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LRO/LAMP Study of the Interstellar Medium Via the Hei 58.4 Nm Resonance Line C

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LRO/LAMP Study of the Interstellar Medium Via the Hei 58.4 Nm Resonance Line C A&A 616, A159 (2018) https://doi.org/10.1051/0004-6361/201731555 Astronomy & © ESO 2018 Astrophysics LRO/LAMP study of the interstellar medium via the HeI 58.4 nm resonance line C. Grava1, W. R. Pryor2, P. D. Feldman3, K. D. Retherford1, G. R. Gladstone1, and T. K. Greathouse1 1 Southwest Research Institute, San Antonio, TX, USA e-mail: [email protected] 2 Central Arizona College, Coolidge, AZ, USA 3 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA Received 12 July 2017 / Accepted 24 April 2018 ABSTRACT Context. Recent measurements by IBEX and detailed modeling have changed our understanding of the flow of the interstellar medium through the solar system. In particular, a time dependence of the direction of the interstellar medium flow has been proposed, and a new population of helium atoms, called the “warm breeze”, has been discovered. Aims. We aim to constrain the structure of the interstellar medium close to the downwind focusing cone using the sensitive LAMP FUV/EUV imaging spectrograph onboard the Lunar Reconnaissance Orbiter. Methods. We measured the brightness of the emission line from interstellar helium atoms resonantly scattering solar photons at 58.4 nm (HeI) and compare it to our “modified cold model” of interstellar HeI sky brightness as a function of ecliptic latitude and lon- gitude. Additionally, we compared LAMP observations to a model with time-dependent inflow direction and a model of the brightness of the “warm breeze”, to see if they can be distinguished by LAMP. Results. We find that the LAMP observations agree within error bars to our “modified cold model”, which in turn is consistent with the latest interstellar helium flow parameters found with IBEX. Our model can therefore be applied to other UV spectroscopic obser- vations of the interstellar helium. However, LAMP observations cannot distinguish between our model and a model with a different inflow direction, since the latter has negligible effect on the 2D brightness of the interstellar HeI emission line. For the same reason, LAMP could not detect the effect of the “warm breeze”. We note a discrepancy between solar irradiances measured by TIMED/SEE and those measured by SDO/EVE. We recommend using values from SDO/EVE. Finally, we derive a value of LAMP sensitivity at the EUV wavelength (58.4 nm) of 0.485 ± 0.014 Hz/Rayleigh. Conclusions. These measurements pave the way to observations of the interstellar wind from lunar orbit. Key words. interplanetary medium – techniques: imaging spectroscopy – Sun: UV radiation – ISM: general – Sun: heliosphere 1. Introduction method used to detect interstellar helium (Meier & Weller 1972), and the method we used and present here. The solar system is moving through the so-called local inter- stellar cloud (LIC), a low density, warm, and partially ionized Advantages of studying the interstellar wind with the HeI 58.4 cloud of gas and plasma (∼9 pc across) which is in turn con- nm resonant line emission. Observations of the interstellar tained within the more diluted Local Bubble (∼90 pc across). medium via the HeI 58.4 nm emission line due to resonant The Sun carves a region within the LIC, called the heliosphere, scattering of helium were carried out first by rockets (Meier & in which ions are precluded from entering by the interplane- Weller 1972; Paresce et al. 1974), then by orbiting satellites such tary magnetic field, but not the neutrals, notably He, H, and O, as STP 72-1 (Weller & Meier 1974), SOLRAD 11B (Weller which are free to travel. These atoms form the local interstellar & Meier 1981), Prognoz 6 (Dalaudier et al. 1984), and EUVE medium (LISM). By studying these atoms, it is possible to infer (Flynn et al. 1998; Vallerga et al. 2004), and also by satellites en the characteristics of the LIC such as direction of motion of the route to other worlds, such as Mariner 10 (Ajello 1978; Broadfoot solar system and density and velocity of incoming neutrals. In & Kumar 1978; Ajello et al. 1979), Nozomi (Yamazaki et al. the ∼40 yr that intervened since the discovery of the interstellar 2006; Nakagawa et al. 2008), and Galileo (Pryor et al. 2014). wind (Bertaux & Blamont 1971; Thomas & Krassa 1971) differ- This technique has several advantages compared to the ent observation techniques have been used to study the motion spectroscopy of interstellar hydrogen, which resonantly scat- of the solar system through the interstellar wind. Indeed, helium ters Lyman-alpha (Ly-α) photons (121.6 nm; Weller & Meier can be detected in three different ways: (1) in situ measurements 1981). Firstly, the interstellar extinction is significantly greater of helium atoms through imaging, e.g. with IBEX (Möbius et al. at 58.4 nm than at 121.6 nm, therefore contamination from the 2009a; McComas et al. 2015b) and Ulysses (Witte et al. 2004); galactic background is negligible at 58.4 nm. Secondly, solar (2) pickup ions in the downwind gravitational focusing cone with radiation pressure for helium is not as important as for hydro- ACE-SWICS (Gloeckler et al. 1998), AMPTE-IRM (Möbius gen, due to the larger mass of helium and to the much lower et al. 1995), Nozomi (Gloeckler et al. 2004), STEREO-PLASTIC solar flux at 58.4 nm. Therefore, helium penetrates much deeper (Drews et al. 2012), and MESSENGER-FIPS (Gershman et al. in the heliosphere (i.e. much closer to the Sun) than hydrogen, 2013); and (3) HeI resonance emission line at 58.4 nm, the first and the downwind focusing cone is more pronounced. Thirdly, Article published by EDP Sciences A159, page 1 of 19 A&A 616, A159 (2018) charge-exchange is negligible for He, but not for H or O, whose of neutral oxygen atoms produced by charge exchange between densities (and brightness) are therefore depleted (Fahr 1991). The the primary population of interstellar hydrogen and hot ionized main loss mechanisms for helium atoms are photoionization and oxygen in the outer heliosphere. electron impact ionization, especially important for heliocentric distances less than 1 Astronomical Unit (AU) (Rucinski & Fahr Our study. The purpose of the analysis of the LAMP obser- 1989; Bzowski et al. 2013; Scherer et al. 2014). Another advan- vations presented here is to test the validity of our “modified tage is that the HeI 58.4 nm line is optically thinner than the Ly-α cold model” of the interstellar wind HeI 58.4 nm brightness, line, so that complex multiple scattering calculations are not which is based on the “standard picture” of a fixed direction of needed in the model. The study of the interstellar wind through motion of the interstellar wind, and to see if the presence of the the HeI 58.4 nm resonance line has therefore the advantage, with “warm breeze” can be detected in the downwind focusing cone. respect to the Ly-α emission line of atomic H, of providing bet- Moreover, these observations are also useful for improving the ter determination of parameters pertinent to the interstellar wind, knowledge of LAMP’s effective area at short wavelengths, since such as bulk flow velocity, ecliptic longitude and latitude of the the LISM is largely opaque to the 58.4 nm radiation from the direction of the interstellar wind, and temperature and density of stars which are usually observed for calibration. As explained neutral helium (for a thorough description of the parameters that in Sect. 2.2, we take advantage of the fact that, being in orbit affect HeI 58.4 nm emission line, see McMullin et al. 2004 and around the Moon, LAMP observations are not affected by the Lallement et al. 2004). Helium is therefore a very useful tracer geocoronal foreground emission (Paresce et al. 1974). of conditions of the LIC. In addition to that, the study of the interstellar medium via 2. Method spectroscopy of the 58.4 nm line has the advantage, compared to in situ pick-up ions measurements or energetic neutral atoms 2.1. LAMP FUV/EUV imaging spectrograph imaging, that it is the one with the longest observation baseline The Lyman-Alpha Mapping Project (LAMP; Gladstone et al. (>45 yr), being implemented since the 1970s. Therefore, an 2010), one of the seven instruments on the Lunar Reconnais- improvement on this method will benefit future attempts to sance Orbiter (LRO; Chin et al. 2007), is a sensitive, photon- study medium- or long-term variations in the interstellar flow counting, imaging FUV spectrograph that covers a bandpass direction. of 57.5–196.5 nm. Its detector is a microchannel plate with a double-delay-line anode that allows 2D position sensing. Wave- The downwind focusing cone. The combination of the grav- length is dispersed along the horizontal direction of the resulting itational attraction of the Sun and the flowing of interstellar 2D data array (1024 columns, of which 776 are illuminated). medium concentrate the helium population in the downwind The vertical direction (32 rows, of which 21 are illuminated) direction, where atoms are channelled into the downwind focus- provides spatial information along the slit. The instrument col- ing cone. The Interstellar Boundary Explorer (IBEX) spacecraft lects data as pixel-list events within 4 ms intervals, and it is (McComas et al. 2009) report “nominal values” for interstellar possible to integrate signals over longer timescales and regions helium of 25:4 km s−1 for velocity, 7500 K for temperature, 75:7◦ of interest. Primary LAMP goals are to identify and localize for ecliptic longitude and −5:1◦ for ecliptic latitude of the down- exposed water frost in permanently shadowed regions (PSRs) wind pristine interstellar flow direction outside the heliosphere of the Moon, characterize landforms and albedos in PSRs, and at a distance of 1000 AU (McComas et al.
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