Test particle acceleration in resistive torsional fan magnetic reconnection using laboratory plasma parameters

Particle acceleration via magnetic reconnection is a fundamental process in astrophysical plasmas. Experimental architectures are able to confirm a wide variety of particle dynamics following the two-dimensional Sweet–Parker model, but are limited in their reproduction of the fan-spine magnetic field topology about three-dimensional (3-D) null points. Specifically, there is not yet an experiment featuring driven 3-D torsional magnetic reconnection. To move in this direction, this paper expands on recent work toward the design of an experimental infrastructure for inducing 3-D torsional fan reconnection by predicting feasible particle acceleration profiles. Solutions to the steady-state, kinematic, resistive magnetohydrodynamic equations are used to numerically calculate particle trajectories from a localized resistivity profile using well-understood laboratory plasma parameters. We confine a thin, 10 eV helium sheath following the snowplough model into the region of this localized resistivity and find that it is accelerated to energies of ${\approx }2$ keV. This sheath is rapidly accelerated and focused along the spine axis propagating a few centimetres from the reconnection region. These dynamics suggest a novel architecture that may hold promise for future experiments studying solar coronal particle acceleration and for technology applications such as spacecraft propulsion.

The Key Role of Cold Ionospheric Ions As a Source of Hot Magnetospheric Plasma and As a Driver of the Dynamics of Substorms and Storms

The solar wind has been seen as the major source of hot magnetospheric plasma since the early 1960’s. More recent theoretical and observational studies have shown that the cold (few eV) polar wind and warmer polar cusp plasma that flow continuously upward from the ionosphere can be a very significant source of ions in the magnetosphere and can become accelerated to the energies characteristic of the plasma sheet, ring current, and warm plasma cloak. Previous studies have also shown the presence of solar wind ions in these magnetospheric regions. These studies are based principally on proxy measurements of the ratios of He++/H+ and the high charge states of O+/H+. The resultant admixture of ionospheric ions and solar wind ions that results has been difficult to quantify, since the dominant H+ ions originating in the ionosphere and solar wind are indistinguishable. The ionospheric ions are already inside the magnetosphere and are filling it from the inside out with direct access from the ionosphere to the center of the magnetotail. The solar wind ions on the other hand must gain access through the outer boundaries of the magnetosphere, filling the magnetosphere from the outside in. These solar wind particles must then diffuse or drift from the flanks of the magnetosphere to the near-midnight reconnection region of the tail which takes more time to reach (hours) than the continuously large outflowing ionospheric polar wind (10’s of min). In this paper we examine the magnetospheric filling using the trajectories of the different ion sources to unravel the intermixing process rather than trying to interpret only the proxy ratios. We compare the timing of the access of the ionospheric and solar wind sources and we use new merged ionosphere-magnetosphere multi-fluid MHD modeling to separate and compare the ionospheric and solar wind H+ source strengths. The rapid access of the initially cold polar wind and warm polar cusp ions flowing down-tail in the lobes into the mid-plane of the magnetotail, suggests that, coupled with a southward turning of the IMF Bz, these ions can play a key triggering role in the onset of substorms and subsequent large storms.

Heavy ion escape from Martian wake enhanced by magnetic reconnection

How ion escape from the near-Mars space is one of the biggest puzzles for understanding the atmospheric evolution of Mars. Ions in the plasma wake region continuously escape from the unmagnetized planet. Although the average ion escape rate in the wake region is relatively low, observations also have revealed the presence of events that contribute bursty and enhanced ion escape fluxes. Boundary instabilities and magnetic reconnection are suggested to be the candidate mechanisms. However, there is a lack of evaluation of ion escape caused by reconnection and comparison of the two mechanisms under a similar plasma environment. Here, we show an exciting reconnection event in the Martian wake. Two types of flux ropes are observed during the event. One was generated by reconnection, while others were produced by dayside boundary instability and convected to tail. The escape rate of oxygen ions in the reconnection region was estimated to be about 53–72% of the total tailward escape. Furthermore, the escape flux in the flux rope produced by reconnection was over twice that caused by dayside instabilities.

Scaling of magnetic reconnection with a limited x-line extent

<p>Magnetic reconnection is a fundamental physical process that is responsible for releasing the magnetic energy during substorms of planetary magnetotails. Previous studies of magnetic reconnection usually take the two-dimensional (2D) approach, which assumes that reconnection is uniform in the 3rd direction out of the 2D reconnection plane. However, observations suggest that reconnection can be limited in the 3rd direction, such as reconnection at Mercury's magnetotail. It turns out that reconnection can be suppressed when reconnection region is very limited in the 3rd direction. An internal x-line asymmetry along the current direction develops because of the transport of reconnected magnetic flux by electrons beneath the ion kinetic scale, resulting in a suppression region identified in Liu et al., 2019. Under the guidance of a series of 3D kinetic simulations, in this work, we incorporate the length-scale of this suppression region ~10d<sub>i</sub> to quantitatively model the reduction of the reconnection rate and the maximum outflow speed observed in the short x-line limit. The average reconnection rate drops because of the limited active region (where the current sheet thins down to the electron inertial scale) within an x-line. The outflow speed reduction correlates with the decrease of the <strong>J</strong>×<strong>B</strong> force, that can be modeled by the phase shift between the <strong>J</strong> and <strong>B</strong> profiles, also as a consequence of the flux transport. Notably, these two quantities are most essential in defining the well-being of magnetic reconnection, which can tell us when reconnection shall be suppressed.</p>

A magnetic reconnection model for hot explosions in the cool atmosphere of the Sun

<p>UV bursts and Ellerman bombs are transient brightenings observed in the low solar atmospheres of emerging flux regions. Observations have discovered the cospatial and cotemporal EBs and UV bursts, and their formation mechanisms are still not clear. The multi-thermal components with a large temperature span in these events challenge our understanding of magnetic reconnection and heating mechanisms in the low solar atmosphere. We have studied magnetic reconnection between the emerging and background magnetic fields. The initial plasma parameters are based on the C7 atmosphere model. After the current sheet with dense photosphere plasma is emerged to <span tabindex="0" role="presentation" data-mathml='<math xmlns="http://www.w3.org/1998/Math/MathML"><mn>0.5</mn></math>'><span><span><span>0.5</span></span></span></span> Mm above the solar surface, plasmoid instability appears. The plasmoids collide and coalesce with each other, which makes the plasmas with different densities and temperatures mixed up in the turbulent reconnection region. Therefore, the hot plasmas corresponding to the UV emissions and colder plasmas corresponding to the emissions from other wavelenghts can move together and occur at about the same height. In the meantime, the hot turbulent structures basically concentrate above <span tabindex="0" role="presentation" data-mathml='<math xmlns="http://www.w3.org/1998/Math/MathML"><mn>0.4</mn></math>'><span><span><span>0.4</span></span></span></span> Mm, whereas the cool plasmas extend to much lower heights to the bottom of the current sheet. These phenomena are consistent with the observations of Chen et al. 2019, ApJL. The synthesized Si IV line profiles are similar to the observed one in UV bursts, the enhanced wing of the line profiles can extend to about <span tabindex="0" role="presentation" data-mathml='<math xmlns="http://www.w3.org/1998/Math/MathML"><mn>100</mn></math>'><span><span><span>100</span></span></span></span> km s<span tabindex="0" role="presentation" data-mathml='<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi></mi><mrow class="MJX-TeXAtom-ORD"><mo>−</mo><mn>1</mn></mrow></msup></math>'><span><span><span><span></span><span><span><span>−</span><span>1</span></span></span></span></span></span></span>. The differences are significant among the numerical results with different resolutions, which indicate that the realistic magnetic diffusivity is crucial to reveal the fine structures and realistic plasmas heating in these reconnection events. Our results also show that the reconnection heating contributed by ambipolar diffusion in the low chromosphere around the temperature minimum region is not efficient.</p>

Energy transport during 3D small-scale reconnection driven by anisotropic turbulence using PIC simulations

<p>Heating and energy dissipation in the solar wind remain important open questions. Turbulence and reconnection are two candidate processes to account for the energy transport to subproton scales at which, in collisionless plasmas, the energy ultimately dissipates. Understanding the effects of small-scale reconnection events in the energy cascade requires the identification of these events in observational data as well as in 3D simulations. We use an explicit fully kinetic particle-in-cell code to simulate 3D small scale magnetic reconnection events forming in anisotropic and Alfvénic decaying turbulence. We define a set of indicators to find reconnection sites in our simulation based on intensity thresholds.  According to the application of these indicators, we identify the occurrence of reconnection events in the simulation domain and analyse one of these events in detail. The event is highly dynamic and asymmetric. We study the profiles of plasma and magnetic-field fluctuations recorded along artificial-spacecraft trajectories passing near and through the reconnection region as well as the energy exchange between particles and fields during this event. Our results suggest the presence of particle heating and acceleration related to asymmetric small-scale reconnection of magnetic flux tubes produced by the anisotropic Alfvénic turbulent cascade in the solar wind. These events are related to current structures of order a few ion inertial lengths in size.</p>

Variability in the energetic electron bombardment of Ganymede

<p>This study examines the bombardment of energetic magnetospheric electrons onto Ganymede as a function of Jovian magnetic latitude. We use the output from a hybrid model to constrain features of the electromagnetic environment during the G1, G8, and G28 Galileo encounters when Ganymede was far above, within, or far below Jupiter's magnetospheric current sheet, respectively. To quantify electron fluxes, we use a test-particle model and trace electrons at discrete energies between 4.5 keV ≤ <em>E</em> ≤ 100 MeV while exposed to these fields. For each location with respect to Jupiter's current sheet, electrons of all energies bombard Ganymede's poles with average number and energy fluxes of 1·10<sup>8</sup> cm<sup>-2</sup> s<sup>-1</sup> and 3·10<sup>9</sup> keV cm<sup>-2</sup> s<sup>-1</sup>, respectively. However, precipitation is inhomogeneous: poleward of the open-closed field line boundary, fluxes are enhanced in the trailing (but reduced in the leading) hemisphere. Within the Jovian current sheet, closed field lines of Ganymede's mini-magnetosphere shield electrons below 40 MeV from accessing the equator. Above these energies, equatorial fluxes are longitudinally inhomogeneous between the sub- and anti-Jovian hemispheres, but the averaged number flux (4·10<sup>3</sup> cm<sup>-2</sup> s<sup>-1</sup>) is comparable to the flux deposited by each of the dominant energetic ion species near Ganymede. When outside of the Jovian current sheet, electrons below 100 keV enter Ganymede's mini-magnetosphere via the downstream reconnection region and bombard the leading apex, while electrons of all energies are shielded from the trailing apex. Averaged over a synodic rotation, electron flux patterns agree with brightness features observed across Ganymede's polar and equatorial surface.</p>