He Pickup Ions in the Inner Heliosphere—Diagnostics of the Local Interstellar Gas and of Interplanetary Conditions

Abstract

The relative motion of the Sun through the Local Interstellar Cloud (LIC) leads to a neutral wind through the heliosphere. Because of its high ionization potential, He remains neutral to well within 1 AU, where it is deflected by the Sun’s gravity and forms a focusing cone on the downwind side. This flow pattern has been studied with UV backscattering, through pickup ions (PUI), and atom imaging. A consolidated set of the physical parameters of He in the LIC has been derived combining all three methods. However, it is still poorly understood why PUI fluxes and velocity distributions vary substantially on temporal scales from hours to many days, which leads among other phenomena to apparent changes in the appearance of the focusing cone, even after averaging over several days. With the combination of PLASTIC on STEREO A and B as well as SWICS on ACE, simultaneous PUI observations over an increasing range of heliospheric longitudes have become possible, for which we have initiated a cross‐calibration effort between the distributed sensors. With these data that feature improved temporal resolution, spatial and temporal variations in the PUI fluxes and spectra can be separated. Therefore, we can probe the effects on PUI distributions and the observed structure of the focusing cone that arise from solar wind structures, such as compressions and rarefactions, from variations in magnetic field strength and direction, and from changes in the ionization rates.