Survival probability and energy modification of hydrogen Energetic Neutral Atoms on their way from the termination shock to Earth orbit

TL;DR

This study models the energy modification and survival probability of hydrogen ENA from the termination shock to Earth orbit, considering solar wind and radiation effects, crucial for interpreting IBEX observations.

Contribution

It provides a detailed numerical simulation of hydrogen ENA transport, incorporating variable solar wind and EUV radiation, to predict energy and survival probability changes en route to Earth.

Findings

Hydrogen ENA reach Earth with minimal energy change due to radiation pressure.

Survival probability varies significantly with energy, solar activity, and position.

Energy and survival probability fluctuations are modulated by solar cycle and geometry.

Abstract

Context: With the forthcoming launch of a NASA SMEX mission IBEX devoted to imaging of heliospheric interface by in-situ detection of Energetic Neutral Atoms (ENA) an important issue becomes recognizing of transport of these atoms from the termination shock of the solar wind to Earth orbit. Aims: Investigate modifications of energy and of survival probability of the H ENA detectable by IBEX (0.01 -- 6 keV) between the termination shock and Earth orbit taking into account the influence of the variable and anisotropic solar wind and solar EUV radiation. Methods: Energy change of the atoms is calculated by numerical simulations of orbits of the H ENA atoms from ~100 AU from the Sun down to Earth orbit, taking into account solar gravity and Lyman-$\alpha$ radiation pressure, which is variable in time and depends on radial velocity of the atom. To calculate survival probabilities of the atoms against onization, a detailed 3D and time-dependent model of H ENA ionization based on observations of the solar wind and EUV ionizing radiation is constructed, and wth the use of this model probabilities of survival of the atoms are calculated by numerical integration along the previously calculated orbits. Results: Owing to the radiation pressure, H ENA reach the Earth orbit practically without energy and direction change except the atoms with energy lower than 0.1 keV during high solar activity. For a given energy at Earth orbit one expects fluctuations of survival probability from ~20% at 0.01 keV down to just a few percent at 6 keV and a modulation of survival probability as a function of the location at Earth orbit, ecliptic latitude of the arrival direction, and the phase of solar cycle with an amplitude of a few dozen percent for 0.1 keV atoms at solar minimum to a few percent for 6 keV atoms at solar maximum.