Akira Sessai Yukimatu

A thermospheric Na layer event observed up to 140 km over Syowa Station (69.0°S, 39.6°E) in Antarctica

We report a thermospheric Na layer event (up to 140u2009km) observed by lidar in the night of 23–24 September 2000 at Syowa (69.0°S, 39.6°E), Antarctica. The thermospheric Na number densities were 2–9u2009cm−3 at 110–140u2009km, 3 orders of magnitude smaller than the peak density of the normal layer at 80–110u2009km. The thermospheric Na layers exhibited a wave-like structure with a period of 1–2u2009h. The colocated ionospheric/auroral observations showed sporadic E layers over Syowa through the night and an enhancement of the ionospheric/auroral activity around south side of Syowa at the event beginning. Adopting the theory by Chu et al. (2011), we hypothesize that the thermospheric Na layers are neutralized from converged Na+ layers. An envelope calculation shows good consistency with the observations.

Omega band pulsating auroras observed onboard THEMIS spacecraft and on the ground

We examined a fortuitous case of an omega band pulsating aurora observed simultaneously on the ground at Sanikiluaq in Canada and onboard the Time History of Events and Macroscale Interactions during Substorm (THEMIS) spacecraft on 1 March 2011. We observed almost the entire process of the generation of the omega band aurora from the initial growth to the declining through expansion period. The omega band aurora grew from a faint seed, not via distortion of a preexisting east-west band aurora. The size scale of the omega band aurora during the maximum period was ~500u2009km in the north-south direction and ~200u2009km in the east-west direction. The mesoscale omega band aurora consisted of more than 15 patches of complex-shaped small-scale auroras. Each patch contained an intense pulsating aurora with a recurrent period of ~9–12u2009s and a poleward moving form. The footprints of the THEMIS D and THEMIS E spacecraft crossed the poleward part of the omega band aurora. THEMIS D observed significant signatures in the electromagnetic fields and particles associated with the time at which the spacecraft crossed the omega band aurora. In particular, it was found that the Y and Z components of the DC electric field intensity, especially the Z component, modulated with almost the same period as that of the optical pulsating auroras. The electrostatic low-frequency waves of less than 30u2009Hz showed quasiperiodic intensity variations similar to those of the DC electric field. These observations suggest that DC electric field variation and low-frequency electrostatic waves may play important roles in the driving mechanism of omega band pulsating auroras.

Morphologies of omega band auroras

We examined the morphological signatures of 315 omega band aurora events observed using the Time History of Events and Macroscale Interactions during Substorm ground-based all-sky imager network over a period of 8xa0years. We find that omega bands can be classified into the following three subtypes: (1) classical (O-type) omega bands, (2) torch or tongue (T-type) omega bands, and (3) combinations of classical and torch or tongue (O/T-type) omega bands. The statistical results show that T-type bands occur the most frequently (45%), followed by O/T-type bands (35%) and O-type bands (18%). We also examined the morphologies of the omega bands during their formation, from the growth period to the declining period through the maximum period. Interestingly, the omega bands are not stable, but rather exhibit dynamic changes in shape, intensity, and motion. They grow from small-scale bumps (seeds) at the poleward boundary of preexisting east–west-aligned auroras, rather than via the rotation or shear motion of preexisting east–west-aligned auroras, and do not exhibit any shear motion during the periods of auroral activity growth. Furthermore, the auroral luminosity is observed to increase during the declining period, and the total time from the start of the growth period to the end of the declining period is found to be about 20xa0min. Such dynamical signatures may be important in determining the mechanism responsible for omega band formation.