S. J. Kanani

University of Iowa, Iowa City, IA, United States

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Publications (15)12.44 Total impact

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    ABSTRACT: Enceladus resides deep within Saturn's magnetosphere. The magnetospheric plasma incident on the satellite is absorbed, forming a thermal plasma wake downstream of the moon, and a cavity extending north-south in higher energy populations. When the Cassini spacecraft crosses Enceladus's L-shell, these cavities are observed as brief dropouts in energetic particle fluxes, termed microsignatures. A survey of thermal plasma observations by the Cassini Plasma Spectrometer's Electron Spectrometer (CAPS-ELS) instrument has revealed the existence of various features in low energy electrons close to and sometimes during the traversal of microsignatures. These features vary in characteristics; some present themselves as a set of discrete spikes, covering electron energies of ~9-15eV and lasting, intermittently, up to tens of minutes. Others, dispersed in time according to energy, show evidence of injection events. We present the results of a survey of these perplexing features, suggest possible causes for their occurrence including important clues from close Enceladus flybys.
    AGU Fall Meeting Abstracts. 12/2010;
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    09/2010;
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    ABSTRACT: The shape and location of a planetary magnetopause can be determined by balancing the solar wind dynamic pressure with the magnetic and thermal pressures found inside the boundary. Previous studies have found the kronian magnetosphere to show rigidity (like that of Earth) as well as compressibility (like that of Jupiter) in terms of its dynamics. In this paper we expand on previous work and present a new model of Saturn's magnetopause. Using a Newtonian form of the pressure balance equation, we estimate the solar wind dynamic pressure at each magnetopause crossing by the Cassini spacecraft between Saturn Orbit Insertion in June 2004 and January 2006. We build on previous findings by including an improved estimate for the solar wind thermal pressure and include low-energy particle pressures from the Cassini plasma spectrometer's electron spectrometer and high-energy particle pressures from the Cassini magnetospheric imaging instrument. Our improved model has a size-pressure dependence described by a power law DP-1/5.0 ± 0.8. This exponent is consistent with that derived from numerical magnetohydrodynamic simulations.
    Journal of Geophysical Research 01/2010; · 3.17 Impact Factor
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    ABSTRACT: We present Cassini observations of a plasma vortex in Saturn's dayside outer magnetosphere. The vortex encounter took place on 13 December 2004 as Cassini was travelling toward the planet. The spacecraft crossed the magnetopause 3 times, before being immersed in the low-latitude boundary layer. During the transition between the boundary layer and the magnetosphere proper, the spacecraft observed deflected boundary layer plasma, a twisted magnetic field topology, and high-energy (>20 keV) directional electron fluxes. These observations are consistent with an encounter with a vortex on the inner edge of the boundary layer, an interface that is expected to be susceptible to the growth of the Kelvin-Helmholtz (K-H) instability due to its low magnetic shear. The size of the vortex is determined to be at least 0.55 RS, and a simple model of the current system resulting from the formation of the vortex is proposed. The possible acceleration mechanisms responsible for the high-energy electrons are discussed. The identification of the structure provides compelling evidence of the operation of the nonlinear K-H instability at Saturn's morning magnetospheric boundaries and has implications for our understanding of the transfer of energy and momentum between the solar wind and Saturn's magnetosphere.
    Journal of Geophysical Research 01/2010; · 3.17 Impact Factor
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    ABSTRACT: We present the first observations of a vortex structure in Saturn’s dayside, outer magnetosphere. The identification of the structure provides conclusive evidence of the operation of the Kelvin-Helmholtz (K-H) instability at Saturn’s magnetospheric boundaries. Cassini observations taken during the inbound pass of the spacecraft’s Revolution B orbit in Decmber 2004 are analysed. Magnetic field conditions during the magnetopause crossings that occurred on this orbital pass suggest that the boundary was highly K-H unstable. Following multiple magnetopause crossings the spacecraft encountered the low-latitude boundary layer. Magnetic field, thermal plasma, and superthermal plasma observations made by Cassini during the spacecraft transition between the boundary layer and magnetosphere proper are consistent with an encounter with a vortex structure on the edge of the boundary layer - this interface is also anticipated to be K-H unstable. High-energy (>20 keV) electrons observed while the spacecraft was within the vortex suggest that the structure was associated with auroral emissions. A model of the coupling between an outer magnetospheric vortex and Saturn’s ionsphere via field-aligned currents is proposed. Estimates based on Knight’s theory imply that field-aligned potentials of a few kV were associated with the region of upward-directed field-aligned current in the northern ionosphere, and that the resulting precipitation of accelerated electrons produced UV auroral emissions with an intensity of a few kR. We propose that K-H vortices in Saturn’s outer magnetosphere produce bright spots of UV aurora. This discovery has implications for our understanding of the interaction between the solar wind and Saturn’s magnetosphere.
    AGU Fall Meeting Abstracts. 11/2009; -1:1633.
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    ABSTRACT: Enceladus resides deep within Saturn’s magnetosphere. The magnetospheric plasma incident on the satellite is absorbed, forming a thermal plasma wake downstream of the moon, and a cavity in higher energy populations. When the Cassini spacecraft crosses Enceladus’s L-shell, these cavities are observed as brief dropouts in energetic particle fluxes, or microsignatures, most clearly observed by the Magnetospheric Imaging Instrument (MIMI). A survey of thermal plasma observations by the Cassini Plasma Spectrometer (CAPS) instrument has revealed the existence of enhancements in low energy electrons close to the times of microsignatures, or immediately before or after their occurrence. These features sometimes appear as a single electron flux enhancement but are often observed to appear as a set of discrete spikes, covering electron energies of 9-15 eV and lasting, intermittently, up to tens of minutes. We present the results of a survey of these perplexing features and suggest possible causes for their occurrence. We rule out dust impacts and spacecraft charging as potential causes, and investigate the possibility of plasma instabilities in Enceladus’s wake being responsible for their presence. We also present possible links between the spikes and coincident fluctuations in the magnetometer data.
    AGU Fall Meeting Abstracts. 11/2009; -1:1142.
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    ABSTRACT: By traversing the plume erupting from high southern latitudes on Saturn's moon Enceladus, Cassini orbiter instruments can directly sample the material therein. Cassini Plasma Spectrometer, CAPS, data show that a major plume component comprises previously-undetected particles of nanometer scales and larger that bridge the mass gap between previously observed gaseous species and solid icy grains. This population is electrically charged both negative and positive, indicating that subsurface triboelectric charging, i.e., contact electrification of condensed plume material may occur through mutual collisions within vents. The electric field of Saturn's magnetosphere controls the jets' morphologies, separating particles according to mass and charge. Fine-scale structuring of these particles' spatial distribution correlates with discrete plume jets' sources, and reveals locations of other possible active regions. The observed plume population likely forms a major component of high velocity nanometer particle streams detected outside Saturn's magnetosphere.
    Geophysical Research Letters 01/2009; · 3.98 Impact Factor
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    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.
    Planetary and Space Science 01/2009; · 2.11 Impact Factor
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    ABSTRACT: Rhea is Saturn’s second-largest moon, and orbits at 8.7 Saturn radii from the planet. The moon is continuously bombarded by magnetospheric plasma: the absorption of thermal plasma that overtakes Rhea in its orbit results in the formation of an upstream plasma wake. High energy electron dropouts - microsignatures, caused by the absorption of more energetic particles by the moon, are also observed. The unusually broad electron microsignatures observed near the moon are suggested to be evidence for the existence of a debris disk orbiting the moon (Jones et al. 2008). We present our current state of knowledge of the Rhea-magnetosphere interaction, based on data obtained by the Cassini CAPS and MIMI instruments during the spacecraft’s two closest encounters with the moon to date, on November 26, 2005, and August 30, 2007. We report on the detection of pickup ions at the moon by the CAPS instrument. This detection agrees with the results of Martens et al. (2008), who previously reported an enhancement in molecular oxygen ion distributions at the L shell of Rhea. We also summarize expectations for the upcoming close encounter on March 2, 2010.
    AGU Fall Meeting Abstracts. 01/2009; 1:1143.
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    ABSTRACT: Crossings of Saturn's magnetopause made by the Cassini spacecraft between 12 and 17 March 2006 are analysed. Magnetic field and plasma data are used to identify excursions into the magnetosheath bounded by crossings of the magnetopause current layer. During most of this period Cassini's trajectory was approximately parallel to the magnetopause boundary given by a model of the surface. Minimum variance analysis (MVA) of the magnetic field vector measurements is used to determine the normal to the boundary for each crossing of the current layer. The normals corresponding to the crossings made on 12, 13 and 17 March oscillate about the normal predicted by the surface model. This suggests the presence of regular boundary waves with a direction of propagation found to be close to parallel to Saturn's rotational equator, and not coincident with the expected solar wind flow direction in the local magnetosheath. Based on this propagation direction and the magnetospheric and magnetosheath magnetic fields we propose that these waves were generated by the Kelvin-Helmholtz instability. In addition we discuss the possibility that on 15 and 16 March nonlinear Kelvin-Helmholtz waves produced a strongly perturbed magnetopause boundary that may have led to local magnetic reconnection.
    AGU Fall Meeting Abstracts. 11/2008; -1:08.
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    ABSTRACT: Photoelectrons are generated in Titan's ionosphere as a result of the strong HeII (30.4nm) solar line ionising N2 [1]. The presence of these photoelectrons is discernible as a discrete peak in the electron energy spectrum observed by the ELS (ELectron Spectrometer) part of the Cassini Plasma Spectrometer (CAPS). They are generally observed in the dayside ionosphere. [2] discussed observations of photoelectrons in Titan's distant tail during the T9 encounter. This study describes other photoelectron observations near Titan. On some encounters photoelectrons are observed at large distances, some of which are near the nightside and in the tail. The origin of these is discussed. In some cases the photoelectrons may have travelled to the observation sites by means of a magnetic connection to lower altitudes on the dayside ionosphere where they could have been produced. To investigate this possibility, T15 observations are compared with different model results ([3], [4], [5]). The prospect of local production is also considered. References: [1] Galand et al (2006), Electron temperature of Titan's sunlit ionosphere, GRL, 33, L21101, doi:10.1029/2006GL027488. [2] Coates, A. J. et al (2007), Ionospheric electrons in Titan's tail: Plasma structure during the Cassini T9 encounter, GRL, 34, L24S05, doi:10.1029/2007GL030919. [3] Sillanpaa, I., Hybrid Modelling of Titan's Interaction with the Magnetosphere of Saturn, Ph.D. dissertation, Yliopistopaino, 2008. [4] Ma, Y. J.,et al (2008), Real-time Global MHD Simulations of Cassini T32 Flyby: from Magnetosphere to Magnetosheath, JGR, submitted. [5] Cravens, T. E. et al (2008), Model-Data Comparisons for Titan's Nightside Ionosphere, Icarus, submitted.
    AGU Fall Meeting Abstracts. 11/2008; -1:1321.
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    ABSTRACT: The shape and location of a planetary magnetopause can be determined by balancing the solar wind dynamic pressure with the magnetic and thermal pressures found inside the magnetopause. Previous studies on the kronian magnetopause have argued that the boundary is Earth-like (Slavin et al. 1985) and Jupiter- like (Arridge et al. 2006) in terms of its dynamics. In this poster we hope to find a solution by presenting a new pressure-dependent model of the magnetopause. We build upon previous findings by including estimated values for the solar wind thermal pressure and including low energy particle pressures from the Cassini plasma spectrometer (CAPS) and high energy particle pressures from the Cassini magnetospheric imaging instrument (MIMI). The results are compared to previous models to see whether the size and the shape of the boundary vary with these additional parameters. Directions for future studies are also outlined.
    AGU Fall Meeting Abstracts. 11/2008; -1:1265.
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    ABSTRACT: On March 12, 2008, the Cassini spacecraft passed through the south polar plume of Enceladus. We present observations from the electron spectrometer (ELS) of the CAPS instrument obtained during this event. Several unanticipated features were observed. We present our interpretations of these features as well as their causes. We also make testable predictions regarding the CAPS-ELS observations within the plume during future flybys.
    09/2008;
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    ABSTRACT: The Electron Spectrometer, ELS, of the Cassini Plasma Spectrometer, CAPS, was oriented for the direct sampling of material inside Enceladus's plume during two of the three close encounters with this moon in 2008, on March 12, and October 9. We present ELS observations obtained during these two encounters, approaching to within 52 and 25 km of the moon's surface, respectively. Several unanticipated features were observed within the plume; we present our interpretations of these features, comparing them with other instruments' observations of the plume and its surface sources.
    AGU Fall Meeting Abstracts. 01/2008;