Vortices in Saturn's Northern Hemisphere (2008–2015) observed by Cassini ISS

Abstract

We use observations from the Imaging Science Subsystem on Cassini to create maps of Saturn's Northern Hemisphere (NH) from 2008 to 2015, a time period including a seasonal transition (i.e., spring equinox in 2009) and the 2010 giant storm. The processed maps are used to investigate vortices in the NH during the period of 2008–2015. All recorded vortices have diameters (east‐west) smaller than 6000 km except for the largest vortex that developed from the 2010 giant storm. The largest vortex decreased its diameter from ~11,000 km in 2011 to ~5000 km in 2015, and its average diameter is ~6500 km during the period of 2011–2015. The largest vortex lasts at least 4 years, which is much longer than the lifetimes of most vortices (less than 1 year). The largest vortex drifts to north, which can be explained by the beta drift effect. The number of vortices displays varying behaviors in the meridional direction, in which the 2010 giant storm significantly affects the generation and development of vortices in the middle latitudes (25–45°N). In the higher latitudes (45–90°N), the number of vortices also displays strong temporal variations. The solar flux and the internal heat do not directly contribute to the vortex activities, leaving the temporal variations of vortices in the higher latitudes (45–90°N) unexplained.

Saturn's Global Zonal Winds Explored by Cassini/VIMS 5‐μm Images

“…On the other hand, the easterly jets around 77°S, 48°S, and 79°N are much sharper than the corresponding parabolas with curvature β , which implies that these jets are very unstable. Basically, all easterly jets are barotropically unstable, which can help explain why Saturn's vortices are concentrated around these latitudes of easterly jets (Trammell et al, , ). The VIMS profile in Figure shows that the VIMS easterly jets have similar sharpness with that of the ISS easterly jets except for the VIMS jets around 46°S and 59°S.…”

Saturn atmospheric dynamics one year after Cassini: Long-lived features and time variations in the drift of the Hexagon

“…During this period, AV reduced in longitude and latitude to half its original size and migrated northwards at least 2 • . Trammell et al (2016) discuss the changes in size and latitude of this vortex from Cassini ISS images in the period 2011-2015 and suggest that its initial drift to the north can be explained by the beta drift effect. This northwards migration made the vortex reach latitudes where zonal winds caused a faster zonal drift.…”

“…This vortex is the same one that formed after the Great White Spot (GWS) convective storm in 2010-2011 Sánchez-Lavega et al, 2012;Sayanagi et al, 2013;Trammell et al, 2016). It was called DS after Dark Spot from its visual aspect in ground based-images in (Sánchez-Lavega et al, 2012) and AV after Anticyclonic Vortex in Sayanagi et al (2013) due to its anticyclonic circulation on Cassini ISS images.…”

“…We will name it AV in this paper. The morphology of the vortex at high resolution in Cassini ISS images can be seen in Trammell et al (2016) and Gunnarson et al (2018). The AV vortex appears in images acquired in the visible as a dark elongated vortex with a faint ring of bright material around it that formed around 2013 (Trammell et al, 2016) and is visible in Figure 2.…”

“…Features of interest visible in the period 2016-2018 are highlighted. Additionally, the CB2 maps can be compared with Cassini ISS maps of the North hemisphere published by (Trammell et al, 2016) from 2008 to 2015 establishing an extense time-line of the morphology of the main features visible in the planet.…”