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International audience; We analyze SOHO/SWAN hydrogen Lyman‐α data collected between 1996 and 2018 to derive the solar wind latitudinal distribution over time. Full‐sky interplanetary Lyman‐α maps are inverted to derive the total hydrogen ionization rate latitude profiles, normalized to proton charge‐exchange and photoionization. Using Interplanetary Scintillation (IPS) velocities to calculate the velocity‐dependent charge‐exchange cross‐sections, we produce the solar wind flux latitudinal profiles. Finally, we compute solar wind velocity latitude profiles, based on the dynamic pressure and energy flux conservation (calculated from OMNI data) over latitude. SWAN reproduces the IPS velocity profiles up to at least ±60°, and also agrees with Ulysses in‐situ measurements for solar minimum periods in 1996‐1997 and 2007. During solar maximum, discrepancies are more frequent because in‐situ data reflect local solar wind conditions, while SWAN data reflect global conditions in the heliosphere.

For more than two decades the SOHO/SWAN instrument has been monitoring the full-sky hydrogen backscattered Lyman-α emission, and the derived three-dimensional solar wind proton flux. We present a comparison of the time series of the latitude-integrated hydrogen ionization rates (β) derived from the inversion of the SWAN full-sky maps with the integrated coronal electron density derived from the inversion of SOHO/LASCO-C2 white light images. The analysis shows a variable time lag of the SWAN β of a few Carrington rotations, correlated with the solar cycle phase (larger delay during solar maxima compared to minima). This is a direct consequence of the variation of the size of the hydrogen ionization cavity and the time it takes for hydrogen atoms to propagate in the inner heliosphere. This effect should be taken into account in studies of the interstellar neutral populations in interplanetary space.

The Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyman-alpha camera on the SOlar and Heliosphere Observer (SOHO) satellite makes daily images of the entire sky to monitor the three-dimensional distribution of solar wind and solar radiation via its imprint on the stream of interstellar hydrogen that flows through the solar system. In the process it also records the distribution of the hydrogen comae of comets. We report here the analyses of six comets originally from the Oort Cloud observed during the 2017-2020 period by SWAN: C/2015 V2 (Johnson), C/2019 Y1 (ATLAS), C/2017 T2 (PanSTARRS), C/2020 F8 (SWAN), C/2019 Y4 (ATLAS), and C/2019 U6 (Lemmon). Of these the nuclei of C/2019 Y4 (ATLAS) and C/2020 F8 (SWAN) both broke up on their inbound orbit before perihelion. The water production rates over the detectable portion of each comet's orbit are determined and discussed in light of the comet's dynamical sub-class of Oort Cloud comets., Comment: 42 pages, 8 figures

C/2020 F3 (NEOWISE) was discovered in images from the Near Earth Object program of the Wide-Field Infrared Survey Explorer (NEOWISE) taken on 27 March 2020 and has become the Great Comet of 2020. The Solar Wind ANisotropies (SWAN) camera on the Solar and Heliospheric Observatory (SOHO) spacecraft, located in a halo orbit around the Earth-Sun L1 Lagrange point, makes daily full-sky images of hydrogen Lyman-alpha. Water production rates were determined from the SWAN hydrogen Lyman-alpha brightness and spatial distribution of the comet measured over a 4-month period of time on either side of the comet's perihelion on 3 July 2020. The water production rate in s^-1 was moderately asymmetric around perihelion and varied with the heliocentric distance, r, in au as (6.9+/-0.5) x 10^28 r^-2.5+/-0.2 and (10.1+/-0.5) x 10^28 r^-3.5+/-0.1 before and after perihelion, respectively. This is consistent with the comet having been through the planetary region of the solar system on one or more previous apparitions. A water production rates as large as 5.27 x 10^30 s^-1 were determined shortly after perihelion, once the comet was outside the solar avoidance area of SWAN, when the comet was 0.324 au from the Sun., Comment: 16 Pages, 4 Figures, 1 Table

The Solar Wind Anisotropies (SWAN) instrument on the SOlar and Heliospheric Observatory (SOHO) satellite has observed 44 long period and new Oort cloud comets and 36 apparitions of 17 short period comets since its launch in December 1995. Water production rates have been determined from the over 3700 images producing a consistent set of activity variations over large parts of each comet's orbit. This has enabled the calculation of exponential power-law variations with heliocentric distance of these comets both before and after perihelion, as well as the absolute values of the water production rates. These various measures of overall water activity including pre- and post-perihelion exponents, absolute water production rates at 1 AU, active surface areas and their variations have been compared with a number of dynamical quantities for each comet including dynamical class, original semi-major axis, nucleus radius (when available), and compositional taxonomic class. Evidence for evolution of cometary nuclei is seen in both long-period and short-period comets., Comment: 43 pages, 9 figures, 6 tables

The Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyman-alpha camera on the SOlar and Heliospheric Observer (SOHO) satellite observed the hydrogen coma of comet C/2017 S3 (PanSTARRS) for the last month of its activity from 2018 July 4 to August 4 and what appears to have been its final disintegration just 11 days before its perihelion on August 15. The hydrogen coma indicated water production had a small outburst on July 8 at a heliocentric distance of 1.1AU and then a much larger one on July 20 at 0.8 AU. Over the following two weeks the water production dropped by more than a factor of ten after which it was no longer detectable. The behavior is reminiscent of comet C/1999 S4 (LINEAR) in 2000, which had a few small outbursts on its inbound orbit and a major outburst at a heliocentric distance of about 0.8 AU, which was close to its perihelion, followed by its complete disintegration that was documented by several sets of observations including SWAN. C/2017 S3 (PanSTARRS) however had a much larger water production rate than C/ 1999 S4 (LINEAR). Here we estimate the size of the nucleus of C/2017 S3 just before its final outburst and apparent disintegration was estimated using the total amount of water produced during its last weeks for a range of values of the refractory/ice ratio in the nucleus. We also determine the size distribution of the disintegrating particles as the comet faded., Comment: 18 pages, 3 tables, 3 figures

International audience; Interplanetary background measurements can be used to derive the solar flux that is illuminating the atoms in the interplanetary medium. We present a new algorithm combining measurements from the SWAN‐SOHO Lyman α photometer with Earth‐based solar flux measurements to derive the solar illuminating flux at Lyman α in any direction along the ecliptic plane. The modulations of the solar flux and the interplanetary background intensity are proportional. The proportionality coefficient is a direct measurement of the fraction of the interplanetary brightness that is due to single scattering in the interplanetary medium. We find that for an observer at 1 AU from the Sun, single scattering photons represent roughly 70% of the total brightness. This new algorithm is tested by a comparison with actual measurements from the EUVM‐MAVEN solar monitor in orbit around Mars since 2014.

Nine recently discovered long-period comets were observed by the Solar Wind Anisotropies (SWAN) Lyman-alpha all-sky camera on board the Solar and Heliosphere Observatory (SOHO) satellite during the period of 2013 to 2016. These were C/2012 K1 (PanSTARRS), C/2013 US10 (Catalina), C/2013 V5 (Oukaimeden), C/2013 R1 (Lovejoy), C/2014 E2 (Jacques), C/2014 Q2 (Lovejoy), C/2015 G2 (MASTER), C/2014 Q1 (PanSTARRS) and C/2013 X1 (PanSTARRS). Of these 9 comets 6 were long-period comets and 3 were possibly dynamically new. Water production rates were calculated from each of the 885 images using our standard time-resolved model that accounts for the whole water photodissociation chain, exothermic velocities and collisional escape of H atoms. For most of these comets there were enough observations over a broad enough range of heliocentric distances to calculate power-law fits to the variation of production rate with heliocentric distances for pre- and post-perihelion portions of the orbits. Comet C/2014 Q1 (PanSTARRS), with a perihelion distance of only ~0.3 AU, showed the most unusual variation of water production rate with heliocentric distance and the resulting active area variation, indicating that when the comet was within 0.7 AU its activity was dominated by the continuous release of icy grains and chunks, greatly increasing the active sublimation area by more than a factor of 10 beyond what it had at larger heliocentric distances. A possible interpretation suggests that a large fraction of the comet's mass was lost during the apparition., Accepted for publication by Icarus, 36 Pages, 3 Tables, 10 Figures, 9 Supplementary Tables

International audience; Coronal mass ejections (CMEs) play a major role in the heliosphere and their contribution to the solar wind mass flux, already considered in the Skylab and Solwind eras with conflicting results, is re-examined in the light of 19 years (1996-2014) of SOHO observations with the Large Angle and Spectroscopic Coronagraph “LASCO-C2” for the CMEs and extended for the first time to all latitudes thanks to the whole-heliosphere data from the Solar Wind ANisotropies “SWAN” instrument supplemented by in-situ data aggregated in the OMNI database. First, several mass estimates reported in the ARTEMIS catalog of LASCO CMEs are compared with determinations based on the combined observations with the twin STEREO/SECCHI coronagraphs in order to ascertain their validity. A simple geometric model of the CMEs is introduced to generate Carrington maps of their mass flux and then to produce annualized synoptic maps. The Lyman-α SWAN data are inverted to similarly produce synoptic maps to be compared with those of the CME flux. The ratio of the annualized CME to solar wind mass flux is found to closely track the solar cycle over the heliosphere. In the near-ecliptic region and at latitudes up to ∼ 55o, this ratio was negligibly small during the solar minima of cycles 22/23 and 23/24 and rose to 6% and 5% respectively at the maximum of solar cycles 23 and 24. These maximum ratios increased at higher latitudes but this result is likely biased by the inherent limitation of determining the true latitude of CMEs.

International audience; The Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyα camera on the SOlar and Heliospheric Observer (SOHO) satellite observed the hydrogen coma of comet C/2012 S1 (ISON) for most of the last month of its activity from 2013 October 24 to November 24, ending just 4 days before perihelion and its final disruption. The water production rate of the comet was determined from these observations. SOHO has been operating in a halo orbit around the Earth-Sun L1 Lagrange point since its launch in late 1995. Most water vapor produced by comets is ultimately photodissociated into two H atoms and one O atom producing a huge hydrogen coma that is routinely observed in the daily SWAN images in comets of sufficient brightness. Water production rates were calculated from 22 images over most of the last month of the pre-perihelion apparition. The water production rate increased very slowly on average from October 24.9 until November 12.9, staying between 1.8 and 3.4 × 1028 s-1, after which it increased dramatically, reaching 1.6 to 2 × 1030 s-1 from November 21.6 to 23.6. It was not detected after perihelion on December 3.7 when it should have been visible. We examine the active surface area necessary to explain the water production rate and its variation and are able to place constraints on the physical size of the original nucleus necessary to account for the large amount of activity from November 12.9 and until just before perihelion.

International audience; Comets C/2011 L4 (PanSTARRS) and C/2012 F6 (Lemmon) were observed throughout their 2012-2013 apparitions with the Solar Wind Anisotropies (SWAN) all-sky hydrogen Lyα camera on board the Solar and Heliosphere Observatory (SOHO) satellite. SOHO has been in a halo orbit around the L1 Earth-Sun Lagrange point since early 1996 and has been observing the interplanetary medium and comets beginning with C/1996 B2 (Hyakutake). The global water production from these comets was determined from an analysis of the SWAN Lyα camera observations. Comet C/2011 L4 (PanSTARRS), which reached its perihelion distance of 0.302 AU on 2013 March 10.17, was observed on 50 days between 2013 January 29 and April 30. Comet C/2012 F6 (Lemmon), which reached its perihelion distance of 0.731 AU on 2013 March 24.51, was observed on 109 days between 2012 November 29 and 2013 June 31. The maximum water production rates were ~1 × 1030 molecules s-1 for both comets. The activities of both comets were asymmetric about perihelion. C/2011 L4 (PanSTARRS) was more active before perihelion than after, but C/2012 F6 (Lemmon) was more active after perihelion than before.

Spectra of astronomical targets acquired from ground-based instruments are affected by the atmospheric transmission. The authors and their institutes are developing a web-based service, TAPAS (Transmissions of the AtmosPhere for AStromomical data). This service, freely available, is developed and maintained within the atmospheric ETHER data center. TAPAS computes the atmospheric transmission in the line-of-sight to the target indicated by the user. The user files a request indicating the time, ground location, and either the equatorial coordinates of the target or the Zenith Angle of the line-of sight (LOS). The actual atmospheric profile (temperature, pressure, humidity, ozone content) at that time and place is retrieved from the ETHER atmospheric data base (from a combination of ECMWF meteorological field and other informations), and the atmospheric transmission is computed from LBLRTM software and HITRAN data base for a number of gases: O2, H2O, O3, CO2, and Rayleigh extinction. The first purpose of TAPAS output is to allow identification of observed spectral features as being from atmospheric or astrophysical origin. The returned transmission may also serve for characterizing the spectrometer in wavelength scale and Instrument Line Spectral Function (ILSF) by comparing one observed spectrum of an atmospheric feature to the transmission. Finally, the TOA (Top Of Atmosphere) spectrum may be obtained either by division of the observed spectrum by the computed transmission or other techniques developed on purpose. The obtention of transmissions for individual species allows more potentialities and better adjustments to the data. In this paper, we describe briefly the mechanism of computation of the atmospheric transmissions, and we show some results for O2 and H2O atmospheric absorption. The wavelength range is presently 500-2500 nm. The address is http://ether.ipsl.jussieu.fr/tapas/, Comment: 12 pages, 10 figures, submitted to Astronomy and Astrophysics

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 Quémerais 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).

International audience; The all-sky hydrogen Lyman-alpha camera, SWAN (Solar Wind Anisotropies), on the SOlar and Heliospheric Observatory (SOHO) satellite made observations of the hydrogen coma of comet C/2009 P1 (Garradd) throughout its apparition from August 15, 2011 through April 6, 2012. SOHO has been operating in a halo orbit around the Earth-Sun L1 Lagrange point since its launch in late 1995. Most water vapor produced by the comet is ultimately photodissociated into two H atoms and one O atom producing a huge atomic hydrogen coma that is routinely observed in the daily full-sky SWAN images in comets of sufficient brightness. Water production rates were calculated from 117 images over 8 months of the apparition yielding about 1 observation every 2 days on the average. The activity during much of the pre-perihelion leg was much larger than the post-perihelion leg and varied rather irregularly, not following the more typical steadily increasing trend with decreasing heliocentric distance. It varied generally between 1 and 3 x 1029 s-1, with a peak value of 4 x 1029 s-1 on November 3, 2011, 50 days before perihelion. The elevated production rate from the very large SWAN field-of-view compared with smaller aperture observations could be consistent with water production from icy particles rather than from gas sublimated directly from the nucleus. Scenarios for the nature and production of the icy grain source are explored. During the post-perihelion leg the water production rate decreased more uniformly and typically from 2 x 1029 s-1 at perihelion, approximately as r-3.2, where r is the heliocentric distance. This is consistent with water sublimation from a reasonably constant total surface area

We update two kinds of results obtained with the SWAN instrument on board SOHO. First, we use H cell data recorded in 2001 and derive the H flow direction in the same way we performed the study at solar minimum. We compare with the Helium flow and doing so we correct for the coordinate system change between the Ulysses and SOHO mission. The deflection plane we obtain is compatible with the previous result within error bars, confirming the predominant role of the interstellar magnetic field. Secondly, we extend the derivation of solar wind ionization temporal evolution as a function of heliolatitude. The pattern for the present solar minimum is strikingly different from the previous minimum, with a much wider slow solar wind equatorial belt which persists until at least 2008. Comparing with synoptic LASCO/C2 electron densities we infer from a preliminary study that the acceleration of the high speed solar wind occurs at a higher altitude during this minimum, a change expansion models should be able to explain., 4 pages, Twelfth International Solar Wind Conference, 21-26 June 2009, Saint-Malo, France

In this study, the possibility that coronal mass ejections (CMEs) may be observed in neutral Lyman-α emission was investigated. An observing campaign was initiated for SWAN (Solar Wind ANisotropies), a Lyman-α scanning photometer on board the Solar and Heliospheric Observatory (SOHO) dedicated to monitoring the latitude distribution of the solar wind from its imprints on the interstellar sky background. This was part of SOHO Joint Observing Program (JOP) 159 and was an exploratory investigation as it was not known how, or even if, CMEs interact with the solar wind and interstellar neutral hydrogen at this distance (≈60 and 120 R S). The study addresses the lack of methods for tracking CMEs beyond the field-of-view of current coronagraphs (30 R S). In our first method we used LASCO, white-light coronagraphs on SOHO, and EIT, an extreme ultraviolet imaging telescope also on SOHO, to identify CME candidates which, subject to certain criteria, should have been observable in SWAN. The criteria included SWAN observation time and location, CME position angle, and extrapolated speed. None of the CME candidates that we discuss were identified in the SWAN data. For our second method we analyzed all of the SWAN data for 184 runs of the observing campaign, and this has yielded one candidate CME detection. The candidate CME appears as a dimming of the background Lyman-α intensity representing ≈10% of the original intensity, moving radially away from the Sun. Multiple candidate CMEs observed by LASCO and EIT were found which may have caused this dimming. Here we discuss the campaign, data analysis technique and statistics, and the results.

[1] We present results of the total hydrogen ionization rate obtained from the inversion of almost 10 years of full-sky maps of the interplanetary Lyman α background measured by the SWAN instrument on SOHO. Thanks to a new estimate of the absolute calibration of the SWAN instrument and its variation during the 10 years of operation of SOHO, we are able to derive absolute values of the ionization rate as well as its latitudinal dependence. We show how the anisotropy of the ionization rate changes from solar minimum to solar maximum. At solar maximum, the so-called ionization groove has completely disappeared and the ionizing fluxes are the same at all heliographic latitudes. We find that the hydrogen ionization cavity which surrounds the Sun increases in size with solar activity. This is evidenced by the low IP Lyman α intensities measured during and after the solar maximum. Our model calculation also shows that the increased radiation pressure is not sufficient to explain the larger cavity observed at solar maximum. We find also that ionization rates derived from in situ solar wind measurements do agree with the SWAN results at solar minimum but are significantly smaller at solar maximum. Derived in situ ionization rates do not show the solar cycle dependence we see from the SWAN data. In conclusion, we discuss possible explanations for this discrepancy.

Using an absorption cell, we measured the Doppler shifts of the interstellar hydrogen resonance glow to show the direction of the neutral hydrogen flow as it enters the inner heliosphere. The neutral hydrogen flow is found to be deflected relative to the helium flow by about 4°. The most likely explanation of this deflection is a distortion of the heliosphere under the action of an ambient interstellar magnetic field. In this case, the helium flow vector and the hydrogen flow vector constrain the direction of the magnetic field and act as an interstellar magnetic compass.