arXiv subjects

E. Moebius

Publications and source records attributed to E. Moebius.

13 recordsLinked to original sources

arXiv2024

IBEX Observations of Elastic Scattering of Interstellar Helium by Solar Wind Particles

The IBEX-Lo instrument on the Interstellar Boundary Explorer (IBEX) mission observes primary and secondary interstellar helium in its 4 lowest energy steps. Observations of these helium populations have been systematically analyzed and compared to simulations using the analytic full integration of neutrals model (aFINM). A systematic difference is observed between the simulations and observations of secondary helium during solar cycle (SC) 24. We show that elastic scattering of primary helium by solar wind protons, which redistributes atoms from the core of the flux distribution, provides an explanation of the observed divergence from simulations. We verify that elastic scattering forms a halo in the wings of the primary He distribution in the spin-angle direction. Correcting the simulation for the effects of elastic scattering requires an increase of the estimated density of primary helium compared to previous estimates by Ulysses/GAS. Thus, based on our analysis of IBEX observations and $\chi ^2$ minimization of simulation data that include the effects of elastic scattering, any estimation of neutral interstellar helium density at 1 AU by direct detection of the peak flux of neutral helium needs to be adjusted by $~\sim$ 10%

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arXiv2023

Science Opportunities for IMAP-Lo Observations of Interstellar Neutral Helium, Neon and Oxygen During a Maximum of Solar Activity

Direct-sampling observations of interstellar neutral (ISN) species and their secondary populations give information about the physical state of the local interstellar medium and processes occuring in the outer heliosheath. Such observations are performed from Earth's orbit by the IBEX-Lo experiment on board the Interstellar Boundary Explorer (IBEX) mission. IBEX ISN viewing is restricted to directions close to perpendicular to the Earth-Sun line, which limits the observations of interstellar species to several months during the year. A greatly improved data set will be possible for the upcoming Interstellar Mapping and Acceleration Probe (IMAP) mission due to a novel concept of putting the IMAP ISN detector, IMAP-Lo, on a pivot platform that varies the angle of observation relative to the Sun-Earth line and the detector boresight. Here we suggest a 2 yr scenario for varying the viewing angle in such a way that all the necessary atom components can be observed sufficiently well to achieve the science goals of the nominal IMAP mission. This scenario facilitates, among others, removal of the correlation of the inflow parameters of interstellar gas, unambiguous analysis of the primary and secondary populations of interstellar helium (He), neon (Ne) and oxygen (O), and determination of the ionization rates of He and Ne free of possible calibration bias. The scheme is operationally simple, provides a good counting statistics, and synergizes observations of interstellar species and heliospheric energetic neutral atoms.

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arXiv2023

Determining the ionization rates of interstellar neutral species using direct-sampling observations of their direct and indirect beams

A good understanding of the ionization rates of neutral species in the heliosphere is important for studies of the heliosphere and planetary atmospheres. So far, the intensities of the ionization reactions have been studied based on observations of the contributing phenomena, such as the solar spectral flux in the EUV band and the flux of the solar wind protons, alpha particles, and electrons. The results strongly depend on absolute calibration of these measurements, which, especially for the EUV measurements, is challenging. Here, we propose a novel method of determining the ionization rate of neutral species based on direct sampling of interstellar neutral gas from two locations in space distant to each other. In particular, we suggest performing observations from the vicinity of Earth's orbit and using ratios of fluxes of ISN He for the direct and indirect orbits of interstellar atoms. We identify the most favorable conditions and observations geometries, suitable for implementation on the forthcoming NASA mission Interstellar Mapping and Acceleration Probe.

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arXiv2019

Interstellar neutral helium in the heliosphere from IBEX observations. VI. The He$^+$ density and the ionization state in the Very Local Interstellar Matter

Interstellar neutral gas atoms penetrate the heliopause and reach 1~au, where they are detected by IBEX. The flow of neutral interstellar helium through the perturbed interstellar plasma in the outer heliosheath (OHS) results in creation of the secondary population of interstellar He atoms, the so-called Warm Breeze, due to charge exchange with perturbed ions. The secondary population brings the imprint of the OHS conditions to the IBEX-Lo instrument. Based on a global simulation of the heliosphere with measurement-based parameters and detailed kinetic simulation of the filtration of He in the OHS, we find the number density of interstellar He$^+$ population at $(8.98\pm 0.12)\times 10^{-3}$~cm$^{-3}$. With this, we obtain the absolute density of interstellar H$^+$ $5.4\times 10^{-2}$~cm$^{-3}$ and electrons $6.3\times 10^{-2}$~cm$^{-3}$, and ionization degrees of H 0.26 and He 0.37. The results agree with estimates of the Very Local Interstellar Matter parameters obtained from fitting the observed spectra of diffuse interstellar EUV and soft X-Ray background.

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arXiv2015

Interstellar neutral helium in the heliosphere from IBEX observations. III. Mach number of the flow, velocity vector, and temperature from the first six years of measurements

We analyzed observations of interstellar neutral helium (ISN~He) obtained from the Interstellar Boundary Explorer (IBEX) satellite during its first six years of operation. We used a refined version of the ISN~He simulation model, presented in the companion paper by Sokol_et al. 2015, and a sophisticated data correlation and uncertainty system and parameter fitting method, described in the companion paper by Swaczyna et al 2015. We analyzed the entire data set together and the yearly subsets, and found the temperature and velocity vector of ISN~He in front of the heliosphere. As seen in the previous studies, the allowable parameters are highly correlated and form a four-dimensional tube in the parameter space. The inflow longitudes obtained from the yearly data subsets show a spread of ~6 degree, with the other parameters varying accordingly along the parameter tube, and the minimum chi-square value is larger than expected. We found, however, that the Mach number of the ISN~He flow shows very little scatter and is thus very tightly constrained. It is in excellent agreement with the original analysis of ISN~He observations from IBEX and recent reanalyses of observations from Ulysses. We identify a possible inaccuracy in the Warm Breeze parameters as the likely cause of the scatter in the ISN~He parameters obtained from the yearly subsets, and we suppose that another component may exist in the signal, or a process that is not accounted for in the current physical model of ISN~He in front of the heliosphere. From our analysis, the inflow velocity vector, temperature, and Mach number of the flow are equal to lambda_ISNHe = 255.8 +/- 0.5 degree, beta_ISNHe = 5.16 +/- 0.10 degree, T_ISNHe = 7440 +/- 260 K, v_ISNHe = 25.8 +/- 0.4$ km/s, and M_ISNHe = 5.079 +/- 0.028, with uncertainties strongly correlated along the parameter tube.

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arXiv2015

Correcting the record on the analysis of IBEX and STEREO data regarding variations in the neutral interstellar wind

The journey of the Sun through space carries the solar system through a dynamic interstellar environment that is presently characterized by Mach 1 motion between the heliosphere and the surrounding interstellar medium (ISM). The interaction between the heliosphere and ISM is an evolving process due to the variable solar wind and to interstellar turbulence. Frisch et al. presented a meta-analysis of the historical data on the interstellar wind flowing through the heliosphere and concluded that temporal changes in the ecliptic longitude of the wind were statistically indicated by the data available in the refereed literature at the time of that writing. Lallement and Bertaux disagree with this result, and suggested, for instance, that a key instrumental response function of IBEX-Lo was incorrect and that the STEREO pickup ion data are unsuitable for diagnosing the flow of interstellar neutrals through the heliosphere. Here we show that temporal variations in the interstellar wind through the heliosphere are consistent with our knowledge of local ISM. The statistical analysis of the historical helium wind data is revisited, and a recent correction of a typographical error in the literature is incorporated into the new fits. With this correction, and including no newer IBEX results, these combined data still indicate that a change in the longitude of the interstellar neutral wind over the past forty years is statistically likely, but that a constant flow longitude is now also statistically possible. It is shown that the IBEX instrumental response function is known, and that the STEREO pickup ion data have been correctly utilized in this analysis.

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arXiv2014

Warm Breeze from the starboard bow: a new population of neutral helium in the heliosphere

We investigate the signals from neutral He atoms observed from Earth orbit in 2010 by IBEX. The full He signal observed during the 2010 observation season can be explained as a superposition of pristine neutral interstellar He gas and an additional population of neutral He that we call the Warm Breeze. The Warm Breeze is approximately two-fold slower and 2.5 times warmer than the primary interstellar He population, and its density in front of the heliosphere is ~7% that of the neutral interstellar helium. The inflow direction of the Warm Breeze differs by ~19deg from the inflow direction of interstellar gas. The Warm Breeze seems a long-term feature of the heliospheric environment. It has not been detected earlier because it is strongly ionized inside the heliosphere, which brings it below the threshold of detection via pickup ion and heliospheric backscatter glow observations, as well as by the direct sampling of GAS/Ulysses. Possible sources for the Warm Breeze include (1) the secondary population of interstellar helium, created via charge exchange and perhaps elastic scattering of neutral interstellar He atoms on interstellar He+ ions in the outer heliosheath, or (2) a gust of interstellar He originating from a hypothetic wave train in the Local Interstellar Cloud. A secondary population is expected from models, but the characteristics of the Warm Breeze do not fully conform to modeling results. If, nevertheless, this is the explanation, IBEX-Lo observations of the Warm Breeze provide key insights into the physical state of plasma in the outer heliosheath. If the second hypothesis is true, the source is likely to be located within a few thousand of AU from the Sun, which is the propagation range of possible gusts of interstellar neutral helium with the Warm Breeze characteristics against dissipation via elastic scattering in the Local Cloud.

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arXiv2012

Neutral interstellar helium parameters based on IBEX-Lo observations and test particle calculations

Neutral Interstellar Helium (NISHe) is almost unaffected at the heliospheric interface with the interstellar medium and freely enters the solar system. It provides some of the best information on the characteristics of the interstellar gas in the Local Interstellar Cloud. The Interstellar Boundary Explorer (IBEX) is the second mission to directly detect NISHe. We present a comparison between recent IBEX NISHe observations and simulations carried out using a well-tested quantitative simulation code. Simulation and observation results compare well for times when measured fluxes are dominated by NISHe (and contributions from other species are small). Differences between simulations and observations indicate a previously undetected secondary population of neutral helium, likely produced by interaction of interstellar helium with plasma in the outer heliosheath. Interstellar neutral parameters are statistically different from previous in situ results obtained mostly from the GAS/Ulysses experiment, but they do agree with the local interstellar flow vector obtained from studies of interstellar absorption: the newly-established flow direction is ecliptic longitude 79.2 deg, latitude -5.1 deg, the velocity is \sim 22.8 km/s, and the temperature is 6200 K. These new results imply a markedly lower absolute velocity of the gas and thus significantly lower dynamic pressure on the boundaries of the heliosphere and different orientation of the Hydrogen Deflection Plane compared to prior results from Ulysses. A different orientation of this plane also suggests a new geometry of the interstellar magnetic field and the lower dynamic pressure calls for a compensation by other components of the pressure balance, most likely a higher density of interstellar plasma and strength of interstellar magnetic field.

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arXiv2010

Can IBEX Identify Variations in the Galactic Environment of the Sun using Energetic Neutral Atom (ENAs)?

The Interstellar Boundary Explorer (IBEX) spacecraft is providing the first all-sky maps of the energetic neutral atoms (ENAs) produced by charge-exchange between interstellar neutral \HI\ atoms and heliospheric solar wind and pickup ions in the heliosphere boundary regions. The 'edge' of the interstellar cloud presently surrounding the heliosphere extends less than 0.1 pc in the upwind direction, terminating at an unknown distance, indicating that the outer boundary conditions of the heliosphere could change during the lifetime of the IBEX satellite. Using reasonable values for future outer heliosphere boundary conditions, ENA fluxes are predicted for one possible source of ENAs coming from outside of the heliopause. The ENA production simulations use three-dimensional MHD plasma models of the heliosphere that include a kinetic description of neutrals and a Lorentzian distribution for ions. Based on this ENA production model, it is then shown that the sensitivities of the IBEX 1.1 keV skymaps are sufficient to detect the variations in ENA fluxes that are expected to accompany the solar transition into the next upwind cloud. Approximately 20% of the IBEX 1.1 keV pixels appear capable of detecting the predicted model differences at the $ 3 \sigma$ level, with these pixels concentrated in the Ribbon region. Regardless of the detailed ENA production model, the success of the modeled \BdotR\ directions in reproducing the Ribbon locus, together with our results, indicate that the Ribbon phenomenon traces the variations in the heliosphere distortion caused by the relative pressures of the interstellar magnetic and gaseous components.

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arXiv2008

Neutral H density at the termination shock: a consolidation of recent results

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.

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arXiv2008

Density of neutral interstellar hydrogen at the termination shock from Ulysses pickup ion observations

By reevaluating a 13-month stretch of Ulysses SWICS H pickup ion measurements near 5 AU close to the ecliptic right after the previous solar minimum, this paper presents a determination of the neutral interstellar H density at the solar wind termination shock and implications for the density and ionization degree of hydrogen in the LIC. The density of neutral interstellar hydrogen at the termination shock was determined from the local pickup ion production rate as obtained close to the cut-off in the distribution function at aphelion of Ulysses. As shown in an analytical treatment for the upwind axis and through kinetic modeling of the pickup ion production rate at the observer location, with variations in the ionization rate, radiation pressure, and the modeling of the particle behavior, this analysis turns out to be very robust against uncertainties in these parameters and the modeling. Analysis using current heliospheric parameters yields the H density at the termination shock equal to $0.087\pm0.022$ cm$^{-3}$, including observational and modeling uncertainties.

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arXiv2004

Observational Evidence of Pitch Angle Isotropization by IMF Waves

A statistical analysis of interstellar He+ pickup ion measurements from SOHO/CTOF combined with magnetic field data from WIND/MFI enable quantitative study of wave-particle interactions in the inner heliosphere for the first time. Magnetic field vector measurements with a time resolu-tion of 3 seconds are used to determine power spectrum char-acteristics of interplanetary magnetic turbulence. These spec-tral characteristics are then compared in superposed epoch and correlation analyses with He+ fluxes and spectra. The observed pickup ion velocity distributions can be explained consistently as a consequence of pitch angle scattering of the interstellar pickup ions by Alfvenic fluctuations.

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arXiv2003

SOHO CTOF Observations of Interstellar He+ Pickup Ion Enhancements in Solar Wind Compression Regions

We present a recent analysis with 1996 SOHO CELIAS CTOF data, which reveals correlations of He+ pickup ion fluxes and spectra with the magnetic field strength and solar wind density. The motivation is to better understand the ubiquitous large variations in both pickup ion fluxes and their velocity distributions found in interstellar pickup ion datasets. We concentrate on time periods of that can be associated with compression regions in the solar wind. Along with enhancements of the overall pickup ion fluxes, adiabatic heating and acceleration of the pickup ions are also observed in these regions. Transport processes that lead to the observed compressions and related heating or acceleration are discussed. A shift in velocity space associated with traveling interplanetary compression regions is observed, and a simple model presented to explain this phenomenon based on the conserved magnetic adiabatic moment.

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