Pick-up Ion Measurements in the Heliosphere – A Review

Abstract

Measurements of the composition and spatial distribution of pick-up ionsinside the heliosphere are reviewed. The first interstellar 4 He + pick-up ions were detected with the SULEICA instrument on the AMPTEspacecraft near Earth's orbit. Most data on pick-up ions were taken in thesolar-wind and suprathermal energy range of SWICS on Ulysses while thespacecraft cruised from 1.4 to 5.4 AU and explored the high-latitudeheliosphere and solar wind from the ecliptic to ± 80 ° heliolatitude.This includes the discovery of H + , 4 He ++ , 3 He + , N + ,O + , and Ne + pick-up ions that originate from the interstellar neutralgas penetrating the heliosphere. From their fluxes properties of theinteraction region between the heliosphere and the Local Interstellar Cloudsuch as the limits on filtration and the strength of the interstellar magneticfield have been revealed. Detailed analysis of the velocity distributions ofpick-up ions led to 1) the discovery of a new distinct source, the so-calledInner Source, consisting of atoms released from interstellar and interplanetarydust inside the heliosphere, 2) the determination of pick-up ion transportparameters such as the long mean free path for pitch-angle scattering of order1 AU, and 3) detailed knowledge on the very preferential injection andacceleration of pick-up ions during interplanetary energetic particle eventssuch as Co-rotating Interaction Regions and Coronal Mass Ejections. SWICSmeasurements have fully confirmed the theory of Fisk, Koslovsky, and Ramatythat pick-up ions derived from the interstellar gas are the dominant sourceof the Anomalous Cosmic Rays; they are pre-accelerated inside the heliosphereand re-accelerated at the solar-wind Termination Shock according to Pesses,Eichler, and Jokipii. The data indicate that the Inner Source of pick-up ionsis largely responsible for the occurence of C + in the Anomalous Cosmic Rays.The abundances of recently discovered Inner-Source Mg + and Si + aresolar-wind like and consistent with their abundances in the energetic particlesassociated with Co-rotating Interaction Regions.Knowledge on the injection and acceleration processes in Co-rotatingInteraction Regions is applied to discuss the current observationalevidence for the Interplanetary Focusing Cone of the interstellarneutral gas due to the Sun's gravitational force. The 25–150 keV/amusuprathermal 4 He + pick-up ion fluxes measured by CELIAS/STOF on boardSOHO over 360 ° of ecliptic longitude represent a `local'ionization and acceleration of interstellar atoms at 1 AU or smallerheliocentric distances. Completing the first limited data set ofSULEICA/AMPTE on 4 He + pick-up ions they indicate a density enhancementin the Interplanetary Focusing Cone which is confirmed by recent SWICS/ACEdata.Clear evidence for signatures in ecliptic longitude are found in thedata on energetic neutral H fluxes observed with the CELIAS/HSTOFsensor on board SOHO. These fluxes are enhanced in the upstream anddownstream directions of the interstellar wind. Detection of energeticH atoms, which propagate unaffected by the Heliospheric Magnetic Field,provided for the first time a diagnostic tool for observations nearEarth to analyze the structure in ecliptic longitude of the interfaceregion between the heliosphere and the Local Interstellar Cloud.