Solar Orbiter/RPW antenna calibration in the radio domain and its application to type III burst observations

¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, Paris, France
e-mail: antonio.vecchio@obspm.fr
² Radboud Radio Lab, Department of Astrophysics, Radboud University Nijmegen, Nijmegen, The Netherlands
³ Goddard Planetary Heliophysics Institute, University of Maryland, Baltimore County, Baltimore, MD, USA
⁴ Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
⁵ Space Sciences Laboratory, University of California, Berkeley, CA, USA
⁶ Physics Department, University of California, Berkeley, CA, USA
⁷ LPP, CNRS, Ecole Polytechnique, Sorbonne Université, Observatoire de Paris, Université Paris-Saclay, Palaiseau, Paris, France
⁸ CNES, 18 Avenue Edouard Belin, 31400 Toulouse, France
⁹ Swedish Institute of Space Physics (IRF), Uppsala, Sweden
¹⁰ LPC2E, UMR7328 CNRS and Université d’Orléans, 3A av. de la Recherche Scientifique, 45071 Orléans cedex, France
¹¹ Technische Universität Dresden, Würzburger Str. 35, 01187 Dresden, Germany
¹² Institute of Atmospheric Physics of the Czech Academy of Sciences, Prague, Czech Republic
¹³ Space Research Institute, Austrian Academy of Sciences, Graz, Austria
¹⁴ Astronomical Institute of the Czech Academy of Sciences, Ondvrejov, Czech Republic
¹⁵ Space and Plasma Physics, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, Stockholm, Sweden

Received: 2 April 2021
Accepted: 20 August 2021

Abstract

Context. In order to allow for a comparison with the measurements from other antenna systems, the voltage power spectral density measured by the Radio and Plasma waves receiver (RPW) on board Solar Orbiter needs to be converted into physical quantities that depend on the intrinsic properties of the radiation itself (e.g., the brightness of the source).

Aims. The main goal of this study is to perform a calibration of the RPW dipole antenna system that allows for the conversion of the voltage power spectral density measured at the receiver’s input into the incoming flux density.

Methods. We used space observations from the Thermal Noise Receiver (TNR) and the High Frequency Receiver (HFR) to perform the calibration of the RPW dipole antenna system. Observations of type III bursts by the Wind spacecraft are used to obtain a reference radio flux density for cross-calibrating the RPW dipole antennas. The analysis of a large sample of HFR observations (over about ten months), carried out jointly with an analysis of TNR-HFR data and prior to the antennas’ deployment, allowed us to estimate the reference system noise of the TNR-HFR receivers.

Results. We obtained the effective length, l_eff, of the RPW dipoles and the reference system noise of TNR-HFR in space, where the antennas and pre-amplifiers are embedded in the solar wind plasma. The obtained l_eff values are in agreement with the simulation and measurements performed on the ground. By investigating the radio flux intensities of 35 type III bursts simultaneously observed by Wind and Solar Orbiter, we found that while the scaling of the decay time as a function of the frequency is the same for the Waves and RPW instruments, their median values are higher for the former. This provides the first observational evidence that Type III radio waves still undergo density scattering, even when they propagate from the source, in a medium with a plasma frequency that is well below their own emission frequency.