The formation of transpolar arcs

1. The formation of transpolar arcs R. C. Fear and S. E. Milan University of Leicester

2. Outline • Introduction • IMAGE survey of transpolar arcs • Dependence of onset MLT on IMF BY • Related ionospheric flows • Conclusions

3. Background Milan 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

4. 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

5. Proposed formation mechanisms Milan 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

6. Proposed formation mechanisms Milan et al. (2005), after Grocott et al. (2004) • 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

7. 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 IMF Clock angle dependence

8. 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 Relationship between IMF BY and onset MLT

9. Relationship between IMF BY and onset MLT • 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 2 hours

10. Ionospheric flows Milan et al. (2005), after Grocott et al. (2004) • 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 BY < 0

11. 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 December 2000

12. Inconsistent flows • 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) 12th December 2002

13. Inconsistent flows • 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 31st December 2001

14. 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 faintearlier arcs – perhaps the chosen onset time is wrong