19th Cluster Workshop on Multi-point Investigations of Magnetosphere-Ionosphere Coupling and Aurora

The Growth Phase Arcsand Substorm Onset

Gerhard Haerendel,

Max Planck Institute for Extraterrestrial Physics,

85748 Garching/Germany

Poiana Braşov, 18-21 May 2010

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Growth Phase Arcs

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Equatorward Moving Arcs (1)

[Akasofu, 1964]

[Deehr and Lummerzheim, 2002]

[Akasofu et al., 2010]]

5577

4278

5861

6300

PBI

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Equatorward Moving Arcs (2)

[Deehr and Lummerzheim, 2001] [Kadokura et al., 2002]

Typically 30–20 minutes before substorm onset, a hydrogen and, somewhat later, an electron arc start moving equatorward. The gap between the two is narrowing until, at substorm onset, the latter is only about 100 km poleward of the former arc. Oguti [1973] associated the substorm onset with this ‘contact’ of the two arcs.

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Two Arcs

There is a hard diffuse arc near the equatorward edge of the oval and a soft arc more poleward, both drifting equatorward. The second arc appears later and is the first to brighten. The first arc may be dominated by protons.

[Robinson and Vondrak, 1990]

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Hydrogen Arc

[Coumans et al., 2007]

Comparison of DMSP ion energy flux with H intensity (curves) from meridional scans. The vertical line is the ion isotropy boundary (IB or b2i) [Donovan et al., 2003]. IB is the equatorward edge of strong pitch angle scattering due to non-adiabatic motions.

max

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Tail Field Stretching

The equatorward displacement may not be a true motion but a progression of tail field stretching. This can shift the region of non-adiabatic ion motions to lower L-values and thus to an equatorward motion of the proton precipitation.

The GOES 8 magnetic field data show enhanced field stretching while the hydrogen aurora at Gillam moves equatorward. The red line is the optically determined IB [Donovan et al. 2003].

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Growth Phase Convection

erosion

Thereby the Near Earth Transition Region (NETR) between trapping region and plasma sheet is eroded and shrinks. The electron and hydrogen arcs are shifted equatorward [Haerendel, 2010].

Dp2 type convection persists during the whole growth phase (e.g. [Grocott et al., 2009].

It returns substantial magnetic flux to the dayside [Coroniti and Kennel, 1972].

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The Role of the Growth Phase Arc

The electron arc moves equatorward at speeds of a few 100 m/s. Kadokura et al. [2002] associate the arc with sunward flow deflection as manifested by the rotation of the equivalent flow vector through the arc.

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Origin and Function of the Growth Phase Arc

The flow out of the plasma sheet, i.e. from the polar cap, moves through the arc system and is deflected in the sunward direction owing to the action of the magnetic shear stresses in the layer between the FAC’s.

Conjecture [Haerendel, 2010]: An outflow from the central tail current sheet creates a high-beta plasma layer of near-dipolar shape at its edge. This layer acts as generator of a Boström Type II current system, with pressure forces pointing away from midnight, mixed with a Type I current system, set up by an earthward directed force. The result is (1) convection along the arc and (2) equatorward plasma flow through the arc.

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A Quantitative Model

CSnCSTzion

Bdt

d,, v

Earcarc v/

kmCS 5000 skmCSn /120v ,

CSCSCSCSionsCS hnWW v5.0

The outflow from the central current sheet as source of the high-beta plasma layer and generator of the growth phase arc has been derived from the properties of an arc [Haerendel, 2010].

The flux ejection rate:

must equal the flux transported along the arc during the transit time:

From and the ionospheric convection speed of 300 m/s the outflow parameters become (with Bz = 1 nT):

kmarc 100

The associated energy release rate: turns out to be sufficient to cover the energy dumped into the arc.

arc

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Poleward Boundary Intensifications

(PBIs)

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Poleward Boundary Intensifications (PBI’s)

[Zesta et al., 2002]

NS-oriented structures have been named auroral streamers [Elphinstone et al., 1997; Sergeev et al., 1999; Nakamura et al., 2001] and have been associated with bursty bulk flows. They are very common features especially during the recovery phase.

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Poleward boundary intensifications (PBIs) followed by north-south propagation often appear a few minutes (~5 min) before substorm onset, possibly activating the breakup [Nishimura et al., 2010].

Pre-onset PBIs

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Low-entropy Flux Tubes

Low-entropy magnetic flux tubes (“bubbles”) propagate along the tail towards the outer edge of the magnetosphere [Pontius and Wolf, 1990; Chen and Wolf, 1999; Birn et al., 2004]. They may be the origin of bursty bulk flows as well as of the pre-onset auroral streamers and their attending field-aligned currents.

Red: downward j||

[Birn et al., 2004]

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Onset Arcs

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The Onset Arc

The initial brightening does not necessarily occur in the growth phase arc but in a new arc appearing slightly equatorward a few minutes prior to onset. The arcs poleward of the onset arc remain unaffected until the expansion phase starts [Lyons et al., 2002].

However, this is not always the case. For instance, Deehr and Lummerzheim [2001] saw the brightening occur in a new arc poleward of the growth phase arc.

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Vortex Structures

The appearance and brightening of the onset arc is usually accompanied by the formation of moving undulations with wavelengths of 70 to 100km. The ripples merge into larger scale features soon becoming vortices. Eventually the westward traveling surge grows out of these structures [Rae et al., 2009].

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Onset Time

Many studies have been performed to proof the outward-in or inward-out hypotheses by satellite and ground-based timings.

But what is the proper definition of the auroral onset time? The start of the exponential brightness rise or the appearance and brightening of the onset arc?

Growth phase and onset arcs [Lyons et al., 2002]

Something must precede the latter by at least one Alfvénic transit time!

Comparison of THEMIS and auroral onset timings [Donovan et al., 2008].

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Near-Earth Onset Scenarios

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Trigger by New Plasma Intrusions

Nishimura et al. [2010]: PBIs (pink star) launch NS-oriented arcs (pink line) moving equatorward along the preexisting ionospheric convection pattern and carrying new plasma from the open-close boundary into the plasma sheet.

When reaching the onset arc(s) (pink wavy lines) the aurora starts brightening. A not identified instability is triggers the substorm onset.

84 % of the studied 249 auroral onset events are preceded by the arrival of either NS- or EW-arcs at the onset location. Average delay time between PBI and breakup: 5.5 min.

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Substorm Onset and Harang Reversal

The splitting of the polar cap convection into the evening- and morningside sunward flows, at the Harang reversal, exists already during the growth phase .

The substorm onset occurs near the center of the Harang reversal. The subauroral plasma flows (SAPS) are enhanced.

[Zou et al., 2009]:

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Slingshot ModelMachida et al. [2009] observed with Geotail the enhancement of Poynting flux towards the plasma sheet center beginning 10 min prior to substorm onset between –10 and –12 RE, with intensification at –4 min and earthward flows in the CPS.

This is attributed to an enhancement of the jxB force over the outward pointing pressure gradient force. Hence the name slingshot/catapult mechanism.

NENL reconnection is seen as a result not cause.

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Current Sheet Collapse

Haerendel [2010]: at near-complete erosion of magnetic flux from the Near Earth Transition Region (NETR), a pressure minimum is building up. The plasma outflow forming the growth phase arc is accelerated and plasma and magnetic flux are massively ejected from the central current sheet. The plasma sheet collapses, not triggered from outside but from the internal convection.

The release of shear stresses in the central current sheet leads to (a) local heating and (b) Poynting flux towards Earth. Process (a) may involve current instabilities, but not disruption. Opposite to ballooning.

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Conclusions

The growth phase is characterized by equatorward moving hydrogen and electron arcs, the latter poleward of the first. Time-scale 30-20 min prior to onset.

In a high number of cases auroral streamers initiated by PBIs appear about 5 min prior to onset. Upon reaching the most equatorward arc(s) breakup occurs.

Brightening of the onset arc, accompanied by the formation of wavy structures and folds, is the earliest optical signature of substorm onset.

Understanding the cause is the clue to the substorm trigger mechanism.

It remains unclear whether the ultimate trigger arrives from the mid-or distant tail (slingshot model versus new plasma intrusions) or from the tail-magnetosphere interface (magnetic flux erosion in the Near Earth Transition Region).