A possible source mechanism for magnetotail current sheet flapping

The origin of the flapping motions of the current sheet in the Earth's magnetotail is one of the most interesting questions of magnetospheric dynamics yet to be solved. We have used a polar plane simulation from the global hybrid-Vlasov model Vlasiator to study the characteristics and source of current sheet flapping in the center of the magnetotail. The characteristics of the simulated signatures agree with observations reported in the literature. The flapping is initiated by a hemispherically asymmetric magnetopause perturbation, created by subsolar magnetopause reconnection, that is capable of displacing the tail current sheet from its nominal position. The current sheet displacement propagates downtail at the same pace as the driving magnetopause perturbation. The initial current sheet displacement launches a standing magnetosonic wave within the tail resonance cavity. The travel time of the wave within the local cavity determines the period of the subsequent flapping signatures. Compression of the tail lobes due to added flux affects the cross-sectional width of the resonance cavity as well as the magnetosonic speed within the cavity. These in turn modify the wave travel time and flapping period. The compression of the resonance cavity may also provide additional energy to the standing wave, which may lead to strengthening of the flapping signature. It may be possible that the suggested mechanism could act as a source of kink-like waves that have been observed to be emitted from the center of the tail and to propagate toward the dawn and dusk flanks.Peer reviewe

Tail reconnection in the global magnetospheric context : Vlasiator first results

The key dynamics of the magnetotail have been researched for decades and have been associated with either three-dimensional (3-D) plasma instabilities and/or magnetic reconnection. We apply a global hybrid-Vlasov code, Vlasiator, to simulate reconnection self-consistently in the ion kinetic scales in the noon-midnight meridional plane, including both dayside and nightside reconnection regions within the same simulation box. Our simulation represents a numerical experiment, which turns off the 3-D instabilities but models ion-scale reconnection physically accurately in 2-D. We demonstrate that many known tail dynamics are present in the simulation without a full description of 3-D instabilities or without the detailed description of the electrons. While multiple reconnection sites can coexist in the plasma sheet, one reconnection point can start a global reconfiguration process, in which magnetic field lines become detached and a plasmoid is released. As the simulation run features temporally steady solar wind input, this global reconfiguration is not associated with sudden changes in the solar wind. Further, we show that lobe density variations originating from dayside reconnection may play an important role in stabilising tail reconnection.Peer reviewe

Contribution of magnetotail reconnection to the cross-polar cap electric potential drop

Since the work of Dungey (1961), the global circulation pattern with two (dayside and nightside) reconnection regions has become a classic concept. However, the contributions of dayside and nightside sources to the cross-polar cap potential (PCP) are not fully understood, particularly, the relative role and specifics of the nightside source are poorly investigated both in quantitative and qualitative terms. To fill this gap, we address the contributions of dayside and nightside sources to the PCP by conducting global MHD simulations with both idealized solar wind input and an observed event input. The dayside source was parameterized by solar wind–based “dayside merging potential” Φd = LeffVBt sin4(θ/2), whereas to characterize the nightside source we integrated across the tail the dawn-dusk electric field in the plasma sheet (to obtain the “cross-tail potential” Φn). For the idealized run we performed simulations using four MHD codes available at the Community Coordinated Modeling Center to show that contribution of the nightside source is a code-independent feature (although there are many differences in the outputs provided by different codes). Particularly, we show that adding a nightside source to the linear fit function for the ionospheric potential (i.e., using the fit function Φfit = KdΦd + KnΦn + Φ0) considerably improves the fitting results both in the idealized events as well as in the simulation of an observed event. According to these simulations the nightside source contribution to the PCP has a fast response time (<5 min) and a modest efficiency (potential transmission factor from tail to the ionosphere is small, Kn < 0.2), which is closely linked to the primarily inductive character of strong electric field generated in the plasma sheet. The latter time intervals are marked by strongly enhanced nightside (lobe) reconnection and can be associated with substorm expansion phases. This association is further strengthened by the simulated patterns of precipitation, the R1-type field-aligned substorm current wedge currents and Hall electrojet currents, which are consistent with the known substorm signatures

Propagation of a shock-related disturbance in the Earth's magnetosphere

The Grand Unified Magnetosphere-Ionosphere Coupling Simulation, version 4, magnetohydrodynamic simulation of the interplanetary shock event on 9 November 2002 is used to determine the shock-associated disturbance propagation characteristics inside the Earth's magnetosphere. Interaction of an interplanetary fast forward shock with the magnetopause caused a shock-related disturbance inside the magnetosphere that propagated at a speed significantly higher than that in the solar wind or magnetosheath. The propagation direction of the disturbance was calculated from the Rankine-Hugoniot conditions, velocity and magnetic coplanarity, and minimum variance analysis and is shown to vary in different regions of the magnetosphere. Furthermore, the impulse disturbance wave mode changes as the plasma and field conditions change inside the magnetosphere. These results bring important new information about the propagation processes that is not directly obtainable from point measurements made by (even several) spacecraft. On the other hand, comparison of ionospheric observations from the IMAGE magnetometer chain with geosynchronous data allow us to also interpret the double step structure observed at dayside geosynchronous orbit, which is below the simulation resolution. This combination provides us with quite a complete view on shock propagation inside the magnetosphere.Peer reviewe

On the characterization of magnetic reconnection in global MHD simulations

The conventional definition of reconnection rate as the electric field parallel to an x-line is problematic in global MHD simulations for several reasons: the x-line itself may be hard to find in a non-trivial geometry such as at the magnetopause, and the lack of realistic resistivity modelling leaves us without reliable non-convective electric field. In this article we describe reconnection characterization methods that avoid those problems and are practical to apply in global MHD simulations. We propose that the reconnection separator line can be identified as the region where magnetic field lines of different topological properties meet, rather than by local considerations. The global convection associated with reconnection is then quantified by calculating the transfer of mass, energy or magnetic field across the boundary of closed and open field line regions. The extent of the diffusion region is determined from the destruction of electromagnetic energy, given by the divergence of the Poynting vector. Integrals of this energy conversion provide a way to estimate the total reconnection efficiency

The magnetotail reconnection region in a global MHD simulation

International audienceThis work investigates the nature and the role of magnetic reconnection in a global magnetohydrodynamic simulation of the magnetosphere. We use the Gumics-4 simulation to study reconnection that occurs in the near-Earth region of the current sheet in the magnetotail. We locate the current sheet surface and the magnetic x-line that appears when reconnection starts. We illustrate the difference between quiet and active states of the reconnection region: variations in such quantities as the current sheet thickness, plasma flow velocities, and Poynting vector divergence are strong. A characteristic feature is strong asymmetry caused by non-perpendicular inflows. We determine the reconnection efficiency by the net rate of Poynting flux into the reconnection region. The reconnection efficiency in the simulation is directly proportional to the energy flux into the magnetosphere through the magnetopause: about half of all energy flowing through the magnetosphere is converted from an electromagnetic into a mechanical form in the reconnection region. Thus, the tail reconnection that is central to the magnetospheric circulation is directly driven; the tail does not exhibit a cycle of storage and rapid release of magnetic energy. We find similar behaviour of the tail in both synthetic and real event runs

Hybrid-Vlasov modeling of three-dimensional dayside magnetopause reconnection

Dayside magnetic reconnection at the magnetopause, which is a major driver of space weather, is studied for the first time in a three-dimensional (3D) realistic setup using a hybrid-Vlasov kinetic model. A noon-midnight meridional plane simulation is extended in the dawn-dusk direction to cover 7 Earth radii. The southward interplanetary magnetic field causes magnetic reconnection to occur at the subsolar magnetopause. Perturbations arising from kinetic instabilities in the magnetosheath appear to modulate the reconnection. Its characteristics are consistent with multiple, bursty, and patchy magnetopause reconnection. It is shown that the kinetic behavior of the plasma, as simulated by the model, has consequences on the applicability of methods such as the four-field junction to identify and analyze magnetic reconnection in 3D kinetic simulations.Peer reviewe

Energy conversion at the Earth's magnetopause using single and multispacecraft methods

We present a small statistical data set, where we investigate energy conversion at the magnetopause using Cluster measurements of magnetopause crossings. The Cluster observations of magnetic field, plasma velocity, current density and magnetopause orientation are needed to infer the energy conversion at the magnetopause. These parameters can be inferred either from accurate multispacecraft methods, or by using single-spacecraft methods. Our final aim is a large statistical study, for which only single-spacecraft methods can be applied. The Cluster mission provides an opportunity to examine and validate single-spacecraft methods against the multispacecraft methods. For single-spacecraft methods, we use the Generic Residue Analysis (GRA) and a standard one-dimensional current density method using magnetic field measurements. For multispacecraft methods, we use triangulation (Constant Velocity Approach - CVA) and the curlometer technique. We find that in some cases the single-spacecraft methods yield a different sign for the energy conversion than compared to the multispacecraft methods. These sign ambiguities arise from the orientation of the magnetopause, choosing the interval to be analyzed, large normal current and time offset of the current density inferred from the two methods. By using the Finnish Meteorological Institute global MHD simulation GUMICS-4, we are able to determine which sign is likely to be correct, introducing an opportunity to correct the ambiguous energy conversion values. After correcting the few ambiguous cases, we find that the energy conversion estimated from single-spacecraft methods is generally lower by 70% compared to the multispacecraft methods.Peer reviewe