2.A Material sources of gas and plasma

Transcription

1 2.A Material sources of gas and plasma The magnetosphere, extending from the top of the Saturn magnetosphere to beyond the magnetopause is dominated by neutral gas. The main components are atomic hydrogen, H I, atomic oxygen, O I, and the hydroxyl radical, OH. O I and OH are evidently the dissociation products of H 2 O. A major source of H 2 O has been identified in observations of what is now described as the Enceladus plume ( Hansen et al., 2006; ) with an estimated ejection rate of a few x molecules s -1. Although the Enceladus plume is a major source it is not clear that Enceladus alone is a strong contributor of the observed water products, based on the spatial distribution and temporal properties of the O I observed in UVIS system scans ( Shemansky et al., 2004; Melin et al., 2009). Identification of other significant specific sources of H 2 O is unresolved at this time. OH was first observed in HST exposures by Shemansky et al.(1993) as predicted by Shemansky and Hall(1992). HST was the last experimental facility with the capability of measuring OH in the Saturn magnetosphere, and this component has not been observed since the epoch of HST capability. The spatial morphology of OH is poorly known, and there is no information on temporal variability in this component. Temporal variability in the source at Enceladus is also essentially unknown. Temporal variability and spatial distribution of O I has been obtained in the Cassini UVIS system scans (Shemansky et al., 2004; Melin et al., 2009), and to date these results raise more questions than answers, pointing to a more complex system. Asymmetry in O I spatial distribution, and temporal effects that are substantially shorter than known rate processes seriously complicate identifying the controlling factors in this system. An example of the observed image of O I is shown in Figure 1_uvis. The distribution of H I in the magnetosphere was first obtained by Shemansky and Hall (1992) using Voyager 1 (V1) post encounter UVS system scans. The V1 observations identified a strong local time asymmetry in noisy images of the H I distribution with a general peak in density in the dusk region, and a minimum in the predawn region of the magnetosphere. Typical densities in the R S region of the sub-solar magnetosphere are O I:OH:H I = 500:700:450 cm -3 (Shemansky et al., 2009; Melin et al., 2009). Shemansky & Hall (1992) argued that the primary source of H I is escaping dissociation products of physical chemistry at the top of the Saturn sunlit atmosphere. Definitive images obtained with Cassini UVIS system scans show distinctive structure in H I escaping from the top of the sunlit atmosphere, with a strong feature creating a propeller shaped distribution originating at -8 o to the ring plane. The density of H I at the rings is of the order 10 4 cm -3. There is no observational evidence for a component of the H I sourced from the rings. At the orbit of Titan the only detectable neutral component is H I at densities ranging from cm -3, with no measurable torus at 20 R S. The image of H I in the close vicinity of Saturn is shown in Figure 2_uvis as a contour map of H Lyα brightness at a resolution of 0.1 X 0.1 R S. The rings in this observation are edge-on so that no scattered ring particle emission is detectable. Figure 3_uvis shows an image of the system in H Lyα extending ± 30 R S from planet center. H I is measurable to beyond ± 45 R S in the orbital plane and ± 30 R S perpendicular to the orbital plane. The temporal variation of total O I population is shown in Figure 4_uvis, indicating a minimum near 2004 DOY 240, with an implied relaxation

2 time scale of ~10 days. Table 1_uvis shows estimated populations and loss rates (Shemansky et al., 2004; Shemansky et al., 2009). The dominant neutral gas population throughout the magnetosphere is the limiting factor for development of the plasma environment. The loss of atomic oxygen from the system is determined mainly by the reaction O+ + H > O + H+ This reaction does not affect the ion population, but is a net loss mechanism for the neutral product. Similarly the reaction H+ + H > H + H+ is a net loss for atomic hydrogen. The primary limiting factor for the development of the plasma is the collisional cooling of the electron population by the neutral gas through excitation of electronic states and momentum transfer (Shemansky and Hall, 1992). If the rates quoted by Shemansky and Hall(1992) (Table 6 of S & H) are used with the current knowledge of neutral densities the energy loss by the ambient electron population is ~3 X ergs cm -3 s -1 in the RS region. Energy deposition from pickup ions is negligible, and most of the energy loss would need to come from heterogenous deposition by inward plasma diffusion from the outer magnetosphere. Table 1 Total system Species Density (cm -3 ) Loss rate (s -1 ) 3-4 R S population O I x ~ OH 700 ~ 4 x ~ H I x x H 2 O ~200* N I minor * Theoretical

3 R S (N - S) R S (E - W) Figure 1. Image of O I nm emission from the Saturn magnetosphere obtained 2004 DOY Sub-spaceraft latitude o. Sub-solar latitude -23 o. Solar flux impacts the system from the right. Brightness is indicated on the contour lines in Rayleighs. The O I emission is entirely forced by fluorescence of the solar O I flux.

4 R S (N - S) R S (E - W) Figure 2. Image of Saturn and inner magnetosphere in H Lya emission showing structured outflow of atomic hydrogen from the top of the atmosphere. The emission is forced by solar flux from the right side of the image, at latitude -17 o. The rings are viewed edge-on from the spacecraft. A ridge of emission at -8 o latitude forms a propeller shaped feature from both sides of the planet, measurable to beyond 4 R S as indicated in the shape of the image contours. Outflow from the sub-solar atmosphere is evident over a broad range of latitudes, apparently affected by ring shadow. Auroral emission is evident at the poles.

5 30 R S (N - S) R S (E - W) 29 Figure 3. Image of the Saturn magnetosphere in H Lyα emission showing the distinct local time asymmetry in distribution. The sub-spacecraft latitude is o. The sun is on the right side of the image at sub-solar latitude -23 o. The separate small bright feature north of the planet is a star.

6 O I total (10 34 atoms) DOY from Figure 4. Temporal variation in measured total O I population in the Saturn inner magnetosphere. See Figure 1 for spatial distribution (also temporally variable), and Table 1 for populations of other neutral species.