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The historical development of ideas and investigations leading to the present understanding of the heliosphere is presented. A summary of the most recent research in plasmas, magnetic fields and accelerated charged particles in the outer heliosphere based on spacecraft investigations, is reviewed. Current plans for investigating the heliosphere in three dimensions are discussed.
The paper presents recent work on evolution of recurrent solar wind structures in the outer heliosphere. Corotating shocks, corotating interaction regions, and merged interaction regions are studied, and an MHD simulation model in which the jump conditions at all shock crossings satisfy the Rankine-Hugoniot solution is examined. Simulation results which describe the evolution of idealized recurrent solar wind structures between 14 AU and the termination shock are reported which show that merged interaction regions belonging to two to four consecutive solar rotations coalesce with each other between 30 AU and the termination shock to reshape the heliospheric structures. In the outer heliosphere, forward and reverse interplanetary shocks which continuously heat the solar wind plasma are investigated.
Early estimates of the radial gradient made in the inner heliosphere are presented. In a discussion of discoveries in the outer heliosphere, consideration is given to low radial gradients and nonradial transport, the identification of different components, the large-scale organization of the heliospheric magnetic field, propagating modulation features, and latitude gradients. Current estimates of the spatial distribution of the cosmic ray intensity are presented as well.
Observations of the Thomson scattering brightness by electrons in the inner heliosphere provide a means of probing the heliospheric electron distributions. An extensive data base of Thomson scattering observations, stretching over many years, is available from the zodiacal light photometers on board the two Helios spacecraft. A survey of these data is in progress, presenting these scattering intensities in the form of synoptic maps for successive Carrington rotations. The Thomson scattering maps reflect conditions at typically several tenths of an astronomical unit from the sun. Some representative examples from the survey in comparison with other solar/heliospheric data, such as in situ observations of the Helios plasma experiment and synoptic maps constructed from magnetic field, H alpha and K-coronameter data are presented. The comparison will provide some information about the extension of solar surface features into the inner heliosphere.
A method for displaying the electron Thomson scattering intensity in the inner heliosphere as observed by the zodiacal light photometers on board the Helios spacecraft in the form of synoptic maps is presented. The method is based on the assumption that the bulk of the scattering electrons along the line of sight is located near the point closest to the sun. Inner-heliospheric structures will generally be represented properly in these synoptic maps only if they are sufficiently long-lived (that is, a significant fraction of a solar rotation period). The examples of Helios synoptic maps discussed (from data in April 1976 and November 1978), indicate that it is possible to identify large-scale, long-lived density enhancements in the inner heliosphere. It is expected that the Helios synoptic maps will be particularly useful in the study of corotating structures (e.g., streamers), and the maps will be most reliable during periods when few transient featurs are present in the corona, i.e., during solar minimum.
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 Cosmic and Heliospheric Branch proposes a bold new program for the years 1995 to 2010 that is centered on the following two themes: (1) the global heliosphere and interstellar space; and (2) cosmic particle acceleration and the evolution of matter. Within these major themes are more specific goals that have been studied and continue to be examined for a better understanding of their processes. These include: origin, structure, and evolution of the solar wind; interaction of the heliosphere, the solar wind, and the interstellar medium; fundamental microscopic and macroscopic plasma processes; acceleration and transport of energetic particles; and the origin and evolution of matter. Finally, the report summarizes a wide variety of proposed small and large space missions.
Recommendations are made for programs to meet high-priority objectives for solar and heliospheric physics. These priority areas include: the physics of the ISM and the heliospheric boundary; solar particle acceleration, transport, and composition; the ultraheavy composition of solar energetic particles; the large-scale structure of the heliosphere and solar modulation; the microphysics of interplanetary acceleration and shock acceleration. Measures that should be taken to continue and enhance ongoing programs are summarized.
Heliospheric current sheet tilt evolves systematically over the solar cycle. Here we show that this evolution is different than the sunspot cycle and that tilt for the period 1992-1996 can be predicted using persistence. That is, the tilt over the coming cycle will be the same as for the past cycle. The Ulysses spacecraft has passed Jupiter and is moving out of the plane of the ecliptic, so we use the prediction of the changing heliospheric current sheet tilt to predict that Ulysses will pass beyond the envelope, or maximum latitude, of the heliospheric current sheet in November 1993.
The paper analyzes the main features of 2- to 3-kHz heliospheric emissions in the context of a general heliospheric 'breathing' as inferred from the Voyager 2 solar wind average ram pressure data. Triggers for the three 3-kHz emission events seen to date are suggested, and good agreement is obtained in timing and expected postshock frequency for termination shock distances of about 90 AU. It is suggested that the visibility of the individual 3-kHz events and their observed upward frequency drift are enhanced when the postulated global heliospheric expansion results in the formation of a transient, compressed external plasma barrier around the heliopause that prevents radiation escape for several months. The average termination shock distance is estimated to be in the range 80-90 AU.
Beginning in 1989, the active phase of the present solar cycle became manifest in the outer heliosphere as large disturbances in solar wind velocity as observed by the Ames plasma analyzers aboard Pioneer 10 (46-50 AU heliocentric distance) and Pioneer 11 (about 28 AU). Inner heliospheric baseline plasma observations from the Pioneer Venus Orbiter (0.7 AU) and IMP 8 (1 AU) are useful for attempts to correlate solar events with the outer heliospheric disturbances. With regard to the onset of activity at Pioneer 11, Pioneer Venus observations are pertinent, and some of these in turn correspond with CMEs (coronal mass ejections) observed in SMM coronagraph data. In particular, enhanced solar wind speeds observed at Pioneer Venus during December 1988 to February 1989 are associated with seven large solar wind shocks (or shock candidates); corresponding CMEs may be identified. Two of these seven shocks were identified as candidates for a precursor to the onset of the disturbances at Pioneer 11. At Pioneer 10 the disturbed period includes two large disturbances, associated with the passage of shocks. There are several candidate CMEs in the SMM observations, one of which may be associated with the second Pioneer 10 shock.
Large transient decreases over 20 percent were observed above 70 MeV cosmic ray intensity in September 1991, at the three spacecraft, V1, V2, and P10, in the outer heliosphere between 35 and 53 AU. These decreases appear to be related to the intense solar activity occurring in late May and early June, which was responsible for a series of rapid transient decreases at the earth that probably reduced the over 70-MeV intensity by more than 50 percent to the 1owest level ever recorded. Average transit speeds of about 600 to 800 km/s are deduced for the propagation of these transients between the earth and the outer heliosphere. The overall picture of the propagation of these transients is consistent with a massive, almost spherical modulating region moving outward in the heliosphere. The smooth almost exponential recovery of intensities at all three spacecraft for over 160 days after the decrease, and the large total modulation beyond P10 at the time of the decrease, suggest that the modulation boundary is well beyond 53 AU and probably beyond 100 AU.
The theory of magnetospheric recirculation for cyclic electron energization and transport may apply in part to heliospheric transport of Jovian electrons if enhanced cross-IMF propagation occurs at heliospheric altitudes near and below the solar wind transition region. Low altitude, ecliptic-to-polar transport would short-circuit conventional interplanetary diffusion, facilitate rapid access to the polar heliosphere with minimal adiabatic energy losses, and provide a seed population for acceleration to 100-1000 MeV energies at the solar wind termination shock and in the heliomagnetotail.
Various papers on measurements of the outer heliosphere are presented. The general topics addressed include: the heliospheric magnetic field, the solar wind, energetic particles from the sun and Jupiter, the interstellar gas, the heliospheric boundary, cosmic ray modulation, and future measurements.
Observations and interpretations of solar wind behavior in the heliosphere are reviewed. The spiral magnetic field, the heliospheric vortex street, multifractals and large-scale fluctuations, and intermittent turbulence are examined. Voyager observations of the outer heliosphere are stressed.
We study a transverse plasma flow induced by the evolution of a Karman vortex street using a Chebyshev-Fourier spectral algorithm to solve both the compressible Navier-Stokes and MHD equations. The evolving vortex street is formed by the nonlinear interaction of two vortex sheets initially in equilibrium. We study spatial profiles of the total plasma velocity, the density, the meridional flow angle and the location of sector boundaries and find generally good agreement with Voyager 2 measurements of quasi-periodic transverse flow in the outer heliosphere. The pressure pulses associated with the meridional flows in the simulation are too small, although they are correctly located, and this may be due to the lack of any 'warp' in the current sheet in this model. A strong, flow-aligned magnetic field, such as would occur in the inner heliosphere, is shown to lead to weak effects that would be masked by the background interplanetary turbulence. We also study the plasma and magnetic transport resulting from the meridional flow and find that deficits of magnetic quantities do occur near the ecliptic. While the effect is relatively small, it is in general agreement with the most recent analysis of 'flux deficit' in the outer heliosphere.
The interaction of the solar wind with the interstellar medium is expected to result in a complex, probably dynamic outer heliospheric boundary region. In increasing distance from the sun the boundary region includes the solar wind termination shock, the heliopause, and perhaps a heliobowshock. The continuing missions of Voyager 1 and 2 and Pioneer 10 and 11 provide a unique opportunity to make in situ particle and field observations of the boundaries of the heliosphere and associated phenomena. Observations already made by these spacecraft suggest that the termination shock may be located 60-100 AU from the sun. Voyager 1 will reach 60 AU in 1995 and 100 AU in 2006. NASA's Space Physics Division is planning with the Voyager and Pioneer Projects to ensure timely and maximum scientific return from spacecraft encounters with the solar wind termination region. Initial emphasis is on being able to reliably determine when an encounter is imminent, so that spacecraft tracking and spacecraft and instrument configurations can be changed to encounter modes. It is anticipated that multiple encounters with heliospheric boundaries will occur due to motion of the boundaries in response to large variations in the solar wind pressure.
The orientation of the heliospheric current sheet predicted from a source surface model is compared with the orientation determined from minimum-variance analysis of International Sun-Earth Explorer (ISEE) 3 magnetic field data at 1 AU near solar maximum. Of the 37 cases analyzed, 28 have minimum variance normals that lie orthogonal to the predicted Parker spiral direction. For these cases, the correlation coefficient between the predicted and measured inclinations is 0.6. However, for the subset of 14 cases for which transient signatures (either interplanetary shocks or bidirectional electrons) are absent, the agreement in inclinations improves dramatically, with a correlation coefficient of 0.96. These results validate not only the use of the source surface model as a predictor but also the previously questioned usefulness of minimum variance analysis across complex sector boundaries. In addition, the results imply that interplanetary dynamics have little effect on current sheet inclination at 1 AU. The dependence of the correlation on transient occurrence suggests that the leading edge of a coronal mass ejection (CME), where transient signatures are detected, disrupts the heliospheric current sheet but that the sheet re-forms between the trailing legs of the CME. In this way the global structure of the heliosphere, reflected both in the source surface maps and in the interplanetary sector structure, can be maintained even when the CME occurrence rate is high.