Probing the Length of the Heliospheric Tail with Energetic Neutral Atoms (ENAs) from 0.52 to 80 keV

Abstract

The shape of the heliosphere is currently under active debate. Energetic neutral atoms (ENAs) offer the best method for investigating the global structure of the heliosphere. To date, the Interstellar Boundary Explorer (IBEX) and the Ion and Neutral Camera (INCA) that was on board Cassini provide the only global ENA observations of the heliosphere. While extensive modeling has been done at IBEX-Hi energies (0.52–6 keV), no global ENA modeling has been conducted for INCA energies (5.2–55 keV). Here, we use an ENA model of the heliosphere based on hybrid results that capture the heating and acceleration of pickup ions (PUIs) at the termination shock to compare modeled global ENA results with IBEX-Hi and INCA observations using both a long- and short-tail model of the heliosphere. We find that the modeled ENA results for the two heliotail configurations produce similar results from the IBEX-Hi through the INCA energies. We conclude from our modeled ENAs, which only include PUI acceleration at the termination shock, that ENA observations in currently available energy ranges are insufficient for probing the shape and length of the heliotail. However, as a prediction for the future IMAP-Ultra mission (3–300 keV) we present modeled ENA maps at 80 keV, where the cooling length (∼600 au) is greater than the distance where the long- and short-heliotail models differ (∼400 au), and find that IMAP-Ultra should be able to identify the shape of the heliotail, predicting differences in the north lobe to downwind flux ratio between the models at 48%.

The Role of Pickup Ions in the Interaction of the Solar Wind with the Local Interstellar Medium. I. Importance of Kinetic Processes at the Heliospheric Termination Shock

“…The energy partition was used to obtain the PUI properties from the plasma mixture instead of solving the equations describing the PUI behavior self-consistently. Recently, Kornbleuth et al (2023) have used the hybrid simulations of Giacalone et al (2021) and the theoretical analysis of Wang et al (2023) to investigate the effect of diffusive shock acceleration on the PUI transmission through the TS in the framework of a global model, which did not include PUIs separately. In addition, rather strong assumptions were made to extrapolate or deduce the properties of PUIs across the entire TS surface and the IHS, with the purpose of fitting the IBEX and Cassini INCA measurements of ENA fluxes in the energy range up to 40 keV.…”

The Discrepancy between Observed and Predicted Heliospheric Energetic Neutral Atoms below Solar Wind Energy

“…The neutral particles are described kinetically using a Monte Carlo method. While two different ENA models are used in the different MHD models, Kornbleuth et al (2021bKornbleuth et al ( , 2023b showed that using the same ENA model in both MHD solutions produced quantitatively similar ENA maps.…”

“…PUIs created in the heliosheath had been previously ignored in the modeling of Kornbleuth et al (2023b) due to their contributions at lower energies. Here, the IHS PUIs are included as well.…”

“…The complex ion energy spectrum inside the heliosheath (Dialynas et al 2020), including the GDF ENA intensities, was used by Gkioulidou et al (2022) showing an energy-dependent discrepancy between the model and the measurements over the energy range of 0.52-55 keV (also see discussion in Galli et al 2022a;Kleimann et al 2022;and references therein). Recently, Kornbleuth et al (2023b) expanded on the basic model from Gkioulidou et al (2022) by introducing a new population of PUIs accelerated via diffusive shock acceleration that become low-energy anomalous cosmic rays (ACRs), and thus eliminated the discrepancy between the modeled and observed GDF ENA fluxes over the ∼0.5-4 keV energy range. However, the discrepancy at ∼4 to ∼20 keV for the Belt ENAs (a broad region of enhanced ENA intensities centered at ∼260°ecliptic longitude; Krimigis et al 2009) is still persistent, suggesting the need for further acceleration of PUIs inside the heliosheath.…”

“…Our investigation revisits several possible sources of the GDF ENAs between 10 eV and 2 keV, presenting new results from two ENA models and discussing the discrepancy between these ENA model predictions and IBEX-Lo observations. The two ENA models are based on two independent heliospheric models developed by and by Kornbleuth et al (2021aKornbleuth et al ( , 2023b. The two models rely on the same interstellar parameters and heliosphere boundary conditions, both modeling the cold solar wind plasma, hot PUIs, and electrons as one MHD fluid and calculating the neutral particle trajectories kinetically.…”