1. Season-dependent magnetotail B y and associated field-aligned currents A.Petrukovich 1 and R. Lukianova 2,1 1 Space Research Institute, Moscow, apetruko@iki.rssi.ru 2 Arctic and Antarctic Research Institute, St. Petersburg Model of quasi-stationary B y based on GEOTAIL Season dependent B y component Associated field aligned currents Acknowledgements: Geotail, CCMC GSFC, RFBR-FWF grant

2. B x and B z : dipole and cross-tail currents B y : IMF B y, 3D tail and FAC Magnetosphere in XZ GSM X GSM Z GSM Y GSM Season change: Dipole tilt

3. Y GSM Z GSM B J x x x xx xxxx x B y ≠ 0 is important Asymmetry of ionosperic footpoints in aurora Decrease of curvature (isotropisation) in the neutral sheet К i = R curv /R ci ~ (B y /B z ) 2 North-south asymmetry of particle trajectories (precipitation) Effect on current sheet instabilities

4. Statistics Geotail 11 years 1995-2005 |Y| < 15 R E, |Y|<|X|, –31 < X < –8 R E, |Z| < 5 R E 12-sec data averaged in 15-min intervals with >75% |B x |< 15 nT ~30 000 points

5. IMF penetration

6. neutral sheet deformation adds a false B y use a Neutral sheet model Tsyganenko & Fairfield JGR 2004 - TF04 Z ~ X, Y, , IMF B y i B z i hinge warptwist Note: it is assumed, that magnetic field follows (on average) the neutral sheet normal.

7. Flaring & hinging However NS hinging affects flaring ! The angle    is taken from TF04, R is determined from data. Tail Flaring results in false B y, odd in Y coordinate Classic estimate: Bx Bx’

8. Warping Neutral sheet profile during winter If magnetic field is coupled with the NS normal  warp is taken from the TF04 model and depends on season Warping creates season-dependent B y, odd in Y coordinate ByBy ByBy

9. NEW even tilt-related B y tilt Regression coeff. between tilt and B y -B y IMF, B y -B y IMF -B y warp After subtraction of B y warp symmetric (even) B y tilt = ±1-2 nT appears

10. a1=0.3247±0:015, a2=0.5827±0.042, a3=0.850±0.066, a4=0.0187±0.032 a5=0.0305±0.0027, a6=0.0614±0.0088 Key coefficients are defined with accuracy 5% Magnetic field in nT, X, Y in R E,  in degrees Final B y model

11. Results – 1: Model 1.Comprehensive B y model includes IMF and dipole tilt driving as well as 3D effects (NS deformation) 2. New B y component, depending on season: B y >0 in summer and B y <0 in winter with max ~1-2 nT (“even” component) Cartoon of season-driven B y nT even 1-2 nT

12. Results – 2: Even tilt effect Earlier hints: Fairfield, JGR 1979 for IMP-6 (winter) By(tail)=0.13 By(ip) – 0.3 auroral breakup asymmetry Østgaard et al, GRL 2005 Liou and Newell, GRL 2010 Reinforcement of IMF in polar cap convection for combinations summer/Byi+ and winter/Byi- (Ruohoniemi & Greenwald JGR 2005) Conductivity gradient “day-night” skews polar convection as IMF B y, Wolf, JGR 1970, Leontiev, G&A,1974, Atkinson & Hutchison, JGR 1978

13. Low-orbiting satellites Ørsted, CHAMP, Magsat low-altitude polar orbit slowly drifting in local time high precision magnetometers 26 million samples above ±45˚ CGL, all local times and seasons in both hemispheres Season-dependent B y is carried by interhemispheric FAC Magnetic storm Quiet time magnetic perturbations minus the main field

14. Decomposition of seasonal effect North South Revealing seasonal interhemispheric FAC summer map - winter map Expected interhemispheric FAC trace – positive at midnight For IMF By=0, Bz=-5 nT

15.  Comprehensive decomposition of the statistical maps of FAC derived from magnetic measurements by Øersted, CHAMP and Magsat satellites shows presence of interhemispheric FAC (IHFAC).  The IHFACs are downward (upward) in the summer (winter) hemisphere being located mainly in the near-noon and premidnight-midnight MLT sectors in the vicinity of the polar cap boundary.  The current density is about 0.1 μА m -2.  The value and direction of IHFAC is consistent with the observed magnetotail B y, depending on the season. Results – 3: interhemispheric FAC

16. warp model is correct for |Y| <10, B y warp < 1-2 nT and has odd profile in Y Warping compare B y warp and observed B y -B y IMF for X <-20 and  < -20 o one more new component of B y = -0.75 nT ??? -0.75 nT, A season-dependent shift warp model zone

17. Statistical model of field-aligned currents parameterized by season and IMF Northern winterSouthern summer IMF clock angle

18. Meridional profiles of the FAC density averaged over 3h MLT sectors.