Abstract
AbstractWe investigate the magnetic perturbations associated with field‐aligned currents observed on 34 Cassini passes over the premidnight northern auroral region during 2008. These are found to be significantly modulated not only by the northern planetary‐period oscillation (PPO) system, similar to the southern currents by the southern PPO system found previously, but also by the southern PPO system as well, thus providing the first clear evidence of PPO‐related interhemispheric current flow. The principal field‐aligned currents of the two PPO systems are found to be co‐located in northern ionospheric colatitude, together with the currents of the PPO‐independent (subcorotation) system, located between the vicinity of the open‐closed field boundary and field lines mapping to ~9 Saturn radius (Rs) in the equatorial plane. All three systems are of comparable magnitude, ~3 MA in each PPO half‐cycle. Smaller PPO‐related field‐aligned currents of opposite polarity also flow in the interior region, mapping between ~6 and ~9 Rs in the equatorial plane, carrying a current of ~ ±2 MA per half‐cycle, which significantly reduce the oscillation amplitudes in the interior region. Within this interior region the amplitudes of the northern and southern oscillations are found to fall continuously with distance along the field lines from the corresponding hemisphere, thus showing the presence of cross‐field currents, with the southern oscillations being dominant in the south, and modestly lower in amplitude than the northern oscillations in the north. As in previous studies, no oscillations related to the opposite hemisphere are found on open field lines in either hemisphere.
Highlights
The means by which momentum is communicated between a planetary atmosphere and the plasma environment beyond is fundamental to the physics of such systems
We begin by examining the field oscillations at small and large colatitudes away from the main current region, representing the overall effect of the planetary-period oscillation (PPO) current system on open field lines and in the quasi-dipolar field region, respectively, in both the northern and southern hemispheres
In the southern hemisphere we use the ranges θiS = 6°–12° for the polar region, and 22.1°–28.8° for the equatorward region, shown by the vertical dotted lines in Figure 3c, the equatorward range being magnetically conjugate to the equatorward range in the northern hemisphere according to our field model. We note that the latter ranges are similar to, but slightly broader than the southern hemisphere ranges employed for similar purposes by Hunt et al [2014]. We note that both polar regions are expected to lie wholly on open field lines according to the average open-closed field boundary (OCB) locations determined by Jinks et al [2014]
Summary
The means by which momentum is communicated between a planetary atmosphere and the plasma environment beyond is fundamental to the physics of such systems. The field perturbations of both systems are superposed and interfere in the equatorial region, producing beats with a period of ~22 days during the interval examined here [Provan et al, 2011], but have been found to be pure northern and southern in the corresponding polar region, to within an ~10% observational limit by amplitude [Andrews et al, 2012] It should be noted for future reference that the azimuthal PPO perturbation field, the component employed here to study the field-aligned currents in the auroral region, varies with azimuth around each system to a first approximation as ~ sin ΨN,S in the equatorial region inside the main field-aligned currents, and as ~ –sin ΨN,S in the outer equatorial and polar regions, the switch between the two being associated with the PPO-related field-aligned current flow through Ampère’s law. We further reexamine the southern data for evidence of northern period currents and show that weaker effects are present
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
