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M. Hesse

Publications and source records attributed to M. Hesse.

12 recordsLinked to original sources
arXiv2020

Major Scientific Challenges and Opportunities in Understanding Magnetic Reconnection and Related Explosive Phenomena in Solar and Heliospheric Plasmas

Magnetic reconnection underlies many explosive phenomena in the heliosphere and in laboratory plasmas. The new research capabilities in theory/simulations, observations, and laboratory experiments provide the opportunity to solve the grand scientific challenges summarized in this whitepaper. Success will require enhanced and sustained investments from relevant funding agencies, increased interagency/international partnerships, and close collaborations of the solar, heliospheric, and laboratory plasma communities. These investments will deliver transformative progress in understanding magnetic reconnection and related explosive phenomena including space weather events.

physics.space-ph↗
arXiv2019

Electron acceleration and thermalization at magnetotail separatrices

In this study we use the Magnetospheric Multiscale (MMS) mission to investigate the electron acceleration and thermalization occurring along the magnetic reconnection separatrices in the magnetotail. We find that initially cold electron lobe populations are accelerated towards the X line forming beams with energies up to a few keV's, corresponding to a substantial fraction of the electron thermal energy inside the exhaust. The accelerated electron populations are unstable to the formation of electrostatic waves which develop into nonlinear electrostatic solitary waves. The waves' amplitudes are large enough to interact efficiently with a large part of the electron population, including the electron beam. The wave-particle interaction gradually thermalizes the beam, transforming directed drift energy to thermal energy.

physics.space-ph↗
arXiv2018

Electron-Scale Dynamics of the Diffusion Region during Symmetric Magnetic Reconnection in Space

Magnetic reconnection is an energy conversion process important in many astrophysical contexts including the Earth's magnetosphere, where the process can be investigated in-situ. Here we present the first encounter of a reconnection site by NASA's Magnetospheric Multiscale (MMS) spacecraft in the magnetotail, where reconnection involves symmetric inflow conditions. The unprecedented electron-scale plasma measurements revealed (1) super-Alfvenic electron jets reaching 20,000 km/s, (2) electron meandering motion and acceleration by the electric field, producing multiple crescent-shaped structures, (3) spatial dimensions of the electron diffusion region implying a reconnection rate of 0.1-0.2. The well-structured multiple layers of electron populations indicate that, despite the presence of turbulence near the reconnection site, the key electron dynamics appears to be largely laminar.

physics.space-ph↗
arXiv2018

On the Role of Separatrix Instabilities in Heating the Reconnection Outflow Region

A study of the role of microinstabilities at the reconnection separatrix can play in heating the electrons during the transition from inflow to outflow is being presented. We find that very strong flow shears at the separatrix layer lead to counterstreaming electron distributions in the region around the separatrix, which become unstable to a beam-type instability. Similar to what has been seen in earlier research, the ensuing instability leads to the formation of propagating electrostatic solitons. We show here that this region of strong electrostatic turbulence is the predominant electron heating site when transiting from inflow to outflow. The heating is the result of heating generated by electrostatic turbulence driven by overlapping beams, and its macroscopic effect is a quasi-viscous contribution to the overall electron energy balance. We suggest that instabilities at the separatrix can play a key role in the overall electron energy balance in magnetic reconnection.

physics.space-ph↗
arXiv2018

How the IMF $\mathit{B}_{y}$ Induces a Local $\mathit{B}_{y}$ Component During Northward IMF $\mathit{B}_{z}$ and Characteristic Timescales

We use the Lyon-Fedder-Mobarry global magnetohydrodynamics model to study the effects of the interplanetary magnetic field (IMF) $\mathit{B}_{y}$ component on the coupling between the solar wind and magnetosphere-ionosphere system when IMF $\mathit{B}_{z}$ $>$0. We describe the evolution of how a magnetospheric $\mathit{B}_{y}$ component is induced on closed field lines during these conditions. Starting from dayside lobe reconnection, the magnetic tension on newly reconnected field lines redistribute the open flux asymmetrically between the two hemispheres. This results in asymmetric magnetic energy density in the lobes. Shear flows are induced to restore equilibrium, and these flows are what effectively induces a local $\mathit{B}_{y}$ component. We show the radial dependence of the induced $\mathit{B}_{y}$ and compare the results to the induced $\mathit{B}_{y}$ during southward IMF conditions. We also show the response and reconfiguration time of the inner magnetosphere to IMF $\mathit{B}_{y}$ reversals during northward IMF $\mathit{B}_{z}$. A superposed epoch analysis of magnetic field measurements from seven Geostationary Operational Environmental Satellite spacecraft at different local times both for negative-to-positive and positive-to-negative IMF $\mathit{B}_{y}$ reversals is presented. We find that the induced $\mathit{B}_{y}$ responds within 16 min of the arrival of IMF $\mathit{B}_{y}$ at the bow shock, and it completely reconfigures within 47 min.

physics.space-ph↗
arXiv2018

The Physical Foundation of the Reconnection Electric Field

We report on computer simulations and analytic theory to provide a self-consistent understanding of the role of the reconnection electric field, which extends substantially beyond the simple change of magnetic connections. Rather, we find that the reconnection electric field is essential to maintaining the current density in the diffusion region, which would otherwise be dissipated by a set of processes. Natural candidates for current dissipation are the average convection of current carriers away from the reconnection region by the outflow of accelerated particles, or the average rotation of the current density by the magnetic field reversal in the vicinity. Instead, we show here that the current dissipation is the result of thermal effects, underlying the statistical interaction of current-carrying particles with the adjacent magnetic field. We find that this interaction serves to redirect the directed acceleration of the reconnection electric field to thermal motion. This thermalization manifests itself in form of quasi-viscous terms in the thermal energy balance of the current layer. These quasi-viscous terms act to increase the average thermal energy. Our predictions regarding current and thermal energy balance are readily amenable to exploration in the laboratory or by satellite missions, in particular, by NASAs Magnetospheric Multiscale mission.

physics.space-ph↗
arXiv2017

Localized Oscillatory Dissipation in Magnetopause Reconnection

Data from the NASA Magnetospheric Multiscale (MMS) mission are used to investigate asymmetric magnetic reconnection at the dayside boundary between the Earth's magnetosphere and the solar wind (the magnetopause). High-resolution measurements of plasmas, electric and magnetic fields, and waves are used to identify highly localized (~15 electron Debye lengths) standing wave structures with large electric-field amplitudes (up to 100 mV/m). These wave structures are associated with spatially oscillatory dissipation, which appears as alternatingly positive and negative values of J dot E (dissipation). For small guide magnetic fields the wave structures occur in the electron stagnation region at the magnetosphere edge of the EDR. For larger guide fields the structures also occur near the reconnection x-line. This difference is explained in terms of channels for the out-of-plane current (agyrotropic electrons at the stagnation point and guide-field-aligned electrons at the x-line).

physics.space-ph↗
arXiv2017

On the collisionless asymmetric magnetic reconnection rate

A prediction of the steady-state reconnection electric field in asymmetric reconnection is obtained by maximizing the reconnection rate as a function of the opening angle made by the upstream magnetic field on the weak magnetic field (magnetosheath) side. The prediction is within a factor of two of the widely examined asymmetric reconnection model [Cassak and Shay, Phys. Plasmas 14, 102114, 2007] in the collisionless limit, and they scale the same over a wide parameter regime. The previous model had the effective aspect ratio of the diffusion region as a free parameter, which simulations and observations suggest is on the order of 0.1, but the present model has no free parameters. In conjunction with the symmetric case [Liu et al., Phys. Rev. Lett. 118, 085101, 2017], this work further suggests that this nearly universal number 0.1, essentially the normalized fast reconnection rate, is a geometrical factor arising from maximizing the reconnection rate within magnetohydrodynamic (MHD)-scale constraints.

physics.plasm-ph↗
arXiv2016

Why does steady-state magnetic reconnection have a maximum local rate of order 0.1?

Simulations suggest collisionless steady-state magnetic reconnection of Harris-type current sheets proceeds with a rate of order 0.1, independent of dissipation mechanism. We argue this long-standing puzzle is a result of constraints at the magnetohydrodynamic (MHD) scale. We perform a scaling analysis of the reconnection rate as a function of the opening angle made by the upstream magnetic fields, finding a maximum reconnection rate close to 0.2. The predictions compare favorably to particle-in-cell simulations of relativistic electron-positron and non-relativistic electron-proton reconnection. The fact that simulated reconnection rates are close to the predicted maximum suggests reconnection proceeds near the most efficient state allowed at the MHD-scale. The rate near the maximum is relatively insensitive to the opening angle, potentially explaining why reconnection has a similar fast rate in differing models.

physics.plasm-ph↗
arXiv2008

Self-regulation of the reconnecting current layer in relativistic pair plasma reconnection

We investigate properties of the reconnecting current layer in relativistic pair plasma reconnection. We found that the current layer self-regulates its thickness when the current layer runs out current carriers, and so relativistic reconnection retains a fast reconnection rate. Constructing a steady state Sweet-Parker model, we discuss conditions for the current sheet expansion. Based on the energy argument, we conclude that the incompressible assumption is invalid in relativistic Sweet-Parker reconnection. The guide field cases are more incompressible than the anti-parallel cases, and we find a more significant current sheet expansion.

astro-ph↗
arXiv2008

The role of the Weibel instability at the reconnection jet front in relativistic pair plasma reconnection

The role of the Weibel instability is investigated for the first time in the context of the large-scale magnetic reconnection problem. A late-time evolution of magnetic reconnection in relativistic pair plasmas is demonstrated by particle-in-cell (PIC) simulations. In the outflow regions, powerful reconnection jet piles up the magnetic fields and then a tangential discontinuity appears there. Further downstream, it is found that the two-dimensional extension of the relativistic Weibel instability generates electro-magnetic fields, which are comparable to the anti-parallel or piled-up fields. In a microscopic viewpoint, the instability allows plasma's multiple interactions with the discontinuity. In a macroscopic viewpoint, the instability leads to rapid expansion of the current sheet and then the reconnection jet front further propagates into the downstream. Possible application to the three-dimensional case is briefly discussed.

astro-ph↗