K. Hosokawa

During a magnetic storm on 14–16 December 2006, a polar cap tongue of ionization (TOI) was detected by an all‐sky imager (ASI) at Resolute Bay, Canada (74.73°N, 265.07°E). We investigate the temporal evolution and spatial structure of the TOI in detail by combining the optical data with other observations (e.g., solar wind, GPS total electron content, SuperDARN, and DMSP and NOAA POES satellites). The TOI was observed as a bright and elongated 630 nm airglow plume for 4 h during the main phase of the storm. This interval corresponded to a period of prolonged stable large‐amplitude southward IMF during a coronal mass ejection (CME). One to one and a half hours before the appearance of TOI, the polar cap boundary expanded rapidly far equatorward, and a positive ionospheric storm occurred. This implies that both the “expansion of the high‐latitude plasma convection” and “build up of the source plasma in the midlatitudes” are necessary conditions for the formation of a TOI. Because both of them were triggered by a major southward turning of the IMF, the prolonged large‐amplitude southward IMF orientation in the trailing part of the CME was primarily responsible for the generation of TOI. After its appearance, the TOI exhibited dynamic motion in the dawn to dusk direction. Simultaneous SuperDARN data suggest that a longitudinal progression of subauroral polarization stream controlled this dynamic motion. The optical TOI was found to be a continuous stream elongated in the noon‐midnight direction although it contained some mesoscale patterns. Absence of large‐scale temporal changes in the cusp plasma flow during the stable IMF period allowed the TOI to remain continuous without being broken into polar cap patches. The mesoscale structures within the TOI were probably produced by small‐scale velocity fluctuations in the cusp plasma flow. The TOI as visualized with the all‐sky airglow imager was found to be much more dynamic and much more complicated than we ever thought. The current study indicates that such a behavior of the TOI was presumably caused by a combination of temporal variations in the global‐scale plasma circulation system, expansion and contraction of the polar cap area, and plasma density changes in the dayside low to midlatitudes.

We present, for the first time, an appearance of Pedersen current layer carried by the electrons in the auroral D‐region. Such a layer was detected by the EISCAT VHF radar in Tromsø, Norway when an intense pulsating aurora (PA) occurred. Due to the high‐energy electron precipitation during the PA, a significant ionization was observed not only in the E‐region but also in the upper part of the D‐region (80–95 km). An altitude profile of the Pedersen conductance derived from EISCAT exhibited two distinct layers of enhanced conductance. The upper one occurred at ≈120 km altitude which corresponded to the normal Pedersen current layer carried by the ions. The lower one appeared as a thin layer between 80 and 95 km in altitude, which was mainly carried by the collisional motion of electrons. Such an electron Pedersen layer is detectable only when the electron density is sufficiently high for allowing an appreciable current to flow in the D‐region. The Pedersen conductance at the peak of the electron Pedersen layer was ≈8 × 10⁻⁵ S m⁻¹, which is only 13% of that at the peak of the ion Pedersen layer (6 × 10⁻⁴ S m⁻¹). However, the electron Pedersen current flows exactly in the altitudes where the pulsating ionization occurs; thus, it would play more important role in the closure of FAC associated with the patch of PA.

We demonstrate a close association between decameter‐scale plasma irregularities in the E region ionosphere and auroral patches in the postmidnight sector. In September 2009, campaign‐based measurements of the aurora were conducted in Iceland with a white light all‐sky camera (ASC) at Tjörnes (66.20°N, 17.12°W) and the SuperDARN radar at þykkvibaer (63.77°N, 20.54°W). On one night during the campaign period, the ASC observed the successive passage of auroral patches in the postmidnight sector after a small substorm‐like activity. The patches were drifting predominantly eastward across the field‐of‐view of the ASC with a speed of approximately 360–450 m s⁻¹, which is consistent with the sunward convection in the postmidnight westward electrojet. The simultaneous radar measurements recorded strong radar backscatter echoes (>15 dB) within the gaps between adjacent auroral patches, while such echoes were not observed or were very weak in the region of the aurora. The Doppler velocity estimation showed that the electric field was clearly reduced within the patches, which was probably the result of the enhanced conductance associated with auroral precipitation. Thus, this reduction in the electric field suppressed the generation of irregularities (i.e., radar echoes) in the regions of auroral patches. This suggests that the conductance enhancement associated with precipitating electrons not only modified the electric field within the aurora but also affected the generation of small‐scale plasma structures in the vicinity of the patch‐type optical auroral forms.

We present, for the first time, a quasiperiodic modulation of ionospheric parameters, associated with the occurrence of pulsating auroras, such as electron density, conductance, and electric field. In March 2008, simultaneous campaign‐based measurements of pulsating auroras were conducted over Tromsø (69.60°N, 19.20°E), Norway, using an all‐sky TV camera (ATV) and the European Incoherent Scatter (EISCAT) UHF system. During an interval within this campaign period, pulsating auroras, with periods of 8–17 s, were observed by the ATV in the morning local time sector (∼0500 MLT). In this interval, quasiperiodic oscillations were identified in the raw electron density obtained by EISCAT. The electron density at lower altitudes in the E region (95–115 km) was enhanced by a factor of 3–4 immediately after the optical pulsation became “on.” The height‐integrated Hall conductance was also elevated, by a factor of 1.5–2, almost in harmony with the electron density variation. The response of the electron density and Hall conductance to the appearance of the pulsating aurora was almost immediate. However, both did not decrease to the background level promptly after optical pulsation ceased. This was primarily because it took a few seconds for the electron density to decrease through recombination with ambient ions at these altitudes. Interestingly, electric field measurements performed by the remote antenna at Kiruna showed that redirection of the electric field occurred when the pulsating aurora was “on.” We propose a model in which the enhancement of Hall conductance within patches of the pulsating aurora caused charge accumulation at the edges of the patches, and the electric field was then modified by the resulting polarization electric field. An estimation of the electric field modulation with this model well reproduced the actual electric field observations carried out by EISCAT, which confirmed the validity of the model. These results imply that the ionization caused by high‐energy electron precipitation associated with a pulsating aurora has a significant effect on the ionospheric conductivity and current system. This modification of the ionosphere may facilitate characterization of the morphological features of pulsating auroras. In particular, modification of the electric field would affect the spatial structure of pulsating aurora patches, such as their motion and shapes.

On the night of December 20, 2006, 630 nm airglow images obtained by an all‐sky camera at Resolute Bay, Canada (74.73°N, 265.07°E; altitude adjusted corrected geomagnetic (AACGM) latitude 82.9°) showed the passage of successive polar cap patches. Shortly after convection came to a temporary halt, one of the patches was reorganized into two substructures in approximately 8 min. The two‐dimensional background ionospheric convection pattern measured using the newly deployed PolarDARN radar at Rankin Inlet (62.82°N, 93.11°W; AACGM latitude 72.96°) showed that a velocity shear of approximately 120 m s⁻¹/340 km suddenly appeared in the vicinity of the patch at the time of reorganization. A qualitative analysis of the relationship between the magnitude of the velocity shear and the distance between the divided patches indicates that the shear in the background plasma convection velocity significantly contributed to the reorganization of the patch. This shear structure appeared soon after a southward turning of the interplanetary magnetic field (IMF) and was probably associated with the reconfiguration of the convection pattern from a pre‐existing northward‐oriented IMF pattern to a southward‐oriented one. The present observations indicate that the reconfiguration/deformation of patches because of a shear in the background convection field, especially reorganization of patches into smaller substructures, may play an important role in the rapid structuring of patches.

A highly sensitive all‐sky airglow imager (part of the Optical Mesosphere Thermosphere Imagers [OMTIs]) has been operating for 3 years at Resolute Bay, Canada (geographic latitude 74.7°N; geomagnetic latitude 82.9°N) since January 2005. One of its major applications is the observation of polar cap patches. Polar cap patches are generated in the vicinity of the cusp during southward interplanetary magnetic field (IMF) conditions, and they exhibit dynamic movement, possibly in association with the changes in the IMF orientation. In order to determine how their motions are controlled by the upstream IMF conditions, we applied a motion‐tracking algorithm that was based on two‐dimensional cross‐correlation analysis to consecutive images obtained from January 2005 to December 2007 by the all‐sky airglow imager at Resolute Bay. We identified and extracted 561 individual patches and then carried out a statistical study of their motion. It is demonstrated that the speed of patches is primarily controlled by the IMF B_z. Furthermore, the dawn‐dusk component of the patch drift velocities is well correlated with the IMF B_y, which is in good agreement with the published B_y dependence of the nightside polar cap convection. This enables us to monitor the plasma convection within the polar cap by using polar cap patches as tracers.