Strength of the Termination Shock Inferred from the Globally Distributed Energetic Neutral Atom Flux from IBEX

Shrestha, Bishwas L.; Zirnstein, E. J.; Heerikhuisen, J.; Zank, G. P.

doi:10.3847/1538-4365/abf659

Cited by 10 publications

(12 citation statements)

References 51 publications

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“…(2010) method (Zirnstein et al 2017;Kornbleuth et al 2020Kornbleuth et al , 2021Shrestha et al 2021), scaling factors ranging from 1.8 to 2.5 were required to match observations. These methods only included two populations of PUIs: transmitted PUIs and reflected PUIs.…”

Section: Simulating the Ena Flux In The Heliosheathmentioning

confidence: 99%

“…This is usually done by partitioning the MHD plasma quantities to represent multiple ion species in postprocessing downstream of the termination shock to properly model ENAs. ENA models that use a combined Maxwellian approach to replicate a kappa distribution in the solar wind plasma for three ion populations at the termination shock (thermal ions, "transmitted PUIs" which have sufficient energy to overcome the cross-shock potential of the termination shock, and "reflected PUIs" which do not initially have sufficient energy to overcome the crossshock potential of the termination shock and are reflected at the shock) require a scaling factor to compare directly with IBEX-Hi observations ranging from 1.8 to 2.5 (Zirnstein et al 2017;Kornbleuth et al 2018Kornbleuth et al , 2020Shrestha et al 2021) regardless of solar wind conditions (steady state or with the inclusion of realistic solar cycle). Baliukin et al (2020) use a kinetic model for PUIs which models the propagation of PUIs in the heliosheath and includes the change in PUI energy due to ∇ • v, where v is the bulk velocity of the plasma, in the heliosheath.…”

Section: Introductionmentioning

confidence: 99%

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An Anomalous Cosmic-Ray Mediated Termination Shock: Implications for Energetic Neutral Atoms

Kornbleuth

Opher

Zank

et al. 2023

ApJL

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The Voyager 2 crossing of the termination shock indicated that most of the upstream energy from the thermal solar wind ions was transferred to pickup ions (PUIs) and other energetic particles downstream of the shock. We use hybrid simulations at the termination shock for the Voyager 2, flank, and tail directions to evaluate the distributions of different ion species downstream of the shock over the energy range of 0.52–55 keV. Here, we extend the work of Gkioulidou et al., which showed an energy-dependent discrepancy between modeled and energetic neutral atom (ENA) observations, and fit distributions to a hybrid model to show that a population of PUIs accelerated via diffusive shock acceleration (DSA) to become low-energy anomalous cosmic rays (ACRs) can bridge the gap between modeled and observed ENA fluxes. Our results with the inclusion of DSA via hybrid fitting give entirely new and novel evidence that DSA at the termination shock is likely to be an important physical process. These ACRs carry a significant fraction of the energy density at the termination shock (22%, 13%, and 19% in the Voyager 2, flank, and tail directions, respectively). Using these ACRs in global ENA modeling of the heliosphere from 0.52 to 55 keV, we find that scaling factors as large as 1.8–2.5 are no longer required to match ENA observations at energies of ∼1–4 keV. Large discrepancies between modeled and observed ENAs only remain over energies of 4–20 keV, indicating that there may be a further acceleration mechanism in the heliosheath at these energies.

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Section: Simulating the Ena Flux In The Heliosheathmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

An Anomalous Cosmic-Ray Mediated Termination Shock: Implications for Energetic Neutral Atoms

Kornbleuth

Opher

Zank

et al. 2023

ApJL

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“…As in Kornbleuth et al (2020), we need to scale modeled ENA maps by a factor of 1.8 to provide an average global agreement with observations. In other works, such as Zirnstein et al (2017) and Shrestha et al (2021) which also use the superposition of Maxwellian distributions for the ions from Zank et al (2010), a scaling factor of 2.5 and ∼3 is required as well, respectively. Baliukin et al (2020) use a kinetic approach to PUI modeling in post-processing and assume that PUIs form a filled shell distribution downstream of the TS.…”

Section: Resultsmentioning

confidence: 99%

Signature of a heliotail organized by the solar magnetic field and the role of non-ideal processes in modeled IBEX ENA maps: a comparison of the BU and Moscow MHD models

Kornbleuth,

Opher,

Baliukin

et al. 2021

Preprint

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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 different 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, pickup 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 organized by the solar magnetic field into two distinct north and south columns that become lobes of high mass flux flowing down the heliotail, though 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 a higher neutral density or some acceleration of PUIs in the heliosheath is required.

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“…We use an SW density of 5.74 and 1.82 cm −3 at 1 au depending on heliolatitude for slow and fast SW, respectively, using Equation (4) (Shrestha et al 2020). We note that this compression ratio is greater than the observed HTS compression ratio from V2 (Richardson et al 2008), and the HTS compression ratio inferred from the IBEX ENA observations (Shrestha et al 2021). But its exact value does not have a significant effect on our results since we are only interested in the qualitative nature of the evolution of simulated ENA spectra.…”

Section: Simulation Of Polar Ena Spectrum Evolutionmentioning

confidence: 96%

Tracking the Rapid Opening and Closing of Polar Coronal Holes through IBEX ENA Observations

Shrestha

Zirnstein

McComas

2023

ApJ

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Fast solar wind (SW) flows outward from polar coronal holes (PCHs). The latitudinal extent of the fast SW varies during different phases of the solar cycle. The fast SW in the inner heliosheath produces a flatter proton spectrum than the slow SW that can be observed through energetic neutral atoms (ENAs) by the Interstellar Boundary Explorer (IBEX). In this study, we investigate the evolution of PCHs as reflected in the high-time resolution ENA flux measurements from IBEX-Hi, where the PCHs are identified by ENA spectral indices <1.8. The ENA spectral index over the poles shows a periodic evolution over the solar cycle 24. The surface area with flatter ENA spectra (<1.8) around the ecliptic south pole increases slightly from 2009–2011 and then decreased gradually from 2012–2014. The PCH completely disappears in 2016 and then starts to appear again starting in 2017, gradually growing until 2019. This evolution shows a clear correlation with the change in the PCH area observed at the Sun once the delay in the ENA observation time is included. In addition, the higher-cadence ENA data at the highest latitudes show a rapid evolution of the ENA spectrum near the south pole in 2014 and 2017. The rapid evolution in 2014 is related to a rapid closing of PCHs in 2012 and that in 2017 is related to a rapid opening of PCHs in late 2014. These results also agree qualitatively with the evolution of the ENA spectral index from simulations using a simple time-dependent heliospheric flow model.

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Strength of the Termination Shock Inferred from the Globally Distributed Energetic Neutral Atom Flux from IBEX

Cited by 10 publications

References 51 publications

An Anomalous Cosmic-Ray Mediated Termination Shock: Implications for Energetic Neutral Atoms

An Anomalous Cosmic-Ray Mediated Termination Shock: Implications for Energetic Neutral Atoms

Signature of a heliotail organized by the solar magnetic field and the role of non-ideal processes in modeled IBEX ENA maps: a comparison of the BU and Moscow MHD models

Tracking the Rapid Opening and Closing of Polar Coronal Holes through IBEX ENA Observations

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