The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) is a five telescope package, which has been developed for the Solar Terrestrial Relation Observatory (STEREO) mission by the Naval Research Laboratory (USA), the Lockheed Solar and Astrophysics Laboratory (USA), the Goddard Space Flight Center (USA), the University of Birmingham (UK), the Rutherford Appleton Laboratory (UK), the Max Planck Institute for Solar System Research (Germany), the Centre Spatiale de Leige (Belgium), the Institut d'Optique (France) and the Institut d'Astrophysique Spatiale (France). SECCHI comprises five telescopes, which together image the solar corona from the solar disk to beyond 1 AU. These telescopes are: an extreme ultraviolet imager (EUVI: 1-1.7 R ), two traditional Lyot coronagraphs (COR1: 1.5-4 R and COR2: 2.5-15 R ) and two new designs of heliospheric imagers (HI-1: 15-84 R and HI-2: 66-318 R ). All the instruments use 2048 × 2048 pixel CCD arrays in a backside-in mode. The EUVI backside surface has been specially processed for EUV sensitivity, while the others have an anti-reflection coating applied. A multi-tasking operating system, running on a PowerPC CPU, receives commands from the spacecraft, controls the instrument operations, acquires the images and compresses them for downlink through the main science channel (at compression factors typically up to 20×) and also through a low bandwidth channel to be used for space weather forecasting (at compression factors up to 200×). An image compression factor of about 10× enable the collection of images at the rate of about one every 2-3 minutes. Identical instruments, except for different sizes of occulters, are included on the STEREO-A and STEREO-B spacecraft.

Abstract.A coordinated effort to combine all three methods that are used to determine the physical parameters of interstellar gas in the heliosphere has been undertaken. In order to arrive at a consistent parameter set that agrees with the observations of neutral gas, pickup ions and UV backscattering we have combined data sets from coordinated observation campaigns over three years from 1998 through 2000. The key observations include pickup ions with ACE and Ulysses SWICS, neutral atoms with Ulysses GAS, as well as UV backscattering at the He focusing cone close to the Sun with SOHO UVCS and at 1 AU with EUVE. For the first time also the solar EUV irradiance that is responsible for photo ionization was monitored with SOHO CELIAS SEM, and the He I 58.4 nm line that illuminates He was observed simultaneously with SOHO SUMER. The solar wind conditions were monitored with SOHO, ACE, and WIND. Based on these data the modeling of the interstellar gas and its secondary products in the heliosphere has resulted in a consistent set of interstellar He parameters with much reduced uncertainties, which satisfy all observations, even extended to earlier data sets. It was also established that a substantial ionization in addition to photo ionization, most likely electron impact, is required, with increasing relative importance closer to the Sun. Furthermore, the total combined ionization rate varies significantly with solar latitude, requiring a fully three dimensional and time dependent treatment of the problem.

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Abstract. The helium gravitational focusing cone has been observed using pickup He + , first during the solar minimum in [1984][1985] with the AMPTE/IRM spacecraft, and again in more detail from 1998 to 2002 with ACE and in 2000 with Nozomi. Five traversals of the cone allow us to obtain an accurate determination of the ecliptic longitude of the interstellar wind flow direction, λ = 74.43 • ± 0.33 • , while observations of pickup He ++ with Ulysses give us an estimate, relatively free of instrumental systematic uncertainties, of the neutral He density, n He = 0.0151 ± 0.0015 cm −3 , in the Local Interstellar Cloud. From best fits to the measured velocity distributions of pickup He + using time-stationary models we deduce the radial dependence and magnitude of electron-impact ionization rates that cannot presently be measured, and find this to be an important ionization process in the inner ( < ∼ 0.5 AU) heliosphere. We obtain excellent model fits to the 1998 cone profile using measured or deduced rates and known interstellar He parameters, and from this conclude that cross-field diffusion of pickup He + is small. Furthermore, we find no evidence for extra sources of He in or near the cone region. Best fits to the velocity distributions of He + are obtained assuming isotropic solar-wind-frame distributions, and we conclude from this that the scattering mean free path for pickup He + in the turbulent slow solar wind is small, probably less than 0.1 AU. We argue that application of 3D, time-dependent models for computation of the spatial distribution of interstellar neutral helium in the inner heliosphere may lead to excellent fits of short-term averaged pickup He + data without assuming loss rates that are significantly different from production rates.

Abstract.This paper shows that the Mg II core-to-wing ratio is a better proxy for Solar Extreme Ultraviolet (EUV) ra-

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Abstract. We present new observations of the diffuse He I 58.4 nm background recorded in 1998 and 2000 by the Extreme Ultraviolet Explorer (EUVE). This emission is due to resonant scattering of the solar EUV radiation by interstellar and geocoronal helium. Depending on the geometry and relative velocity, a fraction of the interstellar helium glow can be absorbed by the line-of-sight geocoronal gas. The new results are combined with measurements obtained in 1992−93 and previously analyzed by Flynn et al. (1998). A kinetic model of the helium flow is now used to analyze the data and reproduce the absorption features due to geocoronal helium. This allows a precise determination of the interstellar flow bulk velocity vector. A model that includes both photoionization and electron impact ionization was fit to the data set. New constraints on the interstellar helium flow temperature and density, as well as on the solar 58.4 nm line width are obtained. The interstellar helium velocity vector parameters, λ = 74.7 ± 0.5• , β = −5.7 ± 0.5• , V He = 24.5 ± 2 km s −1 , are found to be in good agreement with those derived from particle measurements. Using the solar He I 58.4 nm flux and photoionization rate proxies of McMullin et al. (2003), the neutral helium density and temperature derived from the Long Wavelength Spectrometer data is n He = 0.013 ± 0.003 cm −3 , and T He = 6500 ± 2000• respectively, again in good agreement with particle data. However, the width of the downwind cone when scanned across the latitudnal direction tends to be fit better with higher He temperatures, which might indicated latitude anisotropies in the He ionization that we have not included in our models. The solar He I 58.4 nm Doppler width, ∆w D , is found to be =0.0074 nm, (or 38 ± 3 km s −1 ) in 1992−1993, i.e. near solar maximum, and ∆w D = 0.0087 nm (45 ± 3 km s −1 ) in 1998, after solar minimum, in agreement with SOHO SUMER and CDS results, although again, the 1998 fits near solar minimum might suffer from latitudinal anisotropies.

Abstract. The interstellar gas that flows through the heliosphere is strongly affected by ionization close to the Sun, in particular solar photoionization, electron impact, and charge exchange. Therefore, the interpretation of any observation of interstellar gas in the inner heliosphere hinges upon the accurate knowledge of these effects and their variations. In addition, the irradiance and line profile of the relevant solar spectral line are needed to properly interpret resonant backscattering observations of the interstellar neutral gas. With instrumentation on ACE, SOHO and Wind, continuous monitoring of these important environmental conditions simultaneously with a multitude of interstellar gas observations has become possible for the first time. In this paper we present a compilation of the processes and parameters that affect the distribution of interstellar helium inside the heliosphere and their observation, including the irradiance and line profile of the He 58.4 nm line. We also make the connection to proxies for these parameters and evaluate their accuracy in order to expand the time period of coverage wherever possible.