Peter Bochsler

INTERSTELLAR GAS FLOW PARAMETERS DERIVED FROM INTERSTELLAR BOUNDARY EXPLORER-Lo OBSERVATIONS IN 2009 AND 2010: ANALYTICAL ANALYSIS

Neutral atom imaging of the interstellar gas flow in the inner heliosphere provides the most detailed information on physical conditions of the surrounding interstellar medium (ISM) and its interaction with the heliosphere. The Interstellar Boundary Explorer (IBEX) measured neutral H, He, O, and Ne for three years. We compare the He and combined O + Ne flow distributions for two interstellar flow passages in 2009 and 2010 with an analytical calculation, which is simplified because the IBEX orientation provides observations at almost exactly the perihelion of the gas trajectories. This method allows separate determination of the key ISM parameters: inflow speed, longitude, and latitude, as well as temperature. A combined optimization, as in complementary approaches, is thus not necessary. Based on the observed peak position and width in longitude and latitude, inflow speed, latitude, and temperature are found as a function of inflow longitude. The latter is then constrained by the variation of the observed flow latitude as a function of observer longitude and by the ratio of the widths of the distribution in longitude and latitude. Identical results are found for 2009 and 2010: an He flow vector somewhat outside previous determinations (λISM∞ = 79. ◦ 0+3 . 0(−3. ◦ 5), βISM∞ =− 4. 9 ± 0. 2, VISM∞ = 23.5 + 3.0(−2.0) km s −1 , THe = 5000–8200 K), suggesting a larger inflow longitude and lower speed. The O + Ne temperature range, T O+N e = 5300–9000 K, is found to be close to the upper range for He and consistent with an isothermal medium for all species within current uncertainties.

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

Because of its high ionization potential and weak interaction with hydrogen, neutral interstellar helium (NISHe) is almost unaffected at the heliospheric interface with the interstellar medium and freely enters the solar system. This second most abundant species 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 792, latitude –51, the velocity is ~22.8 km s–1, 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.

Unusual composition of the solar wind in the 2–3 May 1998 CME observed with SWICS on ACE

Elemental, isotopic and charge state abundances provide valuable information about the source and acceleration mechanism of Coronal Mass Ejections (CMEs). Even though the kinetic properties of the plasma might be subject to changes because of dynamic effects occurring during the expansion of the CME, the composition of the solar wind remains unchanged after it leaves the low corona. Data from the Solar Wind Ion Composition Spectrometer (SWICS) on ACE are used to study the elemental and charge state composition of He, O, C, N, and Fe as well as the isotopic ratio of He during the very large CME of May 2–3, 1998. We find in this CME anomalously large enrichment of ³He++/4He++, He/O and Fe/O. During the 28 hour long cloud portion of the CME unusually cold material (4He+ and very low charge state heavy ions) was observed together with hot (high charge state ions) and normal solar wind plasma.

Direct Observations of Interstellar H, He, and O by the Interstellar Boundary Explorer

Whats Happening in the Heliosphere The influence of the Sun is felt well beyond the orbits of the planets. The solar wind is a stream of charged particles emanating from the Sun that carves a bubble in interstellar space known as the heliosphere and shrouds the entire solar system. The edge of the heliosphere, the region where the solar wind interacts with interstellar space, is largely unexplored. Voyager 1 and 2 crossed this boundary in 2004 and 2007, respectively, providing detailed but only localized information. In this issue (see the cover), McComas et al. (p. 959, published online 15 October), Fuselier et al. (p. 962, published online 15 October), Funsten et al. (p. 964, published online 15 October), and Möbius et al. (p. 969, published online 15 October) present data taken by NASAs Interstellar Boundary Explorer (IBEX). Since early 2009, IBEX has been building all-sky maps of the emissions of energetic neutral atoms produced at the boundary between the heliosphere and the interstellar medium. These maps have unexpectedly revealed a narrow band of emission that bisects the two Voyager locations at energies ranging from 0.2 to 6 kiloelectron volts. Emissions from the band are two- to threefold brighter than outside the band, in contrast to current models that predict much smaller variations across the sky. By comparing the IBEX observations with models of the heliosphere, Schwadron et al. (p. 966, published online 15 October) show that to date no model fully explains the observations. The model they have developed suggests that the interstellar magnetic field plays a stronger role than previously thought. In addition to the all-sky maps, IBEX measured the signatures of H, He, and O flowing into the heliosphere from the interstellar medium. In a related report, Krimigis et al. (p. 971, published online 15 October) present an all-sky image of energetic neutral atoms with energies ranging between 6 and 13 kiloelectron volts obtained with the Ion and Neutral Camera onboard the Cassini spacecraft orbiting Saturn. It shows that parts of the structure observed by IBEX extend to high energies. These data indicate that the shape of the heliosphere is not consistent with that of a comet aligned in the direction of the Suns travel through the galaxy as was previously thought. Detection of H, He, and O flowing into the heliosphere from the interstellar medium tells us about our local interstellar environment. Neutral gas of the local interstellar medium flows through the inner solar system while being deflected by solar gravity and depleted by ionization. The dominating feature in the energetic neutral atom Interstellar Boundary Explorer (IBEX) all-sky maps at low energies is the hydrogen, helium, and oxygen interstellar gas flow. The He and O flow peaked around 8 February 2009 in accordance with gravitational deflection, whereas H dominated after 26 March 2009, consistent with approximate balance of gravitational attraction by solar radiation pressure. The flow distributions arrive from a few degrees above the ecliptic plane and show the same temperature for He and O. An asymmetric O distribution in ecliptic latitude points to a secondary component from the outer heliosheath.

Isotopic composition of solar wind neon measured by CELIAS/MTOF on board SOHO

We present first results taken from the high-resolution mass time-of-flight spectrometer (MTOF) of the charge, element, and isotope analysis system (CELIAS) experiment on board the Solar and Heliospheric Observatory (SOHO) spacecraft launched in December 1995, concerning the abundance ratios of neon isotopes in the solar wind. We obtain the isotopic ratios 20Ne/22Ne = (13.8 ± 0.7) and 20Ne/21Ne = (440 ± 110), which agree with the values obtained from the Apollo foil solar wind experiments and which have been derived from measurements of solar particles implanted in lunar and meteoritic samples.

ESTIMATION OF THE NEON/OXYGEN ABUNDANCE RATIO AT THE HELIOSPHERIC TERMINATION SHOCK AND IN THE LOCAL INTERSTELLAR MEDIUM FROM IBEX OBSERVATIONS

We report the first direct measurement of the Ne/O abundance ratio of the interstellar neutral gas flowing into the inner heliosphere. From the first year of Interstellar Boundary Explorer IBEX data collected in spring 2009, we derive the fluxes of interstellar neutral oxygen and neon. Using the flux ratio at the location of IBEX at 1 AU at the time of the observations, and using the ionization rates of neon and oxygen prevailing in the heliosphere during the period of solar minimum, we estimate the neon/oxygen ratios at the heliospheric termination shock and in the gas phase of the inflowing local interstellar medium. Our estimate is (Ne/O){sub gas,ISM} = 0.27 {+-} 0.10, which is-within the large given uncertainties-consistent with earlier measurements from pickup ions. Our value is larger than the solar abundance ratio, possibly indicating that a significant fraction of oxygen in the local interstellar medium is hidden in grains and/or ices.

Oxygen 16 to oxygen 18 abundance ratio in the solar wind observed by Wind/MASS

Measurements of the 16O and 18O distribution functions in the solar wind at low to average solar wind speeds from the MASS instrument on the Wind spacecraft are reported. The 16O/18O density ratio is 450 ± 130, a value consistent with terrestrial, solar photospheric, solar energetic particle, and galactic cosmic ray 16O/18O isotopic ratios. This study constitutes the first reported spacecraft measurement of the isotope 18O in the core solar wind and may represent the best determination of the solar 16O/18O density ratio to date.

Solar Parameters for Modeling the Interplanetary Background

The goal of the working group on cross-calibration of past and present ultraviolet (UV) datasets of the International Space Science Institute (ISSI) in Bern, Switzerland was to establish a photometric cross-calibration of various UV and extreme ultraviolet (EUV) heliospheric observations. Realization of this goal required a credible and up-to-date model of the spatial distribution of neutral interstellar hydrogen in the heliosphere, and to that end, a credible model of the radiation pressure and ionization processes was needed. This chapter describes the latter part of the project: the solar factors responsible for shaping the distribution of neutral interstellar H in the heliosphere. In this paper we present the solar Lyman-α flux and the topics of solar Lyman-α resonant radiation pressure force acting on neutral H atoms in the heliosphere. We will also discuss solar EUV radiation and resulting photoionization of heliospheric hydrogen along with their evolution in time and the still hypothetical variation with heliolatitude. Furthermore, solar wind and its evolution with solar activity is presented, mostly in the context of charge exchange ionization of heliospheric neutral hydrogen, and dynamic pressure variations. Also electron-impact ionization of neutral heliospheric hydrogen and its variation with time, heliolatitude, and solar distance is discussed. After a review of the state of the art in all of those topics, we proceed to present an interim model of the solar wind and the other solar factors based on up-to-date in situ and remote sensing observations. This model was used by Izmodenov et al. (2013, this volume) to calculate the distribution of heliospheric hydrogen, which in turn was the basis for intercalibrating the heliospheric UV and EUV measurements discussed in Quemerais et al. (2013, this volume). Results of this joint effort will also be used to improve the model of the solar wind evolution, which will be an invaluable asset in interpretation of all heliospheric measurements, including, among others, the observations of Energetic Neutral Atoms by the Interstellar Boundary Explorer (IBEX).

Oxygen freeze‐in temperatures measured with SOHO/CELIAS/CTOF

We use the charge time-of-flight (CTOF) mass and charge spectrometer of the charge, element, and isotope analysis system (CELIAS) on board the Solar and Heliospheric Observatory (SOHO) to determine the solar wind oxygen freeze-in temperature T76 from the O7+ and O6+ abundance ratios in the period from days 92 to 229 of 1996 (Carrington Rotations 1908 to 1912). The freeze-in temperature is a conserved property of the solar wind because the charge states do not change after a distance of a few solar radii. Therefore it is an ideal in situ diagnostic for remote sensing of the inner solar corona. We determine the mean freeze-in temperature during the selected period to be 1.6 × 106 K. We use it to map coronal regions of different temperatures and to determine the separation between such regions based on our observation of abrupt transitions of the freeze-in temperature. We find a upper limit for the separation in the inner corona of 1000 km.

Lunar soils: A long-term archive for the galactic environment of the heliosphere?

Solar wind implanted in surface layers (≲0.03 μm) of lunar soil grains has often been analyzed to infer the history of the solar wind. In somewhat deeper layers, and thus presumably at higher implantation energies, a mysterious population, dubbed “SEP” for “solar energetic particle,” accounts for the majority of the implanted gas-several orders of magnitude more than expected from the present-day flux of solar energetic particles. In addition, its elemental and isotopic composition is distinct from that of the solar system. While the heavy Ne isotopes are enriched relative to 20Ne, 15N is depleted relative to 14N-a behavior that is hard to explain with acceleration of solar material. N is overabundant with respect to the noble gases (especially Ar). Here we show that interstellar pick-up ions (PUIs) which are ionized and accelerated in the heliosphere and subsequently implanted in lunar regolith grains can account for the properties of the “SEP” population. This implies that lunar soils preserve samples o...