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Energetic Neutral Atom (ENA) observations provide valuable insights into the plasma conditions in the heliosphere and the surrounding interstellar medium. Unlike plasma detectors, which measure charged particles tied to the magnetic fields at their location, ENA detectors capture former ions that were neutralized in distant regions and traverse the heliosphere in straight trajectories. ENA fluxes near the Sun represent line-of-sight integrals of parent ion fluxes multiplied by neutralization (production) rates and reduced by the probability of ENA reionization (loss) processes. So far, most ENA analyses have focused on charge exchange between hydrogen atoms and protons as the primary source of ENAs. Here, we examine various ENA production and loss processes throughout the heliosphere in the broad energy range (5 eV to 500 keV) encompassing the next-generation ENA instruments aboard the Interstellar Mapping and Acceleration Probe (IMAP) mission. Our study considers binary collisions involving the most abundant species: protons, electrons, {\alpha}-particles, He+ ions, photons, as well as hydrogen and helium atoms. Our findings indicate that, in addition to ENAs produced by charge exchange of energetic protons with hydrogen atoms, a significant portion of high-energy ENAs originate from the charge exchange with helium atoms. Below 10 keV, the dominant ENA loss processes are charge exchange collisions with protons and photoionization. However, stripping ionization processes, e.g., from collisions with ambient interstellar neutral hydrogen, become the main loss mechanism for higher energies because the charge exchange rate rapidly decreases., Comment: 27 pages, 7 figures, 3 tables, accepted for publication in ApJ, AAM version

Direct-sampling observations of interstellar neutral gas, including hydrogen and deuterium, have been performed for more than one cycle of solar activity by IBEX. IBEX viewing is restricted to directions perpendicular to the spacecraft--Sun line, which limits the observations to several months each year. This restriction is removed in a forthcoming mission Interstellar Mapping and Acceleration Probe. The IMAP-Lo instrument will have a capability of adjusting the angle of its boresight with the spacecraft rotation axis. We continue a series of studies of resulting science opportunities. We adopt a schedule of adjusting the boresight angle suggested by Kubiak et al. 2023 and focus on interstellar hydrogen and deuterium during solar maximum epoch. Based on extensive set of simulations, we identify the times during calendar year and elongation angles of the boresight needed to measure the abundance of D/H at the termination shock and unambiguously observe interstellar H without contribution from interstellar He. Furthermore, IMAP-Lo will be able to resolve the primary and secondary populations, in particular to view the secondary population with little contribution from the primary. We show that the expected signal is sensitive to details of radiation pressure, particularly its dependence on radial speed of the atoms, and to details of the behavior of the distribution function of the primary and secondary populations at the heliopause. Therefore, IMAP-Lo will be able to provide observations needed to address compelling questions of the heliospheric physics, and even general astrophysics.

Direct observations of solar wind are mostly limited to the vicinity of the ecliptic plane. Retrieving the latitudinal structure of solar wind indirectly based on observations of the backscatter glow of interstellar neutral hydrogen is complex and requires support from theoretical models. The GLOWS instrument, to operate on the planned IMAP mission, will scan the helioglow along circumsolar rings with an angular distance of ~75 degrees. Its objective is to retrieve the latitudinal structure of the ionization rate of interstellar hydrogen and with this, the structure of the solar wind. In preparation for future analysis, we studied the sensitivity of the light curves to temporal and latitudinal variation of the ionization rate of interstellar hydrogen and the solar Lyman-alpha illumination. Based on carefully planned numerical experiments, we analyze the time delay and relaxation time of the system for variations of the ionization rate and solar illumination in heliolatitude and with time. We found that variations in the solar illumination are reflected in the helioglow without delay, but relaxation takes longer than the variation rise time. By contrast, variations in the ionization rate are anti-correlated with the helioglow brightness with a delay of several months. We also found that the helioglow is not sensitive to variations in the ionization rate at the solar poles, so retrieving the ionization rate and solar wind at the poles requires an approximation of the ionization rate profiles with appropriate parametric functions., Comment: Submitted to ApJ

The Interstellar Neutral Helium (ISN He) is an important source of information on the physical state of the Local Interstellar Medium. Radiation pressure acting on the neutral helium atoms in the heliosphere has always been neglected, its effect has been considered insignificant compared to gravitational force. The most advanced numerical models of ISN He take into account more and more subtle effects, therefore it is important to check if the effect of radiation pressure is still negligible. In this paper, we use the most up-to-date version of the Warsaw Test Particle Model (WTPM) to calculate the expected helium distribution in the heliosphere, and simulate the flux of ISN He observed by the Interstellar Boundary Explorer (IBEX) and in the future by the Interstellar Mapping and Acceleration Probe (IMAP). We compare results calculated with and without radiation pressure during low and high solar activity. The results show that in the analysis of IBEX-Lo observations the radiation pressure acting on typical helium causes flux differences at a level of 1-4% and is comparable to the observational errors. For the more sensitive IMAP-Lo instrument, there are some regions in the considered observations configurations where radiation pressure causes potentially statistically significant changes in the calculated fluxes. The effect can be up to 9% for the indirect beam and is likely to be higher than the estimated errors. Therefore, we claim that in the future analysis of the IMAP-Lo observations radiation pressure acting on ISN He should be considered., Comment: Accepted to ApJ

On its journey through the Galaxy, the Sun passes through diverse regions of the interstellar medium. High-resolution spectroscopic measurements of interstellar absorption lines in spectra of nearby stars show absorption components from more than a dozen warm partially ionized clouds within 15 pc of the Sun. The two nearest clouds - the Local Interstellar Cloud (LIC) and Galactic (G) cloud - move toward each other. Their bulk heliocentric velocities can be compared with the interstellar neutral helium flow velocity obtained from space-based experiments. We combine recent results from Ulysses, IBEX, and STEREO observations to find a more accurate estimate of the velocity and temperature of the very local interstellar medium. We find that, contrary to the widespread viewpoint that the Sun resides inside the LIC, the locally observed velocity of the interstellar neutral helium is consistent with a linear combination of the velocities of the LIC and G cloud, but not with either of these two velocities. This finding shows that the Sun travels through a mixed-cloud interstellar medium composed of material from both these clouds. Interactions between these clouds explain the substantially higher density of the interstellar hydrogen near the Sun and toward stars located within the interaction region of these two clouds. The observed asymmetry of the interstellar helium distribution function also supports this interaction. The structure and equilibrium in this region require further studies using in situ and telescopic observations., Comment: 13 pages, 5 figures, 1 table, accepted for publication in ApJL

Absorption of the Lyman-$\alpha$ radiation on Interstellar Neutral Hydrogen (ISN H) atoms in the heliosphere is a potentially important effect to account for in precise gas distribution simulations. In this paper we develop a method to estimate the magnitude of absorption of solar Lyman-$\alpha$ radiation inside the solar wind termination shock and to include absorption effects in the Warsaw Test Particle Model (WTPM) by an appropriate modification of radiation pressure. We perform calculations of absorption effects on a 3D grid in the heliosphere and present a set of parameters to model absorption effects for the mean solar activity conditions. We show that absorption can change up to 3% depending on the solar activity level. Using a modified version of WTPM, we calculate the expected signal from IBEX-Lo and show that absorption may modify the simulated flux up to 8%., Comment: Submited to ApJ

Observations of the Sun's surface suggest a nonuniform radiated flux as related to the presence of bright active regions and darker coronal holes. The variations of the FUV/EUV source radiation can be expected to affect the Lyman-alpha backscatter glow measured by spaceborne instruments. In particular, inferring the heliolatitudinal structure of the solar wind from helioglow variations in the sky can be quite challenging if the heliolatitudinal structure of the solar FUV/EUV radiation is not properly included in the modeling of the heliospheric glow. We present results of analysis of the heliolatitudinal structure of the solar Lyman-alpha radiation as inferred from comparison of SOHO/SWAN satellite observations of the helioglow intensity with modeling results obtained from the recently-developed WawHelioGlow model. We find that in addition to time-dependent heliolatitudinal anisotropy of the solar wind, also time-dependent heliolatitudinal variations of the intensity of the solar Lyman-alpha and photoionizing emissions must be taken into account to reproduce the observed helioglow modulation in the sky. We present a particular latitudinal and temporal dependence of the solar Lyman-alpha flux obtained as a result of our analysis. We analyze also differences between polar-equatorial anisotropies close to the solar surface and seen by an observer located far from the Sun. We discuss the implications of these findings for the interpretation of heliospheric-glow observations., Comment: Accepted for publication in Astrophysical Journal Letters, 11 pages, 5 figures

In observations of diffuse emissions like, e.g., the Lyman-$\alpha$ heliospheric glow, contributions to the observed signal from point sources (e.g., stars) are considered as a contamination. There are relatively few brightest point sources that are usually properly resolved and can be subtracted or masked. We present results of analysis of the distribution of point sources using UV sky-survey maps from the SOHO/SWAN instrument and spectrophotometry data from the IUE satellite. The estimated distribution suggests that the number of these sources increases with decreasing intensity. Below a certain threshold, these sources cannot be resolved against the diffuse signal from the backscatter glow, that results in a certain physical background from unresolved point sources. Detection, understanding and subtraction of the point-source background has implications for proper characterization of diffuse emissions and accurate comparison with models. Stars are also often used as standard candles for in-flight calibration of satellite UV observations, thus proper understanding of signal contributions from the point sources is important for the calibration process. We present a general approach to quantify the background radiation level from unresolved point sources in UV sky-survey maps. In the proposed method, a distribution of point sources as a function of their intensity is properly integrated to compute the background signal level. These general considerations are applied to estimate the unresolved-point-sources background in the SOHO/SWAN observations that on average amounts to $28.9$ R. We discuss also the background radiation anisotropies and general questions related to modeling the point-source contributions to diffuse UV-emission observations., Comment: To appear in Astrophysical Journal, 21 pages, 12 figures

We present a modification of a model of solar cycle evolution of the solar Lyman-alpha line profile, along with a sensitivity study of interstellar neutral H hydrogen to uncertainties in radiation pressure level. The line profile model, originally developed by Kowalska-Leszczynska et al. 2018a, is parametrized by the composite solar Lyman-alpha flux, which recently was revised Machol et al. 2019. We present modified parameters of the previously-developed model of solar radiation pressure for neutral hydrogen and deuterium atoms in the heliosphere. The mathematical function used in the model, as well as the fitting procedure, remain unchanged. We show selected effects of the model modification on ISN H properties in the heliosphere and we discuss the sensitivity of these quantities to uncertainties in the calibration of the composite Lyman-alpha series., Comment: 9 pages, 7 figures, 2 tables Submitted to ApJ

The interstellar neutral (ISN) gas enters the heliosphere and is detected at a few au from the Sun, as demonstrated by Ulysses and the nterstellar Boundary Explorer (IBEX) missions. Ulysses observed ISN gas from different vantage points in a polar orbit from 1994 to 2007, while IBEX has been observing in an Earth orbit in a fixed direction relative to the Sun from 2009. McComas et al. 2018 reported about an IMAP-Lo detector on board the Interstellar Mapping and Acceleration Probe (IMAP), with an ability to track the ISN flux in the sky. We present observation geometries for ISN gas for a detector with the capability to adjust the boresight direction along the Earth orbit over a year within a multichoice ISN observation scheme. We study science opportunities from the observations as a function of time during a year and the phase of solar activity. We identify observation geometries and determine the observation seasons separately for various ISN species and populations. We find that using an adjustable viewing direction allows for ISN gas observations in the upwind hemisphere, where the signal is not distorted by gravitational focusing, in addition to the viewing of ISN species throughout the entire year. Moreover, we demonstrate that with appropriately adjusted observation geometries, primary and secondary populations can be fully separated. Additionally, we show that atoms of ISN gas on indirect trajectories are accessible for detection, and we present their impact on the study of the ionization rates for ISN species., Comment: 15 figures, 3 tables, 26 pages

Interaction of the solar wind with interstellar matter involves, among other, charge exchange between interstellar neutral atoms and plasma, which results in the creation of secondary population of interstellar neutral (ISN) atoms. The secondary population of interstellar He was detected by Interstellar Boundary Explorer (IBEX), but interpretation of these measurements was mostly based on an approximation that the primary interstellar neutral population and the secondary population were non-interacting homogeneous Maxwell-Boltzmann functions in the outer heliosheath. We simulate the distribution function in the outer heliosheath and inside the heliopause using method of characteristics with statistical weights obtained from solutions of the production and loss equations for the secondary atoms due to charge-exchange collisions in the outer heliosheath. We show that the two-Maxwellian approximation for the distribution function of neutral He is not a good approximation within the outer heliosheath but a reasonable one inside the termination shock. This is due to a strong selection effect: the He atoms able to penetrate inside the termination shock are a small, peculiar subset of the entire secondary He population. Nevertheless, the two-Maxwellian approximation reproduces the density distribution of ISN He inside the termination shock well and enables a realistic reproduction of the orientation of the plane defined by the Sun's velocity vector through the local interstellar matter and the vector of unperturbed interstellar magnetic field., Comment: Accepted for ApJ

We study the distribution of the interstellar neutral (ISN) gas density and the pick-up ion (PUI) density of hydrogen, helium, neon, and oxygen in the heliosphere for heliocentric distances from inside 1 au up to the solar wind termination shock (TS), both in and out of the ecliptic plane. We discuss similarities and differences in the large-scale structures of the ISN gas and PUIs formed in the heliosphere between various species. We discuss the distribution of ISN gas and PUI densities for two extreme phases of the solar activity cycle, it is the solar minimum and the solar maximum. We identify the location of the ISN gas density cavity of various species. We study the relative abundance ratios of Ne/O, H/He, Ne/He, and O/He for ISN gas and PUIs densities and their variation with location in the heliosphere. We also discuss the modulation of relative abundance ratios of ISN gas and PUIs along the TS. We conclude that the preferable locations for detection of He$^+$ and Ne$^+$ PUIs are in the downwind hemisphere within 1~au, whereas for H$^+$ and O$^+$ PUIs the preferable locations for detection are for distances from Jupiter to Pluto orbits., Comment: 18 figures, 2 tables, Published in The Astrophysical Journal

Solar ionizing factors are responsible for modulation of interstellar neutral gas and its derivative populations inside the heliosphere. We provide an overview of the current state of knowledge about them for heliospheric particles inside the termination shock. We discuss charge exchange with solar wind particles, photoionization, and electron impact ionization for hydrogen, oxygen, neon, and helium from 1985 to 2018 both in the ecliptic plane and in the polar regions. We discuss ionization rates as a function of time, distance to the Sun, and latitude. We compare the total ionization rates among the species within a consistent and homogeneous system of calculation of the ionization rates. The highest total ionization rates at 1 au in the ecliptic plane are for hydrogen and oxygen, and the lowest are for helium. In the polar regions, the strongest ionization losses are for oxygen, regardless of the solar activity. Photoionization is the dominant ionization reaction for helium and neon, and a reaction of high significance for oxygen. Charge exchange with solar wind particles is the dominant ionization reaction for hydrogen and the second important ionization reaction for oxygen. Electron impact ionization is an important ionization reaction for Ne and He, with the contribution to the total ionization rates stronger within 1 au and smaller outside. The total ionization rates for He and Ne vary in time with the solar activity, whereas the total ionization rates for H and O follow the cyclic solar wind variations out of the ecliptic plane and aperiodic variations in the ecliptic plane., Comment: 12 pages, 8 figures, 2 tables, published in The Astrophysical Journal

Following the derivation of a more accurate model of the evolution of the solar Lyman-$\alpha$ line with the changing solar activity by Kowalska-leszczynska et al. 2018 (IKL18) than the formerly used model by Tarnopolski et al. 2009 (ST09), we investigate potential consequences that adoption of the resulting refined model of radiation pressure has for the model distribution of interstellar neutral (ISN) H in the inner heliosphere and on the interpretation of selected observations. We simulated the ISN H densities using the two alternative radiation pressure models and identical models of all other factors affecting the ISN H distribution. We found that during most of the solar cycle, the IKL18 model predicts larger densities of ISN H and PUIs than ST09 in the inner heliosphere, especially in the downwind hemisphere. However, the density of ISN H at the termination shock estimated by Bzowski et al. 2008 obtained using ST09 does not need revision, and the detection of ISN D by IBEX is supported. However, we point out the existence of a considerable absorption of a portion of the solar Lyman-$\alpha$ spectral flux inside the heliosphere. Therefore, the model of radiation pressure for ISN H is still likely to need revision, and hence the available models of ISN H are not self-consistent., Comment: Accepted to ApJ 17 pages, 10 figures

Direct-sampling observations of interstellar neutral (ISN) He by Interstellar Boundary Explorer (IBEX) provide valuable insight into the physical state of and processes operating in the interstellar medium ahead of the heliosphere. The ISN He atom signals are observed at the four lowest ESA steps of the IBEX-Lo sensor. The observed signal is a mixture of the primary and secondary components of ISN He and H. Previously, only data from one of the ESA steps have been used. Here, we extended the analysis to data collected in the three lowest ESA steps with the strongest ISN He signal, for the observation seasons 2009-2015. The instrument sensitivity is modeled as a linear function of the atom impact speed onto the sensor's conversion surface separately for each ESA step of the instrument. We found that the sensitivity increases from lower to higher ESA steps, but within each of the ESA steps it is a decreasing function of the atom impact speed. This result may be influenced by the hydrogen contribution, which was not included in the adopted model, but seems to exist in the signal. We conclude that the currently accepted temperature of ISN He and velocity of the Sun through the interstellar medium do not need a revision, and we sketch a plan of further data analysis aiming at investigating ISN H and a better understanding of the population of ISN He originating in the outer heliosheath., Comment: 20 pages, 5 figures, 5 tables, accepted for publication in the The Astrophysical Journal

Recent studies of interstellar neutral (ISN) hydrogen observed by the Interstellar Boundary Explorer (IBEX) suggested that the present understanding of the radiation pressure acting on hydrogen atoms in the heliosphere should be revised. There is a significant discrepancy between theoretical predictions of the ISN H signal using the currently used model of the solar Lyman-alpha profile by Tarnopolski et al. 2009 (TB09) and the signal due to ISN H observed by IBEX-Lo. We developed a new model of evolution of the solar Lyman-alpha profile that takes into account all available observations of the full-disk solar Lyman-alpha profiles from SUMER/SOHO, provided by Lemaire et al. 2015 (L15), covering practically the entire 23rd solar cycle. The model has three components that reproduce different features of the profile. The main shape of the emission line that is produced in the chromosphere is modeled by the kappa function; the central reversal due to absorption in the transition region is modeled by the Gauss function; the spectral background is represented by the linear function. The coefficients of all those components are linear functions of the line-integrated full-disk Lyman-alpha irradiance, which is the only free parameter of the model. The new model features potentially important differences in comparison with the model by TB09, which was based on a limited set of observations. This change in the understanding of radiation pressure, especially during low solar activity, may significantly affect the interstellar H and D distributions in the inner heliosphere and their derivative populations., Comment: 15 pages, 11 figures, 2 tables

Observations of energetic neutral atoms (ENAs) are a fruitful tool for remote diagnosis of the plasma in the heliosphere and its vicinity. So far, instruments detecting ENAs from the heliosphere were configured for observations of hydrogen atoms. Here, we estimate emissions of ENAs of the heavy chemical elements helium, oxygen, nitrogen, and neon. A large portion of the heliospheric ENAs is created in the inner heliosheath from neutralized interstellar pick-up ions (PUIs). We modeled this process and calculated full-sky intensities of ENAs for energies 0.2-130 keV/nuc. We found that the largest fluxes among considered species are expected for helium, smaller for oxygen and nitrogen, and smallest for neon. The obtained intensities are 50-10^6 times smaller than the hydrogen ENA intensities observed by IBEX. The detection of heavy ENAs will be possible if a future ENA detector is equipped with the capability to measure the masses of observed atoms. Because of different reaction cross-sections among the different species, observations of heavy ENAs can allow for a better understanding of global structure of the heliosphere as well as the transport and energization of PUIs in the heliosphere., Comment: 21 pages, 8 figures, 1 table, published in The Astrophysical Journal

We simulated the signal due to neutral He atoms, observed by Interstellar Boundary Explorer (IBEX), assuming that charge exchange collisions between neutral He atoms and He+ ions operate everywhere between the heliopause and a distant source region in the local interstellar cloud, where the neutral and charged components are in thermal equilibrium. We simulated several test cases of the plasma flow within the outer heliosheath and investigated the signal generation for plasma flows both in the absence and in the presence of the interstellar magnetic field. We found that a signal in the portion of IBEX data identified as due to the Warm Breeze does not arise when a homogeneous plasma flow in front of the heliopause is assumed, but it appears immediately when any reasonable disturbance in its flow due to the presence of the heliosphere is assumed. We obtained a good qualitative agreement between the data selected for comparison and the simulations for a model flow with the velocity vector of the unperturbed gas and the direction and intensity of magnetic field adopted from recent determinations. We conclude that direct-sampling observations of neutral He atoms at 1~AU from the Sun are a sensitive tool for investigating the flow of interstellar matter in the outer heliosheath, that the Warm Breeze is indeed the secondary population of interstellar helium, as it was hypothesized earlier, and that the WB signal is consistent with the heliosphere distorted from axial symmetry by the interstellar magnetic field., Comment: Submitted to ApJ

Observations of energetic neutral atoms (ENAs) allow for remote sensing of plasma properties in distant regions of the heliosphere. So far, most of the observations have concerned only hydrogen atoms. In this paper, we present perspectives for observations of helium energetic neutral atoms (He ENAs). We calculated the expected intensities of He ENAs created by the neutralization of helium ions in the inner heliosheath and through the secondary ENA mechanism in the outer heliosheath. We found that the dominant source region for He ENAs is the inner heliosheath. The obtained magnitudes of intensity spectra suggest that He ENAs can be observed with future ENA detectors, as those planned on Interstellar Mapping and Acceleration Probe. Observing He ENAs is most likely for energies from a few to a few tens of keV/nuc. Estimates of the expected count rates show that the ratio of helium to hydrogen atoms registered in the detectors can be as low as 1:10^4. Consequently, the detectors need to be equipped with an appropriate mass spectrometer capability, allowing for recognition of chemical elements. Due to the long mean free paths of helium ions in the inner heliosheath, He ENAs are produced also in the distant heliospheric tail. This implies that observations of He ENAs can resolve its structure, which seems challenging from observations of hydrogen ENAs since energetic protons are neutralized before they progress deeper in the heliospheric tail., Comment: 31 pages, 12 figures, 3 tables, published in The Astrophysical Journal

Observations of energetic neutral atoms (ENAs) allow for remote studies of the plasma condition in the heliosphere and the neighboring local interstellar medium. The first IBEX results revealed an arc-like enhancement of the ENA intensity in the sky, known as the ribbon. The ribbon was not expected from the heliospheric models prior to the IBEX launch. One of the proposed explanations of the ribbon is the mechanism of the secondary ENA emission. The ribbon reveals energy-dependent structure in the relative intensity along its circumference and in the position. Namely, the ribbon geometric center varies systematically by about 10$^\circ$ in the energy range 0.7-4.3 keV. Here, we show by analytic modeling that this effect is a consequence of the helio-latitudinal structure of the solar wind reflected in the secondary ENAs. Along with a recently measured distance to the ribbon source just beyond the heliopause, our findings support the connection of the ribbon with the local interstellar magnetic field by the mechanism of the secondary ENA emission. However, the magnitude of the center shift in the highest IBEX energy channel is much larger in the observations than expected from the modeling. This may be due to another, not currently recognized, process of ENA generation., Comment: 14 pages, 8 figures, 1 table, v3: corrected caption to Figure 3

We simulated the modulation of the interstellar neutral (ISN) He, Ne, and O density and pick-up ion (PUI) production rate and count rate along the Earth's orbit over the solar cycle from 2002 to 2013 to verify if solar cycle-related effects may modify the inferred ecliptic longitude of the ISN inflow direction. We adopted the classical PUI model with isotropic distribution function and adiabatic cooling, modified by time- and heliolatitude-dependent ionization rates and non-zero injection speed of PUIs. We found that the ionization losses have a noticeable effect on the derivation of the ISN inflow longitude based on the Gaussian fit to the crescent and cone peak locations. We conclude that the non-zero radial velocity of the ISN flow and the energy range of the PUI distribution function that is accumulated are of importance for a precise reproduction of the PUI count rate along the Earth orbit. However, the temporal and latitudinal variations of the ionization in the heliosphere, and particularly their variation on the solar cycle time-scale, may significantly modify the shape of PUI cone and crescent and also their peak positions from year to year and thus bias by a few degrees the derived longitude of the ISN gas inflow direction., Comment: Published in the Monthly Notices of the Royal Astronomical Society, vol. 458, Issue 4, pp 3691-3704, doi:10.1093/mnras/stw515

We have developed a refined and optimized version of the Warsaw Test Particle Model of interstellar neutral gas in the heliosphere, specially tailored for analysis of IBEX-Lo observations. The former version of the model was used in the analysis of neutral He observed by IBEX that resulted in an unexpected conclusion that the interstellar neutral He flow vector was different than previously thought and that a new population of neutral He, dubbed the Warm Breeze, exists in the heliosphere. It was also used in the reanalysis of Ulysses observations that confirmed the original findings on the flow vector, but suggested a significantly higher temperature. The present version model has two strains targeted for different applications, based on an identical paradigm, but differing in the implementation and in the treatment of ionization losses. We present the model in detail and discuss numerous effects related to the measurement process that potentially modify the resulting flux of ISN He observed by IBEX, and identify those of them that should not be omitted in the simulations to avoid biasing the results. This paper is part of a coordinated series of papers presenting the current state of analysis of IBEX-Lo observations of ISN He. Details of the analysis method are presented by Swaczyna et al. 2015, and results of the analysis are presented by Bzowski et al. 2015., Comment: This paper is a part of the special issue of Astrophysical Journal Supplement Series on interstellar neutrals measured by IBEX

The modeling of the heliosphere requires continuous three-dimensional solar wind data. The in-situ out-of-ecliptic measurements are very rare, so that other methods of solar wind detection are needed. We use the remote-sensing data of the solar wind speed from observations of interplanetary scintillation (IPS) to reconstruct spatial and temporal structures of the solar wind proton speed from 1985 to 2013. We developed a method of filling the data gaps in the IPS observations to obtain continuous and homogeneous solar wind speed records. We also present a method to retrieve the solar wind density from the solar wind speed, utilizing the invariance of the solar wind dynamic pressure and energy flux with latitude. To construct the synoptic maps of the solar wind speed we use the decomposition into spherical harmonics of each of the Carrington rotation map. To fill the gaps in time we apply the singular spectrum analysis to the time series of the coefficients of spherical harmonics. We obtained helio-latitudinal profiles of the solar wind proton speed and density over almost three recent solar cycles. The accuracy in the reconstruction is, due to computational limitations, about 20%. The proposed methods allow us to improve the spatial and temporal resolution of the model of the solar wind parameters presented in our previous paper (Sok\'o{\l} et al. 2013) and give a better insight into the time variations of the solar wind structure. Additionally, the solar wind density is reconstructed more accurately and it fits better to the in-situ measurements from Ulysses., Comment: Accepted to Solar Physics (doi:10.1007/s11207-015-0800-2). 26 pages, 13 figures

The NIS He gas has been observed at a few AU to the Sun almost from the beginning of the space age. To model its flow an estimate of the loss rates due to ionization by solar extreme-ultraviolet (EUV) flux is needed. The EUV irradiance has been measured directly from mid 1990-ties, but with high temporal and spectral resolution only from 2002. Beforehand only EUV proxies are available. A new method of reconstruction of the Carrington rotation averaged photoionization rates for neutral interstellar helium (NIS He) in the ecliptic at 1 AU to the Sun before 2002 is presented. We investigate the relation between the solar rotation averaged time series of the ionization rates for NIS He at 1 AU derived from TIMED measurements of EUV irradiance and the solar 10.7 cm flux (F10.7) only. We perform a weighted iterative fit of a nonlinear model to data split into sectors. The obtained formula allows to reconstruct the solar rotation averages of photoionization rates for He between ~1947 and 2002 with an uncertainty ranging from less than 10% during solar minimum up to 20% for solar maximum., Comment: 12 pages, 4 figures

NASA's Interstellar Boundary Explorer (IBEX) mission has recently opened a new window on the interstellar medium (ISM) by imaging neutral atoms. One "bright" feature in the sky is the interstellar wind flowing into the solar system. Composed of remnants of stellar explosions as well as primordial gas and plasma, the ISM is by no means uniform. The interaction of the local ISM with the solar wind shapes our heliospheric environment with hydrogen being the dominant component of the very local ISM. In this paper, we report on direct sampling of the neutral hydrogen of the local ISM over four years of IBEX observations. The hydrogen wind observed at 1 AU has decreased and nearly disappeared as the solar activity has increased over the last four years; the signal at 1 AU has dropped off in 2012 by a factor of ~8 to near background levels. The longitudinal offset has also increased with time presumably due to greater radiation pressure deflecting the interstellar wind. We present longitudinal and latitudinal arrival direction measurements of the bulk flow as measured over four years beginning at near solar minimum conditions. The H distribution we observe at 1 AU is expected to be different from that outside the heliopause due to ionization, photon pressure, gravity, and filtration by interactions with heliospheric plasma populations. These observations provide an important benchmark for modeling of the global heliospheric interaction. Based on these observations we suggest a further course of scientific action to observe neutral hydrogen over a full solar cycle with IBEX.

Hydrogen gas is the dominant component of the local interstellar medium. However, due to ionization and interaction with the heliosphere, direct sampling of neutral hydrogen in the inner heliosphere is more difficult than sampling the local interstellar neutral helium, which penetrates deep into the heliosphere. In this paper we report on the first detailed analysis of the direct sampling of neutral hydrogen from the local interstellar medium. We confirm that the arrival direction of hydrogen is offset from that of the local Helium component. We further report the discovery of a variation of the penetrating Hydrogen over the first two years of IBEX observations. Observations are consistent with hydrogen experiencing an effective ratio of outward solar radiation pressure to inward gravitational force greater than unity ({\mu}>1); the temporal change observed in the local interstellar hydrogen flux can be explained with solar variability.

Dynamics of H, D, and heavy Energetic Neutral Atoms (ENA) between the termination shock and 1 AU is discussed in the context of the forthcoming NASA SMEX mission IBEX. In particular, effects of the velocity-dependent radiation pressure on atomic trajectories are considered and ionization losses between TS and 1 AU are studied. It is shown, among others, that most of the dynamical effects and ionization losses are induced within a few AU from the Sun, which translates to the time domain into $\sim 1 - 3$ solar rotations before detection. This loosens considerably time requirements for tracking the ionization and radiation pressure history to just prior 3 months. ENA seem excellent tracers of the processes within the heliospheric interface, with the transport effects between the termination shock and detector relatively mild and easy to account for., Comment: submitted to Proceedings of the 5-th IGPP Astrophysics Conference, Honolulu HI, March 2006; 6 pages

It is well known that the Solar System is presently moving through a partially ionized local interstellar medium. This gives rise to a counter-flow situation requiring a consistent description of behaviour of the two fluids -- ions and neutral atoms -- which are dynamically coupled by mutual charge exchange processes. Solutions to this problem have been offered in the literature, all relying on the assumption that the proton fluid, even under evidently nonequilibrium conditions, can be expected to stay in a highly-relaxated distribution function given by mono-Maxwellians shifted by the local proton bulk velocity. Here we check the validity of this assumption, calculating on the basis of a Boltzmann-kinetic approach the actually occurring deviations. As we show, especially for low degrees of ionization, $\xi \le 0.3$, both the H-atoms and protons involved do generate in the heliospheric interface clearly pronounced deviations from shifted Maxwellians with asymmetrically shaped distribution functions giving rise to non-convective transport processes and heat conduction flows. Also in the inner heliosheath region and in the heliotail deviations of the proton distribution from the hydrodynamic one must be expected. This sheds new light on the correctness of current calculations of H-atom distribution functions prevailing in the inner heliosphere and also of the Lyman-$\alpha $ absorption features in stellar spectra due to the presence of the hydrogen wall atoms. Deviations from LTE-functions would be even more pronounced in magnetic interfaces, which via CGL-effects cause temperature anisotropies to arise., Comment: 18 pages, 8 figures, A&A in press