13:20-13:40 UT, when the particle streaming showed very strong anisotropy with the maximum intensity from the direction of the Sun. The second was between 17 UT November 6 and 5 UT November 7, when particle fluxes were dominated by a particle population with a different elemental composition and a different spectral shape of O as compared with the particle population prevailing in the beginning and during the long decay phase of the event. We propose that the source of these particles was associated with a coronal shock wave traveling in the low solar atmosphere.