Saturns dayside ultraviolet auroras: Evidence for morphological dependence on the direction of the upstream interplanetary magnetic field

摘要

We examine a unique data set from seven Hubble Space Telescope (HST) “visits” that imaged Saturn's northern dayside ultraviolet emissions exhibiting usual circumpolar “auroral oval” morphologies, during which Cassini measured the interplanetary magnetic field (IMF) upstream of Saturn's bow shock over intervals of several hours. The auroras generally consist of a dawn arc extending toward noon centered near ∼15° colatitude, together with intermittent patchy forms at ∼10° colatitude and poleward thereof, located between noon and dusk. The dawn arc is a persistent feature, but exhibits variations in position, width, and intensity, which have no clear relationship with the concurrent IMF. However, the patchy postnoon auroras are found to relate to the (suitably lagged and averaged) IMF Bz, being present during all four visits with positive Bz and absent during all three visits with negative Bz. The most continuous such forms occur in the case of strongest positive Bz. These results suggest that the postnoon forms are associated with reconnection and open flux production at Saturn's magnetopause, related to the similarly interpreted bifurcated auroral arc structures previously observed in this local time sector in Cassini Ultraviolet Imaging Spectrograph data, whose details remain unresolved in these HST images. One of the intervals with negative IMF Bz however exhibits a prenoon patch of very high latitude emission extending poleward of the dawn arc to the magnetic/spin pole, suggestive of the occurrence of lobe reconnection. Overall, these data provide evidence of significant IMF dependence in the morphology of Saturn's dayside auroras.Key Points class="unordered" style="list-style-type:disc">We examine seven cases of joint HST Saturn auroral images and Cassini IMF dataThe persistent but variable dawn arc shows no obvious IMF dependencePatchy postnoon auroras are present for northward IMF but not for southward IMF class="kwd-title">Keywords: Auroral phenomena, Hubble Space Telescope, Saturn, Solar wind dependence class="head no_bottom_margin" id="__sec2title">1. IntroductionIt has long been supposed that the dynamics of the magnetospheres of the gas giant planets are dominated by the rotational flows imposed by ion-neutral collisions in the planetary ionosphere [e.g., ; ]. Nevertheless, this expectation does not exclude the possibility that the interaction with the solar wind at the magnetopause boundary produces significant effects involving the outer magnetosphere and magnetospheric tail [e.g., , ]. For the case of Saturn, the topic of the present paper, the estimates of the dayside reconnection rate based on empirical formulae derived from terrestrial experience, indicates typical voltages as low as ∼25 kV during low-field solar wind rarefaction regions, rising to ∼150 kV during high-field compression regions []. Such rates are thus near negligible in the former case, but competitive with ring current region and outer magnetosphere rotational voltages of a few hundred kilovolts each in the latter (the overall rotational voltage at Saturn however, including the inner magnetosphere, is ∼12 MV) []. These dayside reconnection rates nevertheless imply that the time scale governing the solar wind interaction via open flux production is long. To replenish the tail with a typical ∼30 GWb of open flux would take ∼14 days during solar wind rarefaction regions, reducing to ∼2.5 days during compression regions, such time scales being compatible with the rates at which Saturn's auroral oval is observed to increase in size []. Rapid reductions in open flux at Saturn are then found to be excited by strong impulsive compressions of the magnetosphere by the solar wind, resulting in major poleward expansions of the auroras, which are centered in the dawn sector due to the effect of rapid planetary rotation [; ; ; ]. Similar compression-induced closure events are also sometimes observed at Earth, then centered nearer to midnight [e.g., ].Recent work has emphasized however that beyond the simple scaling assumptions employed by for Saturn and ] for Jupiter, dayside reconnection may differ somewhat between the Earth and the gas giants due to the differing physical conditions occurring in the magnetopause regions on both sides of the boundary. In particular, consideration of the effect of flow shear across the boundary related to the interior planetary rotational flow suggests that reconnection may be favored in the dusk sector, where the flow shear is reduced rather than in the dawn sector where it is maximized [], while effects associated with the plasma pressure gradient across the boundary may often restrict the process to loci, where the magnetospheric and magnetosheath fields are quite closely antiparallel []. In addition, href="#b29" rid="b29" class=" bibr popnode">Lai et al. [2012] have confirmed using Cassini data that Earth-like, few minute, flux transfer events (FTEs) are absent at Saturn; these representing the signature of transient magnetopause reconnection events that make an important contribution to the overall open flux transport into the tail at Earth [e.g., href="#b35" rid="b35" class=" bibr popnode">Milan et al., 2000a]. Nevertheless, in addition to the circumstantial evidence for the occurrence of dayside reconnection noted above concerning variations in the size of Saturn's auroral oval, the signatures of reconnection-related plasma heating and magnetic flux connection across the boundary have been reported in Cassini data by href="#b33" rid="b33" class=" bibr popnode">McAndrews et al. [2008] and href="#b5" rid="b5" class=" bibr popnode">Badman et al. [2013], while the presence of magnetosheath plasma in the dayside cusp exhibiting plasma injection signatures has been reported by href="#b44" rid="b44" class=" bibr popnode">Radioti et al. [2013].In addition to these indicators, href="#b43" rid="b43" class=" bibr popnode">Radioti et al. [2011] have presented evidence in Cassini Ultraviolet Imaging Spectrograph (UVIS) data for reconnection-related auroral features in Saturn's dayside ultraviolet (UV) auroras, taking the form of sequential extended bifurcations in the auroral oval in the noon to dusk sector. We noted above that the dusk sector is favored for reconnection at Saturn due to the reduced flow shear across the boundary, between subcorotating magnetospheric plasma on the inside and antisunward flowing magnetosheath plasma on the outside. The auroral bifurcations are found to recur on ∼1–2 h time scales and endure as discrete features for comparable or longer intervals, so that more than one such feature is often present simultaneously, moving slowly poleward and eastward at ∼15% of rigid corotation [href="#b44" rid="b44" class=" bibr popnode">Radioti et al., 2013]. In the events studied by href="#b43" rid="b43" class=" bibr popnode">Radioti et al. [2011], the largest “arcs” were found to contain ∼2 GWb of magnetic flux, corresponding to ∼10% of the preexisting flux lying poleward of the auroral oval, with the oval boundary at other local times expanding equatorward accordingly as the arcs moved poleward. An overall increase in open flux of ∼6 GWb over a ∼4 h interval (see their Figure href="/pmc/articles/PMC4497471/figure/fig04/" target="figure" class="fig-table-link figpopup" rid-figpopup="fig04" rid-ob="ob-fig04" co-legend-rid="lgnd_fig04">4c) corresponds to a substantial averaged dayside reconnection rate of ∼400 kV in this case, rather larger than the typical values estimated by href="#b24" rid="b24" class=" bibr popnode">Jackman et al. [2004] noted above. Similar auroral structures have also been reported by href="#b4" rid="b4" class=" bibr popnode">Badman et al. [2012] in infrared emissions observed by the Cassini visual and infrared mapping spectrometer (VIMS), but in this case centered nearer to noon.href="/pmc/articles/PMC4497471/figure/fig04/" target="figure" rid-figpopup="fig04" rid-ob="ob-fig04">target="object" href="/pmc/articles/PMC4497471/figure/fig04/?report=objectonly">Open in a separate windowclass="figpopup" href="/pmc/articles/PMC4497471/figure/fig04/" target="figure" rid-figpopup="fig04" rid-ob="ob-fig04">Figure 4Set of projected HST images of Saturn's northern UV aurora from (a) the 2011 campaign (visits H1–H3) and (b) the 2012 campaign (visits I5–I8), during which Cassini was located in the solar wind upstream of Saturn's bow shock over intervals of at least several hours. The top rows of Figures 4a and 4b correspond to the five coadded F125LP filtered images at the start of each visit, while the bottom rows similarly correspond to the five coadded F125LP filtered images at the end of each visit. These are termed the “A” and “B” images, respectively, as marked in the figure. HST visit identifiers are given above each image, together with the date (year-month-day) and UT center time (h:min:s). Beneath this are shown the mean and SD values of the three KSM components of the IMF determined from the lagged 2.5 h intervals of Cassini data associated with the center time of each individual image, together with the corresponding mean and SD values of the IMF clock angle. The images are projected onto a polar grid assuming an auroral height of 1100 km above the 1 bar reference spheroid, the grid being shown by white dotted latitude circles at 10° intervals and white dotted longitude lines at 30° intervals (2 h LT), with noon shown at the bottom and dawn to the left. The images have been truncated somewhat past the dawn-dusk meridian to avoid pixels becoming overstretched on approaching the planetary limb. The same color-coded intensity scale, shown upper right, is employed for all images, saturated red at 40 kR.