Ray Fernando Hidalgo Ramírez
The non-uniform rotation of the Earth is caused by various agents such as fluid motions in the core, mass redistribution, ocean and atmosphere motions. This non-uniform rotation results in fluctuations in the Earth Orientation Parameters (EOP), and we are interested in the Length of Day (LOD). LOD is determined by d(UT1 - UTC) and is related with variations in the angular moment of the system Earth-Atmosphere. On other hand, solar activity directly affects the Earth's neutral atmosphere at all altitudes. We analyze the relationship between the solar activity indexes with EOP by comparing the fluctuations in the LOD with the solar parameters. Also, we investigate the periodicities and variations in the LOD time history whose cause is not well identified so far.
After applying the law of conservation of angular momentum, we obtain the Equations 2 and 4 that relate the Atmospheric Angular Moment (AAM) and the LOD (Volland,1996; Gipson and Ma, 1999; Gipson, 2016). Also, Figure 2 shows an excellent short-term agreement between LOD and AAM (Carter and Robertson, 1986).
LOD is estimated using UT1 and UTC. UT1 is determined by from VLBI observations of the diurnal motion of distant radio sources and AAM data is derived from NCEP global circulation model. To analyze the LOD fluctuations and their relationship with solar parameters, we identified the periods in the signals using techniques such as Fast Fourier Transform (FFT), Empirical Mode Decomposition (EMD) and wavelets. Some preliminary results are shown in the poster.
We verified periods found by Sheng and Pen (2016) summarized in the Table 1 and identified periods comparable to solar activity, such as 11-year solar cycle. We found particularly interesting the possible correlation between LOD and the polar magnetic field (PMF) of the Sun. Figure 7 shows a comparison, in an arbitrary scelae, between LOD, PMF and some modes extracted using the EMD (Figure 5).