Low Energy Particles in the Global Heliosphere 2001–2004: 1 to 90 AU

Charged particle instrumentation that will be flying on six spacecraft in the heliosphere between 1 and 90 AU during 2001–2004 will provide a global view of the interplanetary medium that has not heretofore been available. Comparative analyses of the data that will be obtained will provide new understanding of the global evolution of heliospheric features such as traveling shock waves, coronal mass ejections, solar activity-produced particle injections, and anomalous cosmic rays. This revised version was published online in August 2006 with corrections to the Cover Date.     

The interplanetary medium and its interaction with the Earth's magnetosphere

This paper reviews the principal results of direct measurements of the plasma and magnetic field by spacecraft close to the Earth (within the heliocentric distance range 0.7–1.5 AU). The paper gives an interpretation of the results for periods of decrease, minimum and increase of the solar activity. The following problems are discussed: the interplanetary plasma (chemical composition, density, solar wind flow speed, temperature, temporal and spatial variation of these parameters), the interplanetary magnetic field (intensity, direction, fluctuations and its origin), some derived parameters characterizing the physical condition of the interplanetary medium; the quasi-stationary sector structure and its connection with solar and terrestrial phenomena; the magnetohydrodynamic discontinuities in the interplanetary medium (tangential discontinuities and collisionless shock waves); the solar magnetoplasma interaction with the geomagnetic field (the collisionless bow shock wave, the magnetosheath, the magnetopause, the Earth's magnetic tail, the internal magnetosphere characteristics), the connection between the geomagnetic activity and the interplanetary medium and magnetosphere parameters; peculiarities in behaviour of the interplanetary medium and magnetosphere during geomagnetic storms; energetic aspects of the geomagnetic storms.     

Interplanetary shock waves generated by solar flares

Recent observational and theoretical studies of interplanetary shock waves associated with solar flares are reviewed. An attempt is made to outline the framework for the genesis, life and demise of these shocks. Thus, suggestions are made regarding their birth within the flare generation process, MHD wave propagation through the chromosphere and inner corona, and maturity to fully-developed coronal shock waves. Their subsequent propagation into the ambient interplanetary medium and disturbing effects within the solar wind are discussed within the context of theoretical and phenomenological models. The latter — based essentially on observations — are useful for a limited interpretation of shock geometric and kinematic characteristics. The former — upon which ultimate physical understanding depends — are used for clarification and classification of the shocks and their consequences within the solar wind. Classification of limiting cases of blast-produced shocks (as in an explosion) or longer lasting ejecta (or piston-driven shocks) will hopefully be combined with the study of the flare process itself.The theoretical approach, in spite of its contribution to clarification of various concepts, contains some fundamental limitations and requires further study. Numerical simulations, for example, depend upon a non-unique set of multi-parameter initial conditions at or near the Sun. Additionally, the subtle but important influence of magnetic fields upon energy transport processes within the solar wind has not been considered in the numerical simulation approach. Similarity solutions are limited to geometrical symmetries and have not exploited their potential beyond the special cases of the blast and the constant-velocity, piston-driven shock waves. These continuum fluid studies will probably require augmentation or even replacement by plasma kinetic theory in special situations when observations indicate the presence of anomalous transport processes. Presently, for example, efforts are directed toward identification of detailed shock structures (as in the case of Earth's bow shock) and of the disturbed solar wind (such as the piston).Further progress is expected with extensive in situ and remote monitoring of the solar wind over a wide range of heliographic radii, longitudes and latitudes.This paper is a revised and updated version of an invited review originally presented at the IUGG XV General Assembly, Moscow, U.S.S.R., 2–14 August 1971.     

Cosmic-ray scintillations and dynamic processes in space

Cosmic-ray scintillations registered by ground-base observations reflect, as a rule, the action of a whole number of processes proceeding in interplanetary space and Earth's magnetosphere. The study of scintillation phenomena in cosmic rays, is, in fact, divided into a number of problems connected with the interaction of charged particles of cosmic radiation with the matter and fields which they encounter in the entire length of their propagation. The cosmic-ray scintillations established by different authors from the data of ground-base and high-altitude devices for quiet and disturbed periods, as well as the theoretical calculations of different models and mechanisms of the origin and development of cosmic-ray scintillations are analyzed. High-frequency scintillations of f 10^-5 Hz are shown to be precursors of an approaching shock wave, scintillations with periods of the order of 10–20 and 40–50 min being most sensitive to disturbances of interplanetary medium near the Earth. Since cosmic rays of different energies are sensitive to different processes in interplanetary space at different distances from the Earth, one can sound the conditions in interplanetary medium up to 10¹⁵ cm from the Earth by measuring particle fluxes at different energy ranges.     

Fields and plasmas in the outer solar system

The most significant information about fields and plasmas in the outer solar system, based on observations by Pioneer 10 and 11 investigations, is reviewed. The characteristic evolution of solar wind streams beyond 1 AU has been observed. The region within which the velocity increases continuously near 1 AU is replaced at larger distances by a thick interaction region with abrupt jumps in the solar wind speed at the leading and trailing edges. These abrupt increases, accompanied by corresponding jumps in the field magnitude and in the solar wind density and temperature, consist typically of a forward and a reverse shock. The existence of two distinct corotating regions, separated by sharp boundaries, is a characteristic feature of the interplanetary medium in the outer solar system. Within the interaction regions, compression effects are dominant and the field strength, plasma density, plasma temperature and the level of fluctuations are enhanced. Within the intervening quiet regions, rarefaction effects dominate and the field magnitude, solar wind density and fluctuation level are very low. These changes in the structure of interplanetary space have significant consequences for the many energetic particles propagating through the medium. The interaction regions control the access to the inner solar system of relativistic electrons from Jupiter's magnetosphere. The interaction regions and shocks appear to be associated with an acceleration of solar protons to MeV energies. Flare-generated shocks are observed to be propagating through the outer solar system with constant speed, implying that the previously recognized deceleration of flare shocks takes place principally near the Sun. Radial gradients in the solar wind and interplanetary field parameters have been determined. The solar wind speed is nearly constant between 1 and 5 AU with only a slight deceleration of 30 km s⁺¹ on the average. The proton flux follows an r ⁺² dependence reasonably well, however, the proton density shows a larger departure from this dependence. The proton temperature decreases steadily from 1 to 5 AU and the solar wind protons are slightly hotter than anticipated for an adiabatic expansion. The radial component of the interplanetary field falls off like r ⁺² and, on the average, the magnitude and spiral angle also agree reasonably well with theory. However, there is evidence, principally within quiet regions, of a significant departure of the azimuthal field component and the field magnitude from simple theoretical models. Pioneer 11 has obtained information up to heliographic latitudes of 16°. Observations of the interplanetary sector structure show that the polarity of the field becomes gradually more positive, corresponding to outward-directed fields at the Sun, and at the highest latitudes the sector structure disappears. These results confirm a prior suspicion that magnetic sectors are associated with an interplanetary current sheet surrounding the Sun which is inclined slightly to the solar equator.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

Multi-Wavelength Observations of CMEs and Associated Phenomena

This chapter reviews how our knowledge of CMEs and CME-associated phenomena has been improved, since the launch of the SOHO mission, thanks to multi-wavelength analysis. The combination of data obtained from space-based experiments and ground based instruments allows us to follow the space-time development of an event from the bottom of the corona to large distances in the interplanetary medium. Since CMEs originate in the low solar corona, understanding the physical processes that generate them is strongly dependant on coordinated multi-wavelength observations. CMEs display a large diversity in morphology and kinematic properties, but there is presently no statistical evidence that those properties may serve to group them into different classes. When a CME takes place, the coronal magnetic field undergoes restructuring. Much of the current research is focused on understanding how the corona sustains the stresses that allow the magnetic energy to build up and how, later on, this magnetic energy is released during eruptive flares and CMEs. Multi-wavelength observations have confirmed that reconnection plays a key role during the development of CMEs. Frequently, CMEs display a rather simple shape, exhibiting a well known three-part structure (bright leading edge, dark cavity and bright knot). These types of events have led to the proposal of the ‘`standard model’' of the development of a CME, a model which predicts the formation of current sheets. A few recent coronal observations provide some evidence for such sheets. Other more complex events correspond to multiple eruptions taking place on a time scale much shorter than the cadence of coronagraph instruments. They are often associated with large-scale dimming and coronal waves. The exact nature of these waves and the physical link between these different manifestations are not yet elucidated. We also discuss what kind of shocks are produced during a flare or a CME. Several questions remain unanswered. What is the nature of the shocks in the corona (blast-wave or piston-driven?) How they are related to Moreton waves seen in Hα? How they are related to interplanetary shocks? The last section discusses the origin of energetic electrons detected in the corona and in the interplanetary medium. “Complex type III-like events,”which are detected at hectometric wavelengths, high in the corona, and are associated with CMEs, appear to originate from electrons that have been accelerated lower in the corona and not at the bow shock of CMEs. Similarly, impulsive energetic electrons observed in the interplanetary medium are not the exclusive result of electron acceleration at the bow shocks of CMEs; rather they have a coronal origin.     

Ground-based solar radio observations of the August 1972 events

Ground-based observations of the variable solar radio emission ranging from few millimetres to decametres have been used here as a diagnostic tool to gain coherent phenomenological understanding of the great 2, 4 and 7 August, 1972 solar events in terms of dominant physical processes like generation and propagation of shock waves in the solar atmosphere, particle acceleration and trapping.The basic data used in this review have been collected by many workers throughout the world utilizing a variety of instruments such as fixed frequency radiometers, multi-element interferometers, dynamic spectrum analysers and polarimeters. Four major flares are selected for detailed analysis on the basis of their ability to produce energetic protons, shock waves, polar cap absorptions (PCA) and sudden commencement (SC) geomagnetic storms. A comparative study of their radio characteristics is made. Evidence is seen for the pulsations during microwave bursts by the mechanism similar to that proposed by McLean et al. (1971), to explain the pulsations in the metre wavelength continuum radiation. It is suggested that the multiple peaks observed in some microwave bursts may be attributable to individual flares occurring sequentially due to a single initiating flare. Attempts have been made to establish identification of Type II bursts with the interplanetary shock waves and SC geomagnetic storms. Furthermore, it is suggested that it is the mass behind the shock front which is the deciding factor for the detection of shock waves in the interplanetary space. It appears to us that more work is necessary in order to identify which of the three moving Type IV bursts (Wild and Smerd, 1972), namely, advancing shock front, expanding magnetic arch and ejected plasma blob serves as the piston-driver behind the interplanetary shocks. The existing criteria for proton flare prediction have been summarized and two new criteria have been proposed. Observational limitations of the current ground-based experimental techniques have been pointed out and a suggestion has been made to evolve appropriate observational facilities for solar work before the next Solar Maximum Year (SMY).     

Magnetic Flux Emergence, Activity, Eruptions and Magnetic Clouds: Following Magnetic Field from the Sun to the Heliosphere

We present an overview of how the principal physical properties of magnetic flux which emerges from the toroidal fields in the tachocline through the turbulent convection zone to the solar surface are linked to solar activity events, emphasizing the effects of magnetic field evolution and interaction with other magnetic structures on the latter. We compare the results of different approaches using various magnetic observables to evaluate the probability of flare and coronal mass ejection (CME) activity and forecast eruptive activity on the short term (i.e. days). Then, after a brief overview of the observed properties of CMEs and their theoretical models, we discuss the ejecta properties and describe some typical magnetic and composition characteristics of magnetic clouds (MCs) and interplanetary CMEs (ICMEs). We review some individual examples to clarify the link between eruptions from the Sun and the properties of the resulting ejecta. The importance of a synthetic approach to solar and interplanetary magnetic fields and activity is emphasized.     

Magnetohydrodynamic discontinuities in space plasmas: Interrelation between stability and structure

It is believed that shock waves and other discontinuous flows form the basis of a wide range of phenomena in space medium. We review the results concerning the particular property of MHD discontinuities, the interrelation between their stability and structure. Such an interrelation is associated primarily with the requirement of their evolutionarity. For a non-evolutionary discontinuity the amplitudes of reflected and refracted waves caused by a small amplitude incident wave are not determined unambiguously from the conservation laws at the discontinuity surface. Since the problem of the further time evolution of the initial small perturbation does not have a unique solution, such a discontinuity cannot exist in a real medium as a single steady-state configuration. Therefore it is unstable with respect to a disintegration into several discontinuities or to a transition to some more general unsteady flow. This is confirmed by the fact that, in the studied cases, the non-evolutionary shock transitions can, while the evolutionary ones cannot, be represented as a set of several discontinuities and waves of rarefaction. One more fundamental property of the non-evolutionary shocks that argues for their non-existence is that they do not have a unique structure for all values of the dissipative transport coefficients. At the same time, the possibility of their existence cannot be excluded when the shock has a unique structure or when the disintegration is forbidden for some reasons. Besides the non-evolutionarity, which is a direct reason for a disintegration, there is an indirect one. It can be shown that the hydrodynamic shocks without magnetic field that are corrugationally unstable also allow the shock transitions through more than one discontinuity. This suggests that the shocks unstable in the ordinary sense, for which the small perturbation grows with time, do not exist at all, but they disintegrate into stable ones. However, the physical mechanism that distinguishes between these two scenarios remains unclear. The interrelation between the stability and structure of MHD discontinuities thus governs their nonlinear evolution. This fact is essential when describing shock waves and other discontinuous flows in the space medium.     

An Introduction to Theory and Models of CMEs, Shocks, and Solar Energetic Particles

We present a brief introduction to the essential physics of coronal mass ejections as well as a review of theory and models of CME initiation, solar energetic particle (SEP) acceleration, and shock propagation. A brief review of the history of CME models demonstrates steady progress toward an understanding of CME initiation, but it is clear that the question of what initiates CMEs has still not been solved. For illustration, we focus on the flux cancellation model and the breakout model. We contrast the similarities and differences between these models, and we examine how their essential features compare with observations. We review the generation of shocks by CMEs. We also outline the theoretical ideas behind the origin of a gradual SEP event at the evolving CME-driven coronal/interplanetary shock and the origin of “impulsive” SEP events at flare sites of magnetic reconnection below CMEs. We argue that future developments in models require focused study of “campaign events” to best utilize the wealth of available CME and SEP observations.     

Shock Wave Interaction with the Magnetopause

The magnetopause is in continuous motion and shock waves and impulsive acceleration events can occur. As an example, we show that the interaction of an interplanetary shock with the bow shock can generate a shock wave that after passing through the magnetosheath can interact with the magnetopause. In fluid dynamics, when a shock wave encounters a fluid discontinuity, the interface may become unstable and form bubbles and spikes. We consider this Richtmyer-Meshkov instability in magnetohydrodynamics. At the dayside magnetopause, the instability tends to be stabilized by the magnetic field. However, the shock wave interaction can initiate magnetic field reconnection for the southward IMF, which may be important in strong interplanetary shock events. This revised version was published online in August 2006 with corrections to the Cover Date.     

Initial ISEE magnetometer results: Shock observation

ISEE-1 and -2 magnetic field profiles across 6 terrestrial bow shock and one interplanetary shock are examined. The interplanetary shock illustrates the behavior of a low Mach number shock. It had an upstream whistler wave precursor with an apparent wavelength of 180 km. The shock thickness was about 90 km for the thickness of the final field jump or 270 km for the exponential growth of the precursor wave packet. The ion inertial length was 50 km, upstream of the shock.Three examples of low or moderate , high Mach number, quasiperpendicular shocks are examined. These did not have upstream waves, but rather had waves growing in the field gradient. The growth length for these waves and the shock profile was of the order of the ion inertial length.Two examples of high shocks showed little coherence in field variation even though the two vehicles were only a few hundred kilometers apart. Thus, we cannot gauge their velocity and turn the time profiles into distances. The final crossing examined shows clearly the effect of changing the orientation of the interplanetary magnetic field. Initially the upstream magnetic field made an angle of about 80° to the shock normal and the shock position remained fairly steady. Then the field rotated to 45° to the normal and the field profiles became very irregular and the shock position very unstable. Discrete wave packets appeared.Finally, we present the joint behavior of wave, particle and field data across some of these shocks to show some of the myriad of shock features whose behavior we are now beginning to investigate.     

Isotopic Fractionation by Plasma Processes

The Sun is the largest reservoir of matter in the solar system, which formed 4.6 Gy ago from the protosolar nebula. The solar wind carries a nearly unfractionated sample of heavy isotopes at energies of about 1 keV/amu from the Sun into interplanetary space. Data from space missions and theoretical models indicate that the isotopes of the volatile elements N, O, Ne, and Ar are fractionated by at most a few percent per atomic mass unit in different solar wind regimes. In contrast, isotopic abundances of solar and heliospheric energetic particles at energies larger than about 100 keV/amu are observed to strongly vary relative to solar abundances. Processes such as resonant acceleration or pre-acceleration by plasma waves, first-order Fermi acceleration, or propagation in the interplanetary plasma are discussed as causes for charge-to-mass dependent fractionation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Direct correlations between coronal transients and interplanetary disturbances

Major interplanetary shock waves have often been successfully associated with major solar flares. The interplanetary response to weaker solar events, e.g., eruptive prominences (EP) and slow coronal transients, is far less pronounced. Recently, progress has been made by combining the newly-available data of white-light-coronagraph measurements from the earth-orbiting satellite P78/1 (these data show the development of coronal transients between 2.5 and 10 R _bd, in-situ plasma measurements from the HELIOS solar probes positioned mostly above the Sun's limb at solar distances between 60 and 200 R _bd (showing the reactions of the interplanetary plasma), ground based Hα-coronagraphs (showing in a few cases the evolution of EP's from the Sun's limb up to 1.5 A_bd). In the years 1979 to 1981 about 25 uniquely associated events were identified, 19 of which allow some detailed analysis. The events can be sorted into three main categories:

The ‘flare-type’: 13 events, probably all of them flare-related, transient speeds v _t from 560 to 1460 km s^?1, no evidence for post-acceleration of the transient (indicating impulsive injection), all transients followed by drastic interplanetary shock waves, some of them probably involving magnetic clouds.

The ‘EP-type’: 4 events, none of them flare-related, at least one was observed as an Hα-EP, transient speed from 200 to 410 km s^-1, all post-accelerated (indicating ‘driven’ injection), all followed by shocks with at least one magnetic cloud, one showing presence of He⁺ and O²⁺ behind the shock.

The ‘NCDE-type’: 2 events, one observed as an Hα-EP, the other without known solar source, v _t , = 130 and 470 km s^?1, one post-accelerated, the other one not, considerable density increase in interplanetary plasma (however, in pressure equilibrium with surroundings), one event including shock, the other not. These two events may not belong to the same category.

Our results are not completely consistent with previous work which is mainly based on data from the Skylab era, 1973/74. This could be due to the different phase in the solar cycle. The study is being continued.     

CME Theory and Models

This chapter provides an overview of current efforts in the theory and modeling of CMEs. Five key areas are discussed: (1) CME initiation; (2) CME evolution and propagation; (3) the structure of interplanetary CMEs derived from flux rope modeling; (4) CME shock formation in the inner corona; and (5) particle acceleration and transport at CME driven shocks. In the section on CME initiation three contemporary models are highlighted. Two of these focus on how energy stored in the coronal magnetic field can be released violently to drive CMEs. The third model assumes that CMEs can be directly driven by currents from below the photosphere. CMEs evolve considerably as they expand from the magnetically dominated lower corona into the advectively dominated solar wind. The section on evolution and propagation presents two approaches to the problem. One is primarily analytical and focuses on the key physical processes involved. The other is primarily numerical and illustrates the complexity of possible interactions between the CME and the ambient medium. The section on flux rope fitting reviews the accuracy and reliability of various methods. The section on shock formation considers the effect of the rapid decrease in the magnetic field and plasma density with height. Finally, in the section on particle acceleration and transport, some recent developments in the theory of diffusive particle acceleration at CME shocks are discussed. These include efforts to combine self-consistently the process of particle acceleration in the vicinity of the shock with the subsequent escape and transport of particles to distant regions.     

Type II Solar Radio Bursts: Theory and Space Weather Implications

Recent data and theory for type II solar radio bursts are reviewed, focusing on a recent analytic quantitative theory for interplanetary type II bursts. The theory addresses electron reflection and acceleration at the type II shock, formation of electron beams in the foreshock, and generation of Langmuir waves and the type II radiation there. The theory's predictions as functions of the shock and plasma parameters are summarized and discussed in terms of space weather events. The theory is consistent with available data, has explanations for radio-loud/quiet coronal mass ejections (CMEs) and why type IIs are bursty, and can account for empirical correlations between type IIs, CMEs, and interplanetary disturbances. This revised version was published online in August 2006 with corrections to the Cover Date.     

Radio science investigations with Voyager

The planned radio science investigations during the Voyager missions to the outer planets involve: (1) the use of the radio links to and from the spacecraft for occultation measurements of planetary and satellite atmospheres and ionospheres, the rings of Saturn, the solar corona, and the general-relativistic time delay for radiowave propagation through the Sun's gravity field; (2) radio link measurements of true or apparent spacecraft motion caused by the gravity fields of the planets, the masses of their larger satellites, and characteristics of the interplanetary medium; and (3) related measurements which could provide results in other areas, including the possible detection of long-wavelength gravitational radiation propagating through the Solar System. The measurements will be used to study: atmospheric and ionospheric structure, constituents, and dynamics; the sizes, radial distribution, total mass, and other characteristics of the particles in the rings of Saturn; interior models for the major planets and the mean density and bulk composition of a number of their satellites; the plasma density and dynamics of the solar corona and interplanetary medium; and certain fundamental questions involving gravitation and relativity. The instrumentation for these experiments is the same ground-based and spacecraft radio systems as will be used for tracking and communicating with the Voyager spacecraft, although several important features of these systems have been provided primarily for the radio science investigations.     

双弯曲入射激波的可控中心体内收缩基准流场设计

采用有旋特征线法设计了一种双弯曲入射激波的可控中心体内收缩基准流场,两道入射激波交于中心体起始点,入射激波和反射激波通过给定激波径向总压恢复系数分布进行反设计,壁面通过给定轴向马赫数分布规律进行反设计.该基准流场分为“三波四区”且压缩效率较高.基于该基准流场设计了圆形进口内收缩进气道并进行了黏性修正,数值计算结果表明:内收缩进气道设计点核心区的流场特征和激波形状与基准流场基本一致;在来流马赫数为4.0~7.0时进气道具有较高的压缩效率和良好的流量捕获能力,设计点喉道截面增压比和总压恢复系数分别为17.7和0.729;来流马赫数为5.0~7.0时内部总阻力系数变化平缓,从0.23下降为0.22.      

Solar modulation of galactic cosmic radiation

In this review an attempt is made to present an integrated view of the solar modulation process that cause time variation of cosmic ray particles. After briefly surveying the relevant large and small scale properties of the interplanetary magnetic fields and plasma, the motion of cosmic ray particles in the disordered interplanetary magnetic fields is discussed. The experimentally observed long term variations of different species of cosmic ray particles are summarised and compared with the theoretical predictions from the diffusion-convection model. The effect of the energy losses due to decelaration in the expanding solar wind are clearly brought out. The radial density gradient, the modulation parameter and their long term variation are discussed to understand the dynamics of the modulating region. The cosmic ray anisotropy measurements at different energies are summarised. At high energies (E 1 GeV), the average diurnal anisotropy is shown to be energy independent and along the 18.00 h direction consistent with their undergoing partial corotation with the sun. The average semi-diurnal anisotropy seems to vary with energy as E ⁺¹ and incident from a direction perpendicular to the interplanetary field line, consistent with the semi-diurnal component being produced by latitudinal gradients. Both the diurnal and semi-diurnal components are shown to be practically time invariant. On a day to day basis, however, the anisotropy characteristics such as the exponent of variation, the amplitude and the phase show very high variability which are interpreted in terms of convection and variable field aligned diffusion due to the redistribution of the galactic cosmic ray density following transient changes in the interplanetary medium. The anisotropy observation at low energies (E 100 MeV) are, however, not explained by the theory.The rigidity dependence and the anisotropies during short term variations such as Forbush decreases are discussed in terms of the proposed field models for the interplanetary field structure and are compared with the observed rigidity dependence of long term variations. The data pertaining to the 27 day corotating Forbush decreases and their association with enhanced diurnal variation are also presented. The relationship between the energetic storm particle events which are caused by the acceleration of particles in the shock fronts and the Forbush decreases which are caused by the exclusion of galactic particles by the enhanced field structure in the same fronts are clearly brought out. Thus the recurrent increases at low energies and recurrent decreases at high energies may both be caused by the field structure in the shock front. In conclusion, the properties of the very short period fluctuations (18–25 cph) are summarised.