On the nonstationarity of the decadal periodicities of the length of day

Abstract

Abstract. The Earth's rotation rate is not constant, but changes on all observable timescales, from subdaily to decadal and longer. These variations are usually discussed in terms of variations in the length of the day (LoD) and are caused by processes acting within the interior, at the surface and outside of the Earth. Here, we investigate the presence of long-standing decadal variations in yearly LoD data covering the period from 1832 to 2009 by applying the HilbertHuang transform (HHT). The HHT has been slightly modified here to take into account the uncertainty of LoD values that has changed greatly in time due to the use of different LoD measurement techniques. The LoD time series has been completely decomposed into five intrinsic mode functions (IMF) and a residual trend. The estimation of instantaneous frequencies and related amplitudes of the obtained IMFs has allowed us to compute the Hilbert spectrum that has been used as the starting point for studying and discussing the stationarity of typical LoD timescale stationarity. The obtained results while showing the presence of multiple periodicities also indicate the absence of really stationary periodicities. Therefore, rather than considering the processes taking place in the Earth's core as the result of a superposition of oscillations (i.e. stationary mechanisms) occurring on a discrete number of different timescales, it would be better to think of a superposition of fluctuations that are intermittent in both frequency and amplitude.

Brief Communication: Correlation of global earthquake rates with temperature and sunspot cycle

“…The observed 11 year solar induced cyclic variation in the LOD is one of the evidences for such periodic forcing (Challinor, 1971;Curries, 1981). The other slower cycles reported by them are also confirmed in a recent work (Michelis et al, 2013) and probably also observed after very large individual solar flare (Danjon, 1962a, b;Gribbin and Plagemann, 1973;Stothers, 1989). It appears, therefore, that atmospheric Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | disturbances/perturbations might some way or other is involved in catalyzing the small abrupt/instantaneous changes in the earth's rate of rotation.…”

Hints on the Multiscale Nature of Geomagnetic Field Fluctuations During Quiet and Disturbed Periods

“…This method, introduced by Huang and collaborators (Huang et al, 1996(Huang et al, , 1998(Huang et al, , 1999 about 20 years ago, has been successfully applied in different fields ranging from the analysis of acoustic signals to biological, oceanic, seismic, and climatological signals (Gloersen & Huang, 1999;Hu et al, 2002;Loh et al, 2001). It has also been applied to geomagnetic time series for example in order to study the multiscale features of intense geomagnetic storms and their possible sources (De Michelis et al, 2012;Alberti et al, 2016Alberti et al, , 2017, to extract planetary wave modes from magnetic field components (Frühauff et al, 2015) and to investigate the properties of decadal variations in the length of day (De Michelis et al, 2013;Roberts et al, 2007).…”

Intermittent transport events in a cylindrical plasma device: experiment and simulation

“…This can be observed in a variety of physical systems, e.g. in turbulent hydrodynamic flows [2], magnetohydrodynamic turbulence in geophysics [3], in the solar wind [4,5], and in plasma turbulence [6,7]. It is known since decades that intermittency in hydrodynamic turbulence is linked to the formation of turbulent coherent structures (vortices) [8].…”