Imprints from the solar cycle on the helium atom and helium pickup ion distributions

Abstract

Neutral interstellar helium atoms penetrate into the solar system almost unaffected by gas-plasma interactions in the heliospheric interface region, and thus can be considered as carriers of original information on the basic parameters (like density, temperature, bulk velocity) of the Very Local Interstellar Medium (VLISM). Such information can nowadays be derived from analysis of data obtained from different experimental methods: in situ measurements of He atoms (Ulysses), observations of the solar backscattered He 584 Å radiation (EUVE), in situ measurements of He+ pickup ions (AMPTE, Ulysses, Wind, SOHO, ACE). In view of the current coordinated international ISSI campaign devoted to the study of the helium focusing cone structure and its evolution, we analyze expected variations of neutral He density, of He+ pickup fluxes and of their phase space distributions at various phases of the solar activity cycle based on a realistic time-dependent modelling of the neutral helium and He+ pickup ion distributions, which reflect solar cycle-induced variations of the photoinization rate. We show that the neutral helium density values are generally anticorrelated with the solar activity phase and in extreme cases (near the downwind axis) the maximum-to-minimum density ratio may even exceed factors of ∼3 at 1 AU. We also demonstrate that in the upwind hemisphere (at 1 AU and beyond) the He+ fluxes are correlated with the solar cycle activity, whereas on the downwind side the maximum of the expected flux up to distances of ∼3 AU occurs around solar minimum epoch, and only further away does the correlation with solar activity become positive. Finally, we present the response of the phase space distribution spectra of He+ pickup ions (in the solar wind frame) for different epochs of the solar cycle and heliocentric distances from 1 to 5 AU covering the range of Ulysses, Wind and ACE observations.