The Heliosphere

W. I. Axford

S. T. Suess

Space Science Lab/ES82
NASA Marshall Space Flight Center
Huntsville, Alabama 35812 U.S.A.

[This is a slightly expanded version of a paper which appeared in EOS December 1994]

Introduction
The Interstellar Medium
Our Present Understanding of the Heliosphere
Distance to the Termination Shock
Bibliography

Introduction

Larger version of Figure 2 (10K)

The interstellar flow turns at the stagnation point to go around the heliosphere and the subsonic solar wind flow is turned in the "heliosheath" between the termination shock and the heliopause to flow down the "heliotail." The solar wind inside the termination shock is flowing supersonically and radially. The terminology has been chosen to be closely analogous to that for the Earth"s magnetosphere because of the topological similarity of the two configurations.

THE INTERSTELLAR MEDIUM

Until 1972 most observations of the VLISM involved averages over distances of 100 parsecs or more. From the observations of Faraday rotation and dispersion of pulsar emissions, it was deduced that the average electron density is ~ 0.03 per cubic centimeter and the magnetic field strength is ~ 0.3nT, with large fluctuations of the same order. Radio observations of the 21 cm interstellar line suggested density of neutral hydrogen on the order of 0.7 per cubic centimeter, considerably higher than the value obtained from measurements of Lyman alphalpha absorption in the spectra of nearby early type stars, namely about 0.1 per cubic centimeter.

We do not presently have significantly better values for the electron density and magnetic field strength, but the situation with regard to neutral hydrogen and helium has greatly improved. Observations made from the spacecraft OGO 5 led to the remarkable result that interstellar neutral hydrogen is detectable as a consequence of the backscattering of solar Lyman alpha within ~ 5 AU and that the direction of approach is not from the solar apex but close to the projection of the apex direction onto the ecliptic plane. The heliocentric distance of penetration is determined easily by measuring the parallax of the Lyman alpha emission from observations made from the vicinity of the Earth at different times during the year.

The forces acting on a neutral hydrogen atom approaching the Sun are gravity and the radiation pressure. In addition, atoms are lost, mainly as a result of charge exchange with solar wind protons which converts a fast solar wind proton into a fast hydrogen atom. The proton of interstellar origin is "picked up" by the interplanetary electric and magnetic fields. The heliocentric distance to which interstellar atoms having a relative speed of ~ 20 km/s can penetrate before the probability of loss by charge exchange becomes high is about 4 AU, corresponding roughly to the distance obtained from the OGO 5 parallax measurements.

The brightness distribution of scattered solar Lyman alpha corresponding to a given neutral hydrogen density distribution can be computed and compared with observations, provided one knows the shape of the solar Lyman alpha emission line. However, it is not easy to determine the inflow speed of the neutral beam, U, and the loss mean free path, lambda, separately. Hence the value deduced for U is rendered uncertain by the uncertainties in evaluating lambda.

For the case of interstellar neutral helium the situation is different because radiation pressure has negligible effect and the value of lambda is such that the neutral beam penetrates to well within 1 AU if U~ 20 km/s. The interesting conclusion that can be drawn from measurements of scattered solar HeI 584 Å radiation is that the maximum intensity should occur in the downwind direction, looking along the accretion axis, rather than in the upwind direction as in the case of hydrogen. This provides a straightforward means of determining the direction of the interstellar wind.

The early results from observations of scattered Lyman alpha and HeI 584 have been confirmed and refined by the neutral helium experiment on Ulysses and by direct observation of pickup ions on AMPTE. Very accurate values have been found for the speed and direction of the helium wind together with a good value for the temperature. These concensus results are given in Table 1. With regard to the cosmic ray pressure in Table 1, the heliosphere is near a bubble of hot gas, evidently recent and not necessarily in equilibrium as far as locally produced cosmic rays are concerned, so it is possible that a low energy component exists locally and contributes to the total pressure. If this does happen, the shock itslef will be thickened and made more turbulent in the course of the interaction.

Returning to the interstellar pick-up ions, they have an interesting fate. After entering the heliosphere initially as neutral atoms, being ionized, and then picked up, they are carried out to the termination shock. There, a small percentage are accelerated to cosmic ray energies and then propagate back into the inner heliosphere where they are observed as "anomalous cosmic rays." This process has recently been confirmed by the observervation on AMPTE and Ulysses that anomalous cosmic rays are, indeed, singly ionized.

With regard to interstellar plasma, soft x-ray observations suggest that the Sun is located within an irregular region of hot (~ million degree K), low density (~ 0.05 per cubic centimeter) plasma with dimensions of the order of tens to hundreds of parsecs. This "local bubble" of hot plasma must have been produced by a combination of one or more supernovae and stellar winds associated with a group of O and B stars. However, it is quite clear that the VLISM does not have the same characteristics as the local bubble material as a whole since it contains neutrals having a temperature of only ~ 7000 K. The Sun is usually considered to be immersed in a region of "local fluff" that is relatively cool and dense. Since one would expect to have pressure balance between the fluff and the hot medium it would seem to be necessary that the former contains an unobservable component of plasma and magnetic field which would have p/k ~ 10,000 degrees K per cubic centimeter to compensate for the very low pressure of the observed neutrals described above (p/k < 10,000 degrees K per cubic centimeter). This seems rather too large unless the pressure is mainly magnetic which would require BI ~0.6nT. The plasma alone would have to have n >1 per cubic centimeter to be able to produce such a hydrostatic pressure and its ram pressure would put the solar wind termination shock at R ~ 30AU, whereas direct observation places it beyond 50 AU. The relative abundances of hydrogen and helium atoms in the VLISM given in Table 1 suggest that the density of the ionized component (mainly protons) is not greater than about 0.3 per cubic centimeter. If the termination shock is assumed to ie at ~ 100 AU, more reasonable values for the density (~ 0.1per cubic centimeter) can be accomodated if there is also a significant magnetic field (B~ 3-5nT) and this can provide a match to the hydrostatic presure in the local bubble if the latter has been slightly over-estimated by say a factor of 2.

Table 1. Properties of the Very Local Interstellar Medium

  Property                      Value

NEUTRAL COMPONENT
 Flow Speed                  25 +/- 2 km/s
 Flow Direction            75.4  ecliptic longitude
                           -7.5   ecliptic latitude
 Hydrogen density          0.10 +/- 0.01 /cubic cm
 Helium density            0.010 +/-  0.003 /cubic cm
 Hydrogen temperature     (7 +/-  2) x 1000K
 Helium temperature       (7 +/-  2) x 1000K
IONIZED COMPONENT
 Electron density         < 0.3/ cubic cm
 Flow speed               Assumed same as neutral component
 Flow direction           Assumed same as neutral component
 Ion temperature          Assumed same as neutral component
MAGNETIC FIELD
 Magnitude                0.1 - 0.5 nT
 Direction                  Unknown
COSMIC RAYS
 Total pressure          (1.3+/-0.2) x 10^(-12)dynes/square cm
											

The collision time between interstellar neutral atoms and ions is ca. 1000 years and the mean free path is 1000 to 10,000 AU, both of which are small enough so neutral atoms must be moving together with, and have the same temperature as the interstellar plasma in the local fluff. The neutral beam velocity vector gives us the plasma velocity vector. This forces the conclusion that there is an interstellar wind blowing on the heliosphere in the ecliptic plane, at a speed, VI = U = 25 km/s. The plasma density is < 0.3 per cubic centimeter and its temperature is ~ 7000 K. The existence of this wind is the reason the stagnation point flow shown in Figure 2 is accepted as the most likely configuration for the heliosphere.

OUR PRESENT UNDERSTANDING OF THE HELIOSPHERE

Guided by Table 1, the fast mode speed, cf, in the local fluff may be between 10 and 40 km/s (the sound speed is 10 km/s). Thus, VI may be larger than cf which would modify the configuration shown in Figure 2 in three ways. First, a bow shock can form, much like the bow shock in front of the Earth`s magnetosphere. Second, if VI is supersonic, the pressure on the downstream side and flanks of the termination shock is reduced, causing the shock to be further from the Sun in these directions. These two effects have been numerically modeled in a few cases, an example of which is shown in Figure 3 in which VI is assumed to be highly supersonic (The empty circle inside the termination shock represents the inner boundary for the calculation and has no physical significance.). Thirdly, since BI will generally not be aligned with the heliotail, the configuration will no longer be axisymmetric about the heliotail axis. However, effects of an arbitrary BI have not yet been incorporated into numerical simulations.

Figure 3

There are many things that have still been ignored in models. For example, the solar wind dynamic pressure depends both on time and on position. It is likely that the termination shock will consequently move inward and outward over the solar cycle by many AU, and by a few AU over times as short as one month. Thus, during encounters with the shock by Voyages 1 and 2 the shock may move back and forth over the spacecraft several times once the shock is first encountered. Also ignored are the magnetic fields in the solar wind and the interstellar medium. Both magnetic fields will modify the shape and position of the heliopause and these fields control the access and motion of galactic cosmic rays into the heliosphere. With regard to shape of the shock, if the interstellar field is large enough so that VI < cf, the fast mode speed, then it is expected that the shock and heliopause will be more like the configuration shown in Figure 2. In this case, Pioneer 10, which is farther out than Voyagers 1 and 2, but is travelling more slowly in the downstream direction, may be the first spacecraft to reach the shock.

To Table Of Contents

DISTANCE TO THE TERMINATION SHOCK

The distances to the termination shock and to the heliopause are not known. Still, with equation (1) and more sophisticated derivative calculations, it is possible to make estimates. Figure 4 shows such a calculation of the shock distance in the upstream direction for a range of interstellar field strengths and densities. The estimated values for R range between 60 and 150 AU for the parameters from Table 1. This now forces the question of whether there a way of remotely determining R. This influences whether we will continue to collect data from spacecraft in the outer heliosphere. If Pioneer 10 and Voyagers 1/2 have any possibility of passing the shock and the heliopause, it would be a great addition to our knowledge of the heliosphere. Only a finite time exists because Pioneer 10 has about reached its limits and the Voyagers will run out of electrical power by about 2030.

Large version of Fig 4 (9K)

There have, in fact, been several tentative remote detections of the termination shock and heliopause Of these, probably the most tantalizing is the detection of radio emissions coming from the outer heliosphere. It is hypothesized that the emissions are coming from the nose of the heliopause - the stagnation point, and that the distance to the stagnation point is somewhere between 116 and 177 AU. These distances place R between 60 and 160 AU, which is reachable by the Voyager spacecraft. There are also suggestions of a small value for R from cosmic ray modulation over the solar cycle, but this is highly tentative because we still do not have a good understanding of galactic cosmic ray access to and propagation in the heliosphere.

To summarize, the heliosphere is probably well represented as stagnation point flow occuring between the solar wind and an interstellar wind. The resulting configuration is similar to the Earth's magnetosphere. Inside the heliopause, the solar wind passes through a termination shock that is elongated in the downstream direction and which is moving back and forth at speeds up to 100 km/s. The presence of three spacecraft in the outer heliosphere is a unique opportunity to explore the structure and properties of the termination shock, heliosheath, heliopause, and VLISM. This opportunity is realizable based on the indirect evidence we have that R is probably ~ 100, and may be as small as 60-70 AU.

Bibliography

Axford, W. I., The interaction of the solar wind with the interstellar medium, Solar Wind, NASA SP-308 (C. P. Sonett, P. J. Coleman, Jr., and J. M. Wilcox, eds.), pp 609-657, NASA, Washington D.C., 1972.

Baranov, V. B., Gas dynamics of the solar wind interaction with the interstellar medum, Space Sci. Rev., 52, 90-120, 1990.

Grzedzielski, S., and D. E. Page (eds.), Physics of the Outer Heliosphere, COSPAR Colloq. Series V. 1, Pergamon Press, Oxford, 1990.

Holzer, T. E., Interaction between the solar wind and the interstellar medium, Ann. Rev. Astron. Astrophys., 27, 199-234, 1989.

Parker, E. N., Interplanetary Dynamical Processes, Interscience, New York, 1963.

Suess, S. T., The heliopause, Reviews of Geophysics, 28, 1, 97-115, 1990.