In mid-1994, Pioneer 10 was 
60 AU from the Sun, while the solar ranges of
Voyagers 1 and 2 were 
56 and 42 AU, respectively. Even at such great
distances, the solar wind speed, averaged over a solar rotation or longer, is
close to that observed near Earth [ Barnes et al., 1992; Burlaga and
Ness, 1993 a; Belcher et al., 1993; Gazis et al., 1994]. Both
the Voyager and Pioneer data continue to show both solar-cycle variations and
25-day variations associated with solar rotation. Richardson et al.
[1994] have also reported a 1.3-year periodicity in solar wind speed at both
Voyager and Earth over the years 1987-1994; the cause of the 1.3-year variation
is not known. The amplitudes of all these speed variations diminish with
distance from the Sun due to the transfer of momentum when fast wind collides
with slower wind in its path. Figure 1 shows daily averages of the speed and
density of the solar wind observed by Voyager 2 between January, 1990, and June,
1993, when the spacecraft was 31 to 39 AU from the Sun [ Belcher et al.,
1993]. The 
70 km/s variations measured at Voyager in 1993 were considerably
smaller than the 
300 km/s variations measured during the same period near
Earth by IMP 8. (IMP = Interplanetary Monitoring Platform; IMP 8 has monitored
the near-Earth solar wind since 1973.)
The Voyager and Pioneer data have recently been reprocessed using new algorithms
which allow the computation of ion temperature to lower values than previously
possible [ Gazis et al., 1994]. The average proton temperature decreases
with increasing solar distance r as r
out to 20 AU, and is less well
determined but consistent with that power-law cooling rate out to the present
distances [ Gazis et al., 1994]. When compared to the r
power law
expected for adiabatic expansion, it is clear that something continuously heats
the protons, but the energy-transfer mechanisms are not yet identified.
Interplanetary shocks are important in determining the structure, dynamics, and
thermodynamics of the solar wind inside 
10 AU [ Whang, 1991], but both the
number and the strength of shocks are strongly diminished at greater distances
[ Burlaga, 1994]. It is not yet known whether the plasma heating in the
outer heliosphere is caused by turbulent cascades, by instabilities resulting
from the pickup of interstellar ions, or by other mechanisms.
Even less is known about the thermodynamics of electrons in the outer
heliosphere. The instruments on Voyager can measure electron energies only
above a 10 eV threshold and have therefore obtained little electron data beyond
10 AU. Ulysses is the first spacecraft to be flown beyond 1 AU with the
capability of resolving the two components of the interplanetary electron
distribution --- namely the thermalized core population and the nearly
collisionless, higher energy halo population which carries most of the heat flux
from the corona into the wind. McComas et al. [1992] reported that the
core electron temperature observed by Ulysses between 1 and 4 AU decreased, on
the average, as r
, which, perhaps surprisingly, is the same as the
proton temperature profile measured by the Pioneers. The core electron
temperature could also be estimated from noise spectra obtained by the radio
science experiment on Ulysses, and from those data Hoang et al. [1992]
reported that there were different power laws for different types of solar wind
flow and that the average core temperature was nearly constant from 1 to 3 AU.
A power law of r
can, however, be fit through their published data
within the rather large error bars. Other results from McComas et al.
[1992] are that: (1) the ratio of the core temperature to the energy of the
boundary between the core and halo populations remains roughly constant at the
value theoretically predicted by Scudder and Olbert [1979], (2) the ratio
of halo to core densities has roughly a constant value of 0.04, which is not
readily understood theoretically, and (3) the heat flux drops off as
r
, which is consistent with data obtained between 0.3 and 1 AU by the
Helios missions [ Pillip et al., 1990].
Although the direction of the solar-wind flow is usually close to radially away from the Sun, Voyager 2 observed a period of large-scale, systematic north-south flows near solar-activity minimum [ Lazarus et al., 1988]. Two different concepts of the cause of those north-south deflections have been developed. One concept is a vortex street created in the region of strong shear between fast and slow streams at different latitudes [ Veselovsky, 1989; Burlaga, 1990; Siregar et al., 1992; Siregar et al., 1993]. Recent three-dimensional magnetohydrodynamic simulations of the interactions of stationary streams caused by the tilt of the Sun's magnetic dipole with respect to the Sun's spin axis are also able to reproduce the north-south flows seen by Voyager [ Pizzo, 1994 a, b]. Observations farther out in the heliosphere may be able to distinguish between those two concepts because the vortex street would grow with distance while the stream interactions would die out.
The interplanetary magnetic field is, of course, weaker in the outer heliosphere than closer in, but like the plasma speed and temperature, its strength does not decrease monotonically, but shows local maxima and minima related to solar activity as well as to the interaction of fast and slow streams. The heliospheric current sheet (HCS) which divides regions dominated by the magnetic fields of the northern and southern magnetic poles of the Sun can still be discerned at 50 AU and the latitude of the HCS is generally consistent with the latitude inferred from measurements of the magnetic field at the surface of the Sun [ Burlaga and Ness, 1993 b]. The north-south component of the field has an average value of zero, even at the Voyager 1 location some 30 north of the ecliptic plane, in agreement with Parker's spiral model. The average azimuthal components were consistent with the expected spiral directions in 1986-7, but were greatly disturbed during 1989 when solar activity was high and transient flows can distort the HCS [ Burlaga and Ness, 1993 b].
The north-south component of the field has an average value of zero, even at the Voyager 1 location some 30 north of the ecliptic plane, in agreement with Parker's spiral model. The average azimuthal components were consistent with the expected spiral directions in 1986-7, but were greatly disturbed during 1989 when solar activity was high and transient flows can distort the HCS [ Burlaga and Ness, 1993 b].
There is a continuing debate about whether or not there is a flux deficit in the
distant magnetic field which, if it exists, would result from a net plasma flow
from the equator toward the poles [ Winterhalter et al., 1990; Burlaga
and Ness, 1993 a]. When fit to a power-law relation, the Voyager-2 plasma
data yield proton density proportional to r
and speed
proportional to r
[ Belcher et al., 1993], which, if taken
at face value, would indicate a divergence of the plasma out of the equatorial
region, but time (e.g., solar-cycle) variation may alias the radial fits.
The relations between the heliospheric current sheet, magnetic sectors, and stream interaction regions familiar to observers at 1 AU disappear at large distances to be replaced by several types of ``merged interaction regions'' [ Burlaga et al., 1993]. Sometimes transient streams and quasi-steady corotating streams interact to form compound streams even within 1 AU [ Behannon et al., 1991]; sometimes two transient streams collide and interact [ Phillips et al., 1992]; while at other times single streams maintain their identity to distances of at least 6 AU [ Siscoe and Intriligator, 1993].