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Abstract
AbstractWe present the first systematic investigation of the polar cap boundary in Saturn's high‐latitude magnetosphere through a multi‐instrument assessment of various Cassini in situ data sets gathered between 2006 and 2009. We identify 48 polar cap crossings where the polar cap boundary can be clearly observed in the step in upper cutoff of auroral hiss emissions from the plasma wave data, a sudden increase in electron density, an anisotropy of energetic electrons along the magnetic field, and an increase in incidence of higher‐energy electrons from the low‐energy electron spectrometer measurements as we move equatorward from the pole. We determine the average level of coincidence of the polar cap boundary identified in the various in situ data sets to be 0.34° ± 0.05° colatitude. The average location of the boundary in the southern (northern) hemisphere is found to be at 15.6° (13.3°) colatitude. In both hemispheres we identify a consistent equatorward offset between the poleward edge of the auroral upward directed field‐aligned current region of ~1.5–1.8° colatitude to the corresponding polar cap boundary. We identify atypical observations in the boundary region, including observations of approximately hourly periodicities in the auroral hiss emissions close to the pole. We suggest that the position of the southern polar cap boundary is somewhat ordered by the southern planetary period oscillation phase but that it cannot account for the boundary's full latitudinal variability. We find no clear evidence of any ordering of the northern polar cap boundary location with the northern planetary period magnetic field oscillation phase.
Highlights
The question of whether the rapidly rotating gas giant magnetospheres of Jupiter and Saturn are magnetically “open” to the solar wind has been a matter of some debate since the first spacecraft visited them in the 1970s
We find that clear transitions through the northern and southern boundary regions cannot be observed in the remaining high-latitude orbits because, in some cases, the characteristic step up in the frequency of the auroral hiss in the Radio and Plasma Wave Science (RPWS) instrument data is not clear enough to allow for identification of the boundary in that data set
The average poleward edge location of the upward current could be directly compared to the average location of the polar cap boundary to infer the average separation of the main auroral oval and the polar cap boundary
Summary
The question of whether the rapidly rotating gas giant magnetospheres of Jupiter and Saturn are magnetically “open” to the solar wind has been a matter of some debate since the first spacecraft visited them in the 1970s. There has been no systematic study of the “open-closed field line boundary” at either Jupiter or Saturn, and until the NASA Juno spacecraft arrives at Jupiter in mid-2016, there will be no in situ high-latitude observations available at that planet. In the Earth’s magnetosphere, the concept of the open-closed field line boundary or “polar cap boundary” is well understood and has been studied using a variety of instrumental techniques. As the Earth’s magnetosphere is strongly driven by the solar wind interaction, determination of the open-closed field line boundary latitudinal motion provides information on the balance of dayside and nightside magnetic reconnection in the system (as described by Siscoe and Huang [1985] and Cowley and Lockwood [1992, 1996]). A variety of proxies can be used to determine the location of the open-closed field line boundary including the identification of: a cutoff in high-energy (few keV) electron precipitation associated with a trapped population on closed field lines [Evans and Stone, 1972]; a nightside ionosphere boundary between diffuse and discrete auroral emission [e.g., Milan et al, 2003]; a boundary in the spectral width echoes from
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