Radio and Space Plasma Physics Group

The formation of transpolar arcs

R. C. Fear and S. E. Milan

University of Leicester

Outline

• Introduction

• IMAGE survey of transpolar arcs

• Dependence of onset MLT on IMF BY

• Related ionospheric flows

• Conclusions

BackgroundMilan et al. (2005)

• Transpolar arcs (TPAs) are auroral features which extend into the polar cap

• TPAs are a predominantly northward IMF phenomenon (Berkey et al., 1976)

• Results considering IMF BY dependence are more mixed

– Some evidence for northern hemisphere post-midnight arcs when BY < 0 & vice versa (Gussenhoven, 1982)

– Other studies show no significant dependence (Valladares et al. 1994)

– Or mixed results depending on type of arc (Kullen et al., 2002)

• However, different approaches have been taken to evaluating IMF BY

Proposed formation mechanisms

• A range of formation mechanisms have been proposed

• Some place the arcs on open magnetic field lines– Field-aligned currents couple arc to a generator at the

magnetopause through various mechanisms (Burke et al., 1982, Akasofu & Roederer, 1983)

• Others place the arc on closed field lines– Antiparallel reconnection resulting in a tongue of closed flux in the

polar cap (Toffoletto & Hill, 1989) – A change in IMF BY introducing a twist into the magnetotail (Kullen,

2000)

• See Zhu et al. (1996) for a review

Proposed formation mechanismsMilan et al. (2005)

• Milan et al. (2005) studied a TPA (shown left) which formed during an interval of magnetotail reconnection which was not during a substorm

• IMF BY < 0• Onset ~2 MLT

• Milan et al. speculated that the formation of TPAs might be linked to the occurrence of fast ionospheric flows resulting from nightside reconnection

Proposed formation mechanisms• When BY = 0 in the magnetotail, lobe flux is

closed by reconnection and flows back to the dayside in a broadly symmetric manner (a)

• A period of dayside reconnection with a significant IMF BY component leads to a BY perturbation in the magnetotail (Fairfield, 1979; Cowley 1981)

• The flux which crosses the equatorial plane at midnight MLT then has a pre-midnight footprint in one hemisphere and a post-midnight footprint in the other (b)

• Such a field line experiences opposite magnetic tensions in opposite hemispheres, and the return flow is more complicated

• Milan et al. suggested that this leads to a build-up of closed flux which is unable to convect normally, resulting in a tongue of closed flux which protrudes from the plasma sheet, the ionospheric signature of which is a TPA

Milan et al. (2005), after Grocott et al. (2004)

IMF Clock angle dependence

• Examined 5 years of auroral image data from IMAGE FUV WIC and SI12 cameras (June 2000 to October 2005)

• Selected 131 TPAs where:– Onset was observed by IMAGE– TPA persisted for at least 30

minutes– TPA extended protruded mainly

radially into polar cap at some point in its lifetime

• Predominantly occur when IMF is northward over preceding hour, consistent with previous observations

– Only 8 events have a southward component of IMF

Relationship between IMF BY and onset MLT

• Anticorrelation between onset MLT and BY

• Weak if instantaneous IMF is used

• Stronger if IMF is averaged over 1 hour

• Stronger still if averaged over 5 hours

• Still present if averaged over 10 hours

• Correlation between IMF BY at onset time and onset MLT is weak

• If IMF is averaged over hours preceding onset, correlation is stronger

• Peak correlation occurs if averaging period is between ~2 and ~10 hours

• Consistent with Milan et al. (2005) mechanism

Relationship between IMF BY and onset MLT

2 hours

Ionospheric flows

• The Milan et al. (2005) mechanism predicts that the TPA will form at the starting point of an azimuthal flow burst

• Flow should be directed from TPA onset towards midnight MLT (and beyond)

• Examined SuperDARN map potential data averaged over 10 minute intervals for the 30 minutes preceding onset

• Ignored any flows poleward of the auroral oval

• Sufficient scatter in 28 events

• Identified events as consistent or inconsistent with the Milan et al. mechanism within the limits of the available scatter

Milan et al. (2005), after Grocott et al. (2004)

BY < 0

Events consistent with reconnection mechanism

• 21 events had flow patterns (or parts of flow patterns) which are consistent with the reconnection mechanism

• In 13 cases, TPA onset MLT can be confirmed to coincide with start of flow burst (unclear due to lack of scatter in remaining 8)

27th D

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• 7 events had flow patterns which are inconsistent with this mechanism

• Some arcs do not appear any different from many of the ‘consistent’ examples, but clearly inconsistent flows are observed (e.g. above)

Inconsistent flows12

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• 2 events (including above) form parallel to the oval and then swing out into the polar cap – uncertainty in onset local time, or perhaps a different mechanism?

• 2 other events do have some (weak) evidence for high-latitude aurora prior to selected onset – perhaps arc formed earlier and fades in and out of view

Inconsistent flows31

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Summary and conclusions• Surveyed 5 years of IMAGE FUV data and identified 131 transpolar arcs

– Formation observed by IMAGE, allowing the initial MLT to be determined

• Good correlation between onset MLT and IMF BY where IMF is averaged over the hours leading up to the TPA onset– Correlation strongest if IMF is averaged over the 2-10 hours leading up to the

start of the TPA, consistent with the timescale for flux transport into the lobes

• 28 events were formed when SuperDARN observed good enough scatter for us to determine whether the flow patterns were consistent with the reconnection mechanism– 21 consistent– 7 inconsistent

• The 7 inconsistent events include– two events where the arc develops parallel to oval – larger uncertainty in onset

MLT– two other events where there is some evidence for faint

earlier arcs – perhaps the chosen onset time is wrong