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The Growth Phase Arcs and Substorm Onset

This workshop explores the phenomena of growth phase arcs and substorm onset in the magnetosphere-ionosphere coupling, focusing on their characteristics and role in the system. The workshop will feature discussions on topics such as equatorward moving arcs, tail field stretching, growth phase convection, the role of growth phase arcs, and more.

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The Growth Phase Arcs and Substorm Onset

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  1. 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

  2. Growth Phase Arcs Growth Phase Arcs

  3. Equatorward Moving Arcs (1) 5577 4278 5861 6300 PBI [Deehr and Lummerzheim, 2002] [Akasofu, 1964] [Akasofu et al., 2010]] Growth Phase Arcs

  4. 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. Growth Phase Arcs

  5. Two Arcs [Robinson and Vondrak, 1990] 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. Growth Phase Arcs

  6. Hydrogen Arc Comparison of DMSP ion energy flux with Hb intensity (curves) from meridional scans. The vertical line is the ion isotropy boundary (IB or b2iL) [Donovan et al., 2003]. IB is the equatorward edge of strong pitch angle scattering due to non-adiabatic motions. max [Coumans et al., 2007] Growth Phase Arcs

  7. Tail Field Stretching The equatorward displacement maynot 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]. Growth Phase Arcs

  8. Growth Phase Convection 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]. 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]. Growth Phase Arcs

  9. 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. Growth Phase Arcs

  10. Origin and Function of the Growth Phase Arc 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 pointingaway 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. 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. Growth Phase Arcs

  11. A Quantitative Model 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): The associated energy release rate: turns out to be sufficient to cover the energy dumped into the arc. Growth Phase Arcs

  12. Poleward Boundary Intensifications (PBIs) Growth Phase Arcs

  13. 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 commonfeatures especially during the recovery phase. Growth Phase Arcs

  14. Pre-onset PBIs 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]. Growth Phase Arcs

  15. 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] Growth Phase Arcs

  16. Onset Arcs Growth Phase Arcs

  17. 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. Growth Phase Arcs

  18. 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]. Growth Phase Arcs

  19. 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? Comparison of THEMIS and auroral onset timings [Donovan et al., 2008]. Growth phase and onset arcs [Lyons et al., 2002] Something must precede the latter by at least one Alfvénic transit time! Growth Phase Arcs

  20. Near-Earth Onset Scenarios Growth Phase Arcs

  21. 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. Growth Phase Arcs

  22. Substorm Onset and Harang Reversal [Zou et al., 2009]: 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. Growth Phase Arcs

  23. Slingshot Model Machida 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/catapultmechanism. NENL reconnection is seen as a result not cause. Growth Phase Arcs

  24. 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. Growth Phase Arcs

  25. 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. Brighteningof 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). Growth Phase Arcs

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