Abstract
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 A 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 photoionization 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.Key words. Solar physics, astrophysics and astronomy (ultraviolet emissions) – Space plasma physics (ionization processes; numerical simulation studies)
Highlights
Interstellar helium is the second most abundant element in the Local Interstellar Cloud (LIC) and since it does not interact with the heliospheric interface, it allows for the determination of the physical state of interstellar gas in the immediate solar neighbourhood
The information obtained from measurements of the interstellar helium gas has been used for quite some time as a valuable reference to study other interstellar elements inside the heliosphere and some element-specific processes to which these are subjected in the heliospheric interface
The first direct observation of the derivative ion population co-moving with the solar wind, i.e. of the interplanetary He+ pickup ions (PUIs), was performed in 1985 by the AMPTE/ IRM mission (Mobius et al, 1985)
Summary
Interstellar helium is the second most abundant element in the Local Interstellar Cloud (LIC) and since it does not interact with the heliospheric interface, it allows for the determination of the physical state of interstellar gas in the immediate solar neighbourhood. There are plenty of direct observations of the LIC helium atoms presence in the heliosphere and of the derivative helium ions in the interplanetary space. Since 1990, the GAS experiment on board of ULYSSES has been registering in situ neutral interstellar helium atoms in the interplanetary space. From these observations, Witte et al (1996; 2003), derived LIC helium properties with great accuracy. Extremely valuable observations of heliospheric He+ and He++ pickup ions
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