Radio Science Experiment Research Articles

On the ionosphere of Triton: An evaluation of the magnetospheric electron precipitation and photoionization effects

In order to explore the origin and physical nature of the ionosphere of Triton, a simple, 2‐dimensional electron drift model is developed to approximate the ionization effect of magnetospheric electrons. The derived profile of ion production rate is then applied to a steady‐state photochemical model calculation. It is shown that, as a result of the very low CH4 mixing ratio in Triton's atmosphere, the most dominant ion in the upper ionosphere could be N+. There are, however, difficulties in explaining the electron number density profile below 500 km as inferred from the radio science experiment on Voyager 2. One major factor is the increasing importance of the ion chemistry involving N+ and H2 from CH4 photolysis in this region. This problem may be partially resolved if we invoke a more efficient electron transport in the atmosphere or the possible presence of metallic ions like Si+, Ne+ and Na+ which do not react with N2 and H2 in the lower ionosphere.

Remote sensing of mars' ionosphere and solar wind interaction: Lessons from venus

Although the PHOBOS spacecraft will make some limited in-situ measurements of Mars' upper ionosphere during its transfer orbits, the major part of the mission will be limited to remote sensing by radio occultation and topside sounding methods. To “calibrate” the former as a means of studying the ionosphere and solar wind interaction, we have examined the Pioneer Venus Orbiter radio occultation experiment results in light of what we know from in-situ plasma and magnetic field data. This “calibration” can be used to reassess the data from previous Mars missions and to provide a basis for interpreting data from the upcoming PHOBOS mission. The resemblance between the available Mars radio occultation electron density profiles and those of Venus at solar minimum, and at solar maximum when the solar wind dynamic pressure is high, suggest that similar conditions of ionospheric magnetization exist at both planets. A key observation by the PHOBOS radio science experiments, which can be used to infer whether this ionospheric field is intrinsic or arises from the solar wind interaction, will be a determination of the sensitivity of the ionospheric profiles to varying solar wind conditions.

Constraints on Titan's ionosphere

The near flyby of Saturn's moon Titan by Voyager 1 revealed a Venus‐like interaction between the moon and Saturn's magnetospheric plasma. Although neither the radio science experiment occultation observation nor the in situ measurements directly detected the ionosphere, plasma of ionospheric origin was observed as Voyager 1 passed through Titan's wake. Balancing the magnetic pressure in this low‐β region of Saturn's magnetosphere with ionospheric particle pressure yields an upper limit on the ionospheric density. Using an ionospheric temperature equal to the exospheric temperature of 200°K yields a charge density of about 3000 cm−3, which is consistent with the peak ionospheric electron density inferred from a balance of electron impact ionization of molecular nitrogen and recombination loss. Both of these quantities are consistent with limits derived from Voyager 1 observations. Good constraints on these quantities are important in planning the Cassini mission to orbit Saturn and probe Titan's ionosphere and atmosphere at the beginning of the next century.

H3+ in the Jovian ionosphere

Recent measurements and theoretical calculations suggest that H3+ ions in their ground vibrational level recombine much more slowly than in the vibrational levels υ ≥ 3. Using these results as a guide, we have investigated the impact of modified H3+ chemistry on the structure of the Jovian ionosphere for both a solar EUV ionization source and a high‐altitude ionization source, associated with the electroglow. The results are sensitive to the value of the H3+ recombination coefficient, k10, adopted for υ = 0. For the solar EUV source of ionization, and a value of k10 = 2 × 10−8 cm³ s−1 equal to the published experimental upper limit, we find that H+ is the major ion at the peak of the ionosphere, similar to previous results. However, given the current uncertainty in k10, an ionosphere dominated by H3+ cannot be ruled out. A value of k10 ∼ 10−10 cm³ s−1 will produce an ionosphere that consists mainly of H3+. By including a high‐altitude source of ionization we find that H3+ is the major component of the upper ionosphere, for all values of k10 less than or equal to the experimental upper limit. Thus, our calculations suggest that the topside plasma scale height measured by the radio science experiments may be indicative of electron temperatures ∼3000–5000 K or greater, consistent with current models for calculation of electron temperature. Our results may also help shed some light on the Voyager low‐energy charged particle observations of H3+ ions in the magnetosphere. We also reemphasize the fact that H2 molecules in the Jovian upper atmosphere must be vibrationally excited.

Measurement technique for the Giotto radio science experiment

The paper describes the technique used to record time delay and waveform measurements for the Giotto radio science experiment of ESA's mission to comet Halley. The data were taken by using either two-way measurements (during pre- and post-encounter) or one-way measurements (during encounter with comet Halley), the downlink of the radio signal of the Giotto spacecraft being received at 8.4 GHz by the 64 m tracking stations of NASA's Deep Space Network (DSN). The waveform measurements were obtained at a sampling frequency of 50 kHz with an open-loop receiver assembly at DSN station Canberra as recently used for the Voyager/Uranus fly-by. Performance and calibration data are given as relevant to the radio subsystems on the ground and aboard Giotto.

Ultraviolet spectrophotometry of comet Giacobini-Zinner during the ICE encounter

A program of ultraviolet spectrophotometry using the International Ultraviolet Explorer (IUE) Observatory was carried out in support of the International Cometary Explorer (ICE) mission. The H 2O production rate was monitored from 1985 June to October. Between 1985 September 9 and 12, the spatial and temporal variation and abundance (or upper limits) of the remotely detectable species, C, CO, CO +, CO 2 +, CS, H, Mg +, O, OH, and S, were obtained. These observations included the time of the ICE encounter (1985 September 11.46) when the H 2O production rate was 3 × 10 28 sec −1 ± 50%. This rate is consistent with a number of gas production rates derived indirectly from the ICE experiments. The comet was in a nearly steady state around the time of encounter showing no evidence of short-term temporal fluctuations in brightness greater than 6%. A sunward-tailward asymmetry of the OH brightness was observed at 10,000 km from the nucleus. The absence of detected Mg + rules out this species as a possible ion of M Q = 24 which was detected by the Ion Composition Instrument, part of the ICE complement of instruments. Comparison of the abundance of CO 2 + ions with total electron density measured by the plasma electron and radio science experiments on ICE indicates a deficiency of ions relative to electrons. To satisfy charge balance criteria, a major population of ions not detected by remote sensing must be present.

First results from the Giotto Radio-Science Experiment

Doppler and ranging measurements, using the radio signals of the Giotto spacecraft and taken during the encounter with comet Halley, reveal a definite deceleration of the spacecraft due to drag in the cometary atmosphere. The total change in the radial velocity of the spacecraft was measured to be 16.7 cm s−1, occuring over a time interval of ∼100 s and corresponding to a shift in Doppler frequency of 4.7 Hz. Using this velocity change, estimates of the total cometary mass striking the spacecraft range from 0.1 to 1 g.

The Giotto Radio Science Experiment at Parkes

AbstractOn 14 March 1986 the European Space Agency’s Giotto spacecraft passed within 600 km of the nucleus of Halley’s comet. During the encounter a radio science experiment was conducted with the aim of determining the total amount of cometary material which impacted the probe. In this paper the theory and implementation of the experiment are discussed and a summary of experimental results is given.

Preliminary results of the Giotto radio science experiment

Doppler and ranging measurements using the radio signal of the GIOTTO spacecraft were taken before, during, and after the encounter with Comet Halley on 13 14 March 1986. The spacecraft velocity was found to decrease by a total of 23.3 cm s −1 due to impacting gas and (primarily) dust in the cometary atmosphere. A preliminary dust production rate Q d ⋍ 10 × 10 3 kg s −1 is found to be consistent with this deceleration. Power spectra of the carrier phase fluctuations reveal an increase in level and a flattening of the spectrum just prior to encounter, presumably associated with the enhanced dust impact rate. Finally, simulated Doppler time profiles are computed using the radial dependence of plasma density observed by the GIOTTO in situ investigations. It is shown that the cometary electron content profile would have been clearly seen if a dual-frequency downlink radio configuration had been available at encounter.

On plasma transport in the vicinity of the rings of Saturn: A Siphon Flow Mechanism

The unique combination of a rapidly rotating ionosphere and meteoroid impact ionization at the rings of Saturn provides the elements of many interesting plasma transport phenomena. One of the most significant processes might be the upward field‐aligned flow of the impact plasma at the equatorial region inside 1.6252 Rs. Such a siphoning mechanism limits the efficiency of ring plane matter recycling to a minimum and could lead to appreciable loss of ring mass in this region. At the same time, channeling of the heavy ions into the mid‐altitude ionosphere could also cause a large reduction in the ionospheric electron content as observed by the Pioneer 11 and the Voyager radio science experiments. The resulting electrodynamical coupling of the Saturnian rings with the ionosphere thus represents a completely new kind of ionospheric process than studied before.

Halley lookout

Unlike Japan, the Soviet Union, and the European Space Agency, the United States has no plans to send a spacecraft to observe or collect samples from Comet Halley as it whooshes past the earth in late 1985 and early 1986. However, U.S. astronomers are not only searching the skies in the hopes of catching the first glimmer of the famous comet but also are partaking in a coordinated international effort to collect and analyze data on the comet.The International Halley Watch (IHW) will involve an international network of professional and amateur scientists. The organization invites all Comet Halley observers to add their data and photographs to the network's planned collection of observations made with instruments on the ground, in balloons, in earth orbit, and on aircraft. Data compiled by the IHW will form the Halley archive, the largest collection of information ever produced on a single comet, according to the Jet Propulsion Laboratory (JPL). The comet will be studied by using, among other techniques, wide‐angle photography, high‐resolution photographic and electronic imaging, spectroscopy and spectrophotometry, photometry and polarimetry, radio science experiments, infrared spectroscopy and radiometry, and astrometric observations.

An interpretation of the Voyager measurement of jovian electron density profiles

Electron-density profiles measured for the daytime and nighttime Jovian ionosphere by the Voyager 1 radio-science experiment are analyzed. It is found that the measured profiles can be reproduced by using a model appropriate for an exospheric temperature of 1300 K with temperature varying above the homopause and with an eddy diffusion coefficient of 100,000 to 300,000 sq cm/s at the homopause. An overall rate constant of 4.3 x 10 to the -16th cu cm/s is estimated for the reaction H(+) + H2 (v-prime at least 4) yields H2(+) + H.