Vlad Izmodenov

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.

The Interstellar Boundary Explorer (IBEX)

The Interstellar Boundary Explorer (IBEX) mission is exploring the frontiers of the heliosphere where energetic neutral atoms (ENAs) are formed from charge exchange between interstellar neutral hydrogen atoms and solar wind ions and pickup ions. The geography of this frontier is dominated by an unexpected nearly complete arc of ENA emission, now known as the IBEX ‘Ribbon’. While there is no consensus agreement on the Ribbon formation mechanism, it seems certain this feature is seen for sightlines that are perpendicular to the interstellar magnetic field as it drapes over the heliosphere. At the lowest energies, IBEX also measures the flow of interstellar H, He, and O atoms through the inner heliosphere. The asymmetric oxygen profile suggests that a secondary flow of oxygen is present, such as would be expected if some fraction of oxygen is lost through charge exchange in the heliosheath regions. The detailed spectra characterized by the ENAs provide time-tagged samples of the energy distributions of the underlying ion distributions, and provide a wealth of information about the outer heliosphere regions, and beyond.

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.

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

This emission, observed in other galaxies mainly from regions around newborn stars, has now been detected in our Galaxy. Doppler-shifted hydrogen Lyman-alpha (Lyα) emission from galaxies is currently measured and used in cosmology as an indicator of star formation. Until now, the Milky Way emission has not been detected, owing to far brighter local sources, including the H (hydrogen) glow, i.e., solar Lyα radiation backscattered by interstellar atoms that flow within the solar system. Because observations from the Voyager spacecraft, now leaving the heliosphere, are decreasingly affected by the H glow, the ultraviolet spectrographs are detecting Lyα diffuse emission from our Galaxy. The surface brightness toward nearby star-forming regions is about 3 to 4 rayleighs. The escape fraction of the radiation from the brightest H II regions is on the order of 3% and is highly spatially variable. These results will help in constraining models of Lyα radiation transfer in distant galaxies.

glow: applications to the SWAN data

Energetic neutral atoms (ENAs) are recognized as a powerful tool for remote probingof distant hot space plasmas, in particular the plasmas abundant at the heliosphericboundary where the expanding solar wind meets the surrounding local interstellarmedium. We show here for the first time that the heliospheric ENAs originating in theheliospheric sheath between the termination shock and the heliopause and reaching theinner heliosphere also provide an important and heretofore unaccounted source of atomichydrogen in the Sun’s vicinity. These ENAs are a major contributor to the density ofinterplanetary hydrogen at heliocentric distances <1 AU and could dominate in thedownwind (interstellar wind) region under typical solar and interstellar conditions. Masstransport by heliospheric ENAs may become especially important for determining theorigin of the pickup ions attributed to the inner source of neutral particles in the Sun’svicinity and for characterization of the three-dimensional solar wind flow by imaging inextreme ultraviolet.

Voyager Measurements of Hydrogen Lyman-α Diffuse Emission from the Milky Way

The Voyager 1 (V1), Voyager 2 (V2), and Pioneer 10 (P10) Lyα data sets are three of several diagnostic data sets available for the study of the very local interstellar medium (VLISM). Selected V1 data obtained on 1989 day 279 at heliocentric distance of 39.1 AU in the upstream direction relative to the incoming interstellar neutral hydrogen flow and V2 data obtained on 1990 day 143 at heliocentric distance of 32 AU, also in the upstream direction, have been used to estimate the local interstellar neutral hydrogen and proton densities and compared with P10 data obtained in 1981 at distances between 23.39 and 23.87 AU in the downstream direction, respectively. State-of-the-art plasma-neutral and radiative transfer models have been used in the interpretation of the data. It has been found that a VLISM heliospheric model with neutral hydrogen density of 0.18 cm-3 and proton density of 0.06 cm-3 best fits both the V1 data and the V2 data. The P10 data are best fitted by a VLISM model with neutral hydrogen density of 0.15 cm-3 and proton density of 0.05 cm-3. The failure to find a single best-fit stationary heliosphere plasma-neutral model suggests, among other possibilities, that a quantitative interpretation of the heliospheric Lyα glow would require the incorporation of magnetic field and time dependence in the heliospheric model.

Mass transport in the heliosphere by energetic neutral atoms

We present here a study made by two instruments, MASCS on MESSENGER and SWAN on SOHO that observed the interplanetary background in 2010. The combination of these two data sets allows us to perform the first study of the distribution of hydrogen atoms inside the Earths orbit. Triangulation of the position of the Maximum Emissivity Region (MER) was performed for the data of the UVVS channel of the MASCS-MESSENGER instrument. We find that the ecliptic longitude of the MER is 253.2° ±2.0°. This is the same value that was found from the analysis of the SWAN-SOHO H cell data obtained in 1996. This strongly suggests that the direction of the interstellar hydrogen wind has not changed between 1996 and 2010. We have also determined the distance of the MER to the Sun. We find that the volume emission rate peaks at 2.37 AU ±0.2 AU from the Sun. This value is a good test for the solar parameters for total H ionization and radiation pressure used in models. Comparison between the two datasets obtained by the UVVS-MASCS channel and SWAN on SOHO allow to derive the intensity between the two spacecraft at peak emission. Based on the SWAN-SOHO calibration, we find an intensity of 80 R ±36 R. This corresponds to a column density of 1540 m−3 AUx2.3 × 1014 m−3 . When divided by the distance between the two spacecraft, we find an average number density of 2300 m−3.

Voyager 1, Voyager 2, and Pioneer 10 Lyα data and their interpretation

The Pioneer 10 (P10) and Voyager 2 (V2) calibration difference of 4.4 at Lyα has made it difficult to interpret the Lyα data and also to resolve the outer planetary upper atmosphere excess Lyα glow problem. We have carried out radiative transfer calculations using an improved radiative transfer code and six heliosphere neutral-plasma density models to study the calibration of P10 and V2 at Lyα and found that both P10 and V2 intensity measurements are in need of revision. The intercalibration difference is discussed using our model calculations, recent large-distance neutral hydrogen density determinations obtained from pickup-ion and solar wind slow-down data, the recent change in the estimate of the solar Lyα flux values, and Voyager 1 energetic particle measurements. These recent heliospheric measurements and Lyα glow model calculations support the need for an upward revision of P10 and a downward revision of V2 Lyα intensity. It is not yet possible to give a definitive estimate of the required revision because of lack of knowledge of the very local interstellar medium neutral hydrogen density. The calibration revision is found to reduce the range of variation of Jovian dayglow.