Season-dependent magnetotail By and associated field-aligned currents
A.Petrukovich1 and R. Lukianova2,1
1Space Research Institute, Moscow, [email protected] 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.