Showing papers on "Heliosphere published in 1988"
TL;DR: The possibility that the IMF becomes draped around coronal mass ejections (CMEs) propagating rapidly through the quiescent solar wind into the outer heliosphere is investigated theoretically as discussed by the authors.
Abstract: The possibility that the IMF becomes draped around coronal mass ejections (CMEs) propagating rapidly through the quiescent solar wind into the outer heliosphere is investigated theoretically. The results are presented in diagrams and graphs and discussed in detail. It is found that large sunward-directed structures analogous to the Venus and cometary magnetotails should form when the CME velocity exceeds the solar-wind velocity by more than the local Alfven speed; such structures could hang up swept-up IMF flux for as long as several days. Pioneer 11 magnetic-field measurements at 6.9-9.4 AU from three 20-d periods in 1978 are examined and shown to contain some features consistent with CME draping.
81 citations
TL;DR: In this article, the authors used simple models for the coronal field structure, and extrapolated the spacecraft observations of the photospheric field during sunspot cycle 21 were extrapolated for the purpose of modeling quantitatively the long-term behavior of the IMF.
Abstract: Using simple models for the coronal field structure, the spacecraft observations of the photospheric field during sunspot cycle 21 were extrapolated for the purpose of modeling quantitatively the long-term behavior of the IMF during the sunspot cycle 21. Results were compared with the measurements of the radial component of the IMF at earth. The results indicate that the solar source of the IMF can be represented to a first approximation by the dipole component of the photospheric field, whose axis is nearly perpendicular to the ecliptic plane around sunspot minimum, but tilts more strongly toward it around sunspot maximum. It was also found that the average radial IMF strength varies with heliographic latitude; around sunspot minimum, the radial IMF is expected to be roughly twice as strong above the sun's poles as near the ecliptic plane. The average strength of the photospheric field above latitude 55 deg is about 10 G around sunspot minimum.
66 citations
TL;DR: In this article, the authors examined the solar wind data from the plasma science experiment on the Voyager spacecraft to search for the source of the 2 to 3 kHz radio noise detected in the outer heliosphere.
Abstract: The solar wind data from the plasma science experiment on the Voyager spacecraft are examined to search for the source of the 2 to 3 kHz radio noise detected in the outer heliosphere. It is found that two anomalous high speed streams passed Voyager 2 before the noise was initially observed. It is suggested that the interaction of these streams with the terminal shock is responsible for the more intense emission. Using a time-of-flight argument, the distance to the shock is estimated at about 70 AU-140 AU. The larger value is consistent with an estimate using Voyager data and a standard pressure-balance argument.
55 citations
TL;DR: In this paper, it is shown that a steady state model for the combined acceleration and modulation of cosmic rays in the heliosphere is possible, where the acceleration is derived naturally from the shock boundary condition and the low-energy source of particles to be accelerated need not be specified explicitly.
Abstract: In this paper, it is shown that a steady state model for the combined acceleration and modulation of cosmic rays in the heliosphere is possible. In this first of a series of papers, a spherically symmetric heliosphere with a termination shock at 50 AU is adopted. It is shown that the accelerated spectrum develops naturally from the shock boundary condition and that the low-energy source of particles to be accelerated need not be specified explicitly. The demonstration solutions on the shock are the expected power laws, and it is shown where shock curvature and acceleration time scale effects set in. Some of the basic features of the anomalous cosmic-ray component are explained by the solutions, but, as expected, a detailed comparison with observations must await more extensive and realistic models. 25 references.
46 citations
TL;DR: In this article, it is suggested that the heliosphere switches from one physical state to another following the onset of the solar polar field reversal, and that there is an inverse correlation between the amplitude of the radial anisotropy and solar activity.
38 citations
TL;DR: In this article, the solar wind conditions observed from Voyager 2 at approximately 14 AU are extrapolated to the region of the outer heliosphere bounded by the termination shock, using an MHD simulation model.
Abstract: The solar wind conditions observed from Voyager 2 at approximately 14 AU are extrapolated to the region of the outer heliosphere bounded by the termination shock, using an MHD simulation model. Results from two simulation studies are presented for two sets of nearly recurrent solar wind interaction regions, with initial conditions generated from plasma and magnetic field data observed on March 1984 at 13.8 AU, and on November 1984 at 15.4, respectively. Each simulation describes an idealized recurrent solar wind structure in the supersonic region of the outer heliosphere out to the termination shock far beyond the present reaches of the Pioneer and Voyager spacecraft. It is shown that a collision between the forward shock and the reverse shock occurs approximately every 40 AU. When a forward shock interacts with the termination shock, the latter is weakened and moves outward; the termination shock is strengthened and moves inward when a reverse shock interacts with it.
35 citations
TL;DR: Using color-coded plots of the ISEE-3 solar wind electron data and magnetic field data from ISEE 3 for the period from August 1978 through February 1980, evidence was obtained on two transient disturbances which contained reverse shocks in addition to forward shocks as mentioned in this paper.
Abstract: Using color-coded plots of the ISEE-3 solar wind electron data and magnetic field data from ISEE-3 for the period from August 1978 through February 1980, evidence was obtained on two transient disturbances which contained reverse shocks in addition to forward shocks. These disturbances are considered to be associated with coronal mass ejections (CMEs). In the stronger of the two disturbances, the reverse shock was found within the CME and was separated from the forward shock by about 0.2 AU; the pressure between the two shocks was nearly constant. In the weaker disturbance, the reverse shock propagated entirely through the CME, trailing the forward shock by about 0.3-0.4 AU; the pressure between the shocks declined substantially and monotonically. Each disturbance profile can be compared favorably with one of the simple one-dimensional fluid simulations used by Hundhausen (1985) to illustrate the general principles underlying transient disturbance propagation in the solar wind.
34 citations
TL;DR: In this article, the cosmic ray intensity decreases with heliographic latitude and the changing latitudinal gradient observed during this time period can be directly attributed to the changes in the tilt of current sheet.
Abstract: We have used the cosmic ray particle measurements for E > 70 MeV from the Voyager 1 and 2 and IMP 8 detectors together with observations of the tilt of the heliospheric current sheet to determine the latitudinal gradient in 1984–1987 by a new method. We find that the cosmic ray intensity decreases with heliographic latitude and the changing latitudinal gradient observed during this time period can be directly attributed to the changes in the tilt of current sheet. For a radial gradient of 2.0%/AU between IMP 8 and Voyager 2 near the heliographic equator, the latitude gradient varies from ∼ −1.5%/deg to ∼ −0.9%/deg as the current sheet tilt decreases, and the latitudinal gradient that would be measured for a tilt of 0° is −1.5%/deg or −3.3%/AU. This is larger in magnitude than the radial gradient of 2.0%/AU existing at this time. The different methods used to determine the latitudinal gradient are discussed. The measured latitudinal gradients are then compared with the predictions of the drift models for cosmic ray propagation and the effects of the latitudinal variation of the solar wind upon cosmic rays. The measured gradients are generally much larger than predicted, especially in the case of the drift models. In all cases there is an important distinction between the measured average 26-day gradient and the instantaneous or daily average gradient. The 26-day variations of intensity seen at Voyager 1 during the period 1984–1987 are much smaller and more irregular than the predicted variations due to the wavy current sheet. It appears that neither the effects of drifts nor of the solar wind speed variations as a function of distance from the current sheet can fully explain the features of the latitudinal gradient reported here.
33 citations
TL;DR: From approximately July 28 to November 26, 1984, IMP 8 at 1 AU observed quasi-periodic interaction regions in the solar wind characterized by a peak every 13.4 days in the magnetic field strength, plasma density, and temperature, corresponding to an inertial period of 12.5 days as discussed by the authors.
Abstract: From approximately July 28 to November 26, 1984, IMP 8 at 1 AU observed quasi-periodic interaction regions in the solar wind characterized by a peak every 13.4 days in the magnetic field strength, plasma density, and temperature, corresponding to an inertial period of 12.5 days. When the same solar wind reached Voyager 2 (which moved from 15.2 to 16.1 AU during the corresponding time interval, September 27, 1984, to January 27, 1985), the enhancements in the magnetic field strength and the plasma density and temperature recurred with a period of approximately 25 days. Thus the period of the large-scale fluctuations in B, N, and T doubled between 1 AU and 15.2 AU. The magnetic field strength increased linearly with the density and the temperature in the Voyager 2 data. The tails of the distributions of B, N, and T in the Voyager 2 data were approximately exponential.
32 citations
Journal Article•
TL;DR: In this paper, an idealized hydrodynamic model is presented for the mediation of a free-streaming stellar wind by galactic cosmic rays or energetic particles accelerated at the stellar wind termination shock.
Abstract: An idealized hydrodynamic model is presented for the mediation of a free-streaming stellar wind by galactic cosmic rays or energetic particles accelerated at the stellar wind termination shock. The spherically-symmetric stellar wind is taken to be cold; the only body force is the cosmic ray pressure gradient. The cosmic rays are treated as a massless fluid with an effective mean diffusion coefficient k proportional to radial distance r. The structure of the governing equations is investigated both analytically and numerically. Solutions for a range of values of k are presented which describe the deceleration of the stellar wind and a transition to nearly incompressible flow and constant cosmic ray pressure at large r. In the limit of small k the transition steepens to a strong stellar wind termination shock. For large k the stellar wind is decelerated gradually with no shock transition. It is argued that the solutions provide a simple model for the mediation of the solar wind by interstellar ions as both pickup ions and the cosmic ray anomalous component which together dominate the pressure of the solar wind at large r.
32 citations
TL;DR: In this paper, an autocorrelation analysis shows that the flow variation has a period of about 25.5 days in the latter half of 1986, in approximate agreement with the solar rotation period, suggesting that increased pressure in interaction regions remains the best candidate for the driver of the nonradial flows.
Abstract: Voyager 2 observations made in the outer heliosphere near 25 AU and within 2 deg of the heliographic equatorial plane show periodic variations in the meridional (North/South) flow velocities that are much more prominent than the East/West variations. An autocorrelation analysis shows that the flow variation has a period of about 25.5 days in the latter half of 1986, in approximate agreement with the solar rotation period. The results suggest that increased pressure in interaction regions remains the best candidate for the driver of the nonradial flows.
TL;DR: An analysis of 20 years of interplanetary magnetic field (IMF) data reveals that the IMF spiral is more tightly wound north of the heliospheric current sheet than it is south of the current sheet, the difference in mean spiral angle being some 3.1 {plus minus 1.1} at the orbit of Earth.
Abstract: An analysis of 20 years of interplanetary magnetic field (IMF) data reveals that the IMF spiral is more tightly wound north of the heliospheric current sheet than it is south of the current sheet, the difference in mean spiral angle being some 3.1 {plus minus} 1.1 {degree} at the orbit of Earth. A concurrent analysis of plasma data indicates that this asymmetry does not originate with a corresponding asymmetry in solar wind speed. Currently available information suggests that asymmetric solar rotation may give rise to part of the observed spiral asymmetry but probably not all of it. Whatever its origin, the asymmetry of the spiral may help explain north-south differences seen in the solar modulation of galactic cosmic rays.
TL;DR: In this article, the authors examined the meridional plasma flow and its connection with other plasma parameters in the outer heliosphere and compared with observed plasma parameters and a global flow model which predicts such flows in a steady solar wind.
Abstract: Observed meridional plasma flow and its connection with other plasma parameters in the outer heliosphere are discussed. The dynamics of the flow are examined locally and compared with observed plasma parameters and a global flow model which predicts such flows in a steady solar wind. The observational evidence supports stream dynamics and associated pressure gradients as responsible for driving the flow. Such a meridional flow may result in a net transport of magnetic flux from regions near the heliographic equator. The amplitude of the observed meridional component of solar wind flow is consistent with observed magnetic flux deficits in the outer heliosphere. The limited coverage of heliographic latitude by Voyager 2 precludes a direct measurement of the full flow pattern; however, the magnitude of reported magnetic flux deficits and the unambiguous, regular variations in the meridional flow suggest that the stream interactions do produce a net movement of magnetic flux away from the heliographic equator.
Book Chapter•
01 Jan 1988
TL;DR: In the outer and inner heliosphere of the outer solar system, there is evidence that the radial gradient decreases with distance from the Sun as discussed by the authors, which may result from the combination of a latitudinal gradient with respect to magnetic latitude and the excursion of the spacecraft in latitude caused by a wavy current sheet.
Abstract: Recent measurements in the outer and inner heliosphere have provided new information on the composition, spatial distribution, and temporal variations of the anomalous component. Two additional elements, carbon and argon, are now found to be enhanced, and a method has been developed to derive the abundances of the neutral gas flowing into the heliosphere. In addition, latitudinal gradients recently observed in the outer heliosphere for both the anomalous and galactic cosmic-ray components are negative, opposite to the positive latitudinal gradients reported during the 1976 period when the solar magnetic field was reversed, as expected if the latitudinal gradients are due to effects of curvature and gradient drift in the solar magnetic field. Although the radial gradient remains positive in the outer solar system, there is evidence that the gradient decreases with distance from the Sun. Finally, current 26-day variations in the flux of anomalous cosmic-ray oxygen are large and regular in 1986-1987 at Voyager 1 at ~30° latitude but not at Voyager 2 near the ecliptic plane. We suggest that these variations may result from the combination of a latitudinal gradient with respect to magnetic latitude and the excursion of the spacecraft in latitude caused by a wavy current sheet.
TL;DR: The morphology and physical characteristics of the extended solar wind transsonic region, an intrinsically unique regime of the heliosphere, are described in this paper, where the primary acceleration of the solar wind occurs and both subsonic and supersonic plasma flows co-exist and interact.
Abstract: The morphology and physical characteristics of the extended solar wind transsonic region, an intrinsically unique regime of the heliosphere, are described. It is here where the primary acceleration of the solar wind occurs and both subsonic and supersonic plasma flows co-exist and interact. This concept of ‘mixed flow’ has evolved from an analysis of interplanetary scintillation (IPS) data that reveal the solar wind stream structure and anomalous scattering within the solar wind transsonic region.
TL;DR: In this article, the authors investigate the growth rates and eigenmodes of the sausage, kink, and tearing instabilities in the heliospheric current sheet with the observed sheared flow.
Abstract: Satellite observations of the heliospheric current sheet indicate that the plasma flow velocity is low at the center of the current sheet and high on the two sides of current sheet. In this paper, we investigate the growth rates and eigenmodes of the sausage, kink, and tearing instabilities in the heliospheric current sheet with the observed sheared flow. These instabilities may lead to the formation of the plasmoids and kink waves in the solar wind. The results show that both the sausage and kink modes can be excited in the heliospheric current sheet with a growth time about 0.05-5 day. Therefore, these modes can grow during the transit of the solar wind from the Sun to the Earth. The sausage mode grows faster than the kink mode for beta at infinity 1.5. Here beta at infinity is the ratio between the plasma and magnetic pressures away from the current layer. If a finite resistivity is considered, the streaming sausage mode evolves into the streaming tearing mode with the formation of magnetic islands. We suggest that some of the magnetic clouds and plasmoids observedmore » in the solar wind may be associated with the streaming sausage instability. Furthermore, it is found that a large-scale kink wave may develop in the region with a radial distance greater than 0.5-1.5 AU.« less
TL;DR: In this article, a two-fluid model consisting of cosmic rays and thermal stellar wind gas is used in which the cosmic rays are coupled to the background flow via scattering with magnetohydrodynamic waves or irregularities.
Abstract: A numerical iterative method is used to determine the modification of a stellar wind flow with a termination shock by the galactic cosmic rays. A two-fluid model consisting of cosmic rays and thermal stellar wind gas is used in which the cosmic rays are coupled to the background flow via scattering with magnetohydrodynamic waves or irregularities. A polytropic model is used to describe the thermal stellar wind gas, and the cosmic-rays are modeled as a hot, low-density gas with negligible mass flux. The positive galactic cosmic-ray pressure gradient serves to brake the outflowing stellar wind gas, and the cosmic rays modify the location of the critical point of the wind, the location of the shock, the wind fluid velocity profile, and the thermal gas entropy constants on both sides of the shock. The transfer of energy to the cosmic rays results in an outward radial flux of cosmic-ray energy.
TL;DR: In this paper, a detailed photoionization calculation for the local interstellar medium (LISM) is presented based on constraints imposed by H I column density estimates derived from IUE and Copernicus data toward nearby B stars and hot white dwarfs.
Abstract: Detailed photoionization calculations for the local interstellar medium (LISM) are presented based on constraints imposed by H I column density estimates derived from IUE and Copernicus data toward nearby B stars and hot white dwarfs. It is found that the nearby hot white dwarfs dominate the stellar contributions to the EUV radiation field. Considering stellar contribution to the EUV background alone, the resulting lower limits to the hydrogen and helium fractional ionization in the local diffuse cloud, at the sun, are 0.091 and 0.089, respectively. This result is insensitive to the absorbing geometry of the local cloud. The comparable ionizations of H and He near the sun are due to the lack of absorption from photospheric helium in the hot DA white dwarfs, to the difference in optical depths at wavelengths shortward of the H I and He I ionization edges, and to the fact that the hydrogen ionization controls the electron number density. The derived H and He ionization limits have important implications for the interpretation of the He I and H I backscattering results, for which uncertainties in charge-exchange interactions occurring in or near the heliopause reflect additional significant uncertainties in determining the ionization in the LISM.
TL;DR: Voyager 1 and 2 observations of ions accelerated at corotating shocks within about 13-28 AU are discussed in this article, where the ion spectra extend smoothly from at least 30 keV to an energy above about 3 MeV.
Abstract: Voyager 1 and 2 observations of ions accelerated at corotating shocks within about 13-28 AU are discussed. The ion spectra extend smoothly from at least 30 keV to an energy above about 3 MeV. However, these spectra are falling steeply at the other end. The event-avaraged energy spectra during 1984 are similar for both Voyagers, and the event-averaged spectra at Voyager 2 are of similar form both before and after the flux decrease in early 1985. The event-averaged intensities of about 1 MeV protons exhibit an about -3 percent per degree gradient before and after the early 1985 intensity decrease. The significance of these observations for interpreting the shock acceleration is addressed.
TL;DR: In this paper, the authors present observations of a mass ejection that was observed by five different instruments along its way from the solar surface to more than 100 solar radii, including the ground-based Hα coronagraph at Wroclaw, the white-light SOLWIND coronagraph on board the P78-1 satellite, zodiacal light photometers of the HELIOS B spacecraft, in situ plasma detectors and magnetometers on board a spacecraft, and interplanetary scintillation measurements on the ground.
Abstract: We present observations of a mass ejection that was observed by five different instruments along its way from the solar surface to more than 100 solar radii. The instruments involved are the ground-based Hα coronagraph at Wroclaw, the white-light SOLWIND coronagraph on board the P78-1 satellite, zodiacal light photometers of the HELIOS B spacecraft, in situ plasma detectors and magnetometers on board the HELIOS B spacecraft, and interplanetary scintillation measurements on the ground. By using a CAT-scan analysis of the images obtained by the SOLWIND coronagraph near the Earth and HELIOS B photometers placed at 0.3 AU perpendicular to the Earth-Sun line, we have been able to get a three-dimensional density reconstruction of the mass ejection and fit the best velocity curve for its propagation. Although problems exist in smoothly joining the height-time curves (for instance, we had to reduce the brightness of the SOLWIND data by more than a factor of two to make the data sets agree photometrically), both this analysis and direct measurements by the other experiments clearly indicate higher speeds at greater distances from the Sun. The plasma acceleration in this case was obviously not limited only to distances within 3 R 0 , as is usually the case, but continued beyond the outer limit of the coronagraph view at ∼ 8 R 0 .
01 Aug 1988
TL;DR: In this article, a perturbation method is developed to describe the modification of a stellar wind flow with termination shock by the Galactic cosmic rays, which is the ratio of the galactic cosmic-ray pressure to the thermal gas pressure at a large distance from the star.
Abstract: A perturbation method is developed to describe the modification of a stellar wind flow with termination shock by the Galactic cosmic rays. The perturbation parameter epsilon used in the analysis is the ratio of the galactic cosmic-ray pressure to the thermal gas pressure at a large distance from the star. The positive galactic cosmic-ray pressure gradient serves to brake the outflowing stellar wind gas, particularly just upstream of the termination shock of the wind. A one-fluid polytropic model is used to describe the thermal stellar wind gas, with the coupling between the cosmic rays and the thermal gas being determined by an average hydrodynamical diffusion coefficient kappa describing the scattering of cosmic rays by magnetohydrodynamic waves traveling in the background flow. The analysis takes into account cosmic-ray modifications of the critical point of the wind, the thermal gas entropy constants on both sides of the shock, and the fluid velocity profile.
Journal Article•
15 Apr 1988
TL;DR: In this paper, the radial dependence of the peak flux and the fluence of solar flare produced energetic particles under the assumption that they propagate diffusively in the heliosphere was discussed.
Abstract: Researchers discuss the radial dependence of the peak flux and the fluence of solar flare produced energetic particles under the assumption that they propagate diffusively in the heliosphere.
TL;DR: In this paper, the authors used multi-spacecraft observations in the interplanetary space to build up a picture of the distribution of solar wind velocities in heliographic latitude and longitude.
15 Apr 1988
TL;DR: In this paper, a descriptive model of solar particles in the heliosphere was assembled, including the fluence, composition, energy spectra, and spatial and temporal variations of solar particle both within and beyong 1 AU.
Abstract: During a workshop on the interplanetary charged particle environment held in 1987, a descriptive model of solar particles in the heliosphere was assembled. This model includes the fluence, composition, energy spectra, and spatial and temporal variations of solar particles both within and beyong 1 AU. The ability to predict solar particle fluences was also discussed. Suggestions for specific studies designed to improve the basic model were also made.
TL;DR: In this paper, it was shown that the presence of a magnetic quadrupole in addition to a magnetic dipole introduces a north-south asymmetry in the current sheet (HCS) about the heliographic equator.
Abstract: It is shown that in a heliomagnetic field the presence of a magnetic quadrupole in addition to a magnetic dipole introduces a north-south asymmetry in the heliospheric current sheet (HCS) about the heliographic equator. The dominant polarity of the interplanetary magnetic field (IMF) for the above type of current sheet reverses sign at a transition latitude θT, which lies in a heliohemisphere opposite to the one in which the HCS has more heliolatitudinal extension. The position of θT in the heliosphere and the north-south asymmetry introduced in the HCS change with the relative phase of the dipole and quadrupole components present in the solar magnetic field. The effect of the above type of asymmetric HCS in the IMF ‘mean sector width’ is evaluated and the results are in agreement with the observations during the minima of solar cycle 21.
15 Apr 1988
TL;DR: In this article, the authors review the elements that enter into phenomenological models of the composition, energy spectra, and the spatial and temporal variations of galactic cosmic rays, including the so-called anomalous cosmic ray component.
Abstract: Researchers review the elements that enter into phenomenological models of the composition, energy spectra, and the spatial and temporal variations of galactic cosmic rays, including the so-called anomalous cosmic ray component. Starting from an existing model, designed to describe the behavior of cosmic rays in the near-Earth environment, researchers suggest possible updates and improvements to this model, and then propose a quantitative approach for extending such a model into other regions of the heliosphere.
01 Nov 1988
TL;DR: In this paper, a modification of the occultation method was used to measure the radial dependence of the apparent source size, the scattering angle θ(R), which reveals that an anomalous enhancement in the scattering appears at radial distances from about 16 to 30 R⊙, which is associated with the solar wind transsonic region.
Abstract: Radio astronomy experiments designed to probe the interplanetary plasma close to the Sun have been carried out at the wavelength λ = 2.92 m using the occultation method. The experiments are based on a modification of the occultation method by which the sources, in this case quasars, are used to measure the radial dependence of the apparent source size, the scattering angle θ(R). The radial dependence of this source size θ(R) reveals that an anomalous enhancement in the scattering appears at radial distances from about 16 to 30 R⊙, which is associated with the solar wind transsonic region. It is shown by analysis of the theoretical equations that the radial profiles of both the source size θ(R) and the scintillation index m(R) are consistent in the near solar interplanetary medium (R ≲ 40 R⊙). Combining these two independent applications of the occultation method extends the range of the investigation and provides a powerful new diagnostic of the solar wind transsonic region.