| 1. |
- Ågren, Karin, et al.
(author)
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Detection of currents and associated electric fields in Titan's ionosphere from Cassini data
- 2011
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116:4, s. A04313-
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Journal article (peer-reviewed)abstract
- We present observations from three Cassini flybys of Titan using data from the radio and plasma wave science, magnetometer and plasma spectrometer instruments. We combine magnetic field and cold plasma measurements with calculated conductivities and conclude that there are currents of the order of 10 to 100 nA m (2) flowing in the ionosphere of Titan. The currents below the exobase (similar to 1400 km) are principally field parallel and Hall in nature, while the Pedersen current is negligible in comparison. Associated with the currents are perpendicular electric fields ranging from 0.5 to 3 mu V m (1).
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| 2. |
- Ogawa, Y., et al.
(author)
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On the statistical relation between ion upflow and naturally enhanced ion-acoustic lines observed with the EISCAT Svalbard radar
- 2011
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A03313-
-
Journal article (peer-reviewed)abstract
- We have investigated characteristics of ion upflow and naturally enhanced ion-acoustic lines (NEIALs) based on the European Incoherent Scatter (EISCAT) Svalbard radar (ESR) data continuously obtained between March 2007 and February 2008. For the ion upflow study we have used approximately 78,000 field-aligned profiles obtained with the ESR. For the NEIAL study we have identified approximately 1500 NEIALs in the ESR data at altitudes between 100 and 500 km. The occurrence frequency of ion upflow shows two peaks, at about 0800 and 1300 magnetic local time (MLT), while only one strong peak is seen around 0900 MLT for NEIALs. The upward ion flux also has only one peak around 1100-1300 MLT. The occurrence frequency of ion upflow varies strongly over season. It is higher in winter than in summer, whereas NEIALs are more frequent in summer than in winter. NEIALs frequently occur under high geomagnetic activity and also high solar activity conditions. Approximately 10% of NEIALs in the F region ionosphere were accompanied by NEIALs in the E region (occurred at altitudes below 200 km). About half of the E region enhanced echoes did not have an F region counterpart. Upshifted NEIALs dominate in the E region whereas downshifted NEIALs are usually stronger above an altitude of 300 km. The high occurrence frequency of NEIALs in the prenoon region (0800-1000 MLT) might be associated with acceleration of thermal ions to suprathermal ones. At the same MLT and geomagnetic latitude suprathermal ions and broadband extremely low frequency (BBELF) wave activity have been observed, according to previous studies.
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| 3. |
- Ogawa, Y., et al.
(author)
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Relationship between auroral substorm and ion upflow in the nightside polar ionosphere
- 2013
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In: J GEOPHYS RES-SPACE. - 2169-9380. ; 118:11, s. 7426-7437
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Journal article (peer-reviewed)abstract
- We investigated ionospheric ion upflow during an auroral substorm using simultaneous European Incoherent Scatter radar and IMAGE satellite data. Approximately 6 min after an initial brightening identified with data from the IMAGE wideband imaging camera instrument, ion upflow was seen and the electron temperature became enhanced, too. The ion upflow, with a velocity of about 150 m/s, and the electron temperature enhancement lasted for about 25 min. During the poleward expansion phase, surges of large upward ion velocity and flux, and high ion and electron temperatures occurred over Longyearbyen. The upward ion flux reached 2x10(14) m(-2)s(-1). Naturally enhanced ion-acoustic lines (NEIALs) were seen near the poleward edge of the expanded auroral oval both near the end of expansion phase 17 min after onset and also later in the recovery phase. The NEIALs seemed to be accompanied by another type of enhanced echoes, obliquely to the local geomagnetic field. Data from the Low Energy Neutral Atom instrument on the IMAGE satellite show that energetic neutral oxygen reaches the IMAGE satellite about 40 min after the initial brightening, and oxygen continues to get detected during the recovery phase. We propose that ion upflow at the poleward edge of the auroral oval during the expansion phase is related to ion/neutral outflow with energy below 18-27 eV, whereas during the recovery phase of a substorm upward ions are accelerated up to about 60 eV and flow out in the entire polar region.
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| 4. |
- Ogawa, Y., et al.
(author)
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Solar activity dependence of ion upflow in the polar ionosphere observed with the European Incoherent Scatter (EISCAT) Tromso UHF radar
- 2010
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 115, s. A07310-
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Journal article (peer-reviewed)abstract
- The influence of solar activity upon ion upflow in the polar ionosphere was investigated using data obtained by the European Incoherent Scatter (EISCAT) Tromso UHF radar between 1984 and 2008. In agreement with other work we find that the upward ion flux is generally higher when solar activity is high than when it is low. Ion upflow events and also the upward velocity behave the opposite: they are more frequently seen and higher, respectively, at times of low solar activity. In any year about 30-40% ion upflow is accompanied by similar to 500 K higher electron temperature than the background temperature at 400 km altitude. Electron and ion heating in connection with upflow is nearly twice as prevalent during high solar activity as it is at low activity. The acceleration of ions by pressure gradients and ambipolar electric field becomes larger when solar activity is low than when it is high. This variation of the average acceleration is caused by the different shapes of electron density profiles for low and high solar activities. Ions start to flow up at above 450 km altitude when solar activity was high, and lower, at 300-500 km altitude, at low solar activity. It is suggested that the solar activity influences long-term variations of the ion upflow occurrence because it modulates the density of neutral particles, the formation of the F2 density peak, and ion-neutral collision frequencies in the thermosphere and ionosphere.
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