Langmuir Turbulence and Suprathermal Electrons

Charged particle acceleration takes place ubiquitously in the Universe including the near-Earth heliospheric environment. Typical in situ spacecraft measurements made in the solar wind show that the charged particle velocity distribution contains energetic components with quasi scale-free power-law velocity dependence, f～v ^?α , for high velocity range. In this Review a theory of quiet-time solar-wind electrons that contain a suprathermal component is discussed, in which these electrons are taken to be in dynamical equilibrium with Langmuir turbulence. This Review includes an overview of the Langmuir turbulence theory, as well as a discussion on asymptotic equilibrium solution of Langmuir turbulence/suprathermal electron system. Theoretical predictions of high-energy electron velocity power-law distribution index is then compared against the recent observations of the superhalo electron velocity distribution made by instruments onboard WIND and STEREO spacecraft. It is shown that the theoretical prediction of velocity power-law index is intermediate to the observed range.     

Observations of Metals in the Intra-Cluster Medium

Because of their deep gravitational potential wells, clusters of galaxies retain all the metals produced by the stellar populations of the member galaxies. Most of these metals reside in the hot plasma which dominates the baryon content of clusters. This makes them excellent laboratories for the study of the nucleosynthesis and chemical enrichment history of the Universe. Here we review the history, current possibilities and limitations of the abundance studies, and the present observational status of X-ray measurements of the chemical composition of the intra-cluster medium. We summarise the latest progress in using the abundance patterns in clusters to put constraints on theoretical models of supernovae and we show how cluster abundances provide new insights into the star-formation history of the Universe.     

Enrichment of the Hot Intracluster Medium: Observations

Four decades ago, the firm detection of an Fe-K emission feature in the X-ray spectrum of the Perseus cluster revealed the presence of iron in its hot intracluster medium (ICM). With more advanced missions successfully launched over the last 20 years, this discovery has been extended to many other metals and to the hot atmospheres of many other galaxy clusters, groups, and giant elliptical galaxies, as evidence that the elemental bricks of life—synthesized by stars and supernovae—are also found at the largest scales of the Universe. Because the ICM, emitting in X-rays, is in collisional ionisation equilibrium, its elemental abundances can in principle be accurately measured. These abundance measurements, in turn, are valuable to constrain the physics and environmental conditions of the Type Ia and core-collapse supernovae that exploded and enriched the ICM over the entire cluster volume. On the other hand, the spatial distribution of metals across the ICM constitutes a remarkable signature of the chemical history and evolution of clusters, groups, and ellipticals. Here, we summarise the most significant achievements in measuring elemental abundances in the ICM, from the very first attempts up to the era of XMM-Newton, Chandra, and Suzaku and the unprecedented results obtained by Hitomi. We also discuss the current systematic limitations of these measurements and how the future missions XRISM and Athena will further improve our current knowledge of the ICM enrichment.     

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.     

Interplanetary Lyman α Observations: Intensities from Voyagers and Line Profiles from HST/STIS

We present an analysis of Voyager UVS data obtained between 1993 and mid-2007. These data are used to study the interplanetary background and the hydrogen number density in the outer heliosphere. Two types of observations are studied, first the heliospheric scans performed until 2003 and then the fixed line of sight observations, close to the upwind direction, which are still performed at the end of 2007. We make comparisons with models including multiple scattering and hydrogen distributions derived from self-consistent modeling of the interface region. It is found that there is a remaining discrepancy between models and data. The origin of this difference is unknown but it may be linked to a possible tilting of the heliospheric interface due to the presence of an interstellar magnetic field. We should also estimate alternate sources of emission which are not backscattering of solar photons like collisional excitation of hydrogen in the heliosheath and emission after charge transfer or recombination of proton and electron in HII regions. Line profiles from HST/STIS are also presented.     

Interplanetary Shocks and Solar Wind Structure Approaching Solar Maximum: Helios,IMP-8 and Voyager Observations

We studied solar wind observations of five different spacecraft: Helios 1, Helios 2, IMP-8, Voyager 1 and Voyager 2, from November 1977 to February 1978. In this period the large-scale dynamics of the solar wind near of the ecliptic plane was characterized by transient forward shocks (TFSs), ejecta, unstable corotating interaction regions (CIRs), and complex and variable magnetic sector structures. We identified 12 forward shock events of different origin. We did not find any clear tendency of the shock parameters with heliocentric distance nor longitudinal angle, but comparing the observations of each shock event we found local variations in the shock strength and the mean propagation velocities from one spacecraft to another. These unsystematic variations indicate that there were local deformations of the shock fronts, which we attribute to the inhomogenuos solar wind structure that affects the shock propagation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Global Response of Martian Plasma Environment to an Interplanetary Structure: From Ena and Plasma Observations at Mars

As a part of the global plasma environment study of Mars and its response to the solar wind, we have analyzed a peculiar case of the subsolar energetic neutral atom (ENA) jet observed on June 7, 2004 by the Neutral Particle Detector (NPD) on board the Mars Express satellite. The “subsolar ENA jet” is generated by the interaction between the solar wind and the Martian exosphere, and is one of the most intense sources of ENA flux observed in the vicinity of Mars. On June 7, 2004 (orbit 485 of Mars Express), the NPD observed a very intense subsolar ENA jet, which then abruptly decreased within ∼10 sec followed by quasi-periodic (∼1 min) flux variations. Simultaneously, the plasma sensors detected a solar wind structure, which was most likely an interplanetary shock surface. The abrupt decrease of the ENA flux and the quasi-periodic flux variations can be understood in the framework of the global response of the Martian plasma obstacle to the interplanetary shock. The generation region of the subsolar ENA jet was pushed towards the planet by the interplanetary shock; and therefore, Mars Express went out of the ENA jet region. Associated global vibrations of the Martian plasma obstacle may have been the cause of the quasi-periodic flux variations of the ENA flux at the spacecraft location.     

Interplanetary field effect on the magnetosphere

This paper reviews recent developments in the understanding of the solar-wind magnetosphere interaction process in which the interplanetary magnetic field has been found to play a key role. Extensive correlative studies between the interplanetary magnetic field and the magnetospheric parameters have in the past few years yielded detailed information on the nature of the interaction process and have made possible to follow the sequence of events that are produced inside the magnetosphere in consequence of the solar-wind energy transfer. We summarize the observed effects of the interplanetary magnetic field, its north-south and east-west components in particular, found in various domains of the magnetosphere — dayside magnetopause, polar cap, magnetotail, auroral zone —, and present an overall picture of the solar-wind magnetosphere interaction process. Dungey's reconnected magnetosphere model is used as a frame of reference and the basic compatibility of the observations with this model is emphasized. In order to avoid overlap with other review articles in the series discussion on the energy conversion process inside the magnetosphere leading to the substorm phenomenon is kept to the minimal.     

Interplanetary origin of geomagnetic storms

Around solar maximum, the dominant interplanetary phenomena causing intense magnetic storms (Dst<−100 nT) are the interplanetary manifestations of fast coronal mass ejections (CMEs). Two interplanetary structures are important for the development of storms, involving intense southward IMFs: the sheath region just behind the forward shock, and the CME ejecta itself. Whereas the initial phase of a storm is caused by the increase in plasma ram pressure associated with the increase in density and speed at and behind the shock (accompanied by a sudden impulse [SI] at Earth), the storm main phase is due to southward IMFs. If the fields are southward in both of the sheath and solar ejecta, two-step main phase storms can result and the storm intensity can be higher. The storm recovery phase begins when the IMF turns less southward, with delays of ≈1–2 hours, and has typically a decay time of 10 hours. For CMEs involving clouds the intensity of the core magnetic field and the amplitude of the speed of the cloud seems to be related, with a tendency that clouds which move at higher speeds also posses higher core magnetic field strengths, thus both contributing to the development of intense storms since those two parameters are important factors in genering the solar wind-magnetosphere coupling via the reconnection process. During solar minimum, high speed streams from coronal holes dominate the interplanetary medium activity. The high-density, low-speed streams associated with the heliospheric current sheet (HCS) plasma impinging upon the Earth's magnetosphere cause positive Dst values (storm initial phases if followed by main phases). In the absence of shocks, SIs are infrequent during this phase of the solar cycle. High-field regions called Corotating Interaction Regions (CIRs) are mainly created by the fast stream (emanating from a coronal hole) interaction with the HCS plasma sheet. However, because the B_z component is typically highly fluctuating within the CIRs, the main phases of the resultant magnetic storms typically have highly irregular profiles and are weaker. Storm recovery phases during this phase of the solar cycle are also quite different in that they can last from many days to weeks. The southward magnetic field (B_s) component of Alfvén waves in the high speed stream proper cause intermittent reconnection, intermittent substorm activity, and sporadic injections of plasma sheet energy into the outer portion of the ring current, prolonging its final decay to quiet day values. This continuous auroral activity is called High Intensity Long Duration Continuous AE Activity (HILDCAAs). Possible interplanetary mechanisms for the creation of very intense magnetic storms are discussed. We examine the effects of a combination of a long-duration southward sheath magnetic field, followed by a magnetic cloud B_s event. We also consider the effects of interplanetary shock events on the sheath plasma. Examination of profiles of very intense storms from 1957 to the present indicate that double, and sometimes triple, IMF B_s events are important causes of such events. We also discuss evidence that magnetic clouds with very intense core magnetic fields tend to have large velocities, thus implying large amplitude interplanetary electric fields that can drive very intense storms. Finally, we argue that a combination of complex interplanetary structures, involving in rare occasions the interplanetary manifestations of subsequent CMEs, can lead to extremely intense storms. This revised version was published online in June 2006 with corrections to the Cover Date.     

Interplanetary scintillation observations of the high-latitude solar wind

Until the ULYSSES spacecraft reached the polar regions of the solar wind, the only high-latitude measurements available were from indirect techniques. The most productive observations in regions of the solar wind between 5R and 200R have been the family of radio scattering techniques loosely referred to as Interplanetary Scintillation (IPS) (Coles, 1978). Useful observations can be obtained using a variety of radio sources, for example spacecraft beacons, planetary radar echoes and compact cosmic sources (quasars, active galactic nuclei, pulsars, galactic masers, etc.). However for measurement of the high-latitude solar wind cosmic sources provide the widest coverage and this review will be confined to such observations. IPS observations played a very important role in establishing that polar coronal holes (first observed in soft x-ray emission) were sources of fast solar wind streams which occasionally extend down to the equatorial region and are observed by spacecraft. Here I will review the IPS technique and show the variation of both the velocity and the turbulence level with latitude over the last solar cycle. I will also outline recent work and discuss comparisons that we hope to make between IPS and ULYSSES observations.     

Isotopic Signatures of Presolar Materials in Interplanetary Dust

Interplanetary dust particles collected in the stratosphere frequently exhibit enrichments in deuterium (D) and ¹⁵N relative to terrestrial materials. These effects are most likely due to the preservation of presolar interstellar materials. While the elevated D/H ratios probably resulted from mass fractionation during chemical reactions at very low < 100 K temperatures, the origin of the N isotopic anomalies remains unresolved. The bulk of the N-bearing material may have obtained its isotopic signatures from low temperature chemistry, but a nucleosynthetic origin is also possible. This revised version was published online in August 2006 with corrections to the Cover Date.     

Properties of Interplanetary Coronal Mass Ejections

Interplanetary coronal mass ejections (ICMEs) originating from closed field regions on the Sun are the most energetic phenomenon in the heliosphere. They cause intense geomagnetic storms and drive fast mode shocks that accelerate charged particles. ICMEs are the interplanetary manifestations of CMEs typically remote-sensed by coronagraphs. This paper summarizes the observational properties of ICMEs with reference to the ordinary solar wind and the progenitor CMEs.     

Interplanetary streams and their interaction with the earth

Plasma and magnetic field observations of interplanetary streams near 1 AU are summarized. Two types of streams have been identified — corotating streams and flare-associated, and other flow patterns are present due to interactions among streams. The theory of corotating streams, which attributes them to a high temperature region near the Sun, satisfactorily explains many of the effects observed at 1 AU. A correspondingly complete theory of flare-associated streams does not exist. Streams are a key link in the chain that connects solar and geomagnetic activity. The factors that most influence geomagnetic activity are probably related to streams and determined by the dynamics of streams. The evolution of streams on scales of 27 days and 11 years probably determines the corresponding variations of geomagnetic activity.     

Signatures of Energetic Protons and Electrons in the Galaxy

Models of the cosmic-ray, -ray and synchrotron properties of the Galaxy allow conclusions to be drawn about the cosmic-ray injection spectrum and propagation parameters. While the simplest models fail to reproduce the data, reasonable extensions can explain a range of observational facts. Explanations for the diffuse -ray GeV excess found by EGRET are considered; inverse-Compton emission resulting from a hard electron injection spectrum appears most promising. Meanwhile the -ray emission at MeV energies is unlikely to originate entirely from cosmic-ray electrons, and a point source component is required in addition.     

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 Plasma and Suprathermal Ion Composition (PLASTIC) Investigation on the STEREO Observatories

The Plasma and Suprathermal Ion Composition (PLASTIC) investigation provides the in situ solar wind and low energy heliospheric ion measurements for the NASA Solar Terrestrial Relations Observatory Mission, which consists of two spacecraft (STEREO-A, STEREO-B). PLASTIC-A and PLASTIC-B are identical. Each PLASTIC is a time-of-flight/energy mass spectrometer designed to determine the elemental composition, ionic charge states, and bulk flow parameters of major solar wind ions in the mass range from hydrogen to iron. PLASTIC has nearly complete angular coverage in the ecliptic plane and an energy range from ～0.3 to 80 keV/e, from which the distribution functions of suprathermal ions, including those ions created in pick-up and local shock acceleration processes, are also provided.     

The solar WIND and suprathermal ion composition investigation on the WIND spacecraft

The Solar Wind and Suprathermal Ion Composition Experiment (SMS) on WIND is designed to determine uniquely the elemental, isotopic, and ionic-charge composition of the solar wind, the temperatures and mean speeds of all major solar-wind ions, from H through Fe, at solar wind speeds ranging from 175 kms^–1 (protons) to 1280 kms^–1 (Fe⁺⁸), and the composition, charge states as well as the 3-dimensional distribution functions of suprathermal ions, including interstellar pick-up He⁺, of energies up to 230 keV/e. The experiment consists of three instruments with a common Data Processing Unit. Each of the three instruments uses electrostatic analysis followed by a time-of-flight and, as required, an energy measurement. The observations made by SMS will make valuable contributions to the ISTP objectives by providing information regarding the composition and energy distribution of matter entering the magnetosphere. In addition SMS results will have an impact on many areas of solar and heliospheric physics, in particular providing important and unique information on: (i) conditions and processes in the region of the corona where the solar wind is accelerated; (ii) the location of the source regions of the solar wind in the corona; (iii) coronal heating processes; (iv) the extent and causes of variations in the composition of the solar atmosphere; (v) plasma processes in the solar wind; (vi) the acceleration of particles in the solar wind; and (vii) the physics of the pick-up process of interstellar He as well as lunar particles in the solar wind, and the isotopic composition of interstellar helium.     

Solar Origin and Interplanetary Evolution of Stream Interfaces

In a Corotating Interaction Region (CIR) the stream interface is identified as a relatively sharp density drop, temperature rise, and flow shear in the solar wind, and is now generally believed to mark the boundary between solar wind which was originally slow when it left the Sun and solar wind which was originally fast. This paper summarises some important facts and open questions about the origin and nature of the boundary between fast and slow solar wind near the Sun, the evolution of stream interfaces with heliocentric distance in the inner heliosphere, and their relationship. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fe/O Ratios in Interplanetary Shock Accelerated Particles

It is widely accepted that diffusive shock acceleration is an important process in the heliosphere, in particular in producing the energetic particles associated with interplanetary shocks driven by coronal mass ejections. In its simplest formulation shock acceleration is expected to accelerate ions with higher mass to charge ratios less efficiently than those with lower mass to charge. Thus it is anticipated that the Fe/O ratio in shock-accelerated ion populations will decrease with increasing energy above some energy. We examine the circumstances of five interplanetary shocks that have been reported to have associated populations in which Fe/O increases with increasing energy. In each event, the situation is complex, with particle contributions from other sources in addition to the shock. Furthermore, we show that the Fe/O ratio in shock-accelerated ions can decrease even when the shock is traveling through an Fe-rich ambient ion population. Thus, although shock acceleration of an Fe-rich suprathermal population has been proposed to explain large Fe-rich solar particle events, we find no support for this proposal in these observations.