Search

We highlight the importance of magnetic reconnection at the heliopause, both as one of the key processes driving the interaction between solar and interstellar media, but also as an element of the definition of the heliopause itself. We highlight the main observations that have fed the current debates on the definition, location and shape of the heliopause. We explain that discriminating between the current interpretations of plasma and magnetic field structures near the heliopause necessitates appropriate measurements which are lacking on Voyager 1 and 2, and describe some of the ensuing requirements for thermal plasma measurements on a future Interstellar Probe. The content of this article was submitted as a white paper contribution to the Decadal Survey for Solar and Space Physics 2024–2033 of the National Academy of Sciences.

As the Sun moves through the local interstellar medium (LISM), neutral atoms travel through the heliosphere and can be detected by IBEX. We consider interstellar neutral (ISN) hydrogen atoms with a drifting Maxwellian distribution function in the LISM that travel on almost hyperbolic trajectories to the inner heliosphere. They are subject to solar gravity and radiation pressure, as well as ionization processes. For ISN H, the radiation pressure, which exerts an effective force comparable to gravitation, decelerates individual atoms and shifts the longitude of their observed peak relative to that of ISN He. We used the peak longitude of the observed flux in the lowest energy channel of IBEX-Lo to investigate how radiation pressure shifts the ISN H signal over almost an entire solar cycle (2009–2018). Thus, we have created a new methodology to determine the Lyα effective radiation pressure from IBEX ISN H data. The resulting effective ratio of the solar radiation pressure and gravitation (μeff = 1.074 ± 0.038), averaged over cycle 24, appears to agree within the uncertainties with simulations based on total irradiance observations7 while being higher by ∼21%. Our analysis indicates an increase of μeff with solar activity, albeit with substantial uncertainties. Further study of IBEX H response functions and future Interstellar Mapping and Acceleration Probe data should provide significant reduction of the uncertainties and improvements in our understanding of the effects of radiation pressure on ISN atoms. [ABSTRACT FROM AUTHOR]

Interstellar neutral gas atoms penetrate the heliopause and reach 1 au, where they are detected by Interstellar Boundary Explorer (IBEX). The flow of neutral interstellar helium through the perturbed interstellar plasma in the outer heliosheath (OHS) results in the creation of a secondary population of interstellar He atoms, the so-called Warm Breeze, due to charge exchange with perturbed ions. The secondary population brings the imprint of the OHS conditions to the IBEX-Lo instrument. Based on a global simulation of the heliosphere with measurement-based parameters and detailed kinetic simulation of the filtration of He in the OHS, we find the number density of the interstellar He+ population to be (8.98 ± 0.12) × 10−3 cm−3. With this, we obtain the absolute density of interstellar H+ as 5.4 × 10−2 cm−3 and that of electrons as 6.3 × 10−2 cm−3, with ionization degrees of 0.26 for H and 0.37 for He. The results agree with estimates of the parameters of the Very Local Interstellar Matter obtained from fitting the observed spectra of diffuse interstellar EUV and the soft X-ray background. [ABSTRACT FROM AUTHOR]

The Interstellar Boundary Explorer (IBEX) is a NASA satellite in Earth orbit, dedicated to observing interstellar neutral (ISN) atoms entering the heliosphere and energetic neutral atoms from the heliosheath from 11 eV to 6 keV. This work presents comprehensive maps of ISN hydrogen observed with IBEX at energies between 11 and 41 eV, covering almost an entire solar cycle from 2009 to 2018. ISN hydrogen measurements can provide information on the interstellar medium and on the heliosphere that modifies the incoming ISN flow. Whereas hydrogen is the dominant species in the unperturbed interstellar medium, most ISN hydrogen atoms crossing into the heliosphere do not reach the inner solar system: some are filtered out around the heliopause, while others are held off by solar radiation pressure or may be ionized as they approach the Sun. This paper presents and evaluates several approaches for generating model-free maps of ISN hydrogen from IBEX measurements. We discuss the basic implications of our results for ISN hydrogen inflow and outline the remaining discrepancies between observations and model predictions. Our maps show, during weak solar activity from 2009 to 2011, a clear signal of ISN hydrogen for ecliptic longitudes between 240° and 310°, roughly one month after the signal of ISN helium has peaked. When the solar activity approached its maximum around 2014, the ISN hydrogen signal weakened and dropped below the detection threshold because of increasing solar radiation pressure and ionization. The ISN hydrogen signal then reappeared in 2017. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of energetic neutral atoms (ENAs) of 10 eV to 2.5 keV from the downwind hemisphere of the heliosphere. These ENAs are believed to originate mostly from pickup protons and solar-wind protons in the inner heliosheath. This study includes all low-energy observations made with the Interstellar Boundary Explorer over the first eight years. Because the protons around 0.1 keV dominate the plasma pressure within the inner heliosheath in downwind direction, these ENA observations offer the unique opportunity to constrain the plasma properties and dimensions of the heliosheath where no in situ observations are available. We first derive energy spectra of ENA intensities averaged over time for 49 macropixels covering the entire downwind hemisphere. The results confirm previous studies regarding integral intensities and the roll-over around 0.1 keV energy. With the expanded data set, we now find that ENA intensities at 0.2 and 0.1 keV seem to anti-correlate with solar activity. We then derive the product of total plasma pressure and emission thickness of protons in the heliosheath to estimate lower limits on the thickness of the inner heliosheath. The temporally averaged ENA intensities support a rather spherical shape of the termination shock and a heliosheath thickness between 150 and 210 au for most regions of the downwind hemisphere. Around the nominal downwind direction of 76° ecliptic longitude, the heliosheath is at least 280 au thick. There, the neutral hydrogen density seems to be depleted compared to upwind directions by roughly a factor of 2. [ABSTRACT FROM AUTHOR]

Quantitative analysis of the interstellar heavy (oxygen and neon) atom fluxes obtained by the Interstellar Boundary Explorer (IBEX) suggests the existence of the secondary interstellar oxygen component. This component is formed near the heliopause due to charge exchange of interstellar oxygen ions with hydrogen atoms, as was predicted theoretically. A detailed quantitative analysis of the fluxes of interstellar heavy atoms is only possible with a model that takes into account both the filtration of primary and the production of secondary interstellar oxygen in the boundary region of the heliosphere as well as a detailed simulation of the motion of interstellar atoms inside the heliosphere. This simulation must take into account photoionization, charge exchange with the protons of the solar wind and solar gravitational attraction. This paper presents the results of modeling interstellar oxygen and neon atoms through the heliospheric interface and inside the heliosphere based on a three-dimensional kinetic-MHD model of the solar wind interaction with the local interstellar medium and a comparison of these results with the data obtained on the IBEX spacecraft. [ABSTRACT FROM AUTHOR]

The direct measurements of interstellar matter by the Interstellar Boundary Explorer (IBEX) mission have opened a new and important chapter in our study of the interactions that control the boundaries of our heliosphere. Here we derive for the quantitative information about interstellar O flow parameters from IBEX low-energy neutral atom data for the first time. Specifically, we derive a relatively narrow four-dimensional parameter tube along which interstellar O flow parameters must lie. Along the parameter tube, we find a large uncertainty in interstellar O flow longitude, 76.°0 ± 3.°4 from χ2 analysis and 76.°5 ± 6.°2 from a maximum likelihood fit, which is statistically consistent with the flow longitude derived for interstellar He, 75.°6 ± 1.°4. The best-fit O and He temperatures are almost identical at a reference flow longitude of 76°, which provides a strong indication that the local interstellar plasma near the Sun is relatively unaffected by turbulent heating. However, key differences include an oxygen parameter tube for the interstellar speed (relation between speed and longitude) that has higher speeds than those in the corresponding parameter tube for He, and an upstream flow latitude for oxygen that is southward of the upstream flow latitude for helium. Both of these differences are likely the result of enhanced filtration of interstellar oxygen due to its charge-exchange ionization rate, which is higher than that for helium. Furthermore, we derive an interstellar O density near the termination shock of cm−3 that, within uncertainties, is consistent with previous estimates. Thus, we use IBEX data to probe the interstellar properties of oxygen. [ABSTRACT FROM AUTHOR]

We present an improved analysis of the energy spectrum of energetic neutral hydrogen from the heliosheath observed with the IBEX-Lo sensor on the Interstellar Boundary EXplorer from the years 2009 to 2012. This analysis allows us to study the lowest energies between 10 and 100 eV although various background sources are more intense than the targeted signal over broad areas of the sky. The results improve our knowledge of the interaction region between our heliosphere and the interstellar plasma because these neutral atoms are direct messengers from the low-energy plasma in the heliosheath. We find a roll-over of the energy spectrum below 100 eV, which has major implications for the pressure balance of the plasma in the inner heliosheath. The results can also be compared directly with in situ observations of the Voyager 1 and 2 spacecraft. [ABSTRACT FROM AUTHOR]