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In this study, we present an empirical model, named CH-Therm-2018, of the thermospheric mass density derived from 9-year (from August 2000 to July 2009) accelerometer measurements from the CHAllenging Minisatellite Payload (CHAMP) satellite at altitudes from 460 to 310 km. The CHAMP dataset is divided into two 5-year periods with 1-year overlap (from August 2000 to July 2005 and from August 2004 to July 2009) to represent the high-to-moderate and moderate-to-low solar activity conditions, respectively. The CH-Therm-2018 model describes the thermospheric density as a function of seven key parameters, namely the height, solar flux index, season (day of year), magnetic local time, geographic latitude and longitude, as well as magnetic activity represented by the solar wind merging electric field. Predictions of the CH-Therm-2018 model agree well with CHAMP observations (within 20 %) and show different features of thermospheric mass density during the two solar activity levels, e.g., the March-September equinox asymmetry and the longitudinal wave pattern. From the analysis of satellite laser ranging (SLR) observations of the ANDE-Pollux satellite during August-September 2009, we estimate 6 h scaling factors of the thermospheric mass density provided by our model and obtain the median value equal to 1:267±0:60. Subsequently, we scale up our CHTherm-2018 mass density predictions by a scale factor of 1.267. We further compare the CH-Therm-2018 predictions with the Naval Research Laboratory Mass Spectrometer Incoherent Scatter Radar Extended (NRLMSISE-00) model. The result shows that our model better predicts the density evolution during the last solar minimum (2008-2009) than the NRLMSISE-00 model. [ABSTRACT FROM AUTHOR]

Continuous monitoring of the perpetual changing Earth is one of the key topics of present Earth System research. In the past, the satellite mission Gravity Recovery And Climate Experiment (GRACE) was measuring the time-variable gravity field of the Earth with outstanding performance in accuracy and reliability. On October 27 in 2017, GRACE de-orbited and this very successful geodetic satellite mission ended. On May 22 in 2018, the successor mission GRACE-FO (Follow On) was launched and scientific data will be provided soon to the users. Between the two satellite missions, there is a gap of more than six months. Beside the GRACE and other satellite missions, several analysis centers (ACs) worldwide process laser ranging (SLR) data to spherical satellites orbiting the Earth which allow a very accurate estimation of the low-degree spherical harmonics of the Earth's gravity field. Up to now, the available time series are stand-alone solutions which co-exist to each other.In this investigation, we will combine the SLR-derived time series of low-degree spherical harmonics from different SLR analysis centers and, moreover, combine them with the GRACE data in order to extend the continuous time series of the monthly gravity field in the GRACE/GRACE-FO gap and into the past (before the launch of GRACE in 2002). We will compare the combined SLR time series to GRACE monthly solutions and evaluate the impact of different solution setups like the SLR-orbit length, a priori models, etc. on the results. Additionally, we study the benefit of a combined SLR time series compared to the AC stand-alone solutions and motivate the advantages of such a combination in the future! [ABSTRACT FROM AUTHOR]