VOYAGER UVS Lyman-alpha reanalysis: 1993-2011Rosine Lallement1, Eric Quémerais2, Jean-Loup Bertaux2 , Bill R. Sandel3, Vlad Izmodenov4

1:GEPI, Observ. de Paris, CNRS, Univ. Paris Diderot, Meudon, France, 2: LATMOS-IPSL, Univ. Versailles-St Quentin, Guyancourt, France , 3: Lunar and Planetary Laboratory, Univ. of Arizona, Tucson, USA, 4: Lomonosov Moscow State University, Moscow, Russia

Part A: First detection of Lyman-alpha diffuseEmission from the Milky Way (Science, Dec 1st, 2011)

Part B: UVS detector background study: energetic particles from 1993 to 2011

Abstract: Doppler-shifted hydrogen Ly-alpha 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 Glow, solar Ly-alpha radiation backscattered by interstellar H atoms within the heliosphere. Observations from the Voyager spacecraft are decreasingly affected by the H glow, and we showed that the Ultra-Violet Spectrographs (UVS) are detecting Ly-alpha diffuse emission from our Galaxy. To do so we devised new methods of analysis, in particular we estimated the Ly-alpha line emission in the presence of dust-scattered starlight. There is an excess of emission over the heliospheric signal that reaches 3-4 Rayleigh towards H-alpha bright star forming regions along the Galactic Plane. Models of Ly-alpha radiation transfer built for distant galaxies can be tested in the presence of well-known dust, gas and star distributions.

Fig1: (Top) UVS scans of 18 to 20 directions, superimposed on the H-alpha full-sky map of Finkbeiner et al. (2003). The presence of stellar continua (type 2 spectra, at bottom) is clearly linked to H-alpha bright regions. (Bottom) For type 2 spectra the Lya trough in the dust-scattered light must be estimated in order to correctly measure the total Lya line emission. In most UVS spectra only the Lya line is detected (Fig 1, type 1), and its intensity can be derived in a rather straightforward way by summing counts in the 9 Lya channels. At low galactic latitudes the line is superimposed on a spectrally extended signal due to diffuse, dust scattered light of young hot stars (type 2). This emission traces the presence along the line-of-sight of interstellar dust within the H II regions where the UV light of young hot stars is ionizing the neutral hydrogen, and where subsequent recombinations of protons and electrons produce both Ha and Lya. This UV light is thus closely associated with the galactic Lya sources. Using an H-alpha full-sky composite map (Finkbeiner 2003), we clearly see that, for all scans, the directions having the brightest diffuse light clearly coincide with the Galactic Plane and bright Ha regions, which is expected.

We have devised new methods of reduction of the spectra that eliminate bias from possible dust-scattered starlight. We also used a new method of derivation of the Lya line intensity in the presence of the continua. The method has been checked using young star spectra and independently through fitting of the continuum spectral shape. Data are compared with a radiative transfer model of Lya photons throughout the whole heliosphere (Quémerais et al, 2008). The model was applied to time-dependent neutral hydrogen distributions (Izmodenov et al, 2008). Model values computed for each data point were normalized and binned in exactly the same way as the data. An excess is measured that coincides with the bright star-forming regions. The excess is of the order of a few Rayleigh, and is due to galactic Lya photons produced also by recombination, together with Ha photons, but propagating through multiple scattering between galactic H atoms.

Fig2: Comparison of Lya brightness data (black) with a radiative transfer model of the heliospheric glow (red). There is clearly an excess of emission that coincides with the H-alpha-bright areas (green curve) at low latitudes ( violet curve). Data and model are normalized at 60° scan angle, where the data have an intensity of ~45 Rayleigh.

Fig 3: Examples of V2 background spectra recorded at different periods and for the same orientation of the platform. The spectra were derived from raw spectra by subtraction of adjusted « pure » Ly-alpha spectra.

Abstract: In the course of the reanalysis of the Ly a data we have studied in detail the spectral and temporal evolution of the background spectra of the two UVS detectors (1993 to 1999 for V2, 1993 to 2011 for V1). The main source of the signal was previously attributed to MeV particles from the radio-isotope generator (RTG). However, instead of decreasing the signal has constantly increased since 1993 for both S/C. In the case of V1 the signal began to fluctuate in 2000 then again increased smoothly until 2010, reaching then a « plateau » until Feb 2011. The behavior is very similar to V1 CRS 133-242 MeV proton data. Our interpretation is that energetic particles are responsible for this background, adding a new source of information, albeit uncertain about the energy range. After Feb 2011 the signal has continued to increase, potentially indicating that the heliopause is not definitely reached.

Fig 5: Sum of channels 6-40 of V1 background spectra as a function of time for all platform orientations. Note the fluctuations between 2000 and 2004, and the « plateau » that starts in 2010. Before 2003 (blue) data correspond to the scans described in Part A. After 2003 (black points) measurements continued but the platform is fixed (fixed direction of sight). There is a strong similarity with the CRS proton data, another instrument on Voyager (from the NSSDC )

References:Broadfoot, A. L., et al. Space Science Reviews, 21, 183 (1977) Finkbeiner, D. P. , Astrophys. J., 146, 407 (2003)Izmodenov, V., Malama, Y. G., & Ruderman, M. S. , Adv. Space Res., 41, 318 (2008)Lallement R., Quémerais E., Bertaux J.L., Sandel B.R., Izmodenov V.V. , 2011, Science (on line Dec 1st) Manuel , R., Ferreira, S.E.S., Potgieter, M.S., Advances in Space Research 48 (2011) 874–883Quémerais E., Izmodenov V.V., Koutroumpa D.,Malama Y, Astron. Astrophys., 488, 351 (2008)

Fig 4: Sum of channels 8-42 of V2 background spectra as a function of time and azimuth angle of the orientation platform.

Conclusion: Some theoretical models predict that the flux of cosmics should reach a plateau at the heliopause (Manuel et al., 2011). UVS Voyager 1 shows on the contrary a recent (2011) increase in the gradient : according to those models V1 has not yet reached the heliopause.