Journal of Geophysical Research: Space Physics最新文献

Soft X-rays are emitted in the magnetosheath and cusps because of solar wind charge exchange. The soft X-ray Imager (SXI) on board Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) will measure these X-rays. We developed a new method for finding the magnetopause standoff distance from simulations that reproduce the expected X-ray images using software developed by the SXI instrument team. We consider three points near the SMILE apogee. We apply this method to a three-hour interval with an interplanetary shock and a southward interplanetary magnetic field turning when the magnetosphere was moderately compressed. The results show that the magnetopause position can be reconstructed with an accuracy better than 0.5 $��R_E�$ for a five-minute integration time, which matches the SMILE scientific requirements. Moreover, we can even determine the magnetopause position using one-minute integration when the magnetosphere is strongly compressed and the spacecraft's position and SXI's orientation are favorable for magnetopause observations.

The Laschamps geomagnetic excursion (≈41,000 years BP) was a period of significant weakening and incomplete reversal of the Earth's magnetic field. The weakening substantially reduced geomagnetic shielding against cosmic rays (CRs), which contribute to phenomena at Earth, such as cosmogenic isotope production, and atmospheric ionisation and radiation. In this work, we expand upon previous modeling of geomagnetic shielding during excursions and provide a robust methodology for assessing the CR impacts during such an event, focusing on CR-induced atmospheric radiation. This was achieved by updating the open-source OTSO CR trajectory tool to allow for paleomagnetic field models, namely LSMOD.2, to be used as inputs to compute global apparent geomagnetic cut-off rigidities at 100-year intervals throughout the excursion. The CRAC:DOMO model was used to assess the CR-induced atmospheric radiation, and the potential impact on the aviation industry was investigated by computing the effective dose rates for two representative flights, Helsinki to New York and Helsinki to Dubai, under various conditions. Results suggest low-latitude flights, normally well shielded under modern conditions, can experience significant increases in dose rates; in contrast, some high-latitude flight routes may observe decreases in radiation exposure due to the irregular geomagnetic structure during the excursion. These findings reveal that geomagnetic excursions can greatly enhance the levels of CR-induced atmospheric radiation, with wider implications that excursion events can likewise significantly affect other CR-induced processes, such as cosmogenic isotope production and atmospheric ionization. The methodology provided here outlines a framework under which CR impacts can be assessed under non-standard geomagnetic conditions.

The Great American Eclipse occurred on 8 April 2024, and traversed across North and Central America, with solar coverage varying from 80% to 100% in various areas. This presented a unique opportunity to explore the interconnected ionosphere-thermosphere-magnetosphere system through a synchronized multi-instrumental methodology. This research delves into the ionospheric and geomagnetic reactions to the eclipse by examining Total Electron Content (TEC) derived from Global Navigation Satellite System (GNSS) data, variations in Very Low Frequency (VLF) signals, satellite observations from the SWARM and COSMIC-2 missions, and geomagnetic field readings from five INTERMAGNET stations. TEC data from nine GPS stations from the International GNSS Service (IGS) shows a time delay of 11–33 min and a decrease of TEC from 42% to 58%. Longer delays are observed at lower latitudes and stations with less obscuration. The intensity of depletion was found to fluctuate depending on latitude, local time, and the extent of solar obscuration. Additionally, Atmospheric Gravity Waves (AGW) induced by the eclipse were identified in TEC data from various GNSS stations, featuring wave periods ranging from 20 to 80 min, with amplitudes increasing near the path of totality. Dynamic changes in the lower ionosphere were indicated by the significant amplitude and electron density (N_e) changes, with noticeable fluctuations, in the VLF data collected from three transmitters and five receivers located along the eclipse trajectory. Observations from the SWARM and COSMIC-2 satellites corroborated the ground-based results, with SWARM-A identifying a maximum reduction in N_e anomaly of 3.6 × 10⁵ el/cm³, while COSMIC-2 occultation data revealed notable reductions in N_e profiles from D to F regions. Analysis of the geomagnetic field across the five different stations indicated significant aperiodic and quasiperiodic disturbances, especially in the X-component, which correlated with solar obscuration and disruptions in the ionospheric current system within the dynamo region.

Interhemispheric Field-Aligned Currents (IHFACs) serve as the channels connecting the ionospheric current systems over the Northern and Southern Hemispheres and maintaining the balance between them. The directions of the IHFACs represent the inflow and outflow currents from one hemisphere to the other. In this study, we investigated the mechanism for the south-north abnormal reversals of the IHFAC directions at middle and low latitudes by analyzing the field-aligned current density from Swarm, lower thermospheric wind from TIDI and ICON, and ionospheric conductivity derived from IRI-2020 and Nrlmsise-00 models. The results suggest that besides the geomagnetic geometry, the south-north hemispheric asymmetry of ionospheric conductance controls the IHFAC reversal over the American-Atlantic region. By contrast, over the Asian-Pacific region, the IHFACs exhibit multiple reversals across longitudes from ∼60°E to 150°W without being controlled by conductance and geomagnetic geometry. It is surprising that variations in the thermospheric winds are particularly strong over the Asian-Pacific region, playing a significant role in modulating the IHFAC directions. The study reveals the cause of the IHFAC directions in different seasons and regions, and promotes the understanding of the connection between the ionospheric dynamo in the two hemispheres.

Due to the rapid timescales of solar flares, the observed response of the upper atmosphere of Mars during a solar flare event depends on the timing within the flare event at which measurements are made. To accurately constrain the impact that solar flares may cause to the Martian upper atmosphere, the full time-extent of a solar flare's atmospheric effects must as such be understood. In this paper, the full duration of changes to the Martian upper atmosphere due to the 10 September 2017 X8.2-class solar flare are investigated using M-GITM simulations. Analyses characterize the Martian upper atmosphere's electron, ion, and neutral densities and composition over time at the fixed location of MAVEN spacecraft periapsis observations during this flare event. In particular, the effects of the atmospheric processes that drive these changes—which are uniquely accessible during a solar flare's rapid changes due to their differing timescales—are investigated to provide insight into these time-dependent changes. Results indicate rapid initial plasma density changes across the vertical extent of the ionosphere, plasma and neutral density changes with strong altitude- and species-specific dependence, and a long duration of maximum perturbed upper atmospheric conditions due to this flare over time. The neutral thermosphere's thermal expansion over time is found to extend and magnify many of the density changes produced by photochemistry. Constraining solar flares' time-dependent atmospheric effects provides context to future observation-based comparisons of flare events and investigations into the role of solar irradiance in shaping Mars' early climate and impacting technology.

Time-dependent model simulations of the atmosphere of Mars are a necessary tool for providing context to Mars' upper atmospheric conditions during a solar flare event. They extend beyond the short windows of available atmospheric observation to offer temporal context about the different time-dependent stages of the atmosphere's flare-driven response. Yet to interpret this context, a simulation's reproduction of observed atmospheric conditions in the presence and absence of solar flares must first be assessed. In this study, simulations from the Mars Global Ionosphere-Thermosphere Model (M-GITM) during the 10 September 2017 solar flare event and baseline, control conditions are assessed through comparison with observations by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft mass spectrometer. By calculating model bias for electron, ion, and neutral densities and density changes relative to baseline mean conditions, these comparisons offer unique context on the model's accuracy across thermosphere and topside ionosphere altitudes at the timing and aerographic location of observations. Results indicate that the examined M-GITM simulations qualitatively reproduced but quantitatively underestimated observed atmospheric changes for most plasma and neutral species, with closest agreement between 155 and 190 km altitude. Despite large observational noise, compositional trends of these changes were reproduced. Model bias was identified to differ more greatly between different upper atmospheric altitudes and species for absolute densities. This context may aid interpretation of Mars' flare-perturbed atmospheric conditions within M-GITM simulations at timing when observations are not available.

The Qinghai-Tibet Plateau, located on the “Roof of the World,” features complex terrain with the world's largest elevation gradient and is often referred to as the “Third Pole.” It serves as a natural laboratory for investigating coupling mechanisms among different atmospheric layers. Utilizing ionospheric E-layer height (hE) data from the Qinghai-Tibet Plateau and its surrounding regions spanning a full solar activity cycle from 2013 to 2023, this study systematically examines the diurnal, seasonal, and solar-cycle variations of hE as well as regional differences in the area. The results reveal that hE in this region is generally controlled by solar radiation, exhibiting a gradual increase with latitude. However, two stations—Chongqing and Lanzhou—show distinct anomalies: the mean hE at Chongqing is 3–5 km higher than that of stations at similar latitudes, while the mean hE at Lanzhou is 1–3 km lower. The anomalous hE variations in the Qinghai-Tibet Plateau and its surrounding areas may be attributed to the fact that, in addition to solar radiation control, the region's complex topographic structure influences the direction and velocity of lower-atmosphere motions. This, in turn modulates the E-layer height and electron density distribution through the excitation of atmospheric gravity waves and planetary waves. Within this cross-layer atmospheric coupling process, the unique topography of the Qinghai-Tibet Plateau plays a key role. Investigating the variation characteristics of E-layer height over the Qinghai-Tibet Plateau and its surrounding regions is of great significance for revealing the mechanisms of vertical atmospheric coupling and for developing multi-sphere atmospheric coupling models.

The Qinghai-Tibet Plateau, located on the “Roof of the World,” features complex terrain with the world's largest elevation gradient and is often referred to as the “Third Pole.” It serves as a natural laboratory for investigating coupling mechanisms among different atmospheric layers. Utilizing ionospheric E-layer height (hE) data from the Qinghai-Tibet Plateau and its surrounding regions spanning a full solar activity cycle from 2013 to 2023, this study systematically examines the diurnal, seasonal, and solar-cycle variations of hE as well as regional differences in the area. The results reveal that hE in this region is generally controlled by solar radiation, exhibiting a gradual increase with latitude. However, two stations—Chongqing and Lanzhou—show distinct anomalies: the mean hE at Chongqing is 3–5 km higher than that of stations at similar latitudes, while the mean hE at Lanzhou is 1–3 km lower. The anomalous hE variations in the Qinghai-Tibet Plateau and its surrounding areas may be attributed to the fact that, in addition to solar radiation control, the region's complex topographic structure influences the direction and velocity of lower-atmosphere motions. This, in turn modulates the E-layer height and electron density distribution through the excitation of atmospheric gravity waves and planetary waves. Within this cross-layer atmospheric coupling process, the unique topography of the Qinghai-Tibet Plateau plays a key role. Investigating the variation characteristics of E-layer height over the Qinghai-Tibet Plateau and its surrounding regions is of great significance for revealing the mechanisms of vertical atmospheric coupling and for developing multi-sphere atmospheric coupling models.

The low latitude ionosphere exhibits complex day-to-day variability, even during geomagnetically quiet conditions, posing significant challenges in satellite-based applications. These variations arise from a complex interplay between electrodynamic and neutral dynamic processes, including the wave forcing from below, such as planetary waves and tides. Understanding the mechanisms governing day-to-day ionospheric variations is essential for improved characterization of the low latitude ionosphere. This study investigates quiet time day-to-day ionospheric Total Electron Content (TEC) variations using GPS observations at four identified locations from the equator to the EIA crest and beyond in the Indian sector. Strong enhancements and decrements are observed in day time TEC variations under quiet geomagnetic conditions (K_p < 3). A total of 25 deviation events in TEC spanning over 200 days are observed at different latitudes during a 3-year period covering different solar activity conditions. These deviations occur more during summer months of high solar activity with distinct latitudinal dependence. Detailed analysis using equatorial electrojet observations revealed that the electric field variations could not always explain the observed TEC deviations. Wavelet analysis revealed the presence of the periods of 2–6 days in TEC during the events of TEC deviations. Possible drivers of the quiet time day-to-day TEC deviations are explained in terms of the planetary wave forcing through distinct latitude dependent processes. This study provides new insights into the relative contributions of wave forcing from below through coupled ionosphere-thermosphere dynamics and offers a greater understanding of the physical mechanisms governing the quiet time TEC variabilities over the equatorial and low-latitude regions.