Observations of interplanetary shocks: Recent progress

Interplanetary shock observations since the prior Solar Terrestrial Physics Symposium in 1978 are reviewed. Since the interval coincides with the recent solar maximum, emphasis is placed on shocks associated with transient solar phenomena, including coronal transients and eruptive prominences as well as flares. A good correlation between shocks and Storm Sudden Commencements has persisted into the recent maximum. Shocks have been identified that are associated with disappearing filaments and coronal transients rather than with flares. Significant progress has been made in the indirect observation of shocks near the Sun as a result of radio wave measurements in interplanetary space and measurement of the scintillation and spectral broadening of spacecraft radio transmissions. Preliminary results regarding the thickness of interplanetary shocks have appeared. Several quasi-parallel shocks propagating more nearly along, rather than across, the magnetic field have been identified. The plasma drivers accompanying interplanetary shocks have received increased attention and distinctive features have been found in electron, ion and magnetic field data.     

Acceleration of the solar wind

In this review, we discuss critically recent research on the acceleration of the solar wind, giving emphasis to high-speed solar wind streams emanating from solar coronal holes. We first explain why thermally driven wind models constrained by solar and interplanetary observations encounter substantial difficulties in explaining high speed streams. Then, through a general discussion of energy addition to the solar wind above the coronal base, we indicate a possible resolution of these difficulties. Finally, we consider the question of what role MHD waves might play in transporting energy through the solar atmosphere and depositing it in the solar wind, and we conclude by examining, in a simple way, the specific mechanism of solar wind acceleration by Alfvén waves and the related problem of accelerating massive stellar winds with Alfvén waves.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.On leave from the Auroral Observatory, Institute of Mathematical and Physical Sciences, University of Tromsø, N-9001 Tromsø, Norway.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Transient Effects and Disturbed Conditions

In the present phase of the solar cycle no big transients leading to strong modulation had been observed after 1991. Apart from a few minor disturbances cosmic rays were still recovering to a new intensity maximum. It was suggested, therefore, that existing literature from previous cycles should be critically reviewed. The scene was set by the introductory papers on— phenomenology of cosmic ray modulation in successive solar cycles throughout the heliosphere— the present state of models for long term modulation and their shortcomings— the relation between cosmic ray variations and the magnitude of the interplanetary magnetic field (the CR-B-relation)— charge dependent effects.In the discussions, the study of propagating diffusive disturbances and the CR-B-relation played a central role. The difference was stressed between isolated transient disturbances in the inner solar system (Forbush decreases), and the long lasting, step-like decreases caused by merged interaction regions in the outer heliosphere. The recovery rates following the step-like decreases vary with the phase in the 22-year solar cycle. In some cases this requires a modification of existing drift models. In the outer heliosphere, the CR-B-relation leads to the result 1/ between the diffusion coefficient and the field magnitude . This simple result is a challenge for theoreticians to derive the perpendicular diffusion coefficient fromfirst principles. The three articles in this report essentially follow the list of open points and arguments just presented.The article "Observations and Simple Models" is organised around the model of a propagating diffusive barrier, its application to Forbush effects in the inner heliosphere and to decreases caused by merged interaction regions in the outer heliosphere. Acomparison of observed Forbush decreases with model predictions requires a careful separation of the two steps related to the turbulent region behind the shock front and the closed magnetic field regions of the ejecta (the interplanetary counterparts of coronal mass ejections). It is shown that models for propagating disturbances can be used to derive values of the diffusion coefficients phenomenologically, not only during the disturbance, but also in the ambient medium.The "Modeling of Merged Interaction Regions" summarizes the dynamic and time-dependent process of cosmic ray modulation in the heliosphere. Numerical models with only a time-dependent neutral sheet prove to be successful when moderate to low solar activity occurs but fail to describe large and discrete steps in modulated cosmic rays when solar activity is high. To explain this feature of heliospheric modulation, the concept of global merged interaction regions is required. The com-bination of gradient, curvature and neutral sheet drifts with these global merged interaction regions has so far been the most successful approach in explaining the 11-year and 22-year cycles in the long-term modulation of cosmic rays.The "Remarks on the Diffusion Tensor in the Heliosphere" describe available theories of perpen-dicular diffusion and drift, and discuss their relevance to cosmic rays in the heliosphere. In addition, the information about diffusion coefficients and spatial gradients obtained from the analysis of steady state anisotropies at neutron monitor energies is summarized. These topics are intimately related to the other two articles. They are also part of the general discussion about the "Diffusion Tensor throughout the Heliosphere" which played an important role in all working groups.     

SOHO contribution to the understanding of mass supply and flows in the solar corona

We expect a variety of dynamic phenomena in the quiescent non-flaring corona. Plasma flows, such as siphon flows or convective flows of chromospheric material evaporating into the corona, are expected whenever a pressure differences is established either between the footpoints or between the coronal and chromospheric segments of a coronal loop. Such flows can induce phenomena of spatial and temporal brightness variability of the corona. In particular, evaporation induces a net mass input into the corona and consequently coronal density enhancements. Flows are also expected in the regions where energy is released during magnetic reconnection. From the observational point of view the dynamics of the solar atmosphere has been investigated in great detail mostly in the lower transition region with the HRTS, and during flares with theSolar Maximum Mission andYohkoh. The high spectral, temporal and spatial resolution of theSOHO ultraviolet spectrometers should enable us in the near future to fill the gap providing a continuous coverage from the chromosphere to the corona, in the 10⁴–10⁶ K domain, and therefore to best study the dynamics throughout the solar atmosphere.     

The three-dimensional extent of a high speed solar wind stream

A primary goal of the Ulysses mission is to study the 3-dimensional structures making up the interplanetary medium, and example of which is the high speed solar wind stream observedin situ by Ulysses beginning in July 1992. In order to study the longitudinal extent of this stream as a function of Ulysses' increasing heliographic latitude, a second point of reference is required to separate spatial and temporal variations. Such a reference point is provided at Jupiter by a class of Jovian radio bursts, whose occurrence rate varies in a predictable way with solar wind speed. Using thein situ and remote observations from Ulysses, the extent of the high speed stream at 5 AU is mapped and compared to the associated coronal hole boundary on the Sun.     

Coronal disturbances and their terrestrial effects

Coronal disturbances lead to geomagnetic storms, proton showers, auroras and a wide variety of other phenomena at Earth. Yet, attempts to link interplanetary and terrestrial phenomena to specific varieties of coronal disturbances have achieved only limited success. Here, several recent approaches to prediction of interplanetary consequences of coronal disturbances are reviewed. The relationships of shocks and energetic particles to coronal transients, of proton events to γ-ray bursts, of proton events to microwave bursts, of geomagnetic storms to filament eruptions and of solar wind speed increases to the flare site magnetic field direction are explored. A new phenomenon, transient coronal holes, is discussed. These voids in the corona appear astride the long decay enhancements (LDE's) of 2–50 Å X-ray emission that follow Hα filament eruptions. The transient holes are similar to long-lived coronal holes, which are the sources of high speed solar wind streams. There is some evidence that transient coronal holes are associated with transient solar wind speed increases.     

Probing the solar wind acceleration region using spectroscopic techniques

Measurements of the intensities and profiles of UV and EUV spectral lines can provide a powerful tool for probing the physical conditions in the solar corona out to 8 R and beyond. We discuss here how measurements of spectral line radiation in conjunction with measurements of the white light K-corona can provide information on electron, proton and ion temperatures and velocity distribution functions; densities; chemical abundances and mass flow velocities. Because of the fundamental importance of such information, we provide a comprehensive review of the formation of coronal resonance line radiation, with particular emphasis on the H i L line, and discuss observational considerations such as requirements for rejection of stray light and effects of emission from the geocorona and interplanetary dust. Finally, we summarize some results of coronal H i L and white light observations acquired on sounding rocket flights.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

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.     

Relationship of coronal transients to interplanetary shocks: 3D aspects

More than 1000 coronal mass ejections (CMEs) caused by different types of coronal transients have been analyzed up to now, based on the images from white light coronagraphs on board the OSO 7, Skylab, P78-1, and SMM spacecraft. In many cases, the CME images lead us to the impression of loop-like, more planar structures, similar to those of prominence structures often seen in H pictures. There is increasing evidence, though, for a three-dimensional bubble- or cloud-like structure of CMEs. In several cases, CMEs directed toward the earth (or away from it) were identified, as their outer fronts emerged on all sides of the coronagraph's occulting disk, thus suggesting a bubble-like appearance.There now appears to be unanimity about the crucial role that magnetic reconnection plays during the transient process. Recently, direct evidence was found for the pinch-off of CMEs, both from optical observations and from in situ measurements of isolated magnetic clouds' following transient shock waves. However, the detailed sequence of events during the generation of a CME is still unclear.Interplanetary shock waves associated with the CMEs are usually restricted in latitudinal extent to about the angular width of the optically observed CMEs. They may be somewhat less restricted in longitudinal extent. A nearly 1 1 association between CMEs and shock waves measured in situ from spacecraft (Helios 1 and 2, IMP 7 and 8, ISEE 3, Pioneer Venus) can be established, provided the CME and the spacecraft were in the same longitudinal and latitudinal range and the CME speed exceeds 400 km s^–1. Around the past solar activity minimum all CMEs observed were centered at solar latitudes of less than 60°. Around solar maximum, a significant fraction of CMEs also originated from the polar regions. Thus, there is a good chance that the Ulysses spaceprobe will encounter many shocks caused by both low- and high-latitude CMEs, when it finally starts its journey over the Sun's poles.     

Observations of coronal structure during sunspot maximum

This paper presents some of the results that have been obtained from the Kitt Peak observations of coronal holes and the NRL observations of coronal transients during the recent years near sunspot maximum (1979–1981). On the average, low-latitude coronal holes of comparable size contained 3 times more flux near sunspot maximum than near the previous minimum. In the outer corona, transients occurred at the observed rate of at least 2 per day, and quiet conditions persisted during less than 15 % of the observed days. We describe a sample of the more than 800 events that we have observed so far, including the observation of a comet apparently colliding with the Sun.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.Visiting Astronomer, KPNO.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

The Coronal Helium Abundance Experiment on Spacelab 2

The Coronal Helium Abundance Spacelab Experiment, (CHASE), basically consists of a grazing incidence telescope and spectrometer sensitive over the range 150–1335 Å. Whilst aimed primarily at deriving the solar helium abundance from measurements of coronal resonance scattering, its specification has been extended in order to provide a more general purpose solar XUV facility. The instrument will be flown on the Spacelab 2 Mission, currently scheduled for launch in November 1984.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

Interplanetary shock waves: Recent developments

Direct and indirect observations of interplanetary shock waves have been extended to the study of (i) the shock structure itself; (ii) the disturbed solar wind in its wake; (iii) additional discontinuities such as reverse shocks and pistons; and (iv) the shock's kinematic behavior. The last item — the trajectory — has benefited by the procedure (suggested by Pintér) of matching type II radio drift-inferred velocities with indirectly-inferred initial velocities found from at least two successive measurements in space. The significance of making type II observations at hectometric and kilometric wavelengths (as made, for example, by Slysh and Malitson, Feinberg and Stone) cannot be over-emphasized due to this technique's ability to make unambiguous solar terrestrial relationships. More direct and physically-meaningful observations, however, are still dependent uponin situ plasma and magnetic field measurements. Additional emphasis is presently being placed on numerical modeling of shock-induced disturbances in the solar wind as generated by both flares and stream-stream interactions. The former mechanism is emphasized in this review with several recommendations for further research: (a) further numerical modeling for shocks, starting when they are born within relatively low-Alfvén speed coronal regions; (b) expanded synoptic studies by spacecraft at various heliocentric longitudes, radii, and (eventually) latitudes with coordinated diagnostics; and (c) extended patrol of natural probes, such as comets, augmented with theoretical studies of possible shock-induced mechanical and chemical effects.     

Interdisciplinary summary of solar/interplanetary events during August 1972

The solar/interplanetary events in early August 1972 are summarized in Section 1 (Introduction), Section 2 (August 1972 Events in the Solar Cycle 20), Section 3 (Evolution of Solar Active Region: McMath region No. 11976 and its flare-activity), Section 4 (Radio, X-ray, and Proton Characteristics of Four Major Solar Flares: F-1 at 0316 UT on 2 August, F-2 at 1958 UT on 2 August, F-3 at 0626 UT on 4 August, and F-4 at 1522 UT on 7 August), Section 5 (Interplanetary Shock Waves: observations of the shock waves generated from the four major solar flares at several points in interplanetary space, the Earth, Pioneer-9, Pioneer-10, etc.; interplanetary scintillations; shock trajectories in the heliosphere), Section 6 (Variations of Solar and Galactic Cosmic Rays: four solar proton events observed in the vicinity of the earth and at the Pioneer-9 location in the course of interplanetary disturbances; Forbush decreases of cosmic ray intensity; the spikeshaped variation in solar and galactic cosmic rays on 5 August), and Section 7 (Conclusions).     

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.     

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.     

Large-scale propagation properties of interplanetary disturbances revealed from IPS and spacecraft observations

Interplanetary scintillation (IPS) observations may be used to study large-scale propagation properties of transient interplanetary disturbances in a three-dimensional manner, although current IPS observations have several limitations, e.g., poor time resolution and line-of-sight integration. Comparative studies with spacecraft solar wind and white-light coronal mass ejection (CME) observations are quite helpful in interpretation of IPS observations. An interplanetary disturbance apparently in association with a disappearing solar filament, which took place near the central meridian of the Sun on late 22 April, 1979, is discussed to examine previous deductions from IPS observations. Three-station IPS observations of the flow speed and spacecraft observations suggest that a quasi-spherical interplanetary disturbance was formed around the Sun-Earth line, whereas the center of the disturbance derived from the distribution of enhanced IPS across the sky (g-maps) is located to the east of the Sun-Earth line.Permanently at Research Institute of Atmospherics, Nagoya University, Toyokawa 442, Japan.     

Direct observations of solar-wind particles

Direct observations of solar-wind particles are discussed in detail. A well-defined quiet state of the solar wind is indicated by observations made from 1962 to 1967. The plasma properties in this quiet state are compared with those predicted by hydrodynamic models of the coronal expansion. While the basic flow parameters are predicted with reasonable accuracy by these models, the thermal properties of the solar-wind particles remain largely unexplained. As the interplanetary plasma is not in thermodynamic equilibrium, the thermal properties are determined by the specific energy-transfer mechanisms operative in the plasma. The observed magnitude of the magnetic-field-aligned anisotropy of proton random motions is interpreted as evidence for the presence of instability-generated waves; the effect of these waves on the thermal properties is examined. The observed chemical compositon of the solar wind is discussed and related to the solar origin of the inter-planetary material. Finally, the spatial and temporal structure of the medium is investigated through consideration of patterns of variation in the plasma properties.     

Coronal investigations with occulted spacecraft signals

The radio telemetry links between Earth and a spacecraft near superior conjunction penetrate the corona at ranges well within the acceleration regime of the solar wind. Occultation experiments in the solar corona have been performed on many interplanetary missions beginning with the Mariner and Pioneer series and extending up to the more recent data on Helios, Viking, and Voyager. The changes in group and phase velocity of the radio signal are measured to determine the total electron content of the corona and its fluctuations. The broadening of the carrier signal may be used in combination with the electron content data to derive a solar wind velocity profile. The wave number spectrum of electron density fluctuations in the corona may be inferred from amplitude and phase scintillations of the received signal. Linearly polarized signals, which are rotated along the propagation path by the Faraday effect, can provide information on the coronal magnetic field and its variations.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

Energy coupling between the solar wind and the magnetosphere

This paper describes in detail how we are led to the first approximation expression for the solar wind-magnetosphere energy coupling function , which correlates well with the total energy consumption rate U _T of the magnetosphere. It is shown that is the primary factor which controls the time development of magnetospheric substorms and storms. The finding of this particular expression indicates how the solar wind couples its energy to the magnetosphere; the solar wind and the magnetosphere constitute a dynamo. In fact, the power P generated by the dynamo can be identified as by using a dimensional analysis. Furthermore, the finding of indicates that the magnetosphere is closer to a directly driven system than to an unloading system which stores the generated energy before converting it to substorm and storm energies. Therefore, the finding of and its implications have considerably advanced and improved our understanding of magnetospheric processes. The finding of has also led us to a few specific future problems in understanding relationships between solar activity and magnetospheric disturbances, such as a study of distortion of the solar current disk and the accompanying changes of . It is also pointed out that one of the first tasks in the energy coupling study is an improvement of the total energy consumption rate U _T of the magnetosphere. Specific steps to be taken in this study are suggested.     

Modelling of the interstellar hydrogen distribution in the heliosphere

The detailed knowledge of the distribution of neutral interstellar hydrogen in the interplanetary space is necessary for a reliable interpretation of optical and H⁺ pickup ions observations. In the paper, we review the status of the modelling efforts with the emphasis on recent improvements in that field. We discuss in particular the role of the nonstationary, solar cycle-related effects and the consequences of hydrogen filtration through the heliospheric interface region for its distribution in the inner Solar System. We demonstrate also that the use of the simple cold model, neglecting the thermal character of the hydrogen gas (T 8000 K), is generally incorrect for the whole region of the inner heliosphere (R < 5 AU) since it leads to a substantial underestimation of the local hydrogen density and thus influences the derivation of the H properties in the outer heliosphere/LISM. Referring to recent Ulysses measurements, we point out also the need to consider in the modelling the effects of the latitudinal asymmetry of the ionization rate.