94 research outputs found
Spherical Harmonic Representation of Energetic Neutral Atom Flux Components Observed by IBEX
The Interstellar Boundary Explorer (IBEX) images the heliosphere by observing
energetic neutral atoms (ENAs). The IBEX-Hi instrument onboard IBEX provides
full-sky maps of ENA fluxes produced in the heliosphere and very local
interstellar medium (VLISM) through charge exchange of suprathermal ions with
interstellar neutral atoms. The first IBEX-Hi results showed that in addition
to the anticipated globally distributed flux (GDF), a narrow and bright
emission from a circular region in the sky, dubbed the IBEX ribbon, is visible
in all energy steps. While the GDF is mainly produced in the inner heliosheath,
ample evidence indicates that the ribbon forms outside the heliopause in the
regions where the interstellar magnetic field is perpendicular to the lines of
sight. The IBEX maps produced by the mission team distribute the observations
into rectangle pixels in ecliptic coordinates. The overlap
of the GDF and ribbon components complicates qualitative analyses of each
source. Here, we find the spherical harmonic representation of the IBEX maps,
separating the GDF and ribbon components. This representation describes the ENA
flux components in the sky without relying on any pixelization scheme. Using
this separation, we discuss the temporal evolution of each component over the
solar cycle. We find that the GDF is characterized by larger spatial scale
structures than the ribbon. However, we identify two isolated, small-scale
signals in the GDF region that require further study.Comment: 27 pages, 13 figures, v2 accepted for publication in ApJ
Constraining the Evolution of the Proton Distribution Function in the Heliotail
We use Interstellar Boundary Explorer (IBEX) measurements of energetic neutral atoms (ENAs) to constrain the proton (mostly pickup ion, PUI) distribution in the heliotail. In our previous study, we solved the Parker transport equation and found that the velocity diffusion coefficient D(v) for PUIs is approximately D(v) ~ 1.1 × 10⁻⁸ km² s⁻³ (v/v₀)¹̇͘͘·³, assuming the initial proton distribution processed by the termination shock (TS), fp,₀, is a kappa distribution with kappa index κp,₀ = 1.63. In this study, we test different forms for f p,₀. We find that if f p,₀ is kappa-distributed and D(v) = D₀(v/v₀)¹̇͘͘·³, any kappa index in the range 1.5 2 keV compared to IBEX. However, using a fully kinetic particle-in-cell simulation to process a PUI filled-shell across the TS yields ENA spectra consistent with IBEX, reinforcing the significance of self-consistent, preferential PUI heating and diffusion at the TS. Interestingly, an upstream PUI distribution inferred from the particle-in-cell simulation to reproduce Voyager 2 observations of the nose-ward TS is inconsistent with IBEX observations from the heliotail, suggesting differences in the upstream PUI distribution or TS properties
Interstellar Conditions Deduced from Interstellar Neutral Helium Observed by IBEX and Global Heliosphere Modeling
In situ observations of interstellar neutral (ISN) helium atoms by the
IBEX-Lo instrument onboard the Interstellar Boundary Explorer (IBEX) mission
are used to determine the velocity and temperature of the pristine very local
interstellar medium (VLISM). Most ISN helium atoms penetrating the heliosphere,
known as the primary population, originate in the pristine VLISM. As the
primary atoms travel through the outer heliosheath, they charge exchange with
He ions in slowed and compressed plasma creating the secondary population.
With more than 2.4 million ISN helium atoms sampled by IBEX during ISN seasons
2009-2020, we compare the observations with predictions of a parametrized model
of ISN helium transport in the heliosphere. We account for the filtration of
ISN helium atoms at the heliospheric boundaries by charge exchange and elastic
collisions. We examine the sensitivity of the ISN helium fluxes to the
interstellar conditions described by the pristine VLISM velocity, temperature,
magnetic field, and composition. We show that comprehensive modeling of the
filtration processes is critical for interpreting ISN helium observations, as
the change in the derived VLISM conditions exceeds the statistical
uncertainties when accounting for these effects. The pristine VLISM parameters
found by this analysis are the flow speed (26.6 km s), inflow direction
in ecliptic coordinates (255.7, 5.04), temperature (7350 K),
and B-V plane inclination to the ecliptic plane (53.7). The derived
pristine VLISM He density is cm. Additionally, we
show a strong correlation between the interstellar plasma density and magnetic
field strength deduced from these observations.Comment: 13 pages, 3 figures, 2 tables, accepted for publication in Ap
Signature of a Heliotail Organized by the Solar Magnetic Field and the Role of Nonideal Processes in Modeled IBEX ENA Maps: A Comparison of the BU and Moscow MHD Models
Energetic neutral atom (ENA) models typically require post-processing routines to convert the distributions of plasma and H atoms into ENA maps. Here we investigate how two kinetic-MHD models of the heliosphere (the BU and Moscow models) manifest in modeled ENA maps using the same prescription and how they compare with Interstellar Boundary Explorer (IBEX) observations. Both MHD models treat the solar wind as a single-ion plasma for protons, which include thermal solar wind ions, pick-up ions (PUIs), and electrons. Our ENA prescription partitions the plasma into three distinct ion populations (thermal solar wind, PUIs transmitted and ones energized at the termination shock) and models the populations with Maxwellian distributions. Both kinetic-MHD heliospheric models produce a heliotail with heliosheath plasma that is organized by the solar magnetic field into two distinct north and south columns that become lobes of high mass flux flowing down the heliotail; however, in the BU model, the ISM flows between the two lobes at distances in the heliotail larger than 300 au. While our prescription produces similar ENA maps for the two different plasma and H atom solutions at the IBEX-Hi energy range (0.5–6 keV), the modeled ENA maps require a scaling factor of ∼2 to be in agreement with the data. This problem is present in other ENA models with the Maxwellian approximation of multiple ion species and indicates that either a higher neutral density or some acceleration of PUIs in the heliosheath is required
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