Arvydas J. Kliore

Models of the structure of the atmosphere of Venus from the surface to 100 kilometers altitude

From a critical comparison and synthesis of data from the four Pioneer Venus Probes, the Pioneer Venus Orbiter, and the Venera 10, 12, and 13 landers, models of the lower and middle atmosphere of Venus are derived. The models are consistent with the data sets within the measurement uncertainties and established variability of the atmosphere. The models represent the observed variations of state properties with latitude, and preserve the observed static stability. The rationale and the approach used to derive the models are discussed, and the remaining uncertainties are estimated.

Occultation Experiment: Results of the First Direct Measurement of Mars's Atmosphere and Ionosphere

Changes in the frequency, phase, and amplitude of the Mariner IV radio signal, caused by passage through the atmosphere and ionosphere of Mars, were observed immediately before and after occultation by the planet. Preliminary analysis of these effects has yielded estimates of the refractivity and density of the atmosphere near the surface, the scale height in the atmosphere, and the electron density profile of the Martian ionosphere. The atmospheric density, temperature, and scale height are lower than previously predicted, as are the maximum density, temperature, scale height, and altitude of the ionosphere.

The atmosphere of Mars from mariner 9 radio occultation measurements

Abstract The Mariner 9 spacecraft was used to perform 160 radio occultation measurements in orbit about Mars during November and December of 1971. At that time, Mars was experiencing a severely obscuring global dust storm. The effect of dust in the atmosphere was reflected in the reduced temperature gradients that were measured in the daytime near-equatorial atmosphere, indicating heating of the atmosphere by solar radiation being absorbed by dust and a simultaneous cooling of the surface. The temperature gradients increased somewhat with time, possibly indicating a gradual clearing of the atmosphere. Measurements made at 65° latitude near the morning terminator showed atmospheric temperatures consistent with condensation of carbon dioxide at low altitude. The surface pressures in the near equatorial regions ranged from a high of 8.9 mbar in Hellas to a low of 2.8 mbar in the Claritas and Tharsis areas, with a mean pressure of 4.95 mbar. The pressures deduced from measurements at 65° latitude ranged from 7.2 to 10.3 mbar, with a mean of 8.9 mbar. The pressure altitudes, referred to a pressure level of 6.1 mbar, show a range in the equatorial regions from a low of −4.4 km in Hellas to a high of 9.6 km in Claritas, with a net excursion of 14.0 km and a mean altitude of 2.7 km. In contrast, the region at 65° longitude shows uniformly negative altitudes, with a mean of −2.6 km. This disparity in pressures, which is also reflected in the measured radii between the near-equatorial and 65° latitude measurements, strongly suggests that the physical shape of Mars is more oblate than the shape of its gravitational equipotential surface, leading to higher atmospheric pressures near the poles than at the equator. However, more measurements at high latitudes are necessary to support this hypothesis. A daytime ionosphere having a peak density of about 1.5 − 1.7 × 10 5 el/cm 3 at an altitude of 140-134 km over a range of solar zenith angles of 56-47° was measured, showing some correlation between the variations in the peak density and the solar flux measured from the earth. The average topside plasma scale height was 38.5 km, showing little correlation with solar flux and solar zenith angle.

Preliminary Results on the Atmospheres of Io and Jupiter from the Pioneer 10 S-Band Occultation Experiment

The preliminary analysis of data from the Pioneer 10 S-band radio occultation experinment has revealed the presence of an ionosphere on the Jovian satellite Io (JI) having an electron density peak of about 6 x 104 electrons per cubic centimeter at an altitude of approximately 60 to 140 kilometers. This suggests the presence of an atmosphere having a surface number density of about 1010 to 1012 per cubic centimeter, corresponding to an atmospheric surface pressure of between 10-8 and 10-10 bar, at or below the detection threshold of the Beta Scorpii stellar occultation. A measurement of the atmosphere of Jupiter was obtained down to the level of about 80 millibars, indicating a large temperature increase at about the 20 millibar level, which cannot be explained by the absorption of solar radiation by methane alone and can possibly be due to absorption by particulate matter.

Atmosphere and ionosphere of venus from the mariner v s-band radio occultation measurement.

Measurements of the frequency, phase, and amplitude of the S-band radio signal of Mariner V as it passed behind Venus were used to obtain the effects of refraction in its atmosphere and ionosphere. Profiles of refractivity, temperature, pressure, and density in the neutral atmosphere, as well as electron density in the daytime ionosphere, are presented. A constant scale height was observed above the tropopause, and the temperature increased with an approximately linear lapse rate below the tropopause to the level at which signal was lost, presumably because heavy defocusing attenuation occurred as critical refraction was approached. An ionosphere having at least two maxima was observed at only 85 kilometers above the tropopause.

Jupiter's ionosphere: Results from the First Galileo Radio Occultation Experiment

The Galileo spacecraft passed behind Jupiter on December 8, 1995, allowing the first radio occultation measurements of its ionospheric structure in 16 years. At ingress (24°S, 68°W), the principal peak of electron density is located at an altitude of 900 km above the 1-bar pressure level, with a peak density of 105 cm−3 and a thickness of ∼200 km. At egress (43°S, 28°W), the main peak is centered near 2000 km altitude, with a peak density of 2×104 cm−3 and a thickness of ∼1000 km. Two thin layers, possibly forced by upwardly propagating gravity waves, appear at lower altitudes in the ingress profile. This is the first in a two-year series of observations that should help to resolve long-standing questions about Jupiters ionosphere.

Zonal Winds in the Middle Atmosphere of Venus from Pioneer Venus Radio Occultation Data

Abstract Pioneer Venus radio occultation data for four seasons between December 1978 and October 1981 are used to construct meridional cross sections of temperature and zonal wind velocity for the middle atmosphere of Venus in the altitude range of ∼40–80 km and in the latitude range of ∼1 5–85°. Wind speeds are derived assuming cyclostrophic balance. The wind field contains as many as three jets: first, a prominent one above the clouds, situated between 50 and 55° latitude, with a maximum speed of ≈140 m s−1; a second with a speed of 95 m s−1 centered at 70° and at 60 km altitude; and a possible third jet at 15° and 65 km altitude, with a speed of ≈100 m s−1. The notable midlatitude jet is associated with the cold polar collar. Wind speeds above ∼70 km generally decrease with height because of the warm pole at these levels. However, within the jet the speed is 100 m s−1 at 83 km. The jets may be barotropically unstable and inertial instability may occur above the clouds on the equatorial side of the main...

The interaction between the magnetosphere of Saturn and Titan's ionosphere

A three-dimensional (3-D) multi-species magnetohydrodynamic model was used to study the interaction of Titans ionosphere and Saturns magnetosphere. The three generic species which were considered are light (e.g., H + , H 2 + , and H 3 + ), medium (e.g., N + and CH 5 + ), and heavy (e.g., N 2 + and HCNH + ) ion species. The effects of exospheric mass loading, major chemical reactions, and ion-neutral collisions were considered. The upstream parameters were selected to be the nominal values for the case when Titan is in the magnetosphere of Saturn. The simulation results are compared with Voyager measurements as well as related model calculations. The 3-D three-species model results reproduce reasonably well the global features such as magnetic barrier, magnetotail, and the distributions of the major ionospheric species. The outward escape flux of the major ionospheric species (i.e., the heavy ion species) from the tail is calculated to be approximately 6.5 × 10 24 s -1 .

Latitudinal variations in Saturn's ionosphere: Cassini measurements and model comparisons

[1] We present a study of latitudinal variations in Saturn’s ionosphere using Cassini Radio Science Subsystem (RSS) measurements and Saturn‐Thermosphere‐Ionosphere‐Model (STIM) simulations. On the basis of Cassini RSS observations, the peak electron density (NMAX) and the total electron content (TEC) both exhibit a clear increase with latitude, with a minimum at Saturn’s equator. When compared with these RSS trends, current model simulations overestimate NMAX and TEC at low latitudes and underestimate those parameters at middle and high latitudes. STIM is able to reproduce the RSS values for NMAX and TEC at low latitude when an additional low‐latitude loss process, such as a water influx, is introduced near Saturn’s equator. The lack of auroral precipitation processes in the model likely explains some model/data discrepancies at high latitude; however, most of the high‐latitude RSS data are from latitudes outside of Saturn’s typical main auroral oval. Using Cassini RSS electron density altitude profiles combined with ion density fractions and neutral background parameters calculated in STIM, we also present estimates of the latitudinal variations of Saturn’s Pedersen conductance, SP. We find SP to be driven by ion densities in Saturn’s lower ionosphere and to exhibit a latitudinal trend with a peak at mid‐latitude. Model calculations are able to reproduce low‐latitude conductances when an additional loss process is introduced, as before, but consistently underestimate most of the mid‐ and high‐latitude conductances derived from Cassini observations, perhaps indicating a missing ionization source within the model.

MAGNETOSPHERIC AND PLASMA SCIENCE WITH CASSINI-HUYGENS

Magnetospheric and plasma science studies at Saturn offer a unique opportunity to explore in-depth two types of magnetospheres. These are an ‘induced’ magnetosphere generated by the interaction of Titan with the surrounding plasma flow and Saturns ‘intrinsic’ magnetosphere, the magnetic cavity Saturns planetary magnetic field creates inside the solar wind flow. These two objects will be explored using the most advanced and diverse package of instruments for the analysis of plasmas, energetic particles and fields ever flown to a planet. These instruments will make it possible to address and solve a series of key scientific questions concerning the interaction of these two magnetospheres with their environment.The flow of magnetospheric plasma around the obstacle, caused by Titans atmosphere/ionosphere, produces an elongated cavity and wake, which we call an ‘induced magnetosphere’. The Mach number characteristics of this interaction make it unique in the solar system. We first describe Titans ionosphere, which is the obstacle to the external plasma flow. We then study Titans induced magnetosphere, its structure, dynamics and variability, and discuss the possible existence of a small intrinsic magnetic field of Titan.Saturns magnetosphere, which is dynamically and chemically coupled to all other components of Saturns environment in addition to Titan, is then described. We start with a summary of the morphology of magnetospheric plasma and fields. Then we discuss what we know of the magnetospheric interactions in each region. Beginning with the innermost regions and moving outwards, we first describe the region of the main rings and their connection to the low-latitude ionosphere. Next the icy satellites, which develop specific magnetospheric interactions, are imbedded in a relatively dense neutral gas cloud which also overlaps the spatial extent of the diffuse E ring. This region constitutes a very interesting case of direct and mutual coupling between dust, neutral gas and plasma populations. Beyond about twelve Saturn radii is the outer magnetosphere, where the dynamics is dominated by its coupling with the solar wind and a large hydrogen torus. It is a region of intense coupling between the magnetosphere and Saturns upper atmosphere, and the source of Saturns auroral emissions, including the kilometric radiation. For each of these regions we identify the key scientific questions and propose an investigation strategy to address them.Finally, we show how the unique characteristics of the CASSINI spacecraft, instruments and mission profile make it possible to address, and hopefully solve, many of these questions. While the CASSINI orbital tour gives access to most, if not all, of the regions that need to be explored, the unique capabilities of the MAPS instrument suite make it possible to define an efficient strategy in which in situ measurements and remote sensing observations complement each other.Saturns magnetosphere will be extensively studied from the microphysical to the global scale over the four years of the mission. All phases present in this unique environment — extended solid surfaces, dust and gas clouds, plasma and energetic particles — are coupled in an intricate way, very much as they are in planetary formation environments. This is one of the most interesting aspects of Magnetospheric and Plasma Science studies at Saturn. It provides us with a unique opportunity to conduct an in situ investigation of a dynamical system that is in some ways analogous to the dusty plasma environments in which planetary systems form.