Showing papers on "Coronal mass ejection published in 1991"

Geomagnetic activity associated with earth passage of interplanetary shock disturbances and coronal mass ejections

Abstract: Coronal mass ejection events (CMEs) are important occasional sources of plasma and magnetic field in the solar wind at 1 AU, accounting for approximately 10 percent of all solar wind measurements in the ecliptic plane during the last solar activity maximum. Using a recently appreciated capability for distinguishing CMEs in solar wind data in the form of counterstreaming solar wind electron events, this paper explores the overall effectiveness of shock wave disturbances and CMFs in general in stimulating geomagnetic activity. The study is confined to the interval from mid-August 1978 through mid-October 1982, spanning the last solar activity maximum, when ISEE 3 was in orbit about the L1 Lagrange point 220 Re upstream from earth. It is found that all but one of the 37 largest geomagnetic storms in that era were associated with earth passage of CMEs and/or shock disturbances, with the large majority of these storms (27 out of 37) being associated with interplanetary events where earth encountered both a shock and the CME driving the shock (shock/CME events). Although CMEs and/or shock disturbances were increasingly the cause of geomagnetic activity as the level of geomagnetic activity increased, many smaller geomagnetic disturbances were unrelated to these events.

Solar Interior and Atmosphere

Abstract: In this volume, observational data derived from the world's largest solar telescopes are correlated with theoretical discussions in nuclear and atomic physics by contributors representing a wide range of interests in solar research. Focusing both on processes occurring at the interior of the Sun and on complicated features observed at its surface, these chapters offer both basic explanations of solar phenomena and an overview of present controversies. Three areas of research covered in the volume are of particular interest: the pulsating nature of the Sun and how these oscillations facilitate the measurement of temperature, density, and pressure of its interior, thus revealing the depth of the surface convection zone and the composition of central regions; confirmation of the predicted flux of neutrinos via solar oscillation observations, yielding new speculations that they are produced at the solar centre but are converted to unobservable forms in passing through the Sun as they escape; and the interpretation of magnetic surface structures, based on both groundbased and space observations, in terms of chromosphere heating.

The sun in time

Abstract: Various papers on solar science are presented. The optics considered include: variability of solar irradiance, sunspot number, solar diameter, and solar wind properties; theory of luminosity and radius variations; standard solar models; the sun and the IMF; variations of cosmic-ray flux with time; accelerated particles in solar flares; solar cosmic ray fluxes during the last 10 million yrs; solar neutrinos and solar history; time variations of Be-10 and solar activity; solar and terrestrial components of the atmospheric C-14 variation spectrum; solar flare heavy-ion tracks in extraterrestrial objects. Also addressed are: the faint young sun problem; atmospheric responses to solar irradiation; quaternary glaciations; solar-terrestrial relationships in recent sea sediments; magnetic history of the sun; pre- and main-sequence evolution of solar activity; magnetic activity in pre-main-sequence stars; classical T Tauri stars; relict magnetism of meteorites; luminosity variability of solar-type stars; evolution of angular momentum in solar-mass stars; time evolution of magnetic fields on solarlike stars.

Flares on the Sun and Other Stars

Abstract: Research on solar and stellar flares is reviewed. The characteristics of flares in various spectral regions are described, and the physics of flares is discussed, including the two-ribbon flare model, coronal mass ejections, flare dynamics, white light flare production, and electron beams and proton beams in hard X-ray flares. The analysis of the flare energy budget, magnetic field measurements, multispectral stellar observations, plasma emissivity, conditions in flare loops, cooling curves, and proton beam signatures is addressed. Microflaring and coronal heating are examined.

Heating solar coronal holes

Abstract: It has been shown that the coronal hole, and the associated high-speed stream in the solar wind, are powered by a heat input of the order of 500,000 ergs/sq cm s, with most of the heat injected in the first 1-2 solar radii, and perhaps 100,000 ergs/sq cm s introduced at distances of several solar radii to provide the high speed of the issuing solar wind. The traditional view has been that this energy is obtained from Alfven waves generated in the subphotospheric convection, which dissipate as they propagate outward, converting the wave energy into heat. This paper reviews the generation of waves and the known wave dissipation mechanisms, to show that the necessary Alfven waves are not produced under the conditions presently understood to exist in the sun, nor would such waves dissipate significantly in the first 1-2 solar radii if they existed. Wave dissipation occurs only over distances of the order of 5 solar radii or more.

MHD turbulence in the solar wind

Abstract: From the very beginning of in situ observations of the solar wind it was realized that the interplanetary medium by all appearances was usually not quiet but rather turbulent and visibly permeated by sizable fluctuations of the plasma flow velocity and density and of the magnetic field. Fluctuations occurred on all observed spatial and temporal scales, extending from the vast dimensions of the inner heliosphere and the corresponding solar wind transit time, or from the solar rotation period, down to the minute kinetic scales associated with the particles’ gyromotion, where the dissipation was assumed finally to occur. The observational studies often revealed random and nonreproducible behavior of solar wind parameters as a function of time, thus indicating properties typical of a turbulent magnetofluid. The measured fractional variances of the magnetic field components, when normalized to the mean intensity, turned out to be large, suggesting the importance of nonhnear processes that couple a large number of degrees of freedom and turbulent “eddies” of disparate scales.

Solar cycle variation of the interplanetary magnetic field spiral

Abstract: Interplanetary measurements spanning the time interval from 1965 through 1987 and a distance range from 0.7 to 15.9 AU are employed to test the Parker (1958) theory for the large-scale structure of the interplanetary magnetic field. Examination of data recorded by earth-orbiting spacecraft reveals that the interplanetary magnetic field spiral depends upon the phase of the solar cycle, such that the annual mean winding angle in the years surrounding solar maximum is about 10 deg larger than in the years surrounding solar minimum. The observed variation of the solar wind speed with the solar cycle is shown to account for much of the recorded variation in the winding angle. It is suggested that nonzero azimuthal field components arising from differential solar rotation may be convected into the corona to provide a steady source of azimuthal magnetic fields at the source point of the solar wind. A straightforward extension of the Parker theory shows how such seed fields would account for the observed discrepancy in the interplanetary spiral winding. 38 refs.

Evolution of the solar wind structure over a solar cycle: Interplanetary scintillation velocity measurements compared with coronal observations

Abstract: Sixteen years of solar wind observations via the technique of interplanetary scintillation (IPS) are presented. By an ecliptic comparison with in situ spacecraft observations, these data are shown to be valuable estimates of the large-scale slowly evolving structures in the solar wind speed, but to underestimate the speed in small-scale or rapidly evolving structures. These IPS observations allow the large structures to be studied over solar latitudes from 60° north to 60° south over more than a solar cycle. The observations are presented as half-yearly averages in Carrington longitude and latitude. These are compared with plots of coronal density estimated from coronameter observations and radial magnetic field estimated from solar magnetograph observations and the potential field method. In low and declining solar activity, there is the expected relationship between fast wind and low-density open-field regions, and between slow wind, dense corona and proximity to the neutral sheet. The dipolar field component wanders up to 30° from the rotation axis and is followed by dipolar distributions of density and velocity. Near solar maximum, average wind speeds are uniformly slow over the entire range of latitudes covered, the coronal density becomes more spherically distributed (though it retains a somewhat lower density over the solar poles), and the neutral sheet ranges over all latitudes and sometimes forms disconnected surfaces. The relationship between wind speed, density and angular separation from the neutral sheet are then broken. The results confirm the controlling influence of the coronal magnetic field in determining the three-dimensional structure of the solar wind.

Advances in solar system magnetohydrodynamics

Abstract: 1. Introduction to solar system MHD E. R. Priest 2. Dynamo models and Taylor's constraint C. A. Jones 3. Problems of planetary dynamo theory F. H. Buse 4. Convection near the earth's core-mantle boundary D. R. Fearn 5. Magnetic buoyancy D. W. Hughes 6. MHD waves in the Sun B. Roberts 7. MHD waves and wave heating in nonuniform plasmas M. Goossens 8. Weak solutions of magnetostactis T. Amari 9. Magnetic reconnection in solar flares M. Jardine 10. Three-dimensional magnetic reconnection M. Hesse 11. Magnetic heating of the solar corona M. A. Berger 12. Hydromagnetic equilibrium K. Tsinganos 13. Solar prominences P. Demoulin 14. MHD of solar flares A. W. Hood 15. MHD turbulence in the solar wind A. Mangeney 16. The earth's magnetosphere M. Saunders 17. MHD forces in astrophysical disks and jets J. Heyvaerts.

Why fast solar wind originates from slowly expanding coronal flux tubes

Abstract: Empirical studies indicate that the solar wind speed at earth is inversely correlated with the divergence rate of the coronal magnetic field. It is shown that this result is consistent with simple wind acceleration models involving Alfven waves, provided that the wave energy flux at the coronal base is taken to be roughly constant within open field regions.

Activity associated with coronal mass ejections at solar minimum - SMM observations from 1984-1986

Abstract: Seventy-three coronal mass ejections (CMEs) observed by the coronagraph aboard SMM between 1984 and 1986 were examined in order to determine the distribution of various forms of solar activity that were spatially and temporally associated with mass ejections during solar minimum phase. For each coronal mass ejection a speed was measured, and the departure time of the transient from the lower corona estimated. Other forms of solar activity that appeared within 45 deg longitude and 30 deg latitude of the mass ejection and within +/-90 min of its extrapolated departure time were explored. The statistical results of the analysis of these 73 CMEs are presented, and it is found that slightly less than half of them were infrequently associated with other forms of solar activity. It is suggested that the distribution of the various forms of activity related to CMEs does not change at different phases of the solar cycle. For those CMEs with associations, it is found that eruptive prominences and soft X-rays were the most likely forms of activity to accompany the appearance of mass ejections.

Probing the magnetic topologies of magnetic clouds by means of solar energetic particles

Abstract: Solar energetic particles (SEPs) have been used as probes of magnetic cloud topologies. The rapid access of SEPs to the interiors of many clouds indicates that the cloud field lines extend back to the sun and hence are not plasmoids. The small modulation of galactic cosmic rays associated with clouds also suggests that the magnetic fields of clouds are not closed.

Solar cycle variations in the size and shape of the magnetopause

Abstract: The 10 years of the ISEE 1 and 2 mission covering much of solar cycle 21 and the beginning of solar cycle 22 make it possible to study the position, shape, and motion of the magnetopause throughout the course of changing solar activity. The size and shape of the magnetopause were determined for each observing season using the ISEE 1 and 2 magnetometer data IMP 8 data were used to monitor the solar wind changes with the solar cycle. During the 1979-1980 season, at solar maximum, the solar wind dynamic pressure was at its lowest values, and, at solar minimum, the solar wind pressure was at its largest values, more than double the value in the 1979-1980 season. During this solar cycle, the magnetopause was about 0.5 R(E) farther when the interplanetary magnetic field (IMF) was strongly northward, than when strongly southward. Both standoff distance values are fround to be smaller than the value found by Fairfield (1971). The standoff distance of the magnetopause for northward IMF is anticorrelated with the solar wind pressure. However, the standoff distance for southward IMF seems relatively insensitive to solar wind dynamic pressure.

Probing the magnetic topologies of magnetic clouds by means of solar energetic particles. (Reannouncement with new availability information)

Abstract: Magnetic clouds are large (<025 AU) interplanetary regions with topologies consistent with those of magnetic loops They are of interest because they may be an interplanetary signature of coronal mass ejections Clouds have been identified in solar wind data by their magnetic properties and by the presence of bidirectional particle fluxes Two possible closed magnetic topologies have been considered for clouds: (1) an elongated bottle with field lines rooted at both ends in the Sun and (2) a detached magnetic bubble or plasmoid consisting of closed field lines The inferred topologies are also consistent with open field lines that converge beyond 1 AU We have used solar energetic particles (SEPs) as probes of the cloud topologies The rapid access of SEPs to the interiors of many clouds indicates that the cloud field lines extend back to the Sun and hence are not plasmoids The small modulation of galactic cosmic rays associated with clouds also suggests that the magnetic fields of clouds are not closed

A two-component solar cycle

Abstract: From a previous analysis of a long series of geomagnetic data, we came to the conclusion that, during 91.5% of the time, geomagnetic activity is controlled by the solar wind flow at the Earth's orbit.

On the momentum transfer of the solar wind to the Martian topside ionosphere

Abstract: Hot plasma measurements from the Soviet Phobos-2 spacecraft in the Martian magnetosphere suggests that the solar wind interaction with Mars is cometary-like, with mass loading of the solar wind and ion pick-up occuring also outside the subsolar bow-shock. The interaction is characterized by a pronounced decrease of the solar wind speed inside what has been termed the mass-loading boundary (MLB). Well outside the MLB, the ion pick-up process acts in a normal sense. There ions pick up approximately the solar wind velocity - independent of mass. Inside the MLB, the momentum loss of solar wind ions is more pronounced - heavy ions of Martian origin taking up most of the solar wind ion maximum flux. The heavy mass-loading of solar wind ions in the innermost part of the Martian boundary layer (near the magnetopause) leads to a loaded ion pick-up. The process can be understood as internal loading of an MHD-dynamo, propelled by a driver plasma - the solar wind. The ASPERA ion composition and momentum data is consistent with such a pick-up process. Inside the magnetopause ions of Martian origin are accelerated up to energies close to those of the solar wind protons. The authors propose two types ofmore » acceleration processes, one similar to that acting within the Earth's auroral acceleration region (acting in the presence of an ambient magnetic field), another pick-up process acting within a limited spatial region.« less

The solar wind interaction with Mars: Consideration of Phobos 2 mission observations of an ion composition boundary on the dayside

Abstract: This paper describes the features of the boundary in the plasma ion composition near Mars which separates the region dominated by the solar wind protons from the plasma of planetary origin This boundary was detected by the ASPERA experiment on Phobos 2 It is argued that the features of this boundary seem to be similar to those of other composition boundaries detected elsewhere: the cometopause near comet Halley, and a boundary in the ion composition which appears near Venus during periods of high solar wind dynamic pressure Numerical modeling of the solar wind interaction with Mars supports the idea that during solar maximum the interaction of the Martian neutral atmosphere with the solar wind can result in a composition transition from solar wind to planetary ions in the low-altitude magnetosheath This transition occurs because of charge exchange of solar wind protons with the neutral atmosphere and photoionization

Turbulence in the Solar Wind

Abstract: The solar wind is, apparently, a turbulent medium permeated by fluctuations on a broad range of scales characterizing the plasma as a fluid and kinetic entity. Here we briefly review some recent developments and observations on MHD fluctuations in the frequency range between some 10−6 and 10−2 Hz. Spectral analysis reveals that solar wind fluctuations represent ongoing turbulence, often with high Alfvenic correlations of coronal origin, which undergoes considerable spatial and spectral evolution in the radially expanding wind and is coupled to large-scale plasma structures and magnetic field inhomogeneities. Theoretical attempts to model the transport of magnetohydrodynamic fluctuations are discussed. Finally, the dissipation of turbulence and concurrent heating of the wind are addressed.

The magnetic network location of explosive events observed in the solar transition region

Abstract: Compact short-lived explosive events have been observed in solar transition region lines with the High-Resolution Telescope and Spectrograph (HRTS) flown by the Naval Research Laboratory on a series of rockets and on Spacelab 2. Data from Spacelab 2 are coaligned with a simultaneous magnetogram and near-simultaneous He I 10,380 -A spectroheliogram obtained at the National Solar Observatory at Kitt Peak. The comparison shows that the explosive events occur in the solar magnetic network lanes at the boundaries of supergranular convective cells. However, the events occur away from the larger concentrations of magnetic flux in the network, in contradiction to the observed tendency of the more energetic solar phenomena to be associated with the stronger magnetic fields.

Non-steady-state solar wind-magnetosphere interaction

Abstract: Most of the theories proposed to explain the interaction between the solar wind and the geomagnetic field are stationary descriptions based on ideal MHD. In this review an alternative, nonstationary description is discussed. According to this description, most of the plasma-field irregularities, i.e., plasmoids, detected in the solar wind can penetrate inside the geomagnetic field beyond what is considered to be the mean position of the magnetopause. It is the patchy solar wind plasma impinging on the geomagnetic field which imposes rapidly changing and non-uniform boundary conditions over the whole outer magnetospheric surface. This contrasts with the general belief that the observed field variations or ‘events’ arise sporadically near the magnetopause as the result of some plasma instability. A brief historical review is given to illustrate the evolution of the theoretical models proposed to explain the interaction of the solar wind with the magnetosphere. The emergence of the idea of ‘impulsive penetration’ of solar wind plasma irregularities into the magnetosphere is emphasized especially. A kinetic model of the unperturbed magnetopause is described. This model corresponds to a closed magnetosphere whose surface is a tangential discontinuity. This transition layer can sustain plasma jettings and can be traversed by impulsive penetrating plasmoids. This is against the general belief which considers tangential discontinuities as the worse case with respect to impulsive penetration and plasma jettings. The mean features of the theory of impulsive penetration are presented. Gusty penetration of solar wind plasmoids depends on their excess momentum density and on the orientation of the IMF. The motion of plasmoids across non-uniform magnetic field configurations (tangential discontinuities) is discussed theoretically. When the dielectric constant of the streaming plasma is large enough for collective polarization effects to become important, an electric field develops which permits cross-B motions of all charged particles as a whole plasma entity. It is re-emphasized that the value of the integrated Pedersen conductivity is a determining factor in cross-B plasma motion. On the other hand, interconnection of interplanetary magnetic field lines and geomagnetic field lines results from collective diamagnetic effects produced by magnetized plasmoids injected into the magnetosphere. Several consequences of this penetration mechanism are discussed. These are: the escape of energetic particles out of the magnetosphere, the eastward deflection of penetrating plasmoids, the magnetospheric and ionospheric convection patterns, the erosion of plasmoids, and the mass/momentum loading effects. Some significant experimental geophysical observations supporting the impulsive penetration model are also discussed.

Coronal mass ejection.

Abstract: We summarize the observational aspects of the transient solar coronal features known as coronal mass ejections. Recognizing the importance of understanding this form of solar activity, particularly in the light of relations to flare and prominence activity, and geomagnetic effects, we consider the spectrum of models which have been used to describe these events and assess their viability. We find most models to be unphysical and all represent a gross over simplification of solar conditions. In conclusion we set up a cartoon model which best fits the observations and which we feel should be further developed.

On the differences in element abundances of energetic ions from corotating events and from large solar events

Abstract: The abundances of energetic ions accelerated from high-speed solar wind streams by shock waves formed at corotating interaction regions (CIRs) where high-speed streams overtake the lower-speed solar wind are examined. The observed element abundances appear to represent those of the high-speed solar wind, unmodified by the shock acceleration. These abundances, relative to those in the solar photosphere, are organized by the first ionization potential (FIP) of the ions in a way that is different from the FIP effect commonly used to describe differences between abundances in the solar photosphere and those in the solar corona, solar energetic particles (SEPs), and the low-speed solar wind. In contrast, the FIP effect of the ion abundances in the CIR events is characterized by a smaller amplitude of the differences between high-FIP and low-FIP ions and by elevated abundances of He, C, and S.

Solar cycle effects on the structure of the electron density profiles in the dayside ionosphere of venus

Abstract: Results are presented of observations from the changes in the electron density structure of the dayside ionosphere of Venus that were brought about by changing solar activity. The ionopause height is generally low for values of the solar zenith angle below about 50 deg regardless of the phase in the solar cycle. At solar maximum, and at times of intermediate solar activity, the ionopause height for solar zenith angles greater than about 50 deg is highly variable, ranging from a minimum of about 200 km to a maximum of more than 1000 km. At times of solar minimum the great majority of all ionopause heights for all solar zenith angles are uniformly low, lying between 200 and 300 km. It is argued that the compressed nature of the Venus atmosphere at solar minimum is produced by permeation of the ionosphere by the solar wind magnetic field, which occurs when the solar wind dynamic pressure exceeds the ionospheric plasma pressure.

Variations of the Cosmic-Ray Flux with Time

Abstract: The physical foundations of the modulation of the galactic cosmic ray flux by the sun are reviewed and related to heliospheric structure and dynamics. The basic physical effects - diffusion, convection, adiabatic cooling and drifts - are evaluated and shown to be all important. The results of numerical models are briefly presented and compared with observations. Present-day modulation is shown to reflect the combination of diffusion-convection effects and drift effects. The Maunder minimum is conjectured to be a period when a quiet sun and smooth solar wind resulted in the dominance of drift effects.

The physics of cosmic ray modulation: Heliospheric propagation during the 1987 Minimum

Abstract: The period in and near the 1987 solar minimum of modulation is a unique interval in the 11-year solar cycle modulation for determining whether both the heliospheric current sheet and charged particle gradient drift contribute significantly to the modulation of galactic cosmic rays and the anomalous components. Our investigation of this period is based on measurements from neutron monitors and the IMP 8 satellite at 1 AU and the Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 spacecraft distributed throughout the heliosphere to ∼46 AU. Three observations essential for the analysis are as follows: (1) the brief time-intensity profile for cosmic ray minimum could be traced relatively unchanged from 1 AU to ∼41 AU at the average solar wind velocity, (2) the onset of enhanced modulation in the new solar cycle at 1 AU began at the highest magnetic rigidity and progressed in time over a period of months to particles of low magnetic rigidity, and (3) the average tilt angle of the neutral current sheet was available from Hoeksema (1989) as a function of time for the period centered on the 1987 solar minimum at 1 AU. To account for the observations, we propose a model for the onset of the new modulation cycle which includes a change in the average tilt angle of the current sheet as it propagates outward through the heliosphere at the solar wind velocity. Calculations based on our model with an inwardly directed north solar polar magnetic field (which prevailed in the 1985–1988 period) include diffusion, convection, adiabatic deceleration, and drift for the inward propagation of cosmic ray protons in the energy range from 20 to 1500 MeV and reproduced the rigidity phase lag effect observed in the 1987 data. On the other hand, for the reversed solar magnetic field polarity (e.g., 1954, 1977) the model calculations show that an enhanced onset for these solar cycles has a negligible rigidity phase lag at 1 AU, in agreement with the observed 1977 onset of modulation. We point out that transient phenomena (e.g., Forbush decreases and interaction regions) appear to dominate the higher levels of solar modulation following the 1987 period. A consequence of our analysis is that at the time of solar minimum there is a minimum in the amplitude of the 11-year variation of the radial gradient of integral cosmic ray intensity which propagates outward at the solar wind velocity. Therefore the time dependence of the amplitude of the radial gradient is not the result of the inward-outward motion of a modulation boundary.