ALBERT

Description: Cassini field and plasma data measured in the rotating SLS3 coordinate system show positive evidence of structure whose dominant azimuthal wavenumber is m = 1: a long-lived, non-axisymmetric, cam-shaped, global plasma distribution in Saturn's magnetosphere. Previous studies have identified evidence of this plasma cam in wave-derived electron density data, and in Cassini Plasma Spectrometer (CAPS) W + ion counts data. In this paper we report the first comprehensive analysis of CAPS ion moments data to identify the m = 1 density cam. We employ a multi-year, multi-species database of 685,678 CAPS density values, binned into a 1  R S -by-4.8 ∘ discretized grid, spanning 4–19  R S . Fourier (harmonic) analysis shows that at most radial distances the dominant azimuthal mode is m = 1, for both W + and H + ion distributions. The majority (63%) of m = 1 ion peaks are clustered in an SLS3 quadrant centered at 330 ∘ . The plasma cam's existence has important implications for the global interchange-driven convection cycle, and is a clue to solving the mystery of the rotational periodicities in Saturn's magnetosphere.

Description: Observed electron and ion temperatures in planetary ionospheres are higher than the neutral temperature. Instruments on board the Cassini spacecraft have shown this is also true for Titan. The Radio and Plasma Wave Science Langmuir Probe (RPWS-LP) (Wahlund et al., 2005) has measured electron temperatures above 1000 K. Ionospheric ion temperatures were deduced from a combined analysis of data from the Cassini Plasma Spectrometer and Ion and Neutral Mass Spectrometer (INMS) (Crary et al., 2009). Elevated electron temperatures attributed to heating by suprathermal electrons were predicted by pre-Cassini models (e.g., Gan et al., 1992; Roboz and Nagy, 1994) and observed by the Cassini electron spectrometer. Models of the energetic electrons and ions are presented that include Cassini inputs (i.e., measured neutral densities from INMS). The results are compared between 800 and 1800 km with suprathermal electron fluxes and plasma temperatures measured by Cassini instruments emphasizing the thermal electron temperature. Using only solar inputs, the dayside model agrees well with electron temperatures measured by RPWS-LP (Ågren et al., 2009) between 1000 and 1400 km. At higher altitudes energy input from magnetospheric electrons is needed to reproduce the measured temperature. Incorporating typical magnetospheric electron fluxes into the dayside does not noticeably increase ion production near the ionospheric peak; however, effects can be seen near 1350 km. Joule heating effects are shown to be capable of contributing significantly to the ion temperature. Magnetospheric suprathermal electrons are shown to provide sufficient heating for the thermal electron population in the middle to upper ionosphere on the nightside.

Description: Although auroral emissions at giant planets have been observed for decades, the physical mechanisms of aurorae at giant planets remain unclear. One key reason is the lack of simultaneous measurements in the magnetosphere while remote sensing of the aurora. We report a dynamic auroral event identified with the Cassini Ultraviolet Imaging Spectrograph (UVIS) at Saturn on 13 July 2008 with coordinated measurements of the magnetic field and plasma in the magnetosphere. The auroral intensification was transient, only lasting for ∼ 30 minutes. The magnetic field and plasma are perturbed during the auroral intensification period. We suggest that this intensification was caused by wave mode conversion generated field-aligned currents, and we propose two potential mechanisms for the generation of this plasma wave and the transient auroral intensification. A survey of the Cassini-UVIS database reveals that this type of transient auroral intensification is very common (10/11 time sequences, and ∼10 % of the total images).

Description: [1]  We use an analytical model of magnetosphere-ionosphere (M-I) coupling to show that an asymmetric ring current (RC) pressure with an m  = 1 longitudinal dependence can initiate a rotating two-cell interchange potential. The model extends prior similar work by considering both cold plasma interchange and warm plasma pressure. This model predicts that within 7 hours the magnitude of the interchange potential equals the RC seed potential. Within 13 to 26 hours, the model reproduces the degree of cold density nonaxisymmetry at the outer density gradient of the Enceladus plume, as observed by the Cassini Plasma Spectrometer (CAPS). The interchange growth time bears a strong dependence on the particular value of height-integrated ionospheric conductivity, and a weaker dependence on the magnitude of the initial RC perturbation. The model also extends prior work by including an outer boundary. We discuss the qualitative effect of a realistically-shaped magnetopause that is anchored to the non-rotating frame. For high- m interchange, the magnetopause presence has no significant effect. In contrast, low- m interchange modes experience a rotating, asymmetric boundary condition that alternately enhances and inhibits interchange growth each rotation period. Several published studies have proposed interchange-driven, rotating two-cell convection; our results suggest that an asymmetric RC pressure distribution, coupled to Saturn's ionosphere, is one possible means of generating it. Our model predicts that the two-cell interchange potential should be long-lived and relatively insensitive to subsequent ring current injections, because after two Saturnian rotations the interchange potential is an order of magnitude larger than the seed potential that initiated it.

Description: One Titanian year spans over two complete solar cycles and the solar irradiance has a significant effect on ionospheric densities. Solar cycle 24 has been one of the quietest cycles on record. In this paper we show data from the Cassini ion and neutral mass spectrometer (INMS) and the radio and plasma wave science (RPWS) Langmuir probe (LP) spanning the time period from early 2005, at the declining phase of solar cycle 23, to late 2015 at the declining phase of solar cycle 24. Densities of different ion species measured by the INMS show a consistent enhancement for high solar activity, particularly near the ionospheric peak. The density enhancement is best seen in primary ion species such as CH 3 + rather than heavier ion species such as HCNH + . Unlike at Earth, where the ionosphere and atmosphere thermally expand at high solar activity, at Titan the altitude of the ionospheric peak decreases, indicating that the underlying neutral atmosphere was less extensive. Among the major ion species, CH 5 + shows the largest decrease in peak altitude, whereas heavy ions such as C 3 H 5 + show very little decrease. We also calculate the ion production rates using a theoretical model and a simple empirical model using INMS data and show that these effectively predict the increased ion production rates at high solar activity.

Description: We use our fully coupled 3-D Jupiter Thermosphere General Circulation Model (JTGCM) to quantify processes which are responsible for generating neutral winds in Jupiter's oval thermosphere from 20 µbar to 10 -4 nbar self-consistently with the thermal structure and composition. The heat sources in the JTGCM that drive the global circulation of neutral flow are substantial Joule heating produced in the Jovian ovals by imposing high speed anticorotational ion drifts (~3.5 kms -1 ) and charged particle heating from auroral processes responsible for bright oval emissions. We find that the zonal flow of neutral winds in the auroral ovals of both hemispheres is primarily driven by competition between accelerations resulting from Coriolis forcing and ion drag processes near the ionospheric peak. However, above the ionospheric peak (〈0.01 µbar), the acceleration of neutral flow due to pressure gradients is found to be the most effective parameter impacting zonal winds, competing mainly with acceleration due to advection with minor contributions from curvature and Coriolis forces in the southern oval, while in the northern oval it competes alone with considerable Coriolis forcing. The meridional flow of neutral winds in both ovals in the JTGCM is determined by competition between meridional accelerations due to Coriolis forcing and pressure gradients. We find that meridional flow in the lower thermosphere, near the peak of the auroral ionosphere, is poleward, with peak wind speeds of ~0.6 kms -1 and ~0.1 kms -1 in the southern and northern oval, respectively. The corresponding subsiding flow of neutral motion is ~5 ms -1 in the southern oval, while this flow is rising in the northern oval with reduced speed of ~2 ms -1 . We also find that the strength of meridional flow in both auroral ovals is gradually weakened and turned equatorward near 0.08 µbar with wind speeds up to ~250 ms -1 (southern oval) and ~75 ms -1 (northern oval). The corresponding neutral motion in this region is upward, with wind speeds up to 4 ms -1 in both ovals.

Description: Abstract Measurements of electrons and ions in Saturn's ionosphere down to 1500 km altitudes as well as the ring crossing region above the ionosphere obtained by the Langmuir probe onboard the Cassini spacecraft are presented. Five nearly identical deep ionosphere flybys during the Grand Finale orbits and the Final plunge orbit revealed a rapid increase in the plasma densities and discrepancies between the electrons and ions densities (Ne and Ni) near the closest approach. The small Ne/Ni ratio indicates the presence of a dusty plasma, a plasma which charge carrier is dominated by negatively charged heavy particles. Comparison of the LP obtained density with the light ion density obtained by the Ion and Neutral Mass Spectrometer (INMS) confirmed the presence of heavy ions. An unexpected positive floating potential of the probe was also observed when Ne/Ni 〈〈 1. This suggests that Saturn's ionosphere near the density peak is in a dusty plasma state consisting of negatively and positively charged heavy cluster ions. The electron temperature (Te) characteristics in the ionosphere are also investigated and unexpectedly high electron temperature value, up to 5000 K, has been observed below 2500 km altitude in a region where electron‐neutral collisions should be prominent. A well‐defined relationship between Te and Ne/Ni ‐ratio was found, implying that the electron heating at low altitudes is related to the dusty plasma state of the ionosphere.

Description: Pickup ion detection at Titan is challenging because ion cyclotron waves are rarely detected in the vicinity of the moon. In this work, signatures left by freshly produced pick up heavy ions ( m / q ∼ 16 to m / q ∼ 28) as detected in the plasma data by the CAPS/IMS instrument on-board Cassini are analyzed. In order to discern whether these correspond to ions of exospheric origin, one of the flybys during which the reported signatures were observed is investigated in detail. For this purpose, ion composition data from time of flight measurements and test particle simulations to constrain the ions' origin are used. After being validated, the detection method is applied to all the flybys for which the CAPS/IMS instrument gathered valid data, constraining the region around the moon where the signatures are observed. The results reveal an escape region located in the anti-Saturn direction as expected from the nominal corotation electric field direction. These findings provide new constraints for the area of freshly produced pickup ion escape, giving an approximate escape rate of .

Description: The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed an occultation of the Sun by the water vapor plume at the south polar region of Saturn's moon Enceladus. The Extreme Ultraviolet (EUV) spectrum is dominated by the spectral signature of H2O gas, with a nominal line-of-sight column density of 0.90 ± 0.23 × 1016 cm−2 (upper limit of 1.0 × 1016 cm−2). The upper limit for N2 is 5 × 1013 cm−2, or

Description: The Cassini-Huygens mission has been observing Titan since October 2004, resulting in over 70 targeted flybys. Titan's thermosphere is sampled by the Ion and Neutral Mass Spectrometer (INMS) during several of these flybys. The measured upper atmospheric density varies significantly from pass to pass. In order to quantify the processes controlling this variability, we calculate the nitrogen scale height for a variety of parameters related to the solar and plasma environments and, from these, we infer an effective upper atmospheric temperature. In particular, we investigate how these calculated scale heights and temperatures correlate with the plasma environment. Measured densities and inferred temperatures are found to be reduced when INMS samples Titan within Saturn's magnetospheric lobe regions, while they are enhanced when INMS samples Titan in Saturn's plasma sheet. Finally the data analysis is supplemented with Navier-Stokes model calculations using the Titan Global Ionosphere Thermosphere Model. Our analysis indicates that, during the solar minimum conditions prevailing during the Cassini tour, the plasma interaction plays a significant role in determining the thermal structure of the upper atmosphere and, in certain cases, may override the expected solar-driven diurnal variation in temperatures in the upper atmosphere.