Possible Instrumental Effects on Proton Velocity Distribution Functions Due to Plasma Sensor's Finite Sampling Time

The solar wind is a highly turbulent medium in which most of the fluctuating energy is carried by Alfvénic fluctuations. These fluctuations have a wide range of scales whose high frequency end can be relevant for the sampling techniques commonly used to detect the particle velocity distribution function in phase space. We analyze the effect of Alfvénic fluctuations on moments computation of solar wind proton velocity distribution function for a plasma sensor, whose sampling time is comparable or even larger than the time scale of the velocity fluctuations induced by these perturbations. In particular, we numerically simulate the sampling procedure used onboard Helios 2. The Alfvénic fluctuations are constructed by employing the magnetic field fluctuations recorded by Helios 2 magnetometer when the s/c was immersed in Alfvénic fast wind during its primary mission to the Sun, with 1 Hz cadence. Surrogated proton number density values in the time domain are then obtained by extrapolating the low frequency range of the available density spectrum up to 1 Hz and successively inverting this spectrum. We will report about interesting spectral features that have been interpreted in the past literature as due to physical mechanisms while the present study suggests a relevant instrumental effect. SWA-PAS observations will give us the opportunity to discriminate between the different interpretations.