A. J. Kliore

The structure of the venus ionosphere

Our current knowledge of the spatial structure of the Venus ionosphere and its temporal behavior is reviewed, with emphasis on the more recent Pioneer Venus measurements and analysis not covered in earlier reviews. We will stress the ionosphere structure, since other papers in this issue deal with its dynamics, and its magnetic properties. We also discuss some of the limitations that the orbit has placed on the spatial and temporal coverage of the ionosphere. For the benefit of future users of the data some of the factors which affect the measurement accuracies are discussed in an Appendix.

Venus - Mass, gravity field, atmosphere, and ionosphere as measured by the Mariner 10 dual-frequency radio system

Analysis of the Doppler tracking data near encounter yields a value for the ratio of the mass of the sun to that of Venus of 408,523.9 � 1.2, which is in good agreement with prior determinations based on data from Mariner 2 and Mariner 5. Preliminary analysis indicates that the magnitudes of the fractional differences in the principal moments of inertia of Venus are no larger than 10-4, given that the effects of gravity-field harmonics higher than the second are negligible. Additional analysis is needed to determine the influence of the higher order harmonics on this bound. Four distinct temperature inversions exist at altitudes of 56, 58, 61, and 63 kilometers. The X-band signal was much more rapidly attenuated than the S-band signal and disappeared completely at 52-kilometer altitude. The nightside ionosphere consists of two layers having a peak density of 104 electrons per cubic centimeter at altitudes of 140 and 120 kilometers. The dayside ionosphere has a peak density of 3 X 105 electrons per cubic centimeter at an altitude of 145 kilometers. The electron number density observed at higher altitudes was ten times less than that observed by Mariner 5, and no strong evidence for a well-defined plasmapause was found.

The atmosphere of Io from Pioneer 10 radio occultation measurements

Abstract The occultation of the Pioneer 10 spacecraft by Io (JI) provided an opportunity to obtain two S -band radio occultation measurements of its atmosphere. The dayside entry measurements revealed an ionosphere having a peak density of about 6 × 10 4 elcm −3 at an altitude of about 100 km. The topside scale height indicates a plasma temperature of about 406 K if it is composed of Na + and 495 K if N 2 + is principal ion. A thinner and less dense ionosphere was observed on the exit (night side), having a peak density of 9 × 10 3 elcm −3 at an altitude of 50 km. The topside plasma temperature is 160 K for N 2 − and 131 K for Na + . If the ionosphere is produced by photoionization in a manner analogous to the ionospheres of the terrestrial planets, the density of neutral particles at the surface of Io is less than 10 11 −10 12 cm 3 , corresponding to a surface pressure of less than 10 −8 to 10 −9 bars. Two measurements of its radius were also obtained yielding a value of 1830 km for the entry and 192 km for the exit. The discrepancy between these values may indicate an ephemeris uncertainty of about 45 km. The two measurements yield an average radius of 1875 km, which is not in agreement with the results of the Beta Scorpii stellar occultation.

Galileo radio occultation measurements of Io's ionosphere and plasma wake

Six radio occultation experiments were conducted with the Galileo orbiter in 1997, yielding detailed measurements of the distribution and motion of plasma surrounding Io. This distribution has two components. One is highly asymmetric, consisting of a wake or tail that appears only on the downstream side and extends to distances as large as 10 Io radii. The other resembles a bound ionosphere and is present within a few hundred kilometers of Ios surface throughout the upstream and downstream hemispheres. Motion of plasma within the wake was measured through cross correlation of data acquired simultaneously at two widely separated terrestrial antennas. Plasma near Ios equatorial plane is moving away from Io in the downstream direction. Its speed increases from 30 km s−1 at a distance of 3 Io radii from the center of Io to 57 km s−1 at 7 Io radii. The latter corresponds to corotation with Jupiters magnetic field, which suggests that bulk plasma motion rather than wave motion is being observed. Results for the bound ionosphere include vertical profiles of electron density at 10 locations near Ios terminator. The ionosphere is substantial, with the peak density exceeding 50,000 cm−3 at 9 out of 10 locations and reaching a maximum of 277,000 cm−3. The peak density varies systematically with Io longitude, with maxima near the center of the hemispheres facing toward (0°W) and away from (180°W) Jupiter and minima near the center of the downstream (90°W) and upstream (270°W) hemispheres. This pattern may be related to the Alfvenic current system induced by Ios motion through magnetospheric plasma. The vertical extent of the bound ionosphere increases from ∼200 km near the center of the upstream hemisphere to ∼400 km near the boundary between the leading and trailing hemispheres. There is a close resemblance between one ionospheric profile and a Chapman layer, and the topside scale height implies a plasma temperature of 202±14 K if Na+ is the principal ion. Two intense volcanic hot spots, Kanehekili and 9606A, may be influencing the atmospheric structure at this location.

Mariner 9 S-Band Martian Occultation Experiment: Initial Results on the Atmosphere and Topography of Mars

A preliminary analysis of 15 radio occultation measurements taken on the day side of Mars between 40�S and 33�S has revealed that the temperature in the lower 15 to 20 kilometers of the atmosphere of Mars is essentially isothermal and warmer than expected. This result, which is also confirmed by the increased altitude of the ionization peak of the ionosphere, can possibly be caused by the absorption of solar radiation by fine particles of dust suspended in the lower atmosphere. The measurements also revealed elevation differences of 13 kilometers and a range of surface pressures between 2.9 and 8.3 millibars. The floor of the classical bright area of Hellas was found to be about 6 kilometers below its western rim and 4 kilometers below the mean radius of Mars at that latitude. The region between Mare Sirenum and Solis Lacus was found to be relatively high, lying 5 to 8 kilometers above the mean radius. The maximum electron density in the ionosphere (about 1.5 x 105 electrons per cubic centimeter), which was found to be remarkably constant, was somewhat lower than that observed in 1969 but higher than that observed in 1965.

Ionosphere of Callisto from Galileo radio occultation observations

heights of 29.6 and 49.0 km. Four different methods, based on both photoionization and electron impact ionization, were used to obtain estimates of the corresponding neutral densities at the surface. The various assumptions inherent in these methods required using a variety of parameters, (cross sections, rate constants, etc.) all with their associated uncertainties. It was rather surprising and reassuring to find that all of the methods used to estimate the surface neutral density gave very similar results in each of the eight cases. The estimated values fall between 1 and 3 � 10 10 cm � 3 , leading to an estimate for the column density of from 3 to 4 � 10 16 cm � 2 . INDEX TERMS: 6218 Planetology: Solar System Objects: Jovian satellites; 6028 Planetology: Comets and Small Bodies: Ionospheres—structure and dynamics; 6026 Planetology: Comets and Small Bodies: Ionospheres—composition and chemistry; 6025 Planetology: Comets and Small Bodies: Interactions with solar wind plasma and fields; KEYWORDS: Callisto, ionosphere, atmosphere, radio, occultation

The shape of Mars from the Mariner 9 occultations

Abstract The extinction time of the radio signal, as the Mariner 9 spacecraft was occultated by Mars, together with an accurate ephemeris of the spacecraft were used to determine radii from the mass center to the occulting feature. Similarly estimations were made of the radius to a point where the pressure reached a certain fixed value. Several simple models were proposed to fit both sets of radii data.

Small-scale turbulence in the atmosphere of Venus

Abstract Previous studies based on radio scintillation measurements of the atmosphere of Venus have identified two regions of small-scale temperature fluctuations located in the vicinity of 45 and 60 km. A global study of the fluctuations near 60 km, which are consistent with wind-shear-generated turbulence, was conducted using the Pioneer Venus measurements. The structure constants of refractive index fluctuations cn2 and temperature fluctuations cT2 increase poleward, peak near 70° latitude, and decrease over the pole; cn2 varies from 2 × 10−15 to 1.5 × 10−14m 2 3 and cT2 from 4 × 10−3 to 7 × 10−2°K2m −2 3 . These results indicate greater turbulent activity at the higher latitudes. In the region near 45 km the refractive index fluctuations and the corresponding temperature fluctuations are substantially lower. Based on the analysis of one representative occultation measurement, c n 2 = 2 × 10 −16 m −2 3 and c T 2 = 7.3 × 10 −4 ° K 2 m −2 3 in the 45-km region. The fluctuations in this region also appear to be consistent with wind-shear-generated turbulence. The turbulence level is considerably weaker than that at 60 km; the energy dissipation rate e is 4.9 × 10−5m2sec−3 and the small-scale eddy diffusion coefficient K is 2 × 103 cm2 sec−1.

MICROWAVE ABSORPTION CHARACTERISTICS OF THE CLOUDS OF VENUS FROM MARINER 10 RADIO OCCULTATION

Measurements of received signal strength at S-band (13 cm) and X-band (4.8 cm) wavelengths during the radio occultation of Mariner 10 by Venus on February 5, 1974, are examined in order to study the structure and composition of the absorbing medium. The frequency excursions of the signals are determined and used to obtain the structure of the refractive index in the lower atmosphere. Profiles of excess signal attenuation due to atmospheric scattering and absorption are presented which indicate that the X-band signal experienced much more absorption and was extinguished at about 50 km, while the S-band signal penetrated to about 42 km. The optical-depth data are inverted by means of a discrete inversion method to obtain the absorption coefficient for each band as a function of height, and the resulting absorption-coefficient profiles are compared with the attenuation at vertical incidence modeled from planetary radar and passive microwave observations of Venus. The absorption coefficients at the two wavelengths are employed to estimate the liquid content and composition of the microwave-absorbing cloud particles.