G. Provan

Magnetospheric period oscillations at Saturn: Comparison of equatorial and high‐latitude magnetic field periods with north and south Saturn kilometric radiation periods

It has recently been shown using Cassini radio data that Saturn kilometric radiation (SKR) emissions from the Northern and Southern hemispheres of Saturn are modulated at distinctly different periods, similar to 10.6 h in the north and similar to 10.8 h in the south, during the southern summer conditions that prevailed during the interval from 2004 to near-equinox in mid-2009. Here we examine Cassini magnetospheric magnetic field data over the same interval and show that two corresponding systems of magnetic field oscillations that have the same overall periods, as the corresponding SKR modulations, to within similar to 0.01% are also present. Specifically, we show that the rotating quasi-dipolar field perturbations on southern open field lines and the rotating quasi-uniform field in the inner region of closed field lines have the same period as the southern SKR modulations, although with some intervals of slow long-term phase drift of unknown origin, while the rotating quasi-dipolar field perturbations on northern open field lines have the same period as the northern SKR modulations. We also show that while the equatorial quasi-uniform field and effective southern transverse dipole are directed down tail and toward dawn at southern SKR maxima, as found in previous studies, the corresponding northern transverse dipole is directed approximately opposite, pointing sunward and also slightly toward dawn at northern SKR maxima. We discuss these findings in terms of the presence of two independent high-latitude field-aligned current systems that rotate with different periods in the two hemispheres.

Planetary period oscillations in Saturn's magnetosphere: Evolution of magnetic oscillation properties from southern summer to post-equinox

We investigate the evolution of the properties of planetary period magnetic field oscillations observed by the Cassini spacecraft in Saturns magnetosphere over the interval from late 2004 to early 2011, spanning equinox in mid-2009. Oscillations within the inner quasi-dipolar region (L <= 12) consist of two components of close but distinct periods, corresponding essentially to the periods of the northern and southern Saturn kilometric radiation (SKR) modulations. These give rise to modulations of the combined amplitude and phase at the beat period of the two oscillations, from which the individual oscillation amplitudes and phases (and hence periods) can be determined. Phases are also determined from northern and southern polar oscillation data when available. Results indicate that the southern-period amplitude declines modestly over this interval, while the northern-period amplitude approximately doubles to become comparable with the southern-period oscillations during the equinox interval, producing clear effects in pass-to-pass oscillation properties. It is also shown that the periods of the two oscillations strongly converge over the equinox interval, such that the beat period increases significantly from similar to 20 to more than 100 days, but that they do not coalesce or cross during the interval investigated, contrary to recent reports of the behavior of the SKR periods. Examination of polar oscillation data for similar beat phase effects yields a null result within a similar to 10% upper limit on the relative amplitude of northern-period oscillations in the south and vice versa. This result strongly suggests a polar origin for the two oscillation periods.

THEMIS observations of extreme magnetopause motion caused by a hot flow anomaly

[1] On 30 October 2007, the five THEMIS spacecraft observed the cause and consequence of extreme motion of the dawn flank magnetopause, displacing the magnetopause outward by at least 4.8 RE in 59 s, with flow speeds in the direction normal to the model magnetopause reaching 800 km/s. While the THEMIS A, C, D, and E observations allowed the determination of the velocity, size, and shape of a large bulge moving tailward along the magnetopause at a speed of 355 km/s, THEMIS B observed the signatures of a hot flow anomaly (HFA) upstream of the bow shock at the same time, indicating that the pressure perturbation generated by the HFA may be the source of the fast compression and expansion of the magnetosphere. The transient deformation of the magnetopause generated field-aligned currents and created traveling convection vortices which were detected by ground magnetometers. This event demonstrates that kinetic (non-MHD) effects at the bow shock can have global consequences on the magnetosphere.

Field‐aligned currents in Saturn's northern nightside magnetosphere: Evidence for interhemispheric current flow associated with planetary period oscillations

We 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.

The influence of the IMF By component on the location of pulsed flows in the dayside ionosphere observed by an HF radar

A study has been performed on the effect of the By and Bz components of the Interplanetary Magnetic Field (IMF), as measured by the WIND satellite, on the occurrence of pulsed ionospheric flows as observed by the CUTLASS Finland radar. These flows have been suggested as being created at the ionospheric footprint of newly reconnected field lines. The statistical location of the pulsed flows is strongly dependent on the IMF By component, moving to the pre-noon sector during intervals of positive By and into the post-noon sector for intervals of negative By. These results directly confirm previous predictions, demonstrating that during intervals of flux transfer at the magnetopause, pulsed ionospheric convective flows are initiated. In the northern hemisphere these flows are shifted about noon in the opposite sense to the IMF By component, consistent with the average ionospheric convection pattern.

Planetary period oscillations in Saturn's magnetosphere: Comparison of magnetic oscillations and SKR modulations in the postequinox interval

We compare the properties of planetary period oscillations observed in Saturn kilometric radiation (SKR) and magnetospheric magnetic field data from Saturn equinox in August 2009 to July 2013. As shown previously, the southern and northern oscillation periods converged across equinox from ~10.8 h and ~10.6 h, respectively, during southern summer, to closely common values ~10.7 h approximately 1 year after equinox. Near coalescence is judged to have occurred approximately 3 months earlier in the SKR data, centered in late June 2010, than in the magnetic data, in late September, though SKR periods were particularly difficult to determine during this interval due to less clearly modulated emissions. Both data sets agree, however, that by early November 2010 the two periods had separated again but remained closely spaced with a difference in period of ~3 min about a mean of ~10.67 h, with the southern period remaining longer than the northern. Thus, no enduring reversal of the northern and southern periods took place following near coalescence in mid-2010, the periods remaining uncrossed to the end of the interval studied here. The SKR modulations also show effects related to the sharp amplitude changes observed in the magnetic oscillation data at ~100–200 day intervals since February 2011, though the correspondences are not exact, indicating that other factors such as “seeing” effects on the variable Cassini orbit are also involved. Postequinox variations in the relative phase between the magnetic and SKR oscillations are also shown to be related to changes in orbit apoapsis orientation.

Cassini multi-instrument assessment of Saturn's polar cap boundary

We present the first systematic investigation of the polar cap boundary in Saturns 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 degrees 0.05 degrees colatitude. The average location of the boundary in the southern (northern) hemisphere is found to be at 15.6 degrees (13.3 degrees) colatitude. In both hemispheres we identify a consistent equatorward offset between the poleward edge of the auroral upward directed field-aligned current region of similar to 1.5-1.8 degrees 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 boundarys 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.

Planetary period oscillations in Saturn's magnetosphere: Coalescence and reversal of northern and southern periods in late northern spring

Calibrated magnetic field and radio data from the Cassini mission are available from the NASA Planetary Data System at the Jet Propulsion Laboratory (https://pds.jpl.nasa.gov/). Daily sunspot numbers in Figure 11 were obtained from the OMNIWEB Space Physics Data Facility at Goddard Space Flight Center. This work benefitted from discussions held during meetings of the International Space Science Institute team on “Rotational phenomena in Saturns magnetosphere.”

Solar–wind–magnetosphere–ionosphere interactions in the Earth's plasma environment

The properties of the Earths coupled magnetosphere–ionosphere system are dominated by its interaction with the solar–wind plasma, mediated by magnetic reconnection at the magnetopause interface. As a consequence, Earths magnetospheric dynamics depend primarily on the concurrent orientation of the interplanetary magnetic field (IMF). We illustrate current understanding of the system through the results of a number of recent case studies and highlight the remaining issues. The discussion centres on flux–transfer events and substorms during intervals of southward IMF and magnetopause and tail processes during intervals of northward IMF. We emphasize the great diagnostic power of combined in situ and remote–sensing observations from space and on the ground.

Cassini nightside observations of the oscillatory motion of Saturn's northern auroral oval

In recent years we have benefitted greatly from the first in-orbit multi-wavelength images of Saturns polar atmosphere from the Cassini spacecraft. Specifically, images obtained from the Cassini UltraViolet Imaging Spectrograph (UVIS) provide an excellent view of the planets auroral emissions, which in turn give an account of the large-scale magnetosphere-ionosphere coupling and dynamics within the system. However, obtaining near-simultaneous views of the auroral regions with in situ measurements of magnetic field and plasma populations at high latitudes is more difficult to routinely achieve. Here we present an unusual case, during Revolution 99 in January 2009, where UVIS observes the entire northern UV auroral oval during a 2 h interval while Cassini traverses the magnetic flux tubes connecting to the auroral regions near 21 LT, sampling the related magnetic field, particle, and radio and plasma wave signatures. The motion of the auroral oval evident from the UVIS images requires a careful interpretation of the associated latitudinally “oscillating” magnetic field and auroral field-aligned current signatures, whereas previous interpretations have assumed a static current system. Concurrent observations of the auroral hiss (typically generated in regions of downward directed field-aligned current) support this revised interpretation of an oscillating current system. The nature of the motion of the auroral oval evident in the UVIS image sequence, and the simultaneous measured motion of the field-aligned currents (and related plasma boundary) in this interval, is shown to be related to the northern hemisphere magnetosphere oscillation phase. This is in agreement with previous observations of the auroral oval oscillatory motion.