[GEM] THE GEM MESSENGER, Volume 27, Number 39

Peter Chi pchi at igpp.ucla.edu
Fri Sep 1 16:52:54 PDT 2017


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     THE GEM MESSENGER
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Volume 27, Number 39
Aug.27,2017

Announcement submission website:
http://aten.igpp.ucla.edu/gem/messenger_form/

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Table of Contents

1. 2017 GEM Workshop Report: The Magnetic Reconnection in the Magnetosphere
Focus Group

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1. 2017 GEM Workshop Report: The Magnetic Reconnection in the Magnetosphere
Focus Group
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From: Paul Cassak, Andrei Runov, Brian Walsh, and Yi-Hsin Liu (Focus Group
Leaders) (paul.cassak at mail.wvu.edu)

In the final year of the Magnetic Reconnection in the Magnetosphere focus
group, five sessions were convened at the summer workshop.  Two sessions
were individual (with approximately 50 attendees each), one was joint with
the “Dayside Kinetic Processes in Global Solar Wind-Magnetosphere
Interaction” focus groups (approximately 33 attendees), and two were joint
with the “Magnetic Reconnection in the Magnetosphere” focus group jointed
with the “Tail Environment and Dynamics at Lunar Distances” and
“Magnetotail Dipolarization and Its Effects on the Inner Magnetosphere”
focus groups (approximately 83 attendees). Summaries of each session
follow.  The co-chairs thank the members of the GEM community that helped
make the focus group sessions an intellectually stimulating environment
over the course of the last five years.

Session 1 – Joint with the “Tail Environment and Dynamics at Lunar
Distances” and “Magnetotail Dipolarization and Its Effects on the Inner
Magnetosphere” focus groups - Monday, June 19

These two joint sessions encouraged cross-focus group interaction and open
ended discussion on the topics including the onset of tail reconnection,
the role of cross-tail instabilities, the difference between the tailward
and earthward reconnection jets/flux bundles, the interaction of
dipolarization fronts with ambient plasmas.

Vassilis Angelopoulos kicked off the first session with a tutorial talk.
Vassilis provided a broad view of the observation and modeling of the
nightside phenomena and substorms. Topics include the ionospheric
signature, substorm current wedge (SCW), near-Earth-neutral line, current
disruption versus reconnection models, external-driven versus spontaneous
onset, dipolarization fronts, and bursty-bulk flows (BBFs). In particular,
Vassilis challenged global modelers for a quantitative assessment of the
rate and intensity of BBFs, which brought up discussion on the time-scale
difference of BBFs and the SCW. At the end of his talk, Vassilis suggested
employing neural networks to conjoint statistics of occurrence rates and
characteristics from multi-mission datasets.

Misha Sitnov described the internally driven (spontaneous) onset of
magnetotail reconnection, which is only possible - in the case of electrons
magnetized initially by the normal magnetic field - when that field has a
region with a tailward gradient. 3D PIC simulations of the corresponding
ion tearing instability show that its distinctive features are: 1)
spontaneously generated earthward plasma flows that precede the topology
change, 2) new Hall pattern, opposite to the classical quadrupole pattern
near the X-line; and 3) new dissipation region (j*E’>0) at the
dipolarization front that may form before the X-line electron dissipation
region.

Heli Hietala presented ARTEMIS two-spacecraft observations of reconnection
in the presence of density asymmetry in the lunar distance magnetotail. The
observations indicate the reconnection flow channel had a finite width, of
the order of 5 Earth radii.  Andrei Runov discussed kinetic properties of
earthward-contracting dipolarizing flux bundles (DFBs) observed by THEMIS
in the near-Earth tail and tailward progressing rapid flux transport (RFTs)
enhancements observed by THEMIS in the near-tail and by ARTEMIS at lunar
orbit, respectively. The DFBs and RFTs are considered as earthward and
tailward ejecta from near-Earth reconnection. It was shown that whereas
DFBs interacts with near-tail plasma populations and particles within DFBs
gain energy from the increasing magnetic field, the RFT particles do not
interact with ambient field and plasma and keep the energy gained during
reconnection. The plasma state within RFTs is close to isothermal.

Session 2 – Individual Session - Tuesday, June 20

The theme of this session was to discuss aspects of dayside magnetic
reconnection, motivated both by Magnetospheric Multiscale (MMS) data and by
comparisons between THEMIS observations and ionospheric signatures of
dayside reconnection using SuperDARN.  Richard Denton started the session
by spurring discussion of how to determine the boundary normal (LMN)
coordinate system when using MMS data.  Observers should examine eigenvalue
ratios, consistency with results using different time intervals and
methods, and consistency with the geophysical context.  An interesting
discussion followed of how the procedure is affected when there is a guide
field or drift waves and other three-dimensional effects present

Rick Wilder then showed the first direct measurement of an electron jet
from symmetric reconnection in the magnetosheath. Coincident with the jet
was a parallel electric field channel which acted to heat electrons, and
was associated with a streaming instability and electron phase space
holes.  An interesting discussion of the meaning of negative J dot E
followed.

Mike Shay discussed two- and three-dimensional particle-in-cell simulations
of asymmetric guide field reconnection motivated by the 2015 Dec 08 MMS
event (Burch Frontier event).  He showed that the strongest electron
heating is upstream of the magnetospheric side of the x-line, was not due
to particle mixing, and is not due to 3D effects. This is an interesting
contrast to the 2015 Oct 16 (Science paper) event, which was shown by Le et
al. to have significantly more heating in 3D than 2D.  The guide field also
breaks the symmetry between the two exhausts, which affects where drift
waves are expected to arise.

Ying Zou presented two recent results of her study of the ionospheric
signatures of dayside reconnection.  First, she talked about efforts to
infer the azimuthal spreading of magnetopause reconnection. Previous
simulation studies addressed reconnection starting from a localized region
and spreading in the out-of-plane direction, but whether this happens at
Earth's dayside magnetopause has not been observed. Using THEMIS satellite
data at the dayside magnetopause and SuperDARN radars at the conjugate
ionosphere, she observed that reconnection does spread azimuthally in time
and that the spreading speed is closer to the magnitude of magnetopause
current carrier speed than the Alfven speed.  Second, she discussed the
azimuthal width of magnetopause reconnection. She combined in-situ and
remote-sensing observations to reliably determine the azimuthal width of
magnetopause reconnection. She used two THEMIS satellites simultaneously at
the Earth's magnetopause and SuperDARN radars at the conjugate ionosphere
to find that reconnection is often a few-hundred km wide in the ionosphere
and a few Earth radii wide at the magnetopause.

Session 3 – Individual Session - Wednesday, June 21

Approximately half of this session was devoted to new science, and half was
devoted to wrapping up the current focus group and discussing potential
directions for a future focus group.  Mike Shay presented kinetic PIC
simulations of turbulence to determine x-line statistics. Spatial filters
at the Debye length are much more effective than time averaging at removing
spurious x-lines due to numerical noise. The average reconnection rate is
0.1. This could be relevant to magnetosheath reconnection.

Then, Joachim Birn presented a comparison of ion distributions earthward
and tailward of the reconnection site, obtained by a combined MHD/test
particle approach. While ions on the earthward side might experience
multiple, Fermi or betatron-like, acceleration, leading to multiple beams
and ring-like distributions, ions on the tailward side experience only
single direct acceleration, adding a beam to an unperturbed population.

The potential directions for reconnection research within GEM was
broad-based and truly a community effort. Rick Wilder led a discussion of
topics that would be interesting in the next five years, including local
waves and turbulence, kinetic differences as a function of the guide field
strength, global physics including conjunctions with THEMIS, the impact of
heavy ions, and energy partition during reconnection.  In addition to the
new opportunities afforded by MMS and its expected conjunctions with THEMIS
and other spacecraft, advances in capturing kinetic effects in global
magnetospheric simulations will allow new studies and comparisons with the
observational data.

Misha Sitnov led a discussion about another possible topic of interest:
dissipation.  This is relevant to reconnection, but also more broadly to
the GEM community since the magnetosphere is the main natural laboratory of
collisionless plasmas. He said that understanding the collisionless
dissipation remains one of the most compelling and mysterious problems in
plasma physics. There are many questions that could be addressed,
including: Is collisionless dissipation always linked to topology changes,
and if not what are other prospective regions/mechanisms (e.g.,
dipolarization fronts)?  What is the role of turbulence in collisionless
dissipation (e.g., at the magnetopause)?  How does dissipation depend on
scales (ion/electron), composition, kinetic structure (velocity
distributions)?  What is the role of dissipation in the energy conversion
and transport in the geospace?

Session 4 – Joint with the “Dayside Kinetic Processes in Global Solar
Wind-Magnetosphere Interaction” focus groups - Thursday, June 22

This session focused on topics of mutual interest between the two focus
groups.  Andrey Samsonov presented an event in which a directional
discontinuity characterized by a significant IMF Bz change from positive to
negative (a southward turning) impacted the magnetosphere. He showed that
the previously formed magnetic barrier near the magnetopause must be
dissipated due to magnetosheath reconnection and this process increases the
propagation time of the discontinuity through the subsolar magnetosheath up
to 14 minutes.

Sanni Hoilijoki showed that the global hybrid-Vlasov simulation model
Vlasiator can be used to investigate the proton velocity distribution
functions close to the dayside magnetopause reconnection region. In
addition to D-shaped distributions, evolution of more complicated
structures can be studied.

Joo Hwang presented MMS observations of a series of ion-scale flux transfer
events. Field and particle data indicate that the current layer between an
old and a new X-line is unstable to the tearing instability, generating
multiple FTEs.  Heli Hietala presented THEMIS observations of a
magnetosheath high-speed jet triggering (local) magnetopause reconnection,
as the high dynamic pressure jet compressed a thick but high-shear
non-reconnecting magnetopause. These would be the first in situ
observations of a process for which there has only been indirect evidence
so far.

Future Directions

This was the final year for the Magnetic Reconnection in the Magnetosphere
focus group.  The focus group activities and accomplishments will be
summarized in a separate report. Thanks in a large part to the ongoing MMS
mission and the expected conjunctions with other missions including THEMIS
that will take place in the near future, we suspect that a new focus group
on some aspect of magnetic reconnection would have no shortage avenues for
future discussion.


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