The Earth's hydrogen exosphere Lyman‐α radiation was mapped with the Solar Wind Anisotropies/Solar and Heliospheric Observatory (SWAN/SOHO) instrument in January 1996, 1997, and 1998 (low solar ...activity). The use of a hydrogen absorption cell allowed to disentangle the interplanetary emission from the geocoronal one and to assign the absorbed signal almost entirely to the geocorona. The geocorona was found to extend at least up to 100 Earth radii (RE) with an intensity of 5 Rayleigh, an unprecedented distance well exceeding the recent results of Lyman Alpha Imaging Camera (LAICA) imager (∼50 RE), and encompassing the orbit of the Moon (∼60 RE). We developed a numerical kinetic model of the hydrogen atoms distribution in the exosphere, which includes the solar Lyman‐α radiation pressure and the ionization. The radiation pressure compresses the H exosphere on the dayside, producing a bulge of H density between 3 and 20 RE, which fits observed intensities very well. The SWAN Lyman‐α distribution of intensity was compared both to LAICA (2015) and to Orbiting Geophysical Observatory number 5 (1968) measurements. Integrated H densities of SWAN at a tangent distance of 7 RE are larger than LAICA/Orbiting Geophysical Observatory number 5 by factors 1.1–2.5, while we should expect a stronger effect of the radiation pressure at solar max. We discuss the possible role of H atoms in satellite orbits to explain this apparent contradiction. An onion‐peeling technique is used to retrieve hydrogen number density in the exosphere for the three SWAN observations. They show an excess of density versus models at large distances, which is likely due to nonthermal atoms (not in the model).
Key Points
We find that the geocorona extends to almost twice the distance of the Moon
the H exosphere is compressed by solar radiation pressure, forming a bulge on the dayside
this bulge is enhanced at low solar activity, possibly in relation with a population of Hatoms in satellite orbits
ABSTRACT
The interstellar boundary explorer (IBEX) has been measuring fluxes of the energetic neutral atoms (ENAs) using the IBEX-Hi (0.3–6 keV) instrument since 2008. We have developed a numerical ...time-depended code to calculate globally distributed flux (GDF) of hydrogen ENAs employing both (1) 3D kinetic-MHD model of the global heliosphere and (2) reconstruction of atom trajectories from 1 au, where they are observed by IBEX, to the point of their origin in the inner heliosheath (IHS). The key factor in the simulation is a detailed kinetic consideration of the pickup ions (PUIs), the supra-thermal component of protons in the heliosphere, which is ‘parental’ to the ENAs and originates in the region of the supersonic solar wind being picked by the heliospheric magnetic field. As a result of our study, we have concluded that (1) the developed model is able to reproduce the geometry of the multilobe structure seen in the IBEX-Hi GDF maps, (2) the GDF is extremely sensitive to the form of the velocity distribution function of PUIs in the IHS, and the accounting for the existence of an additional energetic population of PUIs is essential to explain the data, (3) despite a relatively good agreement, there are some quantitative differences between the model calculations and IBEX-Hi data. Possible reasons for these differences are discussed.
Abstract
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.
ABSTRACT
Pickup protons originate as a result of the ionization of hydrogen atoms in the supersonic solar wind, forming the suprathermal component of protons in the heliosphere. While they are being ...picked by the heliospheric magnetic field and convected into the heliosheath, the pickup protons may suffer stochastic acceleration from the solar wind turbulence in the region from the Sun up to the heliospheric termination shock, where they can also experience shock-drift acceleration or reflection from the cross-shock potential. These processes create a high-energy tail in the pickup ion energy distribution. The properties of this energetic pickup proton population are still not well defined, in spite of the fact that they are vital for models that simulate energetic neutral atom fluxes. We consider two scenarios for the pickup proton velocity distribution downstream of the heliospheric termination shock (a filled shell with an energetic power-law tail, and bi-Maxwellian). Based on a numerical kinetic model and observations of the energetic neutral atom fluxes from the inner heliosheath by the IBEX-Hi instrument, we characterize the pickup proton distribution and provide estimations of the properties of the energetic pickup proton population downstream of the termination shock.
ABSTRACT
The hydrogen atoms penetrate the heliosphere from the local interstellar medium, and while being ionized, they form the population of pickup protons. The distribution of pickup protons is ...modified by the adiabatic heating (cooling) induced by the solar wind plasma compression (expansion). In this study, we emphasize the importance of the adiabatic energy change in the inner heliosheath that is usually either neglected or considered improperly. The effect of this process on the energy and spatial distributions of pickup protons and energetic neutral atoms (ENAs), which originate in the charge exchange of pickup protons, has been investigated and quantified using a kinetic model. The model employs the global distributions of plasma and hydrogen atoms in the heliosphere from the simulations of a kinetic-magnetohydrodynamic model of solar wind interaction with the local interstellar medium. The findings indicate that the adiabatic energy change is responsible for the broadening of the pickup proton velocity distribution and the significant enhancement of ENA fluxes (up to ∼5 and ∼20 times in the upwind and downwind directions at energies ∼1–2 keV for an observer at 1 au). It sheds light on the role of adiabatic energy change in explaining the discrepancies between the ENA flux observations and the results of numerical simulations.
Abstract
Global models of the heliosphere are critical tools used in the interpretation of heliospheric observations. There are several three-dimensional magnetohydrodynamic (MHD) heliospheric models ...that rely on different strategies and assumptions. Until now only one paper has compared global heliosphere models, but without magnetic field effects. We compare the results of two different MHD models, the BU and Moscow models. Both models use identical boundary conditions to compare how different numerical approaches and physical assumptions contribute to the heliospheric solution. Based on the different numerical treatments of discontinuities, the BU model allows for the presence of magnetic reconnection, while the Moscow model does not. Both models predict collimation of the solar outflow in the heliosheath by the solar magnetic field and produce a split tail where the solar magnetic field confines the charged solar particles into distinct north and south columns that become lobes. In the BU model, the interstellar medium (ISM) flows between the two lobes at large distances due to MHD instabilities and reconnection. Reconnection in the BU model at the port flank affects the draping of the interstellar magnetic field in the immediate vicinity of the heliopause. Different draping in the models cause different ISM pressures, yielding different heliosheath thicknesses and boundary locations, with the largest effects at high latitudes. The BU model heliosheath is 15% thinner and the heliopause is 7% more inwards at the north pole relative to the Moscow model. These differences in the two plasma solutions may manifest themselves in energetic neutral atom measurements of the heliosphere.
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 LEXI and SMILE missions will provide soft X‐ray images of the Earth's magnetosheath and cusps after their anticipated launch in 2023 and 2024, respectively. The IBEX mission showed the potential ...of an Energetic Neutral Atom (ENA) instrument to image dayside magnetosheath and cusps, albeit over the long hours required to raster an image with a single pixel imager. Thus, it is timely to discuss the two imaging techniques and relevant science topics. We simulate soft X‐ray and low‐ENA images that might be observed by a virtual spacecraft during two interesting solar wind scenarios: a southward turning of the interplanetary magnetic field and a sudden enhancement of the solar wind dynamic pressure. We employ the OpenGGCM global magnetohydrodynamics model and a simple exospheric neutral density model for these calculations. Both the magnetosheath and the cusps generate strong soft X‐rays and ENA signals that can be used to extract the locations and motions of the bow shock and magnetopause. Magnetopause erosion corresponds closely to the enhancement of dayside reconnection rate obtained from the OpenGGCM model, indicating that images can be used to understand global‐scale magnetopause reconnection. When dayside imagers are installed with high‐ENA inner‐magnetosphere and FUV/UV aurora imagers, we can trace the solar wind energy flow from the bow shock to the magnetosphere and then to the ionosphere in a self‐standing manner without relying upon other observatories. Soft X‐ray and/or ENA imagers can also unveil the dayside exosphere density structure and its response to space weather.
Key Points
Soft X‐ray and Energetic Neutral Atom (ENA) imaging instruments provide an innovative way to visualize the global solar wind‐magnetosphere interaction
High‐cadence, wide field‐of‐view soft X‐ray, and ENA images can capture the motion of the bow shock and magnetopause
The magnetopause motion can reveal the magnetopause reconnection mode on a global scale
Abstract Determining the magnitude and direction of the interstellar magnetic field ( B ISM ) is a long-standing problem. To date, some methods to infer the direction and magnitude have utilized ...best-fit models to the positions of the termination shock and heliopause measured by Voyager 1 and 2. Other models use the circularity of the Interstellar Boundary Explorer (IBEX) ribbon assuming a secondary energetic neutral atom (ENA) mechanism. Previous studies have revealed that the B ISM organizes the orientation of the heliotail with respect to the solar meridian. Here we propose a new way to infer the direction of the B ISM based on ENA observations of the heliotail. IBEX observations of the heliotail have revealed high-latitude lobes of enhanced ENA flux at energies >2 keV. Analyses showed that the high-latitude lobes are nearly aligned with the solar meridian, while also exhibiting a rotation with solar cycle. We show, using steady-state solar wind conditions, that the inclination of the lobes reproduced with commonly used values for the angle ( α BV ) between B ISM and the interstellar flow in the hydrogen deflection plane (40° < α BV < 60°) is inconsistent with the IBEX ENA observations. We report that 0° < α BV < 20° best replicates the heliotail lobe inclinations observed by IBEX. Additionally, our model results indicate that the variation of the solar magnetic field magnitude with solar cycle causes the longitudinal rotation of the lobes observed by IBEX by affecting the inclination of the lobes.
Abstract
Measuring energetic neutral atoms (ENAs) allows for the remote observation of ion populations from the frontiers of our heliosphere. In this study, we compare the ENAs observed with the ...IBEX-Lo instrument onboard the Interstellar Boundary Explorer with ENA predictions from two heliosphere models. In contrast to previous studies, this paper presents model-data comparisons for the energy range 50 eV–2 keV over one full solar cycle not only in the upwind direction (Voyager 1 and Voyager 2 sky directions), but also for the north pole, south pole, port tail lobe, and downwind directions. The two heliosphere models produce the same basic result: there is a large gap (1 to 2 orders of magnitude in ENA intensity at 100 eV) between ENA data and model predictions between 100 and 500 eV for all sky directions. The reason for this gap is not understood yet. While some explanations are plausible and will be investigated in future studies, other explanations are excluded.
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