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External drivers of decadal variations in the geomagnetic field
and Earth’s rotation
C. Demetrescu (1), V. Dobrica (1), K. Georgieva (2), B. Kirov (2)C. Demetrescu (1), V. Dobrica (1), K. Georgieva (2), B. Kirov (2)(1) Institute of Geodynamics of the Romanian Academy, [email protected]
(2) SRTI, Bulgarian Academy of Science
Acknowledgements: This work was supported by a grant of the Romanian Ministery of Research and
Innovation, CCDI-UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0266/SAFESPACE,
within PNCDI III and the Joint Research Project between Bulgarian and Romanian Academies
Outline
- Earth’s rotation rate/LOD
- Motivation
- Magnetospheric ring-current- Magnetospheric ring-current
- Analysis method
- Decadal (8-13 years) & intra-decadal (6 years)
variations
- Concluding remarks
Earth’s rotation rate (LOD = length of day)
Principle: conservation of the
Fluctuations:
- variable external couples
- internal mass redistribution
- hydrodynamic and
magnetohydrodynamic tensions at
fluid/solid interfaces
Principle: conservation of the
angular momentum of Earth
In general: the transfer of the angular
momentum between the solid part of the
Earth (mantle + inner core) and fluid
volumes in contact:
- Atmosphere + Oceans
- Outer core
0
2
4
LOD (ms)
LOD – Earth’s rotation
1800 1850 1900 1950 2000
Year
-4
-2
•https://www.iers.org/IERS/EN/DataProducts/EarthOrientationData/eop.html
Explanation (sources):- Zonal movements of the outer core fluid (geodynamo) ..... geomagnetic data
Le Mouel et al., Nature, 1981: 1860-1975
Earth rotation rate linked to geomagnetic
field variations (D) (jerks)
Motivation: LOD – Earth’s magnetic field
Gross R., Physical Geodesy,
Treatise on Geophysics, 2007
0
1
2
3
4
0.2
0.16
0.12
0.08
0.04
0
dD/dt
LOD
0.06
0.08
30
40
Motivation: LOD – Earth’s magnetic field
1880 1900 1920 1940 1960 1980 2000 2020
Year
-1 0.24
0
0.02
0.04
0.06
100 10 1
Period (years/cycle)
0
10
20
30
Dobrica&Demetrescu, 2019, submitted
- sub-centennial timescale (60-90 years): 1.5 ms
- inter-decadal timescale (20-35 years): 1 msZatmann&Bloxham, Nature, 1997
Torsional oscillations
0
2
4
LOD (ms)
Motivation: LOD – External sources
1800 1850 1900 1950 2000
Year
-4
-2
•https://www.iers.org/IERS/EN/DataProducts/EarthOrientationData/eop.html
Explanation (sources):- Zonal movements of the outer core fluid (geodynamo) ..... geomagnetic data
- Fluctuations in the magnetospheric ring current
Magnetospheric ring current. Geomagnetic indices
Iyemori (1990); http://wdc.kugi.kyoto-u.ac.jp/
adapted from Shen et al. (2015)
Iyemori (1990); http://wdc.kugi.kyoto-u.ac.jp/
Sugiura (1964); http://wdc.kugi.kyoto-u.ac.jp/
0 10 20 30 40
-40
-30
-20
-10
0
10
Dst (nT)
Dst = -0.939 * aa + 5.78
R2 = 0.64
c.c. = -0.8
Magnetospheric ring current. Dst reconstruction to 1868
aa (nT)
Dst (n
aa
Dst
0 10 20 30 40
aa (nT)
nT)
-4
-2
0
2
4
-0.4
0
0.4
0.8
Analysis of LOD and ring current decadal variations
Data Trend + cyclic HPfilter
Hodrick&Prescott (1997)
1800 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 2020
Year
-0.8
-0.4
Dst (nT)
Dst (n
T)
Decadal variations in LOD, Ring current, dD/dt
LOD
dD/dt
- decadal timescale (9-13 years): 0.5 ms
Intra-decadal variations in LOD, Ring current, dD/dt
dD/dt(6)
decadal
intra-decadal
dD/dt
LOD
- intra-decadal timescale
(6 years): 0.15 ms
Debated causal chain for the 6-year variation
•••• geomagnetic field
- fast torsional waves with a 6-year
variation period (Gillet et al.,
Nature, 2010)
•••• gravitational coupling
- exchange of angular momentum
between the mantle and inner core
MICG (Mound&Buffet, EPS, 2006; Duan
et al., EPS, 2015; PEPI, 2017; EPSL,
2018; Ding&Chao, EPSL, 2018; Ding,
2019)
- geomagnetic jerks (Holme&deViron,
Nature, 2013)
2019)
- we favour the magnetospheric ring current contribution
Concluding remarks
- The contribution of the magnetospheric ring current to variations in
geomagnetic field and LOD at decadal (8-13 years) and intra-decadal (6
years) timescales was investigated;
- We characterized the amplitudes involved in variations at the two timescales:
Decadal (8-13 years) ring current: -15 nT
geomagnetic declination: 1.5 min
LOD: 0.5 ms
Intra-decadal (6 years) ring current: -10 nT
geomagnetic declination: 0.9 min
LOD: 0.15 ms
- Debated causal chain for the 6 years variation. We add the contribution of
external sources of the geomagnetic field (magnetospheric ring current) to
the three previously forwarded possibilities (fast torsional waves,
implication of geomagnetic jerks, mantle – inner core gravitational
coupling).
Thank you for attention !