Ionospheric Convection Response to High-Latitude Reconnection and Electrodynamics of a Split-Transpolar Aurora S. Eriksson 1, G. Provan 2, F. J. Rich 3, C. Mouikis 4, M. W. Dunlop 5, M. Kuznetsova 6, S. Massetti 7, B. Anderson 8, M. Lester 2, J. T. Gosling 1, H. Reme 9, and A. Balogh 10 1 LASP, University of Colorado, Boulder, CO, USA 2 University of Leicester, Leicester, UK 3 AFRL, Hanscom AFB, MA, USA 4 SSC, University of New Hampshire, Durham, NH, USA 5 Rutherford Appleton Laboratory, Chilton, UK 6 NASA/GSFC, Greenbelt, MD, USA 7 Istituto di Fisica dello Spazio Interplanetario, Roma, Italy 8 JHU/APL, Laurel, MD, USA 9 Centre d’Etude Spatiale des Rayonnements, Toulouse, France 10 The Blackett Laboratory, Imperial College, London, UK Contact:

Outline Part I – Global Observations Cluster lobe reconnection observations: 14 February UT BATSRUS MHD simulation UT [c.f. “Stefan”] SuperDARN noon response to IMF UT: Schematic NBZ field-aligned current (FAC) and ExB flow driven by lobe reconnection Iridium Birkeland Currents Summary – Part I

Outline Part II – Electrodynamics Polar UVI & All-sky Camera observations DMSP F13 observations: UT -- ExB drift velocity -- FAC system -- Electron precipitation Summary – Part II

Part I – Global Observations

Lobe Reconnection Schematic Dungey [1963] (courtesy of J. C. Dorelli, UNH)

Cluster C1 Cluster C2 Cluster C3 Cluster C4 Solar Direction View from aboveSide view UT

Cusp Schematic - Cluster FGM Cluster C3 Cluster C1 Direction of magnetic field Lobe field Dayside closed field x z

Bx By Bz Vx Vy Vz

Bx By Bz Vx Vy Vz x-comp y-comp z-comp Walen Test: Quantitative agreement with high-latitude magnetic reconnection

Walen Test: Quantitative agreement with high-latitude magnetic reconnection Bn z x magnetotail lobe magnetosheath

YZ GSM Plane B Jpar Vx Vy

YZ GSM Plane Cluster C1 position ~ UT BVx VyJpar

XZ GSM Plane VxVy P

XZ GSM Plane

Cluster C1 18, 19, 20 UT Cluster C3 18, 19, 20 UT XZ GSM Plane

SuperDARN noon-sector flow in agreement with Cluster C3 observations at 1940 UT and 1950 UT…. one clockwise lobe cell is present in the dayside sector with sunward and dawnward flow across 12 MLT.

How does the sunward flow in the noon sector respond as the IMF clock angle changes? 11 MLT 80 o o 78 o

11 MLT 80 o o 78 o

IMF during SuperDARN high- latitude noon convection changes TPA TPA: Transpolar Aurora (Polar UVI) Red Vertical Line: Time of DMSP F13 TPA Observation

IMF during SuperDARN high- latitude noon convection changes A: Two-cell pattern B: Strong predominantly dawnward flow C: One clockwise global lobe cell D: One counterclockwise postnoon dayside lobe cell E: Two dayside lobe cells (reverse dayside flow) AB CDE

A: Two-cell pattern B: Strong predominantly dawnward flow C: One clockwise global lobe cell D: One counterclockwise postnoon dayside lobe cell E: Two dayside lobe cells (reverse dayside flow)

A: Two-cell pattern B: Strong predominantly dawnward flow C: One clockwise global lobe cell D: One counterclockwise postnoon dayside lobe cell E: Two dayside lobe cells (reverse dayside flow)

A: Two-cell pattern B: Strong predominantly dawnward flow C: One clockwise global lobe cell D: One counterclockwise postnoon dayside lobe cell E: Two dayside lobe cells (reverse dayside flow)

A: Two-cell pattern B: Strong predominantly dawnward flow C: One clockwise global lobe cell D: One counter- clockwise postnoon dayside lobe cell E: Two dayside lobe cells (reverse dayside flow)

A: Two-cell pattern B: Strong predominantly dawnward flow C: One clockwise global lobe cell D: One counterclockwise postnoon dayside lobe cell E: Two dayside lobe cells (reverse dayside flow)

A: Two-cell pattern B: Strong predominantly dawnward flow C: One clockwise global lobe cell D: One counterclockwise postnoon dayside lobe cell E: Two dayside lobe cells (reverse dayside flow)

A: Two-cell pattern B: Strong predominantly dawnward flow C: One clockwise global lobe cell D: One counterclockwise postnoon dayside lobe cell E: Two dayside lobe cells (reverse dayside flow) upward NBZdownward NBZ

Iridium Configuration downwardupward

Iridium Configuration downwardupward R1 NBZ R2 R1 R2

MHD simulation of NBZ development

B: Strong predominantly dawnward flow IMF clock angle +90 C: One clockwise global lobe cell IMF clock angle +45 E: Two dayside lobe cells (reverse dayside flow) IMF clock angle 0 F: One anti-clockwise global lobe cell ??? IMF clock angle -45 Proposed model: The sunward flow and the bounding NBZ FAC system are directly driven by lobe reconnection. As the IMF By changes during positive Bz, so does the lobe reconnection site and thus the location and deflection of the joint sunward flow channel and NBZ system. A TPA is expected within the upward NBZ system. See also: Southwood, 1987; Vennerstrom et al., 2005

Summary Part I The IMF from ACE and Cluster is strongly northward and duskward. The IMF Bx is negative in the solar wind (ACE) and in the magnetosheath (Cluster C1). Lobe reconnection is favored tailward of the northern cusp. Following a southward IMF Bz excursion, the IMF By decreases gradually toward By~0. The Cluster s/c moved through the northern cusp at the beginning of the event. Two s/c (C1 and C3) observed enhanced sunward and dawnward velocity in agreement with high-latitude lobe reconnection tailward of the cusp. MHD simulations confirm the general magnetic field and flow topology consistent with these Cluster observations. NBZ-type FACs are suggested on either side of the MHD lobe reconnection region and in the duskside ionosphere. SuperDARN ExB drift is sunward and dawnward across the 12 MLT meridian at the time of the Cluster C3 flow enhancements. The subsequent direction of SuperDARN noon sector flows (after a southward excursion) tracks the IMF clock angle changes well with different time delays. A faster response time is suggested to the southward (100 to 156 deg) turning (3-6 min) than either the duskward (135 to 34 deg) or due northward (45 to 8 deg) turnings that take 8-9 min and min, respectively.

Part II – Electrodynamics

Polar UVI

All-sky Camera, Daneborg (DNB)

Clockwise Lobe Cell

NBZR1 R2 Clockwise Lobe Cell

DMSP Electron Precipitation

Summary Part I-II SuperDARN verified a sunward flow channel over the TPA as part of a clockwise global lobe cell that covered much of the polar cap. This is consistent with the positive IMF By and northward IMF Bz (~30-50 deg clock angle). A DMSP F13 dusk-to-dawn pass verified a structured sunward lobe cell flow channel over the split-TPA and an NBZ current system on either side of it [Iijima and Shibaji, JGR, 1987; Southwood, 1987]. The TPA was found within the upward NBZ region. Two inverted Vs were detected in agreement with sunward flow shear and local upward FAC filaments at each of the two Sun-aligned arcs of the split- TPA. The high-latitude current system poleward of the duskside R2 system was locally balanced assuming a Pedersen closure. The increased Pedersen conductance at both arcs self-consistently explains the structured sunward drift velocity.

Summary Part I-II The dual arc separation is consistent with a prior Akebono study [Obara et al., 1996]. The structure & dual-arc system is in general agreement with the Zhu et al. [1994, 1996] MI-coupling model. The second (poleward) arc is due to the ionospheric response to an initial magnetospheric flow shear. We do not fully understand the cause and effect of the energy-dependence of the dual-arc separation. It may be related to stronger Hall current system relative to the Pedersen currents. We propose the following response of high-latitude dayside electrodynamics during northward IMF. The sunward flow & the bounding NBZ FAC system are directly driven by lobe reconnection. As the IMF By changes, so does the lobe reconnection site and thus the location and deflection of the joint sunward flow channel & NBZ system. The (dayside) TPA is expected within the upward NBZ system [see also Vennerstrom et al., 2005].

Mach number Plasma Beta Dynamic pressure Northward IMF epsilon: