28 research outputs found
Enhanced ionization of the Martian nightside ionosphere during solar energetic particle events
Electron densities in the Martian nightside ionosphere are more than 90% of time too low to be detected by the Mars Advanced Radar for Subsurface and Ionosphere Sounding radar sounder on board the Mars Express spacecraft. However, the relative number of ionograms with peak electron density high enough to be detected represents a good statistical proxy of the ionospheric density. We focus on solar energetic particle (SEP) events, and we analyze their effects on ionospheric formation. SEP time intervals were identified in situ using the background counts recorded by the ion sensor of the ASPERA-3 instrument on board Mars Express. We show that peak electron densities during the SEP events are large enough to be detected in more than 30% of measurements, and, moreover, the reflections of the sounding signal from the ground almost entirely disappear. Nightside electron densities during SEP events are thus substantially increased as compared to normal nightside conditions
Dynamical and magnetic field time constants for Titan's ionosphere: Empirical estimates and comparisons with Venus
Plasma in TitanÂŽs 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 sâ1 near an altitude of 1000 km and 200 m sâ1 at 1500 km. For comparison, the thermospheric neutral wind speed is about 100 m sâ1. 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 TitanÂŽs ionospheric magnetic field. Comparisons are made between the ionospheric dynamics at Titan and at Venus.Fil: Cravens, T. E.. University of Kansas; Estados UnidosFil: Richard, M.. University of Kansas; Estados UnidosFil: Ma, Y. J.. University of California; Estados UnidosFil: Bertucci, Cesar. Consejo Nacional de InvestigaciĂłnes CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de AstronomĂa y FĂsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomĂa y FĂsica del Espacio; ArgentinaFil: Luhmann, J. G.. University of California; Estados UnidosFil: Ledvina, S.. University of California; Estados UnidosFil: Robertson, I. P.. University of Kansas; Estados UnidosFil: Wahlund, J. E.. Swedish Institute of Space Physics; SueciaFil: Ă
gren, K.. Swedish Institute of Space Physics; SueciaFil: Cui, J.. Imperial College London; Reino UnidoFil: Muller Wodarg, I.. Imperial College London; Reino UnidoFil: Waite, J. H.. Southwest Research Institute; Estados UnidosFil: Dougherty, M.. Imperial College London; Reino UnidoFil: Bell, J.. Southwest Research Institute; Estados UnidosFil: Ulusen, D.. University of California; Estados Unido
Structure of Titan â s induced magnetosphere under varying background magnetic fi eld conditions: Survey of Cassini magnetometer data from fl ybys TA â T85
Cassini magnetic field observations between 2004 and 2012 suggest the ambient field conditions near Titanâs orbit to differ significantly from the frequently applied pre-Cassini picture (background magnetic field homogeneous and perpendicular to Titanâs orbital plane, stationary upstream conditions). In this study, we analyze the impact of these varying background field conditions on the structure of Titanâs induced magnetosphere by conducting a systematic survey of Cassini magnetic field observations in the interaction region during flybys TAâT85 (July 2004âJuly 2012). We introduce a set of criteria that allow to identify deviations in the structure of Titanâs induced magnetosphereâas seen by the Cassini magnetometer (MAG)âfrom the picture of steady-state field line draping. These disruptions are classified as âweakâ, âmoderateâ, or âstrongâ. After applying this classification scheme to all available Titan encounters, we survey the data for a possible correlation between the disruptions of the draping pattern and the ambient magnetospheric field conditions, as characterized by Simon et al. [2010a]. Our major findings are: (1) When Cassini is embedded in the northern or southern lobe of Saturnâs magnetodisk within a ` 3 h interval around closest approach, Titanâs induced magnetosphere shows little or no deviations at all from the steady-state draping picture. (2) Even when Titan is embedded in perturbed current sheet fields during an encounter, the notion of draping the average background field around the moonâs ionosphere is still applicable to explain MAG observations from numerous Titan flybys. (3) Only when Titan is exposed to intense north- south oscillations of Saturnâs current sheet at the time of an encounter, the signatures of the moonâs induced magnetosphere may be completely obscured by the ambient field perturbations. (4) So far, T70 is the only flyby that fully meets the idealized pre-Cassini picture of the Titan interaction (steady background field perpendicular to Titanâs orbital plane, steady upstream flow, unperturbed induced magnetosphere).Fil: Simon, Sven. University of Cologne. Institute of Geophysics and Meteorology; AlemaniaFil: van Treeck, Shari C.. University of Cologne. Institute of Geophysics and Meteorology; AlemaniaFil: Wennmacher, Alexandre. University of Cologne. Institute of Geophysics and Meteorology; AlemaniaFil: Saur, Joachim. University of Cologne. Institute of Geophysics and Meteorology; AlemaniaFil: Neubauer, Fritz M.. University of Cologne. Institute of Geophysics and Meteorology; AlemaniaFil: Bertucci, Cesar. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de AstronomĂa y FĂsica del Espacio(i); ArgentinaFil: Dougherty, Michele K.. Imperial College Of Science And Technology. Space and Atmospheric Physics Group; Reino Unid
Titan's interaction with the supersonic solar wind
After 9 years in the Saturn system, the Cassini spacecraft finally observed Titan in the supersonic and super-AlfvĂ©nic solar wind. These unique observations reveal that Titan?s interaction with the solar wind is in many ways similar to unmagnetized planets Mars and Venus and active comets in spite of the differences in the properties of the solar plasma in the outer solar system. In particular, Cassini detected a collisionless, supercritical bow shock and a well-defined induced magnetosphere filled with mass-loaded interplanetary magnetic field lines, which drape around Titan?s ionosphere. Although the flyby altitude may not allow the detection of an ionopause, Cassini reports enhancements of plasma density compatible with plasma clouds or streamers in the flanks of its induced magnetosphere or due to an expansion of the induced magnetosphere. Because of the upstream conditions, these observations may be also relevant to other bodies in the outer solar system such as Pluto, where kinetic processes are expected to dominate.Fil: Bertucci, Cesar. Consejo Nacional de InvestigaciĂłnes CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de AstronomĂa y FĂsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomĂa y FĂsica del Espacio; ArgentinaFil: Hamilton, D. C.. University of Maryland; Estados UnidosFil: Kurth, W. S.. University of Iowa; Estados UnidosFil: Hospodarsky, G.. University of Iowa; Estados UnidosFil: Mitchell, D.. University Johns Hopkins; Estados UnidosFil: Sergis, N.. Academy of Athens; GreciaFil: Edberg, N. J. T.. Swedish Institute of Space Physics,; SueciaFil: Dougherty, M. K.. Imperial College London; Reino Unid
One-dimensional fluid and hybrid numerical analysis of the plasma properties in the discharge channel of a Hall thruster
One-dimensional fluid and hybrid fluid-kinetic numerical codes are developed and applied to the analysis of axial distributions of plasma properties in the discharge channel of a Hall-effect thruster. Within the hybrid model ions are described by the kinetic Vlasov equation, while electrons and neutral atoms are treated as fluids. The results obtained from fluid and hybrid models are compared. Different operating regimes such as damped, periodic, and aperiodic irregular oscillations about the stationary state are observed and discussed. Thrust and efficiency of the thruster for different input parameters are estimated
Investigation of the force balance in the Titan ionosphere: Cassini T5 flyby model/data comparisons
a b s t r a c t Cassini's Titan flyby on 16 April, 2005 (T5) is the only encounter when the two main ionizing sources of the moon's atmosphere, solar radiation and corotating plasma, align almost anti-parallel. In this paper a single-fluid multi-species 3D MHD model of the magnetospheric plasma interaction for T5 conditions is analyzed. Model results are compared to observations to investigate the ionospheric dynamics at Titan as well as to understand the deviations from a typical solar wind interaction, such as Venus' interaction with the solar wind. Model results suggest that for the T5 interaction configuration, corotating plasma is the dominant driver determining the global interaction features at high altitudes. In the lower ionosphere below 1000 km altitude and by magnetic diffusion below this altitude. Moreover, strong horizontal drag forces due to ion-neutral collisions and comparable drag forces estimated from possible neutral winds in the lower ionosphere below $1400 km altitude oppose over local regions, implying that the Titan interaction must be treated as a 3D problem. Ion and electron densities calculated from the model generally agree with the Cassini Ion Neutral Mass Spectrometer and Langmuir probe measurements; however, there are significant differences between the calculated and measured magnetic fields. We discuss possible explanations for the discrepancy in the magnetic field predictions