V. G. Merkin

Development of large‐scale Birkeland currents determined from the Active Magnetosphere and Planetary Electrodynamics Response Experiment

The Active Magnetosphere and Planetary Electrodynamics Response Experiment uses magnetic field data from the Iridium constellation to derive the global Birkeland current distribution every 10 min. We examine cases in which the interplanetary magnetic field (IMF) rotated from northward to southward resulting in onsets of the Birkeland currents. Dayside Region 1/2 currents, totaling ~25% of the final current, appear within 20 min of the IMF southward turning and remain steady. Onset of nightside currents occurs 40 to 70 min after the dayside currents appear. Thereafter, the currents intensify at dawn, dusk, and on the dayside, yielding a fully formed Region 1/2 system ~30 min after the nightside onset. The results imply that the dayside Birkeland currents are driven by magnetopause reconnection, and the remainder of the system forms as magnetospheric return flows start and progress sunward, ultimately closing the Dungey convection cycle.

Spontaneous formation of dipolarization fronts and reconnection onset in the magnetotail

We present full-particle simulations of 2-D magnetotail current sheet equilibria with open boundaries and zero driving. The simulations show that spontaneous formation of dipolarization fronts and subsequent formation of magnetic islands are possible in equilibria with an accumulation of magnetic flux at the tailward end of a sufficiently thin current sheet. These results confirm recent findings in the linear stability of the ion tearing mode, including the predicted dependence of the tail current sheet stability on the amount of accumulated magnetic flux expressed in terms of the specific destabilization parameter. The initial phase of reconnection onset associated with the front formation represents a process of slippage of magnetic field lines with frozen-in electrons relative to the ion plasma species. This non-MHD process characterized by different motions of ion and electron species generates a substantial charge separation electric field normal to the front.

Magnetic reconnection, buoyancy, and flapping motions in magnetotail explosions

A key process in the interaction of magnetospheres with the solar wind is the explosive release of energy stored in the magnetotail. Based on observational evidence, magnetic reconnection is widely believed to be responsible. However, the very possibility of spontaneous reconnection in collisionless magnetotail plasmas has been questioned in kinetic theory for more than three decades. In addition, in situ observations by multispacecraft missions (e.g., THEMIS) reveal the development of buoyancy and flapping motions coexisting with reconnection. Never before have kinetic simulations reproduced all three primary modes in realistic 2-D configurations with a finite normal magnetic field. Moreover, 3-D simulations with closed boundaries suggest that the tail activity is dominated by buoyancy-driven instabilities, whereas reconnection is a secondary effect strongly localized in the dawn-dusk direction. In this paper, we use massively parallel 3-D fully kinetic simulations with open boundaries to show that sufficiently far from the planet explosive processes in the tail are dominated by reconnection motions. These motions occur in the form of spontaneously generated dipolarization fronts accompanied by changes in magnetic topology which extend in the dawn-dusk direction over the size of the simulation box, suggesting that reconnection onset causes a macroscale reconfiguration of the real magnetotail. In our simulations, buoyancy and flapping motions significantly disturb the primary dipolarization front but neither destroy it nor change the near 2-D picture of the front evolution critically. Consistent with recent multiprobe observations, dipolarization fronts are also found to be the main regions of energy conversion in the magnetotail.

Effects of nightside O+ outflow on magnetospheric dynamics: Results of multifluid MHD modeling

[1] Through spacecraft observations and numerical modeling, it is becoming increasingly well established that ionospheric oxygen is present in the magnetosphere in amounts that can have an effect on magnetospheric dynamics, affecting pressure balance, currents, convection flows, and cross polar cap potential (CPCP). However, an understanding of the various processes through which oxygen may bring about changes in magnetospheric structure and dynamics is still lacking. In this paper, we focus on the role that ionospheric oxygen outflow from the nightside auroral zone plays in determining plasma sheet density, size, and pressure and relate the changes in plasma sheet parameters to changes in CPCP, tail geometry, and magnetospheric convection. We use the Multifluid Lyon-Fedder-Mobarry model as a virtual laboratory to analyze the effects of ionospheric oxygen on magnetospheric parameters. Our simulations use idealized solar wind conditions, constant and uniform ionospheric conductance, and constant nightside oxygen outflow in order to isolate the effects of a change in outflow flux. We concentrate the ionospheric outflow in the nightside auroral region, so as to avoid the possibility of directly loading the distant tail X line with oxygen and hence to avoid a direct modification of the reconnection rate in order to focus on effects within the magnetosphere. We show that the presence of O + outflowing from an isolated patch in the nightside auroral zone increases plasma sheet density and thermal pressure, slows convection, decreases the polar cap potential, and increases the length and width of the nightside magnetosphere. We determine that ionospheric O + mass loading can play a significant role but that strong effects on magnetospheric dynamics require a nightside flux that significantly exceeds statistical observed levels. A more realistic outflow would include both dayside and nightside sources whose total combined flux would be similar to the total fluxes used here.

Rapid acceleration of protons upstream of earthward propagating dipolarization fronts

[1] Transport and acceleration of ions in the magnetotail largely occurs in the form of discrete impulsive events associated with a steep increase of the tail magnetic field normal to the neutral plane (Bz), which are referred to as dipolarization fronts. The goal of this paper is to investigate how protons initially located upstream of earthward moving fronts are accelerated at their encounter. According to our analytical analysis and simplified two-dimensional test-particle simulations of equatorially mirroring particles, there are two regimes of proton acceleration: trapping and quasi-trapping, which are realized depending on whether the front is preceded by a negative depletion in Bz. We then use three-dimensional test-particle simulations to investigate how these acceleration processes operate in a realistic magnetotail geometry. For this purpose we construct an analytical model of the front which is superimposed onto the ambient field of the magnetotail. According to our numerical simulations, both trapping and quasi-trapping can produce rapid acceleration of protons by more than an order of magnitude. In the case of trapping, the acceleration levels depend on the amount of time particles stay in phase with the front which is controlled by the magnetic field curvature ahead of the front and the front width. Quasi-trapping does not cause particle scattering out of the equatorial plane. Energization levels in this case are limited by the number of encounters particles have with the front before they get magnetized behind it.

High-resolution global magnetohydrodynamic simulation of bursty bulk flows

A high-resolution global magnetohydrodynamic simulation is conducted with the Lyon-Fedder-Mobarry (LFM) model for idealized solar wind conditions. Within the simulation results high-speed flows are seen throughout the magnetotail when the interplanetary magnetic field (IMF) is southward. Case study analysis of these flows shows that they have an enhancement in BZ and a decrease in density preceding a peak in the flow velocity. A careful examination of the structure within the magnetotail shows that these features are driven by bursts of magnetic reconnection. In addition to the case study, a superposed epoch analysis of flows occurring during a 90 min interval of southward IMF yields statistical properties that are in qualitative agreement with observational analysis of bursty bulk flows (BBFs). For the idealized simulation conditions, the most significant differences with the observational results are a broader velocity profile in time, which becomes narrower away from the center of the current sheet, and a larger density drop after flow passage. The peak BZ amplitude is larger than in observations and precedes the peak in the flow velocity. We conclude that the LFM simulations are reproducing the statistical features of BBFs and that they are driven by spatially and temporally localized reconnection events within the simulation domain.

Generalized magnetotail equilibria: Effects of the dipole field, thin current sheets, and magnetic flux accumulation

Generalizations of the class of quasi-1-D solutions of the 2-D Grad-Shafranov equation, first considered by Schindler in 1972, are investigated. It is shown that the effect of the dipole field, treated as a perturbation, can be included into the original 1972 class solution by modification of the boundary conditions. Some of the solutions imply the formation of singularly thin current sheets. Equilibrium solutions for such sheets resolving their singular current structure on the scales comparable to the thermal ion gyroradius can be obtained assuming anisotropic and nongyrotropic plasma distributions. It is shown that one class of such equilibria with the dipole-like boundary perturbation describes bifurcation of the near-Earth current sheet. Another class of weakly anisotropic equilibria with thin current sheets embedded into a thicker plasma sheet helps explain the formation of thin current sheets in a relatively distant tail, where such sheets can provide ion Landau dissipation for spontaneous magnetic reconnection. The free energy for spontaneous reconnection can be provided due to accumulation of the magnetic flux at the tailward end of the closed field line region. The corresponding hump in the normal magnetic field profile Bz(x,z = 0) creates a nonzero gradient along the tail. The resulting gradient of the equatorial magnetic field pressure is shown to be balanced by the pressure gradient and the magnetic tension force due to the higher-order correction of the latter in the asymptotic expansion of the tail equilibrium in the ratio of the characteristic tail current sheet variations across and along the tail.

Does the polar cap area saturate

We address the question of how the polar cap area (A PC ) and the open magnetic flux in the Earths ionospheric polar caps depend on the strength of the interplanetary magnetic field (IMF) under conditions of steady driving. We use the Lyon-Fedder-Mobarry (LFM) global magnetohydrodynamic (MHD) model to analyze the relationship between these quantities and compare their behavior to that of the transpolar potential (Φ PC ). In a series of idealized simulation runs we find that in the LFM model A PC saturates faster than Φ PC as the IMF strength increases. The ionospheric conductance and the solar wind ram pressure have similar moderate effects on the saturated A PC , while their influence on Φ PC is very different. It appears that A PC saturates as a result of bulging of the geomagnetic lobes toward the sun, whereby the inner dipole magnetic field is shielded from further reconnection. The process is accompanied by dramatic changes in the global configuration of the magnetic field.

Comparison of predictive estimates of high‐latitude electrodynamics with observations of global‐scale Birkeland currents

Two of the geomagnetic storms for the Space Weather Prediction Center (SWPC) Geospace Environment Modeling (GEM) challenge [cf. Pulkkinen et al., 2013] occurred after data were first acquired by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). We compare Birkeland currents from AMPERE with predictions from four models for the 4-5 April 2010 and 5-6 August 2011 storms. The four models are: the Weimer [2005b] field-aligned current statistical model; the Lyon-Fedder-Mobarry magnetohydrodynamic (MHD) simulation; the Open Global Geospace Circulation Model MHD simulation; and the Space Weather Modeling Framework MHD simulation. The MHD simulations were run as described in Pulkkinen et al. [2013] and the results obtained from the Community Coordinated Modeling Center (CCMC). The total radial Birkeland current, ITotal, and the distribution of radial current density, Jr, for all models are compared with AMPERE results. While the total currents are well correlated, the quantitative agreement varies considerably. The Jr distributions reveal discrepancies between the models and observations related to the latitude distribution, morphologies, and lack of nightside current systems in the models. The results motivate enhancing the simulations first by increasing the simulation resolution, and then by examining the relative merits of implementing more sophisticated ionospheric conductance models, including ionospheric outflows or other omitted physical processes. Some aspects of the system, including substorm timing and location, may remain challenging to simulate, implying a continuing need for real-time specification.

Electrodynamic context of magnetopause dynamics observed by magnetospheric multiscale

Magnetopause observations by Magnetospheric Multiscale (MMS) and Birkeland currents observed by the Active Magnetosphere and Planetary Electrodynamics Response Experiment are used to relate magnetopause encounters to ionospheric electrodynamics. MMS magnetopause crossings on 15 August and 19 September 2015 occurred earthward of expectations due to solar wind ram pressure alone and coincided with equatorward expansion of the Birkeland currents. Magnetopause erosion, consistent with expansion of the polar cap, contributed to the magnetopause crossings. The ionospheric projections of MMS during the events and at times of the magnetopause crossings indicate that MMS observations are related to the main path of flux transport in one case but not in a second. The analysis provides a way to routinely relate in situ observations to the context of in situ convection and flux transport.