| 1. |
- Dieval, C., et al.
(author)
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MARSIS remote sounding of localized density structures in the dayside Martian ionosphere : A study of controlling parameters
- 2015
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In: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 120:9, s. 8125-8145
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Journal article (peer-reviewed)abstract
- Enhanced topside electron densities in the dayside Martian ionosphere have been repetitively observed in areas of near-radial crustal magnetic fields, for periods of tens of days, indicating their long-term spatial and temporal stability despite changing solar wind conditions. We perform a statistical study of these density structures using the ionospheric mode of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) onboard Mars Express. We estimate the apparent extents of these structures relative to the altitude of the surrounding ionosphere. The apex of the density structures often lies higher than the surrounding ionosphere (median vertical extent of 18km), which indicates upwellings. These structures are much wider than they are high, with latitudinal scales of several degrees. The radar reflector regions are observed above both moderate and strong magnetic anomalies, and their precise locations and latitudinal extents match quite well with the locations and latitudinal extents of magnetic structures of given magnetic polarity (oblique to vertical fields), which happen to be regions where the field lines are open part of the time. The majority of the density structures occur in regions where ionospheric plasma is dominant, indicating closed field regions shielded from shocked solar wind plasma.
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| 2. |
- Andrews, David, et al.
(author)
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Control of the topside Martian ionosphere by crustal magnetic fields
- 2015
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In: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 120:4, s. 3042-3058
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Journal article (peer-reviewed)abstract
- We present observations from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument onboard Mars Express of the thermal electron plasma density of the Martian ionosphere and investigate the extent to which it is influenced by the presence of Mars's remnant crustal magnetic fields. We use locally measured electron densities, derived when MARSIS is operating in active ionospheric sounding (AIS) mode, covering an altitude range from approximate to 300km to approximate to 1200km. We compare these measured densities to an empirical model of the dayside ionospheric plasma density in this diffusive transport-dominated regime. We show that small spatial-scale departures from the averaged values are strongly correlated with the pattern of the crustal fields. Persistently elevated densities are seen in regions of relatively stronger crustal fields across the whole altitude range. Comparing these results with measurements of the (scalar) magnetic field also obtained by MARSIS/AIS, we characterize the dayside strength of the draped magnetic fields in the same regions. Finally, we provide a revised empirical model of the plasma density in the Martian ionosphere, including parameterizations for both the crustal field-dominated and draping-dominated regimes.
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| 3. |
- Andrews, David J., et al.
(author)
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MARSIS Observations of Field-Aligned Irregularities and Ducted Radio Propagation in the Martian Ionosphere
- 2018
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In: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 123:8, s. 6251-6263
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Journal article (peer-reviewed)abstract
- Knowledge of Mars's ionosphere has been significantly advanced in recent years by observations from Mars Express and lately Mars Atmosphere and Volatile EvolutioN. A topic of particular interest are the interactions between the planet's ionospheric plasma and its highly structured crustal magnetic fields and how these lead to the redistribution of plasma and affect the propagation of radio waves in the system. In this paper, we elucidate a possible relationship between two anomalous radar signatures previously reported in observations from the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument on Mars Express. Relatively uncommon observations of localized, extreme increases in the ionospheric peak density in regions of radial (cusp-like) magnetic fields and spread echo radar signatures are shown to be coincident with ducting of the same radar pulses at higher altitudes on the same field lines. We suggest that these two observations are both caused by a high electric field (perpendicular to B) having distinctly different effects in two altitude regimes. At lower altitudes, where ions are demagnetized and electrons magnetized, and recombination dominantes, a high electric field causes irregularities, plasma turbulence, electron heating, slower recombination, and ultimately enhanced plasma densities. However, at higher altitudes, where both ions and electrons are magnetized and atomic oxygen ions cannot recombine directly, the high electric field instead causes frictional heating, a faster production of molecular ions by charge exchange, and so a density decrease. The latter enables ducting of radar pulses on closed field lines, in an analogous fashion to interhemispheric ducting in the Earth's ionosphere.
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| 4. |
- Andrews, David J., et al.
(author)
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Oblique reflections in the Mars Express MARSIS data set : Stable density structures in the Martian ionosphere
- 2014
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In: Journal of Geophysical Research-Space Physics. - 2169-9380. ; 119:5, s. 3944-3960
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Journal article (peer-reviewed)abstract
- The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) onboard the European Space Agency's Mars Express (MEX) spacecraft routinely detects evidence of localized plasma density structures in the Martian dayside ionosphere. Such structures, likely taking the form of spatially extended elevations in the plasma density at a given altitude, give rise to oblique reflections in the Active Ionospheric Sounder data. These structures are likely related to the highly varied Martian crustal magnetic field. In this study we use the polar orbit of MEX to investigate the repeatability of the ionospheric structures producing these anomalous reflections, examining data taken in sequences of multiple orbits which pass over the same regions of the Martian surface under similar solar illuminations, within intervals lasting tens of days. Presenting three such examples, or case studies, we show for the first time that these oblique reflections are often incredibly stable, indicating that the underlying ionospheric structures are reliably reformed in the same locations and with qualitatively similar parameters. The visibility, or lack thereof, of a given oblique reflection on a single orbit can generally be attributed to variations in the crustal field within the ionosphere along the spacecraft trajectory. We show that, within these examples, oblique reflections are generally detected whenever the spacecraft passes over regions of intense near-radial crustal magnetic fields (i.e., with a cusp-like configuration). The apparent stability of these structures is an important feature that must be accounted for in models of their origin.
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| 5. |
- 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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| 6. |
- Edberg, Niklas J. T., et al.
(author)
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Effects of Saturn's magnetospheric dynamics on Titan's ionosphere
- 2015
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In: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 120:10, s. 8884-8898
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Journal article (peer-reviewed)abstract
- We use the Cassini Radio and Plasma Wave Science/Langmuir probe measurements of the electron density from the first 110 flybys of Titan to study how Saturn's magnetosphere influences Titan's ionosphere. The data is first corrected for biased sampling due to varying solar zenith angle and solar energy flux (solar cycle effects). We then present results showing that the electron density in Titan's ionosphere, in the altitude range 1600-2400km, is increased by about a factor of 2.5 when Titan is located on the nightside of Saturn (Saturn local time (SLT) 21-03h) compared to when on the dayside (SLT 09-15 h). For lower altitudes (1100-1600km) the main dividing factor for the ionospheric density is the ambient magnetospheric conditions. When Titan is located in the magnetospheric current sheet, the electron density in Titan's ionosphere is about a factor of 1.4 higher compared to when Titan is located in the magnetospheric lobes. The factor of 1.4 increase in between sheet and lobe flybys is interpreted as an effect of increased particle impact ionization from approximate to 200eV sheet electrons. The factor of 2.5 increase in electron density between flybys on Saturn's nightside and dayside is suggested to be an effect of the pressure balance between thermal plus magnetic pressure in Titan's ionosphere against the dynamic pressure and energetic particle pressure in Saturn's magnetosphere.
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| 7. |
- 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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| 8. |
- Farrell, W. M., et al.
(author)
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An estimate of the dust pickup current at Enceladus
- 2014
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In: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 239, s. 217-221
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Journal article (peer-reviewed)abstract
- We demonstrate that the acceleration of submicron dust originating at Enceladus by a reduced co-rotating E-field is capable of creating a dust pickup current perpendicular to the magnetic field with values ranging from 3 to 15 kA (depending upon the effective grain charge). Such a current represents a new contribution to the total pickup current in the region. As such, we suggest that dust pickup currents, along with ion and electron pickup currents, are all active within the plume.
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| 9. |
- 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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| 10. |
- Farrell, W. M., et al.
(author)
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Ion trapping by dust grains : Simulation applications to the Enceladus plume
- 2017
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In: Journal of Geophysical Research - Planets. - : AMER GEOPHYSICAL UNION. - 2169-9097 .- 2169-9100. ; 122:4, s. 729-743
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Journal article (peer-reviewed)abstract
- Using a particle-in-cell electrostatic simulation, we examine the conditions that allow low-energy ions, like those produced in the Enceladus plume, to be attracted and trapped within the sheaths of negatively charged dust grains. The conventional wisdom is that all new ions produced in the Enceladus plume are free to get picked up (i.e., accelerated by the local E field to then undergo vB acceleration). However, we suggest herein that the presence of submicron-charged dust in the plume impedes this pickup process since the local grain electric field greatly exceeds the corotation E fields. The simulations demonstrate that cold ions will tend to accelerate toward the negatively charged grains and become part of the ion plasma sheath. These trapped ions will move with the grains, exiting the plume region at the dust speed. We suggest that Cassini's Langmuir probe is measuring the entire ion population (free and trapped ions), while the Cassini magnetometer detects the magnetic perturbations associated with pickup currents from the smaller population of free ions, with this distinction possibly reconciling the ongoing debate in the literature on the ion density in the plume.
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