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.     

Slow Solar Wind: Observations and Modeling

While it is certain that the fast solar wind originates from coronal holes, where and how the slow solar wind (SSW) is formed remains an outstanding question in solar physics even in the post-SOHO era. The quest for the SSW origin forms a major objective for the planned future missions such as the Solar Orbiter and Solar Probe Plus. Nonetheless, results from spacecraft data, combined with theoretical modeling, have helped to investigate many aspects of the SSW. Fundamental physical properties of the coronal plasma have been derived from spectroscopic and imaging remote-sensing data and in situ data, and these results have provided crucial insights for a deeper understanding of the origin and acceleration of the SSW. Advanced models of the SSW in coronal streamers and other structures have been developed using 3D MHD and multi-fluid equations.However, the following questions remain open: What are the source regions and their contributions to the SSW? What is the role of the magnetic topology in the corona for the origin, acceleration and energy deposition of the SSW? What are the possible acceleration and heating mechanisms for the SSW? The aim of this review is to present insights on the SSW origin and formation gathered from the discussions at the International Space Science Institute (ISSI) by the Team entitled “Slow solar wind sources and acceleration mechanisms in the corona” held in Bern (Switzerland) in March 2014 and 2015.     

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.     

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

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

Cosmic Rays in the Inner Heliosphere: Insights from Observations, Theory and Models

The global modulation of galactic cosmic rays in the inner heliosphere is determined by four major mechanisms: convection, diffusion, particle drifts (gradient, curvature and current sheet drifts), and adiabatic energy losses. When these processes combine to produce modulation, the complexity increases significantly especially when one wants to describe how they evolve spatially in all three dimensions throughout the heliosphere, and with time, as a function of solar activity over at least 22 years. In this context also the global structure and features of the solar wind, the heliospheric magnetic field, the wavy current sheet, and of the heliosphere and its interface with the interstellar medium, play important roles. Space missions have contributed significantly to our knowledge during the past decade. In the inner heliosphere, Ulysses and several other missions have contributed to establish the relative importance of these major mechanisms, leading to renewed interest in developing more sophisticated theories and numerical models to explain these observations, and to understand the underlying physics that determines galactic cosmic ray modulation at Earth. An overview is given of some of the observational and modeling highlights over the past decade.     

Structure of the Solar Wind and Compositional Variations

The composition of the solar wind is largely determined by the composition of the source material, i.e. the present-day composition of the outer convective zone. It is then modified by the processes which operate in the transition region and in the inner corona. In situ measurements of the solar wind composition give a unique opportunity to obtain information on the isotopic and elemental composition of the Sun. However, elemental — and to some degree also isotopic — fractionation can occur in the flow of matter from the outer convective zone into the interplanetary space. The most important examples of elemental fractionation are the well-known FIP/FIT effect (First Ionization Potential/Time) and the sometimes dramatic variations of the helium abundance relative to hydrogen in the solar wind. A thorough investigation of fractionation processes which cause compositional variations in different solar wind regimes is necessary to make inferences about the solar source composition from solar wind observations. Our understanding of these processes is presently improving thanks to the detailed diagnostics offered by the optical instrumentation on SOHO. Correlated observations of particle instruments on Ulysses, WIND, and SOHO, together with optical observations will help to make inferences for the solar composition. Continuous in situ observations of several isotopic species with the particle instruments on WIND and SOHO are currently incorporated into an experimental database to infer isotopic fractionation processes which operate in different solar wind regimes between the solar surface and the interplanetary medium. Except for the relatively minor effects of secular gravitational sedimentation which works at the boundary between the outer convective zone and the radiative zone, refractory elements such as Mg can be used as faithful witnesses to monitor the magnitude of these processes. With theoretical considerations it is possible to make inferences about the importance of isotopic fractionation in the solar wind from a comparison of optical and in situ observations of elemental fractionation with the corresponding models. Theoretical models and preliminary results from particle observations indicate that the combined isotope effects do not exceed a few percent per mass unit. In the worst case, which concerns the astrophysically important 3He/4He ratio, we expect an overall effect of at most several percent in the sense of a systematic depletion of the heavier isotope. Continued observations with WIND, SOHO, and ACE, and, with the revival of the foil technique, with the upcoming Genesis mission will further consolidate our knowledge about the relation between solar wind dynamics and solar wind composition. This revised version was published online in June 2006 with corrections to the Cover Date.     

Physical reasons and consequences of a three-dimensionally structured heliosphere

In this article we have discussed reasons both of solar and of interstellar origin giving rise to a pronounced three-dimensional structure of the expanding solar wind and thus of the global configuration of the heliosphere. Our present observational knowledge on these structurings is reviewed, and all attempts to theoretically model these solar wind structures are critically analysed with respect to their virtues and flaws. It is especially studied here by what mechanisms interstellar imprints on the actual type of solar wind expansion can be envisaged. With concern to this aspect it hereby appears to be of eminent importance that the solar system maintains a relative motion with a submagnetosonic velocity of about 23km/sec with respect to the ambient magnetized interstellar medium corresponding to a magnetosonic Mach number of about 0.5.A heliopause closing the distant heliospheric cavity within a solar distance of about 100AU on the upwind side and opening it into an largely extended tail on the downwind side results as a first consequence from this relative motion. As a second consequence an asymmetric heliospheric shockfront with upwind distances smaller than downwind distances by ratios between 1/3 and 2/3 is most likely provoked which gives rise to at least two important upwind-downwind asymmetric processes influencing the supersonic solar wind expansion downstream from the shock: the anomalous cosmic ray diffusion into the solar wind, and high energetic jet electrons originating at the shock and moving inwards up to an inner critical point at around 20AU. As we shall demonstrate both processes are influencing the solar wind expansion beyond 20AU, however, more efficiently in the upwind hemisphere as compared to the downwind hemisphere. In the region inside 20AU other mechanisms are operating to propagate the interstellar imprint on the solar wind expansion further downstream into the inner heliosphere because here even the original solar wind electrons, in view of the solar wind bulk velocities, behave as a subsonic plasma constituent which can modify the solar wind solutions by means of an appropriate detuning of the circumsolar electric polarisation field. We give quantitative estimates for these effects.What concerns the theory of a solar wind expansion into a counterflowing ambient interstellar medium, some flaws of the present theoretical attempts are identified impeding that the interstellar influence on the actual solar wind solutions can become visible. We thus conclude that there is a clear need for three-dimensional and time-dependent solar wind models with a free outflow geometry taking into account the multisonicity of the solar wind plasma with different eigenmodes for a perturbation propagation.     

Self-Consistent Models of the Solar Wind

The origins of the hot solar corona and the supersonically expanding solar wind are still the subject of much debate. This paper summarizes some of the essential ingredients of realistic and self-consistent models of solar wind acceleration. It also outlines the major issues in the recent debate over what physical processes dominate the mass, momentum, and energy balance in the accelerating wind. A key obstacle in the way of producing realistic simulations of the Sun-heliosphere system is the lack of a physically motivated way of specifying the coronal heating rate. Recent models that assume the energy comes from Alfvén waves that are partially reflected, and then dissipated by magnetohydrodynamic turbulence, have been found to reproduce many of the observed features of the solar wind. This paper discusses results from these models, including detailed comparisons with measured plasma properties as a function of solar wind speed. Some suggestions are also given for future work that could answer the many remaining questions about coronal heating and solar wind acceleration.     

Large-Scale Structure and Dynamics of the Magnetotails of Mercury,Earth, Jupiter and Saturn

Spacecraft observations have established that all known planets with an internal magnetic field, as part of their interaction with the solar wind, possess well-developed magnetic tails, stretching vast distances on the nightside of the planets. In this review paper we focus on the magnetotails of Mercury, Earth, Jupiter and Saturn, four planets which possess well-developed tails and which have been visited by several spacecraft over the years. The fundamental physical processes of reconnection, convection, and charged particle acceleration are common to the magnetic tails of Mercury, Earth, Jupiter and Saturn. The great differences in solar wind conditions, planetary rotation rates, internal plasma sources, ionospheric properties, and physical dimensions from Mercury’s small magnetosphere to the giant magnetospheres of Jupiter and Saturn provide an outstanding opportunity to extend our understanding of the influence of such factors on basic processes. In this review article, we study the four planetary environments of Mercury, Earth, Jupiter and Saturn, comparing their common features and contrasting their unique dynamics.     

Power Law Distributions of Suprathermal Ions in?the?Quiet Solar Wind

At energies above the bulk solar wind and pick-up ion cutoff, observations reveal an interplanetary suprathermal ion population extending to ～1?MeV/nucleon and even higher energies. These suprathermal ions are found under a wide variety of conditions including periods when there are no obvious nearby accelerating shocks. We review the observational properties of these ions in quiet solar wind periods near 1?AU, including transient Corotating Interaction Region (CIR) events, and other, quieter periods in between transient enhancements. The particle energy spectra are power laws close to E ^?1.5 in the range above the solar wind, rolling over at energies of a few hundred keV/nucleon to a few MeV/nucleon. Although the C/O and Fe/O ratios of the tails is close to that of the solar wind, pickup ions and ³He found in the tails indicate sources distinct from the solar wind. We briefly review several mechanisms that have been proposed to explain these ions.     

MHD processes in the outer heliosphere

Conclusion Much has been learned about the structure and dynamics of the outer heliosphere during the last decade as a result observations from the Voyager and Pioneer spacecraft. The large scale of the observations forces one to consider the heliosphere from a new perspective, to think of new dynamical processes, and to introduce new concepts. The early studies of isolated gas dynamic flows must be replaced by MHD dynamics of interacting flows and flow systems. The simple deterministic models that have been dominant in early studies of the solar wind are now seen to have limited applicability, and statistical approaches are being developed. New concepts that have been introduced, such as inverse cascades, filtering, entrainment, etc., must be further explored and clarified, to make them more precise and quantitative. MHD turbulence is probably very important in solar wind dynamics, but the subject is poorly developed from a theoretical point of view. The statistical analysis of solar wind parameters has scarcely begun, but it is clearly necessary for an understanding of complex, large-scale flows. The multitude of possible interactions among shocks and flows of various types needs to be explored systematically with observations, models and analytical theory. Voyagers 1 and 2 and Pioneers 10 and 11 are continuing to move through the outer heliosphere and gather data. The lengthy data reduction procedures require even more care in dealing with the low field strengths, densities and temperatures at large heliocentric distances, and the analysis of the complex flows and fields in the outer heliosphere becomes increasingly difficult. Thus one can expect continued growth of our knowledge of the heliosphere, but comprehensive understanding of the data will take some time. If this review stimulates the specialists in solar wind physics to think critically about the results presented and to remedy the deficiencies of current knowledge of the heliosphere, then it will have served its purpose. It is also hoped that this review will serve to encourage specialists in other fields to bring their talents to bear on heliospheric problems and to transfer results of heliospheric physics to their fields.     

Nuclear processes and accelerated particles in solar flares

Nuclear processes and particle acceleration in solar flares are reviewed. The theory of gamma-ray and neutron production is discussed and results of calculations are compared to gamma-ray, neutron, and charged-particle observations from solar flares. The implications of these comparisons on particle energy spectra, total numbers, anisotropies, electron-to-proton ratios, as well as on acceleration mechanisms and the interaction site, are presented. The information on elemental and isotopic abundances derived from gamma-ray observations is compared to abundances obtained from escaping accelerated particles and other sources.NAS/NRC Resident Research Associate.     

Cosmic-Ray Energy Spectra and Time Variations in the Local Interstellar Medium: Constraints and Uncertainties

The spectra of galactic cosmic rays that are observed inside the heliosphere result from the interaction of the spectra present in the local interstellar medium with the structured but turbulent magnetic field carried by the solar wind. Observational tests of solar modulation theory depend on comparisons between spectra inside and outside the heliosphere. Our knowledge of the local interstellar spectra are indirect, using extrapolations of interplanetary spectra measured at high energies where solar modulation effects are minimal and modeling of the physical processes that occur during particle acceleration and transport in the interstellar medium. The resulting estimates of the interstellar spectra can also be checked against observations of the effects that cosmic rays have on the chemistry of the interstellar medium and on the production of the diffuse galactic gamma-ray background. I review the present understanding of the local galactic cosmic-ray spectra, emphasizing the constraints set by observations and the uncertainties that remain.     

Composition of solar cosmic rays

Summary The general features of the solar particle composition now seem to be clear. The two most abundant components, protons and helium nuclei, have different velocity spectra, similar, but not exactly identical rigidity spectra, and varying relative abundances. The multiply charged nuclei, on the other hand, appear to have the same spectral shape and relative abundances each time measurements are made, at least in the region from 42 to 135 MeV/nucleon. Further, these relative abundances seem to reflect those of the solar atmosphere insofar as comparison can be made. Electrons are rare, but high energy electrons are not expected to be plentiful due to the probable high rate of energy loss caused by synchrotron radiation at the sun. Energetic neutrons were also not expected in large quantity and have not been observed. Finally, there is positive evidence that very small quantities of deuterons exist, probably in an amount which is about 10^-3 or less of the proton abundance.The experimental data indicate that the propagation phenomenon is not purely rigidity dependent. Although the propagation of solar particles is still not well understood, the development of theories which take into account both the general magnetic field and the inhomogeneities in the field seem to hold some promise of explaining the experimental results. The composition data have also established important restraints which any acceleration theory must satisfy, and thereby contributed greatly to the very difficult problem of determining the acceleration mechanism.The similarity of the relative abundance of the energetic solar particles and the nuclei in the sun's photosphere suggested the possibility of having a new means of estimating the solar neon and helium abundances. This very interesting possibility will have to be explored by further testing of the composition of future solar particle events. Finally, it was seen that the composition was a very strong argument against most stars being the principal source of high energy non-solar cosmic rays, and, therefore, special sources, such as supernovae or possibly quasistellar objects, should be considered as much more likely prospects for the origin of cosmic rays.The results which have been obtained thus far on the composition of solar cosmic rays have indicated that further research in this area of study should be very rewarding and of value to many fields of physics. Further data on the composition and relative, as well as absolute, energy spectra of the various components are needed throughout many events. More experiments also should be performed to determine the properties of the rare components, deuterons, tritons, He³ nuclei, electrons, neutrons, and the heavier nuclei. When these experiments are complete, the knowledge which is needed to aid in answering the solar and astrophysical problems discussed in this review should be at hand.     

What Can Be Learned from X-ray Spectroscopy Concerning Hot Gas in the Local Bubble and Charge Exchange Processes?

Both solar wind charge exchange emission and diffuse thermal emission from the Local Bubble are strongly dominated in the soft X-ray band by lines from highly ionized elements. While both processes share many of the same lines, the spectra should differ significantly due to the different production mechanisms, abundances, and ionization states. Despite their distinct spectral signatures, current and past observatories have lacked the spectral resolution to adequately distinguish between the two sources. High-resolution X-ray spectroscopy instrumentation proposed for future missions has the potential to answer fundamental questions such as whether there is any hot plasma in the Local Hot Bubble, and if so what are the abundances of the emitting plasma and whether the plasma is in equilibrium. Such instrumentation will provide dynamic information about the solar wind including data on ion species which are currently difficult to track. It will also make possible remote sensing of the solar wind.     

Integration of Neutron Monitor Data with Spacecraft Observations: a Historical Perspective

Beginning in the early 1950s, data from neutron monitors placed the taxonomy of cosmic ray temporal variations on a firm footing, extended the observations of the Sun as a transient source of high energy particles and laid the foundation of our early concepts of a heliosphere. The first major impact of the arrival of the Space Age in 1957 on our understanding of cosmic rays came from spacecraft operating beyond the confines of our magnetosphere. These new observations showed that Forbush decreases were caused by interplanetary disturbances and not by changes in the geomagnetic field; the existence of both the predicted solar wind and interplanetary magnetic field was confirmed; the Sun was revealed as a frequent source of energetic ions and electrons in the 10–100 MeV range; and a number of new, low-energy particle populations was discovered. Neutron monitor data were of great value in interpreting many of these new results. With the launch of IMP 6 in 1971, followed by a number of other spacecraft, long-term monitoring of low and medium energy galactic and anomalous cosmic rays and solar and interplanetary energetic particles, and the interplanetary medium were available on a continuous basis. Many synoptic studies have been carried out using both neutron monitor and space observations. The data from the Pioneer 10/11 and Voyagers 1/2 deep space missions and the journey of Ulysses over the region of the solar poles have significantly extended our knowledge of the heliosphere and have provided enhanced understanding of many effects that were first identified in the neutron monitor data. Solar observations are a special area of space studies that has had great impact on interpreting results from neutron monitors, in particular the identification of coronal holes as the source of high-speed solar wind streams and the recognition of the importance of coronal mass ejections in producing interplanetary disturbances and accelerating solar energetic particles. In the future, with the new emphasis on carefully intercalibrated networks of neutron monitors and the improved instrumentation for space studies, these symbionic relations should prove to be even more productive in extending our understanding of the acceleration and transport of energetic particles in our heliosphere. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fractionation of SI, NE, and MG Isotopes in the Solar Wind as Measured by Soho/Celias/MTOF

Using the high-resolution mass spectrometer CELIAS/MTOF on board SOHO we have measured the solar wind isotope abundance ratios of Si, Ne, and Mg and their variations in different solar wind regimes with bulk velocities ranging from 330 km/s to 650 km/s. Data indicate a small systematic depletion of the heavier isotopes in the slow solar wind on the order of (1.4±1.3)% per amu (2σ-error) compared to their abundances in the fast solar wind from coronal holes. These variations in the solar wind isotopic composition represent a pure mass-dependent effect because the different isotopes of an element pass the inner corona with the same charge state distribution. The influence of particle mass on the acceleration of minor solar wind ions is discussed in the context of theoretical models and recent optical observations with other SOHO instruments. This revised version was published online in June 2006 with corrections to the Cover Date.