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,leff, 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 obtainedleffvalues 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.
Highlights
The Solar Orbiter (SO) mission (Muller et al 2020; Zouganelis et al 2020) carries ten instruments, including the Radio and Plasma Wave experiment (RPW) (Maksimovic et al 2020), which is designed to measure magnetic and electric fields, plasma wave spectra, and polarization properties, as well as the spacecraft (S/C) floating potential and solar radio emissions in the interplanetary medium
We focus on the two high-frequency receivers of RPW: Thermal Noise Receiver (TNR), producing electric and magnetic power spectral densities in the frequency range from 4 kHz to 1 MHz, and High
The original method implied the use of the galactic background signal as a reference source, the same approach is not entirely suitable for RPW due to the high level of EM pollution suffered
Summary
The Solar Orbiter (SO) mission (Muller et al 2020; Zouganelis et al 2020) carries ten instruments, including the Radio and Plasma Wave experiment (RPW) (Maksimovic et al 2020), which is designed to measure magnetic and electric fields, plasma wave spectra, and polarization properties, as well as the spacecraft (S/C) floating potential and solar radio emissions in the interplanetary medium. The TNR-HFR receiver suffers a strong electromagnetic (EM) contamination due to the central power distribution unit (PCDU) radiated by the Solar Panels, at 120 kHz and harmonics, and the reaction wheel electronic box, at 80 kHz and harmonics (Maksimovic & et al 2021) This makes the observation of the galactic background much more difficult as it is masked by the instrumental noise due to the platform. Type III bursts are among the strongest radio emissions routinely occurring in the heliosphere, reaching flux densities up to 103-104 higher than the galactic background and certainly well above the instrumental TNR-HFR background This strong signal can be used to crosscalibrate the RPW antenna system by comparing the TNR-HFR data with simultaneous measurements of type III radio bursts by the radio receiver band 1 (RAD1, 20-1040 kHz) on Wind/Waves (Bougeret et al 1995). The procedure to derive the system noise background of TNR-HFR by comparing ten months of HFR observations with the expected Galaxy signal and by an analysis of TNR-HFR data before the antennas’ deployment is described in Appendix A
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