Draft version July 21, 2021 Typeset using LATEX twocolumn style in AASTeX63 The far ultra-violet background S. R. Kulkarni1 1Owens Valley Radio Observatory 249-17, Caltech Pasadena, CA 91125, USA ABSTRACT The diffuse far-ultraviolet (FUV) background has received considerable attention from astronomers since the seventies. The initial impetus came from the hope of detecting UV radiation from the hot intergalactic medium. The central importance of the FUV background to the physics (heating and ionization) of the diffuse atomic phases motivated the next generation of experiments. The consensus view is that the diffuse FUV emission at high latitudes has three components: stellar FUV reflected by dust grains (diffuse galactic light or DGL), FUV from other galaxies (extra-galactic background light, EBL) and a component of unknown origin. During the eighties, there was some discussion that decaying dark matter particles produced FUV radiation. In this paper I investigate production of FUV photons by conventional sources: the Galactic Hot Ionized Medium (line emission), two photon emission from the Galactic Warm Ionized Medium and low-velocity shocks, and Lyman-β excitation of hydrogen at several locales in the Solar System (the interplanetary medium, the exosphere and thermosphere of Earth). I conclude that two thirds of the third component can be explained by the sum of the processes listed above. 1. BACKGROUND et al. 2005): FUV (1350{1750 A)˚ and NUV (1750{ ˚ The diffuse background in the range 912{2000 A˚ is of 2800 A). The GALEX FUV band was specifically de- central importance to the physics of the diffuse atomic signed to avoid the two strongest foreground lines in the ˚ phases { the Cold Neutral Medium (CNM) and the FUV band: the geo-coronal Lyα and the 1302{1306 A Warm Neutral Medium (WNM). Historically, the var- O I airglow triplet, both of which are seen even during ious bands of ultra-violet (UV) were defined by wave- the dead of the satellite night. length properties of detectors and mirror coatings. For The diffuse FUV radiation, via photo-electric ioniza- tion of C I and photo-electric heating of dust particles, instance, mirrors coated with MgF2 lose reflectivity be- low 1150 A.˚ With advances in coating technology, along heats the two atomic phases mentioned above. The same with the rapid progress in detector technology, the radiation, via ionization of elements with ionization po- boundaries between the bands started to blur. The In- tential less than that of hydrogen, also provides residual ternational Ultraviolet Observatory (IUE) had two spec- ionization to the two atomic phases. The FUV back- trographs: the short wavelength spectrograph, covering ground is of importance for the molecular medium also. the range 1150{2000 A˚ and the long wavelength spectro- FUV photons excite the Lyman and Werner bands of H2 graph, covering 1850{3300 A.˚ The Far Ultraviolet Spec- (Duley & Williams 1980) which result in UV fluorescent troscopic Explorer (FUSE) focused on the spectral re- line emission and once in ten times leads to dissociations gion between Lyman cutoff and Lyman-α, 905{1195A.˚ (Jura 1974). In the process, the strength of FUV radia- arXiv:2107.09585v1 [astro-ph.GA] 20 Jul 2021 FIMS aboard S. Korea's STSat-1 mission was designed tion determines the transition in diffuse clouds between to explore the far ultraviolet sky, defined by the wave- atomic and molecular phases. length range, 900{1750 A.˚ Separately, scattering of solar photons by zodiacal For this paper we use the Far Ultraviolet (FUV) and dust particles constitute an irreducible background for the Near UV (NUV) bands as defined by the passbands space missions located in the inner Solar system, e.g., of the GALaxy Evolution eXplorer (GALEX; Martin the Hubble Space Telescope (HST) in low earth orbit (LEO) or the planned Russian Spektr-UF mission (aka \World Space Observatory" or WSO) which is expected Corresponding author: S. R. Kulkarni to be in a geo-synchronous orbit (GEO; Boyarchuk et al. [email protected] 2016). However, the Sun is faint in the FUV. Ergo, the sky is dark in the FUV band. As a result, the FUV 2 Kulkarni band is most attractive for low surface brightness imag- ing Spectrometer (FIMS1; Edelstein et al. 2006 provided ing of galaxies (O'Connell 1987). Indeed, it is precisely spectral imaging of the Galactic Hot Ionized Medium this advantage of the FUV band that allowed GALEX to (HIM). discover very faint star-forming complexes well beyond The consensus from all these studies is that much of the optical disk and with sensitivity better than that the diffuse FUV emission is due to reflection of stellar provided by ground-based Hα imaging (Barnes et al. FUV photons by diffuse (\cirrus") clouds and conve- 2011). niently traced by IRAS 100 µm band or fluorescence of In the early seventies it was speculated that the hot stellar FUV photons by molecular hydrogen. Together intergalactic medium (IGM) would be revealed by dif- this emission is called as the Diffuse Galactic Light fuse FUV emission. Searches for diffuse UV emission (DGL). However, some diffuse background emission is were undertaken with great gusto. Separately, this was not correlated with cirrus clouds, a fraction of which also the period when the first pulsar surveys showed that can be reasonably attributed to collective emission from pulsar signals are invariably dispersed (see, for example, other galaxies { the so-called Extragalactic Background Manchester & Taylor 1977; Yao et al. 2017). As a re- Light (EBL). There remains some 120{180 CU emission sult, astronomers became aware of a pervasive Galactic in the FUV band which cannot be clearly attributed to ionized medium. During the eighties, thanks primarily a single source. Here, CU (\continuum unit") stands −1 to the work of Ronald J. Reynolds, this ionized medium for photon cm−2 s−1 A˚ sr−1 with \sr" as a short for was complementarily sensed via Hα recombination emis- steradian. In the literature, this residual component is sion (see Haffner et al. 2009). These two methods { dis- called the \isotropic [offset]” component. The purpose persion of radio signals and diffuse Hα emission { led to of this paper is to investigate possible origin(s) for this the recognition of the Warm Ionized Medium (WIM) as component. a distinct phase of the ISM. The paper is organized as follows. In §2 we review The filling factor of the WIM, by volume, is estimated the measurements of diffuse high-latitude FUV emis- to be between 20% and 40% of the Galactic disk (Haffner sion. This is followed by a summary of the essential et al. 2009). The ionizing power requirement for the physics of hydrogen two-photon decay (§3). We inves- WIM is tremendous. The conventional explanation re- tigate possible two-photon emission from the Galactic quired a significant fraction of Lyman continuum pho- WIM and low-velocity shocks and line emission from the tons from OB stars (Reynolds 1990) and also called for HIM (§4). We then investigate two-photon contribution a porous ISM so as to allow for Lyman continuum pho- from the Solar System (§5), specifically the Interplane- tons to travel significantly away from their parent stars. tary Medium2 (§6), the Earth's atmosphere (\thermo- The observed large vertical scale height, ≈ 1 kpc, of the sphere"; §7) and the exosphere (§8). In §9 we tally the WIM was initially a mystery. This \crisis" led to a res- contributions to the diffuse FUV background and con- urrection of a hypothesis of decaying dark matter as clude that about two thirds of the diffuse FUV back- a major source of ionizing photons (e.g., Sciama 1990 ground can be accounted for by contributions discussed and earlier references therein; more recent references in- in §4-8. We conclude in §10 by discussing experimental clude Kollmeier et al. 2014; Henry et al. 2015). Miller verification of some of the proposed channels of two- & Cox(1993) and Dove & Shull(1994) provided a con- photon production and strategies to take advantage of ventional explanation based on O stars as the principal the dark FUV sky. source of ionizing radiation, radiative transfer modeling and clustering of star-forming regions, \chimneys" and 2. THE DIFFUSE FUV EMISSION \channels". The study of diffuse FUV continued through the The \Interstellar Radiation Field" (ISRF), being a nineties with rocket-borne experiments, UV spectrom- central quantity, is almost assured of a chapter in any eters on Voyager missions, Shuttle-based experiments, serious book on the ISM (e.g., Chapter 12 in Draine FAUST and FUSE. Murthy et al.(2019) provide a com- 2011). The ISRF is composed of both resolved (bright) prehensive list of FUV experiments. Two missions, both stars and diffuse emission. Only the latter component is launched in 2003, greatly advanced the field of diffuse FUV: GALEX (Martin et al. 2005) carrying both an 1 also sometimes referred to as SPEAR FUV and an NUV wide-field imager, each with a field- 2 The term IPM has two different connotations in astronomy. In of-view (FoV) of over a square degree but with pixels radio astronomy, IPM refers to the solar wind that pervades the of few arc-seconds. STSat-1 carrying the Far-UV Imag- solar system out to the heliopause. In planetary studies, the IPM stands for the very local interstellar cloud into which the Solar system is moving. The FUV background 3 of interest to this paper. There have been extensive re- not traced by other indicators". Akshaya et al. views of the diffuse FUV emission (e.g. see Paresce et al. (2018) state \There is an excess emission (over 1980; Henry 1991; Henry et al. 2015). Here we focus on the DGL and the EBL) of 120{180 photon units3 the measurements of the diffuse FUV by GALEX.