Acceleration Of Solar Wind Ions Research Articles

ION INJECTION AT QUASI-PARALLEL SHOCKS SEEN BY THE CLUSTER SPACECRAFT

ABSTRACT Collisionless shocks in space plasma are known to be capable of accelerating ions to very high energies through diffusive shock acceleration (DSA). This process requires an injection of suprathermal ions, but the mechanisms producing such a suprathermal ion seed population are still not fully understood. We study acceleration of solar wind ions resulting from reflection off short large-amplitude magnetic structures (SLAMSs) in the quasi-parallel bow shock of Earth using in situ data from the four Cluster spacecraft. Nearly specularly reflected solar wind ions are observed just upstream of a SLAMS. The reflected ions are undergoing shock drift acceleration (SDA) and obtain energies higher than the solar wind energy upstream of the SLAMS. Our test particle simulations show that solar wind ions with lower energy are more likely to be reflected off the SLAMS, while high-energy ions pass through the SLAMS, which is consistent with the observations. The process of SDA at SLAMSs can provide an effective way of accelerating solar wind ions to suprathermal energies. Therefore, this could be a mechanism of ion injection into DSA in astrophysical plasmas.

Acceleration of solar wind ions to 1 MeV by electromagnetic structures upstream of the Earth's bow shock

We present measurements from the ESA/NASA Cluster mission that show in situ acceleration of ions to energies of 1 MeV outside the bow shock. The observed heating can be associated with the presence of electromagnetic structures with strong spatial gradients of the electric field that lead to ion gyro-phase breaking and to the onset of chaos in ion trajectories. It results in rapid, stochastic acceleration of ions in the direction perpendicular to the ambient magnetic field. The electric potential of the structures can be compared to a field of moguls on a ski slope, capable of accelerating and ejecting the fast running skiers out of piste. This mechanism may represent the universal mechanism for perpendicular acceleration and heating of ions in the magnetosphere, the solar corona and in astrophysical plasmas. This is also a basic mechanism that can limit steepening of nonlinear electromagnetic structures at shocks and foreshocks in collisionless plasmas.

Dispersive MHD waves and alfvenons in charge non-neutral plasmas

Abstract. Dispersive properties of linear and nonlinear MHD waves, including shear, kinetic, electron inertial Alfvén, and slow and fast magnetosonic waves are analyzed using both analytical expansions and a novel technique of dispersion diagrams. The analysis is extended to explicitly include space charge effects in non-neutral plasmas. Nonlinear soliton solutions, here called alfvenons, are found to represent either convergent or divergent electric field structures with electric potentials and spatial dimensions similar to those observed by satellites in auroral regions. Similar solitary structures are postulated to be created in the solar corona, where fast alfvenons can provide acceleration of electrons to hundreds of keV during flares. Slow alfvenons driven by chromospheric convection produce positive potentials that can account for the acceleration of solar wind ions to 300–800 km/s. New results are discussed in the context of observations and other theoretical models for nonlinear Alfvén waves in space plasmas.

First observation of Jupiter by XMM-Newton

We present the first X-ray observation of Jupiter by XMM-Newton. Images taken with the EPIC cameras show prominent emission, essentially all confined to the 0.2-2.0 keV band, from the planet's auroral spots; their spectra can be modelled with a combination of unresolved emission lines of highly ionised oxygen (OVII and OVIII), and a pseudo-continuum which may also be due to the superposition of many weak lines. A 2.8 sigma enhancement in the RGS spectrum at 21-22 A (~0.57 keV) is consistent with an OVII identification. Our spectral analysis supports the hypothesis that Jupiter's auroral emissions originate from the capture and acceleration of solar wind ions in the planet's magnetosphere, followed by X-ray production by charge exchange. The X-ray flux of the North spot is modulated at Jupiter's rotation period. We do not detect evidence for the ~45 min X-ray oscillations observed by Chandra more than two years earlier. Emission from the equatorial regions of the planet's disk is also observed. Its spectrum is consistent with that of scattered solar X-rays.

Acceleration of [TSUP]3[/TSUP]H[CLC]e[/CLC] Nuclei at Interplanetary Shocks

We have surveyed the 0.5-2.0 MeV nucleon-1 ion composition of 56 interplanetary (IP) shocks observed with the Ultra-Low-Energy Isotope Spectrometer on board the Advanced Composition Explorer from 1997 October 1 through 2000 November 30. Our results show the first ever measurement (25 cases) of 3He ions being accelerated at IP shocks. The 3He/4He ratio at the 25 shocks exhibited a wide range of values between 0.0014 and 0.24; the ratios were enhanced between factors of ~3 and 600 over the solar wind value. During the survey period, the occurrence probability of 3He-rich shocks increased with rising solar activity as measured in terms of the daily occurrence rates of sunspots and X-ray flares. The 3He enhancements at IP shocks cannot be attributed to a rigidity-dependent acceleration of solar wind ions and are better explained if the shocks accelerate ions from multiple sources, one being remnant impulsive solar flare material enriched in 3He ions. Our results also indicate that the contribution of impulsive flares to the seed population for IP shocks varies from event to event and that the interplanetary medium is being replenished with impulsive material more frequently during periods of increased solar activity.

On cyclotron wave heating and acceleration of solar wind ions in the outer corona

The preferential heating and acceleration of O+5 ions, as observed by Ultraviolet Coronagraph Spectrometer (UVCS) on Solar and Heliospheric Observatory (SOHO) [Kohl et al., 1998] in the solar coronal holes have been interpreted and modeled by invoking wave‐particle cyclotron resonance [Cranmer et al., 1999a, 1999b]. However, in the model of Cranmer et al. [1999a, 1999b] and in other subsequent models the assumption of a rigid slope of the wave spectrum was made in calculating the wave energy absortion by the different ion species. In the present paper it is shown that a self‐consistent treatment of the wave damping and absorption is necessary and leads to substantially different results. On the basis of quasi‐linear theory, the interaction between the ions and the ion‐cyclotron waves [Marsch et al., 1982a; Marsch, 1998] is studied. The total energy conservation equation, including the kinetic energy of the resonant particles and the wave energy, is derived and discussed in detail. The spectral evolution equation for cyclotron waves, when being controlled by the wave growth/damping rate and WKB effects, is solved self‐consistently together with the full set of anisotropic multifluid equations for the ions including the cyclotron‐resonance wave heating and acceleration rates. From the numerical results we reach the following conclusions: (1) It is physically questionable to use a spectrum with a fixed spectral slope near the cyclotron resonance when one calculates the partition of wave energy among the different ionic species and the kinetic degrees of freedom parallel and perpendicular to the magnetic field. This assumption neglects the important effects of wave absorption and the concurrent reshaping of the wave spectrum, and thus leads in the dissipation domain to extremely low amplitudes of the waves and to difficulties in supplying enough energy to balance the wave absorption at the cyclotron resonances. (2) If the spectrum is allowed to evolve self‐consistently and concurrently with the particles' heating and acceleration through wave absorption, such a high perpendicular temperature and corresponding large temperature anisotropy as observed by UVCS do not occur or cannot be maintained. We conclude that the UVCS oxygen ion observations have not yet been explained satisfactorily by the cyclotron‐resonance theory.

Acceleration of Solar Wind Ions by Nearby Interplanetary Shocks: Comparison of Monte Carlo Simulations withUlyssesObservations

Various theoretical techniques have been devised to determine distribution functions of particles accelerated by the first-order Fermi mechanism at collisionless astrophysical shocks. The most stringent test of these models as descriptors of the phenomenon of diffusive acceleration is a comparison of the theoretical predictions with observational data on particle populations. Such comparisons have yielded good agreement between observations at the quasi-parallel portion of the Earth's bow shock and three theoretical approaches, namely, Monte Carlo kinetic simulations, hybrid plasma simulations, and numerical solution of the diffusion-convection equation. Testing of the Monte Carlo method is extended in this paper to the realm of oblique interplanetary shocks: here observations of proton and He2+ distributions made by the SWICS ion mass spectrometer on Ulysses at nearby interplanetary shocks (less than about 3 AU distant from the Sun) are compared with test-particle Monte Carlo simulation predictions of accelerated populations. The plasma parameters used in the simulation are obtained from measurements of solar wind particles and the magnetic field upstream of individual shocks; pickup ions are omitted from the simulations, since they appear, for the most part, at greater heliospheric distances. Good agreement between downstream spectral measurements and the simulation predictions are obtained for two shocks by allowing the parameter λ/rg, the ratio of the mean-free length to the ionic gyroradius, to vary in an optimization of the fit to the data; generally λ/rg 5, corresponding to the case of strong scattering. Simultaneous H+ and He2+ data, presented only for the 1991 April 7 shock event, indicate that the acceleration process is roughly independent of the mass or charge of the species. This naturally arises if all particles interact elastically with a massive background, as occurs in collisionless scattering off a background magnetic field, and is a patent property of the Monte Carlo technique, since it assumes elastic and quasi-isotropic of particles in the local plasma frame.

Acceleration of solar wind ions by oblique interplanetary shocks

This paper compares observations of proton distributions made by the SWICS ion mass spectrometer on Ulysses at nearby interplanetary shocks with Monte Carlo simulations of particle acceleration at oblique collisionless shocks. The shock parameters are obtained from upstream measurements of the solar wind and magnetic field, and the input particles are drawn from convected Maxwellians, representing solar wind particles. Good agreement between downstream spectral measurements and the simulation predictions are obtained by allowing the parameter λ r g , the ratio of the mean-free scattering length to the ionic gyroradius, to vary in an optimization of the fit to the data. Generally λ r g is found to be less than about 20, which corresponds to the case of strong scattering.

On the origins of energetic ions in the Earth's dayside magnetosheath

Energetic ion events in the Earth's dayside subsolar magnetosheath (0900 ‐ 1300 Local Time) are surveyed using data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) Hot Plasma Composition Experiment. Ion species carrying the signature of their origin (O+ and energetic He2+) are used to distinguish between magnetospheric and solar wind origins for the energetic ion events. The results of this survey indicate that the majority of energetic (10–17 keV/e) H+ and He2+ ions observed in the dayside magnetosheath are accelerated from the solar wind population. The energetic He2+ to H+ density ratio in the magnetosheath is consistent with that predicted from first‐order Fermi acceleration of solar wind ions in the turbulent regions upstream and downstream from the Earth's quasi‐parallel bow shock. Although the majority of the energetic ions appear to be of solar wind origin, magnetospheric O+ is also occasionally present in the magnetosheath. The simultaneous occurrence of both energetic He2+ and magnetospheric O+ indicates that, on occasion, both Fermi acceleration of solar wind ions and leakage of magnetospheric ions occurs in the dayside magnetosheath.

Magnetospheric particle injection and the upstream ion event of September 5, 1984

Energetic particle data from the AMPTE Charge Composition Explorer (CCE) spacecraft in the outer dayside magnetosphere are examined during the period of an upstream ion event observed by the AMPTE Ion Release Module (IRM) spacecraft on September 5, 1984 [Moebius et al., this issue]. The CCE data reveal the following: (a) an ion enhancement was observed at ∼0040 UT in near coincidence with a substorm onset at ∼0035 UT, approximately 15 minutes prior to the onset of the event upstream of the shock; (b) ions of both solar wind (He ++, Fe‐group) and ionospheric (O+) origin over a broad energy range (∼20 keV to > 1350 keV) were injected at substorm onset; (c) the time evolution of the H+, He++, and O+ pitch angle distributions markedly differed, with O+ exhibiting mostly enhancements at off‐90° angles for the first hour after injection; (d) an enhancement in the Fe‐group ions inside the magnetosphere at L ∼ 6.4 occurred simultaneously with the appearance of an O+ burst upstream of the shock. The CCE observations, taken together with the simultaneously observed IRM ion event, suggest that a plausible explanation for the appearance of upstream ions is leakage from the magnetosphere into the upstream region, rather than the alternate explanation which requires in‐situ acceleration of solar wind ions via the Fermi mechanism.

Low energy ions in co-rotating interaction regions at 1 AU: Evidence for statistical ion acceleration

The possibility of the statistical acceleration of solar wind ions to energies above 10 keV in the vicinity of co-rotating high speed solar wind streams by scattering from hydromagnetic waves is considered. We find that this process may occur only in the compressed fast stream plasma within the interaction region between the stream interface and the trailing edge, and may account for the energetic ion enhancements observed in this region by Richardson and Zwickl ( Planet. Space Sci. 32, 1179, 1984). When statistical acceleration occurs in the outer heliosphere, the accelerated ions may provide a source population for acceleration at the co-rotating reverse shock.

Suprathermal ions upstream from interplanetary shocks

Suprathermal ions with energies between solar wind thermal energies and ∼29 keV are occasionally observed ahead of outward propagating interplanetary shocks with the Los Alamos/Garching fast plasma experiment on ISEE 1 and 2. Compared with suprathermal ion velocity distributions observed upstream from the earth's bow shock, the upstream interplanetary shock ion velocity distributions are relatively structureless, and the particle fluxes are less intense. Typically the suprathermal ion distribution emerges smoothly from the solar wind thermal distribution and is nearly isotropic in the solar wind frame. Such distributions are observed with the fast plasma experiment only in association with interplanetary shocks. Field‐aligned beams, kidney‐bean‐shaped distributions, shells of ions in velocity space, and bunches of gyrating ions—all common to the upstream region of the earth's bow shock—have not been observed ahead of interplanetary shocks. Highly structured ion velocity distributions observed upstream of the earth's bow shock apparently are caused directly or indirectly by the nearly specular reflection of solar wind ions at the shock, a consequence of the generally high Mach number of the solar wind flow at 1 AU. By way of contrast, most interplanetary shocks at 1 AU have low, subcritical Mach numbers, and solar wind ion reflection at these shocks does not appear to play a role in producing upstream suprathermal ion distributions at 1 AU. Nevertheless, solar wind ions are accelerated to high energies at interplanetary shocks. Leakage of shocked thermal plasma across these low‐Mach‐number shocks from the downstream region may play an important role in producing upstream suprathermal ion populations and may therefore constitute the first step in the acceleration of solar wind ions to high energies at these shocks.

Wave heating and acceleration of solar wind ions by cyclotron resonance

In order to explain the ion temperature anisotropies and differential speeds, as observed in solar wind high‐speed streams, a fluid type model is presented that takes into account ion heating and acceleration by resonant wave‐particle interactions. Ion‐cyclotron and fast magnetosonic waves propagating away from the sun parallel to the interplanetary magnetic field are considered. The radial evolution of the spectral wave energy density and of the bi‐Maxwellian model ion distributions is calculated self‐consistently for a spherically symmetric solar wind geometry. Numerical results are given for the dependence on heliocentric radial distance for the ion parallel and perpendicular temperatures and ion speeds relative to their center of mass frame. The respective ion flow speeds in the inertial frame are also calculated, based on momentum equations that include the self‐consistent temperature gradients. It is shown that transfer of wave momentum to the ions can lead to a preferential acceleration of the alpha particles with respect to the protons. Owing to the combined action of the ion cyclotron and magnetosonic waves, the alphas are accelerated to a differential speed of about the local Alfvèn speed in close accord with in situ observations. By damping of wave energy the heavier ions are also preferentially heated with the result that alpha particle thermal speeds become equal or slightly larger than proton thermal speeds. Typical signatures in ion temperature anisotropies (like Tp⊥ > Tp∥) as predicted by the model agree fairly well with the observations in fast streams. The results are discussed with respect to the effects of various boundary conditions and the inhomogeneity of the expanding solar wind plasma.