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Impacts of solar flare vary at different parts of the lower ionosphere depending on it’s proximity to the direct exposure of incoming solar radiation. The quantitative analysis of this phenomena can be attributed to ‘solar zenith angle (χ(t))’ profile over ionosphere. We numerically solve the ‘electron continuity equation’ to obtain the lower ionospheric electron density profile (Ne(t)). The electron production rate (q(t)) is governed by the (i) X-ray profile (ϕ(t)) of the flare, (ii) χ(t)-values during the flare occurrence etc. For analyzing the X-ray profile during flares, we use the GOES-15 satellite observations. Since we’re working on electron continuity equation based simplified ionospheric model, we confined our analysis for comparatively stable mid-latitude ionosphere only. We choose three flares each from C, M and X-classes for Ne(t)-profile computation. We observe that temporal Ne(t)-profiles differ when computed for lower ionosphere over different discrete latitudes. Further, we compute the spatial Ne(t)-profile across mid-latitude at the time when ϕ(t)=ϕmax. Now we assume that, these flares repeat themselves every day of a year (DoY) at the same time of a day and we compute Ne(t)-profiles for each day. We found a seasonal effect on Ne(t)-profile due to solar flare. Further, we investigate the response time delay (Δt) of the lower ionosphere, which is the time difference between incidence of X-ray and the respective change in Ne(t)-profiles during solar flares. Strong seasonal effects on Ne(t)-profile and Δt are the unique results of this work.

The electric fields generated by lightning discharges propagate upward and couple with the lower ionosphere, triggering various mesospheric optical emissions. The potential role of local terrain in modulating the lightning-generated electric fields in the lower ionosphere remains poorly understood. To investigate the effect of mountain terrain on the lightning-generated electric fields at high altitudes (70–85 km), a two-dimensional (2D) finite-difference time-domain (FDTD) simulation model was developed. The simplified mountain is parameterized by its height, width, and horizontal distance from the lightning channel. Simulation results show that mountain terrain significantly influences the lightning-driven electric field waveforms in the initiation region of sprite halos. Increased mountain height leads to greater attenuation of the high-altitude electric field amplitudes, thereby suppressing sprite halos initiation. The shielding effect of mountain width on the electric fields is less pronounced than that of mountain height, and it stabilizes when the width exceeds 40 km. When the horizontal distance between the mountain and lightning channel is less than 40 km, the electric field attenuation increases significantly with decreasing distance. The attenuation effect gradually weakens beyond a distance of 40 km, yet the electric field waveforms exhibit considerable fluctuations due to the reflection process.

A key element of magnetosphere‐ionosphere coupling is the precipitation of electrons, which transfers energy from the collisionless, rarefied magnetospheric plasma into the dense, collisional ionosphere. Two distinct types of such precipitation are: auroral electrons, which carry field‐aligned currents and are responsible for auroral arc formation, and energetic electrons, which contribute to ionization in the lower ionosphere. Although these two electron populations are well separated in energy, this study reveals a close connection between them, likely due to the collocation of their equatorial drivers. Using low‐Earth orbit satellite measurements from Electron Losses and Fields Investigation of energetic (50–1,000 keV) electron precipitation and ground‐based all‐sky imager observations of auroral arcs, we demonstrate how the auroral arc structures and locations strongly correlate with the boundaries or gradients of >50${ >} 50$keV precipitation. We identify three typical correlation patterns and discuss their implications for the physics of magnetosphere‐ionosphere coupling.

Terrestrial gamma-ray flashes (TGFs) are transient gamma-ray emissions from thunderstorms, generated by electrons accelerated to relativistic energies in electric fields. Elves are ultraviolet and optical emissions excited in the lower ionosphere by electromagnetic waves radiated from lightning current pulses. We observed a TGF and an associated elve using the Atmosphere-Space Interactions Monitor on the International Space Station. The TGF occurred at the onset of a lightning current pulse that generated an elve, in the early stage of a lightning flash. Our measurements suggest that the current onset is fast and has a high amplitude—a prerequisite for elves—and that the TGF is generated in the electric fields associated with the lightning leader.

On 9 October 2022, a powerful gamma ray burst (GRB), GRB221009A, caused significant changes in the electron density of the lower ionosphere, as evidenced by VLF (3–30 kHz) radio wave observations. However, GRB221009A did not yield any observable signatures at the Schumann resonances (∼8, ∼14 Hz), which are also sensitive to the lower ionosphere. We show that the effects of GRB221009A are observable in a decrease in propagation velocity of ELF (3–1,000 Hz) lightning impulses. Analytical and numerical analysis points to an ionospheric perturbation in which the GRB increased the daytime electron density uniformly over a wide altitude range from 50 to 90 km. The GRB perturbation is markedly different from perturbations from solar flares, which significantly increase the vertical gradient of the electron density. ELF propagation velocity is shown to be a technique that can identify the altitude range of ionospheric perturbations.

Green emissions from excited Oxygen in Sprite Tops (GhOSTs) are due to the 557 nm photons emitted from atomic oxygen (O)$(\\mathrm{O})$excited to the 1S state. In this work we compare the possible contribution of various mechanisms for excitation of O(S1)$\\mathrm{O}({}^{1}S)$under application of a lightning‐induced electric field to the lower ionosphere. The mechanisms considered are informed by O(S1)$\\mathrm{O}({}^{1}S)$studies in the context of airglow and electrical gas discharges. We find that energy transfer from molecular nitrogen N2$\\left({\\mathrm{N}}_{\\mathrm{2}}\\right)$in the A3Σu+${A}^{3}{{\\Sigma }}_{u}^{+}$state to ambient atomic oxygen is the dominant source of O(S1)$\\mathrm{O}({}^{1}S)$above ≈84${\\approx} \\!\\!84$km altitude, while photolysis of molecular oxygen O2$\\left({\\mathrm{O}}_{\\mathrm{2}}\\right)$due to extreme ultraviolet photons emitted from N2${\\mathrm{N}}_{\\mathrm{2}}$singlet states dominates at altitudes below 84 km. The most favorable conditions for the observability of green ghosts are summarized.

The valley region ionosphere spans from approximately 120–200 km in altitude, and hosts a wide range of plasma, neutral, and solar interactions that create and maintain the ionosphere. In this region a ubiquitous and mysterious source of radar echoes called “150‐km echoes” has been observed since the 1960s. Recent work has shown these echoes are the result of solar EUV lines creating a bump‐on‐tail photoelectron distribution, which is unstable to upper hybrid waves. These electron‐scale waves couple to ion‐acoustic waves through a kinetic turbulence process, which we examine in this work. Range‐time‐intensity (RTI) plots of 150‐km echoes are calculated using a kinetic turbulence theory, producing results that qualitatively match radar observations. Calculation of the echo intensity for different radar frequencies shows that the turbulence has a power‐law description for large wavelengths (lower radar frequencies), but the enhancements at smaller wavelengths (sub‐meter) are less predictable but observable.

Sporadic E (Es) layer plays a prominent role in revealing both upward and downward atmosphere‐ionosphere coupling process. This study investigates the responses of Es layers to the May 2024 super geomagnetic storm by using 37 ground‐based ionosondes distributed globally and space‐based COSMIC‐2 radio occultation observations. The results show that Es layers were significantly enhanced during the recovery phase of geomagnetic storm. In addition, the enhanced Es layers mainly occurred over Southeast Asia, Australia, the South Pacific and the East Pacific. The temporal evolution of foEs disturbances over the Asian‐Australian sector clearly shows the “wave propagation” characteristics from high to low latitudes, indicating that the enhancements of the Es layers are most likely caused by the disturbed neutral winds in the E region. This study presents observational evidence for the downward impacts of the geomagnetic storm on the E‐region ionosphere. Plain Language Summary Geomagnetic storms can significantly affect the F‐region electron density, neutral composition, and background neutral winds. However, the disturbances in the E‐region ionosphere are rarely recorded and investigated during geomagnetic storms. The May 2024 super geomagnetic storm with Dst < 400 nT provided a good opportunity to investigate the responses of the lower ionosphere to the super geomagnetic storm. The observations from 37 ionosondes and COSMIC‐2 show that the Es layers were significantly enhanced in some regions during the May 2024 super geomagnetic storm, providing observational evidence for lower ionospheric variations caused by geomagnetic storms. Key Points Significant Es layer enhancements were recorded during the recovery phase of the super geomagnetic storm Enhancements of Es layers mainly appeared over Southeast Asia, Australia and South Pacific, as well as the eastern Pacific region The temporal evolution of enhanced Es layers demonstrates equatorward wave propagation characteristics from high to low latitudes

Earth’s atmosphere, whose ionization stability plays a fundamental role for the evolution and endurance of life, is exposed to the effect of cosmic explosions producing high energy Gamma-ray-bursts. Being able to abruptly increase the atmospheric ionization, they might deplete stratospheric ozone on a global scale. During the last decades, an average of more than one Gamma-ray-burst per day were recorded. Nevertheless, measurable effects on the ionosphere were rarely observed, in any case on its bottom-side (from about 60 km up to about 350 km of altitude). Here, we report evidence of an intense top-side (about 500 km) ionospheric perturbation induced by significant sudden ionospheric disturbance, and a large variation of the ionospheric electric field at 500 km, which are both correlated with the October 9, 2022 Gamma-ray-burst (GRB221009A). Gamma-ray bursts (GRBs) are known to have impact on Earth’s lower ionosphere, but GRB impacts on the upper ionosphere was not observed before. Here, the authors show strong electric field variation at 500 km in the ionosphere caused by GRB221009A.

Energetic electron precipitation (EEP) from the Earth's radiation belts can ionize neutral molecules in the D-region ionosphere (60–90 km altitude), significantly influencing the conductivity and chemical species therein. However, due to the limited resolution of space-borne instruments, the energy and fluxes of electrons that truly precipitate into the atmosphere still remain poorly investigated. To resolve this problem, in this study, we have utilized the wave and particle data measured by the Electric Field Detector (EFD) and High-Energy Particle Detector (HEPP) on board the China Seismo-Electromagnetic Satellite (CSES-01) during nighttime conditions between 2019 and 2021. Using the measurements of extremely low frequency (ELF) waves, we have derived the reflection height of the D-region ionosphere, which turn out to be highly consistent with the electron and X-ray measurements of the CSES. Our results show that the influence of EEP on the two hemispheres is asymmetric: the reflection height in the Northern Hemisphere is in general lowered by 2.5 km, while that in the Southern Hemisphere is lowered by 1.5 km, both of which are consistent with first-principles chemical simulations. We have also found that the decrease in reflection height exhibits strong seasonal variation, which appears to be stronger during wintertime and relatively weaker during summertime. This seasonal difference is likely related to the variation of the background ionospheric electron density. Our findings provide a quantitative understanding of how EEP influences the lower ionosphere during solar minimum periods, which is critical for understanding the magnetosphere–ionosphere coupling and assessing the impact on radio wave propagation.