Saturn's Northern Auroras and Their Modulation by Rotating Current Systems During Late Northern Spring in Early 2014

Abstract

The Hubble Space Telescope imaged Saturn's northern ultraviolet auroras during February–June 2014, when Saturn's northern and southern magnetic perturbation fields were locked in antiphase and matched in rotation period (~10.69 hr). During this coalescence period, we test for evidence of rotational modulation of the auroras using the latest rotating current system model and kilometric radio phases derived from Cassini measurements. While we see modulation of auroral intensity in the rotating frames of the planetary period current systems, the pattern is opposite to that expected and is dominated by an asymmetric local time profile that peaks at dawn. Enhancement of the north emission by rotating upward field aligned currents (FACs) is expected to peak at magnetic longitudes of ~90°, whereas here the intensity increased at ~270°. This unexpected finding is attributed to the presence of nonplanetary period oscillation dynamics having affected the auroral morphology, together with insufficient sampling of the rotational system orientations provided during such Hubble Space Telescope campaigns. Rotational modulation is clearest at dawn regardless of the pattern's orientation, suggesting that the physical relationship between rotating FACs and auroral intensity is not direct, having a local time dependence that is not generally observed in the rotating FAC magnitudes. We also find no statistically significant planetary period oscillation of the auroral circle position, but the mean center was offset from the spin pole by ~3° latitude toward early morning local times. Mean auroral boundaries were located at equatorward and poleward colatitudes of 15.0 ± 2.8° and 12.4 ± 3.0°, respectively.

The Dynamics of Saturn's Main Aurorae

“…The mean and median UV power distributions (Figure c) are in close agreement between noon and midnight but clearly differ near dawn—the mean maximizing here, while the median minimizes. The mean auroral power agrees very well with intensity averages of previous observations which all showed a distinct peak between 6 and 9 LT (e.g., Bader et al, ; Badman et al, ; Carbary, ; Kinrade et al, ; Lamy et al, , ; Nichols et al, )—but, as seen here, the mean UV intensity/power is obviously not a good representation of the typical state of the aurora. The median directly shows that in more cases than not, the dawn aurora is dimmer than the dusk aurora and not brighter; it is the few transient high‐power events subcorotating through dawn which skew the mean power to unrepresentative high values at these LTs.…”

“…A UV power-LT histogram for these images is shown in Figure 2a, with the mean and median power per LT added as line plots; the dawn and dusk slices of this histogram are compared in Figure 2b. We observe similar UV powers through all LTs, disagreeing with previously discussed UV and IR auroral intensity distributions (e.g., Badman et al, 2011;Bader et al, 2018;Carbary, 2012;Kinrade et al, 2018;Lamy et al, 2009Lamy et al, , 2018Nichols et al, 2016) with a brightness peak at dawn probably due to our choice of quiet periods. Centered on roughly 0.5 GW per 40 min LT bin, the powers are more variable and feature a more prominent tail toward lower powers at dawn/noon than at dusk/midnight.…”

“…This agrees with field line mapping of the main aurorae which places the main upward FAC sheet at an equatorial distance beyond 10 R S , outward from the middle ring current (e.g., Belenkaya et al, ; Bradley et al, ; Talboys et al, ). The flow of the solar wind and the associated Dungey cycle activity (e.g., Cowley, Bunce & Prangé ; Jackman & Cowley, ) seem to have little to no impact on this system, since no significant LT asymmetries in auroral FACs (Hunt et al, ) and auroral brightness are observed, contrary to previous findings (e.g., Bader et al, ; Badman et al, ; Carbary, ; Kinrade et al, ; Lamy et al, , ; Nichols et al, ), where observed asymmetries were likely an artifact of small data sets and averaging procedures unsuitable for determining the full variability of Saturn's auroral dynamics. This is supported by earlier studies estimating the Dungey cycle contribution to magnetic flux transport to be roughly an order of magnitude lower than the contribution arising from rotational flows in quiet solar wind conditions such that no asymmetry in auroral brightness is expected (e.g., Badman & Cowley, ; Badman et al, ).…”

“…Previous studies using auroral imagery obtained by the Hubble Space Telescope in the ultraviolet (UV) wavelength band (e.g., Kinrade et al, ; Lamy et al, , ; Nichols et al, ) and by the Cassini spacecraft at infrared (IR) and UV wavelengths (e.g., Bader et al, ; Badman et al, ; Carbary, ) have statistically identified such a brightness asymmetry, seemingly confirming that Saturn's main aurorae are indeed significantly solar wind driven. However, most of these studies used rather small sets of single exposures lacking context and/or short observation series without good time resolution to obtain statistical averages, hence not taking into account the complicated dynamics of Saturn's aurora which had already been observed by the Voyager spacecraft (Sandel & Broadfoot, ; Sandel et al, ).…”

Modulations of Saturn's UV Auroral Oval Location by Planetary Period Oscillations

“…Analyzing HST images from 2007–2008, Nichols et al () found clear evidence of an oscillatory motion of the auroral oval with a period close to the planetary rotation, and Provan et al () showed that the southern oval is generally tilted away from the southern hemisphere's magnetic perturbation field. These findings could be confirmed with 2011–2013 HST data (Nichols et al, ), but not with the 2014 HST data set (Kinrade et al, ). However, many images in the latter data set did not exhibit a dawn arc, preventing reliable circle fits and cutting down the usable data set such that the PPO phase coverage was rather limited.…”