The plasma distributions of the global heliosphere are time-dependent on scales ranging from short solar events to the solar cycle. Neutral atoms from the interstellar medium are crossing this…

This is a $636,766.00 grant award from a civilian federal agency to Trustees of Dartmouth College, a private Ivy League research university, for a research project focused on understanding the relationship between time-dependent neutral fluxes and the underlying time-dependent global heliosphere. The research seeks to establish a comprehensive, detailed understanding of how neutral atoms from the interstellar medium interact with the time-dependent plasma distributions in the global heliosphere as they traverse Keplerian trajectories. The project pursues an ab-initio approach to model these complex interactions, rather than relying on static survival probability maps. The award has an ultimate completion date of August 31, 2026 and does not have a set-aside designation.

The plasma distributions of the global heliosphere are time-dependent on scales ranging from short solar events to the solar cycle. Neutral atoms from the interstellar medium are crossing this time-dependent heliosphere on keplerian trajectories. Some of these primary neutrals are detected by ibex, imap, and ulysses. Throughout the heliosphere, primary neutrals are subject to charge exchange (CX) collisions that produce secondary neutrals in velocity states and in numbers that characterize the underlying plasma distribution these secondaries are observed by the neutral detectors as well. The CX interaction is time dependent. Along their trajectories, neutrals also suffer additional losses in the form of time-dependent photoionization and electron impact ionization. Lastly, variations in the solar intensity create time-dependent perturbations on the neutral trajectory in the form of radiation pressure fluctuations. Neutral fluxes at spacecraft in the innermost heliosphere are therefore inherently time-dependent, and this proposal seeks to establish the relationship between time-dependent neutral fluxes and the underlying time-dependent global heliosphere in a very detailed, comprehensive manner. IT pursues an AB-initio approach and avoids the use of static survival probability maps and similar concepts relying on stationarity. Slow neutrals move many years through the heliosphere so that loss and production processes are time averaged on their outer heliosphere trajectories, whereas this is less the case for enas (fast neutrals). Both categories, however, suffer losses in the last few AU of their journey before detection that imprints the solar and inner heliosphere conditions around the immediate past before detection onto the neutral fluxes. Science goals and objectives: - pursue the idea of neutrals detected by spacecraft in the innermost heliosphere being messengers of remote plasma regions where they were created by charge exchange, but explicitly study and disentangle the effect of three sources of heliosphere time dependence on the neutral fluxes: (i) the time-dependent outer heliosphere (II) the time-dependent solar wind and radiation during the last few AU of a neutral trajectory before detection (III) the time-dependent solar radiation output, manifesting through photoionization rate and radiation pressure variations. - study these effects in a theoretical, systematic way with the help of modeling and assumed solar radiation and wind parameters, to work out the detailed correlation between time-dependence in remote plasma regions and the time-dependent neutral flux signal. The overarching goal is to develop a method to invert this dependence, to allow the study of remote regions by analyzing the measured neutral signals. - apply the developed results to ibex and ulysses maps of neutrals to characterize the time-dependent behavior of the outer heliosheath toward the nose of the heliosphere and characterize the less well-constrained flanks and tail of the heliosphere. Methodology. An existing, very accurate, AB-initio modeling suite of primary and secondary neutrals anywhere in the heliosphere that couples a keplerian test-particle code to global heliosphere models, will be used to calculate primary and secondary neutrals. Time dependent solar conditions will be included, guided by solar observation time series. Fluxes at spacecraft locations will be synthesized and compared to observed data. Relevance. By investigating fundamental plasma-neutral processes, the proposed project is directly relevant to the third and fourth high-level science goals from the heliophysics decadal survey. The project will make use of existing observations of past and ongoing missions. Moreover, the results will be of direct importance to the upcoming imap mission, informing its observation strategy to maximize its scientific return.