ALBERT

Description: Since radio propagation measurements using either natural or spacecraft radio signals are used for probing the solar wind in the vicinity of the sun, they represent a key tool for studying the interplanetary consequences of solar structure and dynamic phenomena. New information on the near sun consequences was obtained from radio scintillation observations of coherent spacecraft signals. The results covering density fluctuations, fractional density fluctuations, coronal streamers, heliospheric current sheets, coronal mass ejections and interplanetary shocks are reviewed. A joint ICE S-band (13 cm wavelength) Doppler scintillation measurement with the SOHO white-light coronograph (LASCO) is described.

Description: Interplanetary disturbances characterized by plasma that is more turbulence and/or moves faster than the background solar wind are readily defected as transients in Doppler scintillation measurements of the near-Sun solar wind. Systematic analysis of over 23,000 hours of Pioneer Venus Orbiter Doppler measurements obtained inside 0.5 AU during 1979-1987 have made it possible for the first time to investigate the frequency of occurrence of Doppler scintillation transients under solar minimum conditions and to determine its dependence on solar cycle. On the basis of a total of 142 transients, Doppler scintillation transient rates vary from a high of 0.22 in 1979 (one every 4.6 days) to a low of 0.077 transients/d in 1986 (one every 13 days), a decrease by almost a factor of 3 from solar maximum to solar minimum. This solar cycle variation, the strongest yet of any solar wind Doppler scintillation property, is highly correlated with both solar activity characterized by sunspot number and the coronal mass ejection rates deduced from Solswind and Solar Maximum Mission (SMM) coronagraph observations. These results indicate that coronal mass ejections and Doppler scintillation transients are closely related not just during solar maximum, as occasional individual comparisons have shown in the past, but throughout the entire solar cycle, and strengthen the notation that the Doppler scintillation and optical transients are different manifestations of the same physical phenomenon. The magnitudes of the transients, as described by the ratio of peak to pretransient scintillation levels (EF for enhancement factor), and their distribution iwth heliocentric distance also vary with solar cycle. While EF tends to diminish with increasing heliocentric distance during high solar activity, it is more evenly distributed during low solar activity. EF is also lower during solar minimum, as 13% of the transients during solar maximum have values exceeding 23, the highest EF observed during solar minimum. These results are consistent with the fact that occasional major fast-moving interplanetary shocks that are observed during solar maximum are very rate during solar minimum.

Description: 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.

Description: Mars Observer radio science investigations focus on two major areas of study: the gravity field and the atmosphere of Mars. Measurement accuracies expressed as an equivalent spacecraft velocity are expected to be of the order of 100 microns/s (for both types of investigations) from use of an improved radio transponder for two-way spacecraft tracking and a highly stable on-board oscillator for atmospheric occultation measurements. Planned gravity investigations include a combination of classical and modern elements. A spherical harmonic (or equivalent) field model of degree and order in the range 30-50 will be obtained, while interpretation will be in terms of internal stress and density models for the planet, using the topography to be obtained from the Mars Observer laser altimeter. Atmospheric investigations will emphasize precision measurement of the thermal structure and dynamics in the polar regions, which are regularly accessible as a result of the highly inclined orbit. Studies based on the measurements will include polar processes, cycling of the atmosphere between the poles, traveling baroclinic disturbances, small-scale waves and turbulence, the planetary boundary layer, and (possibly) the variability and altitude of the ionosphere.

Description: Spectral broadening measurements conducted at S-band (13-cm wavelength) during solar minimum conditions in the heliocentric distance range of 3-8 R(sub O) by Mariner 4, Pioneer 10, Mariner 10, Helios 1, Helios 2, and Viking have been combined to reveal a factor of 2.6 reduction in bandwidth from equator to pole. Since spectral broadening bandwidth depends on electron density fluctuation and solar wind speed, and latitudinal variation of the former is available from coherence bandwidth measurements, the remote sensing spectral broadening measurements provide the first determination of the latitudinal variation of solar wind speed in the acceleration region. When combined with electron density measurements deduced from white-light coronagraphs, this result also leads to the first determination of the latitudinal variation of mass flux in the acceleration region. From equator to pole, solar wind speed increases by a factor of 2.2, while mass flux decreases by a factor of 2.3. These results are consistent with measurements of solar wind speed by multi-station intensity scintillation measurements, as well as measurements of mass flux inferred from Lyman alpha observations, both of which pertain to the solar wind beyond 0.5 AU. The spectral broadening observations, therefore, strengthen earlier conclusions about the latitudinal variation of solar wind speed and mass flux, and reinforce current solar coronal models and their implications for solar wind acceleration and solar wind modeling.

Description: Knowledge of the structure of the Sun's corona is important for our understanding of how this high-temperature plasma is heated, and of the processes involved in the acceleration of the solar wind. The structure can be investigated directly by imaging at optical and shorter wavelengths, or indirectly through the effects of changing electron density on the propagation of radio waves (scattering and scintillation). Radio measurements have established many of the characteristics of the density fluctuations in the corona and solar wind, but the fundamental nature of these structures is not yet fully understood. Two specific features that have proved difficult to explain are an abrupt increase in anisotropy of the irregularities close to the Sun, and a break in the power-law spectrum describing the density fluctuations. Here I argue that these features are the manifestation of a transition from small ray-like or filamentary structures in the corona that rotate with the Sun to turbulent density irregularities convecting with the solar wind. I estimate the size of the smallest filamentary structure within coronal holes to be about I km at the Sun, approximately three orders of magnitude smaller than the smallest filamentary structures observed in images of different wavelengths.

Description: An asymmetry in the radial variation of electron density above the cast and west limbs of the Sun was inferred from centimeter wavelength ranging measurements conducted by Voyager 2 during its 1985 solar conjunction. The Voyager 2 ranging measurements, which took place in the heliocentric distance range of 7-40 solar radius, have been compared with the white-light coronagraph measurements of the underlying corona collected by the Mark 3 K-coronameter located at the Mauna Loa Solar Observatory. It is shown that the disparity in radial profiles is not real but is instead caused by longitudinal variations stemming from the probing of significantly different source regions its revealed in the white-light measurements. These results improve our understanding of the probing abilities of ranging measurements and their relationship to white-light measurements. They reinforce the notion that the high-precision and high-sensitivity features of ranging measurements are more fully exploited in the investigation of density variations across the ubiquitous low-contrast raylike structures that permit the corona, rather than in determining radial density profiles.

Description: This paper conducts the first synoptic study of Doppler scintillation transients caused by interplanetary disturbances. The Doppler scintillation data used are part of the 2.3-GHz navigation data collected by the NASA Deep Space Network when tracking planetary spacecraft during 1979-1983, a period that includes solar maximum. A total of 148 separate transients covering a heliocentric distance range of 5-179 solar radii were detected, including 26 transients detected by more than one spacecraft. The frequency of occurrence was highest near the sun and decreased with radial distance, a reflection of the radial evolution of the transients and the sensitivity of the Doppler scintillation measurements to transients. Since transients can be disruptive, as was demonstrated during the encounter of Saturn by Pioneer 11 in 1979, information on Doppler scintillation transients is essential.

Description: Remote sensing radio occultation measurements are used here to study magnetization of the ionospheres of Venus and Mars. For Venus, the measurements yield results on frequency of occurrence of magnetization during solar maximum that are similar to those obtained from Pioneer Venus in situ magnetic field measurements. During solar minimum, magnetization of the Venus ionosphere is more pervasive than at solar maximum. Magnetization extends to higher solar zenith angles and appears stronger than at solar maximum. These results confirm that during solar minimum the high solar wind dynamic pressure state is more prevalent at Venus because the ionospheric plasma pressure is weaker than at solar maximum. Comparison of a large number of electron density profiles of Mars with those of Venus shows an absence of the ledge and disturbed topside plasma observed in the Venus profiles. These results do not constitute evidence against magnetization of the ionosphere of Mars.

Description: Results are presented of detailed comparisons between Doppler scintillation and in situ plasma measurements to improve the understanding of Doppler scintillation transients. During a combined observing period of nearly 3 mo in 1981-1982 near solar maximum, 22 transients were observed by the Pioneer Venus Orbiter spacecraft and 23 shocks were observed by Helios 1. It is found that at least 84 percent of the transients are shocks, while at least 90 percent of the shocks are transients. Although the temporal profiles of Doppler scintillation and mass flux density are similar, the magnitudes of the Doppler scintillation transients may not simply reflect those of mass flux density. Only one pronounced solar wind event that was observed in the mass flux density measurements showed no signature in the scintillation data; field and particle measurements by Helios 1 suggest that it is a noncompressive density enhancement and/or a magnetic cloud. It is shown that Doppler scintillation measurements can now be used by themselves to detect and locate interplanetary shocks near the sun with a relatively high degree of certainty.