Showing papers on "Heliosphere published in 1991"

Kinetic Physics of the Solar Wind Plasma

Abstract: The interplanetary medium has been continuously explored for more than two decades and substantial progress has been made, both with regard to in situ measurements and a theoretical understanding of the solar wind. Concurrently, measurement techniques for remote sensing of its source regions in the solar corona by means of photons covering the full electromagnetic spectrum from γ-rays to radar have also become mature. The results obtained have greatly improved and corroborated our knowledge about the solar wind from the very coronal base to several solar radii and further out into interplanetary space (see the reviews [8.15, 66–68, 76, 109, 119, 120, 126, 131, 134, 215, 245] and [8.22, 23, 26, 286, 288]). However, even after completion of the Helios mission there remains an important but poorly explored region in the inner heliosphere below 0.3 AU to be investigated and, of course, the wide space out of the ecliptic plane and above the solar poles. Whereas a solar probe, the feasibility of which has already been demonstrated [8.239], may possibly be realized in the far future, the out-of-ecliptic mission [8.199] is soon to become reality and will certainly help to comprehend better the three-dimensional structure of the heliosphere.

Physics of the outer heliosphere

Abstract: Major advances in the physics of the outer heliosphere are reviewed for the 1987-1990 time frame. Emphasis is placed on five broad topics: the detailed structure of the solar wind at large heliocentric distances, the global structure of the interplanetary field, latidudinal variations and meridional flows, radial and temporal variations, and the interaction of the solar wind with the local interstellar medium.

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.

Coupled quasi‐linear wave damping and stochastic acceleration of pickup ions in the solar wind

Abstract: Coupled spatially homogeneous quasilinear kinetic equations are derived which describe the evolution of the energetic ion omnidirectional distribution function and the intensities of magnetohydrodynamic waves propagating parallel and antiparallel to the ambient magnetic field. The energetic ions are assumed to be nearly isotropic and possess speeds much greater than the Alfven speed. For application to pickup ions the equations may also include an energetic ion injection rate and wave excitation or damping caused by isotropization of the newborn ions. The wave kinetic equations may be integrated to yield explicit expressions for the wave intensities, which may be substituted into the ion kinetic equations to yield a single self-consistent energy diffusion equation for the energetic ions. The theory represents the first treatment of stochastic (second-order Fermi) acceleration in which the back reaction of the ions on the turbulence is included self-consistently. Numerical solutions of the kinetic equations are presented for four cases of pickup ions in the solar wind which illustrate the essential features of the evolution: (1) interstellar pickup helium near a heliocentric radial distance of 1 AU; (2) interstellar pickup hydrogen near 10 AU; (3) water group pickup ions downstream of the bow wave of Comet Giacobini-Zinner for parameters observed during the International Cometary Explorer flyby; (4) water group pickup ions downstream of the bow wave of Comet Halley for parameters observed during the Giotto flyby. The helium calculation reveals some modification of the solar wind wave spectrum and energy diffusion of the ions; although adiabatic deceleration is not included, acceleration rates are qualitatively consistent with the observed spectrum at 1 AU (Mobius et al., 1985). The hydrogen calculation shows extreme damping of the solar wind wave spectrum in the cyclotron-resonant frequency range and a reduction in the acceleration rate of most of the ions. It is suggested that this behavior is responsible for an underabundance of hydrogen relative to the minor ions in the anomalous cosmic ray component, which is thought to originate from pickup ions accelerated at the solar wind termination shock. Wave damping is small at comet G-Z, and the calculated energy spectra do not appear to be in quantitative agreement with the observed spectra (Richardson et al., 1987). At Comet Halley, on the other hand, wave damping is substantial and the calculated spectra appear to be in general agreement with the observations (McKenna-Lawlor et al., 1989).

The Influence of the Interface between the Heliosphere and the Local Interstellar Medium on the Penetration of the H Atoms to the Solar System

Abstract: A self-consistent gasdynamic model of the solar wind interaction with the local interstellar medium (LISM), which takes into account the mutual influence of the plasma component (electrons and protons) of LISM and the LISM H atoms that penetrate into the heliosphere, was constructed by Baranov V. B. et al. (1981, Soviet Ast. Letters, 7) in the approximation of axial symmetry. The first step of an iterative method using Monte Carlo simulation is realized. The trajectoires of H atoms are calculated by the Monte Carlo method using the plasma parameter distributions obtained by the model of Baranov et al. for a fully ionized gas.

Multifractal structure of the interplanetary magnetic field: Voyager 2 observations near 25 AU, 1987‐1988

Abstract: Voyager-2 measurements indicate that, for the period day 190, 1987 to day 345, 1988, the large-scale fluctuations of the IMF between 23.3 AU and 27.8 AU have the symmetry properties of a multifractal over scales from 16 hours to 21 days. This suggests the existence of scaling symmetries for the higher moments of the fluctuations of the magnetic field strength.

"Alfvénic" versus "standard" turbulence in the solar wind.

Abstract: We study the variation of the properties of turbulence with stream structure, on time scales of hours and minutes, in the inner heliosphere at solar minimum. Between fast hot streams, this turbulence is found to show many properties typical of standard weakly compressible magnetohydrodynamic (MHD) turbulence such as excess of turbulent magnetic energy and a relative level of density fluctuation approximately equal to the turbulent Mach number squared. We discuss whether or not the more peculiar properties of Alfvenic turbulence, found within fast streams, represent some genuinely different state of MHD turbulence at large distances from the sun and the ecliptic plane. The Ulysses spacecraft data should allow these possibilities to be distinguished

Shock interactions in the outer heliosphere

Abstract: The results of recent simulations of the nonlinear evolution of the solar wind structures are reviewed, emphasizing theoretical development and the shock interactions model (SIM). Models which calculate jumps in flow properties across shocks without using exact Rankine-Hugoniot relations and models which do use them are addressed. The development of a computer code and some basic applications to the SIM are considered. Simulation results for the formation and propagation of forward-reverse shock pairs and the collision and merging of shocks are shown. Two studies which used the SIM to simulate nonlinear evolution of large-scale solar wind structures in the outer heliosphere are examined, and the SIM is then applied to study the heating of the solar wind in the outer heliosphere. The results support the hypothesis that shocks are mainly responsible for the heating of the solar wind plasma in the outer heliosphere at least up to 30 AU.

The role of corotating interaction regions in cosmic-ray modulation

Abstract: The paper presents the first global simulations of the modulation of galactic cosmic rays by a three-dimensional solar wind with corotating interaction regions. The cosmic-ray transport equation is solved in a computed wind and magnetic-field model. The results show both the previously-neglected small-scale response to corotating interaction regions and global, drift-dominated, effects which are similar to previous models. The model predicts a correlation between the local magnetic field and the rate of decrease of the cosmic-ray intensity which is similar to that observed. This is found to be due to inhibited diffusion. It is suggested that such small-scale variations are local and that they do not change significantly the global cosmic-ray structure except, perhaps, near sunspot maximum.

Anisotropy and minimum variance directions of solar wind fluctuations in the outer heliosphere

Abstract: Voyager 2 magnetic field and plasma data are examined over time intervals of 1 to 12 hours in the heliospheric range of 1 to 10 AU to study the evolution of the anisotropy of solar wind fluctuations. Consistent with previous results, the directions of minimum variance vectors of magnetic fluctuations are found to be close to the mean magnetic field direction with an increasing component along the field at larger scales. At large radial distances there is more spread in the minimum variance directions than at smaller radial distances. The power in smaller-scale fluctuations in the magnetic field components perpendicular to the local mean field B(0) is in the ratio of about 5:1 near 1 AU at the scale of 1 hour but decreases to about 3:1 further out. No evidence for selective enhancement of out-of-the-ecliptic components of fluctuations is found. In contrast to results for field fluctuations, analysis of velocity fluctuations shows that the minimum variance direction systematically remains more radially oriented and becomes increasingly less oriented along B(0) with increasing heliocentric distance. The velocity fluctuations are generally more isotropic than the magnetic fluctuations. The observations cannot be explained by a superposed wave picture, and thus are consistent with the view that nonlinear turbulent evolution is responsible for the anisotropy in the fluctuations.

Forbush decreases and interplanetary magnetic field disturbances: Association with magnetic clouds

Abstract: Magnetic clouds have been proposed as a mechanism to produce Forbush decreases in the cosmic radiation. We have examined the temporal association of magnetic clouds and Forbush decreases and find practically no association of the main phase of the Forbush decrease with the arrival of a magnetic cloud. On the other hand, Forbush decreases generally follow the strong interplanetary shocks which sometimes precede magnetic clouds. The main phase of the cosmic ray decrease occurs 2–5 hours after the shock and during the passage of the region in which the magnetic field is disturbed. It appears that a Forbush decrease is more likely to occur following a shock in which the magnetic field and plasma parameters are strongly enhanced. These results indicate that the decrease of the cosmic ray intensity may be produced by the smaller diffusion coefficient in the region behind the shock. The sweeping effect of the enhanced magnetic field associated with the fast shock also probably contributes to the rapid depression of the cosmic ray intensity seen in some decreases.

Propagation of solar flare-associated interplanetary shock waves in the heliospheric meridional plane

Abstract: We have analyzed 149 flare-associated shock wave events based on interplanetary scintillation (IPS) observational data. All of the flare-associated shock waves tend to propagate toward the low latitude region near the solar equator for flares that are located in both the solar northern and southern hemispheres. Also, the fastest propagation directions tend toward the heliospheric current sheet near 1 AU. We suggest that this tendency is caused by the dynamic action of near-Sun magnetic forces on the ejected coronal plasma that traverses the helmet-like magnetic topologies near the Sun outward to the classical topology that is essentially parallel to the heliospheric current sheet.

Cosmic ray modulation: Voyager 2 observations, 1987-1988

Abstract: The intensity profile of cosmic rays above 70 MeV observed by Voyager 2 and its relation to the interplanetary magnetic field and plasma at the beginning of the new modulation cycle from day 190, 1987 to day 345, 1988 in the region from 23.3 AU to 27.8 AU is analyzed. The cosmic ray intensity profile was approximately a series of four plateaus separated by three steps in which the intensity dropped abruptly. Each step was associated with a region in which the magnetic field, density and temperature were higher than average. The plateaus were associated with regions in which the magnetic field was alternately strong and weak. The solar wind within 200 AU during this interval can be roughly pictured as consisting of three shells between which the flow was quasiperiodic with a 26 day periodicity. The latitudinal extent of the shells in the northern hemisphere was probably less than 33 deg, since no steps were observed by Voyager 1. Drift motions might play a role during the recovery phase, just prior to the onset of the new modulation cycle, in the plateau regions between the shells, within the shells where drifts in various directions might mimic diffusion, and close to 1 AU, where large regions of intense magnetic fields have not yet formed. However the principal decreases in the cosmic ray intensity in the outer heliosphere during 1987 and 1988 were associated with the passage of broad regions of intense magnetic fields, consistent with the diffusion/convection model.

The interstellar tunnel of neutral-free gas toward Beta Canis Majoris

Abstract: Using high-resolution sodium absorption observations of early-type stars to determine the distribution of neutral interstellar gas in the direction of the star Beta CMa, the contours of a large feature in the local interstellar medium, some 50 pc in diameter and 300 pc long, that appears to be virtually free of neutral gas have been mapped. This rarefied 'interstellar tunnel' is an extension of a region of very low gas density surrounding the sun called the Local Bubble, which may well have been formed by the interaction of expanding interstellar cavities produced by multiple supernova events. This large region of unusually low gas density will be a prime area for study in the soft X-ray and EUV spectral regions.

Cosmic ray modulation

Abstract: The work expounds on the evolution of the studies of cosmic ray modulation effects and the development of the concepts concerning the processes which occur in interplanetary space and give rise to the 11-year and 22-year cosmic ray variations, to the annual and 22-year variations, to the Forbush effects and to the effects of increases before geomagnetic storms, to the solar anisotropy and scintillations of galactic cosmic ray flux. The role of cosmic rays in restricting and forming the heliosphere and in forming the terminal shock wave is discussed. The theory of cosmic ray propagation in the heliosphere is proposed to generalize for the two-component (solar wind + magnetic clouds) model. The stream kinetic instability is shown to be able of playing an important role in the cosmic ray modulation in the outer heliosphere.

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.

Heliospheric Effects on Cosmic-Ray Electrons

Abstract: Motivated by the success of current modulation models in interpreting the solar modulation of Galactic cosmic-ray nuclei, we consider several effects on cosmic-ray electrons which are new or which have not yet been fully explored. Among these are the contribution of Galactic positrons to the total electron flux, the production of secondary electrons from solar system objects, and the possibility of electrons being accelerated at the solar wind termination shock ("anomalous" cosmic-ray electrons). We find that the first of these provides a natural interpretation of the time variations of Galactic cosmic-ray electrons. Jovian electrons and thermal solar wind electrons may be accelerated to cosmic-ray energies at the termination shock. Such acceleration effects may alter the time-intensity profiles of electrons at Earth, further modifying the interpretation of the electron modulation.

New observations of the low frequency interplanetary radio emissions

Abstract: Recent Voyager 1 observations reveal reoccurrences of the low frequency interplanetary radio emissions. Three of the new events are weak transient events which rise in frequency from the range of 2-2.5 kHz to about 3 kHz with drift rates of approximately 1.5 kHz/year. The first of the transient events begins in mid-1989 and the more recent pair of events both were first detected in late 1991. In addition, there is an apparent onset of a 2-kHz component of the emission beginning near day 70 of 1991. The new transient emissions are barely detectable on Voyager 1 and are below the threshold of detectability on Voyager 2, which is less sensitive than Voyager 1. The new activity provides new opportunities to test various theories of the triggering, generation, and propagation of the outer heliospheric radio emissions and may signal a response of the source of the radio emissions to the increased solar activity associated with the recent peak in the solar cycle.

Comparison of Doppler scintillation and in situ spacecraft plasma measurements of interplanetary disturbances

Abstract: Since 1979 the radio path of the Pioneer Venus orbiter (PVO) spacecraft has spent considerable time probing the near-Sun solar wind off the limbs of the Sun. On occasion, Helios 1 has also been in position above the Sun's limb at the same solar longitude to simultaneously observe in situ the solar wind plasma. These fortuitous circumstances, along with the availability of near-continuous measurements, have made it possible for the first time to carry out detailed comparisons between Doppler scintillation and in situ plasma measurements and to improve our understanding of Doppler scintillation transients. During a combined observing period of nearly 3 months in 1981–1982 near solar maximum, 22 transients were observed by PVO and 23 shocks were observed by Helios 1. On the basis of a comparison of mass flux density and rms Doppler scintillations, we find that at least 84% of the transients are shocks, while at least 90% of the shocks are transients. Hence there is a near one-to-one correspondence between transients and interplanetary shocks. Although the temporal profiles of Doppler scintillation and mass flux density are similar, the magnitudes of the Doppler scintillation transients may not simply reflect those of mass flux density. Only one pronounced solar wind event that was observed in the mass flux density measurements showed no signature in the scintillation data; field and particle measurements by Helios 1 suggest that it is a noncompressive density enhancement and/or a magnetic cloud. One of the scintillation transients that is not a fast-mode shock appears to correspond to a slow shock. However, when scintillations alone are available, slow shocks may be difficult to identify. Shock speeds based on transit times between the PVO radio scintillation path and the Helios 1 spacecraft are consistent with those from the in situ plasma measurements and indicate shock deceleration in essentially all cases. A significant consequence of this investigation is that Doppler scintillation measurements can now be used by themselves to detect and locate interplanetary shocks near the Sun with a relatively high degree of confidence and hence can be used to conduct useful correlative studies in the future with other solar and interplanetary observations.

Cosmic ray unidirectional latitude gradient: Evidence for north-south asymmetric solar modulation

Abstract: The cosmic ray diurnal anisotropy is determined separately in toward and away sectors of the interplanetary magnetic field through analysis of neutron monitor records spanning the period 1953–1988. We suggest that the dominant cause of differences (between sectors) of the anisotropy is a unidirectional latitude gradient, which is indicative of a north-south asymmetry in the solar modulation of cosmic rays. The magnitude of the latitude gradient deduced from the observations is typically of the order of 0.7%/AU, and its direction varies in a manner not obviously related to either the 11-year sunspot cycle or the 22-year solar magnetic cycle. Further support for the reality of a unidirectional gradient is provided by an analysis of the cosmic ray density segregated according to sector polarity. The density is found to differ significantly in the two sector types, and the amount of difference is correlated with the concurrently measured unidirectional latitude gradient in the expected manner. We examine two hypotheses to account for a north-south asymmetry of solar modulation: (1) that the asymmetric modulation results from a north-south asymmetry in the winding angle of the interplanetary magnetic field spiral and (2) that the asymmetric modulation is related to a corresponding north-south asymmetry of solar activity as indicated by sunspot number. Both hypotheses are consistent with the data, suggesting that multiple factors may give rise to north-south asymmetric solar modulation.

Physical reasons and consequences of a three-dimensionally structured heliosphere

Abstract: 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.

Solar wind fluctuations at large scale: A comparison between low and high solar activity conditions

Abstract: The influence of the Sun's activity cycle on the solar wind fluctuations at time scales from 1 hour to 3 days in the inner heliosphere (0.3 to 1 AU) is investigated. Hourly averages of plasma and magnetic field data by Helios spacecraft are used. Since fluctuations behave quite differently with changing scale, the analysis is performed separately for two different ranges in time scale. Between 1 and 6 hours Alfvenic fluctuations and pressure-balanced structures are extensively observed. At low solar activity and close to 0.3 AU, Alfvenic fluctuations are more frequent than pressure-balanced structures. This predominance, however, weakens for rising solar activity and radial distance, to the point that a role exchange, in terms of occurrence rate, is found at the maximum of the cycle close to 1 AU. On the other hand, in all cases Alfvenic fluctuations have a larger amplitude than pressure-balanced structures. On the whole, the Alfvenic contribution to the solar wind energy spectrum comes out to be dominant at all solar activity conditions. At scales from 0.5 to 3 days the most important feature is the growth, as the solar wind expansion develops, of strong positive correlations between magnetic and thermal pressures. These structures are progressivelymore » built up by the interaction between different wind flows. This effect is more pronounced at low than at high activity. Our findings support the conclusion that the solar cycle evolution of the large-scale velocity pattern is the factor governing the observed variations.« less

Plasma wave generation near the inner heliospheric shock

Abstract: There is mounting evidence that the Voyager 1 and 2 and Pioneer 11 spacecraft may approach the inner (termination) heliospheric shock near the end of this century. It is argued here, by analogy with planetary bow shocks, that energetic electrons backstreaming from the heliospheric shock along the magnetic field should be unstable to the generation of Langmuir waves by the electron beam instability. Analytic expressions for the cutoff velocity, corresponding to the beam speed of the electrons backstreaming from the shock, are derived for a standard solar wind model. At the front side of the heliosphere the maximum beam velocity is expected to be at the meridian passing through the nose of the shock, which is assumed to be aligned with the Very Local Inter-Stellar Medium flow. This foreshock region and the associated Langmuir waves are relevant to both the expected in situ observations of the heliospheric boundaries, and to the low-frequency (2-3 kHz) radio emissions observed by the Voyager spacecraft in the outer heliosphere. Provided that these radio emissions are generated by Langmuir waves, the minimum Langmuir wave electric fields at the remote source are estimated to be greater than about 3 - 30 microV/m.

Latitudinal cosmic ray gradients: Their relation to solar activity asymmetry

Abstract: Data obtained by underground cosmic ray telescopes in the northern and southern hemispheres between 1965 and 1985 are analyzed as a function of the interplanetary magnetic field (IMF) sense to examine the long-term nature of the latitudinal cosmic ray gradient, {gradient}N{sub p}. The nature of this gradient is inferred from the B {times} {gradient} N{sub p} contribution to the solar diurnal cosmic ray anisotropy arising from the interaction between the latitudinal gradient and the IMF B. In an earlier paper, the authors noted a correspondence between the characteristics of the latitudinal gradient and the north-south asymmetry in activity on the Sun during the same period and attempted to explain the result in terms of a combination of two gradients: a north-south symmetrical gradient and a north-south asymmetrical gradient. In this paper, the authors present results showing that asymmetry in activity on the Sun displaces the heliospheric neutral sheet either below or above the Earth (on average), according to the direction of the solar activity asymmetry. This displacement, combined with a latitudinal cosmic ray density gradient that is symmetrical in relation to the neutral sheet, produces a latitudinal cosmic ray gradient that is asymmetrical when viewed from the Earth. The cosmicmore » ray data are consistent with a symmetric (in relation to the neutral sheet) latitudinal cosmic ray density gradient in which the density initially decreases with distance from the neutral sheet and then increases again with increasing distance from the sheet. Additional data from inclined underground cosmic ray telescopes, viewing from 60{degree}N to 50{degree}S, give information on latitudinal gradients further from the neutral sheet.« less

Nonthermal ions of interstellar origin at different solar wind conditions

Abstract: The existence of singly ionized interplanetary ions has been observationally well confirmed in past years. In present theories these ion species are ascribed to ionization of neutral interstellar atoms during their traversal through the inner heliosphere. A study of ionization rates and pick-up ion production rates for different solar wind conditions is presented, and the influence of the prevailing solar wind type on the pick-up ion fluxes at the Earth's orbit is studied

Helios spacecraft photometer observation of elongated corotating structures in the interplanetary medium

Abstract: The Helios spacecraft zodiacal light photometers can be used to measure persistent elongated structures at interplanetary distances from the Sun. These data give the heliospheric positions of these structures from their east to west motion with time. An outward motion of the material within each structure can be determined from its curvature. This technique has been used to measure over 40 of these structures from solar minimum to solar maximum (1976 through 1979) using both Helios A and Helios B observations. The positions of the structures measured range from low latitude to as high as 50 {degrees}. The speeds determined have a mean value of 301 {plus minus} 47 km s{sup {minus}1}. The modeling, which determines densities for these structures from 0.15 to over 1.0 AU, indicates that these heliospheric structures can have densities many times above the ambient and that the density in excess of an r{sup {minus}2} ambient can differ greatly along their curved extents at any given time. Most of the structures measured in this sample either remain approximately the same density or decrease in density with height above the Sun. This implies that these structures are generally remnants of features maintained by the physical processes whichmore » formed them near the surface of the Sun (as streamers) and that they are not caused by interplanetary interactions.« less