January 27th, Wed. 3:30PM
Virtual Presentation
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Kevin Genestreti, SwRI
The onset of reconnection in Earth's
magnetotail
The elongated tail of our
nightside magnetosphere stores energy from the solar
wind. The oppositely-directed magnetic fields in the
northern and southern tail are separated by a current
sheet, which periodically becomes very thin then "short
circuits". Understanding the causes of magnetic
reconnection - the "short circuit" mechanism - is an
important yet elusive problem in space physics. We have
learned a tremendous amount about reconnection by
observing it while it is occurring in space, yet it is
still debated how reconnection starts in the first
place. Ambiguity stems from the difficulty of determining
the accurate time history of a reconnection event over the
vast range of relevant spatial scales. The standard
picture is that reconnection of the solar wind and Earth's
dayside magnetic fields drives global (~1015 km3)
magnetospheric convection, which can thin the current
sheet by compressing it or depleting its internal
pressure. However, tail reconnection does not necessarily
follow dayside reconnection and some thin current sheets
remain stable. Microscopic (~107 km3) magnetic
reconnection sites are formed after one of several
proposed instabilities is triggered. While several
instabilities accompany reconnection, causal relationships
have been difficult to verify.
We start the seminar by summarizing the basic concepts of
magnetospheric reconnection and the wide-ranging impacts of
reconnection on near-Earth space. We then report a case study
where many space and ground-based observatories witnessed
magnetotail reconnection being initiated. We find that the
tail current sheet became thin by evacuating its internal
thermal pressure without significant dayside reconnection. The
solar wind prompted the pressure evacuation and, eventually,
initiated reconnection by momentarily compressing the
tail. Reconnection was initiated in multiple locations once
the current sheet surpassed the threshold for the
electron-tearing instability, which requires a sufficiently
thin current sheet with a weak magnetic field and a low
ion-to-electron temperature ratio. One reconnection site
quickly engulfed the others, becoming the dominant region that
shredded the tail's magnetic field, fitting with simple
models.
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February 3rd, Wed. 3:30PM
Virtual Presentation
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Gareth Roberg-Clark,
Max-Planck-Institut fur Plasmaphysik
Calculating the linear critical gradient for the
ion-temperature-gradient mode in magnetically confined
plasmas
A first-principles method to
calculate the critical temperature gradient for the onsetof
the ion-temperature-gradient mode (ITG) in linear
gyrokinetics is presented. We find that conventional notions
of the connection length previously invoked in tokamak
research should be revised and replaced by a generalized
correlation length to explain this onset in
stellarators. Simple numerical experiments and gyrokinetic
theory show that localized "spikes" in shear, a hallmark of
stellarator geometry, are generally insufficient to
constrain the parallel correlation length of the mode. ITG
modes that localize within bad drift curvature wells that
have a critical gradient set by peak drift curvature are
also observed. A case study of nearly helical stellarators
of increasing field period demonstrates that the critical
gradient can indeed be controlled by manipulating magnetic
geometry, but underscores the need for a general framework
to evaluate the critical gradient. We conclude that average
curvature and global shear set the correlation length of
resonant ITG modes near the absolute critical gradient, the
physics of which is included through direct solution of the
gyrokinetic equation. Our method, which handles general
geometry and is more efficient than conventional gyrokinetic
solvers, could be applied to future studies of stellarator
ITG turbulence optimization. |
February 10th, Wed. 3:30PM
Virtual Presentation
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Open
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February 17th, Wed. 3:30PM
Virtual Presentation
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Open
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February 24th, Wed. 3:30PM
Virtual Presentation
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Open
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March 3rd, Wed. 3:30PM
Virtual Presentation
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Open
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March 10th, Wed. 3:30PM
Virtual Presentation
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Open
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March 17th, Wed. 3:30PM
Virtual Presentation
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Spring Break
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March 24th, Wed. 3:30PM
Virtual Presentation
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Marina Battaglia, FHNW,
Switzerland
X-ray diagnostics of accelerated particles in solar
flares
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March 31st, Wed. 3:30PM
Virtual Presentation
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Joe Penano, NRL
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April 7th, Wed. 3:30PM
Virtual Presentation
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Open
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April 14th, Wed. 3:30PM
Virtual Presentation
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Katherine Goodrich,
UC-Berkeley
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April 21st, Wed. 3:30PM
Virtual Presentation
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Open
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April 28th, Wed. 3:30PM
Virtual Presentation
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Open
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May 5th, Wed. 3:30PM
Virtual Presentation
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Open
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