17:15 〜 18:30
[PEM11-P24] Sporadic Fe layer event associated with vertical ion drift based on wind shear theory: simultaneous observation of Fe density and wind at Syowa station (69.0°S, 39.6°E) and simulation
キーワード：中間圏-下部熱圏、金属原子層、スポラディック層、共鳴散乱ライダー
Metallic layers, which ablate from meteoroids, are known to be formed between 80 and 105 km in the terrestrial mesopause region. Meteoric species such as Fe, Mg, and Na exist as atoms in the layers and their dynamical and chemical variability have been investigated by resonance scattering lidars [Plane et al., 2015 and references therein] and satellite-borne measurements. Sporadic E layer, Es layer, is characterized as a thin layer with enhanced electron density and mainly observed by incoherent scatter radars and ionosondes. Mg+ and Fe+ ions are regarded as dominant ion components in Es layers due to their long lifetime, and therefore sporadic metallic layers are believed to play important roles in forming Es layers. Suggested generation mechanisms of sporadic metallic layers in polar regions are mainly as follows: vertical ion converge and neutralization due to wind shear [e.g., Nygrén et al., 1984] and ionospheric electric field [e.g., Kirkwood and von Zahn, 1991].
We identified sporadic Fe, hereafter FeS, event on June 5, 2018 at Syowa station (69.0°S, 39.6°E), Antarctic, that was observed by a resonance scattering lidar. This FeS event can be summarized as follows: a center altitude and FWHM of the FeS layer are 90 km and 5 km, respectively. Duration was about 3 hours. Geomagnetic activity was quiet during this event and co-located ionosonde demonstrated intermittent Es activity. Apparent growth rate is 1.5 %/min implying that development of the FeS is quite slow. During the FeS event, neutral wind data with from an MF radar at Syowa is available. Meridional and zonal wind profiles at the moment of FeS peak density show strong horizontal wind shear (du/dz is positive and dv/dz is negative), which is consistent with Es layer forming in the southern hemisphere.
We tried to explain the observed FeS by wind shear theory since ionospheric electric field (less than 10 mV/m) during the FeS can be neglected. Ion-neutral collision frequency for Fe+ was estimated by an empirical model of Voiculescu and Ignat [2002], NRLMSISE-00, and IGRF-13. In addition, vertical ion velocity and its temporal variations were calculated in consideration of magnetic declination [Yu et al., 2019]. Magnetic declination angle at Syowa station is -51° and therefore should be taken into account. Estimated vertical ion velocity, wi, and vertical ion divergence, dwi/dz, were both negative near 12 UT. In particular, dwi/dz reached at -0.010 m/s/km, that is comparable to simulation result of Yu et al. [2019]. It is also consistent with strong Es (foEs ~ 5 MHz) observed by ionosonde at the moment. However, near 17 UT when FeS peak density was observed, both wi and dwi/dz were positive and favorable for vertical ion divergence. This implies that FeS peak was not caused by vertical ion velocity shear at this moment. Our analysis suggests that Fe+ converge associated with negative dwi/dz might take place about 4 hours earlier than FeS appearance and subsequently neutralization of Fe+ led to the observed FeS forming. Above 90 km Fe+ lifetime ranges from a few minutes to 105 s with altitudes [e.g., Plane et al., 2015]. It seems to be roughly consistent with the delayed appearance of FeS. In addition, we plan to discuss the observed results based on numerical calculations in Fe/Fe+ chemical reactions with vertical ion transportation.
We identified sporadic Fe, hereafter FeS, event on June 5, 2018 at Syowa station (69.0°S, 39.6°E), Antarctic, that was observed by a resonance scattering lidar. This FeS event can be summarized as follows: a center altitude and FWHM of the FeS layer are 90 km and 5 km, respectively. Duration was about 3 hours. Geomagnetic activity was quiet during this event and co-located ionosonde demonstrated intermittent Es activity. Apparent growth rate is 1.5 %/min implying that development of the FeS is quite slow. During the FeS event, neutral wind data with from an MF radar at Syowa is available. Meridional and zonal wind profiles at the moment of FeS peak density show strong horizontal wind shear (du/dz is positive and dv/dz is negative), which is consistent with Es layer forming in the southern hemisphere.
We tried to explain the observed FeS by wind shear theory since ionospheric electric field (less than 10 mV/m) during the FeS can be neglected. Ion-neutral collision frequency for Fe+ was estimated by an empirical model of Voiculescu and Ignat [2002], NRLMSISE-00, and IGRF-13. In addition, vertical ion velocity and its temporal variations were calculated in consideration of magnetic declination [Yu et al., 2019]. Magnetic declination angle at Syowa station is -51° and therefore should be taken into account. Estimated vertical ion velocity, wi, and vertical ion divergence, dwi/dz, were both negative near 12 UT. In particular, dwi/dz reached at -0.010 m/s/km, that is comparable to simulation result of Yu et al. [2019]. It is also consistent with strong Es (foEs ~ 5 MHz) observed by ionosonde at the moment. However, near 17 UT when FeS peak density was observed, both wi and dwi/dz were positive and favorable for vertical ion divergence. This implies that FeS peak was not caused by vertical ion velocity shear at this moment. Our analysis suggests that Fe+ converge associated with negative dwi/dz might take place about 4 hours earlier than FeS appearance and subsequently neutralization of Fe+ led to the observed FeS forming. Above 90 km Fe+ lifetime ranges from a few minutes to 105 s with altitudes [e.g., Plane et al., 2015]. It seems to be roughly consistent with the delayed appearance of FeS. In addition, we plan to discuss the observed results based on numerical calculations in Fe/Fe+ chemical reactions with vertical ion transportation.