Sebastien Lambert

Detailed Analysis of Diurnal Tides and Associated Space Nutation in the Search of the Free Inner Core Nutation Resonance

We propose a comparison of the tidal analysis results obtained from the continuous records of time-varying surface gravity collected by a worldwide network of Superconducting Gravimeters with the analysis results of space nutation observed by the international Very Long Baseline Interferometry (VLBI) network. The length of the surface gravity time series (20 years for the longest) enables now to look for additional diurnal tides that were previously not analyzed. In parallel, we now possess 35 years of VLBI data permitting to look for additional nutation terms. We focus our analysis on the diurnal prograde frequency band in the search for a possible resonance effect linked to the Free Inner Core Nutation. This Earth’s normal mode has never been clearly observed. Its direct deformation effect at the Earth’s surface is theoretically predicted to be too small to be detected. However, the tidal forcing at a frequency close to its eigenfrequency could enhance some tidal or nutation amplitude resulting in the characterization of this mode through its resonance effect.

Testing general relativity with geodetic VLBI: What a single, specially designed experiment can teach us

Context: We highlight the capabilities of geodetic VLBI technique to test general relativity in the classical astrometric style, i.e. measuring the deflection of light in the vicinity of the Sun. Aims: In previous studies, the parameter γ was estimated by global analyses of thousands of geodetic VLBI sessions. Here we estimate γ from a single session where the Sun has approached two strong reference radio sources, 0229+131 and 0235+164, at an elongation angle of 1–3◦. Methods: The AUA020 VLBI session of 1 May 2017 was designed to obtain more than 1000 group delays from the two radio sources. The solar corona effect was effectively calibrated with the dual-frequency observations even at small elongation. Results: We obtained γ with a greater precision (0.9 × 10−4) than has been obtained through global analyses of thousands of standard geodetic sessions over decades. Current results demonstrate that the modern VLBI technology is capable of establishing new limits on observational tests of general relativity.

Atmospheric torques and Earth's rotation: what drove the millisecond-level length-of-day response to the 2015–2016 El Niño?

El Niño–Southern Oscillation (ENSO) events are classically associated with a significant increase in the length of day (LOD), with positive mountain torques arising from an east–west pressure dipole in the Pacific driving a rise of atmospheric angular momentum (AAM) and consequent slowing of the Earth’s rotation. The large 1982–1983 event produced a lengthening of the day of about 0.9 ms, while a major ENSO event during the 2015–2016 winter season produced an LOD excursion reaching 0.81 ms in January 2016. By evaluating the anomaly in mountain and friction torques, we found that (i) as a mixed eastern–central Pacific event, the 2015– 2016 mountain torque was smaller than for the 1982–1983 and 1997–1998 events, which were pure eastern Pacific events, and (ii) the smaller mountain torque was compensated for by positive friction torques arising from an enhanced Hadley-type circulation in the eastern Pacific, leading to similar AAM–LOD signatures for all three extreme ENSO events. The 2015–2016 event thus contradicts the existing paradigm that mountain torques cause the Earth rotation response for extreme El Niño events. Copyright statement. All the authors have agreed to the licence and copyright agreement.