07 July 2013 Substorm onset

Open Questions Concerning the Onset of Magnetospheric Substorms

Gerhard Haerendel,

Max Planck Institute for Extraterrestrial Physics, Garching, Germany

with contributions from Josh Semeter, Harald Frey, Chris Caston, and Jim McFadden.

MPE-JUB Symposium on Auroral Physics and Plasma Boundary Analysis

MPE, 01-05 July 2013

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Program:

Auroral current systems

Growth phase arcs and auroral streamers

A new scenario for the substorm onset

Conclusions

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Auroral Current Systems

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Auroral Current SystemsAnalysis of ground-based observations of magnetic disturbances lead to two types of equivalent ionospheric currents, directly driven DD and unloading UL (DP2 and DP1).

DD is a two-cell and UL a single cell system. The first dominates the solar wind driven perturbations and the growth phase, the second dominates the substorm.

[Akasofu 2011]

Warning: The one-cell system is mainly a 3-D system.

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Two Classes of Auroral Current Systems and Generator Forces

Type I currents are driven by forces, F, acting perpendicular to the field-aligned current sheets and the accompanying arcs. Type I dominates the substorm current wedge.

The generator forces of Type II current systems act parallel to the resulting current sheets and the accompanying auroral arcs. Type II dominates the growth phase.

F

F

[Boström 1964]

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Models for the UL Currents

In disruption models, the current generator lies outside the current wedge. In order to drive plasma from tail into magnetosphere, it must lie inside [Haerendel, 1992].

The current disruption model of [McPherron et al. [1973] has dominated the concepts of the substorm current wedge. The question remained as to where this happens and by which process. Near-Earth current disruption model of Lui [1991].

Flow braking generator [Haerendel 1992].

Magnetic pressure gradient generator [Haerendel 2009].

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First Summary

The most relevant points for the following considerations are:

•The growth phase is connected with a two-cell convection (DD).

•At substorm onset, one-cell convection is initiated (UL).

•The unloading current needs a generator inside substorm wedge.

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Growth phase arcs and auroral streamers

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My first two open questions:

(1) What is the nature of the growth phase arc?

(2) What are the auroral streamers arising from poleward boundary intensifications (PBI’s)?

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Growth Phase Arc

While the growth phase arc is moving equatorward it brightens in 5577 and 4278. The onset occurs when it has come close, but still about 100 km poleward of the equatorward edge of the hydrogen arc. The first brightening was observed slightly poleward of the growth phase arc [Deehr and Lummerzheim 2001].

onset arcElectrons > 1keV

Protons

Electrons< 1 keV

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Convection During Growth Phase

From overhead current and radar observations concurrent with auroral imaging Kadokura et al. [2002] concluded that the return flow of a polar cap two-cell convection is colocated and moves equatorward with the growth phase arc.

Growth phase (electron) arc and hydrogen arc are actually not as much separated.

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Convection Along Growth Phase Arc

420 km

Bsouth

Bwest

Bwest implies a balanced S-N field-aligned current system, northward electric field, and westward convection. Bsouth is consistent with an equatorward drag force acting on the polar cap plasma.

FAST crossing at 22.1 MLT

Colocated diffuse ion and electron precipitations with sharp equatorward edge and narrow inverted-V‘s of about 35 km width interdispersed (also [Lessard et al. 2007]).

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Nature of Growth Phase Arc

The growth phase arc is created dominantly by strong electron pitch angle scattering out of the earthward contracting central plasma sheet with narrow inverted-V arcs interdispersed.

Its sharp equatorward edge and association with a balanced field-aligned current system support the conclusion of Kadokura et al. [2002] that the arc is the location of the sunward diversion of the two-cell polar cap convection system.

E E

The growth phase arc is a manifestation of a barrier preventing the central plasma sheet from penetrating into the dipolar magnetosphere.

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Auroral Streamers

Nishimura et al. [2010] discovered that substorm onset is often preceded by equatorward moving auroral streamers which eventually line up with the growth phase arc, at which time the substorm begins.

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Origin of Auroral Streamers

Streamers separate from poleward boundary intensifications (PBI‘s) with speeds of the order of 600-800 m/s [Zesta et al. 2002].

Halley Bay radar tracking an auroral streamer [Kadokura et al. 2002]

Input from dayside?

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Periodic Structures in Growth Phase Arcs and Auroral Streamers

6 minutes periodicity in splittings of a growth phase arc [Semeter 2013].

The arc becomes narrower while moving equatorward.

Auroral streamers grow out of the polar boundary up to 30 min before onset. They are created by soft electrons.

[Courtesy of J. Semeter] Film!

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Growth Phase Arc Crossing Shortly Before Breakup

Arc crossing by FAST on 14 FEB 2008 over Ft. Simpson/Canada 8 min before breakup.

All sky image at Ft. Simpson at time of FAST crossing. Peak flux of growth phase arc is ~2 mW/m2 , of streamer ~1 mW/m2. However, strong currents connected, .

In three cases so far identified, the arcs poleward of the growth phase

arc had inverted-V signature

Growth phase arc

Auroral streamers

nTB 150

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Streamer and Growth Phase Arc

The magnetic perturbation field suggests opposing flow directions in streamer and growth phase arc.

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Auroral StreamersAuroral streamers seem to follow the two-cell convection pattern and often exhibit a rayed structure [Zesta et al. 2007; Nishimura et al. 2010]. The mean energy is typically below 1 keV (see also Semeter movie).

In the few cases of intercepts by FAST found so far, they appear as inverted-V arcs with peak energy up to 1 keV and energy fluxes not exceeding 1 mW/m2.

Their high movements and large attending currents suggest flow channels.

Since inverted-V arcs extract magnetic energy from sheared fields, these flows may result from the relaxation of strongly extended field lines in the tail which are forced to find their way inside the pre-established slower convection system.

Why do the auroral streamers line up along and poleward of the growth phase arc and do not penetrate into the arc?

Originating in the plasma sheet boundary layer and moving fast equatorwards, auroral streamers cannot enter the contracted central plasma sheet.

The true nature cannot be established at this time.

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Growth Phase Topology

Growth of the tail lobe leads to plasma sheet thinning.

PBI‘s at the Lobe/PSBL interface.

Auroral streamers growing out of PBI‘s move inside the Plasma Sheet Boundary Layer (PSBL), and eventually line up along the outer surface of the CPS.

The central plasma sheet (CPS), source of the onset arc, contracts and its magnetospheric border moves earthward.

At substorm onset, all three boundaries become open and plasma can enter the dipolar magnetosphere.

PSBL

Four regions and three boundaries:

CPS

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Drivers of the Two-Cell Convection

While solar wind drag directly drives the convection across the polar cap, the return flow is driven towards the dipolar magnetosphere by the pressure gradient force of the tail lobe.

Secondary currents are induced along the equatorward boundary. Their sense is opposed to the primary driving currents.

F = force driving the return flow

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Magnetic Pressure Driven Currents

j

B

pjB 2

0

jjjj Bm )2

1(

The solar wind via the tail lobes exerts a force on the plasma sheet. This leads to squeezing the PS plasma towards the Earth, to compression and thinning.

At the boundary between tail lobe and plasma sheet, westward currents, , are flowing balancing the plasma pressure force. They are composed of magnetization and oppositely directed magnetic gradient and curvature currents. Only the latter can connect to field-aligned currents and transfer forces to the ionosphere.

j

Compression and inward motion correspond to a westward electric field, E. While , . The current layer constitutes both a load and a generator.

E

0 jE 0 B

jE

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A possible scenario for the substorm onset

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My third and fourth questions:

(3) What process is behind the correlation between substorm onset and near-contact of an auroral streamer with the growth phase arc? (4) Which type of current system is connected with substorm breakup?

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Auroral Streamer and Substorm OnsetThere are two possibilitis for the frequent coincidence of auroral streamer alignment along the growth phase arc and substorm onset:

(a) Direct interaction between the two arcs.

(b) No direct interaction, but manifestation that onset conditions, i.e. sufficient thinning, have been reached.

Direct interaction is unlikely, since there are onsets without auroral streamers.

The growth phase arc brightening is connected with a sudden change of flow direction from west- to eastward (Film!). This indicates that the arc has become connected to a generator driving a westward current. Either the generator is new or the connection. Two possibilitis:

(α) New connection with a Boström Type I configuration.

(β) New generator, i.e. arrival of dipolarization front.

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New Connection to Generator

F

The magnetic gradient force, F, of the tail lobe on the plasma sheet exists throughout the whole growth phase.

It drives the return flow against the boundary constituted by the outer border of the dipolar magnetosphere.

Thinning of plasma sheet and return flow region leads to a take-over of the primary Type I currents along the source of the growth phase arc.

Current closure along the conducting channel of the growth phase arc.

The Generator The New Current Closure

F

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Break-in of Lobe Field

Towards the end of the growth phase, return flow and penetration of the fast auroral streamers have led to a thinning of the barrier towards the dipolar magnetosphere, especially along the N-S flow divide.

Consequently part of the field lines from the interface Lobe – PSBL are drawn earthward by the normal force, F, applying an east-west closure current to the equatorward boundary of the growth phase arc.

A westward electric field appears and the barrier is opened to the dipolar magnetosphere. The opening grows west- and eastward.

If we consider the tail lobe as the roof on top of the plasma sheet, the process is reminiscent of the break-down of part of the roof because of

failing support. Matter falls into the space below.

The newly applied current system is of Type I

F

The equatorward motions through the just opened gate are controlled by the accompanying conductivity enhancement and are of order 300 m/s.

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Tail Reconnection and Substorm Onset

According to the new scenario, substorm onset and formation of the near-Earth neutral line are not causally related. Both are a consequence of plasma sheet/growth phase arc thinning and start at approximately the same time with a few minutes uncertainty.

Thus both sequences of substorm and reconnection onset appear possible.

However! We have another enigma:

Many investigations, especially with THEMIS data, have shown that NENL formation preceded substorm onset (e.g. [Angelopoulos et al. 2009; Liu et al. 2011; Sergeev et al. 2011]). Other investigations have arrived at the opposite conclusion [Lui 1991; Rae et al. 2009; Lui et al. 2010; Haerendel 2010].

The substorm enigma # 1: Is the substorm onset triggered by a process at the earthward end of the tail or by neutral line formation (reconnection) at 18-30 RE in the tail?

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The Substorm Enigma #2

2009-03-29 06:12: RANK

How can the auroral arcs expand poleward when at the same time an equatorward convection is imposed on the plasma?

This is my present focus of research.

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Arrival of Dipolarization Front

The full substorm follows from arrival of the earthward reconnection flows, i.e. BBF‘s and dipolarization fronts.

However, onsets without a substorm following are also observed (“pseudo break-ups“).

As the substorm develops, plasma is penetrating more deeply into the magnetosphere. The increasing plasma and magnetic field pressure seeks relief by expanding sunward along the oval. This drives convection as well as the morning and evening electrojets.

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Conclusions

Some tentative answers have been given to the four questions raised:

(1) The growth phase arc constitutes an earthward barrier for the polar cap convection.

(2) Auroral streamers are most likely fast flow channels penetrating into the return flow region of the polar cap convection.

(3) Appearance of auroral streamers contributes to the thinning of the barrier towards the dipolar magnetosphere. This may lead to direct application of the lobe force to to the growth phase arc.

(4) The related current system is of Type I. It remains unclear whether it is driven by a new generator related to the arrival of dipolarization fronts or whether it is still the lobe pressure on the plasma sheet. The closure by westward current is consistent with a westward electric field or opening of the barrier.