Solar cycle variation of interstellar neutral He, Ne, O density and pick-up ions along the Earth's orbit

TL;DR

This study models how solar cycle variations affect interstellar neutral gas densities and pick-up ion measurements along Earth's orbit, revealing potential biases in determining the interstellar inflow direction.

Contribution

It introduces a modified PUI model accounting for solar cycle effects, improving the accuracy of interstellar inflow direction measurements.

Findings

Ionization losses impact inflow longitude derivation.

Non-zero radial velocity affects PUI count rate modeling.

Solar cycle variations can bias inflow direction estimates.

Abstract

We simulated the modulation of the interstellar neutral (ISN) He, Ne, and O density and pick-up ion (PUI) production rate and count rate along the Earth's orbit over the solar cycle from 2002 to 2013 to verify if solar cycle-related effects may modify the inferred ecliptic longitude of the ISN inflow direction. We adopted the classical PUI model with isotropic distribution function and adiabatic cooling, modified by time- and heliolatitude-dependent ionization rates and non-zero injection speed of PUIs. We found that the ionization losses have a noticeable effect on the derivation of the ISN inflow longitude based on the Gaussian fit to the crescent and cone peak locations. We conclude that the non-zero radial velocity of the ISN flow and the energy range of the PUI distribution function that is accumulated are of importance for a precise reproduction of the PUI count rate along the Earth orbit. However, the temporal and latitudinal variations of the ionization in the heliosphere, and particularly their variation on the solar cycle time-scale, may significantly modify the shape of PUI cone and crescent and also their peak positions from year to year and thus bias by a few degrees the derived longitude of the ISN gas inflow direction.