| 1. |
- Sanchez-Cano, B., et al.
(author)
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Total electron content in the Martian atmosphere : A critical assessment of the Mars Express MARSIS data sets
- 2015
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In: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 120:3, s. 2166-2182
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Journal article (peer-reviewed)abstract
- The total electron content (TEC) is one of the most useful parameters to evaluate the behavior of the Martian ionosphere because it contains information on the total amount of free electrons, the main component of the Martian ionospheric plasma. The Mars Express Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) radar is able to derive TEC from both of its operation modes: (1) the active ionospheric sounding (AIS) mode and (2) the subsurface mode. TEC estimates from the subsurface sounding mode can be computed from the same raw data independently using different algorithms, which should yield similar results. Significant differences on the dayside, however, have been found from two of the algorithms. Moreover, both algorithms seem also to disagree with the TEC results from the AIS mode. This paper gives a critical, quantitative, and independent assessment of these discrepancies and indicates the possible uncertainty of these databases. In addition, a comparison between the results given by the empirical model of the Martian ionosphere developed by Sanchez-Cano et al. (2013) and the different data sets has been performed. The main result is that for solar zenith angles higher than 75 degrees, where the maximum plasma frequency is typically small compared with the radar frequencies, the two subsurface algorithms can be confidently used. For solar zenith angles less than 75 degrees, where the maximum plasma frequency is very close to the radar frequencies, both algorithms suffer limitations. Nevertheless, despite the solar zenith angle restrictions, the dayside TEC of one of the two algorithms is consistent with the modeled TEC.
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| 2. |
- Wahlund, J. E., et al.
(author)
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Detection of dusty plasma near the E-ring of Saturn
- 2009
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In: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 57:14-15, s. 1795-1806
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Journal article (peer-reviewed)abstract
- We present several independent in-situ measurements, which provide evidence that charged dust in the E-ring interacts collectively with the dense surrounding plasma disk of Saturn, i.e., form a system of dust-plasma interaction. The results are based on data sampled by the Radio and Plasma Wave Science (RPWS) investigation onboard Cassini, which allows for interferometry of plasma density inhomogeneities (delta n/n) with two antenna elements and a Langmuir probe sensor. The interferometer experiment detects two ion populations: one co-rotating with the planetary magnetic field and another moving with near Keplerian speed around Saturn. The full range of RPWS measurements indicates that the Keplerian population consists of colder ions (T-i
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| 3. |
- Engelhardt, Ilka. A. D., et al.
(author)
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Plasma regions, charged dust and field-aligned currents near Enceladus
- 2015
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In: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 117, s. 453-469
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Journal article (peer-reviewed)abstract
- We use data from several instruments on board Cassini to determine the characteristics of the plasma and dust regions around Saturn's moon Enceladus. For this we utilize the Langmuir probe and the electric antenna connected to the wideband receiver of the radio and plasma wave science (RPWS) instrument package as well as the magnetometer (MAG). We show that there are several distinct plasma and dust regions around Enceladus. Specifically they are the plume filled with neutral gas, plasma, and charged dust, with a distinct edge boundary region. Here we present observations of a new distinct plasma region, being a dust trail on the downstream side. This is seen both as a difference in ion and electron densities, indicating the presence of charged dust, and directly from the signals created on RPWS antennas by the dust impacts on the spacecraft. Furthermore, we show a very good scaling of these two independent dust density measurement methods over four orders of magnitude in dust density, thereby for the first time cross-validating them. To establish equilibrium with the surrounding plasma the dust becomes negatively charged by attracting free electrons. The dust distribution follows a simple power law and the smallest dust particles in the dust trail region are found to be 10 nm in size as well as in the edge region around the plume. Inside the plume the presence of even smaller particles of about 1 nm is inferred. From the magnetic field measurements we infer strong field-aligned currents at the geometrical edge of Enceladus.
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| 4. |
- Farrell, W. M., et al.
(author)
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Mass unloading along the inner edge of the Enceladus plasma torus
- 2008
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:2, s. L02203-
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Journal article (peer-reviewed)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.
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| 5. |
- Gurnett, D. A., et al.
(author)
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A plasmapause-like density boundary at high latitudes in Saturn's magnetosphere
- 2010
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 37, s. L16806-
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Journal article (peer-reviewed)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.
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| 6. |
- Morooka, Michiko, et al.
(author)
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Saturn's Dusty Ionosphere
- 2019
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In: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 124:3, s. 1679-1697
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Journal article (peer-reviewed)abstract
- Measurements of electrons and ions in Saturn's ionosphere down to 1,500-km altitudes as well as the ring crossing region above the ionosphere obtained by the Langmuir probe onboard the Cassini spacecraft are presented. Five nearly identical deep ionosphere flybys during the Grand Finale orbits and the Final plunge orbit revealed a rapid increase in the plasma densities and discrepancies between the electrons and ions densities (N-e and N-i) near the closest approach. The small N-e/N-i ratio indicates the presence of a dusty plasma, a plasma which charge carrier is dominated by negatively charged heavy particles. Comparison of the Langmuir probe obtained density with the light ion density obtained by the Ion and Neutral Mass Spectrometer confirmed the presence of heavy ions. An unexpected positive floating potential of the probe was also observed when N-e/N-i << 1. This suggests that Saturn's ionosphere near the density peak is in a dusty plasma state consisting of negatively and positively charged heavy cluster ions. The electron temperature (T-e) characteristics in the ionosphere are also investigated and unexpectedly high electron temperature value, up to 5000 K, has been observed below 2,500-km altitude in a region where electron-neutral collisions should be prominent. A well-defined relationship between T-e and N-e/N-i ratio was found, implying that the electron heating at low altitudes is related to the dusty plasma state of the ionosphere.
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| 7. |
- Ye, S. -Y, et al.
(author)
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Dust Observations by the Radio and Plasma Wave Science Instrument During Cassini's Grand Finale
- 2018
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In: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 45:19, s. 10101-10109
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Journal article (peer-reviewed)abstract
- Dust particles in the Saturn system can be detected by the Radio and Plasma Wave Science (RPWS) instrument on board Cassini via antenna voltage signals induced by dust impacts. These impact signals have been simulated in the laboratory by accelerating dust particles onto a Cassini model with electric field antennas. RPWS dust measurements have been shown to be consistent with the Cosmic Dust Analyzer. During the Grand Finale orbits, Cassini flew through the gap between the D ring and Saturn's atmosphere 22 times. In situ measurements by RPWS helped quantify the hazards posed to the spacecraft and instruments on board, which showed a micron-sized dust density orders of magnitude lower than that observed during the Ring Grazing orbits. Close inspection of the waveforms indicated a possible dependence of the impact signal decay time on ambient plasma density. Plain Language Summary Cassini flew through the gap between Saturn and its rings for 22 times before plunging into the atmosphere of Saturn, ending its 20-year mission. The radio and plasma waves instrument on board Cassini helped quantify the dust hazard in this previously unexplored region. The measured density of large dust particles was much lower than expected, allowing high-value science observations during the subsequent Grand Finale orbits.
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| 8. |
- Andrews, David J., et al.
(author)
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Plasma observations during the Mars atmospheric "plume" event of March-April 2012
- 2016
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In: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 121:4, s. 3139-3154
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Journal article (peer-reviewed)abstract
- We present initial analyses and conclusions from plasma observations made during the reported "Mars plume event" of March-April 2012. During this period, multiple independent amateur observers detected a localized, high-altitude "plume" over the Martian dawn terminator, the cause of which remains to be explained. The estimated brightness of the plume exceeds that expected for auroral emissions, and its projected altitude greatly exceeds that at which clouds are expected to form. We report on in situ measurements of ionospheric plasma density and solar wind parameters throughout this interval made by Mars Express, obtained over the same surface region but at the opposing terminator. Measurements in the ionosphere at the corresponding location frequently show a disturbed structure, though this is not atypical for such regions with intense crustal magnetic fields. We tentatively conclude that the formation and/or transport of this plume to the altitudes where it was observed could be due in part to the result of a large interplanetary coronal mass ejection (ICME) encountering the Martian system. Interestingly, we note that the only similar plume detection in May 1997 may also have been associated with a large ICME impact at Mars.
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| 9. |
- Cheng, J. C., et al.
(author)
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Adolescent idiopathic scoliosis
- 2015
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In: Nature Reviews Disease Primers. - : Springer Science and Business Media LLC. - 2056-676X. ; 1
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Journal article (peer-reviewed)abstract
- Adolescent idiopathic scoliosis (AIS) is the most common form of structural spinal deformities that have a radiological lateral Cobb angle - a measure of spinal curvature - of >= 10 degrees. AIS affects between 1% and 4% of adolescents in the early stages of puberty and is more common in young women than in young men. The condition occurs in otherwise healthy individuals and currently has no recognizable cause. In the past few decades, considerable progress has been made towards understanding the clinical patterns and the three-dimensional pathoanatomy of AIS. Advances in biomechanics and technology and their clinical application, supported by limited evidence-based research, have led to improvements in the safety and outcomes of surgical and non-surgical treatments. However, the definite aetiology and aetiopathogenetic mechanisms that underlie AIS are still unclear. Thus, at present, both the prevention of AIS and the treatment of its direct underlying cause are not possible.
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| 10. |
- Farrell, W. M., et al.
(author)
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Electron density dropout near Enceladus in the context of water-vapor and water-ice
- 2009
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 36:10, s. L10203-
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Journal article (peer-reviewed)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.
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