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

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)

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

Substorms

• 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• 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?

• 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_xMiddle row: J, E_parBottom 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:• 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:

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?

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.

Papers at: http://artemis.sr.unh.edu/~jraeder/Home/index.php?n=Main.OnlinePapers?action=browseNumbers 64 (Raeder et al., ICS10), 63 (Siscoe et al., 2009), and 62 (Zhu et al., AG, 2009)

Papers at: http://artemis.sr.unh.edu/~jraeder/Home/index.php?n=Main.OnlinePapers?action=browseNumbers 64 (Raeder et al., ICS10), 63 (Siscoe et al., 2009), and 62 (Zhu et al., AG, 2009)

Extra Slides

Reconnection Rate in 3D?

Hesse, Forbes & Birn, (ApJ, 2006): reconnection rate ~ maximum integrated parallel electric field (,). Consistent with traditional 2d picture.

Describe field with Euler potentials and define a pseudo potential (,) (= integral of E_parallel along field lines):

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• 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

• 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 ky0 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 Bz0 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.