M. Palmroth

Supermagnetosonic Jets behind a Collisionless Quasiparallel Shock

The downstream region of a collisionless quasiparallel shock is structured containing bulk flows with high kinetic energy density from a previously unidentified source. We present Cluster multispacecraft measurements of this type of supermagnetosonic jet as well as of a weak secondary shock front within the sheath, that allow us to propose the following generation mechanism for the jets: The local curvature variations inherent to quasiparallel shocks can create fast, deflected jets accompanied by density variations in the downstream region. If the speed of the jet is super(magneto)sonic in the reference frame of the obstacle, a second shock front forms in the sheath closer to the obstacle. Our results can be applied to collisionless quasiparallel shocks in many plasma environments.

Compression of the Earth's magnetotail by interplanetary shocks directly drives transient magnetic flux closure

We use a novel method to evaluate the global opening and closure of magnetic flux in the terrestrial system, and to analyse two interplanetary shock passages that occurred during magnetically quiet periods. We find that, even under these quiet conditions, where the amount of open flux was already low, the compression of the magnetotail by the shocks still created intense but short-lived bursts of flux closure reaching ∼130 kV, comparable to values obtained shortly after a substorm onset, although no expansion phase developed. The results, supported by a global MHD simulation of the space environment, point to a trigger mechanism of flux closure directly driven by the solar wind compression, independent of the usual substorm expansion phase process.

Cusp and magnetopause locations in global MHD simulation

We use the global MHD code Grand Unified Magnetosphere Ionosphere Coupling Simulation (GUMICS-4) to simulate the location and motion of the magnetospheric cusp and the subsolar magnetopause under various interplanetary magnetic field (IMF) directions and solar wind dynamic pressures. We identify the cusp in the simulation from the location of the open/closed field line boundary (OC boundary, equatorward edge of the cusp), determined by direct field line tracing, and several MHD parameters that are expected to react to the entry of the magnetosheath fluid. These parameters include the location of the maximum internal energy density and maximum diamagnetic depression and are collectively called the plasma proxies of the cusp. The simulation results show that with increasingly southward IMF the OC boundary and the plasma proxies move equatorward, as expected. During northward IMF, however, the plasma proxies are located equatorward of the OC boundary and thus on closed field lines. The GUMICS-4 OC boundary location and the statistical plasma cusp location from Polar satellite observations are in good quantitative agreement during southward IMF, but during northward IMF the observed cusp is again several degrees equatorward from the simulated OC boundary. Therefore we conclude that during northward IMF the cusp identification from either the OC boundary or the plasma proxies may become problematic in MHD simulations and discuss the possible physical reasons. The simulation results further indicate that increasing solar wind dynamic pressure shifts the high-altitude cusp slightly equatorward. Furthermore, the increasing pressure also pushes the magnetopause clearly earthward, as expected, while the same field line in the ionosphere shifts only slightly equatorward, implying changes in the field line shape as the pressure changes. In other respects, the simulated magnetopause is in quantitative agreement with the empirical model of Shue et al. [1998].

Location of highhaltitude cusp during steady solar wind conditions

We have investigated the highhaltitude cusp location and its dependence on the interplanetary magnetic field lIMFr Bz, solar wind Ey, and solar wind dynamic pressure during steady solar wind conditions. We have adopted strict event selection criteria, where we have studied only events during stationary IMF Bz, when the cusp latitude is supposed to be stationary. Prior to this study, the statistical studies have shown a great deal of scatter between the cusp latitude and the IMF Bz; however, with the strict event selection criteria, we have been able to reduce the scatter. The strongest source of scatter comes from large and largely fluctuating IMF By. When the events are selected during small IMF By, the scattering between the cusp latitude and the IMF Bz is reduced. When IMF is southward, also the fluctuation of IMF By has strong influence on the cusp latitude, whereas for northward IMF, the IMF By fluctuation has less influence on the cusp latitude. For larger southward IMF the cusp appears further equatorward than with smaller southward IMF. When the scatter is reduced by introducing the IMF By control, the larger northward IMF brings the cusp to lower latitudes than the smaller northward IMF, i.e., the maximum cusp latitude is obtained for small positive IMF Bz. For increasing solar wind Ez the cusp moves equatorward. Furthermore, with increasing solar wind dynamic pressure, the cusp appears at lower latitudes. This behavior depends on the orientation of the IMF Bz, with southward IMF the cusp moves faster equatorward than with northward IMF.

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.

Magnetopause reconnection and energy conversion as influenced by the dipole tilt and the IMF Bx

We study the effect of Earths dipole tilt angle and interplanetary magnetic field (IMF) Bx and By components on the location of reconnection and the energy conversion at the magnetopause. We simulate southward IMF satisfying both inward- and outward-type Parker spiral conditions during three different dipole tilt angles using a global magnetohydrodynamic model GUMICS-4. We find that positive (negative) Bx contributes to the magnetopause reconnection line location by moving northward (southward) and positive (negative) dipole tilt angle by moving it southward (northward). The tilt shifts the dayside load region toward the winter hemisphere and the summer cusp toward the equatorial plane. Magnetic flux hence piles effectively in the summer hemisphere leading to increased magnetopause currents that enhance the Poynting flux through the magnetopause. We find that the intensity of the energy conversion in the generators is strongly affected by the dipole tilt angle, whereas intensity in the load region is mainly affected by IMF Bx.

Assessing the performance of community‐available global MHD models using key system parameters and empirical relationships

Global magnetohydrodynamic (MHD) modeling is a powerful tool in space weather research and predictions. There are several advanced and still developing global MHD (GMHD) models that are publicly available via Community Coordinated Modeling Centers (CCMC) Run on Request system, which allows the users to simulate the magnetospheric response to different solar wind conditions including extraordinary events, like geomagnetic storms. Systematic validation of GMHD models against observations still continues to be a challenge, as well as comparative benchmarking of different models against each other. In this paper we describe and test a new approach in which (i) a set of critical large-scale system parameters is explored/tested, which are produced by (ii) specially designed set of computer runs to simulate realistic statistical distributions of critical solar wind parameters and are compared to (iii) observation-based empirical relationships for these parameters. Being tested in approximately similar conditions (similar inputs, comparable grid resolution, etc.), the four models publicly available at the CCMC predict rather well the absolute values and variations of those key parameters (magnetospheric size, magnetic field, and pressure) which are directly related to the large-scale magnetospheric equilibrium in the outer magnetosphere, for which the MHD is supposed to be a valid approach. At the same time, the models have systematic differences in other parameters, being especially different in predicting the global convection rate, total field-aligned current, and magnetic flux loading into the magnetotail after the north-south interplanetary magnetic field turning. According to validation results, none of the models emerges as an absolute leader. The new approach suggested for the evaluation of the models performance against reality may be used by model users while planning their investigations, as well as by model developers and those interesting to quantitatively evaluate progress in magnetospheric modeling.

Reconnection rates and X line motion at the magnetopause: Global 2D‐3V hybrid‐Vlasov simulation results

We present results from a first study of the local reconnection rate and reconnection site motion in a 2D-3V global magnetospheric self-consistent hybrid-Vlasov simulation with due southward interplanetary magnetic field. We observe magnetic reconnection at multiple locations at the dayside magnetopause and the existence of magnetic islands, which are the 2-D representations of flux transfer events. The reconnection locations (the X lines) propagate over significant distances along the magnetopause, and reconnection does not reach a steady state. We calculate the reconnection rate at the location of the X lines and find a good correlation with an analytical model of local 2-D asymmetric reconnection. We find that despite the solar wind conditions being constant, the reconnection rate and location of the X lines are highly variable. These variations are caused by magnetosheath fluctuations, the effects of neighboring X lines, and the motion of passing magnetic islands.

Mirror modes in the Earth's magnetosheath: Results from a global hybrid‐Vlasov simulation

We investigate mirror mode structures in the Earths magnetosheath using our global kinetic Vlasiator simulation, which models ion behavior through their distribution function and treats electrons as a charge-neutralizing fluid. We follow the evolution of waves as they advect along velocity streamlines through the magnetosheath. We find that mirror mode waves are observed preferentially in the quasi-perpendicular magnetosheath along velocity streamlines that enter the sheath in the vicinity of the foreshock ULF wave boundary where there are enough initial perturbations in the plasma for the mirror modes to grow, and the plasma properties fulfill the mirror instability condition better than elsewhere in the magnetosheath. We test selection criteria defined by previous studies and show that the spatial extent over which mirror modes occur ranges from much of the magnetosheath on the quasi-perpendicular side of the subsolar point to very small isolated regions depending on the criteria.

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 ...