ALBERT

Description: We propose a possible explanation for the second wake event observed by Cassini during the T9 encounter with Titan. As shown in Hartle and Sittler [2007a], ions will emanate from Titan's upper atmosphere as ion beams when the ion gyroradii are large compared to the neutral scale height. Furthermore, Sittler and Hartle [2007] and Hartle and Sittler [2007b] showed that when this condition is satisfied and the electric field of the external flow is not reduced significantly due to draping field lines, the heavier pickup ions will be highly localized in space and velocity, or beam-like, in Titan's wake. This can cause these ion bunches to jump across the spacecraft trajectory and not be observed except for the lighter ions such as H+ and H2+, which have smaller gyroradii. These heavy ions will form a large pickup current which can deflect the tail position away from Saturn. We will discuss this model for the T9 encounter, which was a wake pass, and also explore its possible application for T5.

Description: With the recent discovery of heavy ions, positive and negative, by the Cassini Plasma Spectrometer (CAPS) instrument in Titan's ionosphere, it reveals new possibilities for aerosol formation at Titan and the introduction of free oxygen to the aerosol chemistry from Saturn's magnetosphere with Enceladus as the primary oxygen source. One can estimate whether the heavy ions in the ionosphere are of sufficient number to account for all the aerosols, under what conditions are favorable for heavy ion formation and how they are introduced as seed particles deeper in Titan's atmosphere where the aerosols form and eventually find themselves on Titan's surface where unknown chemical processes can take place. Finally, what are the possibilities with regard to their chemistry on the surface with some free oxygen present in their seed particles?

Description: This first analysis of Pioneer Venus Orbiter (PVO) plasma analyzer electron measurements obtained in early 1992 during the PVO entry phase of the mission indicates the presence downstream from the terminator of a depletion or "bite out" of energetic ionosheath electrons similar to that observed on Mariner 10. There is more than one possible explanation for this energetic electron depletion. If it is due to atmospheric scattering, the electrons traveling along draped magnetic flux tubes that thread through the Venus neutral atmosphere would lose energy from impact ionization with oxygen. The cross-section for such electron impact ionization of oxygen has a peak near 100 eV, and it remains high above this energy, so atmospheric loss could provide a natural process for electrons at these energies to be selectively removed. In this case, our results are consistent with the Kar et al. (1994) study of PVO atmospheric entry ion mass spectrometer data which indicates that electron impact plays a significant role in maintaining the nightside ionosphere. Although it is appealing to interpret the energetic electron depletion in terms of direct atmospheric scattering, alternatively it could result from strong draping which connects the depletion region magnetically to the weak downstream bow shock and thereby reduces the electron source strength.

Description: During the final, low solar activity phase of the Pioneer Venus mission, the Orbiter Ion Mass Spectrometer measurements found all ion species, in the midnight-dusk sector, reduced in concentration relative to that observed at solar maximum. Molecular ion species comprised a greater part of the total ion concentration as O(+) and H(+) had the greatest depletions. The nightside ionospheric states were strikingly similar to the isolated solar maximum "disappearing" ionospheres. Both are very dynamic states characterized by a rapidly drifting plasma and 30-100 eV superthermal O(+) ions.

Description: Composition and structure of neutral constituents in the lunar exosphere can be determined through measurements of phase space distributions of pickup ions borne from the exosphere [1]. An essential point made in an early study [ 1 ] and inferred by recent pickup ion measurements [2, 3] is that much lower neutral exosphere densities can be derived from ion mass spectrometer measurements of pickup ions than can be determined by conventional neutral mass spectrometers or remote sensing instruments. One approach for deriving properties of neutral exospheric source gasses is to first compare observed ion spectra with pickup ion model phase space distributions. Neutral exosphere properties are then inferred by adjusting exosphere model parameters to obtain the best fit between the resulting model pickup ion distributions and the observed ion spectra. Adopting this path, we obtain ion distributions from a new general pickup ion model, an extension of a simpler analytic description obtained from the Vlasov equation with an ion source [4]. In turn, the ion source is formed from a three-dimensional exospheric density distribution, which can range from the classical Chamberlain type distribution to one with variable exobase temperatures and nonthermal constituents as well as those empirically derived. The initial stage of this approach uses the Moon's known neutral He and Na exospheres to deriv e He+ and Na+ pickup ion exospheres, including their phase space distributions, densities and fluxes. The neutral exospheres used are those based on existing models and remote sensing studies. As mentioned, future ion measurements can be used to constrain the pickup ion model and subsequently improve the neutral exosphere descriptions. The pickup ion model is also used to estimate the exosphere sources of recently observed pickup ions on KAGUYA [3]. Future missions carrying ion spectrometers (e.g., ARTEMIS) will be able to study the lunar neutral exosphere with great sensitivity, yielding the necessary ion velocity spectra needed to further analysis of parent neutral exosphere properties.

Description: A considerable fraction of atmospheric loss at Venus and Titan is in the form of plasma escape. This is due in part to the fact that the ionospheres of these unmagnetized bodies interact directly with the high speed plasmas flowing around them. The similarities of the interactions help reinforce interpretations of measurements made at each body, especially when instruments and measurement sites differ. For example, it is well established through this method that ions born in the exospheres above the ionopauses are picked up and carried away by the solar wind at Venus and the rotating plasma in Saturn's magnetosphere. On the other hand, it is more difficult to relate the observations associated with escape of cooler ionospheric plasma down the ionotails of each body. A clear example of ionospheric plasma escaping Titan was observed as it flowed down its ionotail (1). Measurements at Venus have not as yet clearly distinguished between ionospheric and pickup ion escape in the ionotail; however, cold ions detected in the distant wake at 1 AU by the CELIAS/CTOF instrument on SOHO have been interpreted as ionospheric in origin (2). An algorithm to determine ionospheric flow from Pioneer Venus aeronomical measurements is used to show that escape of cold ionospheric plasma is likely to occur. These results along with plasma flow measurements made in the ionotail of Venus are combined and compared to the corresponding flow at Titan.

Description: Convergence of suprathermal keV-MeV proton and ion spectra approximately to the Fisk-Gloeckler (F-G) form j(E) = j(sub 0) E(sup -1.5) in Voyager land 2 heliosheath measurements is suggestive of distributed acceleration in Kolmogorov turbulence which may extend well beyond the heliopause into the local interstellar medium (LISM). Turbulence of this type is already indicated by interstellar radio scintillation measurements of electron density power spectra. Previously published extrapolations (Cooper et al., 2003, 2006) of the LISM proton spectrum from eV to GeV energies are highly consistent with the F-G power-law and further indicative of such turbulence and LISM effectiveness of the F-G cascade acceleration process. The LISM pressure computed from this spectrum well exceeds that from current estimates for the LISM magnetic field, so exchange of energy between the protons and the magnetic field would likely have a strong role in evolution of the turbulence as per the F-G theory and as long ago proposed for cosmic ray energies by Parker and others. Pressure-dependent estimates of the LISM field strength should not ignore this potentially strong and even dominant contribution from the plasma. Presence of high-beta suprathermal plasma on LISM field lines could significantly affect interactions with the heliospheric outer boundary region and might potentially account for distributed and more discrete features in ongoing measurements of energetic neutral emission from the Interstellar Boundary Explorer (IBEX) mission.

Description: Using Cassini Plasma Spectrometer (CAPS) Ion Mass Spectrometer (IMS) measurements, we present the ion fluid properties and its ion composition of the upstream flow for Titan's interaction with Saturn's magnetosphere. A 3D ion moments algorithm is used which is essentially model independent with only requirement is that ion flow is within the CAPS IMS 2(pi) steradian field-of-view (FOV) and that the ion 'velocity distribution function (VDF) be gyrotropic. These results cover the period from TA flyby (2004 day 300) to T22 flyby (2006 363). Cassini's in situ measurements of Saturn's magnetic field show it is stretched out into a magnetodisc configuration for Saturn Local Times (SLT) centered about midnight local time. Under those circumstances the field is confined near the equatorial plane with Titan either above or below the magnetosphere current sheet. Similar to Jupiter's outer magnetosphere where a magnetodisc configuration applies, one expects the heavy ions within Saturn's outer magnetosphere to be confined within a few degrees of the current sheet while at higher magnetic latitudes protons should dominate. We show that when Cassini is between dusk-midnight-dawn local time and spacecraft is not within the current sheet that light ions (H, 142) tend to dominate the ion composition for the upstream flow. If true, one may expect the interaction between Saturn's magnetosphere, locally devoid of heavy ions and Titan's upper atmosphere and exosphere to be significantly different from that for Voyager 1, TA and TB when heavy ions were present in the upstream flow. We also present observational evidence for Saturn's magnetosphere interaction with Titan's extended H and H2 corona which can extend approx. 1 Rs from Titan.