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Register a new userInterstellar Neutral Gas Species And Their Pickup Ions Inside The Heliospheric Termination Shock. The Large-scale Structures
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Justyna M. Sok\\'o{\\l}
Publication date
2019
fields
Physics
and research's language is
English
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We study the distribution of the interstellar neutral (ISN) gas density and the pick-up ion (PUI) density of hydrogen, helium, neon, and oxygen in the heliosphere for heliocentric distances from inside 1 au up to the solar wind termination shock (TS), both in and out of the ecliptic plane. We discuss similarities and differences in the large-scale structures of the ISN gas and PUIs formed in the heliosphere between various species. We discuss the distribution of ISN gas and PUI densities for two extreme phases of the solar activity cycle, it is the solar minimum and the solar maximum. We identify the location of the ISN gas density cavity of various species. We study the relative abundance ratios of Ne/O, H/He, Ne/He, and O/He for ISN gas and PUIs densities and their variation with location in the heliosphere. We also discuss the modulation of relative abundance ratios of ISN gas and PUIs along the TS. We conclude that the preferable locations for detection of He$^+$ and Ne$^+$ PUIs are in the downwind hemisphere within 1~au, whereas for H$^+$ and O$^+$ PUIs the preferable locations for detection are for distances from Jupiter to Pluto orbits.
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Solar ionizing factors are responsible for modulation of interstellar neutral gas and its derivative populations inside the heliosphere. We provide an overview of the current state of knowledge about them for heliospheric particles inside the termination shock. We discuss charge exchange with solar wind particles, photoionization, and electron impact ionization for hydrogen, oxygen, neon, and helium from 1985 to 2018 both in the ecliptic plane and in the polar regions. We discuss ionization rates as a function of time, distance to the Sun, and latitude. We compare the total ionization rates among the species within a consistent and homogeneous system of calculation of the ionization rates. The highest total ionization rates at 1 au in the ecliptic plane are for hydrogen and oxygen, and the lowest are for helium. In the polar regions, the strongest ionization losses are for oxygen, regardless of the solar activity. Photoionization is the dominant ionization reaction for helium and neon, and a reaction of high significance for oxygen. Charge exchange with solar wind particles is the dominant ionization reaction for hydrogen and the second important ionization reaction for oxygen. Electron impact ionization is an important ionization reaction for Ne and He, with the contribution to the total ionization rates stronger within 1 au and smaller outside. The total ionization rates for He and Ne vary in time with the solar activity, whereas the total ionization rates for H and O follow the cyclic solar wind variations out of the ecliptic plane and aperiodic variations in the ecliptic plane.
We discuss a consolidation of determinations of the density of neutral interstellar H at the nose of the termination shock carried out with the use of various data sets, techniques, and modeling approaches. In particular, we focus on the determination of this density based on observations of H pickup ions on Ulysses during its aphelion passage through the ecliptic plane. We discuss in greater detail a novel method of determination of the density from these measurements and review the results from its application to actual data. The H density at TS derived from this analysis is equal to 0.087 pm 0.022 cm-3, and when all relevant determinations are taken into account, the consolidated density is obtained at 0.09 pm 0.022 cm-3. The density of H in CHISM based on literature values of filtration factor is then calculated at 0.16 pm 0.04 cm-3.
We study the properties of cosmological shock waves identified in high-resolution, N-body/hydrodynamic simulations of a $Lambda$CDM universe and their role on thermalization of gas and acceleration of nonthermal, cosmic ray (CR) particles. External shocks form around sheets, filaments and knots of mass distribution when the gas in void regions accretes onto them. Within those nonlinear structures, internal shocks are produced by infall of previously shocked gas to filaments and knots, and during subclump mergers, as well as by chaotic flow motions. Due to the low temperature of the accreting gas, the Mach number of external shocks is high, extending up to $Msim 100$ or higher. In contrast, internal shocks have mostly low Mach numbers. For all shocks of $Mge1.5$ the mean distance between shock surfaces over the entire computed volume is $sim4 h^{-1}$ Mpc at present, or $sim 1 h^{-1}$ Mpc for internal shocks within nonlinear structures. Identified external shocks are more extensive, with their surface area $sim2$ times larger than that of identified internal shocks at present. However, especially because of higher preshock densities, but also due to higher shock speeds, internal shocks dissipate more energy. Hence, the internal shocks are mainly responsible for gas thermalization as well as CR acceleration. In fact, internal shocks with $2 la M la 4$ contribute $sim 1/2$ of the total dissipation. Using a nonlinear diffusive shock acceleration model for CR protons, we estimate the ratio of CR energy to gas thermal energy dissipated at cosmological shock waves to be $sim1/2$ through the history of the universe. Our result supports scenarios in which the intracluster medium contains energetically significant populations of CRs.
The interstellar neutral (ISN) gas enters the heliosphere and is detected at a few au from the Sun, as demonstrated by Ulysses and the nterstellar Boundary Explorer (IBEX) missions. Ulysses observed ISN gas from different vantage points in a polar orbit from 1994 to 2007, while IBEX has been observing in an Earth orbit in a fixed direction relative to the Sun from 2009. McComas et al. 2018 reported about an IMAP-Lo detector on board the Interstellar Mapping and Acceleration Probe (IMAP), with an ability to track the ISN flux in the sky. We present observation geometries for ISN gas for a detector with the capability to adjust the boresight direction along the Earth orbit over a year within a multichoice ISN observation scheme. We study science opportunities from the observations as a function of time during a year and the phase of solar activity. We identify observation geometries and determine the observation seasons separately for various ISN species and populations. We find that using an adjustable viewing direction allows for ISN gas observations in the upwind hemisphere, where the signal is not distorted by gravitational focusing, in addition to the viewing of ISN species throughout the entire year. Moreover, we demonstrate that with appropriately adjusted observation geometries, primary and secondary populations can be fully separated. Additionally, we show that atoms of ISN gas on indirect trajectories are accessible for detection, and we present their impact on the study of the ionization rates for ISN species.
In recent years, a phenomenological solar wind heating model based on a turbulent energy cascade prescribed by the Kolmogorov theory has produced reasonably good agreement with observations on proton temperatures out to distances around 70 AU, provided the effect of turbulence generation due to pickup ions is included in the model. In a recent study [Ng et al., J. Geophys. Res., 115, A02101 (2010)], we have incorporated in the heating model the energy cascade rate based on Iroshnikov-Kraichnan (IK) scaling. We showed that the IK cascade rate can also produce good agreement with observations, with or without the inclusion of pickup ions. This effect was confirmed both by integrating the model using average boundary conditions at 1 AU, and by applying a method [Smith et al., Astrophys. J., 638, 508 (2006)] that uses directly observed values as boundary conditions. The effects due to pickup ions is found to be less important for the IK spectrum, which is shallower than the Kolmogorov spectrum. In this paper, we will present calculations of the pickup ions effect in more details, and discuss the physical reason why a shallower spectrum generates less waves and turbulence.
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