630-nm Airglow Images Research Articles

Westward Drift of Ionospheric Plasma Irregularities over a Low to Mid-latitude Transition Region in Indian Sector

We report the observation of plasma depletions/plumes in the F region ionosphere over a low to middle latitude transition region in the Indian sector. The observation of these plasma depletions is based on the data obtained in May 2019 through the all-sky airglow CCD imager installed in the campus of University of Kashmir, Srinagar (34.12 °N, 74.83 °E, magnetic latitude 25.91 °N). The depletions on the two consecutive nights of 05 and 06 May 2019 are aligned along the North-South (N-S) direction and drift westward. Several depletion bands along with some enhancement bands are seen in the 630-nm airglow images throughout the two nights. The observed structures show certain characteristics similar to Medium Scale Traveling Ionospheric Disturbances (MSTIDs) but these airglow features are not completely periodic. Further, in the observed depletion bands some East-West asymmetries are observed along with the structured tree-like branches of the airglow depletions. Some depletion bands even bifurcate leading to the inference that the structures are signatures of plasma irregularities rather than the usual MSTIDs observed in low-mid latitude transition region. The westward drift of the depletions especially during geomagnetic quiet times over this region makes this study significant since it offers a possible evidence that shows extension of spread F irregularities from the mid latitude region to the low-mid latitude transition region. In this paper, we point out some possible mechanisms related to the occurrence of plasma depletions at this region and their westward movement during geomagnetic quiet times.

What controls the luminosity of polar cap airglow patches?: Implication from airglow measurements in Eureka, Canada in comparison with SuperDARN convection pattern

Polar cap patches are known as islands of enhanced electron density in the polar cap F region ionosphere. To observe the airglow signature of polar cap patches continuously at a fixed point near the center of the MLAT/MLT coordinate system, we started operating an all-sky imager in Eureka, Canada since 2015, where the magnetic latitude is ~87°. By statistically analyzing the 630-nm airglow images from Eureka, it was identified that the luminosity of patches, which is proportional to the electron density in the F region, increases during specific UT interval from 16 to 22 UT. This outstanding UT variation of patch luminosity can be explained simply by the systematic shift of the terminator in the MLAT/MLT coordinate system due to the offset between the geographic and geomagnetic pole. That is, the spatial relationship between the dense source plasma in the sunlit area and the high-latitude convection system controls the UT variation. We also found that when the IMF By is positive, the number of polar cap patches was twice of that in the negative IMF By. By deriving average convection patterns from the archived SuperDARN map potential data, we confirmed that the configuration of plasma convection is more appropriate for patches to be transported toward the magnetic pole during the positive IMF By condition.

Geomagnetically conjugate observations of ionospheric and thermospheric variations accompanied by a midnight brightness wave at low latitudes

We conducted geomagnetically conjugate observations of 630-nm airglow for a midnight brightness wave (MBW) at Kototabang, Indonesia [geomagnetic latitude (MLAT): 10.0°S], and Chiang Mai, Thailand (MLAT: 8.9°N), which are geomagnetically conjugate points at low latitudes. An airglow enhancement that was considered to be an MBW was observed in OI (630-nm) airglow images at Kototabang around local midnight from 2240 to 2430 LT on February 7, 2011. This MBW propagated south-southwestward, which is geomagnetically poleward, at a velocity of 290 m/s. However, a similar wave was not observed in the 630-nm airglow images at Chiang Mai. This is the first evidence of an MBW that does not have geomagnetic conjugacy, which also implies generation of MBW only in one side of the hemisphere from the equator. We simultaneously observed thermospheric neutral winds observed by a co-located Fabry–Perot interferometer at Kototabang. The observed meridional winds turned from northward (geomagnetically equatorward) to southward (geomagnetically poleward) just before the wave was observed. This indicates that the observed MBW was generated by the poleward winds which push ionospheric plasma down along geomagnetic field lines, thereby increasing the 630-nm airglow intensity. The bottomside ionospheric heights observed by ionosondes rapidly decreased at Kototabang and slightly increased at Chiang Mai. We suggest that the polarization electric field inside the observed MBW is projected to the northern hemisphere, causing the small height increase observed at Chiang Mai. This implies that electromagnetic coupling between hemispheres can occur even though the original disturbance is caused purely by the neutral wind.Graphical abstractA schematic picture of the possible ionospheric variations during the MBW event on February 7, 2011. u is the thermospheric neutral winds at Kototabang, J is an ionospheric current, E_{mathrm{P}} is a polarization electric field, B_{mathrm{Z}} and B are the vertical and total components of the geomagnetic field, respectively, v_{mathrm{Z}} is an upward {mathbf{E}}times {mathbf{B}} drift, and Sigma _{mathrm{in}} and Sigma _{mathrm{out}} are height-integrated Pedersen conductivities inside and outside MBW, respectively. We suggest that the polarization electric field inside the observed midnight brightness wave in the southern hemisphere is projected to the northern hemisphere, causing the observed ionospheric height increase in the northern hemisphere.

Airglow-imaging observation of plasma bubble disappearance at geomagnetically conjugate points

We report the first observation of the disappearance of a plasma bubble over geomagnetically conjugate points. It was observed by airglow imagers at Darwin, Australia (magnetic latitude: −22°N) and Sata, Japan (21°N) on 8 August 2002. The plasma bubble was observed in 630-nm airglow images from 1530 (0030 LT) to 1800 UT (0300 LT) and disappeared equatorward at 1800 to 1900 UT (0300 to 0400 LT) in the field of view. The ionograms at Darwin and Yamagawa (20 km north of Sata) show strong spread-F signatures at approximately 16 to 21 UT. At Darwin, the F-layer virtual height suddenly increased from approximately 200 to approximately 260 km at the time of bubble disappearance. However, a similar F-layer height increase was not observed over the conjugate point at Yamagawa, indicating that this F-layer rise was caused not by an eastward electric field but by enhancement of the equatorward thermospheric wind over Darwin. We think that this enhancement of the equatorward neutral wind was caused by an equatorward-propagating large-scale traveling ionospheric disturbance, which was identified in the north-south keogram of 630-nm airglow images. We speculate that polarization electric field associated with this equatorward neutral wind drive plasma drift across the magnetic field line to cause the observed bubble disappearance.

Imaging observations of the equatorward limit of midlatitude traveling ionospheric disturbances

This paper reports the first attempt to observe the equatorward limit of medium-scale traveling ionospheric disturbances (TIDs) in the middle latitudes. The TIDs usually propagate southwestward in the northern hemisphere. An all-sky cooled-CCD imager measured 630-nm airglow at a southern island of Japan, Okinawa (26.9°N, 128.3°E, geomagnetic latitude (MLAT) = 17.0°), during the FRONT-2 campaign of August 4–15, 1999. The TIDs were detected at the mainland of Japan (∼21°–36° MLAT) by the total electron content (TEC) observations of more than 1000 GPS receivers. In the August 4 event, the TIDs moving southwestward was seen only in the northern sky of Okinawa as a depletion band in the 630-nm airglow images. In the August 6 event, the TIDs were not seen in the 630-nm images at Okinawa, although weak TID activity was observed by the GPS network at the mainland of Japan. The TEC data also showed weakening of the TID activity below 18° MLAT. Based on these observations, we suggest that there is a possible limit of medium-scale TID propagation around ∼18° MLAT.