Interplanetary propagation of relativistic solar protons

Particle fluxes and pitch angle distributions of relativistic solar protons at Earth's orbit have been determined by Monte Carlo calculations. The analysis covers two hours after the release of the particles from the Sun and total of 8 × 10⁶ particle trajectories were simulated. The pitch angle scattering was assumed to be isotropic and the scattering mean free path was varied from 0.1 to 4 AU.The intensity-time profiles after a delta-like injection from the Sun show that the interplanetary propagation is clearly non-diffusive at scattering mean-free paths above 0.5 AU. All pitch angle distributions have a steady minimum at 90 °, and they become similar about 20 min after the arrival of first particles.As an application, the solar injection profile and the interplanetary scattering mean-free path of particles that gave rise to the GLE on 7 May, 1978 were determined. In contrast to the values of 3–5 AU published by other authors, the average scattering mean-free path was found to be about 1 AU.     

Angular distributions of solar protons and electrons

High angular-resolution measurements of directional fluxes of solar particles in space have been obtained with detectors aboard OGO-5 during the cosmic ray event of 18 November 1968. This is the only case on record for which sharply-defined directional observations of protons and electrons covering a wide rigidity range (0.3 MV to 1.5 GV) are available.The satellite experiment provided data for determining pitch-angle distributions with respect to the direction of the local interplanetary magnetic field lines during the lengthy highly anisotropic phase of the event. It was found that the unidirectional differential intensities j(θ) of 3- to 25-MeV protons varied in accordance with the relationship j(θ) = b₀ + b₁cosθ + b₂cos²θ, where b₀ and b₁ ? 0, and b₂, is positive, zero or negative. Soon after onset, 79–266-keV electrons arriving from the direction of the Sun displayed an anisotropic component with the intensity varying as cos θ. Later, a double-peaked distribution appeared at the lower energies, whereas the flux at the upper end of the range covered by the experiment became isotropic. These results have been interpreted in the light of the temporal flux profiles and the state of the interplanetary medium.The observation of the unusually large and long-lasting anisotropies lead to several conclusions including: (1) If injection of the solar particles was instantaneous, the diffusion coefficient was either constant or increasing with distance from the Sun. (2) If the solar source emitted particles over an extended period, and there is evidence to that effect, there was weak scattering in the region between the Sun and the Earth and a strong scattering region beyond the Earth's orbit. (3) Solar electrons were stored near the Sun. (4) The observed angular distribution of 200-MV protons in the magnetosheath was in good agreement with that deduced in an earlier analysis of polar orbiting satellite observations and trajectory calculations.     

Cosmic ray anisotropies observed late in the decay phase of solar flare events

Concurrent interplanetary magnetic field and 0.7–7.6 MeV proton cosmic-ray anisotropy data obtained from instrumentation on Explorers 34 and 41 are examined for five cosmic-ray events in which we observe a persistent eastern-anisotropy phase late in the event (t ? 4 days). The direction of the anisotropy at such times shows remarkable invariance with respect to the direction of the magnetic field (which generally varies throughout the event) and it is also independent of particle species (electrons and protons) and particle speed over the range 0.06 ? β ? 0.56. The anisotropy is from the direction 38.3° ± 2.4° E of the solar radius vector, and is inferred to be orthogonal to the long term, mean interplanetary field direction. Both the amplitude of the anisotropy and the decay time constant show a strong dependence on the magnetic field azimuth. Detailed comparison of the anisotropy and the magnetic field data shows that the simple model of convection plus diffusion parallel to the magnetic field is applicable for this phase of the flare effect. It is demonstrated that contemporary theories do not predict the invariance of the direction as observed, even when the magnetic field is steady; these theories need extension to take into account the magnetic field direction ψ varying from its mean direction ψ _o. It is shown that the late phase anisotropy vector is not expected to be everywhere perpendicular to the mean magnetic field. The suggestion that we are observing kinks in the magnetic field moving radially outwards from the Sun leads to the conclusion that the parallel diffusion coefficient varies as 1/cos² (ψ ? ψ _o). Density gradients in the late decay phase are estimated to be ≈ 700%∣AU for 0.7–7.6 MeV protons. A simple theory reproduces the dependence of the decay time constant on anisotropy; it also leads to a radial density gradient of about 1000%∣AU and diffusion coefficient of 1.3 × 10²⁰ cm² s^?1.     

Solar flare track densities in interplanetary dust particles: The determination of an asteroidal versus cometary source of the zodiacal dust cloud

Dust particles that are larger than 1 μm, when injected into the Solar System from comets and asteroids, will spiral into the Sun due to the Poynting-Robertson effect. During the process of spiraling in, such dust particles accumulate solar flare tracks in their component minerals. The accumulated track density for a given dust grain is a function of the duration of its space exposure and its distance from the Sun. Using a computer model, it was determined that the expected track density distributions from grains produced by comets are very different from those produced by asteroids. Individual asteroids produce populations of particles that arrive at 1 AU with scaled track density distributions containing “spikes,” while comets supply particles with a flatter and wider distribution of track densities. Particles with track densities above 3 × 10⁷ (sϱA/v) tracks/cm² have probably been exposed to solar flare tracks prior to injection into the interplanetary medium and are therefore likely to be asteroidal. Particles with track densities below 0.7 × 10⁷(sϱA/v) tracks/cm² must be derived from comets or Earth-crossing asteroids. Earth-crossing asteroids are not responsible for all the dust collected at 1 AU since they cannot produce the large track densities observed in some of the interplanetary dust particles collected in the stratosphere. The track densities observed in the stratospheric dust fall within the predicted range, but there is at present an insufficient number of carefully determined densities to make strong statements about the sources of the present dust population.     

Spatial Distribution of Solar Energetic Particles in the Inner Heliosphere

We study the spatial distribution of solar energetic particles (SEPs) throughout the inner heliosphere during six large SEP events from the period 1977 through 1979, as deduced from observations on the Helios 1 and 2, IMP 7 and 8, ISEE 3, and Voyager 1 and 2 spacecraft. Evidence of intensity maxima associated with the expanding shock wave is commonly seen along its central and western flanks, although the region of peak acceleration or “nose” of the shock is sometimes highly localized in longitude. In one event (1 January 1978) a sharp peak in 20?–?30 MeV proton intensities is seen more strongly by Voyager at ～?2 AU than it is by spacecraft at nearby longitudes at ～?1 AU. Large spatial regions, or “reservoirs,” often exist behind the shocks with spatially uniform SEP intensities and invariant spectra that decrease adiabatically with time as their containment volume expands. Reservoirs are seen to sweep past 0.3 AU and can extend out many AU. Boundaries of the reservoirs can vary with time and with particle velocity, rather than rigidity. In one case, a second shock wave from the Sun reaccelerates protons that retain the same hard spectrum as protons in the reservoir from the preceding SEP event. Thus reservoirs can provide not only seed particles but also a “seed spectrum” with a spectral shape that is unchanged by a weaker second shock.     

Albedos and size distribution of meteoroids from 0.3 to 4.8 AU

Comparison is made between the run of number density of meteoroids from penetration detectors aboard Helios A (masses below 10^?8 g) and Pioneer 10 (masses near and above 3 × 10^?9 g), the source function of the zodiacal light deduced from photometric observations aboard Helios A and Pioneer 10, counts versus brightness of objects passing by Pioneer 10 from the Sisyphus experiment and the distribution of meteoroids deduced from radar and optical meteors at the Earth. The Sisyphus experiment on Pioneer 10 observed reflecting glints on meteoroids rather than the meteoroids themselves and the counting statistics refer not to the effective radii of the meteoroids but to the effective radii of curvature of the reflecting glints on the meteoroids. The penetration detectors appear to find some increase in number density toward the Sun and a flat distribution outward to 5.2 AU. The overall behavior of the zodiacal light is that the relative distribution over direction is unchanged while the source scattering function diminishes as the inverse 1.4 power of distance from the Sun. The fit to the brightness of the zodiacal light obtained from these statistics can be combined with the mass distribution results from the optical meteors to deduce a mean geometric albedo of meteoroids of 0.006 at 1 AU from the Sun. Combination of the space distribution from radar meteors with the scattering source function of the zodiacal light yields geometric albedos for meteoroids running from 0.07 at 0.1 AU, from the Sun through 0.006 at 1 AU down to about 0.0001 at 3.3 AU which may run flat thence outward. This result is imposed by the indicated modest increase in density of meteoroids very near the Sun, a minimum between the Sun and the Earth near 0.4 AU and rising density outward to somewhere beyond 3.3 AU which is very different from the inverse 1.4 power of the distance shown for scatterers (product of number density and albedo) by the zodiacal light. A check on the distribution at very large sizes is possible if a search is made for fireballs in Jupiter's atmosphere by the Mariner Jupiter Saturn 1977 television cameras during the two encounters with Jupiter in 1979. An easy detection of such activity would put the maximum in the meteoroid distribution out near Jupiter and lend further confirmation to the indicated drop in albedo.     

Implications of Proton Anisotropy Development Observed by the Erne Instrument During the 9 July 1996 Solar Particle Event

We analysed the solar particle event following the 9 July 1996 solar flare. High-energy protons were detected by the ERNE instrument on board SOHO. Anisotropy of arriving protons revealed very peculiar non-monotonic development. A short period of almost isotropic distribution was imbedded into the prolonged period of beam-like distribution of 14–17 MeV protons. This implies the existence of a narrow magnetic channel with a much smaller mean free path than in the surrounding quiet solar wind plasma. We used Monte Carlo simulations of interplanetary transport to fit the observed anisotropies and intensity–time profiles. Proton injection and transport parameters are estimated. The injection scenario is found to be very close to the scenario of the 24 May 1990 event, but the intensity and the interplanetary transport parameters are different. The extreme anisotropy observed implies prolonged injection of high-energy protons at the Sun and at the interplanetary shock front, and either a very large mean free path (≥ 5 AU) outside the slow transport channel, or alternatively, a somewhat smaller mean free path (≈2 AU) and enhanced focusing between the Sun and the Earth.     

Balloon observations of solar protons on September 29–30, 1968, over Iceland

On September 29, 1968 a proton event has been recorded during three balloon flights performed at Reykjavik, Iceland (64.2 N, 21.7 W) with GM telescopes and scintillation detector. Solar X-rays have been recorded at 1620 UT when a flare of Importance 2B occurred at N 16, W 52. A comparison between X-rays and microwave emissions is made; the time of the maximum of X-ray intensity is taken as the time of the acceleration and ejection of the particles. The beginning of the proton event is at 1650 UT, and particles were observed for almost 24 h. The spectrum of solar protons E>120 MeV is given for several periods between 7 and 20 h after the flare using three independent methods. The solar particle source spectrum is found as: 321-01 (particles/MeV ster), which implies that (1.2±0.1) × 10³¹ protons (E>120 MeV)/ster have been ejected by the Sun.The time behaviour of the event fits well with Krimigis' model for solar particles diffusion in the interplanetary space. Comparison with other events shows that the radial dependence of the diffusion coefficient is the same (1) on September 28, 1961, July 7, 1966 and September 29, 1968. The diffusion mean free path at 1 AU is 0.11 AU for 1966, period of low solar activity, and decreases with solar activity (0.08 AU for 1961 and 1968). The fit of the time behaviour of the event with Burlaga's ADB model is also discussed.     

Three years of Ulysses dust data: 1993–1995

The Ulysses spacecraft is orbiting the Sun on a highly inclined ellipse (i = 79°). After its Jupiter flyby in 1992 at a heliocentric distance of 5.4 AU, the spacecraftreapproached the inner solar system, flew over the Suns south polar region in September 1994,crossed the ecliptic plane at a distance of 1.3 AU in March 1995, and flew over the Suns northpolar region in July 1995. We report on dust impact data obtained with the dust detector onboardUlysses between January 1993 and December 1995. We publish and analyse the complete dataset of 509 recorded impacts of dust particles with masses between 10⁻¹⁶ g–10⁻⁷ g. Together with 968 dust impacts from launch until the end of 1992 published earlier (Grün et al., 1995c), information about 1477 particles detected with theUlysses sensor between October 1990 and December 1995 is now available. The impact ratemeasured between 1993 and 1995 stayed relatively constant at about 0.4 impacts per day andvaried by less than a factor of ten. Most of the impacts recorded outside about 3.5 AU arecompatible with particles of interstellar origin. Two populations of interplanetary particles havebeen recognized: big micrometer-sized particles close to the ecliptic plane and smallsub-micrometer-sized particles at high ecliptic latitudes. The observed impact rate is comparedwith a model for the flux of interstellar dust particles which gives relatively good agreement withthe observed impact rate. No change in the instruments noise characteristics or degradation of thechanneltron could be revealed during the three-year period.     

The frequency and intensity of comet showers from the Oort cloud

We simulate the Oort comet cloud to study the rate and properties of new comets and the intensity and frequency of comet showers. An ensemble of ∼10⁶ comets is perturbed at random times by a population of main sequence stars and white dwarfs that is described by the Bahcall-Soneira Galaxy model. A cloning procedure allows us to model a large ensemble of comets efficiently, without wasting computer time following a large number of low eccentricity orbits. For comets at semimajor axis a = 20,000 AU, about every 100 myr a star with mass in the range 1M_⊙−2M_⊙ passes within ∼10,000 AU of the Sun and triggers a shower that enhances the flux of new comets by more than a factor of 10. The time-integrated flux is dominated by the showers for comets with semimajor axes less than ∼30,000 AU. For semimajor axes greater than ∼30,000 AU the comet loss rate is roughly constant and strong showers do not occur. In some of our simulations, comets are also perturbed by the Galactic tidal field. The inclusion of tidal effects increases the loss rate of comets with semimajor axes between 10,000 and 20,000 AU by about a factor of 4. Thus the Galactic tide, rather than individual stellar perturbations, is the dominant mechanism which drives the evolution of the Oort cloud.     

On Fields and Mass Constraints for the Uniform Propagation of Magnetic-Flux Ropes Undergoing Isotropic Expansion

An analytical 3-D magnetohydrodynamic (MHD) solution of a magnetic-flux rope (FR) is presented. This FR solution may explain the uniform propagation, beyond ～?0.05 AU, of coronal mass ejections (CMEs) commonly observed by today’s missions like The Solar Mass Ejection Imager (SMEI), Solar and Heliospheric Observatory (SOHO) and Solar Terrestrial Relations Observatory (STEREO), tracked to tens of times the radius of the Sun, and in some cases up to 1 AU, and/or beyond. Once a CME occurs, we present arguments regarding its evolution based on its mass and linear momentum conservation. Here, we require that the gravitational and magnetic forces balance each other in the framework of the MHD theory for a simple model of the evolution of a CME, assuming it interacts weakly with the steady solar wind. When satisfying these ansätze we identify a relation between the transported mechanical mass of the interplanetary CME with its geometrical parameters and the intensity of the magnetic field carried by the structure. In this way we are able to estimate the mass of the interplanetary CME (ICME) for a list of cases, from the Wind mission records of ICME encountered near Earth, at 1 AU. We obtain a range for masses of ～?10⁹ to 10¹³ kg, or assuming a uniform distribution, of ～?0.5 to 500 cm^?3 for the hadron density of these structures, a result that appears to be consistent with observations.     

Interplanetary scattering of fast solar electrons deduced from type III bursts observed at low frequencies

Observations of low frequency solar type III radio bursts and the associated fast solar electrons show that the total path length traveled by the particles between the Sun and the Earth is significantly greater than the length of the smooth Archimedean spiral trajectory followed by the centroid of the type III exciter (Alvarez et al., 1975). Here we assume that the ratio of electron path length and the spiral length increases approximately as r ⁿ, where r is heliocentric distance, and then compute the radio bursts arrival time at 1 AU for different values of n. A comparison with the radio observations indicates that the best fit occurs for n = 1.5 ± 1.0. We interpret these results in terms of the variation of electron scattering with heliocentric distance.     

Motion of dust particles under the influence of a latitudinally dependent force

The Kelperian motion of dust particles in the solar system is mainly influenced by the electromagnetic and plasma Poynting-Robertson drag. The first force is isotropic while the second one shows latitudinal variations due to the observed differences of the solar wind parameters in the ecliptic plane and over the solar poles. Close to the Sun other effects become important, e.g. sublimation and sputtering, as well as for submicron particles Lorentz scattering has to be taken into account. These forces are very weak for dust grains of moderate size (10–100 µ) not too close (>0.03 AU) to the Sun and are neglected here. Assuming that the general form of the latidudinally dependent force is a series expansion in Legendre polynomials, we have studied the averaged equations of motion for the classical elements and found the first integral of them. The general character of motion is the same as for the classical Poynting-Robertson drag: particles spiral towards the Sun. The new features in the orbital evolution under the latitudinally dependent force as compared with the isotropic Poynting-Robertson drag are:

1. not only the semimajor axisa and the eccentricity ε but also the argument of the perihelion ω varies with time,
2. the rate of change ofa, ε, ω depends on the inclination.

An example of particle trajectories in the phase space of elements is presented.     

Outward transport of CAIs during FU‐Orionis events

Abstract— Evidence from meteorites shows that the first solids to form in the solar system, calcium‐aluminum‐rich inclusions (CAIs), were transported outward from the Sun by several AU in the early solar system. We introduce a new concept of levitation and outward transport of CAIs at the surface of protoplanetary disks. Thermal radiation from the disk and the Sun can cause particles to levitate above the disk and drift outward through a process known as photophoresis. During normal conditions this process only works for dust‐sized particles but during high luminosity events like FU‐Orionis outbursts, the process can provide an efficient lift and transport of CAIs from within the inner 1 AU to a distance of several AU from the Sun. This might explain why CAIs, believed to have formed close to the Sun, are common in meteorites believed to come from the outer asteroid belt but are rare or absent in samples from the inner solar system. Since the process only works during the FU‐Orionis event and only for particles up to cm‐size, it may also explain why the CAIs we find in meteorites appear to have formed within a short period of time and why they rarely exceed cm size.     

The Large Longitudinal Spread of Solar Energetic Particles During the 17 January 2010 Solar Event

We investigate multi-spacecraft observations of the 17 January 2010 solar energetic particle event. Energetic electrons and protons have been observed over a remarkable large longitudinal range at the two STEREO spacecraft and SOHO, suggesting a longitudinal spread of nearly 360 degrees at 1?AU. The flaring active region, which was on the backside of the Sun as seen from Earth, was separated by more than 100 degrees in longitude from the magnetic footpoints of each of the three spacecraft. The event is characterized by strongly delayed energetic particle onsets with respect to the flare and only small or no anisotropies in the intensity measurements at all three locations. The presence of a coronal shock is evidenced by the observation of a type II radio burst from the Earth and STEREO-B. In order to describe the observations in terms of particle transport in the interplanetary medium, including perpendicular diffusion, a 1D model describing the propagation along a magnetic field line (model 1) (Dr?ge, Astrophys. J. 589, 1027??C?1039, 2003) and the 3D propagation model (model 2) by Dr?ge et?al. (Astrophys. J. 709, 912??C?919, 2010) including perpendicular diffusion in the interplanetary medium have been applied. While both models are capable of reproducing the observations, model 1 requires injection functions at the Sun of several hours. Model 2, which includes lateral transport in the solar wind, reveals high values for the ratio of perpendicular to parallel diffusion. Because we do not find evidence for unusual long injection functions at the Sun, we favor a scenario with strong perpendicular transport in the interplanetary medium as an explanation for the observations.     

Collisional balance of the meteoritic complex

Taking into account meteoroid measurements by in situ experiments, zodiacal light observations, and oblique angle hypervelocity impact studies, it is found that the observed size distributions of lunar microcraters usually do not represent the interplanetary meteoroid flux for particles with masses ?10^?10g. From the steepest observed lunar crater size distribution a “lunar flux” is derived which is up to 2 orders of magnitude higher than the interplanetary flux at the smallest particle masses. New models of the “lunar” and “interplanetary” meteoroid fluxes are presented. The spatial mass density of interplanetary meteoritic material at 1 AU is ～10^?16g/m³. A large fraction of this mass is in particles of 10^?6 to 10^?4 g. A detailed analysis of the effects of mutual collisions (i.e., destruction of meteoroids and production of fragment particles) and of radiation pressure has been performed which yielded a new picture of the balance of the meteoritic complex. It has been found that the collisional lifetime at 1 AU is shortest (～10⁴years) for meteoroids of 10^?4 to 1 g mass. For particles with masses m > 10^?5g, Poynting-Robertson lifetimes are considerably larger than collisional lifetimes. The collisional destruction rate of meteoroids with masses $\text{m ? 10}{}^{\mathrm{?3}}\text{g}$ is about 10 times larger than the rate of collisional production of fragment particles in the same mass range. About 9 tons/sec of these “meteor-sized” (m > 10^?5g) particles are lost inside 1 AU due to collisions and have to be replenished by other sources, e.g., comets. Under steady-state conditions, most of these large particles are “young”; i.e., they have not been fragmented by collisions and their initial orbits are not altered much by radiation pressure drag. Many more micrometeoroids of masses m ? 10^?5g are generated by collisions from more massive particles than are destroyed by collisions. The net collisional production rate of intermediate-sized particles 10^?10g ? m ? 10^?5g is found to be about 16 times larger at 1 AU than the Poynting-Robertson loss rate. The total Poynting-Robertson loss rate inside 1 AU is only about 0.26 tons/sec. The smallest fragment particles (m ? 10^?10g) will be largely injected into hyperbolic trajectories under the influence of radiation pressure (β meteoroids). These particles provide the most effecient loss mechanism from the meteoritic complex. When it is assumed that meteoroids fragment similarly to experimental impact studies with basalt, then it is found that interplanetary meteoroids in the mass range 10^?10g ? m ? 10^?5g cannot be in temporal balance under collisions and Poynting-Robertson drag but their spatial density is presently increasing with time.     

2-Gyr Simulation of the Oort-cloud Formation II. A Close View of the Inner Oort cloud after the First Two Giga-years

We simulate the formation of the Oort cloud (OC) till the age of 2 Gyr starting from an initial disc of planetesimals made by 10 038 test particles. The results on the outer part of the distant comet reservoir are reported by Neslu?an et al. (this issue). Here we deal with the evolution of the population and structure at 2 Gyr of the complementary inner part of the Oort cloud. The dynamical evolution of the massless test particles was followed via the numerical integration of their orbits. We considered the perturbations produced by four giant planets assuming they have their current orbits and masses, as well as the perturbations caused by the Galactic tide and passing stars. The efficiency of the formation of inner OC is found to be very low: only about 1.1% of all considered particles ended in this part of the OC. At 2 Gyr, the dynamics of the inner cloud is mainly governed by the dominant z-term of the Galactic tide. The number density of the bodies is proportional to the heliocentric distance, r, as r ^?3.53. The directional distribution of orbits is still strongly inhomogeneous. There are large empty regions in the space angles around the Galactic Equator points with the galactic longitude 90 and 270° (non-rotating frame), or there are only few bodies having the ecliptical latitude higher than +60° or lower than 60°. A strong concentration of objects at the Ecliptic is apparent up to ≈1,000 AU, with a possible—but still not proved—extension to ≈1,500 AU. Beyond r ≈ 6,000 AU, bodies directly above and below the Sun, with respect to the Ecliptic, are absent.     

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.     

Solar Wind Electron Strahls Associated with a High-Latitude CME: Ulysses Observations

Counterstreaming beams of electrons are ubiquitous in coronal mass ejections (CMEs) – although their existence is not unanimously accepted as a necessary and/or sufficient signature of these events. We continue the investigation of a high-latitude CME registered by the Ulysses spacecraft on 18?–?19 January 2002 (Dumitrache, Popescu, and Oncica, Solar Phys. 272, 137, 2011), by surveying the solar-wind electron distributions associated with this event. The temporal evolution of the pitch-angle distributions reveals populations of electrons that are distinguishable through their anisotropy, with clear signatures of i) electron strahls, ii) counter-streaming in the magnetic clouds and their precursors, and iii) unidirectionality in the fast wind preceding the CME. The analysis of the counter-streams inside the CME allows us to elucidate the complexity of the magnetic-cloud structures embedded in the CME and to refine the borders of the event. Identifying such strahls in CMEs, which preserve properties of the low β [<1] coronal plasma, gives more support to the hypothesis that these populations are remnants of the hot coronal electrons that escape from the electrostatic potential of the Sun into the heliosphere.