Yongliang Zhang

The NASA Artemis II rocket was successfully launched at 06:35 p.m. ET (22:35 UTC) on 1 April 2026 from the Kennedy Space Center (28°35′28″N 80°39′03″W). The rocket plume caused enhanced Lyman α emission (up to ∼3 kR) observed by TIMED/GUVI on 2–6 April (5 days). Its peak intensity decreased linearly from April 3 at a rate ∼740 R/day. The half‐maximum width of the zonal Lyman α intensities at 20°–30° latitude (at ∼01:30 p.m. local solar time) increased from ∼3,000 km on April 2–∼9,000 km on April 6. GUVI also detected significant reductions in O‐130.4 nm and O‐135.6 nm radiances, indicating O density reduction. Weakly enhanced NO ε band intensity (NO density) was also observed on April 2 and 3. These features are due to photochemical processes associated with water vapor in the rocket plume and diffusion.

The Daily Atmospheric Ionospheric Limb Imager (DAILI) (Daily Atmospheric and Ionospheric Limb Imager) mission utilized a 6U CubeSat, deployed at about 410 km altitude in a 51‐degree orbit, to make simultaneous daytime limb measurements in two colors; at 630 nm to measure the OI (630 nm) redline and at 765 nm to measure the Atmospheric (0,0) band (O2A band). The ratio of the O2A band to the redline allows a retrieval of the absolute daytime density from ∼140–200 km altitude using a technique demonstrated on the Remote Atmosphere and Ionosphere Detection System (RAIDS) experiment in 2009 (Hecht et al., 2015, https://doi.org/10.1002/2014gl062355 ). Daily Atmospheric Ionospheric Limb Imager required the development of a new Earthshade that would fit into the 6U form factor and would, while observing the limb at 140 km and above, suppress scattering by a factor at or above 1 million from both the daylit Earth's surface and from clouds, and this capability is demonstrated on‐orbit. Over 7200 limb profiles were obtained, many of which were geomagnetically quiet but several of which occurred during a Kp geomagnetic storm where the NASA GUVI (Global UV Imager) instrument obtained O/ data. This allowed a determination of the extent of variations of compared to the GUVI O/ data during a large storm at altitudes below 200 km. The DAILI data for this storm are not reproduced by current models. Both the DAILI and GUVI data are consistent with significant equatorward transport of heated air from the auroral zone compared to model predictions.

It is well‐known that the auroral cusp appears poleward of the dayside oval during northward IMF, and that transpolar arcs (TPAs) are typically cusp‐aligned. Using Lyman‐Birge‐Hopfield band short (LBHs) images from the Defense Meteorological Satellite Program, we investigate electron aurora that occur approximately in the same region as the ion cusp. For this purpose, we examine 140 LBHs images during which multiple TPAs appear simultaneously in the polar cap, and a region of bright electron precipitation is visible close to the tip of the TPAs. In eight cases, the ion cusp could be identified from particle data. We find that the electron precipitation region in most cases overlaps with the cusp, but also extends dawnward of it. This dawnside part is identified as the auroral signature of upward flowing northward Bz currents in the polar cap. Studying the spatial distribution of the 140 events, a clear dipole tilt dependence of cusp and connected electron precipitation region is observed. In the summer hemisphere, they appear on higher latitudes and in winter on lower latitudes close to the dayside oval. While the electron precipitation region extends nearly always to the dawnside, the winter hemisphere cusp appears in the northern (southern) hemisphere at postnoon (prenoon). IMF By controls the dawn‐dusk location of cusp and electron precipitation region only in the summer hemisphere. In the winter By influences instead its latitude. As TPAs typically merge with the electron precipitation region, these observations might also explain the recently reported explicit By dependence of initial TPA location.

During northward interplanetary magnetic field (IMF) conditions, magnetic reconnection between the Earth's magnetosphere and the IMF takes place on high latitudes, causing the auroral oval to contract. During those times several auroral structures may appear inside the polar cap, such as cusp aurora, high latitude dayside aurora (HiLDA) and transpolar auroral arcs. Concurrently, a field‐aligned current system appears in the dayside polar cap, called Northward B _z (NBZ) currents. In this work, we use Defense Meteorological Satellite Program images and particle data, as well as Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) global field‐aligned current maps in order to investigate the connection between dayside polar cap aurora and NBZ currents. We analyze 10 events during northward IMF for different IMF B _y and dipole tilts, the majority of which are taken from the companion paper by Kullen et al. (2026, https://doi.org/10.1029/2026JA035276 ). We observe that when cusp aurora occurs in the middle of the polar cap, electron aurora consistently appears dawnward of the cusp aurora. The extent of the dawnside expansion depends on the IMF clock angle. This dawnside electron aurora region overlaps in all cases with the upward NBZ current region measured by AMPERE, but may appear as broadband, discrete arcs or HiLDA. This shows that electron aurora occurring dawnward of the cusp during northward IMF is a signature of the upward NBZ currents.

This study investigates midlatitude ionospheric and thermospheric variations during the 16 April 2025 geomagnetic storm through multi‐instrument analysis and numerical simulation. The analysis uses a comprehensive data set comprising ground‐based Global Navigation Satellite System Total Electron Content (GNSS TEC) and ionosonde measurements, along with space‐based observations from the Swarm, Defense Meteorological Satellite Program (DMSP), Global‐scale Observations of the Limb and Disk (GOLD), and TIMED Global Ultraviolet Imager (GUVI), as well as the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). We found distinct midlatitude density gradient structures in both the ionosphere and thermosphere during the storm's main phase as follows: (a) An unusual storm‐enhanced density (SED) was observed originating in the dawn sector, which persisted over North American longitudes for 8–9 hr. In contrast, the local afternoon and dusk sectors over Europe showed no SED signature but instead exhibited a negative ionospheric storm phase. (b) Coincident with the morning SED, we observed a distinct tongue‐like structure in neutral composition with O/ enhancement of 20%–40% in the North American sector, which was bounded by areas of O/ depletion. This formed a coherent thermosphere‐ionosphere structure with concurrent enhancement and depletion. (c) The dawnside SED can be largely attributed to two synergistic mechanisms: a chemically driven increase in plasma density from an Oxygen‐rich neutral tongue, coupled with an electrodynamically driven zonal electric field increased by the sunrise enhancement of the penetration electric field. These results underscore the critical role of neutral composition and electrodynamic forcing in the formation of storm‐time midlatitude ionospheric gradient structures, offering new insights into the dynamics of the coupled ionosphere‐thermosphere system.

We investigate how the location, size, and intensity of the auroral oval is affected by the combination of the tilt of the Earth's magnetic dipole axis and a strong, stable dawn/dusk component of the interplanetary magnetic field (IMF B_y) during northward IMF. Sunlit auroral observations are contaminated by dayglow, and its impact on average intensity estimates remains unclear. Dayglow modelling is also accompanied by significant uncertainties that increase with increasing sunlight intensity. These difficulties motivate us to develop a new technique for isolating the auroral contribution in dayglow-subtracted UV images. This technique assumes the observed distribution of intensities consists of separate contributions from dayglow and aurora, and that the dayglow subtraction process to which the observations are subjected is imperfect and leaves a residual error. By performing a nonlinear fit to dayglow-subtracted count distributions one may extract the best-fit parameters that describe the dayglow residual error and auroral sources separately. We apply this isolation technique to dayglow-subtracted UV images derived from measurements made by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) onboard the Defense Meteorological Satellite Program’s F16–19 satellites during 2005–2018. The isolation technique produces 30 %–40 % higher auroral intensities than a simple calculation of the distribution mean. Statistics of the auroral component show a clear and substantial (∼500 km in NH, ∼430 km in SH) dawn-dusk shift in the polar cap location depending on the sign of IMF B_y during local summer in both Hemispheres. This shift is absent during local winter. We propose that the cause of this seasonally dependent shift in the polar cap location is likely to be related to seasonal differences in lobe reconnection rates. We also demonstrate how the heteroskedasticity of distributions of dayglow residual intensity can influence quantitative estimates of the average auroral intensities.

Transpolar arcs (TPAs) are auroral structures spanning the polar cap. One type appears in conjugate hemispheres and is thought to be located on closed field lines, but how closed flux enters the open polar cap remains debated. We address this using observation and simulation of conjugate TPAs after an interplanetary magnetic field (IMF) By sign reversal. The reversal displaces the reconnection point, redirects magnetospheric convection, and segregates closed field lines that originally mapped to the auroral oval into the lobe. In the ionosphere, this segregation builds new polar caps rooted in the dawnside (Northern Hemisphere) and duskside (Southern Hemisphere) auroral ovals. TPAs form along the old–new polar cap boundary and drift with the new polar cap's expansion. This expansion replaces the old counterclockwise circulation with a new clockwise circulation in the Northern Hemisphere, and vice versa in the Southern Hemisphere, on the timescale of antisunward IMF propagation.

On 23 November 2022, during exceptionally quiet geomagnetic conditions, a distinct ultra‐low frequency (ULF) wave in the Pc5 band (∼2.4 mHz) was observed in the duskside sub‐auroral region by two Canadian SuperDARN (Super Dual Auroral Radar Network) radars. The wave exhibited a periodic Doppler velocity signature resembling a “caterpillar,” characterized by anti‐sunward propagation and an azimuthal wave number of ∼12. Ground‐based magnetometer data revealed latitudinal variations in wave amplitude and phase consistent with field line resonance, peaking at ∼66° magnetic latitude. The Arase satellite, whose ionospheric footprint traversed the region of the caterpillar ULF wave, detected toroidal oscillations in electric and magnetic fields at magnetically conjugate locations in the inner magnetosphere, with high coherence and phase consistency with ground‐based observations. Arase observed the electric field variation preceding the magnetic field variation by ∼45°, which is unusual for a pure standing mode and suggests a departure from a simple standing‐wave interpretation. Instead, the observations can be interpreted as mixed standing and propagating characteristics along the field line. Simultaneously, the clear anti‐sunward azimuthal phase propagation and sustained wave activity suggest a driving mechanism by a large‐scale surface perturbation at the magnetospheric flank. These properties are consistent with excitation by the Kelvin‐Helmholtz instability. A minor enhancement in solar wind dynamic pressure was observed, potentially contributing to the wave onset. This study highlights SuperDARN's capability to detect ULF waves during quiet intervals and demonstrates the utility of multi‐instrumental conjugate observations in revealing the spatiotemporal variations of ULF waves and their generation mechanisms.

The afternoon detached auroral arc is an important phenomenon in the subauroral region, reflecting coupling processes between the Earth's magnetosphere and ionosphere. Previous studies have not identified fine‐scale structures in such arcs, leaving the dynamics underlying their formation poorly understood. Here we report an afternoon detached auroral arc event on 13 September 2017 during the recovery phase of a storm. For the first time, the sawtooth‐like undulations were observed along the equatorward boundary of the afternoon detached arc in the Lyman‐Birge‐Hopfield Long (LBHL) wavelength band of Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager (DMSP/SSUSI). This auroral structure is accompanied by >10 keV ion precipitation and by tens to hundreds of eV electron precipitation at higher latitudes. Detailed analyses based on coordinated observations from the Arase satellite indicate that the structure is associated with a plasmaspheric plume, with surface waves occurring along its boundary. Joint observations from ground‐based magnetometer stations indicate that magnetic pulsations in the Pc1‐2 band were also distinctly detected. We suggest that surface waves perturb the cold plasma density within the plume, thereby modulating Electromagnetic Ion Cyclotron (EMIC) waves. The modulated EMIC waves resonate with energetic ions, producing precipitation that contributes to the formation of the sawtooth‐like undulations in afternoon detached auroral arc.