Bifurcations of the main auroral ring at Saturn: ionospheric signatures of consecutive reconnection events at the magnetopause

Abstract

[1] This work reports for the first time on bifurcations of the main auroral ring at Saturn observed with the UVIS instrument onboard Cassini. The observation sequence starts with an intensification on the main oval, close to noon, which is possibly associated with dayside reconnection. Consecutive bifurcations appear with the onset of dayside reconnection, between 11 and 18 magnetic local time, while the area poleward of the main emission expands to lower latitudes. The bifurcations depart with time from the main ring of emission, which is related to the open-closed field line boundary. The augmentation of the area poleward of the main emission following its expansion is balanced by the area occupied by the bifurcations, suggesting that these auroral features represent the amount of newly open flux and could be related to consecutive reconnection events at the flank of the magnetopause. The observations show that the open flux along the sequence increases when bifurcations appear. Magnetopause reconnection can lead to significant augmentation of the open flux within a couple of days and each reconnection event opens ∼10% of the flux contained within the polar cap. Additionally, the observations imply an overall length of the reconnection line of ∼4 hours of local time and suggest that dayside reconnection at Saturn can occur at several positions on the magnetopause consecutively or simultaneously.

Magnetospheric magnetic field modelling for the 2011 and 2012 HST Saturn aurora campaigns – implications for auroral source regions

“…Since these emissions straddle the open-closed boundary, they map along model field lines to the outer magnetosphere close to the magnetopause, and onto adjacent open field lines. These results are consistent with the suggestion that these emissions are associated with reconnection phenomena and open-flux production at the dayside magnetopause, related to the similarly interpreted emissions observed in Cassini UVIS data at higher spatial resolution by Radioti et al (2011Radioti et al ( , 2013 and Badman et al (2013). The auroras are likely pro-duced through field-aligned currents associated with related momentum exchange between the magnetosphere and magnetosheath, together possibly with precipitation of reconnection-heated plasma.…”

“…Grodent et al, 2005), implying typical reconnection (open-flux production) rates ∼ 50-100 kV, comparable to estimates based on simple scaling of empirical Earth values (Jackman et al, 2004;Badman et al, 2005). Indeed, in the auroral events studied by Radioti et al (2011), the expansion in open-flux observed implies mean reconnection rates of ∼ 400 kV over an interval of several hours.…”

“…In a parallel study of these images, Meredith et al (2014) found no clear IMF response in the dawn emissions, while patchy auroras observed at higher latitudes in the noon to dusk sector were found to occur for northward but not for southward IMF, thus suggesting a connection with reconnection-related processes at the magnetopause as discussed previously by Radioti et al (2011 and Badman et al (2013). In addition, a prominent patch of polar emissions observed in one image for southward IMF was suggested to be due to high-latitude "lobe reconnection" between the IMF and open tail flux tubes poleward of the dayside cusp (e.g.…”

“…Nevertheless, this does not preclude the solar wind interaction from playing an important role in outer magnetosphere dynamics at the giant planets; furthermore, it is known, in particular, that bright auroral emissions extend to very high latitudes in the dawn sector at Saturn in response to strong magnetospheric compressions by the solar wind ). In addition, structured dayside auroral features observed in the post-noon sector have been interpreted as the signatures of time-dependent reconnection and openflux production at the dayside magnetopause, similar to "flux transfer events" at Earth (Radioti et al, 2011Badman et al, 2013;Meredith et al, 2014).…”

Open and partially closed models of the solar wind interaction with outer planet magnetospheres: the case of Saturn

“…For related reasons, Masters et al (2014) have expressed doubt about the importance of reconnection at Saturn's magnetopause. They noted that despite reconnection signatures being observed in Saturn's magnetosphere (e.g., McAndrews et al, 2008;Radioti et al, 2011Radioti et al, , 2013Badman et al, 2013), the reconnection efficiency at Saturn may be much lower than at Earth due to the different solar wind conditions at these planets. Investigation of this subject is also significant for other magnetic planets and exoplanets located in the flow of magnetized plasma.…”