377 results found
Heyner, Auster, Fornacon, et al., 2021, The BepiColombo Planetary Magnetometer MPO-MAG: what can we Learn from the Hermean magnetic field?, Space Science Reviews, Vol: 217, ISSN: 0038-6308
The magnetometer instrument MPO-MAG on-board the Mercury Planetary Orbiter (MPO) of the BepiColombo mission en-route to Mercury is introduced, with its instrument design, its calibration and scientific targets. The instrument is comprised of two tri-axial fluxgate magnetometers mounted on a 2.9 m boom and are 0.8 m apart. They monitor the magnetic field with up to 128 Hz in a ±2048 nT range. The MPO will be injected into an initial 480×1500 km polar orbit (2.3 h orbital period). At Mercury, we will map the planetary magnetic field and determine the dynamo generated field and constrain the secular variation. In this paper, we also discuss the effect of the instrument calibration on the ability to improve the knowledge on the internal field. Furthermore, the study of induced magnetic fields and field-aligned currents will help to constrain the interior structure in concert with other geophysical instruments. The orbit is also well-suited to study dynamical phenomena at the Hermean magnetopause and magnetospheric cusps. Together with its sister instrument Mio-MGF on-board the second satellite of the BepiColombo mission, the magnetometers at Mercury will study the reaction of the highly dynamic magnetosphere to changes in the solar wind. In the extreme case, the solar wind might even collapse the entire dayside magnetosphere. During cruise, MPO-MAG will contribute to studies of solar wind turbulence and transient phenomena.
Staniland NR, Dougherty MK, Masters A, et al., 2021, The cushion region and dayside magnetodisc structure at Saturn, Geophysical Research Letters, Vol: 48, Pages: 1-9, ISSN: 0094-8276
A sustained quasi‐dipolar magnetic field between the current sheet outer edge and the magnetopause, known as a cushion region, has previously been observed at Jupiter, but not yet at Saturn. Using the complete Cassini magnetometer data, the first evidence of a cushion region forming at Saturn is shown. Only five examples of a sustained cushion are found, revealing this phenomenon to be rare. Four of the cushion regions are identified at dusk and one pre‐noon. It is suggested that greater heating of plasma post‐noon coupled with the expansion of the field through the afternoon sector makes the disc more unstable in this region. These results highlight a key difference between the Saturn and Jupiter systems.
Provan G, Bradley TJ, Bunce EJ, et al., 2021, Saturn's Nightside Ring Current During Cassini's Grand Finale, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 126, ISSN: 2169-9380
Southwood DJ, Cao H, Shebanits O, et al., 2021, Discovery of Alfven Waves Planetward of Saturn's Rings, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 126, ISSN: 2169-9380
Agiwal O, Cao H, Cowley SWH, et al., 2021, Constraining the Temporal Variability of Neutral Winds in Saturn's Low-Latitude Ionosphere Using Magnetic Field Measurements, JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS, Vol: 126, ISSN: 2169-9097
Staniland NR, Dougherty MK, Masters A, et al., 2020, The cushion region and dayside magnetodisc structure at Saturn, Publisher: ESSOAr
A sustained dipolar magnetic field between the current sheet outer edge and the magnetopause, known as a cushion region, has yet to be observed at Saturn. Whilst some signatures of reconnection occurring in the dayside magnetodisc have been identified, the presence of this large-scale structure has not been seen. Using the complete Cassini magnetometer data, the first evidence of a cushion region forming at Saturn is shown. Only five potential examples of a sustained cushion are found, revealing this phenomenon to be rare. This feature more commonly occurs at dusk compared to dawn, where it is found at Jupiter. It is suggested that due to greater heating and expansion of the field through the afternoon sector the disc is more unstable in this region. We show that magnetodisc breakdown is more likely to occur within the magnetosphere of Jupiter compared to Saturn.
Baumjohann W, Matsuoka A, Narita Y, et al., 2020, The BepiColombo-Mio magnetometer en route to Mercury, Space Science Reviews, Vol: 216, Pages: 1-33, ISSN: 0038-6308
The fluxgate magnetometer MGF on board the Mio spacecraft of the BepiColombo mission is introduced with its science targets, instrument design, calibration report, and scientific expectations. The MGF instrument consists of two tri-axial fluxgate magnetometers. Both sensors are mounted on a 4.8-m long mast to measure the magnetic field around Mercury at distances from near surface (initial peri-center altitude is 590 km) to 6 planetary radii (11640 km). The two sensors of MGF are operated in a fully redundant way, each with its own electronics, data processing and power supply units. The MGF instrument samples the magnetic field at a rate of up to 128 Hz to reveal rapidly-evolving magnetospheric dynamics, among them magnetic reconnection causing substorm-like disturbances, field-aligned currents, and ultra-low-frequency waves. The high time resolution of MGF is also helpful to study solar wind processes (through measurements of the interplanetary magnetic field) in the inner heliosphere. The MGF instrument firmly corroborates measurements of its companion, the MPO magnetometer, by performing multi-point observations to determine the planetary internal field at higher multi-pole orders and to separate temporal fluctuations from spatial variations.
Bradley TJ, Cowley SWH, Bunce EJ, et al., 2020, Saturn's Nightside Dynamics During Cassini's F Ring and Proximal Orbits: Response to Solar Wind and Planetary Period Oscillation Modulations, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380
Cao H, Dougherty MK, Hunt GJ, et al., 2020, The landscape of Saturn's internal magnetic field from the Cassini Grand Finale, ICARUS, Vol: 344, ISSN: 0019-1035
Shebanits O, Hadid LZ, Cao H, et al., 2020, Saturn’s near-equatorial ionospheric conductivities from in situ measurements, Scientific Reports, Vol: 10, ISSN: 2045-2322
Cassini’s Grand Finale orbits provided for the first time in-situ measurements of Saturn’s topside ionosphere. We present the Pedersen and Hall conductivities of the top near-equatorial dayside ionosphere, derived from the in-situ measurements by the Cassini Radio and Wave Plasma Science Langmuir Probe, the Ion and Neutral Mass Spectrometer and the fluxgate magnetometer. The Pedersen and Hall conductivities are constrained to at least 10−5–10−4 S/m at (or close to) the ionospheric peak, a factor 10–100 higher than estimated previously. We show that this is due to the presence of dusty plasma in the near-equatorial ionosphere. We also show the conductive ionospheric region to be extensive, with thickness of 300–800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities from orbit 288 to 292.
Hunt GJ, Bunce EJ, Cao H, et al., 2020, Saturn's auroral field-aligned currents: observations from the Northern Hemisphere dawn sector during cassini's proximal orbits, Journal of Geophysical Research: Space Physics, Vol: 125, ISSN: 2169-9380
We examine the azimuthal magnetic field signatures associated with Saturn's northern hemisphere auroral field‐aligned currents observed in the dawn sector during Cassini's Proximal orbits (April 2017 and September 2017). We compare these currents with observations of the auroral currents from near noon taken during the F‐ring orbits prior to the Proximal orbits. First, we show that the position of the main auroral upward current is displaced poleward between the two local times (LT). This is consistent with the statistical position of the ultraviolet auroral oval for the same time interval. Second, we show the overall average ionospheric meridional current profile differs significantly on the equatorward boundary of the upward current with a swept‐forward configuration with respect to planetary rotation present at dawn. We separate the planetary period oscillation (PPO) currents from the PPO‐independent currents and show their positional relationship is maintained as the latitude of the current shifts in LT implying an intrinsic link between the two systems. Focusing on the individual upward current sheets pass‐by‐pass we find that the main upward current at dawn is stronger compared to near‐noon. This results in the current density been ~1.4 times higher in the dawn sector. We determine a proxy for the precipitating electron power and show that the dawn PPO‐independent upward current electron power ~1.9 times higher than at noon. These new observations of the dawn auroral region from the Proximal orbits may show evidence of an additional upward current at dawn likely associated with strong flows in the outer magnetosphere.
Staniland N, Dougherty M, Masters A, et al., 2020, Determining the nominal thickness and variability of the magnetodisc current sheet at saturn, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-15, ISSN: 2169-9380
The thickness and variability of the Saturnian magnetodisc current sheet is investigated using the Cassini magnetometer data set. Cassini performed 66 fast, steep crossings of the equatorial current sheet where a clear signature in the magnetic field data allowed for a direct determination of its thickness and the offset of its center. The average, or nominal, current sheet half‐thickness is 1.3 R S , where R S is the equatorial radius of Saturn, equal to 60,268 km. This is thinner than previously calculated, but both spatial and temporal dependencies are identified. The current sheet is thicker and more variable by a factor ∼2 on the nightside compared to the dayside, ranging from 0.5–3 R S . The current sheet is on average 50% thicker in the nightside quasi‐dipolar region (≤15 R S ) compared to the dayside. These results are consistent with the presence of a noon‐midnight electric field at Saturn that produces a hotter plasma population on the nightside compared to the dayside. It is also shown that the current sheet becomes significantly thinner in the outer region of the nightside, while staying approximately constant with radial distance on the dayside, reflecting the dayside compression of the magnetosphere by the solar wind. Some of the variability is well characterized by the planetary period oscillations (PPOs). However, we also find evidence for non‐PPO drivers of variability.
Agiwal O, Hunt GJ, Dougherty MK, et al., 2020, Modeling the Temporal Variability in Saturn's Magnetotail Current Sheet From the Cassini F-ring Orbits, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380
Cao Y, Wellbrock A, Coates AJ, et al., 2020, Field‐aligned photoelectron energy peaks at high altitude and on the nightside of titan, Journal of Geophysical Research: Planets, Vol: 125, Pages: 1-13, ISSN: 2169-9097
The ionization of N urn:x-wiley:jgre:media:jgre21272:jgre21272-math-0001 by strong solar He II 30.4‐nm photons produces distinctive spectral peaks near 24.1 eV in Titan's upper atmosphere, which have been observed by the Electron Spectrometer (ELS) as part of the Cassini Plasma Spectrometer. The ELS observations reveal that, in addition to the dayside, photoelectron peaks were also detected on the deep nightside where photoionization is switched off, as well as at sufficiently high altitudes where the ambient neutral density is low. These photoelectron peaks are unlikely to be produced locally but instead must be contributed by transport along the magnetic field lines from their dayside source regions. In this study, we present a statistical survey of all photoelectron peaks identified with an automatic finite impulse response algorithm based on the available ELS data accumulated during 56 Titan flybys. The spatial distribution of photoelectron peaks indicates that most photoelectrons detected at an altitude above 4,000 km and a solar zenith angle above 100° are field aligned, which is consistent with the scenario of photoelectron transport along the magnetic field lines. Our analysis also reveals the presence of a photoelectron gap in the deep nightside ionosphere where almost no photoelectrons were detected. It appears to be very difficult for photoelectrons to travel to this region, and such a feature may not be driven by the changes in the orientation between the solar and corotation wakes.
Jackman CM, Thomsen MF, Dougherty MK, 2019, Survey of Saturn's Magnetopause and Bow Shock Positions Over the Entire Cassini Mission: Boundary Statistical Properties and Exploration of Associated Upstream Conditions, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 8865-8883, ISSN: 2169-9380
Provan G, Cowley SWH, Bradley TJ, et al., 2019, Magnetic Field Observations on Cassini's Proximal Periapsis Passes: Planetary Period Oscillations and Mean Residual Fields, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 8814-8864, ISSN: 2169-9380
Guo RL, Yao ZH, Sergis N, et al., 2019, Long-standing Small-scale Reconnection Processes at Saturn Revealed by Cassini, ASTROPHYSICAL JOURNAL LETTERS, Vol: 884, ISSN: 2041-8205
Sorba AM, Achilleos NA, Sergis N, et al., 2019, Local Time Variation in the Large-Scale Structure of Saturn's Magnetosphere, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 7425-7441, ISSN: 2169-9380
Hunt G, Cowley S, Provan G, et al., 2019, Currents associated with Saturn's intra-D ring azimuthal field perturbations, Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 5675-5691, ISSN: 2169-9380
During the final 22 full revolutions of the Cassini mission in 2017, the spacecraft passed at periapsis near the noon meridian through the gap between the inner edge of Saturn’s D ring and the denser layers of the planet’s atmosphere, revealing the presence of an unanticipated low-latitude current system via the associated azimuthal perturbation field peaking typically at ~10-30 nT. Assuming approximate axisymmetry, here we use the field data to calculate the associated horizontal meridional currents flowing in the ionosphere at the feet of the field lines traversed, together with the exterior field-aligned currents required by current continuity. We show that the ionospheric currents are typically~0.5–1.5 MA per radian of azimuth, similar to auroral region currents, while the field-aligned current densities above the ionosphere are typically ~5-10 nA m-2 , more than an order less than auroral values. The principal factor involved in this difference is the ionospheric areas into which the currents map. While around a third of passes exhibit unidirectional currents flowing northward in the ionosphere closing southward along exterior field lines, many passes also display layers of reversed northward field-aligned current of comparable or larger magnitude in the region interior to the D ring, which may reverse sign again on the innermost field lines traversed. Overall, however, the currents generally show a high degree of north-south conjugacy indicative of an interhemispheric system, certainly on the larger overall spatial scales involved, if less so for the smaller-scale structures, possibly due to rapid temporal or local time variations.
Sulaiman AH, Farrell WM, Ye S-Y, et al., 2019, A Persistent, Large-Scale, and Ordered Electrodynamic Connection Between Saturn and Its Main Rings, GEOPHYSICAL RESEARCH LETTERS, Vol: 46, Pages: 7166-7172, ISSN: 0094-8276
Provan G, Cowley SWH, Bunce EJ, et al., 2019, Variability of Intra-D Ring Azimuthal Magnetic Field Profiles Observed on Cassini's Proximal Periapsis Passes, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 379-404, ISSN: 2169-9380
Brown P, Auster U, Bergman JES, et al., 2019, MEETING THE MAGNETIC EMC CHALLENGES FOR THE IN-SITU FIELD MEASUREMENTS ON THE JUICE MISSION, ESA Workshop on Aerospace EMC (Aerospace EMC), Publisher: IEEE
Dougherty M, Christensen U, Cao H, et al., 2019, Saturn's Magnetic Field and Dynamo, Saturn in the 21st Century, Editors: Baines, Flasar, Krupp, Stallard, Publisher: Cambridge University Press, Pages: 69-96, ISBN: 978-1-107-10677-2
Guo RL, Yao ZH, Sergis N, et al., 2018, Reconnection Acceleration in Saturn's Dayside Magnetodisk: A Multicase Study with Cassini, ASTROPHYSICAL JOURNAL LETTERS, Vol: 868, ISSN: 2041-8205
Krupp N, Roussos E, Kollmann P, et al., 2018, Energetic Neutral and Charged Particle Measurements in the Inner Saturnian Magnetosphere During the Grand Finale Orbits of Cassini 2016/2017, GEOPHYSICAL RESEARCH LETTERS, Vol: 45, Pages: 10847-10854, ISSN: 0094-8276
Dougherty M, Buratti BJ, Seidelmann PK, et al., 2018, Enceladus as an active world: History and discovery. In Enceladus and the Icy, Enceladus and the Icy Moons of Saturn, Editors: Schenk, Clark, Howett, Verbiscer, Waite, Publisher: University of Arizona Press, Pages: 3-16, ISBN: 9780816537075
Dougherty M. K., Buratti B. J., Seidelmann P. K., and Spencer J. R. (2018) Enceladus as an active world: History and discovery. In Enceladus and the Icy Moons of Saturn (P. M. Schenk et al., eds.), pp. 3–16. Univ. of Arizona, Tucson, DOI: ...
Sorba AM, Achilleos NA, Guio P, et al., 2018, The Periodic Flapping and Breathing of Saturn's Magnetodisk During Equinox, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 8292-8316, ISSN: 2169-9380
Yao ZH, Radioti A, Grodent D, et al., 2018, Recurrent Magnetic Dipolarization at Saturn: Revealed by Cassini, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 8502-8517, ISSN: 2169-9380
Khurana KK, Dougherty MK, Provan G, et al., 2018, Discovery of atmospheric-wind-driven electric currents in Saturn's magnetosphere in the gap between Saturn and its rings, Geophysical Research Letters, Vol: 45, Pages: 10068-10074, ISSN: 0094-8276
Magnetic field observations obtained by the Cassini spacecraft as it traversed regions inside of Saturn's D ring packed a genuine surprise. The azimuthal component of the magnetic field recorded a consistent positive perturbation with a strength of 15–25 nT near closest approach. The closest approaches were near the equatorial plane of Saturn and were distributed narrowly around local noon and brought the spacecraft to within 2,550 km of Saturn's cloud tops. Modeling of this perturbation shows that it is not of internal origin but is produced by external currents that couple the low‐latitude northern ionosphere to the low‐latitude southern ionosphere. The azimuthal perturbations diminish at higher latitudes on field lines that connect to Saturn's icy rings. The sense of the current system suggests that the southern feet of the field lines in the ionosphere leads their northern counterparts. We show that the observed field perturbations are consistent with a field‐aligned current whose strength is ~1 MA/radian, that is, comparable in strength to the planetary‐period‐oscillation‐related current systems observed in the auroral zone. We show that the Lorentz force in the ionosphere extracts momentum from the faster moving low‐latitude zonal belt and delivers it to the northern ionosphere. We further show that the electric current is generated when the two ends of a field line are embedded in zonal flows with differing wind speeds in the low‐latitude thermosphere. The wind‐generated currents dissipate 2 × 1011W of thermal power, similar to the input from the solar extreme ultraviolet flux in this region.
Dougherty MK, Cao H, Khurana KK, et al., 2018, Erratum for the Research Article “Saturn’s magnetic field revealed by the Cassini Grand Finale” by M. K. Dougherty, H. Cao, K. K. Khurana, G. J. Hunt, G. Provan, S. Kellock, M. E. Burton, T. A. Burk, E. J. Bunce, S. W. H. Cowley, M. G. Kivelson, C. T. Russell, D. J. Southwood, Science, Vol: 362, ISSN: 0036-8075
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