MFjsr2001

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Published on November 15, 2007

Author: Natalia

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Slide1:  How much may one « cheat » the non-rigid Earth nutation theory to make it match VLBI results? M. Feissel (1, 3), M. Yseboodt (2), V. Dehant (2), O. de Viron (2), C. Bizouard (1) (1) Observatoire de Paris (2) Royal Observatory of Belgium (3) Institut Géographique National - MHB2000 methodology and environment - Excitation of nutations by the atmosphere - Stability of the VLBI celestial reference frame - Discrepancies between observation and theory - How much can one change a part of the parameters in the model and still match the observed values of others? MHB2000 methodology and environment:  MHB2000 methodology and environment Adopt a rigid Earth nutation model (astronomical) oceanic tidal excitations mean values for atmospheric and non-tidal oceanic excitations Construct a theoretical transfer function of the non-rigid Earth based on realistic physical properties Adjust (least squares) parameters in the theory to fit theoretical predictions to VLBI-derived amplitudes Adjusted parameters concern: Dynamical ellipticities of mantle and fluid core Magnetic couplings at Mantle-Core and Core-Inner Core boundaries Free nutations amplitudes (FCN, FICN) ... Atmospheric excitation of nutation (AER, pressure term) Wavelet transform of the Prograde component:  Atmospheric excitation of nutation (AER, pressure term) Wavelet transform of the Prograde component Atmospheric excitation of nutation (AER, pressure term) Wavelet transform of the Retrograde component:  Atmospheric excitation of nutation (AER, pressure term) Wavelet transform of the Retrograde component The atmospheric excitation of nutation:  The atmospheric excitation of nutation Origin: diurnal waves (solar heating) AAM series (1958 =>) corrected for non-IB at diurnal frequencies Results for the Retrograde Annual nutation Maximum excitation frequency changes in time Amplitude is variable Stability of the extragalactic reference frame Source position time series at 0.5-year intervals, 1984-1999 (from Eubanks, 1999):  Stability of the extragalactic reference frame Source position time series at 0.5-year intervals, 1984-1999 (from Eubanks, 1999) Stability of the extragalactic reference frame:  Stability of the extragalactic reference frame Slide8:  Analysis of VLBI time series - VLBI data (available at the IERS/EOP Product Center): time series of celestial pole offsets <= 24-hour sessions ------------------------------------------------------------ Institute Data span No of StError Stdev (mas) No of Series points (mas) /MHB2000 sources ------------------------------------------------------------- BKG 01 R 01 1984.0-2001.0 2273 0.19 0.22 578 GSFC 01 R 01 1980.0-2000.9 2696 0.27 0.23 552 IAA 01 R 01 1980.0-2001.2 2155 0.17 0.25 667 SHA 01 R 01 1980.0-2001.2 2735 0.24 0.23 675 USNO 99 R 03 1980.0-2001.0 2489 0.30 0.25 652 ------------------------------------------------------------- - Form differences of the five VLBI series with the MHB2000 model - Estimate linear (precession, obliquity rate) + 18.6 year terms on the total data span - Estimate 1025d + 430d + annual + semi-annual nutations on 6-year data spans - « VLBI result »: weighted mean of the five solutions Estimation of the linear and 18.6-year nutation terms based on 13- through 21-year time series of the celestial pole offsets obtained by GSFC (o) and IAA(star):  Estimation of the linear and 18.6-year nutation terms based on 13- through 21-year time series of the celestial pole offsets obtained by GSFC (o) and IAA(star) Precession correction and obliquity rate: MHB2000 and VLBI values :  Precession correction and obliquity rate: MHB2000 and VLBI values ---------------------------------------------------------------- Data Precession corr. Obliquity rate ---------------------------------------------------------------- MHB2000 -2.997 -0.255 VLBI-MHB +0.024 +-.002 +0.015 +-.002 DCRF +0.010 +-.002 -0.008 +-.001 ---------------------------------------------------------------- Corrections to the MHB2000 18.6 years nutation term:  Corrections to the MHB2000 18.6 years nutation term - VLBI : obtained from the analysis of the VLBI series -  +CRF : corrected for the celestial pole motion effect - +Atmo: corrected for the atmospheric excitation (+Atmo) Unit: 0.001'' Prograde and retrograde components of the annual nutation derived from VLBI observations. Unit: 0.001'':  Prograde and retrograde components of the annual nutation derived from VLBI observations. Unit: 0.001'' Celestial frame unstability effect < 10 mas Sensitivity of the theoretical FCN and FICN periods to perturbation of the MHB2000 transfer function:  Sensitivity of the theoretical FCN and FICN periods to perturbation of the MHB2000 transfer function Perturb the MHB2000 transfer function by introducing plausible changes : in the retrograde annual term in the prograde & retrograde 18.6-year term Evaluate departure of the FCN and FICN periods that are allowed by the size of actual discrepancies (VLBI + atmospheric) Slide14:  432.4 431.8 431.3 430.7 430.2 429.7 429.1 428.6 428.1 1071.0 1059.9 1049.1 1038.5 1028.1 1017.9 1008.0 988.2 13.6 days prograde 13.6 days retrograde 1 year retrograde 18.6 years prograde 18.6 years retrograde FICN period (days) FCN period (days) VLBI-derived amplitudes at the FCN and FICN frequencies :  VLBI-derived amplitudes at the FCN and FICN frequencies Origin of phases: J2000.0 Celestial frame unstability effect < 10 mas Frequency of atmospheric excitation and VLBI-derived amplitudes of FCN and annual term:  Frequency of atmospheric excitation and VLBI-derived amplitudes of FCN and annual term Period of maximum atmospheric excitation and observed amplitudes of the FCN and Retrograde Annual term Summary – 1. Metrology:  Summary – 1. Metrology VLBI-MHB2000 (precision ~10 mas) long term: 10-50 mas medium term: observed variations <50 mas Atmospheric excitation via diurnal waves has varying dominant period Celestial reference frame unstability: long term: 10-30 mas medium term: < 10 mas Summary – 2. Physics:  Summary – 2. Physics Atmospheric excitation of annual nutation is observable The atmosphere may have excited the retrograde FCN (430d) in the 1980 ’s (250 mas) The prograde FICN (1025d) may have been excited around 1984 (100 mas) Future work:  Future work In order to understand still unmodelled nutation signals at the level 10-50 mas: Validate atmospheric data from various meteorological centres Monitor atmospheric excitation Monitor long-term and medium-term stability of the VLBI celestial reference frame

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