3:30 PM - 5:00 PM
[SGD01-P15] Simple estimation of the upper-bound of the Q of Chandler Wobble taking advantage of its non-excitation
Keywords:Chandler wobble, Q, non-excitation
The quality factor (Q) of the Chandler Wobble (CW) is a unique parameter that can constrain the frequency dependent deformation response of solid Earth, particularly the lower mantle, and bridge the orders-of-magnitude gap between the seismological, semi-diurnal, and long-period (18.6 year) tidal frequencies. However, there is still a three-fold variation in the estimated Q of the CW from 50 to 179, which indicates our incomplete understanding of the excitation sources. Yamaguchi and Furuya (2021, Geod. Soc. Jpn.; 2022, JpGU; 2022, AGU) discovered that the CW has been absent since ~2015, which was the first-time in the observation history of CW.
We can exploit the recent non-excitation of the CW to more simply estimate the Chandler period (P) and Q without using any geophysical excitation data. While we can compute "geodetic CW" by re-integrating the non-seasonal "geodetic excitation" derived with the prescribed P and Q, we should note that the CW could also be derived without assuming any pairs of the P and Q but by simply taking out the annual wobble (AW) from the polar motion time-series data; we here assume that the amplitude changes of AW can be negligible. Denoting the geodetic CW as the CWGEOD and the latter as the CWOBS, we compare a series of CWGEOD with CWOBS. Because the two CW are in disagreement during the early- to mid-period due to the lack of the freely damping term in the CWGEOD, we focus on to what extent the CWGEOD and the CWOBS are matching during the recent non-excited period. While this analysis is not strongly sensitive to the P, it turns out that the larger Q (>100) is unlikely and should be below 50. Although the lower Q as much as 25 also gives good agreement, we will have to include the excitation sources to constrain the lower bound of Q. Indeed, Yamaguchi and Furuya showed that the Q~25 is too low to account for the CW in 1980-2000s.
We can exploit the recent non-excitation of the CW to more simply estimate the Chandler period (P) and Q without using any geophysical excitation data. While we can compute "geodetic CW" by re-integrating the non-seasonal "geodetic excitation" derived with the prescribed P and Q, we should note that the CW could also be derived without assuming any pairs of the P and Q but by simply taking out the annual wobble (AW) from the polar motion time-series data; we here assume that the amplitude changes of AW can be negligible. Denoting the geodetic CW as the CWGEOD and the latter as the CWOBS, we compare a series of CWGEOD with CWOBS. Because the two CW are in disagreement during the early- to mid-period due to the lack of the freely damping term in the CWGEOD, we focus on to what extent the CWGEOD and the CWOBS are matching during the recent non-excited period. While this analysis is not strongly sensitive to the P, it turns out that the larger Q (>100) is unlikely and should be below 50. Although the lower Q as much as 25 also gives good agreement, we will have to include the excitation sources to constrain the lower bound of Q. Indeed, Yamaguchi and Furuya showed that the Q~25 is too low to account for the CW in 1980-2000s.