•We derived a global 3D shape model and the rotational parameters of comet 67P/C-G from high-resolution visible images collected aboard the Rosetta spacecraft.•Using this model, we could calculate ...accurate nucleus parameters including a volume of 18.8 ± 0.3 km3 and a density of 532 ± 7 kg m−3.•We could also retrieve the shape and the geometry of the two lobes using computer graphics tools.•A slight excitation of the spin state was detected, with a precession period of 11.5 ± 0.5 day.•The coordinates of the spin axis indicates a slight inhomogeneity of the density distribution.
The Rosetta spacecraft reached Comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) in August 2014 at an heliocentric distance of 3.6 a.u. and was then put in orbit around its nucleus to perform detailed observations. Among the collected data are the images acquired by the OSIRIS instrument up to the perihelion passage of the comet in August 2015, which allowed us to map the entire nucleus surface at high-resolution in the visible. Stereophotoclinometry methods have been used to reconstruct a global high-resolution shape model and to monitor its rotational parameters using data collected up to perihelion.
The nucleus has a conspicuous bilobate shape with overall dimensions along its principal axes of (4.34 ± 0.02) × (2.60 ± 0.02) × (2.12 ± 0.06) km. The best-fit ellipsoid dimensions of the individual lobes along their principal axes of inertia are found to be 4.10 × 3.52 × 1.63 km and 2.50 × 2.14 × 1.64 km. Their volume amounts to 66% and 27% of the total volume of the nucleus. The two lobes are connected by a “neck” whose volume has been estimated to represent ∼7% of the total volume of the comet. Combining the derived volume of 18.8 ± 0.3 km3 with the mass of 9.982 ± 0.003 × 1012 kg determined by the Rosetta/RSI experiment, we obtained a bulk density of the nucleus of 532±7kgm−3. Together with the companion value of 535±35kgm−3 deduced from the stereophotogrammetry shape model of the nucleus (Preusker et al. 2015 Astron. Astrophys. 583, A33), these constitute the first reliable and most accurate determination of the density of a cometary nucleus to date. The calculated porosity is quite large, ranging approximately from 70% to 75% depending upon the assumed density of the dust grains and the dust-to-ice mass ratio. The nature of the porosity, either micro or macro or both, remains unconstrained. The coordinates of the center of gravity are not compatible with a uniform nucleus density. The direction of the offset between the center of gravity and the center of figure suggests that the big lobe has a slightly higher bulk density compared to the small one. the center of mass position cannot be explained by different, but homogenous densities in the two lobes.
The initial rotational period of 12.4041 ± 0.0001 h of the nucleus persisted until October 2014. It then slightly increased to a maximum of 12.4304 h reached on 19 May 2015 and finally dropped to 12.305 h just before perihelion on August 10, 2015. A periodogram analysis of the (RA, Dec) direction of the Z-axis of the comet obtained in parallel with the shape reconstruction exhibits a highly significant minima at 11.5 ± 0.5 day clearly indicating an excited rotational state with an amplitude of 0.15 ± 0.03°.
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We present a new quantitative technique that determines the times and durations of substorm expansion and recovery phases and possible growth phases based on percentiles of the rate of change of ...auroral electrojet indices. By being able to prescribe different percentile values, we can determine the onset and duration of substorm phases for smaller or larger variations of the auroral index or indeed any auroral zone ground‐based magnetometer data. We apply this technique to the SuperMAG AL (SML) index and compare our expansion phase onset times with previous lists of substorm onsets. We find that more than 50% of events in previous lists occur within 20 min of our identified onsets. We also present a comparison of superposed epoch analyses of SML based on our onsets identified by our technique and existing onset lists and find that the general characteristics of the substorm bay are comparable. By prescribing user‐defined thresholds, this automated, quantitative technique represents an improvement over any visual identification of substorm onsets or indeed any fixed threshold method.
Key Points
Identifies substorm expansion, recovery, and possible growth phases from magnetic indices
Phases identified from percentiles of the index rate change not fixed values
Technique provides greater information about substorms than existing lists
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Magnetic reconnection is a fundamental physical process in planetary magnetospheres, in which plasma can be exchanged between the solar wind and a planetary magnetosphere, and material can be ...disconnected and ultimately lost from a magnetosphere. Magnetic reconnection in a planetary magnetotail can result in the release of plasmoids downtail and dipolarizations planetward of an x‐line. The signatures of these products include characteristic deflections in the north‐south component of the magnetic field which can be detected by in‐situ spacecraft. These signatures have been identified by eye, semi‐automated algorithms, and recently machine learning (ML) methods. Here, we apply statistical analysis to the most thorough catalog of Kronian magnetospheric reconnection signatures created through ML methods to improve understanding of magnetospheric evolution. This research concludes that no quasi‐steady position of the magnetotail x‐line exists within 70 RS. This research introduces prediction equations to estimate the distribution of duration of plasmoid passage over the spacecraft (N=300Δt−1.3, bin width = 1 min) and north‐south field deflection (N=52ΔBθ−2.1, bin width = 0.25 nT) expected to be identified by an orbiting spacecraft across a year of observations. Furthermore, this research finds a local time asymmetry for reconnection identifications, with a preference for dusk‐side over dawn‐side. This may indicate a preference for Vasyliunas style reconnection over Dungey style for Saturn. Finally, through these distributions, the reconnection rate of Saturn’s magnetotail can be estimated as 3.22 reconnection events per day, with a resulting maximum mass loss from plasmoids of 44.87 kg s−1 on average, which is comparable with the magnetospheric mass loading from Enceladus (8–250 kg s−1).
Key Points
Machine learning classifications in previously unobserved environments can be validated through iterative runs
The Cassini data do not reveal a quasi‐steady magnetotail reconnection x‐line inside of <70 RS
Cassini observations indicate a mass loss rate between 1.50–44.87 kg s−1 due to magnetotail plasmoid release
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An analytical method is developed by which measurements made by the Cassini spacecraft in Saturn's magnetosheath can be used to infer the upstream solar wind parameters, specifically the solar wind ...speed (Vsw) and the dynamic pressure (Pd). The method is validated by comparing the results with other estimates of these parameters, including the mSWiM MHD model and magnetopause and bow shock models applied to observed boundary crossings. The comparisons suggest that the new inferred Vsw are on average ~40 km/s lower than the mSWiM values, and the dynamic pressure values are slightly lower as well. We find few of the lower Pd values predicted by mSWiM, probably because Cassini would have been inside the expanded magnetosphere under such conditions. Systematic temporal variations such as interplanetary shocks do seem to be captured well, with arrival times within several days of the MHD prediction. Compared to dynamic pressures estimated from boundary crossings with well‐known magnetopause and bow shock models, the magnetosheath‐inferred dynamic pressure tends to be somewhat lower, but within the uncertainties of the analytical derivation. Comparison of the inferred dynamic pressure with observed Saturn's kilometric radiation (SKR) activity reveals several episodes of very good temporal tracking between dynamic pressure and SKR intensity, with relatively short time delays (4–5 hr), suggesting rather direct driving. Such good tracking intervals occur almost exclusively on the dawnside of the magnetosphere, where the dominant SKR source is visible. When the tracking is good, the SKR fluxes vary roughly as the square of the dynamic pressure.
Plain Language Summary
The extent to which Saturn's magnetosphere may be driven by variations in the upstream solar wind is not well understood, in part because single‐spacecraft missions provide no monitor of the upstream plasma to accompany the in situ magnetospheric measurements. One global measure of magnetospheric activity is Saturn's kilometric radiation (SKR). Remote observations of SKR can be combined with solar wind measurements to explore the solar wind's influence on the magnetosphere. We propose a method for greatly increasing the amount of time for which the upstream conditions can be known by using the large database of measurements made by Cassini when it was in Saturn's magnetosheath, the shocked solar wind that coats the sunward side of the magnetosphere. The resulting parameters are validated by comparison with other estimates. Then comparison with Cassini SKR observations reveals that at some times the SKR fluxes clearly track the variations in the solar wind dynamic pressure, suggesting a rather direct solar wind driving of at least some of the magnetospheric processes that produce SKR.
Key Points
Analytical development of method to estimate upstream solar wind parameters from Cassini measurements in Saturn's magnetosheath
Results in generally good agreement with solar wind speed and dynamic pressure estimated by other means
Some several‐day intervals are found where there is good tracking between the inferred dynamic pressure and SKR intensities
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OSIRIS-REx is the first NASA mission to return a sample of an asteroid to Earth. Navigation and flight dynamics for the mission to acquire and return a sample of asteroid 101955 Bennu establish many ...firsts for space exploration. These include relatively small orbital maneuvers that are precise to ∼1 mm/s, close-up operations in a captured orbit about an asteroid that is small in size and mass, and planning and orbit phasing to revisit the same spot on Bennu in similar lighting conditions. After preliminary surveys and close approach flyovers of Bennu, the sample site will be scientifically characterized and selected. A robotic shock-absorbing arm with an attached sample collection head mounted on the main spacecraft bus acquires the sample, requiring navigation to Bennu’s surface. A touch-and-go sample acquisition maneuver will result in the retrieval of at least 60 grams of regolith, and up to several kilograms. The flight activity concludes with a return cruise to Earth and delivery of the sample return capsule (SRC) for landing and sample recovery at the Utah Test and Training Range (UTTR).
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
We study 10 years (1995–2004 inclusive) of auroral kilometric radiation (AKR) radio emission data from the Wind spacecraft to examine the link between AKR and terrestrial substorms. We use substorm ...lists based on parameters including ground magnetometer signatures and geosynchronous particle injections as a basis for superposed epoch analyses of the AKR data. The results for each list show a similar, clear response of the AKR power around substorm onset. For nearly all event lists, the average response shows that the AKR power begins to increase around 20 min prior to expansion phase onset, as defined by the respective lists. The analysis of the spectral parameters of AKR bursts show that this increase in power is due to an extension of the source region to higher altitudes, which also precedes expansion phase onset by 20 min. Our observations show that the minimum frequency channel that observes AKR at this time, on average, is 60 kHz. AKR visibility is highly sensitive to observing spacecraft location, and the biggest radio response to substorm onset is seen in the 21:00–03:00 hr local time sector.
Plain Language Summary
Substorms are an energetic disturbance to the magnetic environment of the Earth. They represent the driving of the terrestrial magnetosphere by particles from the Sun and the subsequent response in various parts of this environment, in both its inner and outer boundaries. These effects are mostly constrained to the nightside of Earth, and can be observed by both ground‐based and remote sensing instruments. In this work, we select auroral kilometric radiation (AKR) observations from 10 years (from 1995 to 2004 inclusive) of radio data from the Wind/WAVES instrument, and compare this with lists of substorm expansion phase onsets that are derived from various observational signatures. After accounting for visibility of the radio sources, we show that the AKR response correlates with the size/strength of the substorm, based on the sensitivity of the list. Our results show that the AKR source region tends to increase in size along magnetic field lines while the emission intensifies, using a longer data set to corroborate previous results.
Key Points
Auroral kilometric radiation (AKR) observations made over 10 years are compared with four event lists of substorm onsets using superposed epoch analyses
On average, AKR power increases and source region extends to higher altitudes in the 20 min prior to onset
The occurrence of AKR power at higher altitudes is sensitive to the substorm size
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The Cassini spacecraft orbited the planet Saturn from July 2004 to September 2017, and its varied orbital trajectory took it across the magnetopause and bow shock boundaries multiple times, at ...varying radial distances, local times, latitudes, and phases of the solar cycle. Here we present a comprehensive list of these boundary crossings, derived primarily using data from the Cassini magnetometer instrument, with cross‐validation against the electron spectrometer data where available. There are a multitude of scientific avenues for exploitation of this list. In this work, we examine the variability in boundary location and use the crossing times in concert with models of the bow shock and magnetopause to infer the upstream solar wind dynamic pressure at the times of crossings. This analysis allows us to understand the limitations of the Cassini trajectory for studying boundary physics under a range of solar wind driving conditions. In addition, rapid traversals of the magnetosheath are used to estimate the range of speeds of boundary motion.
Key Points
Comprehensive set of magnetopause and bow shock crossings spanning the entire Cassini mission presented
Crossings used in conjunction with boundary models to infer upstream solar wind dynamic pressure and range of standoff distances
Rapid traversals of magnetosheath explored in context of rapidly changing external conditions
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Purpose: This study aimed to extend understanding of goal setting in physical activity (PA) by exploring qualitatively active and insufficiently active individuals' experiences of pursuing specific ...and non-specific goals. Methods: Twelve active (M
age
= 25.00 years) and nine insufficiently active (M
age
= 24.33 years) adults were interviewed after participating in three 6-minute walking tests, during which they pursued specific and non-specific (open and DYB) goals and completed a no-goal control condition. Content analysis was used to examine each subgroup independently, before a between-group comparison to explore similarities and differences in experiences was undertaken. Results: Several strategies were employed to enhance trustworthiness. Our analysis was organised into seven categories: (1) perception of challenge; (2) perceived control; (3) performance satisfaction; (4) motivational intensity; (5) enjoyment; (6) self-efficacy; and (7) strategies for pursuing goals. Our findings suggest that active participants responded more positively to specific goals, with differences in the cognitions experienced before, during, and after the goal conditions. Conversely, insufficiently active participants responded more positively to non-specific goals, with differences in the cognitions experienced before, during, and after the goal conditions and more negatively to specific goals. Conclusion: Findings extend understanding of specific and non-specific goals in PA and underline the need to consider goal specificity in the prescription of PA for certain population groups most in need of intervention.
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BFBNIB, GIS, IJS, KISLJ, NUK, PNG, UL, UM, UPUK
Auroral Kilometric Radiation (AKR) is terrestrial radio emission that originates in particle acceleration regions along magnetic field lines, coinciding with discrete auroral arcs. AKR viewing ...geometry is complex due to the confinement of the source regions to nightside local times (LTs) and the anisotropy of the beaming pattern, so observations are highly dependent on spacecraft viewing position. We present a novel, empirical technique that selects AKR emission from observations made with the spin‐axis aligned antenna of the Wind/WAVES instrument, based on the rapidly varying amplitude of AKR across spacecraft spin timescales. We apply the technique to Wind/WAVES data during 1999 day of year 227–257, when the Cassini spacecraft flew past Earth and provided an opportunity to observe AKR from two remote locations. We examine the AKR flux and power, with observations made from LTs of 1700–0300 hr having an average power up to 104 Wsr‐1 larger than those on the dayside and an increasing AKR power observed at higher magnetic latitudes. We perform a linear cross‐correlation between the Wind AKR power and the spacecraft magnetic latitude, showing positive then negative correlation as Wind travels from the Northern to Southern magnetic hemisphere. Statistically significant diurnal modulations are found in the whole 30‐day period and in subsets of the data covering different local time sectors, indicative of a predominantly geometrical effect for remote AKR viewing. The reproduction of well‐known features of the AKR verifies the empirical selection and shows the promise of its application to Wind/WAVES observations.
Plain Language Summary
Auroral Kilometric Radiation (AKR) is naturally occurring radio emission from the Earth's Northern and Southern polar regions, which becomes more intense as the aurora brightens. In this work, we examine data from the Wind spacecraft WAVES instrument from a 30‐ day interval in 1999 when a second spacecraft, Cassini, was also flying near Earth and measuring the AKR from a different viewpoint. In this work, we select the AKR using an empirical measure of the variability observed by the WAVES instrument, and compare the distribution and time profile of AKR intensity. Comparing measurements of this radio emission from different spacecraft positions help us to understand how the AKR is best viewed and illustrate the constrained beaming of the emission. This information is important for anyone wanting to attempt to interpret measurements of the AKR.
Key Points
Novel, empirically based method to extract Auroral Kilometric Radiation (AKR) from Wind/WAVES is presented and applied to observations made during the Cassini flyby
Selected data show a distribution of AKR power with expected longitudinal and latitudinal visibility constraints
Diurnal temporal modulation observed, suggesting the dominance of a geometric viewing effect and agreeing with previous AKR observations
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Saturn Kilometric Radiation (SKR) is a non‐thermal auroral emission with peak emission occurring at 100–400 kHz. Its properties have been extensively studied since Cassini's arrival at Saturn until ...mission end with its Radio and Plasma Wave Science (RPWS) experiment. Low Frequency Extensions (LFEs) of SKR which consist of global intensifications of SKR accompanied by extensions of the main SKR band down to lower frequencies have been studied in particular. Low Frequency Extensions result from internally driven tail reconnection and from solar wind compressions of the magnetosphere, which also trigger tail reconnection. They have been cataloged through visual inspection with two approaches, using an intensity threshold for LFEs in 2006 (Reed et al., 2018, https://doi.org/10.1002/2017ja024499) and more recently O’Dwyer et al. (2023a, https://doi.org/10.25546/103103) produced a sample of LFEs detected by Cassini/RPWS by fitting their exact frequency‐time coordinates with polygons. In this study we use the latter catalog of LFEs as a training set for an image based machine learning algorithm to classify all LFEs detected by Cassini/RPWS. The inputs to the model are multi‐channel images consisting of spectrogram images in flux density and degree of circular polarization. The outputs of the model are binary masks showing the exact location of the LFE in frequency‐time space. The median Intersection Over Union across the testing and training set were calculated to be 0.97 and 0.98, respectively. The output of this study is a list of all 4,874 LFEs detected using this method. The list of LFE frequency‐time coordinates is available for use amongst the scientific community.
Plain Language Summary
We are using radio observations from the Cassini spacecraft that was in orbit around the planet Saturn for 13 years. We want to search for characteristic features of Saturn's auroral radio emissions (called Saturn Kilometric Radiation or SKR) in the data stream from the radio instrument—specifically events called Low Frequency Extensions (LFEs). The edges of these events can be tracked in time‐frequency spectrograms of Cassini radio observations. We find several hundred examples of the LFEs that we're looking for, and feed these into a computer algorithm which learns what they look like. The algorithm can then be applied to new/unseen data and we allow it to search for similar events. The end result is an extensive catalog of all the LFEs observed throughout the 13‐year near‐Saturn mission by the radio instrument of Cassini. This catalog can be used by the scientific community as a basis for statistical studies of Saturn's radio emissions. The machine learning aspect of this work can be adapted through something known as transfer learning to other planets where we look for similar features in data.
Key Points
Supervised learning applied to database of labeled polygons marked on radio spectrograms
Focus on Low Frequency Extensions of Saturn Kilometric Radiation to return a full catalog from the Cassini mission
A modified U‐Net architecture achieved median Intersection over Union values of 0.98 and 0.97 across the training and testing set
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