Enhancement of equatorial OI(1D) emissions at midnight

Abstract

A number of recent studies have highlighted observational evidence of midnight brightness of the 630.0 nm nightglow, which is usually related to the midnight temperature maximum (MTM) effect. In this report, MTM-related enhancements of the 630.0 nm airglow around midnight are observed through images from the ISUAL/FORMOSAT-2 satellite. The data statistics are classified into three specific types (no-peak, single-peaked, and double-peaked events) and separated into the different seasons. In order to understand the influences of geomagnetic conditions, the statistical analyses are also separated into two regions. One is collected from the region whose geomagnetic equator is north of the geographic equator, and the other is collected from the region whose geomagnetic equator is south of the geographic equator. The results show that the single-peaked brightness often appears between the geographic equator and the geomagnetic equator. The double-peaked brightness appears simultaneously on the two sides of the region sandwiched by the two equators. Coupled with the summer-to-winter neutral wind generated by seasonal effects, one side of brightness could be enhanced or disappear due to the plasma moving along the field line. The no-peak events mainly occur close to May–July, which may be due to the effect of ionospheric annual anomalies. Overall, the statistical results for the occurrence rate show strong seasonal variations with different cycles at different longitudinal regions for all three types of events. Graphical Abstract