| 1. |
- Ogawa, Y., et al.
(författare)
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Relationship between auroral substorm and ion upflow in the nightside polar ionosphere
- 2013
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Ingår i: J GEOPHYS RES-SPACE. - 2169-9380. ; 118:11, s. 7426-7437
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Tidskriftsartikel (refereegranskat)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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| 2. |
- Motoba, T., et al.
(författare)
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In situ evidence for interplanetary magnetic field induced tail twisting associated with relative displacement of conjugate auroral features
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A04209-
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Tidskriftsartikel (refereegranskat)abstract
- We provide in situ evidence for a twisted near-Earth tail configuration that is responsible for the time sequence of conjugate auroral features associated with relative interhemispheric displacement during a weak substorm, as reported by Motoba et al. (2010). We analyzed the magnetic field data observed using four Cluster satellites in the vicinity of 11-14 R-E central downtail, in close conjunction with the Iceland-Syowa conjugate optical auroral features. Interestingly, we found that the variations in the magnetic field y component (B-y) at all satellites correlated moderately well with the variations in the time-shifted interplanetary magnetic field (IMF) clock angle (theta(CA)). The correlation coefficients (0.56 similar to 0.61) between the B-y field at Cluster and IMF theta(CA) peaked at a time delay of 52 +/- 1 min from the dayside magnetopause, probably corresponding to the time scale for the reconfiguration of the IMF theta(CA) related B-y field in the near-Earth tail. The IMF theta(CA) related B-y variation at Cluster, regarded as a manifestation of the twisting magnetotail configuration, also roughly coincided with the relative magnetic local time displacement of nightside conjugate auroral forms. These results provide strong evidence that the reconfiguration (twisting) process of the near-Earth tail on a relatively longer time scale controls the nightside conjugate auroral locations in both ionospheres.
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| 3. |
- Ogawa, Y., et al.
(författare)
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On the statistical relation between ion upflow and naturally enhanced ion-acoustic lines observed with the EISCAT Svalbard radar
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A03313-
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Ogawa, Y., et al.
(författare)
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Upper atmosphere cooling over the past 33 years
- 2014
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 41:15, s. 5629-5635
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Tidskriftsartikel (refereegranskat)abstract
- Theoretical models and observations have suggested that the increasing greenhouse gas concentration in the troposphere causes the upper atmosphere to cool and contract. However, our understanding of the long-term trends in the upper atmosphere is still quite incomplete, due to a limited amount of available and well-calibrated data. The European Incoherent Scatter radar has gathered data in the polar ionosphere above TromsO for over 33years. Using this long-term data set, we have estimated the first significant trends of ion temperature at altitudes between 200 and 450km. The estimated trends indicate a cooling of 10-15K/decade near the F region peak (220-380km altitude), whereas above 400km the trend is nearly zero or even warming. The height profiles of the observed trends are close to those predicted by recent atmospheric general circulation models. Our results are the first quantitative confirmation of the simulations and of the qualitative expectations.
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