Cone angle control of the interaction of magnetic clouds with the Earth's bow shock

Transcription

1 Cone angle control of the interaction of magnetic clouds with the Earth's bow shock L. Turc1, P. Escoubet1, D. Fontaine2, E. Kilpua3 1 ESA/ESTEC, Noordwijk, The Netherlands 2 LPP-CNRS-Ecole Polytechnique-UPMC, Palaiseau, France 3 Department of Physics, University of Helsinki, Helsinki, Finland

2 Magnetic Clouds Nuages magnétiques Flux rope Zurbuchen & Richardson (2006) Magnetic clouds = subset of coronal mass ejections Properties in the solar wind (Burlaga et al., 1981): - Enhanced magnetic field - Smooth rotation of the magnetic field direction - Lower proton temperature Induce strong disturbances in Earth's environment: geomagnetic storms interest for space weather 2

3 Importance of the Bz component Southward Bz in the solar wind favourable to reconnection at the subsolar magnetopause Magnetic clouds possible long intervals of southward fields can be very geoeffective 3

4 Classification of magnetic clouds Li et al., 2011, Sol. Phys. Bipolar magnetic clouds Unipolar magnetic clouds Based on the North-South (Bz) component 4

5 Geoeffectivity of magnetic clouds Correlations between the clouds' parameters and the magnetic activity measured on the ground Gopalswamy et al., 2008, JASTP However, no one to one correlation can be found Zhang et al., 2004, JGR - clouds with southward fields no storm - large scatter for extreme events - temporal evolution? We should look in more detail at the interaction of the magnetic clouds with the Earth's environment 5

6 Bow shock and magnetosheath Magnetopause Magnetic cloud Magnetosphere Bow shock Magnetosheath The magnetosheath plasma interacts with the magnetosphere 6

7 Bow shock and magnetosheath Magnetopause Magnetic cloud Magnetosphere Enhanced magnetic field strength Modification of the cloud's magnetic structure Bow shock Slowed down and diverted flow Enhanced density and temperature Magnetosheath 7

8 Bow shock and magnetosheath Magnetopause Magnetic cloud Magnetosphere Bow shock Draping of the field lines Dawn-dusk asymmetries Accelerated flows Magnetosheath Magnetosheath properties upstream conditions for reconnection, KelvinHelmholtz instability influence the solar wind-magnetospheric coupling 8

9 A key parameter: ΘBn ΘBn: angle between the normal to the shock surface and the interplanetary magnetic field direction ΘBn> 45 : quasi-perpendicular shock ΘBn< 45 : quasi-parallel shock 9

10 A key parameter: ΘBn ΘBn controls: - the magnetosheath properties dawn-dusk asymmetries (Walsh et al., 2012, Dimmock et al. 2013, Walsh et al., 2014) influences the reconnection rate (Lavraud & Borovsky, 2008), the development of the Kelvin-Helmholtz instability (Nykyri, 2013) - the alteration of the magnetic structure of magnetic clouds across the bow shock (Turc et al., 2014, 2015) influences where reconnection occurs Can we know in advance the shock configuration? Classical picture: Parker-spiral IMF quasi-parallel on the dawnside and quasiperpendicular on the duskside. Does that still hold during magnetic clouds? How is their magnetic field oriented? 10

11 Event database From January 2000 to December magnetic clouds at Earth Year # of events Interball Geotail Cluster TC-1 Themis Orientation of the magnetic field during magnetic clouds Each event divided into 5-min samples > samples 11

12 Magnetic cone angle Nuages field magnétiques 45% of the time: cone angle between 70 and 110 Peak around 90 magnetic field ~ in the plane perpendicular to the Sun-Earth line Bx component often negligible 12

13 Magnetic clock angle Nuagesfield magnétiques One standard deviation Mean Are the variations statistically significant? More evenly distributed than the cone angle no strongly predominant direction in the YZ plane 13

14 Magnetic clock angle Nuagesfield magnétiques Orientation of the flux ropes - More SN or NS, depending on odd/even solar cycle (e.g., Li & Luhmann, 2004, Li et al., 2011) - No trend for the chirality (e.g., Li et al., 2011) Can we relate this -weak- trend to observations at the Sun? 14

15 Bow shock configuration Nuages magnétiques Bow shock model (Jerab et al., 2005) + Interplanetary magnetic field (IMF) direction Geoeffective bow shock area = from which originate all the flowlines passing within 2 Re of the dayside magnetopause ΘBn at the bow shock's surface Typical solar wind: Parker-spiral IMF Duskside: quasi-perpendicular Dawnside: quasi-parallel 15

16 Bow shock configuration Nuages magnétiques Bow shock model (Jerab et al., 2005) + Interplanetary magnetic field direction ΘBn at the bow shock's surface Most frequent orientation during magnetic clouds: B in the YZ plane Quasi-perpendicular shock configuration 16

17 Bow shock configuration Nuages magnétiques Bow shock model (Jerab et al., 2005) + Interplanetary magnetic field direction ΘBn at the bow shock's surface Radial interplanetary magnetic field Quasi-parallel shock configuration 17

18 Link with the cone angle Nuages magnétiques ~ 54% of the time: exclusively quasi-perpendicular shock Magnetosheath observations ΘBn downstream of the geoeffective bow nshock B shock solar wind Bsolar wind ~ 6.6% of the time: exclusively quasi-parallel shock Cone angle x Dashed lines: upper and lower limits of ΘBn in the geoeffective bow shock area 18

19 Nuages magnétiques Variation of the magnetic cloud's magnetic structure Bsolar wind Ψ BMsheath Ψ quantifies the variation of the cloud's structure Bsolar wind Cone angle x Cone angle ~ 90 <Ψ> ~ 20 Cone angle < 30 or > 150 <Ψ> ~ 40 19

20 Conclusions Cone angle in the solar wind range of ΘBn values on the bow shock geoeffective area shock configuration extent of the alteration of the magnetic cloud's magnetic structure 54% of the time: quasi-perpendicular shock only more favourable to accelerated flows in the magnetosheath (pile-up) Any clock angle for the magnetic field accelerated flows at any latitude influence on the development of the Kelvin-Helmholtz instability? 6.6% of the time: quasi-parallel shock only Less frequent but still significant Strong alteration of the magnetic cloud's magnetic structure Increased turbulence Influence of the foreshock? 20