Season-dependent magnetotail By and associated field-aligned currents

A.Petrukovich1 and R. Lukianova2,1

1Space Research Institute, Moscow, apetruko@iki.rssi.ru2Arctic and Antarctic Research Institute, St. Petersburg

Model of quasi-stationary By based on GEOTAIL

Season dependent By component

Associated field aligned currents

Acknowledgements: Geotail, CCMC GSFC , RFBR-FWF grant

Bx and Bz : dipole and cross-tail currents

By: IMF By, 3D tail and FAC

Magnetosphere in XZ GSM

XGSM

ZGSM

YGSM

Season change:Dipole tilt

YGSM

ZGSM

B J

xx x x x

x x x x x

By ≠ 0 is important

Asymmetry of ionosperic footpoints in aurora

Decrease of curvature (isotropisation) in the neutral sheet Кi = Rcurv/Rci ~ (By/Bz)2

North-south asymmetry of particle trajectories (precipitation)

Effect on current sheet instabilities

Statistics Geotail

11 years 1995-2005 |Y| < 15 RE , |Y|<|X|, –31 < X < –8 RE , |Z| < 5 RE 12-sec data averaged in 15-min intervals

with >75% |Bx|<15 nT ~30 000 points

IMF penetration

iy

IMFy B)10

Y78.0cos())30|X|

1(54.033.0(B -+=

neutral sheet deformation adds a false By

use a Neutral sheet model Tsyganenko & Fairfield JGR 2004 - TF04

Z ~ X, Y, , IMF Byi Bz

i

hinge warp twist

Note: it is assumed, that magnetic field follows (on average) the neutral sheet normal.

Flaring & hinging

)BΨtanB(RB z*

xflaringy -=

However NS hinging affects flaring !

The angle is taken from TF04,R is determined from data.

Tail Flaring results in false By, odd in Y coordinate

Classic estimate: xflaringy RBB ~

Bx

Bx’

Warping

Neutral sheet profile during winter

)Θtan(BB warpz

warpy =If magnetic field is coupled with

the NS normal

warp is taken from the TF04 model and depends on season

warpΘ

Warping creates season-dependent By, odd in Y coordinate

By By

NEW even tilt-related Bytilt

Regression coeff. between tilt and By-ByIMF, By-By

IMF-Bywarp

After subtraction of Bywarp symmetric (even) By

tilt = ±1-2 nT appears

))||((

)cos())||((

30-1

1030-1

65

4321

XaaB

aBYaXaaB

BBBBB

tilty

iy

IMFy

flaringy

tilty

IMFy

warpy

My

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 RE, in degrees

Final By model

Results – 1: Model

1. Comprehensive By model includes IMF and dipole tilt driving

as well as 3D effects (NS deformation)

2 . New By component, depending on season: By>0 in summer and By<0 in winter with max ~1-2 nT (“even” component)

Cartoon of season-driven By

nT nT

nTnTeven 1-2 nT

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 2005Liou 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 By, Wolf, JGR 1970, Leontiev, G&A,1974, Atkinson & Hutchison, JGR 1978

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 By is carried by interhemispheric FAC

Magnetic storm Quiet time

magnetic perturbationsminus the main field

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

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 By, depending on the season.

Results – 3: interhemispheric FAC

warp model is correct for |Y| <10, Bywarp < 1-2 nT and has odd profile in Y

Warping

compare Bywarp and observed By-By

IMF for X<-20 and < -20o

one more new component of By = -0.75 nT ???

-0.75 nT, A season-dependent shift

warp model zone

Statistical model of field-aligned currents parameterized by season and IMF

Northern winter Southern summer

IMF clock angle

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