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This paper describes properties and behavior of magnetic reconnection and flux transfer events (FTEs) on the dayside magnetopause using the global hybrid‐Vlasov code Vlasiator. We investigate two simulation runs with and without a sunward (positive) Bx component of the interplanetary magnetic field (IMF) when the IMF is southward. The runs are two‐dimensional in real space in the noon‐midnight meridional (polar) plane and three‐dimensional in velocity space. Solar wind input parameters are identical in the two simulations with the exception that the IMF is purely southward in one but tilted 45° toward the Sun in the other. In the purely southward case (i.e., without Bx) the magnitude of the magnetosheath magnetic field component tangential to the magnetopause is larger than in the run with a sunward tilt. This is because the shock normal is perpendicular to the IMF at the equatorial plane, whereas in the other run the shock configuration is oblique and a smaller fraction of the total IMF strength is compressed at the shock crossing. Hence, the measured average and maximum reconnection rate are larger in the purely southward run. The run with tilted IMF also exhibits a north‐south asymmetry in the tangential magnetic field caused by the different angle between the IMF and the bow shock normal north and south of the equator. Greater north‐south asymmetries are seen in the FTE occurrence rate, size, and velocity as well; FTEs moving toward the Southern Hemisphere are larger in size and observed less frequently than FTEs in the Northern Hemisphere. Key Points: Sunward IMF tilt results in a smaller tangential field at the magnetopause, slowing dayside reconnectionSmaller tangential field results from a decrease in the IMF component that is shocked up at the bow shockA positive IMF Bx component introduces north‐south asymmetries, where fewer (but larger) FTEs appear on the Southern Hemisphere [ABSTRACT FROM AUTHOR]

In this study we examine the recovery of relativistic radiation belt electrons on 15-16 November 2014, after a previous reduction in the electron flux resulting from the passage of a corotating interaction region (CIR). Following the CIR, there was a period of high-speed streams characterized by large, nonlinear fluctuations in the interplanetary magnetic field (IMF) components. However, the outer radiation belt electron flux remained at a low level for several days before it increased in two major steps. The first increase is associated with the IMF background field turning from slightly northward on average to slightly southward on average. The second major increase is associated with an increase in the solar wind velocity during a period of southward average IMF background field. We present evidence that when the IMF B z is negative on average, the whistler mode chorus wave power is enhanced in the outer radiation belt, and the amplification of magnetic integrated power spectral density in the ULF frequency range, in the nightside magnetosphere, is more efficient as compared to cases in which the mean IMF B z is positive. Preliminary analysis of the time evolution of phase space density radial profiles did not provide conclusive evidence on which electron acceleration mechanism is the dominant. We argue that the acceleration of radiation belt electrons requires (i) a seed population of keV electrons injected into the inner magnetosphere by substorms and both (ii) enhanced whistler mode chorus waves activity as well as (iii) large-amplitude MHD waves. [ABSTRACT FROM AUTHOR]

Previous observations have shown a ∼10-15 min time delay in the ionospheric response to solar wind directional discontinuities marked by either southward or northward interplanetary magnetic field (IMF) turnings. We have studied one southward IMF turning observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) and GOES in the dayside magnetosphere. Using a global MHD model, we have reproduced the magnetopause motion in this event. We find that the observed delay in the ground response can be completely explained by deceleration of the directional discontinuity in the subsolar magnetosheath. We show that the speed of the discontinuity significantly decreases in the vicinity of the magnetopause where the magnetic barrier formed during the previous northward IMF interval. The southward turning can reach the magnetopause only after complete disruption of the magnetic barrier. The disruption or dissipation occurs via magnetosheath reconnection, as confirmed by high-speed jets in the magnetosheath. The magnetopause moves sunward as the directional discontinuity transits the magnetosheath. This sunward motion is followed by the earthward motion when the discontinuity strikes the magnetopause and magnetopause reconnection begins. [ABSTRACT FROM AUTHOR]

The plasma of the solar wind incident upon the Earth's magnetosphere can produce several types of geoeffective events. Among them, an important phenomenon consists of the interrelation of the magnetospheric-ionospheric current systems and the chargedparticle population of the Earth's Van Allen radiation belts. Ultra-low-frequency (ULF) waves resonantly interacting with such particles have been claimed to play a major role in the energetic particle flux changes, particularly at the outer radiation belt, which is mainly composed of electrons at relativistic energies. In this article, we use global magnetohydrodynamic simulations along with in situ and ground-based observations to evaluate the ability of two different solar wind transient (SWT) events to generate ULF (few to tens of mHz) waves in the equatorial region of the inner magnetosphere. Magnetic field and plasma data from the Advanced Composition Explorer (ACE) satellite were used to characterize these two SWT events as being a sector boundary crossing (SBC) on 24 September 2013, and an interplanetary coronal mass ejection (ICME) in conjunction with a shock on 2 October 2013. Associated with these events, the twin Van Allen Probes measured a depletion of the outer belt relativistic electron flux concurrent with magnetic and electric field power spectra consistent with ULF waves. Two ground-based observatories apart in 90° longitude also showed evidence of ULF-wave activity for the two SWT events. Magnetohydrodynamic (MHD) simulation results show that the ULF-like oscillations in the modeled electric and magnetic fields observed during both events are a result from the SWT coupling to the magnetosphere. The analysis of theMHD simulation results together with the observations leads to the conclusion that the two SWT structures analyzed in this article can be geoeffective on different levels, with each one leading to distinct ring current intensities, but both SWTs are related to the same disturbance in the outer radiation belt, i.e. a dropout in the relativistic electron fluxes. Therefore, minor disturbances in the solar wind parameters, such as those related to an SBC, may initiate physical processes that are able to be geoeffective for the outer radiation belt. [ABSTRACT FROM AUTHOR]

We use the Fokker-Planck-Kolmogorov equation to study the statistical aspects of stochastic dynamics of the radiation belt (RB) protons driven by nonlinear electromagnetic ion-cyclotron (EMIC) wave packets. We obtain the spectra of keV protons scattered by these waves that show steeping near the gyroresonance, the signature of resonant wave-particle interaction that cannot be described by a simple power law. The most likely mechanism for proton precipitation events in RBs is shown to be nonlinear wave-particle interaction, namely, the scattering of RB protons into the loss cone by EMIC waves. [ABSTRACT FROM AUTHOR]