The Substorm Current Wedge: Further Insights from MHD Simulations

Using a recent magnetohydrodynamic simulation of magnetotail dynamics, we further investigate the buildup and evolution of the substorm current wedge (SCW), resulting from flow bursts generated by near-tail reconnection. Each flow burst generates an individual current wedge, which includes the reduction of cross-tail current and the diversion to region 1 (R1)-type field-aligned currents (earthward on the dawn and tailward on the duskside), connecting the tail with the ionosphere. Multiple flow bursts generate initially multiple SCW patterns, which at later times combine to a wider single SCW pattern. The standard SCWmodel is modified by the addition of several current loops, related to particular magnetic field changes: the increase of Bz in a local equatorial region (dipolarization), the decrease of |Bx| away from the equator (current disruption), and increases in |By| resulting from azimuthally deflected flows. The associated loop currents are found to be of similar magnitude, 0.1-0.3 MA. The combined effect requires the addition of region 2 (R2)-type currents closing in the near tail through dawnward currents but also connecting radially with the R1 currents. The current closure at the inner boundary, taken as a crude proxy of an idealized ionosphere, demonstrates westward currents as postulated in the original SCW picture as well as North-South currents connecting R1- and R2-type currents, which were larger than the westward currents by a factor of almost 2. However, this result should be applied with caution to the ionosphere because of our neglect of finite resistance and Hall effects

New Measure of the Dissipation Region in Collisionless Magnetic Reconnection

A new measure to identify a small-scale dissipation region in collisionless magnetic reconnection is proposed. The energy transfer from the electromagnetic field to plasmas in the electron's rest frame is formulated as a Lorentz-invariant scalar quantity. The measure is tested by two-dimensional particle-in-cell simulations in typical configurations: symmetric and asymmetric reconnection, with and without the guide field. The innermost region surrounding the reconnection site is accurately located in all cases. We further discuss implications for nonideal MHD dissipation

Energetic Electrons in Dipolarization Events: Spatial Properties and Anisotropy

Using the electromagnetic fields of an MHD simulation of magnetotail reconnection, flow bursts, and dipolarization, we further investigate the acceleration of electrons to suprathermal energies. Particular emphasis is on spatial properties and anisotropies as functions of energy and time. The simulation results are compared with Time History of Events and Macroscale Interactions during Substorms observations. The test particle approach successfully reproduces several observed injection features and puts them into a context of spatial maps of the injection region(s): a dominance of perpendicular anisotropies farther down the tail and closer to the equatorial plane, an increasing importance of parallel anisotropy closer to Earth and at higher latitudes, a drop in energy fluxes at energies below approximately 10 keV, coinciding with the plasma density drop, together with increases at higher energy, a triple peak structure of flux increases near 0 deg, 90 deg, and 180 deg, and a tendency of flux increases to extend to higher energy closer to Earth and at lower latitudes. We identified the plasma sheet boundary layers and adjacent lobes as a main source region for both increased and decreased energetic electron fluxes, related to the different effects of adiabatic acceleration at high and low energies. The simulated anisotropies tend to exceed the observed ones, particularly for perpendicular fluxes at high energies. The most plausible reason is that the MHD simulation lacks the effects of anisotropy-driven microinstabilities and waves, which would reduce anisotropies

Don't break a leg: Running birds from quail to ostrich prioritise leg safety and economy in uneven terrain

Cursorial ground birds are paragons of bipedal running that span a 500-fold mass range from quail to ostrich. Here we investigate the task-level control priorities of cursorial birds by analysing how they negotiate single-step obstacles that create a conflict between body stability (attenuating deviations in body motion) and consistent leg force–length dynamics (for economy and leg safety). We also test the hypothesis that control priorities shift between body stability and leg safety with increasing body size, reflecting use of active control to overcome size-related challenges. Weight-support demands lead to a shift towards straighter legs and stiffer steady gait with increasing body size, but it remains unknown whether non-steady locomotor priorities diverge with size. We found that all measured species used a consistent obstacle negotiation strategy, involving unsteady body dynamics to minimise fluctuations in leg posture and loading across multiple steps, not directly prioritising body stability. Peak leg forces remained remarkably consistent across obstacle terrain, within 0.35 body weights of level running for obstacle heights from 0.1 to 0.5 times leg length. All species used similar stance leg actuation patterns, involving asymmetric force–length trajectories and posture-dependent actuation to add or remove energy depending on landing conditions. We present a simple stance leg model that explains key features of avian bipedal locomotion, and suggests economy as a key priority on both level and uneven terrain. We suggest that running ground birds target the closely coupled priorities of economy and leg safety as the direct imperatives of control, with adequate stability achieved through appropriately tuned intrinsic dynamics

The Role of Compressibility in Energy Release by Magnetic Reconnection

Using resistive compressible magnetohydrodynamics, we investigate the energy release and transfer by magnetic reconnection in finite (closed or periodic) systems. The emphasis is on the magnitude of energy released and transferred to plasma heating in configurations that range from highly compressible to incompressible, based on the magnitude of the background beta (ratio of plasma pressure over magnetic pressure) and of a guide field in two-dimensional reconnection. As expected, the system becomes more incompressible, and the role of compressional heating diminishes, with increasing beta or increasing guide field. Nevertheless, compressional heating may dominate over Joule heating for values of the guide field of 2 or 3 (in relation to the reconnecting magnetic field component) and beta of 5-10. This result stems from the strong localization of the dissipation near the reconnection site, which is modeled based on particle simulation results. Imposing uniform resistivity, corresponding to a Lundquist number of 10(exp 3) to 10(exp 4), leads to significantly larger Ohmic heating. Increasing incompressibility greatly reduces the magnetic flux transfer and the amount of energy released, from approx. 10% of the energy associated with the reconnecting field component, for zero guide field and low beta, to approx. 0.2%-0.4% for large values of the guide field B(sub y0) > 5 or large beta. The results demonstrate the importance of taking into account plasma compressibility and localization of dissipation in investigations of heating by turbulent reconnection, possibly relevant for solar wind or coronal heating

Electron Power-Law Spectra in Solar and Space Plasmas

Particles are accelerated to very high, non-thermal energies in solar and space plasma environments. While energy spectra of accelerated electrons often exhibit a power law, it remains unclear how electrons are accelerated to high energies and what processes determine the power-law index $\delta$. Here, we review previous observations of the power-law index $\delta$ in a variety of different plasma environments with a particular focus on sub-relativistic electrons. It appears that in regions more closely related to magnetic reconnection (such as the `above-the-looptop' solar hard X-ray source and the plasma sheet in Earth's magnetotail), the spectra are typically soft ($\delta \gtrsim$ 4). This is in contrast to the typically hard spectra ($\delta \lesssim$ 4) that are observed in coincidence with shocks. The difference implies that shocks are more efficient in producing a larger non-thermal fraction of electron energies when compared to magnetic reconnection. A caveat is that during active times in Earth's magnetotail, $\delta$ values seem spatially uniform in the plasma sheet, while power-law distributions still exist even in quiet times. The role of magnetotail reconnection in the electron power-law formation could therefore be confounded with these background conditions. Because different regions have been studied with different instrumentations and methodologies, we point out a need for more systematic and coordinated studies of power-law distributions for a better understanding of possible scaling laws in particle acceleration as well as their universality.Comment: 67 pages, 15 figures; submitted to Space Science Reviews; comments welcom

`Island Surfing' Mechanism of Electron Acceleration During Magnetic Reconnection

One of the key unresolved problems in the study of space plasmas is to explain the production of energetic electrons as magnetic field lines `reconnect' and release energy in a exposive manner. Recent observations suggest possible roles played by small scale magnetic islands in the reconnection region, but their precise roles and the exact mechanism of electron energization have remained unclear. Here we show that secondary islands generated in the reconnection region are indeed efficient electron accelerators. We found that, when electrons are trapped inside the islands, they are energized continuously by the reconnection electric field prevalent in the reconnection diffusion region. The size and the propagation speed of the secondary islands are similar to those of islands observed in the magnetotail containing energertic electrons.Comment: 5 pages, 4 figures, submitted to J. Geophys. Res

On the propagation of bubbles in the geomagnetic tail

Using three-dimensional magnetohydrodynamic simulations, we investigate the propagation of low-entropy magnetic flux tubes ("bubbles") in the magnetotail. Our simulations address fundamental properties of the propagation and dynamics of such flux tubes rather than the actual formation process. We find that the early evolution, after a sudden reduction of pressure and entropy on a localized flux tube, is governed by re-establishing the balance of the total pressure in the dawn-dusk and north-south directions through compression on a time scale less than about 20s for the typical magnetotail. The compression returns the equatorial pressure to its original unperturbed value, due to the fact that the magnetic field contributes only little to the total pressure, while farther away from the equatorial plane the magnetic field compression dominates. As a consequence the pressure is no longer constant along a flux tube. The subsequent evolution is characterized by earthward propagation at speeds of the order of 200-400km/s, depending on the initial amount of depletion and the cross-tail extent of a bubble. Simple acceleration without depletion does not lead to significant earthward propagation. It hence seems that both the entropy reduction and the plasma acceleration play an important role in the generation of fast plasma flows and their propagation into the near tail. Earthward moving bubbles are found to be associated with field-aligned current systems, directed earthward on the dawnward edge and tailward on the duskward edge. This is consistent with current systems attributed to observed bursty bulk flows and their auroral effects.<br><br><b>Key words.</b> Magnetospheric physics (magnetospheric configuration and dynamics; magnetotail; plasma sheet)nguage