[GEM] THE GEM MESSENGER, Volume 25, Number 35

[Newsletter Editor]

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     THE GEM MESSENGER
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Volume 25, Number 35
September 4, 2015

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1. 2015 Summer Workshop Report: The Magnetic Reconnection in the Magnetosphere Focus Group
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From: Paul Cassak (WVU, paul.cassak at mail.wvu.edu), Andrei Runov (UCLA), Brian Walsh (Boston University) and Yi-Hsin Liu (NASA-GSFC)

In year three for the focus group on Magnetic Reconnection in the Magnetosphere, we welcomed two new co-chairs – Yi-Hsin Liu and Brian Walsh.  We thank Homa Karimabadi for his efforts the first two years of the focus group.  At the summer workshop, four sessions were convened. Three sessions were joint with other focus groups - one with the “Tail-Inner Magnetosphere Interactions” (TIMI) focus group (approximately 60 attendees), one with the Community Coordinated Modeling Center (CCMC) (40 attendees), and one with the “Geospace Systems Science” (GSS) focus group (60 attendees).  The TIMI and GSS sessions had two “scene-setting” talks to provide an overview of the relevant topics, followed by discussion and shorter contributed presentations on both observations and theory/simulations.  The individual session had a number of short presentations.  The purpose of the CCMC joint session was to solicit community input on developing infrastructure for particle-in-cell simulations at CCMC.  One session was independent with close to 30 people in attendance.  Summaries of each session follow, followed by a brief discussion of future plans.

Session 1 – Joint session with Tail-Inner Magnetosphere Interactions (TIMI) - Tuesday, June 16, 1:30pm

The first two presentations were scene setting talks.  Jim Drake presented an overview of the properties and issues related to magnetotail flow bursts, emphasizing that ion reflection at the front leads to dissipation of bulk flow kinetic energy.  Open issues include the role of electron dissipation at the front, whether and how the front breaks up, the potential for secondary reconnection along the flanks of the flow burst and the mechanisms that initiate and terminate flow bursts.  Joachim Birn first presented a brief summary of particle-in-cell (PIC) and magnetohydrodynamic (MHD) modeling results concerning the competition between tearing and the ballooning/interchange instability related to substorm onset, suggesting that adiabatic deformation of the magnetotail does not change stability against ballooning modes, whereas the onset and progression of reconnection causes entropy loss, enabling ballooning which in turn can accelerate reconnection and cause cross-tail structures.   He also presented results from test-particle tracing in an MHD simulation of tail reconnection and field collapse, demonstrating the formation of ion beams in the plasma sheet boundary layer (PSBL) as a consequence of near-tail reconnection; the test-particle simulations, consistent with PIC simulations, indicate that the crescent shape of the ion distribution results from propagation toward higher magnetic field, rather than from the direct acceleration mechanism.

In other talks, Phil Pritchett addressed two aspects of magnetotail reconnection using PIC simulations. In 2D he showed that the B_z hump configuration considered by Sitnov and Schindler (2010) is indeed unstable, but the growth rate is an order of magnitude smaller than seen in previous open simulations and does not have the characteristics of a tearing instability. In 3D he discussed the structure of exhaust jets from a finite length (in y) X-line and found that the dawnward portion travels faster than the remaining wider duskward portion of the front; both parts tend to break up on scales of several d_i.  Then, Misha Sitnov showed 3D PIC simulations with open boundaries of dipolarization fronts, where the ion and electron temperatures increase by the factors less than ~2 and 4, respectively, consistent with THEMIS observations published by Andrei Runov. In contrast to ions, electrons are heated within a relatively short region (~ 5 ion inertial scales) behind the front, and variations of their temperature along the front correlate with the northward magnetic field B_z and anti-correlate with the corresponding ion temperature variations.  

Further on the topic of dipolarization fronts (DFs), David Sibeck, presenting for Joo Hwang, identified both typical (earthward-propagating) and atypical (tailward-propagating) dipolarization fronts observed by MMS.  Four MMS spacecraft provide exciting observations of tail magnetic topology changes.  Hoaming Liang argued that pre-existing current sheets having a significant concentration of O+ can determine the thickness of DFs and encumber the propagation of DFs. The force contribution on DFs provided by the pre-existing current sheet O+ is not only determined by the density gradient but also ion temperatures, which means that ion heating, acceleration, and reflection near DFs need to be considered self-consistently for the force balance.  

Mike Wiltberger presented results from high resolution LFM simulations to examine BBFs in the magnetotail.  They found that, in the simulation, the BBFs are caused by magnetic reconnection and have statistical properties in agreement with observations made by Geotail.  Yu Lin presented a 3D global hybrid simulation of magnetotail reconnection under a southward IMF. The evolution of 3D flux ropes as dipolarization fronts, the ion acceleration and injection, and their impacts to wave turbulence at global diplolarizations were discussed. 

Ying Zou discussed ground-based observations revealing nightside magnetic reconnection and subsequent disturbances in the magnetotail are preceded by localized and fast flows in the polar cap. These flows are well traced by polar cap arcs and as they impinge on the auroral poleward boundary, they are followed by and spatially connected to major intensifications within the auroral oval.  Toshi Nishimura presented MHD simulation results of driven reconnection by localized flow channels impinging on the plasma sheet. He suggested that magnetotail reconnection can be triggered by fast flow channels originating at the dayside.

Session 2 – Joint with Community Coordinated Modeling Center - Wednesday, June 17, 3:30pm

Lutz Rastaetter and Masha Kuznetsova announced plans to incorporate particle-in-cell simulations at CCMC.  The goal is to make it possible for researchers to look at distribution functions on request for comparison with data from existing missions or especially the Magnetospheric Multiscale (MMS) mission.  This session was to brainstorming to see what community members desired in such resources and what issues need to be overcome.  

Two-dimensional cuts of three-dimensional distribution functions should be readily possible.  The desire is to allow the user to choose where to take data and to output the distribution functions.  A number of logistic issues were discussed, including normalization and how much data can be made available given storage constraints.  Andrei Runov showed observations of distribution functions and Jim Drake and Jason Shuster showed distribution functions in simulations to get a feel for the comparison.

Finally, Alex Glocer and Colin Komar spoke about the development of separator (X-line) finding techniques in global magnetospheric simulations and their implementation at CCMC.  They have begun the task of allowing the user to request separator locations at run time.

Session 3 – Joint with Geospace Systems Science - Thursday, June 18, 1:30pm

The first two presentations were scene settings talks.  Bin Zhang showed global simulation results based on multi-fluid MHD revealing that: 1) a small amount of mass loading at the dayside magnetopause only redistributes local reconnection rate without a significant change in the integrated reconnection rate and 2) a large amount of mass loading reduces both local reconnection rates and the integrated reconnection rate on the dayside, with corresponding changes in the shape of the magnetopause, in the properties of the magnetosheath and the geoeffective length in the solar wind. Simulation results suggests that the dayside reconnection potential may be controlled by both local and global processes.  Then, Colin Komar discussed the longstanding debate over whether reconnection is a local or global process.  Advances in locating three-dimensional X-lines, or separators, in global simulations were discussed with an emphasis on quantifying reconnection local to these structures in order to develop a comprehensive understanding of magnetospheric reconnection.

In other talks, Bill Lotko discussed LFM simulations including the effects of auroral field-aligned potential drops which show that they affect nightside reconnection by 1) enhancing the rate (and accompanying fast exhaust flows) along the pre-midnight X-line relative to that in the post-midnight plasma sheet and 2) cause the pre-midnight X-line to move earthward relative to that in the post-midnight sector. The enhanced pre-midnight reconnection is evidently required to power the extra Joule dissipation associated with the low-altitude, field-aligned potential drops occurring there and is enabled by the earthward displacement of the X-line where the lobe flux of the reconnection inflow and, therefore, the reconnection rate, is larger.  Yi-Hsin Liu discussed 3D PIC simulations demonstrating that the X-line bisects the total magnetic shear angle in the large ion-to-electron mass ratio limit, consistent with the idea of maximizing the peak reconnection rate in corresponding 2D oblique planes.  

Slava Merkin analytically investigated the kinetic and MHD tearing stability of 2D magnetotail current sheets as a function of their length, level of stretching and amplitude, as well as the scale size and direction of the magnetic field B_z gradient. He showed that not only “hump” equilibria have a potential for instability, but also equilibria possessing a tailward B_z gradient; however, instability requires the current sheet to be sufficiently long and stretched and tailward B_z gradient to be sufficiently steep.  Misha Sitnov emphasized the inherently multi-scale nature of the Lembege-Pellat ion tearing stability criterion that depends on the global parameter, the flux tube volume, and local parameters, particularly the current sheet thickness. It was shown that the formation of thin current sheets further from Earth can be provided by a generalization of the 1972 class of equilibrium models described by Schindler if the effect of the dipole field is taken into account.

Session 4 –Individual Reconnection Session - Thursday, June 18, 3:30pm

In this session, there were a number of contributed talks as well as discussion.  Colby Haggerty showed that long range, weak amplitude parallel electric fields are present in magnetic reconnection exhausts as a result of electron pressure balance along a magnetic field line. These parallel electric fields form a repulsive potential for ions entering the exhaust, reducing their counter-streaming velocities and thus reducing ion heating during reconnection.  Shan Wang proposed an electron velocity distribution function model based on the electron diffusion region energization process to obtain the electron bulk heating coefficient. The coefficient is found to be ~2% in magnetotail observations, where the substorm unloading causes large variations in upstream conditions.  Jason Shuster discussed PIC simulations of collisionless magnetic reconnection, showing the spatiotemporal evolution of electron velocity distributions in the electron diffusion region (EDR) to elucidate how electrons are accelerated and heated. The multicomponent non-gyrotropic distributions may serve as “smoking-gun” observables to identify the EDR based on spacecraft measurements.

Liang Wang suggested a multi-fluid moment model to address several needs of modern global codes: 1) including full electron pressure tensor in a fluid-based framework, 2) operation without an artificially specified resistivity, 3) efficient incorporation of the Hall term, and 4) straightforward handling of multi-ion species. This model has been coupled to OpenGGCM as an optional time integrator, and realistic problems like Ganymede and Earth's magnetosphere are in progress.  In the context of a discussion on temperature anisotropy and ion-to-electron temperature ratio within reconnection jets, Andrei Runov showed recent statistical results from THEMIS that revealed the absence of the pronounced anisotropy and vanishing correlations between ion and electron temperatures in the dipolarizing flux bundles detected in the near-Earth plasma sheet (R < 15 R_E). 


Rick Wilder showed dual-spacecraft observations of a high-latitude moving X-line using data from the Cluster mission. He observed that the X-line retreats at approximately the magnetosheath speed.  Paul Cassak discussed theory and simulations of asymmetric reconnection with flow shear.  A new prediction of the drift speed of isolated X-lines and the reconnection rate for arbitrary upstream flow speeds and upstream parameters was given and confirmed with simulations and comparisons to Rick Wilder’s observations.

Future Directions

Undoubtedly, next year’s sessions will include discussion of data from MMS and what it means for reconnection in the magnetosphere.  As the observations will be at the dayside, this will likely dominate some of the discussion, including kinetic dissipation physics and particle acceleration in asymmetric systems, and the 3D structure of dayside reconnection.  As warranted, conjunctions with THEMIS will be discussed.  For the magnetotail, we expect continued interest in the nature of dipolarization fronts and what causes reconnection onset.


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