Abstract
Interstellar neutral (ISN) helium atoms penetrating the heliosphere are used to find the flow velocity and temperature of the very local interstellar medium near the heliosphere. Recently, ...it was found that, in addition to charge exchange collisions, elastic collisions contribute to the filtration of these atoms outside the heliopause. Momentum exchange between colliding particles related to their angular scattering modifies the properties of the primary and secondary ISN helium populations before the atoms enter the heliosphere. Here, we calculate the transport of ISN helium atoms using plasma and neutral flows from a global three-dimensional heliosphere model. We confirm earlier results based on one-dimensional calculations that the primary population is slowed down and heated by the momentum exchange. Moreover, accounting for momentum exchange in charge exchange collisions results in a faster and warmer secondary population. The paper presents how the velocity and density of these populations vary over the entrance position to the heliosphere. We point out that Maxwell distributions cannot correctly describe these populations. Finally, we calculate the expected Interstellar Boundary Explorer (IBEX) count rates and show that the filtration processes change them significantly. Consequently, future studies of IBEX or Interstellar Mapping and Acceleration Probe (IMAP) observations of ISN atoms should account for these processes.
Models play an important role in our understanding of the global structure of the solar wind and its interaction with the interstellar medium. A critical ingredient in many types of models is the ...charge-exchange collisions between ions and neutrals. Some ambiguity exists in the charge-exchange cross-section for protons and hydrogen atoms, depending on which experimental data is used. The differences are greatest at low energies, and for the plasma-neutral interaction in the outer heliosheath may exceed 50%. In this paper we assess a number of existing data sets and formulae for proton-hydrogen charge exchange. We use a global simulation of the heliosphere to quantify the differences between the currently favored cross-section, and we suggest a formulation that more closely matches the majority of available data. We find that in order to make the resulting two heliospheres the same size, the interstellar proton and hydrogen densities need to be adjusted by 10%-15%, which provides a way to link the uncertainty in the cross-section to the uncertainty in the parameters of the pristine interstellar plasma.
The interaction between the solar wind and the interstellar medium represents a collision between two plasma flows, resulting in a heliosphere with an extended tail. While the solar wind is mostly ...ionized material from the corona, the interstellar medium is only partially ionized. The ion and neutral populations are coupled through charge‐exchange collisions that operate on length scales of tens to hundreds of astronomical units. About half the interstellar hydrogen flows into the heliosphere where it may charge‐exchange with solar wind protons. This process gives rise to a nonthermal proton, known as a pickup ion, which joins the plasma. In this paper we investigate the effects of approximating the total ion distribution of the subsonic solar wind as a generalized Lorentzian, or κ distribution, using an MHD neutral code. We illustrate the effect different values of the κ parameter have on both the structure of the heliosphere and the energetic neutral atom flux at 1 AU. We find that using a κ distribution in our simulations yields levels of energetic neutral atom flux that are within a factor of about 2 or 3 over the IBEX‐Hi range of energies from 0.5 to 6 keV. While the presence of a suprathermal tail in the proton distribution leads to the production of high‐energy neutrals, the sharp decline in the charge‐exchange cross section around 10 keV mitigates the enhanced transfer of energy from the ions to the neutrals that might otherwise be expected.
Key Points
Show how varying kappa in the inner heliosheath affects heliospheric structure
Show that assuming the cross section is constant is not valid for small kappa
Show how varying kappa in the inner heliosheath affects the ENA flux at Earth
ABSTRACT The nearly circular band of energetic neutral atom emission dominating the field of view of the Interplanetary Boundary Explorer (IBEX ) satellite, is most commonly attributed to the effect ...of charge exchange of secondary pickup ions (PUIs) gyrating about the magnetic field in the outer heliosheath and the interstellar space beyond. Several models for the PUI dynamics of this mechanism have been proposed, each requiring either strong or weak scattering of the initial pitch angle. Conventional wisdom states that ring distributions tend to generate waves and scatter onto a shell on timescales too short for charge exchange to occur. We performed a careful study of ring and thin shell proton distribution stability using theoretical tools and hybrid plasma simulations. We show that the kinetic behavior of a freshly injected proton ring is a far more complicated process than previously thought. In the presence of a warm Maxwellian core, narrower rings could be more stable than broader toroidal distributions. The scattered rings possess a fine structure that can only be revealed using very large numbers of macroparticles in a simulation. It is demonstrated that a "stability gap" in ring temperature exists where the protons could retain large gyrating anisotropies for years, and the wave activity could remain below the level of the ambient magnetic fluctuations in interstellar space. In the directions away from the ribbon, however, a partial shell distribution is more likely to be unstable, leading to significant scattering into one hemisphere in velocity space. The process is accompanied by turbulence production, which is puzzling given the very low level of magnetic fluctuations measured in the outer heliosheath by Voyager 1.
The heliosphere is formed due to interaction between the solar wind (SW) and local interstellar medium (LISM). The shape and position of the heliospheric boundary, the heliopause, in space depend on ...the parameters of interacting plasma flows. The interplay between the asymmetrizing effect of the interstellar magnetic field and charge exchange between ions and neutral atoms plays an important role in the SW-LISM interaction. By performing three-dimensional, MHD plasma/kinetic neutral atom simulations, we determine the width of the outer heliosheath-the LISM plasma region affected by the presence of the heliosphere-and analyze quantitatively the distributions in front of the heliopause. It is shown that charge exchange modifies the LISM plasma to such extent that the contribution of a shock transition to the total variation of plasma parameters becomes small even if the LISM velocity exceeds the fast magnetosonic speed in the unperturbed medium. By performing adaptive mesh refinement simulations, we show that a distinct boundary layer of decreased plasma density and enhanced magnetic field should be observed on the interstellar side of the heliopause. We show that this behavior is in agreement with the plasma oscillations of increasing frequency observed by the plasma wave instrument onboard Voyager 1. We also demonstrate that Voyager observations in the inner heliosheath between the heliospheric termination shock and the heliopause are consistent with dissipation of the heliospheric magnetic field. The choice of LISM parameters in this analysis is based on the simulations that fit observations of energetic neutral atoms performed by Interstellar Boundary Explorer.
Abstract
In 2009, the Interstellar Boundary Explorer (IBEX) discovered a narrow “ribbon” of energetic neutral atom emissions across the sky with properties correlated with the solar wind latitudinal ...structure and the interstellar magnetic field draped around the heliosphere. It is widely believed that the ribbon is formed from the escape of heliospheric ENAs into the local interstellar medium and their eventual return as secondary ENAs. However, there is no consensus on the rate of pitch angle scattering of these PUIs before they become secondary ENAs. We test two opposing limits of scattering rates (“weak” versus “strong”) by solving a time-dependent model of the ribbon that evolves with the solar cycle, and we compare them to IBEX observations over 2009–2019. First, we find that both models qualitatively reproduce the evolution of IBEX fluxes for most of the data set, with a few exceptions, although the strong (or “spatial retention”) scattering model greatly underestimates the observed fluxes. Regardless, time dependence of fluxes cannot distinguish these models. Second, the ribbon’s geometric properties, i.e., its center and radius, are significantly different between the models. The spatial retention model reproduces the observed ribbon centers as a function of energy and time slightly better than the weak scattering model, and the spatial retention model reproduces the observed ribbon radius over energy and time almost perfectly, whereas the weak scattering model compares poorly. Our analysis favors the spatial retention mechanism as the source of the IBEX ribbon, but it requires modification to increase the flux of ENAs observed at 1 au.
ABSTRACT The solar wind emanating from the Sun interacts with the local interstellar medium (LISM), forming the heliosphere. Hydrogen energetic neutral atoms (ENAs) produced by the solar-interstellar ...interaction carry important information about plasma properties from the boundaries of the heliosphere, and are currently being measured by NASA's Interstellar Boundary Explorer (IBEX). IBEX observations show the existence of a "ribbon" of intense ENA emission projecting a circle on the celestial sphere that is centered near the local interstellar magnetic field (ISMF) vector. Here we show that the source of the IBEX ribbon as a function of ENA energy outside the heliosphere, uniquely coupled to the draping of the ISMF around the heliopause, can be used to precisely determine the magnitude (2.93 0.08 G) and direction (227 28 0 69, 34 62 0 45 in ecliptic longitude and latitude) of the pristine ISMF far (∼1000 AU) from the Sun. We find that the ISMF vector is offset from the ribbon center by ∼8 3 toward the direction of motion of the heliosphere through the LISM, and their vectors form a plane that is consistent with the direction of deflected interstellar neutral hydrogen, thought to be controlled by the ISMF. Our results yield draped ISMF properties close to that observed by Voyager 1, the only spacecraft to directly measure the ISMF close to the heliosphere, and give predictions of the pristine ISMF that Voyager 1 has yet to sample.
Recent IBEX observations indicate that the local interstellar medium (LISM) flow speed is less than previously thought. Reasonable LISM plasma parameters indicate that the LISM flow may be either ...marginally super-fast magnetosonic or sub-fast magnetosonic. A theoretical analysis shows that the transition from a super-fast to a sub-fast magnetosonic downstream state is due to the charge exchange of fast neutral H and hot neutral H created in the supersonic solar wind and hot inner heliosheath, respectively. The charge exchange of fast and hot heliospheric neutral H therefore provides a primary dissipation mechanism at the weak heliospheric bow shock, in some cases effectively creating a one-shock heliosphere. Both super-fast magnetosonic models produce a sizeable H-wall. Subject to further modeling and comparison against further lines of sight, we conclude with the tantalizing possibility that IBEX may have discovered a class of interstellar shocks mediated by neutral H.
Interstellar Boundary Explorer (IBEX) measurements of energetic neutral atoms (ENAs) from the heliotail show a multi-lobe structure of ENA fluxes as a function of energy between ∼0.71 and 4.29 keV. ...Below ∼2 keV, there is a single structure of enhanced ENA fluxes centered near the downwind direction. Above ∼2 keV, this structure separates into two lobes, one north and one south of the solar equatorial plane. ENA flux from these two lobes can be interpreted as originating from the fast solar wind (SW) propagating through the inner heliosheath (IHS). Alternatively, a recently published model of the heliosphere suggests that the heliotail may split into a "croissant-like" shape, and that such a geometry could be responsible for the heliotail ENA feature. Here we present results from a time-dependent simulation of the heliosphere that produces a comet-like heliotail, and show that the 11-year solar cycle leads to the formation of ENA lobes with properties remarkably similar to those observed by IBEX. The ENA energy at which the north and south lobes appear suggests that the pickup ion (PUI) temperature in the slow SW of the IHS is ∼107 K. Moreover, we demonstrate that the extinction of PUIs by charge-exchange is an essential process required to create the observed global ENA structure. While the shape and locations of the ENA lobes as a function of energy are well reproduced by PUIs that cross the termination shock, the results appear to be sensitive to the form of the distribution of PUIs injected in the IHS.
Pickup Ions beyond the Heliopause Roytershteyn, V.; Pogorelov, N. V.; Heerikhuisen, J.
The Astrophysical journal,
08/2019, Letnik:
881, Številka:
1
Journal Article
Recenzirano
Fluxes of energetic neutral atoms (ENAs) with energies greater than 1 KeV measured by the Interstellar Boundary Explorer (IBEX) are predominantly created by charge exchange of interstellar neutral ...atoms with hot, nonthermal (pickup) ions. Since the properties of pickup ions (PUIs) depend on the place of their birth, they bear an imprint of the interaction between the solar wind (SW) and the local interstellar medium (LISM). Of special importance is a narrow "ribbon" of an enhanced ENA flux discovered by IBEX. While the origin of this ribbon is still under debate, various models attribute it to charge exchange between the LISM neutral atoms and PUIs just beyond the heliopause. In this work, we investigate the stability of PUI distributions inferred from global modeling of neutral atoms in the heliosphere using fully kinetic particle-in-cell modeling. The initial distribution consists of PUIs created from high-speed, low-temperature neutral atoms born in the supersonic SW ahead of the heliospheric termination shock (TS) and PUIs created from low-speed, high-temperature H atoms born between the TS and the heliopause. The simulations indicate that such distributions are unstable against quasi-parallel modes with fast growth timescales (of the order of one hour or less). The development of these instabilities may strongly modify the initial PUI distribution, leading, for example, to formation of a plateau in the direction parallel to the local magnetic field. These modes, however, saturate at relatively small amplitudes and do not lead to complete isotropization of the distribution on the timescales considered.
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