Introduction.

1. Night-side effects on the plasma convection in thepolar ionosphere due to a Sudden Impulse (SI)of solar wind dynamic pressure Coco, I.(1,2,3); Amata, E.(2); Marcucci, M.F.(2); Villain, J.-P.(3); Hanuise, C.(3); Cerisier, J.-C.(4); St. Maurice, J.-P.(5); Sato, N.(6) • Earth Science Dept., University of Siena – Italy. (2) IFSI – INAF, Rome – Italy, (3) LPCE – CNRS, Orléans – France. (4) IPSL – CETP, Saint Maur – France. (5) Dept. of Physics and Astronomy, Univ. of Western Ontario, London – Canada. (6) NIPR, Tokyo – Japan. • Introduction. • The Sudden Impulse (SI) of solar wind dynamic pressure of 2000/02/20, 21:00 UT, is investigated by making use of data from: WIND GEOTAIL, POLAR and GOES; ground magnetometer chains (Greenland, IMAGE, CANOPUS); SuperDARNHF radars in both Northern and Southern hemispheres. • This event is interesting for the following reasons: • Bz and By are positive during a long period before and after the event, and the solar wind is very quiet prior to the dynamic pressure jump; • in the SuperDARN data the effect of the SI is more clearly seen in the night side, where an enhancement of the anti-solar polar ionospheric convection is observed.

2. The SuperDARN HF-radar network (1). Fig. 1 The backscattered wave of the SuperDARN signal kr, is generated when the incident radio wave from the radar encounters an ionospheric irregularity, propagating with wave vector k =  2 kr, and k  B (B is the Earth’s magnetic field). For the typical frequencies of SuperDARN this condition is reached in the E and F regions of the ionosphere. • Each radar operates through an array of 16 log-periodic antennae, which permits to steer electronically the emitted signals over 16 azimuthal beams covering roughly 52°. • For each azimuthal beam the radars collect echoes from 75 spatial gates (Ranges), from 180 to 3550 km from the radar. • All SuperDARN radars operate sinchronously and a complete scan in range and azimuth is performed by each radar in 2 minutes. • An interferometric array of 4 antennae is used at most SuperDARN radars to infer the elevation angle of the backscattered signals. Fig. 2: The Kerguelen SuperDARN radar.

3. The SuperDARN HF-radar network (2). Fig. 5 SuperDARN is a network of nine HF Coherent radars in the Northern hemisphere (Fig.3) and six in the Southern hemisphere (Fig.4), which continously scan the high-latitude ionosphere in the frequency range 8 – 20 MHz. Fig. 5 shows an example of 2-min scan for the Kerguelen radar. Velocities to (from) the radar are coded in blue (red). Other physical quantities (not shown) deduced from the analysis of backscattered signals are the reflected power and the spectral width of the signal. Moreover the radars work in pairs, providing crossed fields-of-views (See Figs. 3,4) from which it is possible to reconstruct the velocity vectors of plasma convection. Fig. 3: Northern Hemisphere Coverage Fig. 4: Southern Hemisphere Coverage

4. Fig. 6 Fig. 7 Fig. 8 Fig. 8. Magnetic field observed close to Earth between 21:00 and 23:00 UT. From top to bottom: GEOTAIL Bx; Polar |B|, Bx, By and Bz; GOES-8 and GOES-10 Ht. At 21:39 UT (vertical red dotted line) the effect of compression starts at GEOTAIL and Bx increases by 20 nT over 10 min. At 21:40 UT, Polar observed a sharp polarity reversal of By and Bz. The two bottom panels show the increase of the total magnetic field seen by GOES 10 (about noon MLT) and by GOES 8 (late afternoon MLT) roughly 2 min later. Courtesy of S. Kokobun (MGF - Geotail) and C. T. Russell (MFE - Polar) through CDAWEB, and courtesy of Space Physics Interactive Data Resource, NGDC – NOAA, for GOES data. GOES-8 GOES-10 Polar Shock front, 21:00 UT y Wind GOES-10 GOES-8 GEOTAIL Shock front, 21:38 UT x 38 min later… Fig. 7. The IMF as observed by WIND for the same event. From top to bottom: |B|, Bx, By and Bz. Courtesy of R. Lepping (MFI-WIND), and CDAWEB, GSFC, NASA. Fig. 6. WIND observations on February 20, 2000, from 20:30 to 21:30 UT. From top to bottom: ion density, GSM x component of the SW velocity, dynamic pressure. Courtesy of K. Ogilvie (SWE- WIND), and CDAWEB, GSFC, NASA. Spacecraft GSM positions (RE) at 21:38 UT: WIND:X=162, Y=22, Z=11 POLAR: X=2.1, Y=2.1, Z=8.2 GEOTAIL: X=-26, Y=9.2, Z=3.4 GOES 8: 75° W GOES 10: 135° W

5. Fig. 10 Fig. 11 Fig. 10 shows selected magnetograms from the CANOPUS chain, for stations located at about 15 MLT, and between 61° and 69°L (L increases from bottom to top). At 21:39 UT the 4 lower L stations display a Preliminary Impulse (PI-), followed at about 21:40 UT by a Main Impulse (MI+) of the kind described by Araki (1994). The field then takes more than half an hour to reach the pre-pulse level. On the contrary at Resolute Bay, 83°L (top panel), at 21:38 UT one can see a positive pulse. Fig. 9 shows magnetograms from some stations located at middle and equatorial latitudes in both hemispheres (L increases from bottom to top). At 21:40 UT (vertical red line), a step-like world-wide response is noticed, which is usually attributed to the increase in the magnetopause current. Fig. 9 Fig. 11 shows three selected magnetograms from stations located in the morning MLT: starting at 21:37 UT, two stations (Dawson and College) notice a reverse response (consistent with the Araki (1994) picture) with respect to the stations located in the afternoon, i.e. with the same notation as above, a (PI+) at about 21:39 UT followed by a (MI-). In the early morning, the Kotel’nyy station shows a positive pulse of the H component. (Courtesy of CANOPUS staff and WDC for Geomagnetism, Kyoto).

6. SuperDARN: magnetic conjugacy. Fig. 12a Fig. 13a Fig. 12a: range vs time plots for the beam 6 of Stokkseyri (upper panel), and the beam 4 of Syowa South (lower panel). Poleward moving structures are seen in the first part of the figure at both radars. At about 21:38 UT, a strong increase of the LOS speed shows up close to 70 L in both hemispheres lasting for 2 min. It seems that a large convection is added to the main convection, driving the plasma poleward. At 21:43 UT, at both radars a sharp polarity reversal of the LOS velocity is clearly seen (~ 19 MLT). Fig 12: Conjugate fields of view of the Stokkseyri (red), and Syowa South (blue) radars. The radars both operated in special mode: beam 6 for Stokkseyri and beam 4 for Syowa South acquired data at high resolution (3 s). A region where similar convective structures are observed in both hemispheres is circled in black (~ 68°-73° L (N or S), ~ 19 MLT). Fig. 12 to Sun Fig. 13a: range vs time plots for the beam 10 of Thykkvibaer (upper panel), and the beam 14 of Syowa East (lower panel). The main convection points generally towards the radars in both hemispheres. Between 21:39 and 21:47 UT, in a region located at about 70 L in the late evening side, at 21:30 MLT, we observe high velocity fluxes directed away from the radar sites. The inspection of the other beams leads to think to a reconfiguration of the global convection near the Harang discontinuity. Fig. 13 to Sun Fig 13: Conjugate fields of view of the Thykkvibaer (red) and Syowa East (blue) radars. Beam 10 from Thykkvibaer (1 min resolution) and beam 14 from Syowa East (2 min resolution) appear to almost coincide. A region where similar convective structures are observed in both hemispheres is circled in black (~ 68°-73° L (N or S), ~ 22 MLT).

7. SuperDARN Global Convection Maps: Northern Hemisphere. Fig.14a Fig.14b SuperDARN Northern Global Convection Maps (APL RST software) for 21:40 – 21:42 UT (Fig. 14a) and 21:42 - 21:44 UT (Fig. 14b). Around 21:42 UT the morning convection cell expands towards the West across MLT midnight, while the Cross Polar Cap Potential does not vary much.

8. 1. 2. 3. 4. 5. 6. 7. 8. SuperDARN Global Convection Maps: Southern Hemisphere. Fig. 15 SuperDARN Southern Global Convection Maps obtained with the APL RST program for the time interval 21:34 UT – 21:50 UT. The southern radars operating during the period under study were: Kerguelen, Syowa East, Syowa South, SANAE and Halley Bay. However, the data actually available for generating the maps extend for about 6 hours MLT around midnight. Between 21:40 and 21:48 UT (panels 4 to 7) the tailward velocity flux increases around the local midnight.This period coincides with the Bx increase by 20 nT over 10 min at GEOTAIL.

9. Summary of observations • Solar wind observations. Bz and By are positive during a long period before and after the event, and the solar wind is very quiet prior to the dynamic pressure jump. • Magnetometers show clear signatures of vortex-like structures of the kind predicted by Araki (1994). These signatures are observed all over the world almost simultaneously: at about 21:39 UT the preliminary impulse starts, followed within two min by the main impulse. However, such structures are not evident in the SuperDARN northern hemisphere convection maps. • The conjugate couple Stokkseyri-Syowa South shows: • an increase of the LOS speed at 21:38 lasting for 2 min close to 70° L; • a sharp polarity reversal of the LOS velocity at 21:43 UT, corresponding to 19 MLT. • The conjugate couple Thykkvibaer-Syowa East shows high velocity fluxes directed away from the radar sites between 21:39 and 21:47 UT, in a region located at about 70° L in the late evening side, at 22 MLT. • At 21:42 UT the global northern convection map shows a sudden expansion, close to midnight MLT, of the morning convection cell towards the afternoon side. At the same time, an enhancement of the antisolar convection is seen near 24:00 MLT in the southern hemisphere. This occurs in good agreement with the observation by GEOTAIL of a 20 nT Bx increase over 10 min.

10. Conclusions The SI event we have discussed yields a reconfiguration of the ionospheric convection. Two are the main features of this reconfiguration: An expansion of the morning cell to the afternoon sector (Northern Hemisp.): this could be a local effect due to the increase of the By component of the IMF around the SI time (see Fig. 7). An increase of the antisolar convection speed around midnight (both Hemisp.): the passage of the SI along the magnetotail causes a stretch of the field lines which mainly affects the Bx component, as seen by GEOTAIL at 21:41 TU (see Fig. 8). In order to maintain the neutrality in the Neutral Plasma Sheet, the dawn-to-dusk electric field in this region must increase; the projection of the electric field upon the night side ionosphere along the magnetic field lines leads to an increase of the ionospheric E x B convection velocity, in the antisunward direction. To our knowlegde, it is the first time that such an effect is reported.