[Show abstract][Hide abstract] ABSTRACT: 1] One major objective of the Cassini mission is the analysis of Saturnian radio emissions of magnetospheric (auroral) as well as atmospheric (lightning) origin. The Radio and Plasma Wave Science (RPWS) experiment is designed to measure the full polarization and the wave vector of the incoming radio waves, allowing us to retrieve information on source locations and emission modes. For that purpose, RPWS uses a two-channel receiver, connected to two electric monopoles (selected among three), which measures the voltages induced by the electric field of the incident waves and their various correlations. The accuracy of retrieved source locations depends directly on the precise knowledge of the orientation of the three effective monopole axes and lengths, which do not coincide with the physical ones owing to interaction with the spacecraft body. Antenna calibration aims at determining the so-called effective length vector of each antenna (combining orientation and length information). For that purpose, roll maneuvers of the Cassini spacecraft were performed before and after the Jupiter flyby, at distances such that Jovian radio sources can be identified with the planet's center but still provide a high signal-to-noise ratio. The resulting modulations of the measured signals allow us to derive the orientation and length of the effective antennas. The analysis is performed in two steps: first, the Stokes parameters (wave polarization) are determined using approximate antenna orientations derived from laboratory measurements on a scale model of the spacecraft. Second, measurements with high signal-to-noise ratio and pure circular polarization are selected and used for the determination of the effective length vectors of the RPWS antennas. Two methods have been developed for inverting the system of equations relating antenna parameters, wave parameters, and measurements (least squares fit and analytical inversion), both of which provide consistent results and present different advantages and limitations which are discussed. A final set of antenna parameters to be used for direction finding studies with the RPWS experiment is obtained. Citation: Vogl, D. F., et al. (2004), In-flight calibration of the Cassini-Radio and Plasma Wave Science (RPWS) antenna system for direction-finding and polarization measurements, J. Geophys. Res., 109, A09S17, doi:10.1029/2003JA010261.
Journal of Geophysical Research Atmospheres 01/2004; 109:A09S17. · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Within the magnetohydrodynamic (MHD) approach, the interchange instability is studied for the subsolar magnetosheath of Venus. The instability analysis considers the profiles of magnetic field and plasma parameters between the bow shock and the ionopause which are obtained from the numerical MHD solution of the solar wind flow around the ionosphere. With the Fourier transformations, the linearized MHD equations are reduced to a second-order differential equation for the total pressure perturbation as a function of the normal distance from the ionopause. This equation is integrated numerically, and the interchange instability growth rate is obtained as a function of the wave number. The instability growth time is found to be smaller than the time scale of magnetic barrier formation.
Advances in Space Research 01/2004; · 1.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: During periods of a persistent southward interplanetary magnetic field, reconnection is initiated at the dayside magnetopause and flux is transported away to the nightside magnetosphere. This is the so-called phenomenon of magnetosphere erosion. In this paper, erosion at the terrestrial magnetopause is studied. A new theoretical approach to study erosion at the magnetopause on the basis of time-dependent reconnection is presented. We calculate the earthward motion of the magnetopause resulting from each reconnection pulse, together with the associated bow shock motion. Thus, the displacement of both boundary layers is presented.
Advances in Space Research 01/2004; 33(11):2103-2107. · 1.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Martian neutral gas temperatures estimated from topside plasma scale heights of electron density distributions and Lyman-α dayglow observations obtained by several Mars missions yield an exospheric temperature in the order of 350 ± 100 K. However, this value would imply a much higher dependence on the F10.7 radio flux than found for Venus and does not appear to be consistent with the observed solar cycle dependence of the Martian ionospheric peak plasma densities. On the other hand, observations on Venus indicate that photochemically produced very low energetic H atoms, as well as part of the solar wind protons and accelerated planetary H+ ions, which can be transformed into energetic H atoms (ENAs) may also contribute to the observed neutral H atoms. By calculating the flux of solar wind ENAs we find that they may give rise to an increase of the exospheric temperature of up to several tens of Kelvin.
Advances in Space Research 01/2004; · 1.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, we concentrate on the analysis of the anisotropic Rankine–Hugoniot equations for perpendicular and oblique fast shocks. In particular, as additional information to the anisotropic set of equations, the threshold conditions of the fire-hose and mirror instability are used to bound the range of the pressure anisotropy downstream of the discontinuity. These anisotropic threshold conditions of the plasma instabilities are obtained via a kinetic approach using a generalized Lorentzian distribution function, the so-called kappa distribution function. Depending on up-stream conditions, these instabilities further define stable and unstable regions with regard to the pressure anisotropy downstream of the shock. The calculations are done for different upstream Alfvén Mach numbers. We found that low values of the parameter kappa reduce the pressure anisotropy downstream of the shock.
Planetary and Space Science 10/2003; 51(12):715-722. · 2.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper we focus on the variation of the plasma and field parameters across a parallel fast shock geometry for anisotropic plasma conditions on either side of the discontinuity. In particular we derive the physical quantities as functions of the pressure anisotropy downstream of the shock for various upstream Alfven and Sonic Mach numbers as well as for different upstream pressure anisotropies. In studying the shock environment we a) include upstream a magnetic energy term regarding the whistler mode in the energy equation and b) assume a so--called switch--on shock in the downstream region.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we report on the Rankine--Hugoniot equations for a perpendicular fast shock geometry taking into account anisotropic plasma conditions on either side of the shock. To provide a physical limit to the pressure anisotropy downstream of the discontinuity, we introduce the threshold condition of the mirror instability and that of the ion--cyclotron instability as bounding conditions to the set of equations. For various upstream conditions and wave frequencies, we study the changes of the density across the shock taking into account these two threshold conditions and compare the two different approaches. Assuming a given wave frequency, we analyze further all relevant plasma parameter in the downstream region in order to proof the ion--cyclotron wave mode amplification.
[Show abstract][Hide abstract] ABSTRACT: Based on realistic solar wind kinetic characteristics one finds
macroscopic quantities, as parallel and perpendicular pressures, via the
second moments of the velocity space distribution, serving along with
magnetic field, flow velocity and density as upstream bow shock input
parameters in the set of the Rankine-Hugoniot equations. The solution of
the system of nonlinear equations returns the modified downstream
physical quantities where a decomposition yields the corresponding
magnetosheath distributions. This process allows deducing directly
kinetic magnetosheath plasma characteristics from undisturbed solar wind
conditions. Based on well-established observations of anisotropic solar
wind core-halo structures we model the proton component by the family of
kappa distributions and allow in view of full generality also double
humped conditions, appropriate for high-speed solar wind streams. The
numerical simulation permits arbitrary upstream solar wind flow and IMF
directions with respect to the shock transition layer and provides all
characteristics of the velocity space distributions downstream, as
anisotropic heating or change of the fraction of suprathermal particle
[Show abstract][Hide abstract] ABSTRACT: In this paper, we concentrate on the solution of the anisotropic Rankine-Hugoniot equations for inclined fast shocks taking into account a new approach in closing the set of equations. In particular, the threshold conditions of the fire-hose and that of the mirror instability, obtained in a kinetic approach using the so-called kappa distribution function, are used to bound the range of the pressure anisotropy downstream of the discontinuity. We study the variation of the density across the shock for a given Alfvén Mach number and upstream pressure anisotropy and find that the parameter kappa is most sensitive to stable plasma conditions, i.e. low values of kappa reduce the pressure anisotropy downstream of the discontinuity.
Advances in Space Research 01/2003; 32(4):519-523. · 1.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Reconnection of magnetic field lines is a very important coupling mechanism in space for configurations with considerable skew in the magnetic field. In a magnetospheric context such configurations occur at the dayside magnetopause and in the magnetotail. Thus, reconnection couples phenomena prevailing in the solar wind with ionospheric phenomena. We report on the most important signatures of reconnection. Recent developments of reconnection theory are presented. Reconnection phenomena in the tail are briefly discussed
[Show abstract][Hide abstract] ABSTRACT: In this paper, we concentrate on the analysis of a perpendicular fast shock includ- ing upstream and downstream pressure anisotropy as well as the concept of the so- called kappa distribution function and apply it to conditions at the Earth's bow shock. For closing the set of equations we use the threshold condition of the mirror insta- bility to bound the range of the pressure anisotropy downstream of the shock wave. This anisotropic instability is obtained a) via the well known bi-Maxwellian distribu- tion function and b) from a generalized Lorentzian distribution function, the so-called kappa distribution function. We first study the variations of the physical parameters and the magnetic field strength across the shock wave as functions of upstream so- lar wind conditions. These variations are mostly influenced by the upstream Alfvèn Mach number. We further compare the two different expressions of the mirror insta- bilites and find that low values of the kappa-parameter reduce the pressure anisotropy downstream of the perpendicular fast shock wave.
[Show abstract][Hide abstract] ABSTRACT: Many observations show a difference between the pressure parallel and perpendicular to the magnetic field. Thus it is reasonable to study the Petschek reconnection mechanism for the set of equations allowing for pressure anisotropy. We study the simplest case of so—called Petschek shocks, where the Alfvén discontinuity and the slow shock degenerate to one discontinuity. More specifically, we consider switch—off shocks, where the magnetic field on the downstream side vanishes in lowest order, so that the plasma is isotropic on that side.
Advances in Space Research 01/2002; · 1.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Magnetic reconnection is a process which allows topological different magnetic fields to interconnect. Thus, in magnetospheric context, reconnection is strongly associated with substorm phenomena. Because many observations show a difference between the pressure parallel and perpendicular to the magnetic field, it is reasonable to study the reconnection mechanism for the set of equations, involving a pressure tensor. Existing theoretical work for isotropic weak reconnection is extended for anisotropic theory. In particular, the reconnection associated discontinuities as the Alfvén discontinuity, the slow shock, and the contact discontinuity are generalized for anisotropic pressure.
Advances in Space Research 01/2002; 29:1113-1118. · 1.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To study magnetosphere-ionosphere interactions, appropriate considerations on the solar wind, the bow shock, the magnetosheath, and the outer ionosphere are of importance. In this study, we concentrate on the analysis of an inclined fast shock including upstream and downstream pressure anisotropy and apply it to conditions at the Earth's bow shock. It is the main goal of this work to perform a parameter study of the magnetic field strength and plasma parameters downstream of an inclined fast shock as functions of upstream parameters and downstream pressure anisotropy. For closing the set of equations we use two threshold conditions of plasma instabilities as additional equations to bound the range of the pressure anisotropy, i.e., the criterion of the fire-hose instability and the criterion of the mirror instability. We found that the pressure anisotropy in the solar wind has a small influence on the changes of the relevant physical quantities across the shock wave. We further show that the variations of the plasma and field parameters are strongly influenced by the upstream Alfven Mach number and the angle between the normal vector of the discontinuity and the upstream magnetic field.
[Show abstract][Hide abstract] ABSTRACT: During periods of southward interplanetary field and basically constant dynamic pressure, the magnetopause can move earthward due to the so-called phenomenon of magnetopause erosion. In this study, we present several erosion events monitored at geostationary orbit by the GOES spacecrafts underlying WIND measurements in the solar wind. We selected a number of events using 4 years of WIND observations (1996-1999). Specific selection criteria are based on obtaining a progressively decreasing IMF Bz negative, to have various levels of erosion, with and without dynamic pressure changes and of different durations in time. To figure out the erosion effect on geostationary orbit, we have to compare the measured depression in the geostationary magnetic field strength with the magnetic field strength on the well known May 11, 1999, the day the solar wind almost disappeared.