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Radio occultations are commonly used to determine vertical profiles of ionospheric electron density from measurements of frequency. “Frequency residuals” are important intermediate data products in radio occultation experiments. Frequency residuals are defined as the difference between observed and predicted values of the received frequency, where the predicted frequency includes all effects except refraction at the target object. However, relationships between properties of a vertical profile of ionospheric electron density and properties of time series of frequency residuals are not widely known by the broad community of scientists who may work with frequency residuals. Here we illustrate and explain how properties of a vertical profile of ionospheric electron density affect time series of frequency residuals. We also develop four quantitative relationships between electron density values and frequency residuals. These provide predictions for the topside plasma scale height, topside electron densities, peak altitude, and peak electron density that can be generated directly from a set of frequency residuals without the need to implement the full Abel transform analysis that is usually required to determine electron densities from frequency residuals. They can also be generated prior to the implementation of a high-quality baseline correction, to which the Abel transform analysis is sensitive. The development of these predictions assumed that the ionosphere has an exponential topside and a Chapman-like peak. These predictions are successfully tested on two-way radio occultation observations from the MAVEN Radio Occultation Science Experiment (ROSE) at Mars.

Conditions in the ionosphere of Mars vary from solar minimum to solar maximum due to changes in the ionizing solar irradiance. Although changes in the main ionospheric layers, namely the M2 layer and the lower M1 layer, over the solar cycle have been well-characterized by previous work, corresponding changes in the upper regions of the photochemical ionosphere ( ∼ 140–200 km) have not. Here we investigate those changes using ionospheric electron density profiles acquired at solar zenith angles of 70°–90° under solar minimum conditions by the MAVEN Radio Occultation Science Experiment (ROSE) and under solar maximum conditions by a similar radio occultation experiment on Mars Global Surveyor. Due to the increase in ionizing irradiance from solar minimum to solar maximum, we expect that densities at all altitudes will increase from solar minimum to solar maximum. Due to the eccentricity of Mars’s orbit, observed electron density profiles must be adjusted to a common Mars–Sun distance to highlight solar cycle effects. This process involves increasing observed electron density values by a factor proportional to the Mars–Sun distance, which mitigates for variations in incident irradiance, and expressing altitude relative to the main ionospheric peak, which mitigates for variations in heating and expansion of the neutral thermosphere. We find that the scale height of the upper regions of the photochemical ionosphere is remarkably constant between solar minimum and solar maximum, whereas the corresponding neutral temperature increases by 50%. Yet these two quantities are thought to be proportional. Electron density values in the upper regions of the photochemical ionosphere and peak electron density values increase similarly from solar minimum to solar maximum.

We present the results of the remaining ten soundings of Titan's atmosphere by radio occultations using the Cassini spacecraft that have not been previously reported. Three were in 2008 and 2009, and used the Ultra Stable Oscillator onboard the spacecraft. The rest were in 2014 and 2016 after the USO had failed and were executed in two-way mode. Comparison of the later soundings with those earlier in the Cassini mission provide an indication of the seasonal change at mid latitudes. The southern hemisphere has shown a dramatic cooling during the onset of autumn; change in the northern hemisphere is noticeable, but more subdued. Changes at low latitudes are even smaller. The additional retrievals from 2008 to 2009 add a better constraint on the latitude structure of the previously reported destabilization of the temperature profiles at northern latitudes during winter and early spring. They show that the most pronounced effect is at polar latitudes (>70 N), suggesting that they are associated with the descending circulation over the winter pole that has been proposed by several authors. The full set of occultations indicates minimal variation with latitude and time in the troposphere, likely attributable to the large atmospheric radiative damping time and efficient meridional heat transports, but somewhat more variation near the surface, which has a smaller thermal inertia.

We present the results of six soundings of the atmosphere of Titan by the radio occultation technique using the Cassini spacecraft currently in orbit around Saturn. These occultations occurred during four separate targeted Titan encounters in both the Prime and Equinox missions of Cassini over 3 years. They cover a wide range of latitude from 75°S to 79°N, split so that three soundings are in the northern hemisphere and three are in the southern hemisphere. Techniques and error analysis are similar to Schinder et al. (2011) . The six temperature-altitude profiles presented here are compared to those earlier results. Of special interest is the sudden cooling observed at altitudes of ∼80–100 km in the two high northern (winter) soundings at 74°N and 80°N, where the temperature drops by about 10 K over the course of 20 km. The northern profiles also exhibit a transition between the troposphere and stratosphere that is much more abrupt than in the south, and the northern tropopause temperatures are much cooler.

We present results from the two radio occultations of the Cassini spacecraft by Titan in 2006, which probed mid-southern latitudes. Three of the ingress and egress soundings occurred within a narrow latitude range, 31.34 deg S near the surface, and the fourth at 52.8 deg S. Temperature - altitude profiles for all four occultation soundings are presented, and compared with the results of the Voyager 1 radio occultation (Lindal et al., 1983), the HASI instrument on the Huygens descent probe (Fulchignoni et al., 2005), and Cassini CIRS results (Flasar et al., 2005; Achterberg et al., 2008b). Sources of error in the retrieved temperature - altitude profiles are also discussed, and a major contribution is from spacecraft velocity errors in the reconstructed ephemeris. These can be reduced by using CIRS data at 300 km to make along-track adjustments of the spacecraft timing. The occultation soundings indicate that the temperatures just above the surface at 31-34 deg S are about 93 K, while that at 53 deg S is about 1 K colder. At the tropopause, the temperatures at the lower latitudes are all about 70 K, while the 53 deg S profile is again 1 K colder. The temperature lapse rate in the lowest 2 km for the two ingress (dawn) profiles at 31 and 33 deg S lie along a dry adiabat except within approximately 200m of the surface, where a small stable inversion occurs. This could be explained by turbulent mixing with low viscosity near the surface. The egress profile near 34 deg S shows a more complex structure in the lowest 2 km, while the egress profile at 53 deg S is more stable.

[1] Nineteen new radio occultations of the ionosphere of Saturn have been obtained since 2006. Sixteen of these occultations were from midlatitude and high latitudes and thus provided important, new information of the ionosphere for these regions. A high degree of variability in the electron densities were observed, but grouping and averaging the observations as low-, middle-, and high-latitude ones clearly showed that the electron densities increase with latitude. The topside scale heights also indicate small increases with latitude, but these changes are small enough so these increases may not be statistically significant.

[1] The first four sets of radio occultations of the Titan's ionosphere were obtained by the Cassini spacecraft between March 2006 and May 2007. These occultations occurred at middle and high latitudes, at solar zenith angles from about 86° to 96°. The main ionospheric peak was seen, as expected from modeling and previous observations, near 1200 km, with a density of about 1-3 x 10 3 cm -3 . A consistent ledge near 1000 km was also seen, and one of the polar observations found a significant (∼3 x 10 3 cm -3 ) layer in the region of 500-600 km. This layer also is seen in other observations with a density varying from about 0.7 to 1.7 x 10 cm -3 , suggesting a variable production source (or sources) for this peak.

[1] The first set of near-equatorial occultations of the Saturn ionosphere was obtained by the Cassini spacecraft between May and September of 2005. The occultations occurred at near-equatorial latitudes, between 10°N and 10°S, at solar zenith angles from about 84° to 96°. The entry observations correspond to dusk conditions and the exit ones to dawn. An initial look at the data indicates that the average peak densities are lower and the peak altitude higher at dawn than at dusk, possibly the result of ionospheric decay during the night hours. There are also significant differences between individual dawn and dusk occultations; the initial thought is that this variation must be connected to changes in the water inflow into the upper atmosphere and/or variations in the particle impact ionization rates.

On December 7, 1995, the Galileo spacecraft will begin observations of the Jovian system with orbit insertion and a satellite tour of 10 orbits. The Galilean satellites will be observed with four remote sensing instruments spanning the ultraviolet, visible, near infrared, and thermal infrared regions of the spectrum. The Galileo near-infrared mapping spectrometer will observe the satellites in the wavelength range 0.7–5.2 μm, a region particularly well suited for analyzing volatile components. Planned observations include mapping most available longitudes at about 100-km resolution per pixel at full wavelength resolution, together with observing limited regions at high spatial resolution. The opportunity to affect the choice and design of observations for the Galileo tour extends until June 1996.

Galileo's Near-Infrared Mapping Spectrometer (NIMS) observed Io during the spacecraft's three flybys in October 1999, November 1999, and February 2000. The observations, which are summarized here, were used to map the detailed thermal structure of active volcanic regions and the surface distribution of SO2 and to investigate the origin of a yet unidentified compound showing an absorption feature at ---1 m. We present a summary of the observations and results, focusing on the distribution of thermal emission and of SO2 deposits. We find high eruption temperatures, consistent with ultramafic volcanism, at Pele. Such temperatures may be present at other hot spots, but the hottest areas may be too small for those temperatures to be detected at the spatial resolution of our observations. Loki is the site of frequent eruptions, and the low thermal emission may represent lavas cooling on the caldera's surface or the cooling crust of a lava lake. High- resolution spectral observations of Emakong caldera show thermal emission and SO2 within the same pixels, implying that patches of SO2 frost and patches of cooling lavas or sulfur flows are present within a few kilometers from one another. Thermal maps of Prometheus and Amirani show that these two hot spots are characterized by long lava flows. The thermal profiles of flows at both locations are consistent with insulated flows, with the Amirani flow field having more breakouts of fresh lava along its length. Prometheus and Amirani each show a white ring at visible wavelengths, while SO2 distribution maps show that the highest concentration of SO2 in both ring deposits lies outside the white portion. Visible measurements at high phase angles show that the white deposit around Prometheus extends into the SO2 ring. This suggests that the deposits are thin and that compositional or grain size variations may occur in the radial direction. SO2 mapping of the Chaac region shows that the interior of a caldera adjacent to Chaac has almost pure SO2. The deposit appears to be topographically controlled, suggesting a possible origin by liquid flow.