First dust measurements with the Solar Orbiter Radio and Plasma Wave instrument

Abstract

Context. Impacts of dust grains on spacecraft are known to produce typical impulsive signals in the voltage waveform recorded at the terminals of electric antennas. Such signals (as may be expected) are routinely detected by the Time Domain Sampler (TDS) system of the Radio and Plasma Waves (RPW) instrument on board Solar Orbiter. Aims. We investigate the capabilities of RPW in terms of interplanetary dust studies and present the first analysis of dust impacts recorded by this instrument. Our purpose is to characterize the dust population observed in terms of size, flux, and velocity. Methods. We briefly discuss previously developed models of voltage pulse generation after a dust impact onto a spacecraft and present the relevant technical parameters for Solar Orbiter RPW as a dust detector. Then we present the statistical analysis of the dust impacts recorded by RPW/TDS from April 20, 2020 to February 27, 2021 between 0.5 AU and 1 AU. Results. The study of the dust impact rate along Solar Orbiter’s orbit shows that the dust population studied presents a radial velocity component directed outward from the Sun. Its order of magnitude can be roughly estimated as vr, dust ≃ 50 km s−1, which is consistent with the flux of impactors being dominated by β-meteoroids. We estimate the cumulative flux of these grains at 1 AU to be roughly Fβ ≃ 8 × 10−5 m−2 s−1 for particles of a radius r ≳ 100 nm. The power law index δ of the cumulative mass flux of the impactors is evaluated by two differents methods, namely: direct observations of voltage pulses and indirect effect on the impact rate dependency on the impact speed. Both methods give the following result: δ ≃ 0.3 − 0.4. Conclusions. Solar Orbiter RPW proves to be a suitable instrument for interplanetary dust studies, and the dust detection algorithm implemented in the TDS subsystem an efficient tool for fluxes estimation. These first results are promising for the continuation of the mission, in particular, for the in situ study of the inner Solar System dust cloud outside of the ecliptic plane, which Solar Orbiter will be the first spacecraft to explore.

Machine learning detection of dust impact signals observed by the Solar Orbiter

“…The dust production was modeled by collisional fragmentation near the Sun and the dust trajectories were calculated with included radiation pressure and Lorentz force terms. Mann and Czechowski (2021) showed that the observed impact rates largely agree with the model calculations for dust > 100 nm and proposed that the differences may be explained by the influence of smaller particles and of other dust components, such as dust in bound orbits and interstellar dust.…”

“…Systematic studies of the dust flux near 1 AU are conducted with the Solar Terrestrial Relations Observatory (STEREO) (Zaslavsky et al, 2012) and Wind (Malaspina et al, 2014). The first analyses show that a large fraction of the observed dust particles are repelled from the Sun, i.e., the dust particles are in unbound orbits (Zaslavsky et al, 2021;Szalay et al, 2020;Malaspina et al, 2020). Mann and Czechowski (2021) used model calculations to explain the impact rates observed by the Parker Solar Probe.…”

“…The first analyses show that a large fraction of the observed dust particles are repelled from the Sun, i.e., the dust particles are in unbound orbits (Zaslavsky et al, 2021;Szalay et al, 2020;Malaspina et al, 2020). Mann and Czechowski (2021) used model calculations to explain the impact rates observed by the Parker Solar Probe. The dust production was modeled by collisional fragmentation near the Sun and the dust trajectories were calculated with included radiation pressure and Lorentz force terms.…”

“…The Solar Orbiter will thus provide long-term, in situ observations of the environment in the inner solar system with multiple instruments. One of these instruments is the Radio and Plasma Waves instrument, allowing observations of the cosmic dust flux with typical diameters ranging from ∼ 100 to ∼ 500 nm (Zaslavsky et al, 2021).…”

An analytical model for dust impact voltage signals, and its application to STEREO/WAVES data

“…For particles orbiting at Keplerian speeds, we can assume a typical impact velocity of 30 km.s −1 ; the obtained mass range is then 20 − 340 × 10 −17 kg, which corresponds to the size interval 2 -5 µm (we assume a mass density ρ = 2.5 g.cm −3 ). On the other hand, it has appeared that the fluxes observed on several spacecraft, including STEREO (Zaslavsky et al 2012), but also Parker Solar Probe (Pusack et al 2021) and Solar Orbiter (Zaslavsky et al 2021), are dominated by impacts from a population of dust particles produced close to the Sun and pushed away along hyperbolic orbits by the radiation pressure, the β meteoroids. The velocity of these particles at 1 AU depends quite importantly on their origin and composition, through the value of the β parameter equal to the ratio of the radiation pressure force to the gravitational force on the dust grain.…”