Yury G. Malama

Solar cycle influence on the interaction of the solar wind with Local Interstellar Cloud

We present results of a new time-dependent kinetic model of the H atom penetration through the solar wind - interstellar medium interaction region. A kinetic 6D (time, two dimensions in space, and three dimensions in velocity-space) equation for interstellar H atoms was solved self-consistently with time-dependent Euler equations for the solar wind and interstellar charged components. We study the response of the interaction region to 11-year solar cycle variations of the solar wind dynamic pressure. It is shown that the termination shock location varies within ±7 AU, the heliopause variation is ∼ 4A U, and the bow shock variation is negligible. At large heliocentric distances, the solar cycle induces 10-12% fluctuations in the number density of both primary and secondary interstellar H atoms and atoms created in the inner heliosheath. We underline the kinetic behavior of the fluctuations of the H atom populations. Closer to the Sun the fluctuations increase up to 30-35% at 5 AU due to solar cycle variation of the charge exchange rate. Solar cycle variations of interstellar H atoms in the heliospheric interface and within the heliosphere may have major importance for the interpretation of H atom observations inside the heliosphere.

Modeling of the heliospheric interface: multi-component nature of the heliospheric plasma

We present a new model of the heliospheric interface - the region of the solar wind interaction with the local interstellar medium. This new model performs a multi-component treatment of charged particles in the heliosphere. All charged particles are divided into several co-moving types. The coldest type, with parameters typical of original solar wind protons, is considered in the framework of fluid approximation. The hot pickup proton components created from interstellar H atoms and heliospheric ENAs by charge exchange, electron impact ionization and photoionization are treated kinetically. The charged components are considered self-consistently with interstellar H atoms, which are described kinetically as well. To solve the kinetic equation for H atoms we use the Monte Carlo method with splitting of trajectories, which allows us 1) to reduce statistical uncertainties allowing correct interpretation of observational data; 2) to separate all H atoms in the heliosphere into several populations depending on the place of their birth and on the type of parent protons.

On the distribution function of H Atoms in the problem of the solar wind interaction with the local interstellar medium

This paper is devoted to the analysis of the models of the solar wind interaction with the local interstellar medium (LISM) self-consistently taking into account mutual influence of plasma (electrons and protons) and neutral (H atoms) components of the LISM. The axisymmetric interaction model of Baranov and Malama [1993], that used the kinetic description of H atoms motion together with the hydrodynamic approximation for the plasma component, showed that the distribution functions of the all H atom populations are essentially not Maxwellian. The hydrodynamic model by Zank et al. [1996] is based on the Maxwellian distribution functions of the LISM and solar wind hydrogen atoms. The comparisons of the results obtained on the basis of the kinetic model by Baranov and Malama [1993] and multifluid model by Zank et al. [1996] show that the distributions of H atom parameters have large quantitative as well as qualitative distinctions. A number of theoretical predictions and relevant, experimental data are analyzed.

Interstellar hydrogen atom distribution function in the outer heliosphere

We study the evolution of the velocity distribution function of interstellar hydrogen atoms in the heliospheric interface, the region of solar and interstellar wind interaction. The velocity distribution is a key tool to evaluate uncertainties introduced by various simplified models of the interface. We numerically solve the kinetic equation for neutral gas self-consistently with the hydrodynamical equations for plasma. Neutral and plasma components efficiently coupled by charge exchange. This interaction disturbs the atom velocity distribution, which is assumed to be Maxwellian in the circumsolar local interstellar medium. It is shown that besides “original” interstellar atoms, there are three other important atom populations originating in the heliospheric interface. We present velocity distribution functions of these populations in various locations of the interface region and discuss their kinetic properties.

Effects of interstellar and solar wind ionized helium on the interaction of the solar wind with the local interstellar medium

The Sun is moving through a warm (~6500 K) and partly ionized local interstellar cloud (LIC) with a velocity of ~26 km s-1. Recent measurements of the ionization of the LIC (Wolff, Koester, & Lallement) suggest that interstellar helium in the vicinity of the Sun is 30%-40% ionized and that interstellar hydrogen is less ionized. Consequently, interstellar helium ions contribute up to 50% of the total dynamic pressure of the ionized interstellar component. Up to now, interstellar helium ions have been ignored in existing models of the heliospheric interface. In this Letter, we present results of a new model of the interaction of the solar wind with the interstellar medium; this model takes into account interstellar helium ions. Using the results of this model, we find that the heliopause, termination, and bow shocks are closer to the Sun than the model results that ignore He ions. The influence of interstellar helium ions is partially compensated by solar wind α-particles, which are taken into account in our new model as well. Finally, with our new model, we place constraints on the plausible location of the termination shock.

Filtration of interstellar H, O, N atoms through the heliospheric interface: Inferences on local interstellar abundances of the elements

In this Letter we report on our study of the filtration of interstellar atoms of hydrogen, oxygen and nitrogen in the interaction region between the solar wind and the local interstellar medium. The filtration has great importance for the deter- mination of local interstellar abundances of these elements, which becomes now possible due to measurements of interstellar pickup ions by Ulysses and ACE, and anomalous cosmic rays by Voyagers, Ulysses, ACE, SAMPEX and Wind. The filtra- tion of the different elements depends on the level of their coupling with the plasma in the interaction region. We study the dependence of the filtration on the local interstellar proton and H atom number densities and evaluate the effects of charge exchange and electron impact ionization on the filtration. We explore the influence of electron temperature in the inner he- liosheath on the filtration process. Using our filtration coefficients and SWICS/Ulysses pickup ion measurements we conclude nOI,LIC = (7.8 ± 1.3) × 10 −5 cm −3 and nNI,LIC = (1.1 ± 0.2) × 10 −5 cm −3 .

Radiation transport of heliospheric Lyman-alpha from combined Cassini and Voyager data sets

Aims. Heliospheric neutral hydrogen scatters solar Lyman-α radiation from the Sun with “27-day” intensity modulations observed near Earth due to the Sun’s rotation combined with Earth’s orbital motion. These modulations are increasingly damped in amplitude at larger distances from the Sun due to multiple scattering in the heliosphere, providing a diagnostic of the interplanetary neutral hydrogen density independent of instrument calibration. Methods. This paper presents Cassini data from 2003−2004 obtained downwind near Saturn at ∼10 AU that at times show undamped “27-day” waves in good agreement with the single-scattering models of Pryor et al. (1992, ApJ, 394, 363). Simultaneous Voyager 1 data from 2003−2004 obtained upwind at a distance of 88.8−92.6 AU from the Sun show waves damped by a factor of ∼0.21. The observed degree of damping is interpreted in terms of Monte Carlo multiple-scattering calculations (e.g., Keller et al. 1981, AA Izmodenov et al. 2001, J. Geophys. Res., 106, 10681; Baranov & Izmodenov 2006, Fluid Dyn., 41, 689). Results. We conclude that multiple scattering is definitely occurring in the outer heliosphere. Both models compare favorably to the data, using heliospheric neutral H densities at the termination shock of 0.085 cm −3 and 0.095 cm −3 . This work generally agrees with earlier discussions of Voyager data in Quemerais et al. (1996, ApJ, 463, 349) showing the importance of multiple scattering but is based on Voyager data obtained at larger distances from the Sun (with larger damping) simultaneously with Cassini data obtained closer to the Sun.

Lyα Absorption from Heliosheath Neutrals

We assess the information that HST observations of stellar Lyα lines can provide on the heliosheath, the region of the heliosphere between the termination shock and heliopause. To search for evidence of heliosheath absorption, we conduct a systematic inspection of stellar Lyα lines reconstructed after correcting for ISM absorption (and heliospheric/astrospheric absorption, if present). Most of the stellar lines are well centered on the stellar radial velocity, as expected, but the three lines of sight with the most downwind orientations relative to the ISM flow (χ1 Ori, HD 28205, and HD 28568) have significantly blueshifted Lyα lines. Since it is in downwind directions that heliosheath absorption should be strongest, the blueshifts are almost certainly caused by previously undetected heliosheath absorption. We make an initial comparison between the heliosheath absorption and the predictions of a pair of heliospheric models. A model with a complex multicomponent treatment of plasma within the heliosphere predicts less absorption than a model with a simple single-fluid treatment, which leads to better agreement with the data. Finally, we find that nonplanetary energetic neutral atom (ENA) fluxes measured by the ASPERA-3 instrument on board Mars Express, which have been interpreted as being from the heliosheath, are probably too high to be consistent with the relative lack of heliosheath absorption seen by HST. This would argue for a local interplanetary source for these ENAs instead of a heliosheath source.

Time dependent model of the interplanetary Lyman

Aims. Previous results of the study of interplanetary Lyman α background data obtained by the SWAN-SOHO between 1996 and 2005 clearly show that the solar cycle variations of the solar parameters deeply affect the interplanetary background emission. In this work, we compare these observational results with a time-dependent modeling of the interplanetary background. The hydrogen distributions in the model are one-year averages. Methods. The solar wind input in the model is derived from the omniweb dataset. The solar Lyman α flux values used to compute the radiation pressure are derived from the dataset of the SOLSTICE instrument. The hydrogen photo-ionization rate is extrapolated from the solar UV flux. These inputs are used to compute the hydrogen distribution in the heliosphere for two solar cycles. The resulting yearly averages of the interplanetary H distribution are then used as input for a radiative transfer model, which allows us to compute interplanetary background intensities, lineshifts, and linewidths for the geometries of the observations. Results. We find that the upwind intensities computed from the model do not follow variations observed by SWAN-SOHO between 1996 and 2005. On the other hand, the lineshift variations during the solar cycle are correctly reproduced. Comparison of observed linewidths with model results show that we can reproduce the general trend of the linewidth data. Time-dependent variations are not fully reproduced. Conclusions. The agreements obtained with the lineshifts and linewidths suggest that the velocity distribution of hydrogen is adequately represented by the model. On the other hand, we find that the temporal variations of the brightness data are not well reproduced by the model. To explain this, we suggest that the interface effects and radiation pressure are correctly represented whereas the ionization rate used as input in the model needs to be corrected. Further studies including anisotropy of the solar wind will be necessary to check this result.

\sf \alpha

We compare new results of models of the interplanetary H Lyα intensity background in the outer heliosphere with scans performed by the Voyager 1/2 UV spectrometer (UVS) instruments between 1993 and 2003. This study shows that the excess intensity initially reported by Quemerais et al. can be explained by models of the hydrogen atom distribution including effects of the heliospheric interface. The models of the hydrogen atom distribution in the interplanetary medium used in this work have been developed following the numerical scheme presented by Baranov & Malama. Recent improvements are described by Izmodenov et al. Radiative transfer computations of the interplanetary Lyα intensity are made following a Monte Carlo approach presented by Quemerais and Quemerais & Izmodenov. We find that the upwind intensity excess observed in the outer heliosphere initially reported by Quemerais et al. can be explained by a full radiative transfer computation. This computation must include a full description of the velocity distributions of the different hydrogen populations that enter the heliosphere after crossing the interface. The excess upwind intensity observed by UVS on Voyager 1 and Voyager 2 can be explained as an emission of the decelerated hydrogen atoms near the stagnation point of the heliopause. Because those atoms are slowed down relative to the main hydrogen flow, photons they scatter suffer less absorption and are visible at a much larger distance than is the case for photons scattered by atoms in the main flow. The shape and extent of the excess emission gives information about the decelerated population near the heliopause stagnation point. A detailed comparison between the data and our present model does not show a complete agreement. The modeled intensity excess is larger than the observed one. We discuss possible improvements to the H distribution model in order to decrease the size of the excess in the model, for example, by decreasing the density of H atoms in the hydrogen wall.