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Season-dependent magnetotail By and
associated field-aligned currents
A.Petrukovich1 and R. Lukianova2,1
1Space
2Arctic
Research Institute, Moscow, apetruko@iki.rssi.ru
and Antarctic Research Institute, St. Petersburg
пѓјModel of quasi-stationary By based on GEOTAIL
пѓјSeason dependent By component
пѓјAssociated field aligned currents
Acknowledgements: Geotail, CCMC GSFC , RFBR-FWF grant
Magnetosphere in XZ GSM
ZGSM
XGSM
YGSM
Season change:
Dipole tilt
Bx and Bz : dipole and cross-tail currents
By: IMF By, 3D tail and FAC
By в‰ 0 is important
пѓј
Asymmetry of ionosperic footpoints in aurora
пѓј Decrease of curvature (isotropisation) in the neutral sheet
пѓј
Рљi = Rcurv/Rci ~ (By/Bz)2
пѓј North-south asymmetry of particle trajectories (precipitation)
пѓј Effect on current sheet instabilities
B x Jx
x
x
x
ZGSM
YGSM
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
B
IMF
y
= ( 0 . 33 + 0 . 54
(1 -
|X |
Y
))
cos(
0
.
78
30
10 ) B y
i
neutral sheet deformation adds a false By
use a Neutral sheet model Tsyganenko & Fairfield JGR 2004 - TF04
Z ~ X, Y, Y, IMF Byi Bzi
hinge
warp
twist
Note: it is assumed, that magnetic field follows (on average)
the neutral sheet normal.
Flaring & hinging
Tail Flaring results in false By, odd in Y coordinate
flaring
Classic estimate: B y
~ RB x
However NS hinging affects flaring !
Bx
Bx’
flaring
By
*
= R ( B x - tan ОЁ B z )
The angle Y* is taken from TF04,
R is determined from data.
Warping
Warping creates season-dependent By, odd in Y coordinate
О�
By
warp
Neutral sheet profile during winter
By
warp
If magnetic field is coupled with
By
= B z tan(
the NS normal
Qwarp is taken from the TF04 model and depends on season
О�
warp
)
NEW even tilt-related Bytilt
Regression coeff. between tilt and
By-ByIMF,
By-ByIMF-Bywarp
After subtraction of Bywarp symmetric (even) Bytilt = В±1-2 nT appears
Final By model
By пЂЅ By
M
IMF
By
B
tilt
y
warp
пЂ« By
IMF
пЂЅ ( a 1 пЂ« a 2 (1 пЂЅ ( a 5 пЂ« a 6 (1 -
пЂ« By
пЂ« By
tilt
flaring
|X |
30
|X |
30
Magnetic field in nT, X, Y in RE,
)) cos( a 3 Y
)B y пЂ« a 4
i
10
)) Y
Y
in degrees
a1=0.3247В±0:015, a2=0.5827В±0.042, a3=0.850В±0.066, a4=0.0187В±0.032
a5=0.0305В±0.0027, a6=0.0614В±0.0088
Key coefficients are defined with accuracy 5%
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
nT
even
1-2 nT
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
Г�stgaard 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 By,
Wolf, JGR 1970, Leontiev, G&A,1974, Atkinson & Hutchison, JGR 1978
Season-dependent By
is carried by interhemispheric FAC
Low-orbiting satellites Г�rsted, CHAMP, Magsat
пѓј
low-altitude polar orbit slowly drifting in local time
пѓј
high precision magnetometers
пѓј 26 million samples above В±45Лљ CGL,
all local times and seasons in both hemispheres
Magnetic storm
Quiet time
magnetic perturbations
minus the main field
Decomposition of seasonal effect
For IMF By=0, Bz=-5 nT
North
South
Revealing seasonal
interhemispheric FAC
summer map - winter map
Expected
interhemispheric FAC trace
– positive at midnight
Results – 3: interhemispheric FAC
пѓ�
Comprehensive decomposition of the statistical maps of FAC
derived from magnetic measurements by Г�ersted, CHAMP
and Magsat satellites shows presence of interhemispheric
FAC (IHFAC).
пѓ�
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.
пѓ�
The current density is about 0.1 ОјРђ m-2.
пѓ�
The value and direction of IHFAC is consistent with the observed
magnetotail By, depending on the season.
Warping
compare Bywarp and observed By-ByIMF for X<-20 and Y < -20o
warp model zone
-0.75 nT,
A season-dependent shift
warp model is correct for |Y| <10, Bywarp < 1-2 nT and has odd profile in Y
one more new component of By = -0.75 nT ???
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.
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