Detection of Time Variable Gravity Signals using Terrestrial Clock Networks

Local gravity potential variations can be determined from the frequency differences of high-performance optical clocks at different locations. Case studies for three regions affected by different mass change processes - Himalaya, Amazon and Greenland - provide promising results. Time-varying gravity signals can be observed with clocks that achieve fractional frequency uncertainties of 10^-18 corresponding to 0.1 m²/s² in gravity potential variation. As the clocks rest on the deformable earth surface, clock observations do not only include potential variations due to mass changes but also associated variations due to the vertical deformation of the land. For the simulations, vertical displacements were derived from real GNSS measurements, and mass variations were computed from GRACE solutions. In the Himalayan region, seasonal variations with a maximum range of [-0.2 0.2] m²/s² were obtained. There, early and long-lasting precipitation patterns in North East India and the gradual spreading towards the West can be potentially observed by a dedicated clock network. In the case study for the Amazon region, seasonal variations with a maximum range of [-0.5 0.5] m²/s² to be observed by clocks also reveals the Amazon’s seasonal secrets of annual rainfall variability at the north and south of the equator. The rainy season in the north of the equator is during the summer season from June to August, but from November to April in the south of the equator. The long-term trend of the ice mass loss in Greenland between 2004 and 2015 causes signals of potential variations of 1 m²/s² that again can be observed by clock measurements. Especially, the higher rates of potential mass variations in the west and south parts of Greenland can well be observed. These examples illustrate impressively that terrestrial clock networks can be used as a modern tool for detecting various time-variable gravity signals for understanding the local patterns of the variations and for providing complementary information.    

Acknowledgment: 

This study has been funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy EXC 2123 Quantum Frontiers - Project-ID 90837967 and the SFB 1464 TerraQ - Project-ID 434617780 within project C02.