| 1. |
- Bertucci, C., et al.
(författare)
-
Structure of Titan's mid-range magnetic tail : Cassini magnetometer observations during the T9 flyby
- 2007
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 34:24, s. L24S02-
-
Tidskriftsartikel (refereegranskat)abstract
- We analyze the magnetic structure of Titan's mid-range magnetic tail (5-6 Titan radii downstream from the moon) during Cassini's T9 flyby. Cassini magnetometer (MAG) measurements reveal a well-defined, induced magnetic tail consisting of two lobes and a distinct central current sheet. MAG observations also indicate that Saturn's background magnetic field is close to the moon's orbital plane and that the magnetospheric flow has a significant component in the Saturn-Titan direction. The analysis of MAG data in a coordinate system based on the orientation of the background magnetic field and an estimation of the incoming flow direction suggests that Titan's magnetic tail is extremely asymmetric. An important source of these asymmetries is the connection of the inbound tail lobe and the outbound tail lobe to the dayside and nightside hemispheres of Titan, respectively. Another source could be the perturbations generated by changes in the upstream conditions.
|
|
| 2. |
- Bertucci, C., et al.
(författare)
-
Titan's interaction with the supersonic solar wind
- 2015
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 42:2, s. 193-200
-
Tidskriftsartikel (refereegranskat)abstract
- After 9years in the Saturn system, the Cassini spacecraft finally observed Titan in the supersonic and super-Alfvenic solar wind. These unique observations reveal that Titan's interaction with the solar wind is in many ways similar to unmagnetized planets Mars and Venus and active comets in spite of the differences in the properties of the solar plasma in the outer solar system. In particular, Cassini detected a collisionless, supercritical bow shock and a well-defined induced magnetosphere filled with mass-loaded interplanetary magnetic field lines, which drape around Titan's ionosphere. Although the flyby altitude may not allow the detection of an ionopause, Cassini reports enhancements of plasma density compatible with plasma clouds or streamers in the flanks of its induced magnetosphere or due to an expansion of the induced magnetosphere. Because of the upstream conditions, these observations may be also relevant to other bodies in the outer solar system such as Pluto, where kinetic processes are expected to dominate.
|
|
| 3. |
- Cravens, T. E., et al.
(författare)
-
The Ion Composition of Saturn's Equatorial Ionosphere as Observed by Cassini
- 2019
-
Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 46:12, s. 6315-6321
-
Tidskriftsartikel (refereegranskat)abstract
- The Cassini Orbiter made the first in situ measurements of the upper atmosphere and ionosphere of Saturn in 2017. The Ion and Neutral Mass Spectrometer (INMS) found molecular hydrogen and helium as well as minor species including water, methane, ammonia, and organics. INMS ion mode measurements of light ion species (H+, H-2(+), H-3(+), and He+) and Radio and Plasma Wave Science instrument measurements of electron densities are presented. A photochemical analysis of the INMS and Radio and Plasma Wave Science data indicates that the major ion species near the ionospheric peak must be heavy and molecular with a short chemical lifetime. A quantitative explanation of measured H+ and H-3(+) densities requires that they chemically react with one or more heavy neutral molecular species that have mixing ratios of about 100 ppm.
|
|
| 4. |
- Farrell, W. M., et al.
(författare)
-
Electron density dropout near Enceladus in the context of water-vapor and water-ice
- 2009
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 36:10, s. L10203-
-
Tidskriftsartikel (refereegranskat)abstract
- On 12 March 2008, the Cassini spacecraft made a close encounter with the Saturnian moon Enceladus, passing within 52 km of the moon. The spacecraft trajectory was intentionally-oriented in a southerly direction to create a close alignment with the intense water-dominated plumes emitted from the south polar region. During the passage, the Cassini Radio and Plasma Wave System (RPWS) detected two distinct radio signatures: 1) Impulses associated with small water-ice dust grain impacts and 2) an upper hybrid (UH) resonance emission that both intensified and displayed a sharp frequency decrease in the near-vicinity of the moon. The frequency decrease of the UH emission is associated with an unexpectedly sharp decrease in electron density from similar to 90 cl/cm(3) to below 20 cl/cm(3) that occurs on a time scale of a minute near the closest encounter with the moon. In this work, we consider a number of scenarios to explain this sharp electron dropout, but surmise that electron absorption by ice grains is the most likely process.
|
|
| 5. |
- Farrell, W. M., et al.
(författare)
-
Mass unloading along the inner edge of the Enceladus plasma torus
- 2008
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:2, s. L02203-
-
Tidskriftsartikel (refereegranskat)abstract
- A major discovery made by the Cassini spacecraft at Saturn was the substantial mass ejection from the south pole of Enceladus. Previous studies show that this ejected gas can become ionized and subsequently load mass onto the connecting magnetic field lines near the moon. Radial diffusion then allows the mass-loaded field lines to move outward to similar to 15 R-s and inward to similar to 2 R-s, forming a plasma torus. We demonstrate herein that the mass is also '' unloaded '' along the inner edge of this plasma torus the edge incident with the plasma-absorbing A-ring. Interpreting down-drifting z-mode tones from active sites along the inner edge of the ion torus as emission near the local electron plasma frequency, f(pe), we can remotely-monitor this reduction in plasma density along the torus inner edge as a function time. We find that the down-drift of the z-mode tones corresponds typically to a plasma density change dn/dt similar to - 5x10(-4)/cm(3)-s and when integrated over an annulus defined by the outer edge of the A-ring, corresponds to a mass loss of similar to 40 kg/s. Using the z-mode tones, we also find locations where plasma mass from the ring-ionosphere is possibly loaded at 1 - 2 kg/s onto field lines near the Cassini gap.
|
|
| 6. |
- Farrell, W. M., et al.
(författare)
-
Modification of the plasma in the near-vicinity of Enceladus by the enveloping dust
- 2010
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 37:20, s. L20202-
-
Tidskriftsartikel (refereegranskat)abstract
- The plasma near Saturn's equator is quasi-corotating, but those fluid elements entering the near-vicinity of the moon Enceladus become uniquely modified. Besides the solid body, the Moon has a surrounding dust envelop that we show herein to be detected similar to 20 Enceladus radii (1 R-E = 252 km) both north and south of the body. Previous reports indicate that corotating plasma slows down substantially in the near-vicinity of Enceladus. We show herein that the commencement of this plasma slow down matches closely with Cassini's entry into the dense portions of the enveloping dust in the northern hemisphere above the Moon. We also examine in detail the source of the dust about 400 km above the south polar fissures. We find that a large positive potential must exist between the south pole of the moon and the spacecraft to account for ions streaming away from the pole on connecting magnetic field lines.
|
|
| 7. |
- Farrell, W. M., et al.
(författare)
-
Saturn's Plasma Density Depletions Along Magnetic Field Lines Connected to the Main Rings
- 2018
-
Ingår i: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 45:16, s. 8104-8110
-
Tidskriftsartikel (refereegranskat)abstract
- We report on a set of clear and abrupt decreases in the high-frequency boundary of whistler mode emissions detected by Cassini at high latitudes (about +/- 40 degrees) during the low-altitude proximal flybys of Saturn. These abrupt decreases or dropouts have start and stop locations that correspond to L shells at the edges of the A and B rings. Langmuir probe measurements can confirm, in some cases, that the abrupt decrease in the high-frequency whistler mode boundary is associated with a corresponding abrupt electron density dropout over evacuated field lines connected to the A and B rings. Wideband data also reveal electron plasma oscillations and whistler mode cutoffs consistent with a low-density plasma in the region. The observation of the electron density dropout along ring-connecting field lines suggests that strong ambipolar forces are operating, drawing cold ionospheric ions outward to fill the flux tubes. There is an analog with the refilling of flux tubes in the terrestrial plasmasphere. We suggest that the ring-connected electron density dropouts observed between 1.1 and 1.3 R-s are connected to the low-density ring plasma cavity observed overtop the A and B rings during the 2004 Saturn orbital insertion pass.Plain Language Summary We present Cassini observations during the close passes by the planet Saturn indicating that plasma on magnetic field lines that pass through the A and B rings is of anomalously low density. These observations are consistent with the Saturn orbit insertion observations of a plasma cavity located at equatorial regions overtop the dense B ring. Using a terrestrial analogy, we suggest that the low-density conditions overtop the rings create an electrical force, called an ambipolar electric field that draws plasma out of the ionosphere in an attempt to replenish the plasma void found at equatorial regions.
|
|
| 8. |
- Gurnett, D. A., et al.
(författare)
-
A plasmapause-like density boundary at high latitudes in Saturn's magnetosphere
- 2010
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 37, s. L16806-
-
Tidskriftsartikel (refereegranskat)abstract
- Here we report the discovery of a well-defined plasma density boundary at high latitudes in Saturn's magnetosphere. The boundary separates a region of relatively high density at L less than about 8 to 15 from a region with densities nearly three orders of magnitude lower at higher L values. Magnetic field measurements show that strong field-aligned currents, probably associated with the aurora, are located just inside the boundary. Analyses of the anisotropy of energetic electrons show that the magnetic field lines are usually closed inside the boundary and open outside the boundary, although exceptions sometimes occur. The location of the boundary is also modulated at the similar to 10.6 to 10.8 hr rotational period of the planet. Many of these characteristics are similar to those predicted by Brice and Ioannidis for the plasmapause at a strongly magnetized, rapidly rotating planet such as Saturn.
|
|
| 9. |
- Gurnett, D. A., et al.
(författare)
-
The rotation of the plasmapause-like boundary at high latitudes in Saturn's magnetosphere and its relation to the eccentric rotation of the northern and southern auroral ovals
- 2011
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 38, s. L21203-
-
Tidskriftsartikel (refereegranskat)abstract
- Here we present a study of the rotation of the plasmapause-like density boundary discovered by the Cassini spacecraft at high latitudes in the Saturnian magnetosphere, and compare the results with previously published studies of high-latitude magnetic field perturbations and the eccentric rotation of the auroral ovals. Near the planet the density boundary is located at dipole L values ranging from about 8 to 15, and separates a region of very low densities at high latitudes from a region of higher densities at lower latitudes. We show that the density boundary rotates at different rates in the northern and southern hemispheres, and that the periods are the same as the modulation periods of Saturn kilometric radiation in those hemispheres. We also show that the phase of rotation in a given hemisphere is closely correlated with the phase of the high-latitude magnetic field perturbations observed by Cassini in that hemisphere, and also with the phase of the eccentric rotation of the auroral oval observed by the Hubble Space Telescope.
|
|
| 10. |
- Hadid, Lina Z, et al.
(författare)
-
Ring Shadowing Effects on Saturn's Ionosphere : Implications for Ring Opacity and Plasma Transport
- 2018
-
Ingår i: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 45:19, s. 10084-10092
-
Tidskriftsartikel (refereegranskat)abstract
- We present new results obtained by the Radio and Plasma Wave Science Langmuir probe on board Cassini during the Grand Finale. The total direct current sampled by the Langmuir probe at negative bias voltage is used to study the effect of the ring shadows on the structure of the Kronian topside ionosphere. The D and C rings and the Cassini Division are confirmed to be optically thin to extreme ultraviolet solar radiation. However, different responses from the opaque A and B rings are observed. The edges of the A ring shadow are shown to match the A ring boundaries, unlike the B ring, which indicates variable responses to the B ring shadow. We show that the variable responses are due to the ionospheric plasma, more precisely to the longer chemical lifetime of H+ compared to H-2(+) and H-3(+), suggesting that the plasma is transported from the sunlit region to the shadowed one in the ionosphere. Plain Language Summary As Saturn's northern hemisphere experienced summer during the Grand Finale, the planet's northern dayside hemisphere and its rings were fully illuminated by the Sun. However, the southern hemisphere was partly obscured because of the shadows cast by the A and B rings. Using the in situ measurements of the Langmuir probe part of the Radio and Plasma Wave Science investigation on board the Cassini spacecraft, we study for the first time the effect of the ring shadows on Saturn's ionosphere. From the ring shadows signatures on the total ion current collected by the Langmuir probe, we show that the A and B rings are optically thicker (to the solar extreme ultraviolet radiation) than the inner C and D rings and the Cassini Division to the solar extreme ultraviolet radiation. Moreover, we reproduce the boundaries of the A ring and the outer edge of the B ring. Furthermore, observed variations with respect to the inner edge of the B ring imply a delayed response of the ionospheric H+ because of its long lifetime and suggest that the ionospheric plasma is transported from an unshadowed region to a shadowed one in the ionosphere.
|
|
