Magnetosphere–Ionosphere Coupling

The process of magnetosphere-ionosphere coupling involves the transport of vast quantities of energy and momentum between a magnetized planet and its space environment, or magnetosphere. This transport involves extended, global sheets of electrical current, which flows along magnetic field lines. Some of the charged particles, which carry this current rain down onto the planet’s upper atmosphere and excite aurorae–beautiful displays of light close to the magnetic poles, which are an important signature of the physics of the coupling process. The Earth, Jupiter, and Saturn all have magnetospheres, but the detailed physical origin of their auroral emissions differs from planet to planet. The Earth’s aurora is principally driven by the interaction of its magnetosphere with the upstream solar wind—a flow of plasma continually emanating from the Sun. This interaction imposes a particular pattern of flow on the plasma within the magnetosphere, which in turn determines the morphology and intensity of the currents and aurorae. Jupiter, on the other hand, is a giant rapid rotator, whose main auroral oval is thought to arise from the transport of angular momentum between the upper atmosphere and the rotating, disc-like plasma in the magnetosphere. Saturn exhibits auroral behavior consistent with a solar wind–related mechanism, but there is also regular variability in Saturn’s auroral emissions, which is consistent with rotating current systems that transport energy between the magnetospheric plasma and localized vortices of flow in the upper atmosphere/ionosphere.</p

Whistler mode waves upstream of Saturn

Whistler-mode waves are generated within and can propagate upstream of collisionless shocks. They are known to play a role in electron thermodynamics/acceleration and, under certain conditions, are markedly observed as wave trains preceding the shock ramp. In this paper, we take advantage of Cassini's presence at ~10 AU to explore the importance of whistler-mode waves in a parameter regime typically characterized by higher Mach number (median of ~14) shocks, as well as a significantly different IMF structure, compared to near Earth. We identify electromagnetic precursors preceding a small subset of bow shock crossings with properties which are consistent with whistler-mode waves. We find these monochromatic, low-frequency, circularly-polarized waves to have a typical frequency range of 0.2 - 0.4 Hz in the spacecraft frame. This is due to the lower ion and electron cyclotron frequencies near Saturn, between which whistler waves can develop. The waves are also observed as predominantly right-handed in the spacecraft frame, the opposite sense to what is typically observed near Earth. This is attributed to the weaker Doppler shift, owing to the large angle between the solar wind velocity and magnetic field vectors at 10 AU. Our results on the low occurrence of whistler waves upstream of Saturn also underpins the predominantly supercritical bow shock of Saturn.Comment: Published in Journal of Geophysical Research: Space Physics (January 2017) 21 pages, 4 figure

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