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Upcoming Mars Surveyor lander missions will include extensive spectroscopic capabilities designed to improve interpretations of the mineralogy and geology of landing sites on Mars. The 1999 Marsokhod Field Experiment (MFE) was a Mars rover simulation designed in part to investigate the utility of visible/near-infrared and thermal infrared field spectrometers to contribute to the remote geological exploration of a Mars analog field site in the California Mojave Desert. The experiment simultaneously investigated the abilities of an off-site science team to effectively analyze and acquire useful imaging and spectroscopic data and to communicate efficiently with rover engineers and an on-site field team to provide meaningful input to rover operations and traverse planning. Experiences gained during the MFE regarding effective communication between different mission operation teams will be useful to upcoming Mars mission teams. Field spectra acquired during the MFE mission exhibited features interpreted at the time as indicative of carbonates (both dolomitic and calcitic), mafic rocks and associated weathering products, and silicic rocks with desert varnish-like coatings. The visible/near-infrared spectra also suggested the presence of organic compounds, including chlorophyll in one rock. Postmission laboratory petrologic and spectral analyses of returned samples confirmed that all rocks identified as carbonates using field measurements alone were calc-silicates and that chlorophyll associated with endolithic organisms was present in the one rock for which it was predicted. Rocks classified from field spectra as silicics and weathered mafics were recognized in the laboratory as metamorphosed monzonites and diorite schists. This discrepancy was likely due to rock coatings sampled by the field spectrometers compared to fresh rock interiors analyzed petrographically, in addition to somewhat different surfaces analyzed by laboratory thermal spectroscopy compared to field spectra.

An autonomous system has been developed to identify the presence of carbonates in near-IR reflectance spectra (0.35–2.5 μm). This system consists of a set of feature-extraction algorithms that operate in conjunction with a rule-based system to identify carbonates on the basis of presence and characteristics of absorption bands near 2.33 and 2.5 μm. This autonomous system was tested during the February 1999 Marsokhod field operations. While performance was compromised to some extent by noise problems associated with the spectrometer, the autonomous system successfully rejected all noisy spectra. Among the nonnoisy spectra that it accepted, it identified carbonates with a success rate of 70% compared with the performance of the field team, a success rate of 82–90% compared with the performance of the remote science team, and a false positive rate of 0%.

Corotating interaction regions (CIRs) and their successors play an important role in the structure and dynamics of the outer heliosphere. CIRs, consisting of a forward and reverse shock pair in association with a region of enhanced solar wind density and temperature, are a dominant structure in the solar wind at low heliographic latitudes at heliocentric distances between 2 and 8 AU. At larger heliocentric distances, CIRs spread, merge, and interact to form merged interaction regions (MIRs). MIRs are a dominant structure at low latitudes between 8 and 12 AU. At even larger heliocentric distances, considerable uncertainty remains regarding the small-scale structure of the solar wind. Solar wind and interplanetary magnetic field observations from the Pioneer 10 and Voyager 2 spacecraft were used to conduct a large-scale survey of CIRs and their successors over solar cycles 21 and 22 between heliocentric distances of 1 and 50 AU to categorize changes that occur as CIRs were convected outward from the Sun. At heliocentric distances less than 37 AU, similar changes were observed at similar heliocentric distances at different spacecraft during different portions of the solar cycle. These changes may reflect qualitative changes in the dynamics of the solar wind as CIRs spread and merge and the flux of kinetic energy available in solar wind streams declines with respect to the flux of thermal energy. At heliocentric distances greater than 37 AU, Pioneer 10 and Voyager 2 observed different types of structure. Interpretation of these differences is complicated by differences in the trajectories of the two spacecraft, and it remains to be determined whether they are associated with latitudinal gradients, solar cycle variations or the effects of interstellar pickup ions at Voyager 2.

Possible signatures of interstellar pickup hydrogen have been identified in data from the Pioneer 10 and 11 Ames plasma analyzers at locations out to beyond the orbit of Saturn, and at a wide range of longitudes in the heliosphere, both upstream and downstream in the interstellar wind. These signatures provide an extension of pickup ion observations out to beyond 15 AU heliocentric distance, including near where the maximum pickup hydrogen flux is expected. A depletion of interstellar pickup hydrogen downstream from the Sun in the interstellar flow may be indicated. These signatures are consistent with Ulysses pickup ion measurements.

Interstellar pickup ions, if they are thermalized, could produce a longitudinal asymmetry in solar wind temperature in the outer heliosphere. Between 1978 and mid-1992, the Pioneer 10 and Voyager 2 spacecraft were well situated to search for such an asymmetry. Pioneer 10 was headed downstream with respect to the local interstellar medium while Voyager 2 was headed upstream. Both spacecraft were at large heliocentric distances; by mid-1992, Pioneer 10 was at 55 AU, while Voyager 2 was at 37 AU. Measurements from these spacecraft and the Pioneer Venus Orbiter are complicated by solar cycle effects but suggest an upper limit of 5000 K for any asymmetry that might be present. These measurements are also consistent with no asymmetry. There is also some suggestion that the radial profile of solar wind temperature in the vicinity of the solar equator was steeper during the declining phase of the last solar cycle. The solar wind temperature has increased at Voyager 2 after mid-1992, but Voyager 2 has also been moving to higher Heliographic latitudes, where such an increase should be expected from the well-known latitudinal gradient in solar wind temperature that has been observed by Pioneer 11 and Ulysses.

In situ observations by different spacecraft have shown that the heliospheric current sheet (HCS) is associated with a narrow band of low speed high density solar wind. The inclination of the HCS and the three-dimensional structure of solar wind streams is related to the inclination of the coronal neutral line. Observations from the Ulysses spacecraft during the descending phase of solar cycle 22, when Wilcox Solar Observatory coronal field models suggest that the inclination of the coronal neutral line was between 50° and 70°, showed that the inclination of the HCS was ≃20° while solar wind streams were confined to heliographic latitudes less than 35°. At higher latitudes, low and medium speed solar wind disappeared and the solar wind speed was uniformly high. Observations from the Pioneer 11 - Voyager 2 spacecraft pair near the time of the 1986 solar minimum revealed a different structure. At that time the inclination of the coronal neutral line was unusually low (

For over three decades a succession of spacecraft have provided in situ measurements of interplanetary plasma and magnetic field parameters. These measurements span a range of heliocentric distances from 0.3 to 61 AU, and provide an explicit picture of the three-dimensional structure of the inner and outer heliosphere in the vicinity of the ecliptic plane, while ground-based interplanetary scintillation (IPS) measurements and observations from the Ulysses spacecraft extend our knowledge of the inner and outer heliosphere to higher latitudes. The structure of the heliosphere varies dramatically over the course of a solar cycle. Much of this variation can be related to changes in the structure and inclination of the coronal magnetic field.

Long-term enhancements in solar wind speed over timescales on the order of a year appear to be a common feature throughout the heliosphere over heliocentric distances that range from less than 0.72 AU to greater than 60. The origin of these events remains to be determined, but they are almost certainly associated with long-term variations at the solar wind source, in contrast with smaller-scale structures such as CIRs, MIRs, and GMIRs which are dynamical in origin. We present a survey of the long-term speed enhancements observed at the Pioneer Venus Orbiter (PVO), IMP 8, Voyager 2, and Pioneer 10 between 1974 and 1994 and compare this with published reports of smaller-scale events such as MIRs. We examine several of these long-term speed enhancements in detail to identify and characterize aspects of their structure, then describe how that structure evolves with heliocentric distance. Finally we discuss some of the implications of these events.

It has been suggested by Richardson et al. [1995] that the solar wind shows signs of deceleration due to mass-loading by interstellar pickup ions. They based their conclusion on observations from the Voyager 2 and IMP 8 spacecraft. Observations from the Pioneer Venus Orbiter (PVO), IMP 8, Voyager 2, and Pioneer 10 spacecraft are compared to look for this deceleration. While these comparisons suggest the possibility of deceleration at Voyager 2, the magnitude of this deceleration appears to be significantly less than the deceleration reported by Richardson et al. [1995]. Some of this difference could be explained by a north-south asymmetry in solar wind speed during solar cycle 21. The implications of these results are discussed, along with possibilities for further investigations.

Solar wind measurements from the Pioneer 10, Pioneer 11, Voyager 2, IMP 8, and Pioneer Venus Orbiter (PVO) spacecraft were examined to search for long-term periodicities during the last three solar cycles. For the time of the last solar maximum, these measurements confirm the existence of the periodic 1.3-year enhancements in solar wind velocity reported by Richardson et al. (1994). For most of the preceding two solar cycles, long-term velocity enhancements occurred that were similar in structure but lacked the 1.3-year periodicity. It appears that long-term enhancements in solar wind velocity, with durations on the order of a few months to a year, are a common feature throughout the heliosphere.

Solar wind observations from the IMP 8 and Pioneer Venus Orbiter (PVO) spacecraft from 1982 until 1988 are combined to construct synoptic maps of solar wind parameters near 1 AU. Each map consists of 6 months of hourly averaged solar wind data, binned by heliographic latitude and Carrington longitude and projected back to the Sun. These maps show the structure and time evolution of solar wind streams near 1 AU in the heliographic latitudes of ±7.25° and provide an explicit picture of several phenomena, such as gradients, changes in the inclination of the heliospheric current sheet, and the relative positions of various structures in the inner heliosphere, that is difficult to obtain from single-spacecraft observations. The stream structure varied significantly during the last solar cycle. Between 1982 and early 1985, solar wind parameters did not depend strongly on heliographic latitude. During the last solar minimum, the solar wind developed significant latitudinal structure, and high-speed streams were excluded from the vicinity of the solar equator. The interplanetary magnetic field was strongly correlated with the coronal field, and the current sheet tended to coincide with the coronal neutral line. The solar wind speed showed the expected correlations with temperature, interplanetary magnetic field, and distance from the current sheet. The solar wind speed was anticorrelated with density, but the regions of highest density occurred east of the heliospheric current sheet and the regions of lowest solar wind speed. This is consistent with compression at the leading edge of high-speed streams.

Solar wind measurements from the Pioneer 10, Pioneer 11, and Voyager 2 spacecraft are now available through mid-1993. These measurements extend our knowledge of the outer heliosphere to heliographic latitudes that range between -10 deg and 17.5 deg, and provide insight into the variation with solar cycle of the structure of the distant solar wind. The average temperature, mass flux density, dynamic pressure, and kinetic and thermal energy flux densities varied strongly with solar cycle at the latitude of Pioneer 11 (10 deg to 17 deg N), but were almost constant in the vicinity of the solar equator. These parameters may have increased with latitude between the solar equator and 17 deg N. There was also a short-term variation in average solar wind parameters near the time of the 1986 solar minimum, when the inclination of the heliospheric current sheet dropped below the latitude of Pioneer 11.

During the late declining phase of the solar cycle, the tilt of the solar magnetic dipole with respect to the Sun's rotation axis leads to large-scale organization of the solar wind, such that alternating regions of high- and low-speed solar wind are observed in the ecliptic plane. In this paper, we use Doppler scintillation measurements to investigate mass flux of these two types of solar wind in the ecliptic plane and inside 0.3 AU, where in situ measurements have not been possible. To the extent that Doppler scintillation reflects mass flux, we find that mass flux in high-speed streams: (1) is lower (by a factor of approximately 2.2) than the mass flux of the average solar wind in the heliocentric distance range of 0.3-0.5 AU; (2) is lower still (by as much as a factor of about 4) than the mass flux of the slow solar wind associated with the streamer belt; and (3) appears to grow with heliocentric distance. These Doppler scintillation results are consistent with the equator to pole decrease in mass flux observed in earlier spectral broadening measurements, and with trends and differences between high- and low-speed solar wind observed by in situ measurements in the range of 0.3-0.1 AU. The mass flux results suggest that the solar wind flow in high-speed streams is convergent towards the ecliptic near the Sun, becoming less convergent and approaching radial with increasing heliocentric distance beyond 0.3 AU. The variability of mass flux observed within equatorial and polar high-speed streams close to the Sun is strikingly low. This low variability implies that, as Ulysses currently ascends to higher latitudes and spends more time in the south polar high-speed stream after crossing the heliocentric current sheet, it can expect to observe a marked decrease in variations of both mass flux and solar wind speed, a trend that appears to have started already.

Magnetometer data obtained over the course of the previous solar cycle by the Pioneer Venus orbiter at about 0.7 AU and IMP 8 at 1.0 AU are used to compare the long-term behavior of the IMF at these two heliocentric distances. Similarities include an enhancement in the typical or median field magnitude during the declining phase of the solar cycle (as compared to solar maximum or minimum), slight decreases in the Parker spiral angle from the declining phase through solar minimum, similar trends in the Alfvenic and magnetosonic Mach numbers, and a remarkably consistent sector structure. Differences include the temporal behavior of the high-field tail of the field distribution, showing that high fields are most frequently observed during solar maximum at the Earth but during the declining phase of activity at Venus. This feature suggests that the perceived occurrence history of large fields from transient disturbances such as coronal mass ejections is a sensitive function of position within the heliosphere.

The mass flux density and velocity of the solar wind at polar latitudes can provide strong constraints on solar wind acceleration mechanisms. We use plasma observations from the first polar passage of the Ulysses spacecraft to investigate this question. We find that the mass flux density and velocity are too high to reconcile with acceleration of the solar wind by classical thermal conduction alone. Therefore acceleration of the high-speed wind must involve extended deposition of energy by some other mechanism, either as heat or as a direct effective pressure, due possibly to waves and/or turbulence, or completely non-classical heat transport.

[1] Potential field source surface models of the coronal magnetic field, based on Mt. Wilson Observatory synoptic magnetograms, are used to infer the coronal hole sources of low-heliolatitude solar wind over approximately the last three solar cycles. Related key parameters like interplanetary magnetic field and bulk velocity are also calculated. The results illustrate how the evolving contribution of the polar hole sources relative to that from low-latitude and midlatitude active region hole sources can explain solar magnetic field control of long-term interplanetary variations. In particular, the enduring consistent magnetogram record and continuous model displays produce a useful overview of the solar control of interplanetary cycles and trends that affect space weather.

At the end of 1992, the Pioneer 10, Pioneer 11, and Voyager 2 spacecraft were at heliocentric distances of 56.0, 37.3, and 39.0 AU and heliographic latitudes of 3.3 deg N, 17.4 deg N, and 8.6 deg S, respectively. Pioneer 11 and Voyager 2 are at similar celestial longitudes, while Pioneer 10 is on the opposite side of the Sun. All three spacecraft have working plasma analyzers, so intercomparison of data from these spacecraft provides important information about the global character of the solar wind in the outer heliosphere. The averaged solar wind speed continued to exhibit its well-known variation with solar cycle: Even at heliocentric distances greater than 50 AU, the average speed is highest during the declining phase of the solar cycle and lowest near solar minimum. There was a strong latitudinal gradient in solar wind speed between 3 deg and 17 deg N during the last solar minimum, but this gradient has since disappeared. The solar wind temperature declined with increasing heliocentric distance out to a heliocentric distance of at least 20 AU; this decline appeared to continue at larger heliocentric distances, but temperatures in the outer heliosphere were suprisingly high. While Pioneer 10 and Voyager 2 observed comparable solar wind temperatures, the temperature at Pioneer 11 was significantly higher, which suggests the existence of a large-scale variation of temperature with heliographic longitude. There was also some suggestion that solar wind temperatures were higher near solar minimum.

In order to understand the solar cycle variation of interplanetary shocks and their driving source at 0.72 AU, a survey of Pioneer Venus Orbiter (PVO) magnetometer and plasma data from 1979-1988 has been conducted. Known shock drivers at 1.0 AU include coronal mass ejections (CMEs) and fast/slow stream interactions. In our analysis, CMEs were identified by a decrease in plasma temperature to background or below accompanied by an increase in plasma density and dynamic pressure. It was also required that the magnetic field exhibit a coherent rotation over about a day and an increase and decline in magnitude on a timescale of hours to days. Stream interactions were identified by a characteristic increase in ion temperature and velocity coincident with a decrease in density and a coincident increase in the total magnetic field magnitude. These signatures were usually preceded within 24 hours by a change in flow angle. In all, 45 shocks were identified: 36 driven by CMEs, 6 resulting from fast/slow stream interactions, and 3 with sources that could not be defined. The shocks driven by CMEs show a solar cycle variation that roughly follows the sunspot number. These shocks all have normals consistent with radial propagation of the shock fronts from the sun. In contrast, the few stream interaction related shocks show a tendency to occur later in the solar cycle and have a broader distribution of shock normals.

Solar wind speeds and magnetic field data from the Pioneer Venus Orbiter (0.7 AU) and IMP 8 (1 AU) have been compared to infer the latitudinal structure of the solar wind in the inner heliosphere between 1984 and 1987. The solar wind in the inner heliosphere was found to exhibit an unusual structure during the last solar minimum (1986-1987). High-speed streams were excluded from the vicinity of the solar equator, and the solar wind at low heliographic latitudes (less than 3 deg) was characterized by low-speed solar wind with irregular fluctuations in velocity. At higher latitudes the solar wind had a conventional stream structure with two high-speed streams per solar rotation. While the velocities of these high-speed streams did not appear to vary significantly with latitude, the latitudinal gradients at the equatorward boundaries of high-speed streams were high.

A radial alignment of three solar wind stream structures observed by IMP-7 and -8 (at 1.0 AU) and Voyager 1 and 2 (in the range 1.4 to 1.8 AU) in late 1977 is presented. It is demonstrated that several important aspects of the observed dynamical evolution can be both qualitatively and quantitatively described with a single-fluid 2-D MHD numerical model of quasi-steady corotating flow, including accurate prediction of: (1) the formation of a corotating shock pair at 1.75 AU in the case of a simple, quasi-steady stream; (2) the coalescence of the thermodynamic and magnetic structures associated with the compression regions of two neighboring, interacting, corotating streams; and (3) the dynamical destruction of a small (i.e., low velocity-amplitude, short spatial-scale) stream by its overtaking of a slower moving, high-density region associated with a preceding transient flow. The evolution of these flow systems is discussed in terms of the concepts of filtering and entrainment.

Simultaneous measurements are made of the solar wind proton temperatures by the Voyager 1 and 2 spacecraft, far from earth, and the IMP 8 spacecraft in earth orbit. This technique permits a separation of radial and temporal variations of solar wind parameters. The average value of the proton temperature between 1 and 9 AU is observed to decrease as r (the heliocentric radius) to the -(0.7 + or - 0.2). This is slower than would be expected for adiabatic expansion. A detailed examination of the solar wind stream structure shows that considerable heating occurs at the interface between high and low speed streams.

Voyager 1/2 observations are used to confirm and quantify the unusual bluntness of the dayside Saturn magnetopause suggested by the earlier Pioneer 11 measurements. Once corrected for variations in solar wind pressure, the Saturn observations are found to indicate a magnetospheric radius in the terminator plane equal to 1.8 times the nose distance. This ratio is 30% greater than the terrestrial value and is corroborated by the very blunt shape of the Saturn bow shock and large width of the forward magnetosheath. Possible causes and ramifications of these results for the solar wind interaction with Saturn are discussed.

Time profiles and histograms of plasma data from Pioneers 10 and 11 are examined for the period between 1975 and 1983. During this time, Pioneer 10 traveled between a heliocentric distance of 8.7 and 30.4 AU. The velocity structure of the solar wind at these heliocentric distances is found to have one of two distinct forms: approximately 70 percent of the time the solar wind has a nearly flat velocity profile. Occasionally, this flat velocity profile is accompanied by quasi-periodic variations in density and in thermal speed consistent with the concept that the 'corotating interaction regions' which are produced by the interaction of high- and low-speed streams at intermediate heliocentric distances are replaced by 'pressure regions' in the outer heliosphere. The remaining 30 percent of the time the solar wind is marked by large (50-200 km/s) long-term (30-120 days) shifts in the average solar wind velocity.

Observations are reported of 35 interplanetary shocks detected at heliocentric distances between 6.5 and 9.4 AU in mid-1980 by the Voyager 1, Voyager 2, and Pioneer 11 spacecraft. These shocks were all evidently generated by the interaction of corotating streams. Measurements of the pre- and postshock plasma parameters are used to determine the shock normals and speeds for each shock. Twelve of these 35 events are observed at all three spacecraft. The shock parameters at each spacecraft are compared to determine the time history for the 12 shocks. The single-spacecraft determinations of shock normal and speed are compared wih the results of several techniques for determining shock velocities using all three spacecraft. It is found that shocks undergo significant evolution as they travel past the three spacecraft. Every shock seen by both end spacecraft was also seen by the middle spacecraft. Shock formation possibly may be occurring at a heliocentric distance as great as 7 AU. It appears that the shock surfaces have a complex three-dimensional structure with 'ripples' on a scale of 0.001-1.0 AU.

The Venus bow shock location has been measured at nearly 2000 shock crossings, and its dependence on solar EUV, solar wind conditions, and the interplanetary magnetic field determined. The shock position at the terminator varies from about 2.14 Venus radii at solar minimum to 2.40 Venus radii at solar maximum. The location of the shock varies little with solar wind dynamic pressure but strongly with solar wind Mach number. The shock is farthest from Venus on the side of the planet in which newly created ions gyrate away from the ionosphere. When the interplanetary magnetic field is perpendicular to the flow, the cross section of the shock is quite elliptical. This effect appears to be due to the anisotropic propagation of the fast magnetosonic wave. When the interplanetary magnetic field is aligned with the flow, the bow shock cross section is circular and only weakly sensitive to changing EUV flux.