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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

Abstract Substorm-associated electron injection, starting on Apr. 5, 2017, was observed by the ERG (Arase), GOES-15 and GOES-13 spacecraft. ERG successfully observed a clear and sufficient extent of manifestations of the dispersionless injection and the successive drift echoes at radial distances shorter than geosynchronous orbit (GEO) during a unique period of the satellite mission. The GOES-15 and GOES-13 measured the drift echoes of the event as well. The observations provided constraints to study the event and opportunities to make adjustments to the previous substorm injection models. Models built on an impulsive earthward-propagating electromagnetic field have been proposed to simulate substorm injections. So far such models showed good results of dispersionless features compared to spacecraft observations, but could only produce drift echoes with periods somewhat different from geosynchronous observations. To study the substorm injection event and produce drift echoes with better periods, we modify an existing model in the literature. ERG and GOES spacecraft measured tens to a few hundred keV electrons injected during the substorm, providing important seed population for ring current and radiation belts. Since the electron energies of interest are comparable to the rest mass energy, our work further provides the relativistic form of the previous model and employs a semiempirical model as background field instead of a dipole-based one in the previous study. Our work shows that the main features of the substorm injection event are successfully reproduced with the drift echoes periods showing a better fit to the observations of this event when relativistic effects are considered. Despite possible deviation of the model magnetic fields from reality, the relativistic computations still show dominant effect on the drift echoes periods. The substorm injection expanding earthward farther than GEO was observed by ERG, and the event can be better simulated by the further-developed model shown in this work.

Abstract In this report, we describe the low-energy electron instrument LEPe (low-energy particle experiments–electron analyzer) onboard the Arase (ERG) spacecraft. The instrument measures a three-dimensional distribution function of electrons with energies of $$\sim 19$$ ∼ 19 eV–19 keV. Electrons in this energy range dominate in the inner magnetosphere, and measurement of such electrons is important in terms of understanding the magnetospheric dynamics and wave–particle interaction. The instrument employs a toroidal tophat electrostatic energy analyzer with a passive 6-mm aluminum shield. To minimize background radiation effects, the analyzer has a background channel, which monitors counts produced by background radiation. Background counts are then subtracted from measured counts. Electronic components are radiation tolerant, and 5-mm-thick shielding of the electronics housing ensures that the total dose is less than 100 kRad for the one-year nominal mission lifetime. The first in-space measurement test was done on February 12, 2017, showing that the instrument functions well. On February 27, the first all-instrument run test was done, and the LEPe instrument measured an energy dispersion event probably related to a substorm injection occurring immediately before the instrument turn-on. These initial results indicate that the instrument works fine in space, and the measurement performance is good for science purposes.