A multi-scale magnetotail reconnection event at Saturn and associated flows: Cassini/UVIS observations (original) (raw)
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Mechanisms of Saturn's Near‐Noon Transient Aurora: In Situ Evidence From Cassini Measurements
Geophysical Research Letters, 2017
Although auroral emissions at giant planets have been observed for decades, the physical mechanisms of aurorae at giant planets remain unclear. One key reason is the lack of simultaneous measurements in the magnetosphere while remote sensing of the aurora. We report a dynamic auroral event identified with the Cassini Ultraviolet Imaging Spectrograph (UVIS) at Saturn on 13 July 2008 with coordinated measurements of the magnetic field and plasma in the magnetosphere. The auroral intensification was transient, only lasting for ∼30 min. The magnetic field and plasma are perturbed during the auroral intensification period. We suggest that this intensification was caused by wave mode conversion generated field-aligned currents, and we propose two potential mechanisms for the generation of this plasma wave and the transient auroral intensification. A survey of the Cassini UVIS database reveals that this type of transient auroral intensification is very common (10/11 time sequences, and ∼10% of the total images).
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 Saturn's polar atmosphere from the Cassini spacecraft. Specifically, images obtained from the Cassini UltraViolet Imaging Spectrograph (UVIS) provide an excellent view of the planet's 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. BUNCE ET AL.
Origin of Saturn's aurora: Simultaneous observations by Cassini and the Hubble Space Telescope
Journal of Geophysical Research, 2008
1] Outer planet auroras have been imaged for more than a decade, yet understanding their physical origin requires simultaneous remote and in situ observations. The first such measurements at Saturn were obtained in January 2007, when the Hubble Space Telescope imaged the ultraviolet aurora, while the Cassini spacecraft crossed field lines connected to the auroral oval in the high-latitude magnetosphere near noon. The Cassini data indicate that the noon aurora lies in the boundary between open-and closed-field lines, where a layer of upward-directed field-aligned current flows whose density requires downward acceleration of magnetospheric electrons sufficient to produce the aurora. These observations indicate that the quasi-continuous main oval is produced by the magnetosphere-solar wind interaction through the shear in rotational flow across the openclosed-field line boundary.
Saturn’s aurora observed by the Cassini camera at visible wavelengths
Icarus, 2016
The first observations of Saturn's visible-wavelength aurora were made by the Cassini camera. The aurora was observed between 2006 and 2013 in the northern and southern hemispheres. The color of the aurora changes from pink at a few hundred km above the horizon to purple at 1000-1500 km above the horizon. The spectrum observed in 9 filters spanning wavelengths from 250 nm to 1000 nm has a prominent H-alpha line and roughly agrees with laboratory simulated auroras. Auroras in both hemispheres vary dramatically with longitude. Auroras form bright arcs between 70 • and 80 • latitude north and between 65 • and 80 • latitude south, which sometimes spiral around the pole, and sometimes form double arcs. A large 10,000-km-scale longitudinal brightness structure persists for more than 100 hours. This structure rotates approximately together with Saturn. On top of the large steady structure, the auroras brighten suddenly on the timescales of a few minutes. These brightenings repeat with a period of ∼1 hour. Smaller, 1000-km-scale structures may move faster or lag behind Saturn's rotation on timescales of tens of minutes. The persistence of nearly-corotating large bright longitudinal structure in the auroral oval seen in two movies spanning 8 and 11 rotations gives an estimate on the period of 10.65 ± 0.15 h for 2009 in the northern oval and 10.8± 0.1 h for 2012 in the southern oval. The 2009 north aurora period is close to the north branch of Saturn Kilometric Radiation (SKR) detected at that time.
Auroral signatures of multiple magnetopause reconnection at Saturn
Geophysical Research Letters, 2013
Auroral observations capture the ionospheric response to dynamics of the whole magnetosphere and may provide evidence of the significance of reconnection at Saturn. Bifurcations of the main dayside auroral emission have been related to reconnection at the magnetopause and their surface is suggested to represent the amount of newly opened flux. This work is the first presentation of multiple brightenings of these auroral features based on Cassini ultraviolet auroral observations. In analogy to the terrestrial case, we propose a process, in which a magnetic flux tube reconnects with other flux tubes at multiple sites. This scenario predicts the observed multiple brightenings, it is consistent with subcorotating auroral features which separate from the main emission, and it suggests north-south auroral asymmetries. We demonstrate that the conditions for multiple magnetopause reconnection can be satisfied at Saturn, like at Earth.
Journal of Geophysical Research, 2011
This work reports for the first time on bifurcations of the main auroral ring at Saturn observed with the UVIS instrument onboard Cassini. The observation sequence starts with an intensification on the main oval, close to noon, which is possibly associated with dayside reconnection. Consecutive bifurcations appear with the onset of dayside reconnection, between 11 and 18 magnetic local time, while the area poleward of the main emission expands to lower latitudes. The bifurcations depart with time from the main ring of emission, which is related to the open-closed field line boundary. The augmentation of the area poleward of the main emission following its expansion is balanced by the area occupied by the bifurcations, suggesting that these auroral features represent the amount of newly open flux and could be related to consecutive reconnection events at the flank of the magnetopause. The observations show that the open flux along the sequence increases when bifurcations appear. Magnetopause reconnection can lead to significant augmentation of the open flux within a couple of days and each reconnection event opens ∼10% of the flux contained within the polar cap. Additionally, the observations imply an overall length of the reconnection line of ∼4 hours of local time and suggest that dayside reconnection at Saturn can occur at several positions on the magnetopause consecutively or simultaneously.
Saturn's auroral morphology and activity during quiet magnetospheric conditions
Journal of Geophysical Research, 2006
1] We report the results of a coordinated Hubble Space Telescope-Cassini campaign that took place between 26 October and 2 November 2005. During this period, Saturn's magnetosphere was in an expanded state and the solar wind was quiet, as indicated by the location of the magnetopause, in situ particle measurements, weak auroral SKR emission, and the generally low brightness of the aurora. We describe the morphology and dynamics of the aurora during this period in parallel with concurrent Cassini measurements. We show that the aurora exhibits considerable longitudinal structure and time variations over intervals of a few hours, in spite of the absence of observable external triggers and generally low intensity. In particular, enhancements of the dawn-morning oval are seen while no apparent indication of solar wind activity is observed. These features rotate at a speed corresponding to about 65% of the planet's angular velocity. We also describe energetic neutral atom measurements indicating that an ENA acceleration event occurred in the magnetotail on 26 October without any measured signature of solar wind activation. These observations suggest an intrinsically dynamical magnetosphere where injection of hot plasma occasionally takes place in the night or dawn sector during quiet magnetospheric conditions, possibly connected with either the Dungey or the Vasyliunas convection cycle. Citation: Gérard, J.-C., et al. (2006), Saturn's auroral morphology and activity during quiet magnetospheric conditions,
The Astrophysical Journal Letters, 2021
Stunning aurorae are mainly produced when accelerated electrons travel along magnetic field lines to collide with the atmosphere. The motion of electrons often corresponds to the evolution of a magnetic field-aligned current system. In the terrestrial magnetosphere, the current system is formed at the night-side sector, and thus produces an auroral bulge at night. Due to the different energy sources between Saturn and the Earth, it is expected that their auroral current systems are fundamentally different, although the specific auroral driver at Saturn is poorly understood. Using simultaneous measurements of the aurora, particles, magnetic fields, and energetic neutral atoms, we reveal that a chain of paired currents, each of which includes a downward and an upward current branch, is formed in Saturnʼs magnetosphere, which generates separated auroral patches. These findings inform similar auroral current structures between the Earth and Saturn, while the difference is that Saturnʼs unique mass and energy sources lead to a rotational characteristic.
Geophysical Research Letters, 2009
We present the first images of Saturn's conjugate equinoctial auroras, obtained in early 2009 using the Hubble Space Telescope. We show that the radius of the northern auroral oval is 1.5°smallerthanthesouthern,indicatingthatSaturn′spolarionosphericmagneticfield,measuredforthefirsttimeintheionosphere,is1.5°smaller than the southern, indicating that Saturn's polar ionospheric magnetic field, measured for the first time in the ionosphere, is 1.5°smallerthanthesouthern,indicatingthatSaturn′spolarionosphericmagneticfield,measuredforthefirsttimeintheionosphere,is17% larger in the north than the south. Despite this, the total emitted UV power is on average $17% larger in the north than the south, suggesting that field-aligned currents (FACs) are responsible for the emission. Finally, we show that individual auroral features can exhibit distinct hemispheric asymmetries. These observations will provide important context for Cassini observations as Saturn moves from southern to northern summer.