ABSTRACT
Cassini's Grand Finale orbits provided for the first time in-situ measurements of Saturn's topside ionosphere. We present the Pedersen and Hall conductivities of the top near-equatorial dayside ionosphere, derived from the in-situ measurements by the Cassini Radio and Wave Plasma Science Langmuir Probe, the Ion and Neutral Mass Spectrometer and the fluxgate magnetometer. The Pedersen and Hall conductivities are constrained to at least 10-5-10-4 S/m at (or close to) the ionospheric peak, a factor 10-100 higher than estimated previously. We show that this is due to the presence of dusty plasma in the near-equatorial ionosphere. We also show the conductive ionospheric region to be extensive, with thickness of 300-800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities from orbit 288 to 292.
ABSTRACT
Previous modeling studies of Titan's dayside ionosphere predict electron number densities that are roughly a factor of 2 higher than those observed by the RPWS/Langmuir probe. The issue can equivalently be described as the ratio between the calculated electron production rates and the square of the observed electron number densities resulting in roughly a factor of 4 higher effective recombination coefficient than expected from the ion composition and the electron temperature. Here we make an extended reassessment of Titan's dayside ionization balance, focusing on 34 flybys between TA and T120. Using a recalibrated data set and by taking the presence of negative ions into account, we arrive at lower effective recombination coefficients compared with earlier studies. The values are still higher than expected from the ion composition and the electron temperature, but by a factor of ~2-3 instead of a factor of ~4. We have also investigated whether the derived effective recombination coefficients display dependencies on the solar zenith angle (SZA), the integrated solar EUV intensity (<80 nm), and the corotational plasma ram direction (RAM), and found statistically significant trends, which may be explained by a declining photoionization against the background ionization by magnetospheric particles (trends in SZA and RAM) and altered photochemistry (trend in EUV). We find that a series of flybys that occurred during solar minimum (2008) and with similar flyby geometries are associated with enhanced values of the effective recombination coefficient compared with the remaining data set, which also suggests a chemistry dependence on the sunlight conditions.
ABSTRACT
Effects of solar EUV on positive ions and heavy negative charge carriers (molecular ions, aerosol, and/or dust) in Titan's ionosphere are studied over the course of almost 12 years, including 78 flybys below 1400 km altitude between TA (October 2004) and T120 (June 2016). The Radio and Plasma Wave Science/Langmuir Probe-measured ion charge densities (normalized by the solar zenith angle) show statistically significant variations with respect to the solar EUV flux. Dayside charge densities increase by a factor of ≈2 from solar minimum to maximum, while nightside charge densities are found to anticorrelate with the EUV flux and decrease by a factor of ≈3-4. The overall EUV dependence of the ion charge densities suggest inapplicability of the idealized Chapman theory below 1200 km in Titan's ionosphere. Nightside charge densities are also found to vary along Titan's orbit, with higher values in the sunward magnetosphere of Saturn compared to the magnetotail.
ABSTRACT
The importance of the heavy ions and dust grains for the chemistry and aerosol formation in Titan's ionosphere has been well established in the recent years of the Cassini mission. In this study we combine independent in situ plasma (Radio Plasma and Wave Science Langmuir Probe (RPWS/LP)) and particle (Cassini Plasma Science Electron Spectrometer, Cassini Plasma Science Ion Beam Spectrometer, and Ion and Neutral Mass Spectrometer) measurements of Titan's ionosphere for selected flybys (T16, T29, T40, and T56) to produce altitude profiles of mean ion masses including heavy ions and develop a Titan-specific method for detailed analysis of the RPWS/LP measurements (applicable to all flybys) to further constrain ion charge densities and produce the first empirical estimate of the average charge of negative ions and/or dust grains. Our results reveal the presence of an ion-ion (dusty) plasma below ~1100 km altitude, with charge densities exceeding the primary ionization peak densities by a factor ≥2 in the terminator and nightside ionosphere (n e /n i ≤ 0.1). We suggest that ion-ion (dusty) plasma may also be present in the dayside ionosphere below 900 km (n e /n i < 0.5 at 1000 km altitude). The average charge of the dust grains (≥1000 amu) is estimated to be between -2.5 and -1.5 elementary charges, increasing toward lower altitudes.
