Electron density distribution and solar plasma correction of radio signals using MGS, MEX, and VEX spacecraft navigation data and its application to planetary ephemerides^⋆

¹ Observatoire de Besançon, CNRS UMR6213, 41bis Av. de l’Observatoire, 25000 Besançon, France
² CNES, Toulouse, France
e-mail: ashok@obs-besancon.fr
³ Astronomie et Systèmes Dynamiques, IMCCE-CNRS UMR 8028, 77 Av. Denfert-Rochereau, 75014 Paris, France
⁴ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France

Received: 25 June 2012
Accepted: 19 December 2012

Abstract

The Mars Global Surveyor (MGS), Mars Express (MEX), and Venus Express (VEX) experienced several superior solar conjunctions. These conjunctions cause severe degradations of radio signals when the line of sight between the Earth and the spacecraft passes near to the solar corona region. The primary objective of this work is to deduce a solar corona model from the spacecraft navigation data acquired at the time of solar conjunctions and to estimate its average electron density. The corrected or improved data are then used to fit the dynamical modeling of the planet motions, called planetary ephemerides. We analyzed the radio science raw data of the MGS spacecraft using the orbit determination software GINS. The range bias, obtained from GINS and provided by ESA for MEX and VEX, are then used to derive the electron density profile. These profiles are obtained for different intervals of solar distances: from 12 R_⊙ to 215 R_⊙ for MGS, 6 R_⊙ to 152 R_⊙ for MEX, and from 12 R_⊙ to 154 R_⊙ for VEX. They are acquired for each spacecraft individually, for ingress and egress phases separately and both phases together, for different types of solar winds (fast, slow), and for solar activity phases (minimum, maximum). We compared our results with the previous estimations that were based on in situ measurements, and on solar type III radio and radio science studies made at different phases of solar activity and at different solar wind states. Our results are consistent with estimations obtained by these different methods. Moreover, fitting the planetary ephemerides including complementary data that were corrected for the solar corona perturbations, noticeably improves the extrapolation capability of the planetary ephemerides and the estimation of the asteroids masses.