Olga Katushkina

LOCAL INTERSTELLAR MEDIUM: SIX YEARS OF DIRECT SAMPLING BY IBEX

The Interstellar Boundary Explorer (IBEX) has been directly observing neutral atoms from the local interstellar medium for the last six years (2009–2014). This paper ties together the 14 studies in this Astrophysical Journal Supplement Series Special Issue, which collectively describe the IBEX interstellar neutral results from this epoch and provide a number of other relevant theoretical and observational results. Interstellar neutrals interact with each other and with the ionized portion of the interstellar population in the “pristine” interstellar medium ahead of the heliosphere. Then, in the heliosphereʼs close vicinity, the interstellar medium begins to interact with escaping heliospheric neutrals. In this study, we compare the results from two major analysis approaches led by IBEX groups in New Hampshire and Warsaw. We also directly address the question of the distance upstream to the pristine interstellar medium and adjust both sets of results to a common distance of ~1000 AU. The two analysis approaches are quite different, but yield fully consistent measurements of the interstellar He flow properties, further validating our findings. While detailed error bars are given for both approaches, we recommend that for most purposes, the community use “working values” of ~25.4 km s⁻¹, ~75°7 ecliptic inflow longitude, ~−5°1 ecliptic inflow latitude, and ~7500 K temperature at ~1000 AU upstream. Finally, we briefly address future opportunities for even better interstellar neutral observations to be provided by the Interstellar Mapping and Acceleration Probe mission, which was recommended as the next major Heliophysics mission by the NRCʼs 2013 Decadal Survey.

Neutral interstellar helium parameters based on Ulysses/GAS and IBEX-LO observations: What are the reasons for the differences?

Recent analysis of the interstellar helium fluxes measured in 2009-2010 at Earths orbit by the Interstellar Boundary Explorer (IBEX) has suggested that the interstellar velocity (both direction and magnitude) is inconsistent with that derived previously from Ulysses/GAS observations made in the period from 1990 to 2002 at 1.5-5.5 AU from the Sun. Both results are model dependent, and models that were used in the analyses are different. In this paper, we perform an analysis of the Ulysses/GAS and IBEX-Lo data using our state-of-the-art three-dimensional time-dependent kinetic model of interstellar atoms in the heliosphere. For the first time, we analyze Ulysses/GAS data from year 2007, the closest available Ulysses/GAS observations in time to the IBEX observations. We show that the interstellar velocity derived from the Ulysses 2007 data is consistent with previous Ulysses results and does not agree with the velocity derived from IBEX. This conclusion is very robust since, as is shown in the paper, it does not depend on the ionization rates adopted in theoretical models. We conclude that Ulysses data are not consistent with the new local interstellar medium (LISM) velocity vector from IBEX. In contrast, IBEX data, in principle, could be explained with the LISM velocity vector derived from the Ulysses data. This is possible for the models where the interstellar temperature increased from 6300 K to 9000 K. There is a need to perform further studies of possible reasons for the broadening of the helium signal core measured by IBEX, which could be an instrumental effect or could be due to unconsidered physical processes.

Effect of the heliospheric interface on the distribution of interstellar hydrogen atom inside the heliosphere

This paper deals with the modeling of the interstellar hydrogen atoms (H atoms) distribution in the heliosphere. We study influence of the heliospheric interface, that is the region of the interaction between solar wind and local interstellar medium, on the distribution of the hydrogen atoms in vicinity of the Sun. The distribution of Hatoms obtained in the frame of the self-consistent kinetic-gasdynamicmodel of the heliospheric interface is compared with a simplified model which assumes Maxwellian distribution of H atoms at the termination shock and is called often as “hot” model. This comparison shows that the distribution of H atoms is significantly affected by the heliospheric interface not only at large heliocentric distances, but also in vicinity of the Sun at ∼1–5 AU. Hence, for analysis of experimental data connected with direct or undirect measurements of the interstellar atoms one necessarily needs to take into account effects of the heliospheric interface. In this paper we propose a new model that is relatively simple but takes into account all major effects of the heliospheric interface. This model can be applied for analysis of backscattered La-alpha radiation data obtained on board of different spacecraft.

Distribution of Interstellar Hydrogen Atoms in the Heliosphere and Backscattered Solar Lyman-α

We review the modern concepts of penetration of interstellar atoms of hydrogen into the heliosphere up to 1 AU. Before entering into the heliosphere the atoms penetrate through the region of the solar wind (SW) interaction with the local interstellar medium (LISM). In the interaction region the atoms can exchange charge with both solar wind and interstellar protons disturbed in the SW/LISM interaction region. Charge exchange results in a disturbance of the pristine interstellar atom flow in the interaction region, and, therefore, the parameters of interstellar gas inside the heliosphere are different from their interstellar values. This makes it more difficult to determine local interstellar parameters from measurements of the interstellar atoms inside the heliosphere, but, on the other side, opens possibilities to study the SW/LISM interaction region remotely. This paper overviews the main physical phenomena and modern models of the SW/LISM interaction and presents a state-of-art 3D kinetic model of the interstellar hydrogen gas inside the heliosphere. The distributions of the gas parameters are compared with the distributions obtained in the context of the classical hot model. Quantitative and qualitative differences are discussed. The state-of-art model is employed to calculate spectra of the backscattered Lyman-\(\alpha \) radiation as they would be measured at 1 AU and the zero, first and second moments of the spectra. It is shown that the SW/LISM interaction imprints in the spatial and velocity distribution of the interstellar atoms are revealed in the intensities, line-shifts, and line-widths of the distribution functions. A qualitative comparison of the model results with SOHO/SWAN data are presented.

OBSERVATIONS OF THE INTERPLANETARY HYDROGEN DURING SOLAR CYCLES 23 AND 24. WHAT CAN WE DEDUCE ABOUT THE LOCAL INTERSTELLAR MEDIUM

Observations of interstellar helium atoms by the Interstellar Boundary Explorer (IBEX) spacecraft in 2009 reported a local interstellar medium (LISM) velocity vector different from the results of the Ulysses spacecraft between 1991 and 2002. The interplanetary hydrogen (IPH), a population of neutrals that fills the space between planets inside the heliosphere, carries the signatures of the LISM and its interaction with the solar wind. More than 40 yr of space-based studies of the backscattered solar Lyα emission from the IPH provided limited access to the velocity distribution, with the first temporal evolution map of the IPH line-shift during solar cycle 23. This work presents the results of the latest IPH observations made by the Hubble Space Telescopes Space Telescope Imaging Spectrograph during solar cycle 24. These results have been compiled with previous measurements, including data from the Solar Wind Anisotropies instrument on the Solar and Heliospheric Observatory. The whole set has been compared to physically realistic models to test both sets of LISM physical parameters as measured by Ulysses and IBEX, respectively. This comparison shows that the LISM velocity vector has not changed significantly since Ulysses measurements.

IUVS echelle‐mode observations of interplanetary hydrogen: Standard for calibration and reference for cavity variations between Earth and Mars during MAVEN cruise

The high-resolution echelle mode of the Imaging Ultraviolet Spectrograph (IUVS) instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) mission has been designed to measure D and H Lyman-α emissions from the martian atmosphere to obtain key information about the physical processes by which water escapes into space. Toward this goal, the absolute calibration of the instrument is critical for determining the D and H densities, the D/H ratio, and the escape flux of water. The instrument made observations of interplanetary hydrogen (IPH) along multiple look directions and conducted several post-launch calibration campaigns during cruise as well as during orbit around Mars. The calibration efforts monitored instrument degradation and produced a consistent calibration factor at the hydrogen Lyman-α wavelength (121.567 nm). The instrument was calibrated with the diffuse emission of interplanetary hydrogen (IPH) as a standard candle using measurements and model results from the Solar Wind Anisotropies (SWAN) instrument. Validation of the calibrated instrument was made by (1) comparisons to simultaneous observations of the IPH made with the lower resolution FUV-mode of the IUVS instrument that were independently calibrated using standard stars as well as by (2) comparisons to same-day observations of Mars at hydrogen Lyman-α made with the Hubble Space Telescope that were calculated with a radiative transfer model. The FUV-mode stellar calibrated values and HST-based model results agreed with the echelle SWAN calibrated values to within 6% and 4%, respectively. The calibrated IUVS instrument can be used to interpret emissions of atmospheric species at Mars for insights into water evolution at the planet, as well as observed IPH measurements made during cruise for further insights into dynamics of the inner heliosphere.

INTERSTELLAR HYDROGEN FLUXES MEASURED BY IBEX-LO IN 2009: NUMERICAL MODELING AND COMPARISON WITH THE DATA

In this paper, we perform numerical modeling of the interstellar hydrogen fluxes measured by IBEX-Lo during orbit 23 (spring 2009) using a state-of-the-art kinetic model of the interstellar neutral hydrogen distribution in the heliosphere. This model takes into account the temporal and heliolatitudinal variations of the solar parameters as well as non-Maxwellian kinetic properties of the hydrogen distribution due to charge exchange in the heliospheric interface. We found that there is a qualitative difference between the IBEX-Lo data and the modeling results obtained with the three-dimensional, time-dependent model. Namely, the model predicts a larger count rate in energy bin~2 (20-41 eV) than in energy bin~1 (11-21 eV), while the data shows the opposite case. We perform study of the model parameter effects on the IBEX-Lo fluxes and the ratio of fluxes in two energy channels. We shown that the most important parameter, which has a major influence on the ratio of the fluxes in the two energy bins, is the solar radiation pressure. The parameter fitting procedure shows that the best agreement between the model result and the data occurs in the case when the ratio of the solar radiation pressure to the solar gravitation,

Hydrogen atoms in the inner heliosphere: SWAN‐SOHO and MASCS‐MESSENGER observations

\mu_0

Stability of the interstellar hydrogen inflow longitude from 20 years of SOHO/SWAN observations

, is 1.26

Non-monotonic spatial distribution of the interstellar dust in astrospheres: finite gyroradius effect

^{+0.06}_{-0.076}