| 1. |
- Andrews, David J., et al.
(författare)
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The Structure of Planetary Period Oscillations in Saturn's Equatorial Magnetosphere : Results From the Cassini Mission
- 2019
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Ingår i: Journal of Geophysical Research - Space Physics. - : AMER GEOPHYSICAL UNION. - 2169-9380 .- 2169-9402. ; 124:11, s. 8361-8395
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Tidskriftsartikel (refereegranskat)abstract
- Saturn's magnetospheric magnetic field, planetary radio emissions, plasma populations, and magnetospheric structure are all known to be modulated at periods close to the assumed rotation period of the planetary interior. These oscillations are readily apparent despite the high degree of axisymmetry in the internally produced magnetic field of the planet and have different rotation periods in the northern and southern hemispheres. In this paper we study the spatial structure of (near-)planetary period magnetic field oscillations in Saturn's equatorial magnetosphere. Extending previous analyses of these phenomena, we include all suitable data from the entire Cassini mission during its orbital tour of the planet so as to be able to quantify both the amplitude and phase of these field oscillations throughout Saturn's equatorial plane, to distances of 30 planetary radii. We study the structure of these field oscillations in view of both independently rotating northern and southern systems, finding spatial variations in both magnetic fields and inferred currents flowing north-south that are common to both systems. With the greatly expanded coverage of the equatorial plane achieved during the latter years of the mission, we are able to present a complete survey of dawn-dusk and day-night asymmetries in the structure of the oscillating fields and currents. We show that the general structure of the rotating currents is simpler than previously reported and that the relatively enhanced nightside equatorial fields and currents are due in part to related periodic vertical motion of Saturn's magnetotail current sheet.
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| 2. |
- Backrud-Ivgren, Marie, et al.
(författare)
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Cluster observations and theoretical identification of broadband waves in the auroral region
- 2005
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 23:12, s. 3739-3752
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Tidskriftsartikel (refereegranskat)abstract
- Broadband waves are common on auroral field lines. We use two different methods to study the polarization of the waves at 10 to 180 Hz observed by the Cluster spacecraft at altitudes of about 4 Earth radii in the nightside auroral region. Observations of electric and magnetic wave fields, together with electron and ion data, are used as input to the methods. We find that much of the wave emissions are consistent with linear waves in homogeneous plasma. Observed waves with a large electric field perpendicular to the geomagnetic field are more common (electrostatic ion cyclotron waves), while ion acoustic waves with a large parallel electric field appear in smaller regions without suprathermal (tens of eV) plasma. The regions void of suprathermal plasma are interpreted as parallel potential drops of a few hundred volts.
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| 3. |
- Blomberg, Lars, et al.
(författare)
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Electric Field Diagnostics in the Jovian System : Brief Scientific Case and Instrumentation Overview
- 2006
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Ingår i: Proceedings of the 6th IAA International Conference on Low-Cost Planetary Missions. ; , s. 335-340
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The Jovian plasma environment exhibits a variety of plasma flow interactions with magnetised as well as unmagnetised bodies, making it a good venue for furthering our understanding of solar wind - magnetosphere / ionosphere interactions. On an overall scale the solar wind interacts with the Jovian magnetosphere, much like at Earth but with vastly different temporal and spatial scales. Inside the Jovian magnetosphere the co-rotating plasma interacts with the inner moons. The latter interaction is slower and more stable than the corresponding interaction between the solar wind and the planets, and can thus provide additional information on the principles of the interaction mechanisms. Because of the wealth of expected low-frequency waves, as well as the predicted quasi-static electric fields and plasma drifts in the interaction regions between different parts of the Jovian system, a most valuable component in future payloads would be a double-probe electric field instrument. Recent developments in low-mass instrumentation facilitate electric field measurements on spinning planetary spacecraft, which we here exemplify.
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| 4. |
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| 5. |
- Chatain, A., et al.
(författare)
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Re-Analysis of the Cassini RPWS/LP Data in Titan's Ionosphere : 2. Statistics on 57 Flybys
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 126:8
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Tidskriftsartikel (refereegranskat)abstract
- The ionosphere of Titan hosts a complex ion chemistry leading to the formation of organic dust below 1,200 km. Current models cannot fully explain the observed electron temperature in this dusty environment. To achieve new insight, we have re-analyzed the data taken in the ionosphere of Titan by the Cassini Langmuir probe (LP), part of the Radio and Plasma Wave Science package. A first paper (Chatain et al., 2021) introduces the new analysis method and discusses the identification of four electron populations produced by different ionization mechanisms. In this second paper, we present a statistical study of the whole LP dataset below 1,200 km which gives clues on the origin of the four populations. One small population is attributed to photo- or secondary electrons emitted from the surface of the probe boom. A second population is systematically observed, at a constant density (similar to 500 cm(-3)), and is attributed to background thermalized electrons from the ionization process of precipitating particles from the surrounding magnetosphere. The two last populations increase in density with pressure, solar illumination and Extreme ultraviolet flux. The third population is observed with varying densities at all altitudes and solar zenith angles (SZA) except on the far nightside (SZA > similar to 140 degrees), with a maximum density of 2,700 cm(-3). It is therefore certainly related to the photo-ionization of the atmospheric molecules. Finally, a fourth population detected only on the dayside and below 1,200 km reaching up to 2000 cm(-3) could be photo- or thermo-emitted from dust grains.
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| 6. |
- Chatain, A., et al.
(författare)
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Re-Analysis of the Cassini RPWS/LP Data in Titan's Ionosphere : 1. Detection of Several Electron Populations
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 126:8
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Tidskriftsartikel (refereegranskat)abstract
- Current models of Titan's ionosphere have difficulties in explaining the observed electron density and/or temperature. In order to get new insights, we re-analyzed the data taken in the ionosphere of Titan by the Cassini Langmuir probe (LP), part of the Radio and Plasma Wave Science (RPWS) instrument. This is the first of two papers that present the new analysis method (current paper) and statistics on the whole data set. We suggest that between two and four electron populations are necessary to fit the data. Each population is defined by a potential, an electron density and an electron temperature and is easily visualized by a distinct peak in the second derivative of the electron current, which is physically related to the electron energy distribution function (Druyvesteyn method). The detected populations vary with solar illumination and altitude. We suggest that the four electron populations are due to photo-ionization, magnetospheric particles, dusty plasma and electron emission from the probe boom, respectively.
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| 7. |
- Cravens, T. E., et al.
(författare)
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Plasma Transport in Saturn's Low-Latitude Ionosphere : Cassini Data
- 2019
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 124:6, s. 4881-4888
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Tidskriftsartikel (refereegranskat)abstract
- In 2017 the Cassini Orbiter made the first in situ measurements of the upper atmosphere and ionosphere of Saturn. The Ion and Neutral Mass Spectrometer in its ion mode measured densities of light ion species (H+, H-2(+), H-3(+), and He+), and the Radio and Plasma Wave Science instrument measured electron densities. During proximal orbit 287 (denoted P287), Cassini reached down to an altitude of about 3,000 km above the 1 bar atmospheric pressure level. The topside ionosphere plasma densities measured for P287 were consistent with ionospheric measurements during other proximal orbits. Spacecraft potentials were measured by the Radio and Plasma Wave Science Langmuir probe and are typically about negative 0.3 V. Also, for this one orbit, Ion and Neutral Mass Spectrometer was operated in an instrument mode allowing the energies of incident H+ ions to be measured. H+ is the major ion species in the topside ionosphere. Ion flow speeds relative to Saturn's atmosphere were determined. In the southern hemisphere, including near closest approach, the measured ion speeds were close to zero relative to Saturn's corotating atmosphere, but for northern latitudes, southward ion flow of about 3 km/s was observed. One possible interpretation is that the ring shadowing of the southern hemisphere sets up an interhemispheric plasma pressure gradient driving this flow.
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| 8. |
- Dreyer, Joshua, et al.
(författare)
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Constraining the Positive Ion Composition in Saturn's Lower Ionosphere with the Effective Recombination Coefficient
- 2021
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Ingår i: The Planetary Science Journal. - : American Astronomical Society. - 2632-3338. ; 2:1
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Tidskriftsartikel (refereegranskat)abstract
- The present study combines Radio and Plasma Wave Science/Langmuir Probe and Ion and Neutral Mass Spectrometer data from Cassini's last four orbits into Saturn's lower ionosphere to constrain the effective recombination coefficient α300 from measured number densities and electron temperatures at a reference electron temperature of 300 K. Previous studies have shown an influx of ring material causes a state of electron depletion due to grain charging, which will subsequently affect the ionospheric chemistry. The requirement to take grain charging into account limits the derivation of α300 to upper limits. Assuming photochemical equilibrium and using an established method to calculate the electron production rate, we derive upper limits for α300 of ≲ 3 × 10−7 cm3 s−1 for altitudes below 2000 km. This suggests that Saturn's ionospheric positive ions are dominated by species with low recombination rate coefficients like HCO+. An ionosphere dominated by water group ions or complex hydrocarbons, as previously suggested, is incompatible with this result, as these species have recombination rate coefficients > 5 × 10−7 cm3 s−1 at an electron temperature of 300 K.
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| 9. |
- Dreyer, Joshua, 1993-
(författare)
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Diving Deep into Saturn's Equatorial Ionosphere with Cassini : Insights from the Grand Finale
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the summer of 2017, the Cassini mission concluded its nearly 13 years orbiting Saturn with a series of daring dives between the rings and the upper reaches of Saturn's atmosphere. This last phase of the mission, called the Grand Finale, revealed a highly variable equatorial ionosphere dominated by a large influx of ring material from Saturn's D ring. The papers included in this thesis utilize data gathered during these proximal orbits to gain insights into the nature and effects of the infalling ring material.Initially, we derive upper limits for the effective recombination coefficient in Saturn's equatorial ionosphere at altitudes below 2500 km, where photochemical equilibrium can be assumed, to constrain the composition of the positive ion species. Our inceptive results indicate that ion species with low recombination coefficients are dominant.We follow up on this by developing a photochemical model, incorporating grain charging, to investigate the effects of the ring influx on the plasma composition. The model results at an altitude of 1700 km yield vastly different abundances of two types of neutral species when compared to those derived from measurements, ultimately representing the difficulty of reconciling the observed H+ and H3+ densities with our and other model results.Exploring the nature of narrow decreases in the ionospheric H2+ densities reveals a time shift in the ion data. After correcting for this, the decreases line up very well with calculated shadows for substructures in Saturn's C ring. We can further estimate the optical depths of these substructures and investigate at which altitudes photochemical equilibrium for H2+ is applicable.The direct measurement of heavier neutral species during the proximal orbits is complicated by the high spacecraft speed. We devise a method to utilize helium ion chemistry to independently derive the mixing ratios of these heavier neutrals in Saturn's ionosphere. Our results show considerable variability, which may suggest temporal and/or spatial changes in the ring influx. A comparison with other studies indicates that potentially only the most volatile ring-sourced species significantly ablate to enter the gas phase in this region of Saturn's ionosphere.Finally, we compare the fixed-bias Langmuir probe electron densities and the light ion densities. They exhibit a strong positive correlation for most parts of the proximal orbits even on short timescales. We find three distinct regions in the proximal orbits, which can provide further insight into the ionospheric composition, connection to the rings, and measurement uncertainties.
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| 10. |
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