Acceleration and Composition of Solar Wind Suprathermal Tails

Observations in the solar wind have revealed important insights into how energetic particles are accelerated in astrophysical plasmas. In circumstances where stochastic acceleration is expected, a suprathermal tail on the distribution function is formed with a common spectral shape: the spectrum is a power law in particle speed with a spectral index of −5. Recent theories for this phenomenon, in which thermodynamic constraints are applied to explain the common spectral shape, are reviewed. As an example of potential extensions of this theoretical work, consideration is given to the acceleration of Anomalous Cosmic Rays in the heliosheath.     

Helios: Evolution of Distribution Functions 0.3–1 AU

The radial evolution of the velocity distribution functions of the protons, electrons and ions, as they were measured during the Helios mission in the solar wind between 0.3 and 1.0 AU, is discussed and analysed. Emphasis is placed on the detailed plasma measurements, and on the non-thermal features of the particles and the kinetic processes they undergo in the expanding solar wind. As the plasma is multi-component and nonuniform, complexity prevails and the observed distributions exhibit, owing to their low number densities, significant deviations from local thermal equilibrium, and reveal such suprathermal particles as the strahl electrons, as well as ion beams and temperature anisotropies. The distribution functions still carry imprints of their solar boundaries that are reflected locally, but also have ample free energy driving in situ plasma instabilities which are triggered and modulated by wave-particle interactions. The ion temperatures and their anisotropies and the non-adiabatic radial evolution of the solar wind internal energy are discussed in detail.     

Topics in Microphysics of Relativistic Plasmas

Astrophysical plasmas can have parameters vastly different from the more studied laboratory and space plasmas. In particular, the magnetic fields can be the dominant component of the plasma, with energy-density exceeding the particle rest-mass energy density. Magnetic fields then determine the plasma dynamical evolution, energy dissipation and acceleration of non-thermal particles. Recent data coming from astrophysical high energy missions, like magnetar bursts and Crab nebula flares, point to the importance of magnetic reconnection in these objects. In this review we outline a broad spectrum of problems related to the astrophysical relevant processes in magnetically dominated relativistic plasmas. We discuss the problems of large scale dynamics of relativistic plasmas, relativistic reconnection and particle acceleration at reconnecting layers, turbulent cascade in force-fee plasmas. A number of astrophysical applications are also discussed.     

Microphysics in Astrophysical Plasmas

Although macroscale features dominate astrophysical images and energetics, the physics is controlled through microscale transport processes (conduction, diffusion) that mediate the flow of mass, momentum, energy, and charge. These microphysical processes manifest themselves in key (all) boundary layers and also operate within the body of the plasma. Crucially, most plasmas of interest are rarefied to the extent that classical particle collision length- and time-scales are long. Collective plasma kinetic phenomena then serve to scatter or otherwise modify the particle distribution functions and in so-doing govern the transport at the microscale level. Thus collisionless plasmas are capable of supporting thin shocks, current sheets which may be prone to magnetic reconnection, and the dissipation of turbulence cascades at kinetic scales. This paper lays the foundation for the accompanying collection that explores the current state of knowledge in this subject. The richness of plasma kinetic phenomena brings with it a rich diversity of microphysics that does not always, if ever, simply mimic classical collision-dominated transport. This can couple the macro- and microscale physics in profound ways, and in ways which thus depend on the astrophysical context.     

Heliospheric Lessons for Galactic Cosmic-ray Acceleration

The heliosphere is bathed in the supersonic solar wind, which generally creates shocks at any obstacles it encounters: magnetic structures such as coronal mass ejections and planetary magnetospheres, or fast-slow stream interactions such as corotating interaction regions (CIRs) or the termination shock. Each of these shock structures has an associated energetic particle population whose spectra and composition contain clues to the acceleration process and the sources of the particles. Over the past several years, the solar wind composition has been systematically studied, and the long-standing gap between high energy (>1 MeV amu^–1) and the plasma ion populations has been closed by instruments capable of measuring the suprathermal ion composition. In CIRs, where it has been possible to observe all the relevant populations, it turns out that the suprathermal ion population near 1.8–2.5 times the solar wind speed is the seed population that gets accelerated, not the bulk particles near the solar wind peak. These new results are of interest to the problem of Galactic Cosmic-Ray (GCR) Acceleration, since the injection and acceleration of GCRs to modest energies is likely to share many features with processes we can observe in detail in the heliosphere.     

The Seed Population for Energetic Particles Accelerated by CME-Driven Shocks

Understanding properties of solar energetic particle (SEP) events associated with coronal mass ejections has been identified as a key problem in solar-terrestrial physics. Although recent CME shock acceleration models are highly promising, detailed agreement between theoretical predictions and observations has remained elusive. Recent observations from ACE have shown substantial enrichments in the abundances of ³He and He⁺ ions which are extremely rare in the thermal solar wind plasma. Consequently, these ions act as tracers of their source material, i.e., ³He ions are flare suprathermals and He⁺ ions are interstellar pickup ions. The average heavy ion composition also exhibits unsystematic differences when compared with the solar wind values, but correlates significantly with the ambient suprathermal material abundances. Taken together these results provide compelling evidence that CME-driven shocks draw their source material from the ubiquitous but largely unexplored suprathermal tail rather than from the more abundant solar wind peak. However, the suprathermal energy regime has many more contributors and exhibits much larger variability than the solar wind, and as such needs to be investigated more thoroughly. Answers to fundamental new questions regarding the preferred injection of the suprathermal ions, the spatial and temporal dependence of the various sources, and the causes of their variability and their effects on the SEP properties are needed to improve agreement between the simulations and observations.     

Plasma kinetics in the solar wind

An account is given of the observations and theoretical ideas concerning the role of kinetic processes in the solar wind. This includes, first of all, the measurements on distribution functions of plasma electrons and protons, the relation of the observed non-thermal electron features with the concept of an exospheric expansion of the solar corona, and the connection of non-thermal proton distributions with bulk flow inhomogeneities of the wind. A discussion is given of the present understanding of the connection between observed features of the particle distributions and anomalous values of some plasma transport coefficients, which in turn determine the actual values of macroscopic plasma parameters.A further topic of the review is that of possible kinetic processes occurring within small scale structures in the solar wind, like collisionless shocks, various types of discontinuities and D-sheets.     

Transonic Flows in Two-Fluid Plasmas

Transonically rotating toroidal plasmas occur at all scales in the plasma universe and, recently, also in laboratory tokamak plasmas. This offers great opportunities for new insights of the effects of transonic transitions on the background equilibrium flows, and on the waves and instabilities excited. Transfer of knowledge and computational methods on MHD and two-fluid waves and instabilities in magnetically confined laboratory fusion plasmas to space and astrophysical plasmas is seriously hampered though by two related difficulties:

1. in contrast to laboratory plasmas, astrophysical plasmas always have sizeable plasma flows so that they can never be described as a static equilibrium;
2. these flows are usually ‘transonic’, i.e., surpass one of the critical speeds related to the different flow regimes with quite different physical characteristics.

Based on previously obtained MHD results on the stationary states and instabilities of transonically rotating accretion disks about compact objects, the extension to two-fluid plasmas is initiated: A variational principle for the computation of two-fluid stationary states is constructed which involves seven fields determining the different physical variables, and six arbitrary stream functions that should be determined by spatially resolved astrophysical observations. It exhibits all the intricacies due to the electron and ion flow excursions from the magnetic flux surfaces. New hyperbolic flow regimes are found with quite different properties than the MHD ones.     

Notes on Magnetohydrodynamics of Magnetic Reconnection in Turbulent Media

Astrophysical fluids have very large Reynolds numbers and therefore turbulence is their natural state. Magnetic reconnection is an important process in many astrophysical plasmas, which allows restructuring of magnetic fields and conversion of stored magnetic energy into heat and kinetic energy. Turbulence is known to dramatically change different transport processes and therefore it is not unexpected that turbulence can alter the dynamics of magnetic field lines within the reconnection process. We shall review the interaction between turbulence and reconnection at different scales, showing how a state of turbulent reconnection is natural in astrophysical plasmas, with implications for a range of phenomena across astrophysics. We consider the process of magnetic reconnection that is fast in magnetohydrodynamic (MHD) limit and discuss how turbulence—both externally driven and generated in the reconnecting system—can make reconnection independent on the microphysical properties of plasmas. We will also show how relaxation theory can be used to calculate the energy dissipated in turbulent reconnecting fields. As well as heating the plasma, the energy dissipated by turbulent reconnection may cause acceleration of non-thermal particles, which is briefly discussed here.     

Solar flares as sources of energetic particles

In this paper a review is presented of the present status of our knowledge of solar flare phenomena with special emphasis on the production of suprathermal particles and their solar effects. Of these energetic particles electrons play an important role since they produce the X-ray and radiobursts observed during many flares. Also, during their slowing down to thermal energies they contribute to the heating of localized regions in the solar atmosphere, through energy exchange with the ambient electrons. Observable radiations of energetic protons, and other nuclei, are produced through nuclear interactions leading to the emissions of gamma-ray lines. Detectable fluxes of these gamma-ray lines are produced only in the most powerful flares. Also the nuclei that enter into deeper layers of the solar atmosphere transfer most of their kinetic energy to the ambient plasma.     

Wave particle interactions as an energy transfer mechanism between different particle species

Recent improvements in experimental techniques and cooperative data analysis efforts have brought a lot of information on the basic mechanisms by which energy can be exchanged between different particle species in the collisionless magnetospheric or solar wind plasmas. Some of these mechanisms are reviewed. A particular emphasis is put on interactions which occur in the equatorial magnetosphere between energetic protons and electromagnetic ultra low frequency (ULF) waves and which are linked with He⁺ ion trapping and heating as well as with field-aligned suprathermal electron beam generation. The process by which ion conic distributions are produced by electrostatic ion cyclotron waves generated at high altitude along auroral field lines by drifting electrons is also discussed.     

Particle Acceleration in the Heliosphere: Implications for Astrophysics

There has been a remarkable discovery concerning particles that are accelerated in the solar wind. At low energies, in the region where the particles are being accelerated, the spectrum of the accelerated particles is always the same: when expressed as a distribution function, the spectrum is a power law in particle speed with a spectral index of ?5, and a rollover at higher particle speeds that can often be described as exponential. This common spectral shape cannot be accounted for by any conventional acceleration mechanism, such as diffusive shock acceleration or traditional stochastic acceleration. It has thus been necessary to invent a new acceleration mechanism to account for these observations, a pump mechanism in which particles are pumped up in energy through a series of adiabatic compressions and expansions. The conditions under which the pump acceleration is the dominant acceleration mechanism are quite general and are likely to occur in other astrophysical plasmas. In this paper, the most compelling observations of the ?5 spectra are reviewed; the governing equation of the pump acceleration mechanism is derived in detail; the pump acceleration mechanism is applied to acceleration at shocks; and, as an illustration of the potential applicability of the pump acceleration mechanism to other astrophysical plasmas, the pump mechanism is applied to the acceleration of galactic cosmic rays in the interstellar medium.     

Kinetic Excitation Mechanisms for ION-Cyclotron Kinetic Alfvén Waves in Sun-Earth ConnectionI

We study kinetic excitation mechanisms for high-frequency dispersive Alfvén waves in the solar corona, solar wind, and Earth's magnetosphere. The ion-cyclotron and Cherenkov kinetic effects are important for these waves which we call the ion-cyclotron kinetic Alfvén waves (ICKAWs). Ion beams, anisotropic particles distributions and currents provide free energy for the excitation of ICKAWs in space plasmas. As particular examples we consider ICKAW instabilities in the coronal magnetic reconnection events, in the fast solar wind, and in the Earth's magnetopause. Energy conversion and transport initiated by ICKAW instabilities is significant for the whole dynamics of Sun-Earth connection chain, and observations of ICKAW activity could provide a diagnostic/predictive tool in the space environment research. This revised version was published online in August 2006 with corrections to the Cover Date.     

Observation of Injection and Pre-Acceleration Processes in the Slow Solar Wind

Knowledge of injection and pre-acceleration mechanisms of ions is of fundamental importance for understanding particle acceleration that takes place in various astrophysical settings. The heliosphere offers the best chance to study these poorly understood processes experimentally. We examine ion injection and pre-acceleration using measurements of the bulk and suprathermal solar wind, and pickup ions. Our most puzzling observation is that high-velocity tails, extending to at least 60 keV/e - the upper limit of measurements -, are omnipresent in the slow, in-ecliptic solar wind; these tails exist even in the absence of any shocks. The cause of these tails is unknown. In the disturbed solar wind inside CIRs and downstream of shocks and waves these high-speed tails in the distributions of H⁺, He⁺ and He⁺⁺ become more pronounced and more complex, but with the shapes of the tails showing the same dependence on ion speed for the different species. Pickup hydrogen and helium are found to be readily injected for subsequent acceleration to MeV energies, and thus are the dominant source of CIR-accelerated energetic ions. Competing sources of MeV ions heavier than He are: (1) heated suprathermal solar wind observed downstream of CIR shocks, (2) interstellar N, O and Ne, and (3) the newly discovered heavy pickup ions from an extended inner source inside 1 AU. Our main conclusion is that mechanisms other than the traditional first-order shock acceleration process produce most of the modestly accelerated ions seen in the slow solar wind. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Suprathermal Ion Telescope (SIT) For the IMPACT/SEP Investigation

The Solar-Terrestrial Relations Observatory (STEREO) mission addresses critical problems of the physics of explosive disturbances in the solar corona, and their propagation and interactions in the interplanetary medium between the Sun and Earth. The In-Situ-Measurements of Particles and CME Transients (IMPACT) investigation observes the consequences of these disturbances and other transients at 1 AU. The generation of energetic particles is a fundamentally important feature of shock-associated Coronal Mass Ejections (CMEs) and other transients in the interplanetary medium. Multiple sensors within the IMPACT suite measure the particle population from energies just above the solar wind up to hundreds of MeV/nucleon. This paper describes a portion of the IMPACT Solar Energetic Particles (SEP) package, the Suprathermal Ion Telescope (SIT) which identifies the heavy ion composition from the suprathermal through the energetic particle range (～few 10 s of keV/nucleon to several MeV/nucleon). SIT will trace and identify processes that energize low energy ions, and characterize their transport in the interplanetary medium. SIT is a time-of-flight mass spectrometer with high sensitivity designed to derive detailed multi-species particle spectra with a cadence of 60 s, thereby enabling detailed studies of shock-accelerated and other energetic particle populations observed at 1 AU.     

The Solar Wind in the Outer Heliosphere

The solar wind evolves as it moves outward due to interactions with both itself and with the circum-heliospheric interstellar medium. The speed is, on average, constant out to 30 AU, then starts a slow decrease due to the pickup of interstellar neutrals. These neutrals reduce the solar wind speed by about 20% before the termination shock (TS). The pickup ions heat the thermal plasma so that the solar wind temperature increases outside 20–30 AU. Solar cycle effects are important; the solar wind pressure changes by a factor of 2 over a solar cycle and the structure of the solar wind is modified by interplanetary coronal mass ejections (ICMEs) near solar maximum. The first direct evidences of the TS were the observations of streaming energetic particles by both Voyagers 1 and 2 beginning about 2 years before their respective TS crossings. The second evidence was a slowdown in solar wind speed commencing 80 days before Voyager 2 crossed the TS. The TS was a weak, quasi-perpendicular shock which transferred the solar wind flow energy mainly to the pickup ions. The heliosheath has large fluctuations in the plasma and magnetic field on time scales of minutes to days.     

Thermophysical and transport properties of multicomponent gas plasmas at multiple temperatures

For a gas plasma, with the electrons at an elevated temperature with respect to the heavy particles temperature, a general method is presented using the rigorous kinetic theory to calculate the equilibrium composition and transport properties such as the viscosity, the thermal conductivity, the electrical conductivity, the electron diffusion coefficient and the mobility coefficient. The values for the noble gas plasmas and air plasma are determined for various pressures, electron temperature up to 50,000 K and electron to heavy particles temperature ratio up to two. However, the two-temperature calculations are made only for the cases where there are no molecular species present in the gas mixture.     

Energy flux in the Earth's magnetosphere: Storm – substorm relationship

Three ways of the energy transfer in the Earth's magnetosphere are studied. The solar wind MHD generator is an unique energy source for all magnetospheric processes. Field-aligned currents directly transport the energy and momentum of the solar wind plasma to the Earth's ionosphere. The magnetospheric lobe and plasma sheet convection generated by the solar wind is another magnetospheric energy source. Plasma sheet particles and cold ionospheric polar wind ions are accelerated by convection electric field. After energetic particle precipitation into the upper atmosphere the solar wind energy is transferred into the ionosphere and atmosphere. This way of the energy transfer can include the tail lobe magnetic field energy storage connected with the increase of the tail current during the southward IMF. After that the magnetospheric substorm occurs. The model calculations of the magnetospheric energy give possibility to determine the ground state of the magnetosphere, and to calculate relative contributions of the tail current, ring current and field-aligned currents to the magnetospheric energy. The magnetospheric substorms and storms manifest that the permanent solar wind energy transfer ways are not enough for the covering of the solar wind energy input into the magnetosphere. Nonlinear explosive processes are necessary for the energy transmission into the ionosphere and atmosphere. For understanding a relation between substorm and storm it is necessary to take into account that they are the concurrent energy transferring ways. This revised version was published online in August 2006 with corrections to the Cover Date.     

Physical Processes in the Outer Heliosphere

This chapter covers the theory of physical processes in the outer heliosphere that are particularly important for the IBEX Mission, excluding global magnetohydrodynamic/Boltzmann modeling of the entire heliosphere. Topics addressed include the structure and parameters of the solar wind termination shock, the transmission of ions through the termination shock including possible reflections at the shock electrostatic potential, the acceleration and transport of suprathermal ions and anomalous cosmic rays at the termination shock and in the heliosheath, charge-exchange interactions in the outer heliosphere including mass and momentum loading of the solar wind, the transport of interstellar pickup ions, and the production and anticipated intensities of energetic neutral atoms (ENAs) in the heliosphere.