![]() If no magnetic field values were available for any individual distribution function, it was still possible to locate the peak of the distribution, but the rotational symmetry axis of the bi-Maxwellian could not be determined. Only the component of the temperature tensor in the radial direction ( \(T _)\). If the total proton number density is of interest, then a moment is accurate, but to obtain the difference between the number density of the core or beam, an analytical fitting process must be used. This does not distinguish between the proton core and beam populations. Only the total number density is calculated. Although taking numerical moments is computationally fast, it has a number of differences when compared with a bi-Maxwellian analysis of the full 3D distribution function: Footnote 1 This set of parameters was calculated in the 1970s and 1980s by taking numerical moments of 1D energy spectra, obtained by integrating the 3D distributions over all solid angles. In Section 5 we use the new dataset to provide a new set of radial trends for the proton core population of the solar wind.Īs far as we know, the only other publicly available set of proton plasma parameters available from the Helios mission is the “merged” dataset. In Section 4 we compare the dataset to the previously available data. We provide in Section 3.1 an overview of the plasma instrumentation, and in Section 3.2, we describe the data processing and summarise the new dataset. In Section 2 we give a brief overview of the data that were already widely available to researchers. Here we present systematic bi-Maxwellian fits to the proton core population for the entire duration the Helios mission. The proton population can be further split into two: the proton core, which accounts for \(\approx 90\%\) of the protons, and the smaller proton beam, which travels at a different velocity to the core (Feldman et al., 1973 Marsch et al., 1982b). The solar wind primarily consists of protons, with a smaller fraction of alpha particles ( \(\sim 1\%\) – 5% by positive ion number density), a series of other minor ions ( \(\ll 1\%\) by positive ion number density), and neutralising electrons (Neugebauer and Snyder, 1962 Marsch et al., 1982a Pilipp et al., 1987 Bochsler, 2007). This makes the dataset easily reproducible and reusable. Marsch, Ao, and Tu, 2004 Matteini et al., 2007 Hellinger et al., 2011), but importantly, the new dataset and the code used to generate it are openly available to researchers. The plasma data have been reprocessed before ( e.g. ![]() In this paper we revisit the plasma measurements made on board the two Helios spacecraft. The data returned by these spacecraft provided a wealth of information, but the computational resources available to process the data were limited at the time, and there is no publicly available dataset containing reliable and reproducible moments derived from the full three-dimensional (3D) distribution functions. To date, the most comprehensive mission to visit the inner heliosphere and make in-situ measurements of the solar wind was the Helios mission, consisting of two spacecraft, which explored the heliosphere from 0.29 – 1 AU in the 1970s and 1980s, covering solar minimum between Solar Cycles 20 and 21 and the maximum of Solar Cycle 21 (Porsche, 1977). With the imminent launches of Parker Solar Probe (Fox et al., 2016) and Solar Orbiter (Müller et al., 2013), heliospheric and solar physics are about to enter a new age of discovery.
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