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SuperDARN interhemispheric observations of reconnection signatures: a case study.

Super Dual Auroral Radar Network ANNUAL WORKSHOP Chincoteague (VA) – USA, June, 5 – 9, 2006. SuperDARN interhemispheric observations of reconnection signatures: a case study. Coco, I. (1) , S. Massetti (1) , E. Amata (1) , M. F. Marcucci (1) , and J.-P. Villain (2).

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SuperDARN interhemispheric observations of reconnection signatures: a case study.

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  1. Super Dual Auroral Radar Network ANNUAL WORKSHOP Chincoteague (VA) – USA, June, 5 – 9, 2006 SuperDARN interhemispheric observations of reconnection signatures: a case study. Coco, I.(1), S. Massetti(1), E. Amata(1), M. F. Marcucci(1), and J.-P. Villain(2) • Istituto di Fisica dello Spazio Interplanetario (IFSI) – INAF, Rome, Italy • (2) Laboratoire de Physique et Chimie de l’Environnement (LPCE) – CNRS, Orléans, France

  2. Summary • On December 19, 2002, btw 09:00 and 11:00 UT, after a sharp turning of the IMF Bz from negative to positive, the IMF clock angle varies from 0° (Bz > 0 and By 0) to 90° (Bz > 0 and |By| >> Bz), and keeps the 90° value almost steadily for about one hour. • The italian twin all-sky-camera system (ITACA2) composed by two auroral cameras at the Svalbard and at Daneborg (Greenland), observed intense red emission (630 nm) across noon, characteristic of soft electron precipitation during the whole period. Latitudinal and longitudinal cusp bifurcations have been evidenced and a train of TCVs over Greenland has been reported btw 10 and 11 UT (Massetti, JGR, 110, A07304, 2005 and Massetti, Ann. Geo. 2006, submitted). • SuperDARN dayside coverage is particularly abundant in both hemispheres. Thus, we try to follow the ionospheric convection patterns evolution in such a very interesting period of IMF By-dominated and almost positive IMF Bz. Such preliminary study concerns, for the moment, only observations in the dayside, where SuperDARN convection maps and aurora images overlap in the Northern hemisphere.

  3. Geotail data in Dec. 19, 2002, 08 – 12 UT. Spacecraft pos.: (8, -27, 5) Re GSE. No delay applied to data. SW dyn. pressure • Period 1: 9:06 – 9:34 UT • 0° < θ < 45° • SW pressure growing • Period 2: 9:36 – 11:00 UT • 45° < θ< 90° • Huge fall of SW pressure, followed by several “buffetings” IMF θ

  4. 1° period overview (9:06 – 9:34): convection maps for N and S hemispheres are shown, along with the ITACA2 aurora images taken one minute after each scan (because of the 120 s delay of the 630 nm red line emission).

  5. 1° period (9:06 – 9:34) features: Northern Hemisphere • When θ < 45° a sunward flow prevails in the postnoon sector, evidencing a double reverse cell structure, as we may expect for tailward-of-the-cusp reconnection during positive Bz periods. • A counterclockwise cell is almost always present in the morning sector, with centre around 75° MLAT, accompanied at times with a clockwise one at lower latitudes (viscous cells?) • At about 9:22 the clock angle has a quick jump up to 45°-50°. Btw 9:24 and 9:34, antisunward flows around 11 MLT are observed. They seem to start around 66° - 67° MLAT and, though they are not much intense, the distorsion of the equipotential contours is noteworthy (“nose”). Could this observation be interpreted as an evidence of “component merging” at the subsolar magnetopause? Up to 9:30 the aurora was characterized by a bright and wide spot around noon, somewhat westward of the Svalbard, interpreted as the footprint of the cusp (Massetti, 2005). At 9:30 aurora takes the form of two spots, one above Svalbards and one above Greenland (bifurcation). This effect is usually observed for θ > 135° (Bz < 0, By dominant), as the result of precipitation coming from a reconnection site in the opposite hemisphere. Why are we observing the same for our conditions? (component merging, high SW pressure...)

  6. 1° period (9:06 – 9:34) features: Southern Hemisphere • During this first period the convection patterns in the Southern hemisphere show different behaviours with respect to the Northern hemisphere patterns. There are two cells in the morning, and hints of antisunward velocity fluxes in both morning and afternoon sectors.

  7. 2° period overview (9:36 – 11:00): convection maps for N and S hemispheres are shown, along with the ITACA2 aurora images taken one minute after each scan (because of the 120 s delay of the 630 nm red line emission).

  8. 2° period (9:36 – 11:00) features (1) 9:40 – 9:50: the pressure suffers a strong decrease. • The ionospheric convection is strongly reduced (or, at least, radars measure less intense velocities) • The aurora faints away almost desappearing around 9:50. • Btw 9:44 and 9:46 the pressure has a little spike: immediately the convection seems to react, and new antisunward fluxes appear in the morning close to noon. • Traveling vortices displacing north-eastward are seen in the southern hemisphere at about 11 MLT. 9:42-9:44 9:44-9:46 9:46-9:48

  9. 2° period (9:36 – 11:00) features (2) • After 9:50 UT the clock angle reaches 90° and keeps this configuration for at least one hour. • For most of this time, the dayside convection shows the characteristic patterns that we expect when By is positive: a large convection cell in the afternoon Northern hemispere with strong velocity fluxes from dusk to dawn, and an opposite situation in the Southern hemisphere, with strong fluxes from dawn to dusk. • Antisunward velocity fluxes are often present around noon in both hemispheres, possibly driven by the SW pressure variations. • Aurora and Greenland magnetometers in the North show sequences of TCVs moving in the prenoon sector (Massetti, 2005). Though SuperDARN time resolution is not high enough to closely follow their evolution, traces of their motion are abserved at times in both hemispheres.

  10. Conclusions • When Bz is positive, but not By – dominated, the convection patterns look different in the two hemispheres: while in the North sunward fluxes and typical auroral spots seem to indicate the occurrence of cusp reconnection, in the South the pattern behaviour is more complex, and requires further analysis. • Clock - angle correlations: there are evidences in both hemispheres of antisunward convection fluxes injection around noon and from latitudes somewhat lower (~ 68° - 69° MLAT) than the average convection patterns for that period. These fluxes appear when θ passes 45°, and show up recurrently hereafter. In our opinion, that could be an evidence of component merging taking place at the subsolar magnetopause. • Clock - angle correlations: When θreaches 90°, the ionospheric convection reacts very quickly. The dayside convection patterns assume the characteristic By – dominated simmetry, opposite for the two hemisphere. To our knowledge, such effect has never been shown so clearly before, for a single event. • SW pressure correlations: During the whole period, the variations of the pressure seem to influence some convection characteristics, especially the intensity of the antisunward fluxes around noon: when the pressure has sudden enhancements, also the fluxes increase. • SW pressure correlations: During an abrupt pressure fall-down we have seen a noticeable reduction of the global convection and the aurora. Is it a reduction of the ambient electric field, or somewhat is affecting ionospheric density and propagation characteristics causing a temporary radar and aurora “blinding”?

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