Abstract:

In order to analyze varying plasma conditions upstream of Titan, we have combined a physical model of Saturn?s plasma disk with a geometrical model of the oscillating current sheet. During modeled oscillation phases where Titan is farthest from the current sheet, the main sources of plasma pressure in the near-Titan space are the magnetic pressure and, for disturbed conditions, the hot plasma pressure. When Titan is at the center of the sheet, the main sources are the dynamic pressure associated with Saturn?s cold, subcorotating plasma and the hot plasma pressure under disturbed conditions. Total pressure at Titan (dynamic plus thermal plus magnetic) typically increases by a factor of up to about 3 as the current sheet center is approached. The predicted incident plasma flow direction deviates from the orbital plane of Titan by ≲ 10◦ . These results suggest a correlation between the location of magnetic pressure maxima and the oscillation phase of the plasma sheet. Our model may be used to predict near-Titan conditions from ?far-field? in situ measurements.

Author affiliation: Achilleos, N.. University College London; Reino Unido

Author affiliation: Arridge, C. S.. University College London; Reino Unido

Author affiliation: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Guio, P.. University College London; Reino Unido

Author affiliation: Romanelli, Norberto Julio. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Sergis, N.. Academy Of Athens. Office for Space Research and Technology; Grecia

Abstract:

onization of neutrals by precipitating electrons and ions is the main source of Titan's nightside ionosphere. This paper has two goals: (1) characterization of the role of electron impact ionization on the nightside ionosphere for different magnetospheric conditions and (2) presentation of empirical ion production rates determined using densities measured by the Cassini Ion and Neutral Mass Spectrometer on the nightside. The ionosphere between 1000 and 1400 km is emphasized. We adopt electron fluxes measured by the Cassini Plasma Spectrometer-Electron Spectrometer and the Magnetospheric Imaging Instrument as classified by Rymer et al. (2009). The current paper follows an earlier paper (Paper I), in which we investigated sources of Titan's dayside ionosphere and demonstrated that the photoionization process is well understood. The current paper (Paper II) demonstrates that modeled and empirical ionization rates on the nightside are in agreement with an electron precipitation source above 1100 km. Ion production rate profiles appropriate for different Saturnian magnetospheric conditions, as outlined by Rymer et al., are constructed for various magnetic field topologies. Empirical production rate profiles are generated for deep nightside flybys of Titan. The results also suggest that at lower altitudes (below 1100 km) another source, such as ion precipitation, is probably needed.

Author affiliation: Richard, M. S.. University of Kansas; Estados Unidos. Benedictine College; Estados Unidos

Author affiliation: Cravens, T. E.. University of Kansas; Estados Unidos

Author affiliation: Wylie, C.. University of Kansas; Estados Unidos

Author affiliation: Webb, D.. University of Kansas; Estados Unidos

Author affiliation: Chediak, Q.. University of Kansas; Estados Unidos

Author affiliation: Mandt, K.. Southwest Research Institute; Estados Unidos

Author affiliation: Waite Jr, J. H.. Southwest Research Institute; Estados Unidos

Author affiliation: Rymer, A.. University Johns Hopkins; Estados Unidos

Author affiliation: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Wellbrock, A.. University College London; Reino Unido

Author affiliation: Windsor, A.. Benedictine College. Department of Physics and Astronomy; Estados Unidos

Author affiliation: Coates, A. J.. University College London; Reino Unido

Abstract:

A través del estudio de los datos provistos por el magnetómetro VHM, a bordo de la sonda espacial Cassini-Huygens en órbita alrededor del planeta desde el año 2004, realizamos un relevamiento y análisis detallados de las ondas de ultra-baja frecuencia asociadas al choque de Saturno. Más específicamente, identificamos ondas lineales y no lineales que se generan en la región conectada magnéticamente al choque (foreshock). Asimismo, realizamos un análisis del choque de un planeta gigante como Saturno, sus límites y dependencias, y estudiamos como varían las componentes de campo magnético al atravesarlo. Además se estudió la correlación existente entre la presencia-ausencia de ondas y el ángulo entre la línea de campo magnético y la normal al choque en el punto de intersección.

By studying the data provided by the VHM magnetometer on board of the Cassini-Huygens spacecraft orbiting the planet since 2004, we conducted a global survey and detailed analysis of ultra-low frequency (ULF) waves associated with Saturn’s bow shock. More specifically, we identify linear and nonlinear waves generated in the region magnetically connected to the bow shock (i.e. the foreshock). In addition, we analyzed the bow shock of a giant planet like Saturn and study the variation of the magnetic field components as we pass through the bow shock. Also, we studied the correlation between the presence (or absence) of waves and the angle between the interplanetary magnetic field and the shock normal at the point of intersection.

Author affiliation: Andrés, Nahuel. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Gomez, Daniel Osvaldo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Abstract:

During the Cassini Titan flyby on 2 July 2006 (T15), Titan was surrounded by a magnetospheric plasma flow with density about 0.1 cm‑3 as measured by Cassini Plasma Spectrometer (CAPS). A very low fraction of water group ions (O+) was detected in the flow dominated by hydrogen ions. We show that Titan's plasma interaction can be highly sensitive to the small fraction of oxygen ions in the magnetospheric flow. The ion quantities of the magnetospheric flow during the flyby were obtained from numerical moments calculated from the CAPS measurements; the average ambient magnetic field was determined using the Cassini magnetometer data. We simulated the flyby using a global hybrid model; the water group abundance in the flow was varied in three simulation runs. Based on the simulation results, the oxygen content has an especially notable effect on the extent of Titan's induced magnetosphere. A multi-instrument analysis was performed comparing with the simulations, whereby a comprehensive picture of the plasma properties around Titan during this flyby was obtained. Comparisons between the hybrid model simulations and Cassini measurements during the flyby point toward O+ density in the undisturbed magnetospheric flow having been around 0.008 cm‑3, which would have accounted for one half of the dynamic pressure of the flow.

Author affiliation: Sillanpää, I.. Southwest Research Institute; Estados Unidos

Author affiliation: Young, D. T.. Southwest Research Institute; Estados Unidos

Author affiliation: Crary, F.. Southwest Research Institute; Estados Unidos

Author affiliation: Thomsen, M.. Los Alamos National Laboratory,; Estados Unidos

Author affiliation: Reisenfelc, D.. University of Montana; Estados Unidos

Author affiliation: Wahlund, J. E.. Swedish Institute of Space Physics; Suecia

Author affiliation: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Kallio, E.. Finnish Meteorological Institute,; Finlandia

Author affiliation: Jarvinen, R.. Finnish Meteorological Institute; Finlandia

Author affiliation: Janhunen, P.. Finnish Meteorological Institute,; Finlandia

Abstract:

Crossings of Saturn´s magnetopause made by the Cassini spacecraft on 12, 13 and 17 March 2006 are analysed. During this period Cassini´s trajectory was approximately parallel to the magnetopause boundary given by a model of the surface. Magnetic field and electron data are used to identify excursions into the magnetosheath bounded by crossings of the magnetopause current layer. Minimum variance analysis of the magnetic field vector measurements is used to determine the normal to the boundary for each crossing. The normals corresponding to the crossings oscillate about an average orientation that is consistent with the unperturbed normal predicted by the surface model. This reveals the presence of regular boundary waves with a direction of propagation found to be within 24° of Saturn´s rotational equator. Two categories of boundary wave are identified: the first with a period of the order of hours, and the second with a period of 45±9 min. Based on the propagation direction and a comparison of magnetospheric and magnetosheath magnetic fields, we conclude that both types of wave were driven by the Kelvin-Helmholtz instability. The observed boundary perturbations are consistent with a superposition of different types of surface wave activity.

Author affiliation: Masters, A.. Imperial College London; Reino Unido

Author affiliation: Arridge, C. S.. University College London; Estados Unidos

Author affiliation: Dougherty, M. K.. Imperial College London; Reino Unido

Author affiliation: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Billingham, L.. Imperial College London; Reino Unido

Author affiliation: Schwartz, S. J.. Imperial College London; Reino Unido

Author affiliation: Jackman, C. M.. Imperial College London; Reino Unido

Author affiliation: Bebesi, Z.. Research Institute for Particle and Nuclear Physics; Hungría

Author affiliation: Coates, A. J.. University College London; Estados Unidos

Author affiliation: Thomsen, M. F.. Los Alamos National Laboratory; Estados Unidos

Abstract:

The structure and variability of Saturn's magnetic field in the vicinity of Titan's orbit is studied. In the dawn magnetosphere, the magnetic field presents a significant radial component directed towards Saturn, suggesting that Titan is usually located below the planet's warped and dynamic magnetodisc. Also, a non-negligible component along the co-rotation direction suggests that Saturn's magnetic field lines close to the magnetodisc are being swept back from their respective magnetic meridians. In the noon sector, Titan seems to be closer to the magnetodisc central current sheet, as the field lines in this region seem to be more dipolar. The distance between the central current sheet and Titan depends mainly on the solar wind pressure. Also, δ|B|/|B|∼0.5 amplitude waveforms at periods close to Saturn's kilometric radiation period are present in the background magnetic field. This modulation in the field is ubiquitous in Saturn's magnetosphere and associated with the presence of a rotating asymmetry in the planet's magnetic field.

Author affiliation: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina. Imperial College London; Reino Unido

Abstract:

We use magnetometer data from the Mars Global Surveyor (MGS) spacecraft during portions of the premapping orbits of the mission to study the variability of the Martian-induced magnetotail as a function of the orientation of the interplanetary magnetic field (IMF). The time spent by MGS in the magnetotail lobes during periods with positive solar wind flow-aligned IMF component B IMF suggests that their location as well as the position of the central polarity reversal layer (PRL) are displaced in the direction antiparallel to the IMF cross-flow component B IMF. Analogously, in the cases where B IMF is negative, IMF the lobes are displaced in the direction of B ⟂ . This behavior is compatible with a previously published analytical model of the IMF draping, where for the first time, the displacement of a complementary reversal layer (denoted as IPRL for inverse polarity reversal layer) is deduced from first principles.

Author affiliation: Romanelli, Norberto Julio. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Gomez, Daniel Osvaldo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Mazelle, C.. Université Paul Sabatier; Francia

Abstract:

Plasma in Titan´s ionosphere flows in response to forcing from thermal pressure gradients, magnetic forces, gravity, and ion-neutral collisions. This paper takes an empirical approach to the ionospheric dynamics by using data from Cassini instruments to estimate pressures, flow speeds, and time constants on the dayside and nightside. The plasma flow speed relative to the neutral gas speed is approximately 1 m s^‑1 near an altitude of 1000 km and 200 m s^‑1 at 1500 km. For comparison, the thermospheric neutral wind speed is about 100 m s^‑1. The ionospheric plasma is strongly coupled to the neutrals below an altitude of about 1300 km. Transport, vertical or horizontal, becomes more important than chemistry in controlling ionospheric densities above about 1200-1500 km, depending on the ion species. Empirical estimates are used to demonstrate that the structure of the ionospheric magnetic field is determined by plasma transport (including neutral wind effects) for altitudes above about 1000 km and by magnetic diffusion at lower altitudes. The paper suggests that a velocity shear layer near 1300 km could exist at some locations and could affect the structure of the magnetic field. Both Hall and polarization electric field terms in the magnetic induction equation are shown to be locally important in controlling the structure of Titan´s ionospheric magnetic field. Comparisons are made between the ionospheric dynamics at Titan and at Venus.

Author affiliation: Cravens, T. E.. University of Kansas; Estados Unidos

Author affiliation: Richard, M.. University of Kansas; Estados Unidos

Author affiliation: Ma, Y. J.. University of California; Estados Unidos

Author affiliation: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Luhmann, J. G.. University of California; Estados Unidos

Author affiliation: Ledvina, S.. University of California; Estados Unidos

Author affiliation: Robertson, I. P.. University of Kansas; Estados Unidos

Author affiliation: Wahlund, J. E.. Swedish Institute of Space Physics; Suecia

Author affiliation: Ågren, K.. Swedish Institute of Space Physics; Suecia

Author affiliation: Cui, J.. Imperial College London; Reino Unido

Author affiliation: Muller Wodarg, I.. Imperial College London; Reino Unido

Author affiliation: Waite, J. H.. Southwest Research Institute; Estados Unidos

Author affiliation: Dougherty, M.. Imperial College London; Reino Unido

Author affiliation: Bell, J.. Southwest Research Institute; Estados Unidos

Author affiliation: Ulusen, D.. University of California; Estados Unidos

Abstract:

We analyze the variability of the ambient magnetospheric field along Titan’s orbit at 20.3 Saturn radii. However, while our preceding study (Simon et al., 2010) focused on Cassini magnetometer observations from the 62 Titan flybys (TA–T62) between October 2004 and October 2009, the present work discusses magnetic field data that were collected near Titan’s orbit when the moon was far away. In analogy to the observations during TA–T62, the magnetospheric fields detected during these 79 ‘‘virtual’’ Titan flybys are strongly affected by the presence of Saturn’s bowl-shaped and highly dynamic magnetodisk current sheet. We therefore provide a systematic classification of the magnetic field observations as magnetodisk current sheet or lobe-type scenarios. Among the 141 (62 real+79 virtual) crossings of Titan’s orbit between July 2004 and December 2009, only 17 encounters (9 real+8 virtual) took place within quiet, magnetodisk lobe-type fields. During another 50 encounters (21 real+29 virtual), rapid transitions between current sheet and lobe fields were observed around the moon’s orbital plane. Most of the encounters (54¼22 real+32 virtual) occurred when Titan’s orbit was embedded in highly distorted current sheet fields, thereby invalidating the frequently applied idealized picture of Titan interacting with a homogeneous and stationary magnetospheric background field. The locations of real and virtual Titan flybys are correlated to each other. Each of the 62 real Titan flybys possesses at least one virtual counterpart that occurred shortly before or after the real encounter and at nearly the same orbital position. A systematic comparison between Cassini magnetometer observations from the real Titan flybys and their virtual companions suggests that there is no clear evidence of Titan exerting a significant level of control on the vertical oscillatory motion of the magnetodisk near its orbit.

Author affiliation: Simon, Sven. University of Cologne; Alemania

Author affiliation: Wennmacher, Alexandre. University of Cologne; Alemania

Author affiliation: Neubauer, Fritz M.. University of Cologne; Alemania

Author affiliation: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Kriegel, Hendrik. Institute for Theoretical Physics; Alemania

Author affiliation: Russell, Christopher T.. University of California; Estados Unidos

Author affiliation: Dougherty, Michele K.. Imperial College London; Reino Unido

Abstract:

We use the Cassini Radio and Plasma Wave Science/Langmuir probe measurements of the electron density from the first 110 flybys of Titan to study how Saturn´s magnetosphere influences Titan´s ionosphere. The data is first corrected for biased sampling due to varying solar zenith angle and solar energy flux (solar cycle effects). We then present results showing that the electron density in Titan´s ionosphere, in the altitude range 1600-2400 km, is increased by about a factor of 2.5 when Titan is located on the nightside of Saturn (Saturn local time (SLT) 21-03 h) compared to when on the dayside (SLT 09-15 h). For lower altitudes (1100-1600 km) the main dividing factor for the ionospheric density is the ambient magnetospheric conditions. When Titan is located in the magnetospheric current sheet, the electron density in Titan´s ionosphere is about a factor of 1.4 higher compared to when Titan is located in the magnetospheric lobes. The factor of 1.4 increase in between sheet and lobe flybys is interpreted as an effect of increased particle impact ionization from 200 eV sheet electrons. The factor of 2.5 increase in electron density between flybys on Saturn´s nightside and dayside is suggested to be an effect of the pressure balance between thermal plus magnetic pressure in Titan´s ionosphere against the dynamic pressure and energetic particle pressure in Saturn´s magnetosphere.

Author affiliation: Edberg, N. J. T.. University of Iowa; Estados Unidos. Swedish Institute of Space Physics; Suecia

Author affiliation: Andrews, D. J.. Swedish Institute of Space Physics; Suecia

Author affiliation: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Author affiliation: Gurnett, D. A.. University of Iowa; Estados Unidos

Author affiliation: Holmberg, M. K. G.. Swedish Institute of Space Physics; Suecia

Author affiliation: Jackman, C. M.. University Of Southampton; Reino Unido

Author affiliation: Kurth, W. S.. University of Iowa; Estados Unidos

Author affiliation: Menietti, J. D.. University Of Iowa; Estados Unidos

Author affiliation: Opgenoorth, H. J.. Swedish Institute of Space Physics; Suecia

Author affiliation: Shebanits, O.. Swedish Institute of Space Physics; Suecia

Author affiliation: Vigren, E.. Swedish Institute of Space Physics; Suecia

Author affiliation: Wahlund, J. E.. Swedish Institute of Space Physics; Suecia