Inflow direction of interstellar neutrals deduced from pickup ion measurements at 1 AU

Abstract

[1] Observations of interstellar pickup ions inside the heliosphere provide an indirect method to access information on the surrounding interstellar medium. The so-called pickup ion focusing cone and pickup ion crescent, which show an imprint of the related longitudinal distribution of interstellar neutrals in form of two overabundances on the down-and upwind side of the sun, are both believed to be aligned along the inflow vector of the interstellar medium. By finding their longitudinal positions, we can give an accurate value for the inflow direction l ISM of interstellar matter.

Challenges in the determination of the interstellar flow longitude from the pickup ion cutoff

“…Hence, the focusing cone is only observed for elements with a high first ionization potential, such as helium or neon (Drews et al 2010). On the upwind side of the Sun, called the crescent, forms for elements with a low first ionization potential like oxygen (Drews et al 2012). At ecliptic longitudes, λ, which corresponds to the interstellar upwind direction, the neutrals have a smaller pathlength to reach a distance of 1 AU from the Sun.…”

“…In this way, the crescent also becomes visible for species with low ionization rates, such as helium. Hence, PUI measurements can also be utilized to determine λ flow , as was performed in Drews et al (2012). Although a careful error estimation was performed, it is likely that these results are affected by systematic errors.…”

“…By discarding two orbits of data each time, we get 21 different combinations of five out of seven orbits with sufficient statistics that we can utilize to estimate the error of our analysis that is introduced by the slight variation of the cutoff due to varying solar wind conditions. This method is closely related to the error estimation for λ flow utilized by Drews et al (2012). In the approach applied in this work, variations of the interplanetary conditions introduce stochastically distributed fluctuations in the PUI flux, which is utilized to determine the location of the focusing cone and crescent.…”

“…Using this information one can extract the He + data of PLASTIC's solar wind section, as in Drews et al (2010Drews et al ( , 2012. Once an ion is identified, one can calculate the ion's speed in the spacecraft frame of reference from the energy-per-charge information, which makes it possible to derive the VDF of an ion species.…”

“…Furthermore, we propose improved methods to derive the cutoff speed and we present an approach to exclude acceleration sites from the data that may have an influence on the cutoff speed measurements. The final improvement of the original method that we introduce here is an error estimation that is based on a bootstrap Monte Carlo approach, similar to that utilized in Drews et al (2012).…”