Open questions in the present heliospheric research and challenges for upcoming space mission like IMAP

Abstract:

Since the appearance of the first IBEX ENA fullsky maps in 2009 up to the present time it remained an open question what ENA sources are reponsible for the famous "IBEX-ribbon" feature. Both innerheliospheric and outerheliospheric ENA sources still today compete as reponsible candidates for that feature. Whether the ribbon-producing ENA`s are due to charge exchange processes inside or outside the heliopause is an outstanding question up to the present days. The answer, as I will show, will surely come with an extended energy range of the ENA detector and with a better time resolution in ENA maps, helping to control the reflection of temporal variations of solar wind flux patterns in ENA features as it all will be realized with IMAP.

A second open question concerns the compressibility of the heliosheath plasma. Incompressibility prevails under very low Mach number flows or very high effective sound speeds, as could be realized by hot PUI`s or hot electrons downstream of the termination shock. Hot suprathermal electrons have been predicted, but not yet confirmed by VOYAGER measurements. At the low energy side of the spectrum this is explained by the negative charge-up of the spacecraft, at the medium energy range (0.5-1.0 MeV) shock-processed suprathermal electrons are likely to in fact explain the observations.

Furthermore these electrons would directly influence the electrostatic wave characteristics in the heliosheath leading to unexpected propagation properties of such waves. To clarify on some of these points IMAP should have a sensitive electron detector which could monitor solar wind electron properties at passages of travelling shocks and thus prove the efficiency of shock processing.

If in fact hot electrons have to be expected in the heliosheath, then electron impact ionization processes have to be studied in new details, and the question may arise whether also LISM helium under these new auspices becomes partially ionized when crossing the heliosheath.