EGU General Assembly 2020
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Length of day fluctuations at long and short timescales. Geomagnetic drivers

Crisan Demetrescu and Venera Dobrica
Crisan Demetrescu and Venera Dobrica
  • Institute of Geodynamics, Romanian Academy, Bucharest, Romania (

We decompose the well known LOD time series provided by IERS, that shows so-called decadal variations, in fluctuations at several timescales, namely: sub-centennial (60-90 years), inter-decadal (20-35 years), decennial (~11 years) and intra-decennial (~6 years). A Hodrick and Prescott (1977) type of analysis is used, followed by the decomposition of the trend and oscillatory parts at the mentioned timescales using Butterworth filtering. Comparing the results to previously (e.g. Dobrica et al., 2018) known oscillations of the geomagnetic field (dD/dt), and carrying out a similar analysis for parameters describing the evolution of the magnetospheric ring current, suggest the latter is the ultimate driver of both geomagnetic and LOD variations. The probable mechanisms are discussed as well: Alfvén torsional oscillations in the outer core, triggered by variations in the magnetospheric ring current, or a direct control of geomagnetic declination by variations in the magnetospheric ring current. While the first one is long accepted for the long-term variations in D and LOD, a similar possibility for the 6-year variation is out of question due to the implied value of the geomagnetic field within the outer core (Gillet et al., 2010); for the latter we suggest the second mechanism.       

How to cite: Demetrescu, C. and Dobrica, V.: Length of day fluctuations at long and short timescales. Geomagnetic drivers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13127,, 2020

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Display material version 1 – uploaded on 29 Apr 2020
  • CC1: Comment on EGU2020-13127, Paul PUKITE, 07 May 2020

    Since the dLOD is dominated by variations caused by tidal/gravitational forcing, why isn't the  6-year cycle traced to the interaction between the nodal (18.6y) and perigean (8.85y) cycle?

    • AC1: Reply to CC1, Crisan Demetrescu, 07 May 2020

      It was shown by Gillet et al. (2010) that the 6-year varaition in LOD can be linked to variations in the geomagnetic field produced in the Earth's fluid outer core by Alfven waves and torssional oscillations of the appropriate period. We do not reject the possibility that two lunar cycles contribute as well. Thanks.

      • CC3: Reply to AC1, Paul PUKITE, 07 May 2020

        Thanks. I have fit the derivative of the dLOD signal very effectively to tidal signals, to the point that the 18.6 year nodal cycle is well resolved (see figure).  If the 6-year signal is also in the fit, it is fairly weak in comparison to the obvious 18.6 year cycle, so good work in being able to extract it!

  • CC2: Comment on EGU2020-13127, Christian Bizouard, 07 May 2020

    What mean notations Dst, D, aa? I guess that D is the geomagnetic declination? 

    • AC2: Reply to CC2, Crisan Demetrescu, 07 May 2020

      Dst and aa are geomagnetic indices and D, as you guessed, is geomagnetic declination. Dst accounts for storm time geomagnetic disturbance and aa accounts for global geomagnetic activity at mid-latitudes.