Plasma dynamics in Saturn's middle-latitude ionosphere and implications for magnetosphere-ionosphere coupling

Sakai, Shotaro; Watanabe, Shigeto

doi:10.1016/j.icarus.2016.03.009

Cited by 4 publications

(6 citation statements)

References 57 publications

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“…Heat transport by the magnetospheric plasma is commonly observed in the planetary ionospheres and moons (Edberg et al, ; Hoegy et al, ; McDonald & Williams, ; Nagy et al, ). In the ionospheric model by Sakai and Watanabe () the electron temperature T e was found to be heated up to 10 4 [K] at an altitude of 5,000 km due to the heat flow from the magnetospheric electrons with energy of 2 eV. This value is much larger than the observed value (~1500 K), and it requires less energy input via heat conduction from the magnetosphere to be consistent with our observation.…”

Section: The Electron Temperature In the Low‐altitude Ionospheresupporting

confidence: 87%

“…A positive probe floating potential has been found as N e /N i becomes smaller, which suggests that the positively and negatively charged heavy ions are the dominant charge carriers in Saturn's ionosphere. We present also the electron temperature characteristics and compare them with the models by Moore et al () and Sakai and Watanabe ().…”

Section: Introductionmentioning

confidence: 84%

“…This value is much larger than the observed value (~1500 K), and it requires less energy input via heat conduction from the magnetosphere to be consistent with our observation. Since Sakai and Watanabe () considers the ionosphere in the middle latitude (~50°), different temperature of the magnetospheric electrons in different L shell can exist here. In the left panel, the outbound temperatures (dots) seem to be ~0.05 eV warmer than the inbound temperatures (crosses).…”

Section: The Electron Temperature In the Low‐altitude Ionospherementioning

confidence: 99%

“…Their results predicted a maximum electron temperature of around 500 K in the altitude range 1,500–3,000 km. Sakai and Watanabe () used a parameterized heating/cooling rate to model the altitude profile of the electron temperature. Their model considered the heat flow from the magnetosphere in addition to the chemistry of the atmosphere and therefore predicted the temperatures of up to 10,000 K at high altitudes, above 6,000 km.…”

Section: Introductionmentioning

confidence: 99%

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Saturn's Dusty Ionosphere

et al. 2019

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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.Plain Language Summary Cassini Langmuir probe measurements revealed ion densities in excess of the electron densities, indicative of a dusty plasma, in Saturn's ionosphere below 2,500-km altitude. Comparison of the Langmuir probe measurements with those of the Ion and Neutral Mass Spectrometer, sensitive to only lighter ions during this period, showed that heavy ions dominate in this region. Positive spacecraft potentials were also found, suggesting that Saturn's ionosphere contains dusty plasma of negatively and positively charged heavy ions.

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Section: The Electron Temperature In the Low‐altitude Ionospheresupporting

confidence: 87%

Section: Introductionmentioning

confidence: 84%

Section: The Electron Temperature In the Low‐altitude Ionospherementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Saturn's Dusty Ionosphere

et al. 2019

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“…This poses a problem as long as a viable mechanism for enhanced H + production (lacking in the model) has not been identified. Theoretical predictions (Moore et al 2008;Sakai & Watanabe 2016) suggest that the electron temperature is closer to the neutral temperature at the pressure level of interest. It can be noted that a model run utilizing an electron temperature of T e = 370 K (≈neutral temperature; see, e.g., Yelle et al 2018) instead of 1500 K causes a reduction in the calculated n i from >10,000 cm −3 to ∼6000 cm −3 , which is a consequence of the fact that dissociative recombination is more efficient at lower T e .…”

Section: Grain Size Conundrummentioning

confidence: 94%

Empirical Photochemical Modeling of Saturn’s Ionization Balance Including Grain Charging

et al. 2022

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We present a semianalytical photochemical model of Saturn’s near-equatorial ionosphere and adapt it to two regions (∼2200 and ∼1700 km above the 1 bar level) probed during the inbound portion of Cassini’s orbit 292 (2017 September 9). The model uses as input the measured concentrations of molecular hydrogen, hydrogen ion species, and free electrons, as well as the measured electron temperature. The output includes upper limits, or constraints, on the mixing ratios of two families of molecules, on ion concentrations, and on the attachment rates of electrons and ions onto dust grains. The model suggests mixing ratios of the two molecular families that, particularly near ∼1700 km, differ notably from what independent measurements by the Ion Neutral Mass Spectrometer suggest. Possibly connected to this, the model suggests an electron-depleted plasma with a level of electron depletion of around 50%. This is in qualitative agreement with interpretations of Radio Plasma Wave Science/Langmuir Probe measurements, but an additional conundrum arises in the fact that a coherent photochemical equilibrium scenario then relies on a dust component with typical grain radii smaller than 3 Å.

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Cassini Radio Occultation Observations of Saturn's Ionosphere: Electron Density Profiles From 2005 to 2013

et al. 2023

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A set of electron density profiles of Saturn's ionosphere acquired by Cassini radio occultations is archived at the NASA Planetary Data System. However, the reference surface that defines zero altitude in these profiles is unknown and appears to vary by 1,500 km at fixed latitude. These profiles are immensely valuable for addressing questions pertaining to the vertical structure, meridional structure, diurnal variations, or solar cycle dependence of Saturn's ionosphere, but their value is severely limited by their questionable altitude scales. Here we have resolved this problem by independently generating the set of 60 electron density profiles. These profiles confirm that, as noted by previous authors, the structure of Saturn's ionosphere is highly variable. Nevertheless, the profiles suggest an underlying morphology of a broad layer at relatively high altitude (often at approximately 2,000–3,000 km altitude) and a series of narrower layers at lower altitude (often at approximately 1,000 km altitude). The vertical structure of Saturn's ionosphere depends on latitude and local time. At low latitudes, densities are greater at dusk than at dawn. Conversely, at mid latitudes, densities are greater at dawn than at dusk. The plasma scale height of the topside ionosphere is relatively small at low latitudes. The high, broad ionospheric layer is apparent at mid and high latitudes at both dawn and dusk, but is not present at low latitudes at either dawn or dusk. Total electron content also shows a strong dependence on latitude, with high latitudes having greater values than low and mid latitudes.

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Plasma dynamics in Saturn's middle-latitude ionosphere and implications for magnetosphere-ionosphere coupling

Cited by 4 publications

References 57 publications

Saturn's Dusty Ionosphere

Saturn's Dusty Ionosphere

Empirical Photochemical Modeling of Saturn’s Ionization Balance Including Grain Charging

Cassini Radio Occultation Observations of Saturn's Ionosphere: Electron Density Profiles From 2005 to 2013

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