Daniel Langmayr

The solution of the Rankine–Hugoniot equations for fast shocks in an anisotropic kappa distributed medium

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 Alfven Mach numbers. We found that low values of the parameter kappa reduce the pressure anisotropy downstream of the shock.

Propagation of nonlinear slow waves produced by pressure pulses along the Io flux tube

Abstract A pressure enhancement in the vicinity of Io can be created in the course of the torus plasma flow around Io due to mass loading or it can be produced by volcanic outbursts on Io. For a given magnetic flux tube crossed by Io, a pressure pulse generates two slow magnetosonic waves propagating along the tube to the southern and northern ionosphere of Jupiter. These slow waves evolve rather quickly into shocks due to a steepening mechanism with accelerated plasma flow behind the shock front. This plasma flow streaming along the Io flux tube generates a field aligned potential difference, which can reach values of 1 kV for sufficiently strong pressure pulses. Therefore, this slow mode scenario seems to contribute to the Io controlled aurora as well as to the Io controlled Jovian decameter radiation (DAM) together with the generally accepted Alfven wings model.

Dayside magnetopause erosion on geostationary orbit using WIND and GOES data (1996-1999)

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.

Variations of magnetic field and plasma parameters in the magnetosheath related to reconnection pulses

Abstract The interplanetary magnetic field is enhanced in a thin layer near the magnetopause which is called the magnetic barrier or plasma depletion layer. The magnetic energy stored in the magnetic barrier can be released during the process of magnetic field reconnection. Using ideal magnetohydrodynamics and assuming a sudden decrease of the magnetic field near the magnetopause due to the reconnection pulse, we analyze the model variations of the plasma parameters and the magnetic field at the magnetosheath. For a given reconnection rate and calculated parameters of the magnetic barrier, we derive the duration of a reconnection pulse as a function of the solar wind parameters.

Analysis of a pressure disturbance in a homogeneous magnetic field

Abstract It is well known that in contrast to the Alfven wave, which is propagating strictly along the direction of the magnetic field, a slow mode wave shows a deviation from the ambient magnetic field. This deviation is determined by the dispersion equation for the slow mode wave. With the help of this dispersion equation we present a theoretical study of the spatial and temporal evolution of an initial pressure disturbance in a homogeneous and constant background magnetic field. The main factor determining the amount of the deviation is the so-called plasma beta, i.e., the ratio of magnetic to thermal energy, which is investigated quantitatively. We obtain that for a low beta plasma, the disturbance propagates more or less strictly along the magnetic field. However, for increasing beta the disturbances across the magnetic field gets stronger. These results can be applied to magnetospheric phenomena, where slow shocks may play a role as a kind of energy carrier as in the case of the Io–Jupiter interaction or magnetic field line reconnection.

Analysis of an inclined fast shock including pressure anisotropy

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 Earths 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.

Electric potential difference due to MHD slow shocks propagating along the Io flux tube

Many ionospheric and magnetospheric phenomena, e.g., the northern lights, require the existence of accelerated particle populations. One possible explanation for the development of such particles is an electric field directed along magnetic field lines. The main aim of this paper is to investigate the physical mechanisms leading to an electric potential difference along the Jo flux tube with special emphasis on the processes acting in the outer ionosphere of Jupiter. As a starting point, we assume a pressure perturbation at the position of Ιo and follow the evolution of this pressure perturbation from To towards Jupiter. Initially, the pressure pulse produces two slow mode waves propagating along the Ιo flux tube. These slow mode waves are converted into slow shocks traveling towards Jupiter, and are accompanied by a supersonic flow behind the shock front. The crucial point is now that due to the propagation into a more narrow flux tube, the flow velocity behind the shock increases, in particular fast near the surface of Jupiter. Such a strong plasma flow generates an electric potential difference along the magnetic field. We estimate this potential difference using well-known techniques of kinetic theory. It turns out that the strength of the potential drop is directly proportional to the flow energy of ions. Thus, the very heavy ion populations in the Ιo torus plasma provide an appropriate environment in order to generate an electric potential difference of the order of 1 kV. Therefore, the pressure pulse mechanism can contribute to the explanation of aurora and planetary radio emissions together with the generally accepted Alfven wings model.