Yann Kempf

Ion distributions in the Earth's foreshock: Hybrid-Vlasov simulation and THEMIS observations

We present the ion distribution functions in the ion foreshock upstream of the terrestrial bow shock obtained with Vlasiator, a new hybrid-Vlasov simulation geared toward large-scale simulations of the Earths magnetosphere (http://vlasiator.fmi.fi). They are compared with the distribution functions measured by the multispacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. The known types of ion distributions in the foreshock are well reproduced by the hybrid-Vlasov model. We show that Vlasiator reproduces the decrease of the backstreaming beam speed with increasing distance from the foreshock edge, as well as the beam speed increase and density decrease with increasing radial distance from the bow shock, which have been reported before and are visible in the THEMIS data presented here. We also discuss the process by which wave-particle interactions cause intermediate foreshock distributions to lose their gyrotropy. This paper demonstrates the strength of the hybrid-Vlasov approach which lies in producing uniformly sampled ion distribution functions with good resolution in velocity space, at every spatial grid point of the simulation and at any instant. The limitations of the hybrid-Vlasov approach are also discussed.

Wave dispersion in the hybrid-Vlasov model: Verification of Vlasiator

Vlasiator is a new hybrid-Vlasov plasma simulation code aimed at simulating the entire magnetosphere of the Earth. The code treats ions (protons) kinetically through Vlasovs equation in the six-dimensional phase space while electrons are a massless charge-neutralizing fluid [M. Palmroth et al., J. Atmos. Sol.-Terr. Phys. 99, 41 (2013); A. Sandroos et al., Parallel Comput. 39, 306 (2013)]. For first global simulations of the magnetosphere, it is critical to verify and validate the model by established methods. Here, as part of the verification of Vlasiator, we characterize the low-β plasma wave modes described by this model and compare with the solution computed by the Waves in Homogeneous, Anisotropic Multicomponent Plasmas (WHAMP) code [K. Ronnmark, Kiruna Geophysical Institute Reports No. 179, 1982], using dispersion curves and surfaces produced with both programs. The match between the two fundamentally different approaches is excellent in the low-frequency, long wavelength range which is of interest ...

Helical Ion Beams from Fluctuating Shock Structures

Using two separate simulation methods, kinetic-scale test-particle simulations and hybrid-Vlasov simulations, we have obtained evidence of nongyrotropic, helical beam structures in foreshocks of wavy shock structures. Artificial satellite observations with a simulated electrostatic analyzer modeled after the THEMIS ESA instrument show that in typical 2D cuts of velocity space, these structures are not properly visible. We instead propose a method to visualize the temporal development of the relevant part of phase space in order to obtain signatures of velocity-space spirals.

Particle acceleration and foreshock evolution in heliospheric shocks from self-consistent Monte Carlo simulations

Self-consistent Monte Carlo simulations have been a fruitful approach to model particle acceleration dynamically coupled with the foreshock development in quasi-parallel shocks. There is the global Coronal Shock Acceleration (CSA) Monte Carlo simulation code that is capable of modelling self-consistent shock acceleration from the inner corona to the solar wind. However, in CSA, the resonant interactions of particles with the foreshock Alfvén waves are not modelled using the full resonance condition. The used simplified condition implies that particles of a given energy interact with only one particular spectral component of the wave spectrum. In contrast, the exact (within quasi-linear theory) treatment implies that such particles due to scattering in pitch angle can interact with different wave spectrum components. This changes the modelled particle acceleration efficiency of the shock and the wave spectrum evolution in the foreshock. We have developed a new self-consistent Monte Carlo simulation code, in which we overcome the previous simplification, and applied the two codes to model acceleration of protons in a parallel coronal shock. We also used the new code to simulate proton acceleration in an interplanetary shock. Due to the choice of the plasma and shock parameters, this simulation is applicable to quasi-parallel bow shock of the Earth. Comparison of the results shows that the resonant wave-particle interactions governed by the full resonance condition yield less efficient particle acceleration at the shock and the opposite energy dependence of the proton mean free path in the foreshock than the simplified treatment. Moreover, the Alfvén wave intensity spectrum resulting from the new code exhibits a k−2 dependence at large wavenumbers, characteristic for both the coronal shock and the interplanetary one. This result is in agreement with that of a hybrid-Vlasov simulation (Vlasiator) of the foreshock evolution of the Earth’s bow shock.

Atmospheric Drag, Occultation ‘N’ Ionospheric Scintillation (ADONIS) mission proposal - Alpbach Summer School 2013 Team Orange

The Atmospheric Drag, Occultation ‘N’ Ionospheric Scintillation mission (ADONIS) studies the dynamics of the terrestrial thermosphere and ionosphere in dependency of solar events over a full solar cycle in Low Earth Orbit (LEO). The objectives are to investigate satellite drag with in-situ measurements and the ionospheric electron density profiles with radio occultation and scintillation measurements. A constellation of two satellites provides the possibility to gain near real-time data (NRT) about ionospheric conditions over the Arctic region where current coverage is insufficient. The mission shall also provide global high-resolution data to improve assimilative ionospheric models. The low-cost constellation can be launched using a single Vega rocket and most of the instruments are already space-proven allowing for rapid development and good reliability.From July 16 to 25, 2013, the Alpbach Summer School 2013 was organised by the Austrian Research Promotion Agency (FFG), the European Space Agency (ESA), the International Space Science Institute (ISSI) and the association of Austrian space industries Austrospace in Alpbach, Austria. During the workshop, four teams of 15 students each independently developed four different space mission proposals on the topic of “Space Weather: Science, Missions and Systems”, supported by a team of tutors. The present work is based on the mission proposal that resulted from one of these teams’ efforts.

Hybrid-Vlasov simulations of the Earth's collisionless bow shock and foreshock region

We present magnetospheric simulation results obtained with the hybrid-Vlasov code Vlasiator. In this model, the ion distribution function is propagated in up to three spatial and three velocity dimensions with Vlasovs equation, while electrons are treated as a massless charge-neutralising fluid. Therefore ion kinetic effects are fully included in the description. One major strength of the model is the unprecedented quality of the ion distribution functions due to the uniform sampling and the absence of statistical noise. Another strength lies in the robust finite volume algorithm which yields good numerical stability especially in shock modelling.

Ion distributions in the Earth's foreshock region: Hybrid-Vlasov simulations and spacecraft observations

Summary form only given. We present the ion distribution functions in the terrestrial foreshock, simulated by the hybrid-Vlasov model called Vlasiator and observed by the THEMIS and Cluster spacecraft. In the hybrid-Vlasov description, the ion distribution function is propagated in up to three spatial and three velocity dimensions and electrons are modelled as a massless charge-neutralising fluid. Vlasiator was used to model self-consistently the terrestrial bow shock and foreshock regions in the ecliptic plane (two spatial, three velocity dimensions). The simulations were run for tens of ion gyroperiods over hundreds of ion inertial lengths. Vlasiator provides, for the first time, a large-scale picture of the ion distribution in the foreshock with a quality comparable to or even better than spacecraft data thanks to the uniform velocity space sampling and the absence of statistical noise. This allows us to study the interaction between the backstreaming ions and the solar wind, which can trigger instabilities leading to waves in the foreshock. This poster will feature a simulation snapshot on a large scale including the simulated ion distribution functions at high spatial resolution, as well as comparison to spacecraft data. Beam and ring-beam distributions are present near the foreshock edge. The distributions become progressively more intermediate/cap-shaped with increasing distance from the foreshock edge, including occurrences of multiple-cap distributions. At the same time the drift speed relative to the solar wind decreases deeper in the foreshock. Diffuse distributions are present near the quasi-parallel bow shock.