Determination of Forced Diurnal Polar Motion Using Large Ring Lasers Gyroscopes

In October 2001, a very large ring laser gyroscope with an area of 16~m² was inaugurated at the Fundamental Station in Wettzell, Bavaria. Large gyroscopes with areas of 1~m², 16~m², and 367~m² are also operational at the University of Canterbury in Christchurch, New Zealand. These ring lasers, measuring the frequency difference between two counter-rotating laser beams, are now approaching a precision of 10~ppb for geophysical signals with periods of less than 2~days and therefore allow the monitoring of variations in Earth rotation (polar motion and length of day) in the high-frequency spectrum. For the first time, forced diurnal polar motion (also called "Oppolzer terms", caused by the gravitational attraction of Sun and Moon) with variations up to 20~mas (60~cm on the Earth's surface) can directly be measured. This is a novelty, because with the space geodetic techniques like VLBI, GPS, SLR/LLR, and DORIS, nutation and forced diurnal polar motion cannot be separated. With a few months of data stemming from the two most accurate ring lasers, the amplitudes of the largest terms of forced diurnal polar motion have been determined with a precision of about 1~mas. The results are discussed and compared to theoretical models.