Transcript Ballooning Instability in the Near

Ballooning and other funny modes just
before, and after, substorm expansion onset
J. Raeder
Space Science Center, University of New Hampshire, Durham, NH 03824, USA
Ping Zhu, U. Wisconsin
Yasong Ge, UNH
George Siscoe, Boston University
THEMIS telecon, July 6, 2010, 2010
Introduction
• The common wisdom holds that the onset of the
substorm expansion phase occurs when near-Earth
reconnection reaches open field lines (NENL model).
• A competing model holds that “current disruption”
initiates the onset and that reconnection occurs later.
• Others believe that tail oscillations and coupling with
the ionosphere makes it happen.
• Maybe they are all wrong (or right).
• Arguments in this talk are based on OpenGGCM
simulations.
OpenGGCM: Global Magnetosphere Modeling
The Open Geospace General
Circulation Model:
• Coupled global magnetosphere - ionosphere thermosphere model.
• 3d Magnetohydrodynamic magnetosphere
model.
• Coupled with NOAA/SEC 3d dynamic/chemistry
ionosphere - thermosphere model (CTIM).
• Coupled with inner magnetosphere / ring current
models: Rice U. RCM, NASA/GSFC CRCM.
• Model runs on demand (>300 so far) provided at
the Community Coordinated Modeling Center
(CCMC at NASA/GSFC).
http://ccmc.gsfc.nasa.gov/
• Fully parallelized code, real-time capable. Runs
on IBM/datastar, IA32/I64 based clusters, PS3
clusters, and other hardware.
• Used for basic research, numerical experiments,
hypothesis testing, data analysis support,
NASA/THEMIS mission support, mission
planning, space weather studies, and Numerical
Space Weather Forecasting in the future.
• Funding from NASA/LWS, NASA/TR&T,
NSF/GEM, NSF/ITR, NSF/PetaApps, AF/MURI
programs.
Aurora
Ionosphere Potential
Personnel: J. Raeder, D. Larson, W. Li, A. Vapirev, K. Germaschewski, Y. Ge, H.-J. Kim, M. Gilson, B. Larsen, (UNH), T. Fuller-Rowell,
N. Muriyama (NOAA/SEC), F. Toffoletto, A. Chan, B. Hu (Rice U.), M.-C. Fok, A. Glocer (GSFC), A. Richmond, A. Maute (NCAR)
Substorms
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Substorms are a consequence of
reconnection rate imbalance: the
nightside rate must balance dayside
rate, at least on long scales (>1h) to
return flux back to the dayside.
During a substorm, first the dayside
reconnection rate exceeds the
nightside rate: growth phase.
Explosive reconnection in the
nightside signals the expansion
phase with auroral brightening and
westward traveling surge.
It remains an open question what
triggers the expansion phase onset: 2
minute question.
If rates balance: steady
magnetospheric convection (SMC).
Russell, 1993
March 23, 2007 substorm
• Northward IMF turn at ~1000
UT at Wind
• Wind located at L1, 200RE
upstream, ~1h time delay.
Aurora in the Simulation
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OpenGGCM discrete e- precipitation energy flux.
First onset near midnight, just as observed, but a bit too early, ~1045 UT.
Second onset, ~2100 MLT, 11:20 UT.
WTS expansion all the way to 1800 MLT, not quite right.
 Yes, it is a substorm.
What triggers the substorm?
• Ignoring the red and green surfaces:
• Obviously a new x-line forms ~15RE.
• Near-Earth fields dipolarizes, plasmoid is ejected.
What happens before reconnection?
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Different simulation,
idealized (no dipole
tilt, constant
SW/IMF, run at
CCMC), symmetric.
Recently published,
and all credit to:
Siscoe,
Kusnetzova, and
Raeder, Annales
Geophysicae, 27,
3142, 2009.
Red lines: before
equilibrium loss:
dp/dx (solid) and
JxB (dashed)
match.
Black lines: after
equilibrium loss:
both forces
reduced, but JxB
more so.
What happens before reconnection?
• Red iso-surface: force imbalance |grad(p)-JxB|.
• Green iso-surface: parallel E (what is reconnection?).
•  loss of equilibrium before reconnection sets in!
X-Z cuts in the onset meridian, top row: Bx and Vx, middle row: E_parallel and
pressure, bottom row: Bz and net force (grad(p)-JxB)_x, blue is taiward.
Tail Keograms: top row: Vx, Bz, E_par; bottom row: F_bal_x, pressure, |J|.
Taken at Bx=0 line.
Tail Keograms:
Top row: Vx, Fbal_x
Middle row: J, E_par
Bottom row: Bz, P
Taken at Bx=0 line.
Substorm simulation synopsis I
• Growth phase adds flux to the tail and squeezes PS/CS.
Distribution of p, J, and Bz changes, but JxB ~ grad(p) in
equilibrium.
• At some point JxB ~ grad(p) equilibrium is no longer possible.
Plasma accelerates, but only tailward (that distinguishes it
from tearing mode). CS thins further, and much quicker than
during the growth phase.
• After ~2 min significant tailward flow emanating from
X~13RE. Bz decreases.
• After ~4 min Bz  0, signs of tearing mode, earthward
acceleration and significant E_par.
• After ~6 min tearing mode fully developed. Strong tailward
AND earthward flows.
Wait, there is more:
• Careful look at the center of the current sheet, defined by
Bx=0:
• Clear finger-like structures in radial direction, but well aligned
with numerical grid:
If the
movie
does not
work:
Higher resolution:
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More ballooning!
Wavelength does not change, not always aligned with grid  resolved.
However, ballooning does not seem to initiate onset, but appears on the edge of
the dipolarizing inner magnetosphere during expansion.
If the movie does not work:
Findings
• A new ky=0 ideal instability (KY0 mode), likely precursor to tearing.
• Classical ballooning mode, consistent with Zhu et al. (2009) prediction
from theory. Wavelength ~ 0.5 RE. Apparently the first time this has
been seen in global simulations.
A new can of worms: questions
• Does the ballooning mode help the tail become unstable?
• How does the ballooning mode relate to the “blowout” and tearing
modes?
• What determines the ky of ballooning?
• Is ballooning of any consequence? Are there possibly other cases
where ballooning plays a critical role?
• Is it really resolved in the simulation (pretty sure, though)?
• Kinetic effects? Hall MHD effects?
• Ionosphere signature?
• Observations?
Papers at:
http://artemis.sr.unh.edu/~jraeder/Home/index.php?n=Main.OnlinePapers?action=browse
Numbers 64 (Raeder et al., ICS10), 63 (Siscoe et al., 2009), and 62 (Zhu et al., AG, 2009)
Papers at:
http://artemis.sr.unh.edu/~jra
eder/Home/
index.php?n=Main.OnlinePapers?
action=browse
Numbers 64 (Raeder et al.,
ICS10), 63 (Siscoe et al.,
2009), and 62 (Zhu et al., AG,
2009)
Extra Slides
Reconnection Rate in 3D?
Describe field with Euler potentials and define a pseudo
potential (,) (= integral of E_parallel along field lines):
Hesse, Forbes & Birn, (ApJ, 2006):
reconnection rate ~ maximum
integrated parallel electric field (,).
Consistent with traditional 2d picture.
What is Reconnection in 3D?
Non-ideal terms break
frozen-in condition in a
limited region of space
--> plasma parcels can
move to other field lines.
Necessary process for
reconnection, but not
sufficient, it also
describes magnetic
diffusion.
Reconnection seen in the ionosphere
• Left: auroral emisions.
• Middle: reconnection pseudo-potential.
• Right: VX in the equatorial plane.
Time Series Comparisons
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THEMIS: black, OpenGGCM: red (from top: Vx, Vy, T, N, Bx, By, Bz).
Sunward flow burst (top panel).
B dipolarization (Bx, Bz).
strong By deflection from tailward, stretching to more radial.
N up, T down (CDPS material?).
Aurora and WTS
• Both Polar UVI and VIS observe onset.
• First onset near midnight, expands to ~21.5 MLT
Assessment
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Substorm current wedge and dipolarization. Check.
Auroral brightening and expansion, westward traveling surge (WTS).
Check.
Particle injections (coming with RCM/CRCM coupling). Not yet, but MHD
heating. Update: now in coupled code w/RCM.
Electrojet, A-indices. Not checked here, but seen in other simulations.
Tail flows. Check.
PS thinning/thickening. Check.
plasmoids, TCRs, lobe field (energy storage). Not checked here, but
seen in other simulations.
Mid/high latitude Pi2. Possibly, needs work.
More subtle features (maybe): dimming before onset, initial arc
brightening, proton aurora (see Matt Gilson poster Tuesday).
Beyond MHD/fluid: AKR, Pi1B, O+ beams, dispersed injections, PSBL
beams,…. Maybe never in OpenGGCM.
OVERALL: looks like reconnection powers the substorm, But….. 
Substorm discussion I
• Growth phase adds flux to the tail and squeezes PS/CS. Distribution of p,
J, and Bz changes, but JxB ~ grad(p) equilibrium remains. Consistent
with equilibrium solutions by Birn et al. and others.
• At some point JxB ~ grad(p) equilibrium is no longer possible. Plasma
accelerates, but only tailward (that distinguishes it from tearing mode).
CS thins further, and much quicker than during the growth phase. This
must be an ideal MHD instability. It looks like an ballooning or an
interchange mode. If it is a balloning mode it would be in the ky0 limit,
opposite to what is usually assumed for ballooning. Interchange is
unlikely because there is no earthward motion nearby. The nature of the
instability still needs to be resolved. Again, this result is consistent with
Birn’s equilibria, which become singular when the CS becomes too thin.
Also consistent an “exposive growth phase”, and with RCM-E, which
experiences breakdown at this stage.
• After ~2 min significant tailward flow emanating from X~13RE. Bz
decreases. The unstable mode drives a tailward flow, but no earthward
flow. Bz and plasma is transported away. Bz diminishes because of
DBz/Dt=Bz*div(v). As plasma is transported away pressure balance in
the z-direction requires that the CS thins.
Substorm discussion II
• After ~4 min Bz  0, signs of tearing mode, earthward acceleration and
significant E_par. With Bz0 the tearing mode is unstable and grows.
The emerging x-line must produce roughly symmetric earthward and
tailward force. Earthward flows are slower because a strong pressure
gradient builds up on the Earthward side.
• After ~6 min tearing mode fully developed. Strong tailward AND
earthward flows. There is still no need that the reconnection occurs on
open field lines at this stage. Thus, by standard NENL picture, onset may
not yet have occurred.
• Caveat: numerical codes tend to produce too much E_par and thus over
emphasize reconnection. It is thus difficult to separate tearing from an
ideal instability. However, this strengthens the preceding scenario since
in the real world the ideal instability may grow even longer before tearing
sets in.