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, crisan@geodin.ro

(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 !