Andreas Kopp

On the star-magnetosphere interaction of close-in exoplanets

Numerical simulations using a resistive MHD code are performed in order to investigate the interaction of the magnetospheres of hot Jupiters (or close-in extrasolar giant planets) with the central host stars. Because of the sub-Alfvenic nature of the stellar wind outflow at the orbital positions of these close-in exoplanets, no bow shock would form. When the orientation of the stellar coronal magnetic field is favorable to strong coupling with magnetic reconnection, the power (~1027 ergs s-1) generated could reach the level of a typical solar flare. As a particular type of star-planet atmospheric interaction, as investigated by Cuntz, Saar, & Musielak, magnetospheric interaction as studied in this Letter could lead to extensive energy injection into the auroral zones of the exoplanets, producing massive atmospheric escape process as recently detected.

Generalized magnetohydrodynamic equations for partially ionized dusty magnetoplasmas: Derivation and applications

A comprehensive investigation of electromagnetic wave and instability phenomena in partially ionized, magnetized dusty plasmas has been carried out. By employing the multi‐fluid balance equations along with the Maxwell equations, a compact set of coupled field equations for the cases in which the dust grains are either robust (vd=0) or dynamic is derived. These systems of partial differential equations are used to study wave phenomena and resistive tearing mode instabilities analytically as well as by means of numerical simulations. For robust dust grains, it is shown that coupled sound‐Alfven waves can appear even in the absence of ion‐neutral collisions. The unstable tearing modes are coupled to convective drift modes, if the dust number density is inhomogeneous. In the induction equation two new source terms for self‐generation of magnetic fields can be identified. In parameter regimes that are characterized by dynamic dust grains, the low‐frequency phenomena develop on timescales that are governed by ...

Cosmic-ray propagation properties for an origin in Supernova remnants

We have studied the impact of cosmic-ray (CR) acceleration in supernova remnants on the spectra of CR nuclei in the Galaxy using a series expansion of the propagation equation, which allows us to use analytical solutions for part of the problem and to perform an efficient numerical treatment of the remaining equations and thus accurately describes the cosmic-ray propagation on small scales around their sources in three spatial dimensions and time. We found strong variations of the cosmic-ray nuclei flux by typically 20%, with occasional spikes of much higher amplitude, but only minor changes in the spectral distribution. The locally measured spectra of primary cosmic rays fit well into the obtained range of possible spectra. We furthermore showed that the spectra of the secondary element boron show almost no variations, so that the above findings also imply significant fluctuations of the boron-to-carbon ratio. Therefore, the commonly used method of determining CR propagation parameters by fitting secondary-to-primary ratios appears flawed on account of the variations that these ratios would show throughout the Galaxy.

Fluid equations governing the dynamics and energetics of partially ionized dusty magnetoplasmas

A set of nonlinear equations for multi-component partially ionized dusty magnetoplasmas is developed, taking into account the contributions of frictional thermalization, collisional heating in the energy equations on one hand, and those of ionization and recombination in the continuity equations on the other hand. The present nonlinear equations form a tool for numerical simulation studies of various collective effects in the presence of sources and sinks. However, in order to illustrate possible applications of the present system of equations, two examples of thermal condensation instabilities are discussed.

Asymmetric mass loading effect at Titan's ionosphere

Because of the fact that Titan does not possess an intrinsic magnetic field, the corotating plasma in the Saturnian magnetosphere can interact directly with the nitrogen-rich atmosphere of Titan. In this work we perform three-dimensional resistive MHD simulations of the interaction between Titan and the Saturnian magnetosphere. The magnetic field of Saturn is modeled as a dipole field located at the origin of a cylindrical coordinate system. Of particular interest is the mass loading effect in the vicinity of Titan. The pickup of atmospheric ions is strongly influenced by the ionization rate and scale lengths of the system. The fact that the ion gyroradius of the system is of the order of the radius of Titan has important consequences. Ions emerging from Titans ionosphere at the hemisphere facing toward Saturn get lost back into the ionosphere, whereas ions at the opposite hemisphere may contribute to the satellites mass loading by enhanced atmospheric sputtering. This results in an asymmetry of the mass loading patterns between the planet-facing hemisphere and the antiplanet hemisphere. For different mass loading ratios we investigate the effect of this asymmetry. By comparing our results with Voyager 1 measurements we conclude that a mass loading asymmetry could in principle provide an explanation of the observed deflection in the magnetic fields as well as the asymmetry of the plasma flow in the wake region.

A novel code for numerical 3-D MHD studies of CME expansion

Abstract. A recent third-order, essentially non-oscillatory central scheme to advance the equations of single-fluid magnetohydrodynamics (MHD) in time has been implemented into a new numerical code. This code operates on a 3-D Cartesian, non-staggered grid, and is able to handle shock-like gradients without producing spurious oscillations. To demonstrate the suitability of our code for the simulation of coronal mass ejections (CMEs) and similar heliospheric transients, we present selected results from test cases and perform studies of the solar wind expansion during phases of minimum solar activity. We can demonstrate convergence of the system into a stable Parker-like steady state for both hydrodynamic and MHD winds. The model is subsequently applied to expansion studies of CME-like plasma bubbles, and their evolution is monitored until a stationary state similar to the initial one is achieved. In spite of the models (current) simplicity, we can confirm the CMEs nearly self-similar evolution close to the Sun, thus highlighting the importance of detailed modelling especially at small heliospheric radii. Additionally, alternative methods to implement boundary conditions at the coronal base, as well as strategies to ensure a solenoidal magnetic field, are discussed and evaluated.

Modifications of the electrodynamic interaction between Jupiter and Io due to mass loading effects

The electrodynamic interaction between Jupiter and Io is investigated by means of three-dimensional resistive MHD simulations with Jupiter being modeled as an aligned rotator situated in the origin of a cylindrical coordinate system. For numerical reasons, however, only qualitative statements can be made, which do not allow quantitative comparisons with observations. The main topic is the generation of electric currents parallel to the magnetic field. In consideration of the fact that some observational features cannot be qualitatively understood with present theories, the role of the plasma production from los atmosphere as well as by ionization in the plasma torus is investigated. If these two plasma sources are significantly cooler than the plasma environment, they can be shown to cause a torus corotational lag, in consequence of which a field-aligned current system develops. If microinstabilities occur along this system, anomalous resistivity regions may be built up where the electric field attains a component parallel to the magnetic field. Thus ionization within the torus might explain the observed decametric radiation upstream of Ios current position. Furthermore, plasma production from Ios atmosphere shows a strong influence on the MHD modes excited by the Io/torus interaction. The pressure signatures generated by slow mode perturbations disclose the formation of a tail-like structure in the wake behind Io. Thus previous conceptions about a new current systems in Ios wake can be confirmed by self-consistent MHD simulations. Finally, the correlations between this current and gradients in angular velocity and density are studied in detail.

Magnetohydrodynamic Simulations of the Magnetic Interaction of Hot Jupiters with Their Host Stars: A Numerical Experiment

Three-dimensional resistive magnetohydrodynamic simulations of the magnetic interaction of extrasolar planets with their host stars are performed on the basis of a Weber-Davis model of stellar winds. The free parameters of this model are the stellar magnetic field, the temperature of the corona, and the mass flux and have been fitted in order to theoretically explain the observed phase shifts between the so-called hot spots in the stellar chromospheres and the substellar points for the planets HD 179949 b and υ And b. The relative motion between planet and stellar wind causes perturbations of the stellar magnetic field, which propagate along the Alfven characteristics toward the star. In a first step it is investigated whether the planet has to be magnetized in order to perturb the magnetic field. The second step consists of time-dependent simulations of the propagation, where the component of the electric current density parallel to the magnetic field is used to trace the perturbation. The simulations confirm the theoretical model for the explanation of the observed phase shifts for both planets.

POSSIBLE EVIDENCE FOR A FISK-TYPE HELIOSPHERIC MAGNETIC FIELD. I. ANALYZING ULYSSES/KET ELECTRON OBSERVATIONS

The propagation of energetic charged particles in the heliospheric magnetic field is one of the fundamental problems in heliophysics. In particular, the structure of the heliospheric magnetic field remains an unsolved problem and is discussed as a controversial topic. The first successful analytic approach to the structure of the heliospheric magnetic field was the Parker field. However, the measurements of the Ulysses spacecraft at high latitudes revealed the possible need for refinements of the existing magnetic field model during solar minimum. Among other reasons, this led to the development of the Fisk field. This approach is highly debated and could not be ruled out with magnetic field measurements so far. A promising method to trace this magnetic field structure is to model the propagation of electrons in the energy range of a few MeV. Employing three-dimensional and time-dependent simulations of the propagation of energetic electrons, this work shows that the influence of a Fisk-type field on the particle transport in the heliosphere leads to characteristic variations of the electron intensities on the timescale of a solar rotation. For the first time it is shown that the Ulysses count rates of 2.5–7 MeV electrons contain the imprint of a Fisk-type heliospheric magnetic field structure. From a comparison of simulation results and the Ulysses count rates, realistic parameters for the Fisk theory are derived. Furthermore, these parameters are used to investigate the modeled relative amplitudes of protons and electrons, including the effects of drifts.

Pluto's ionospheric models and solar wind interaction

Abstract Chemical model calculations are carried out to describe the ion composition and density profile of Plutos ionosphere. With the adopted surface pressure (≈ 10 microbars) of the N 2 gas and the mixing ratio of CH 4 (≈7.4×10 −3 ), the major ion is found to be H 2 CN + and the peak ion number density is estimated to be about 3×10 3 cm −3 . If Pluto does not have an intrinsic magnetic field, the ionospheric penetration of the interplanetary magnetic field and solar wind plasma would be stopped at an altitude of about 1400 km because of collisional friction of the neutral atmosphere.