Signature of Saturn's auroral cusp: Simultaneous Hubble Space Telescope FUV observations and upstream solar wind monitoring

Abstract

Model simulations by Bunce et al. (2005a) have shown that direct precipitation of electrons in Saturn's dayside cusp regions is not capable of producing significant FUV aurora. Instead, they suggested the possibility that the FUV bright emissions sometimes observed near noon are associated with reconnection occurring at the dayside magnetopause, possibly pulsed, analogous to flux transfer events seen at the Earth. Pulsed reconnection at the low‐latitude dayside magnetopause when the IMF is directed northward (antiparallel to Saturn's magnetic field lines) is expected to give rise to pulsed twin‐vortical flows in the magnetosphere and hence to bipolar field‐aligned currents centered in the vortical flows closing in ionospheric Pedersen current. In the case of southward IMF and high‐latitude lobe reconnection the model predicts that the vortical flows are displaced poleward of the open‐closed field line boundary with reversed field‐aligned currents compared with the former case. During January 2004, a unique campaign took place during which magnetic field and plasma instruments on board the Cassini‐Huygens spacecraft measured the in situ solar wind and embedded interplanetary magnetic field while the Hubble Space Telescope simultaneously observed the far ultraviolet aurora in Saturn's southern hemisphere. The IMF was highly structured during this interval. The electric potential at Cassini is estimated from solar wind magnetic field and velocity measurements for the case of low‐latitude or lobe reconnection. We show that a dayside FUV signature of intense electron precipitation is found poleward of or along the main oval during a period of minor compression period when the dayside reconnection voltage is estimated to be ∼30–100 kV. Overall, we find that the conceptual model of Bunce et al. (2005a) provides a good estimate of the UV brightness and power for the case of northward IMF but somewhat underestimates the power for the southward IMF case, except if the speed of the vortical flow is larger than its value in the nominal model.