The Dynamics of Saturn's Ultraviolet Aurorae

Saturn’s aurorae are highly dynamic, controlled from within Saturn’s magnetosphere and by its interaction with the solar wind. This thesis investigates ultraviolet observations of these auroral emissions and corresponding in situ measurements of fields and particles, both mostly obtained by the Cassini mission, in order to separate different components of the aurorae and determine their origin. The brightest emissions are found to be generated by recurring magnetotail reconnection, the occurrence of which is controlled by solar wind conditions and the phasing of Saturn’s planetary period oscillation systems of rotating magnetic field perturbations and electric currents. The auroral signature resembles a series of bright patches emerging near local midnight and subcorotating with the planet’s rotation. Underlying these is a steady auroral band which may be driven by flow shears in the outer magnetosphere and is modulated in intensity and location by the rotating planetary period oscillation systems, accompanied by a dim equatorward outer emission which is suggested to be related to wave scattering of electrons in the inner ring current. Observations further show various small-scale transients such as short-lived ∼ 1 h quasiperiodic flashes possibly indicative of magnetodisc reconnection occurring predominantly near dusk, or numerous fine arcs only visible in the highest resolution imagery obtained by Cassini which may be related to interchange injection events. The relation between the source of auroral particles in the magnetosphere and the auroral emissions they generate upon impacting the atmosphere was investigated, with in situ measurements close above the aurorae revealing the presence of energetic field-aligned ion beams and conics as well as complex wave-particle interactions which may be responsible for their energization. While this thesis uncovers much unknown detail on the workings of Saturn’s aurorae, many questions remain to be answered in future research

Magnetosphere imager science definition team interim report

For three decades, magnetospheric field and plasma measurements have been made by diverse instruments flown on spacecraft in may different orbits, widely separated in space and time, and under various solar and magnetospheric conditions. Scientists have used this information to piece together an intricate, yet incomplete view of the magnetosphere. A simultaneous global view, using various light wavelengths and energetic neutral atoms, could reveal exciting new data nd help explain complex magnetospheric processes, thus providing a clear picture of this region of space. This report documents the scientific rational for such a magnetospheric imaging mission and provides a mission concept for its implementation

Tracking counterpart signatures in Saturn's auroras and ENA imagery during large-scale plasma injection events

Saturn's morningside auroras consist mainly of rotating, transient emission patches, following periodic reconnection in the magnetotail. Simultaneous responses in global energetic neutral atom (ENA) emissions have been observed at similar local times, suggesting a link between the auroras and large‐scale injections of hot ions in the outer magnetosphere. In this study, we use Cassini's remote sensing instruments to observe multiple plasma injection signatures within coincident auroral and ENA imagery, captured during 9 April 2014. Kilometric radio emissions also indicate clear injection activity. We track the motion of rotating signatures in the auroras and ENAs to test their local time relationship. Two successive auroral signatures—separated by ~4 hr UT—form postmidnight before rotating to the dayside while moving equatorward. The first has a clear ENA counterpart, maintaining a similar local time mapping throughout ~9 hr observation. Mapping of the ionospheric equatorward motion post‐dawn indicates a factor ~5 reduction of the magnetospheric source region's radial speed at a distance of ~14‐20 RS, possibly a plasma or magnetic boundary. The second auroral signature has no clear ENA counterpart; viewing geometry was relatively unchanged, so the ENAs were likely too weak to detect by this time. A third, older injection signature, seen in both auroral and ENA imagery on the nightside, may have been sustained by field‐aligned currents linked with the southern planetary period oscillation system, or the re‐energization of ENAs around midnight local times. The ENA injection signatures form near magnetic longitudes associated with magnetotail thinning

Using ultra-low frequency waves and their characteristics to diagnose key physics of substorm onset

Substorm onset is marked in the ionosphere by the sudden brightening of an existing auroral arc or the creation of a new auroral arc. Also present is the formation of auroral beads, proposed to play a key role in the detonation of the substorm, as well as the development of the large-scale substorm current wedge (SCW ), invoked to carry the current diversion. Both these phenomena, auroral beads and the SCW, have been intimately related to ultra-low frequency (ULF) waves of specific frequencies as observed by ground-based magnetometers. We present a case study of the absolute and relative timing of Pi1 and Pi2 ULF wave bands with regard to a small substorm expansion phase onset. We find that there is both a location and frequency dependence for the onset of ULF waves. A clear epicentre is observed in specific wave frequencies concurrent with the brightening of the substorm onset arc and the presence of “auroral beads”. At higher and lower wave frequencies, different epicentre patterns are revealed, which we conclude demonstrate different characteristics of the onset process; at higher frequencies, this epicentre may demonstrate phase mixing, and at intermediate and lower frequencies these epicentres are characteristic of auroral beads and cold plasma approximation of the “Tamao travel time” from near-earth neutral line reconnection and formation of the SCW

Background removal from global auroral images: Data-driven dayglow modeling

Global images of auroras obtained by cameras on spacecraft are a key tool for studying the near-Earth environment. However, the cameras are sensitive not only to auroral emissions produced by precipitating particles, but also to dayglow emissions produced by photoelectrons induced by sunlight. Nightglow emissions and scattered sunlight can contribute to the background signal. To fully utilize such images in space science, background contamination must be removed to isolate the auroral signal. Here we outline a data-driven approach to modeling the background intensity in multiple images by formulating linear inverse problems based on B-splines and spherical harmonics. The approach is robust, flexible, and iteratively deselects outliers, such as auroral emissions. The final model is smooth across the terminator and accounts for slow temporal variations and large-scale asymmetries in the dayglow. We demonstrate the model by using the three far ultraviolet cameras on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission. The method can be applied to historical missions and is relevant for upcoming missions, such as the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) mission

Evaluating auroral forecasts against satellite observations

The aurora is a readily visible phenomenon of interest to many members of the public. However, the aurora and associated phenomena can also significantly impact communications, ground-based infrastructure, and high-altitude radiation exposure. Forecasting the location of the auroral oval is therefore a key component of space weather forecast operations. A version of the OVATION-Prime 2013 auroral precipitation model (Newell et al., 2014, https://doi.org/10.1002/2014sw001056) was used by the UK Met Office Space Weather Operations Centre (MOSWOC). The operational implementation of the OVATION-Prime 2013 model at the UK Met Office delivered a 30-min forecast of the location of the auroral oval and the probability of observing the aurora. Using weather forecast evaluation techniques, we evaluate the ability of the OVATION-Prime 2013 model forecasts to predict the location and probability of the aurora occurring by comparing the forecasts with auroral boundaries determined from data from the IMAGE satellite between 2000 and 2002. Our analysis shows that the operational model performs well at predicting the location of the auroral oval, with a relative operating characteristic (ROC) score of 0.82. The model performance is reduced in the dayside local time sectors (ROC score = 0.59) and during periods of higher geomagnetic activity (ROC score of 0.55 for Kp = 8). As a probabilistic forecast, OVATION-Prime 2013 tends to underpredict the occurrence of aurora by a factor of 1.1–6, while probabilities of over 90% are overpredicted

An isolated, bright cusp aurora at Saturn

Saturn's dayside aurora displays a number of morphological features poleward of the main emission region. We present an unusual morphology captured by the Hubble Space Telescope on 14 June 2014 (day 165), where for 2 h, Saturn's FUV aurora faded almost entirely, with the exception of a distinct emission spot at high latitude. The spot remained fixed in local time between 10 and 15 LT and moved poleward to a minimum colatitude of ~4°. It was bright and persistent, displaying intensities of up to 49 kR over a lifetime of 2 h. Interestingly, the spot constituted the entirety of the northern auroral emission, with no emissions present at any other local time—including Saturn's characteristic dawn arc, the complete absence of which is rarely observed. Solar wind parameters from propagation models, together with a Cassini magnetopause crossing and solar wind encounter, indicate that Saturn's magnetosphere was likely to have been embedded in a rarefaction region, resulting in an expanded magnetosphere configuration during the interval. We infer that the spot was sustained by reconnection either poleward of the cusp or at low latitudes under a strong component of interplanetary magnetic field transverse to the solar wind flow. The subsequent poleward motion could then arise from either reconfiguration of successive open field lines across the polar cap or convection of newly opened field lines. We also consider the possible modulation of the feature by planetary period rotating current systems

Study of meso-scale reversed flow events in the polar ionosphere by SuperDARN radars

Postponed access: the file will be accessible after 2018-06-13In this study we have investigated Reversed Flow Events (RFEs) in the northern hemisphere polar cap. A RFE is an ∼50-250 km wide flow channel that opposes the large scale background flow. A total of 57 new RFEs were discovered using data from the Super Dual Auroral Radar Network (SuperDARN) for primarily December of 2014-2016. We found RFEs lasting up to 97 minutes, with an average duration of 11.4 minutes. Most RFEs were found in the dawn and dusk region with 26 events in the 4-10 MLT dawn region (46%) and 14 in the 14-20 MLT dusk region (25%). 12 RFEs were identified within the 10-14 MLT dayside region (21%) and only 5 in the 20-04 MLT nightside region (9%). There was no significant spread in MLT based on IMF Bz, but in our study 79% of the RFEs with stable IMF prior to onset were observed during positive IMF Bz. For By there was a strong preference towards dawn and night for negative values, and day and dusk for positive values. Most RFEs were seen stationary during their existence, while at least one RFE moved poleward. Depending on their location we have classified the reversed flow channels as either dayside RFEs, lobe cell RFEs (dawn and dusk) or nightside RFEs. Our findings agree with previous studies of dayside reconnection generated RFEs, but expand our knowledge of the phenomenon to a wider area of the polar cap.Masteroppgåve i fysikkMAMN-PHYSPHYS39

Study of meso-scale reversed flow events in the polar ionosphere by SuperDARN radars

In this study we have investigated Reversed Flow Events (RFEs) in the northern hemisphere polar cap. A RFE is an ∼50-250 km wide flow channel that opposes the large scale background flow. A total of 57 new RFEs were discovered using data from the Super Dual Auroral Radar Network (SuperDARN) for primarily December of 2014-2016. We found RFEs lasting up to 97 minutes, with an average duration of 11.4 minutes. Most RFEs were found in the dawn and dusk region with 26 events in the 4-10 MLT dawn region (46%) and 14 in the 14-20 MLT dusk region (25%). 12 RFEs were identified within the 10-14 MLT dayside region (21%) and only 5 in the 20-04 MLT nightside region (9%). There was no significant spread in MLT based on IMF Bz, but in our study 79% of the RFEs with stable IMF prior to onset were observed during positive IMF Bz. For By there was a strong preference towards dawn and night for negative values, and day and dusk for positive values. Most RFEs were seen stationary during their existence, while at least one RFE moved poleward. Depending on their location we have classified the reversed flow channels as either dayside RFEs, lobe cell RFEs (dawn and dusk) or nightside RFEs. Our findings agree with previous studies of dayside reconnection generated RFEs, but expand our knowledge of the phenomenon to a wider area of the polar cap.Masteroppgåve i fysikkMAMN-PHYSPHYS39

Modulations of Saturn's UV Auroral Oval Location by Planetary Period Oscillations

It is well known that Saturn's magnetospheric dynamics are greatly influenced by the so-called planetary period oscillations (PPOs). Based on Cassini Ultraviolet Imaging Spectrograph (UVIS) imagery, it has been shown previously that the UV auroral intensity is clearly modulated in phase with rotating field-aligned current (FAC) systems associated with the PPOs. Here we expand upon this investigation by using the same data set to examine the PPO-induced spatial modulation of the main auroral oval. We present a robust algorithm used for determining the location of the main emission in Cassini-UVIS images. The location markers obtained are then used to calculate the statistical location of the auroral oval and its periodic displacement due to the PPO FACs and the related ionospheric flows. We find that the largest equatorward displacement of the main arc lags behind the PPO-dependent statistical brightening of the UV aurora by roughly 45–90° in both hemispheres and is not colocated with it as the present model based on magnetometer observations suggests. We furthermore find the center of the auroral oval by fitting circles to the main emission and analyze its elliptic motion as the entire oval is displaced in phase with the PPO phases. It is demonstrated that the periodic displacements of both the auroral oval arc and its center are larger when the two PPO systems rotate in relative antiphase than when they are in phase, clearly indicating that interhemispheric PPO FAC closure modulates not only the intensity but also the location of the main UV auroral emission