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EISCAT-Cluster observations of quiet-time near-Earth magnetotail fast flows

EISCAT-Cluster observations of quiet-time near-Earth magnetotail fast flows and their signatures in the ionosphere. T. Pitkänen (1) , A. T. Aikio (1) , O. Amm (2) , K. Kauristie (2) , H. Nilsson (3) , and K. U. Kaila (1). Department of Physics, University of Oulu, Oulu, Finland

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EISCAT-Cluster observations of quiet-time near-Earth magnetotail fast flows

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  1. EISCAT-Cluster observations of quiet-time near-Earth magnetotail fast flows and their signatures in the ionosphere T. Pitkänen(1), A. T. Aikio(1), O. Amm(2), K. Kauristie(2), H. Nilsson(3), and K. U. Kaila(1) • Department of Physics, University of Oulu, Oulu, Finland • Finnish Meteorological Institute, Helsinki, Finland • Swedish Institute of Space Physics, Kiruna, Sweden Nordic Cluster Meeting, Uppsala, Sweden, 24 – 25 August 2011

  2. Introduction • Case study of quiet-time bursty bulk flows (BBFs) on 17 Oct 2005 • Focus on signatures of BBFs in the ionosphere • In this talk: plasma flow patterns • Observations: • Cluster • EISCAT • MIRACLE all-sky camera (557 nm) and magnetometers • Meridian-scanning photometer (Hβ 486 nm) Sergeev et al., 2000

  3. Bursty bulk flows (BBFs) Baumjohann et al., 1990; Angelopoulos et al., 1992, 1994 • Transient, localized, fast, Earthward plasma flows • Convective in the central plasma sheet, field-aligned in the outer plasma sheet • Typical duration of ~ 10 min with • superimposed flow bursts of ~ 1 min • V ~ 100 – 800 km/s (max. ~ 2000 km/s), • cross-tail width 1 – 5 RE • Only ~ 5 % contribution to the plasma • flows, but 70 – 80 % to the total plasma • and magnetic flux transport in the • plasma sheet • Tend to occur during substorm expansions, but appear during all levels of geomagnetic • activity • Ionospheric signatures of BBFs include auroral streamers, which are considered to represent • upward field-aligned current flowing to the duskside flank of a BBF Sergeev et al., 2000

  4. 17 Oct 2005 Geomagnetic conditions • AE indices indicate gradually weakening convection electrojets after 17 UT decrease of Bz After Walsh et al., 2009  Between 18 – 19 UT, the near-Earth space was on quiet state

  5. 17 Oct 2005 Cluster satellites 18:00 – 19:00 UT Cluster in the evening sector southern plasma sheet (plasma β > 0.5) XGSM ~ –12 to –14 RE 06 00 12 18 After Walsh et al., 2009 Sun

  6. 17 Oct 2005 Ground-based measurements Svalbard geomagn. north • Red: The mainland EISCAT VHF near Tromsø; • The ESR 32m and 42m antennas on • Svalbard • Yellow:MIRACLE Kevo all-sky camera • (KEV ASC) • Blue:MIRACLE magnetometer stations • Green: Meridian-scanning photometer • (KIL MSP) operated at Kilpisjärvi Footpoints of Cluster (T96) Field-of-view of the KEV ASC KIL MSP

  7. 17 Oct 2005 Cluster C1Observations BBF proper flows Earthward CIS HIA data duskward • After 18:15 UT a series of • 3 flow events identified • Deflection and compression of ambient plasma in front of an approaching plasma bubble • (e.g. Sergeev et al., 1996; Snekvik et al., 2007) • Tailward flows are consistent with return flow patterns around edges of a bubble (Pontius and Wolf, 1990) tailward dawnward dawnside Deflection bursts duskside Return flows FGM data Walsh et al., 2009 CIS HIA data Plasma compression non reduction during dawnside return flows n reduction during duskside return flows, which could be signature of a wake behind the moving bubble suggested by Walsh et al. (2009)

  8. Kevo all-sky camera 17 Oct 2005 18:25:00 – 18:57:00 UT, 3 frames/min UT MLT

  9. 17 Oct 2005 17 Oct 2005 EISCAT vs.Cluster C1 CIS HIA • Auroral streamers seen by VHF in • Te can be directly associated to • the BBFs by Cluster • Streamers are associated with • - sharp velocity shear at the equat- • orward edge of a streamer • - equatorward component in the • plasma flow within the streamer • - reduced Ne and enhanced Ti in • poleward flows C4 CIS CODIF (proton) equatorwards equatorwards line-of- sight pole- wards polewards streamers PCB polar cap boundary (PCB) Erec (mV/m) PCB UT MLT 21:00 21:30 UT MLT

  10. BBF-streamer ( 2) geomagnetic north velocity shear 20:50 MLT streamer 2-D equivalent currents from MIRACLE data (Amm, 1997; Amm and Viljanen, 1999) proton oval 22:20 MLT ~18:31 – 18:34 UT (frame d) C1 Vx, Vy Earthward duskward tailward dawnward • The streamer evolution in the optical EISCAT and ASC data is consistent with the Cluster data 18:30 18:35 • Cluster and EISCAT see dawnside and duskside return flows, respectively (frame e) • Cluster in the tail and EISCAT in the ionosphere see the same BBF proper flow (frame d) • Cluster sees the plasma deflection when mapped roughly in front of the streamer (frame c)

  11. BBF-streamer ( 3) geomagnetic north velocity shear 21:00 MLT streamer 22:30 MLT ~18:42 – 18:45 UT (frame b) Earthward tailward 18:40 18:45 18:50

  12. 17 Oct 2005 Summary and conclusions • The BBFs at Cluster • agree with the bubble model (Pontius and Wolf, 1990), including tailward return flows • tailward return flows show an interesting asymmetry in plasma density The duskside return flows were associated with decrease in plasma density. However, no similar feature was seen for the dawnside return flows, but rather an increase in density • decrease in plasma density in duskside return flows gives some support to the recent • suggestion of formation of a depleted wake (Walsh et al., 2009) • The BBFs in the ionosphere • auroral streamers and associated plasma flows agree with the simultaneous Cluster data • (T96 mapping), which suggests that • EISCAT saw the ionospheric counterpart of BBFproper and return flows observed by Cluster • in the magnetotail • asoutheastward streamer-aligned plasma flow roughly poleward of the streamer is the ion- • ospheric manifestation of the BBF proper flow channel. A northwestward low-density plasma • flow on the equatorward side of a velocity shear corresponds to the duskside return flow After Walsh et al., 2009 Walsh et al., 2009 See: Pitkänen et al., Ann. Geophys. 29, pp. 299–319, 2011

  13. 17 Oct 2005 Summary and conclusions • The BBFs at Cluster • agree with the bubble model (Pontius and Wolf, 1990), including tailward return flows • tailward return flows show an interesting asymmetry in plasma density The duskside return flows were associated with decrease in plasma density. However, no similar feature was seen for the dawnside return flows, but rather an increase in density • decrease in plasma density in duskside return flows gives some support to the recent • suggestion of formation of a depleted wake (Walsh et al., 2009) • The BBFs in the ionosphere • auroral streamers and associated plasma flows agree with the simultaneous Cluster data • (T96 mapping), which suggests that • EISCAT saw the ionospheric counterpart of BBFproper and return flows observed by Cluster • in the magnetotail • asoutheastward streamer-aligned plasma flow roughly poleward of the streamer is the ion- • ospheric manifestation of the BBF proper flow channel. A northwestward low-density plasma • flow on the equatorward side of a velocity shear corresponds to the duskside return flow After Walsh et al., 2009 See: Pitkänen et al., Ann. Geophys. 29, pp. 299–319, 2011

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