Yun‐Ju Chen

Equatorial plasma bubbles are ionospheric structures characterized by plasma densities below the background level and are influenced by season, local time, longitude, solar and geomagnetic activity, and the offset between geographic and magnetic poles. Using density measurements from the Ionospheric Connection Explorer (ICON) satellite between 2020 and 2022, together with a rolling‐ball algorithm and irregularity index criteria, we examine the climatology and evolution of bubbles, along with the corresponding vertical drift velocity. Bubble occurrence is generally correlated with upward drift, which lifts the F‐layer and creates favorable conditions for bubble development. An exception is observed in the Asian sector during June, where weakened downward drifts likely promote post‐midnight bubble generation. High pre‐midnight occurrences occur in the American and African sectors during equinoxes under both low and high solar activity, consistent with alignment of the sunset terminator and magnetic meridian. During low solar activity, the presence of post‐midnight bubbles in the Pacific and Asian sectors without pre‐midnight activity suggests possible local generation. A key finding is the local‐time variations in bubble occurrence rates, suggesting different evolution mechanisms between low and high solar activity. During high solar activity, rates exhibit a rise‐and‐decay pattern, peaking in the pre‐midnight sector, consistent with rapid post‐sunset generation. In contrast, during low solar activity, rates gradually increase into the post‐midnight sector, implying a combination of drifting fossil remnants and locally generated bubbles. Overall, while largely consistent with established climatological features, our results provide new insight into the solar activity dependence of bubble evolution in the topside ionosphere.

Plasma density enhancements in the polar region are localized ionospheric structures with densities exceeding the background level, influenced by season, Universal time (UT), longitude, interplanetary magnetic field (IMF), and the offset between the geographic and magnetic poles. In this study, we use in situ density measurements from the Defense Meteorological Satellite Program F15–F18 satellites to examine their spatial scale size, distribution, and occurrence rate in the topside ionosphere of both hemispheres, and their dependency on the IMF B _y , season, and UT for different peak‐to‐background density ratios (N/N ₀ ) under southward IMF. Our observations indicate that enhancements with higher N/N ₀ exhibit larger scale sizes, and those with N/N ₀ > 2 have larger scale sizes in the Northern Hemisphere. Preferential elongation is observed along the dawn‐dusk direction on the dayside in the north and noon‐midnight direction in the south. Spatial distributions strongly depend on IMF B _y regardless of N/N ₀ , highlighting the role of plasma convection morphology. Occurrence rates are generally higher in the Southern Hemisphere, varying with season, UT, N/N ₀ , and background density. Higher N/N ₀ enhancements occur more frequently during southern winter and equinoxes at 12–24 UT, and during northern winter at 12–24 UT. Lower N/N ₀ enhancements are more common during December than June solstices in both hemispheres. Applying a background density threshold significantly alters occurrence rate patterns, especially in the south, due to the larger offset between the geographic and magnetic poles. Our results align with previous studies and reinforce the importance of the plasma convection pattern and its relative position to the solar terminator.

In this case study, we focus on how hemispheric differences in the electric potential distribution in the auroral zones are related to hemispheric differences in the energetic particle precipitation that indicate the presence of field‐aligned potential drops. Utilizing data during the spring and fall equinox in 2013 and 2014 from the Defense Meteorological Satellite Program (DMSP) F16 and F17 satellites, we systematically examine simultaneous measurements of plasma drift velocity and particle precipitation in magnetically conjugate regions across the auroral zone in both hemispheres. This allows us to establish the degree to which the interhemispheric differences in ionospheric electric potential may be accounted for by field‐aligned potential drops. Thirty‐seven eligible cases examined show accelerated electron energy spectra at conjugate locations in the northern and/or southern hemispheres. Of these 37 cases, 23 show conjugate differences in electric potential that agree qualitatively with the observed electron acceleration and average energy of precipitating electrons in each hemisphere. This result suggests that the hemispheric differences in ionospheric electric potential in the auroral zone may be most frequently accounted for by field‐aligned potential drops along the magnetic flux tubes connecting conjugate points to the equatorial plane. The spatial and temporal variability of the potential distribution, electron precipitation, and magnetic field configuration could account for deviations from the normally observed situation in the remaining cases. More magnetically conjugate observations at various altitudes in both hemispheres are needed to investigate this hemispheric coupling in more detail.

Total electron content (TEC) and L‐band scintillations measured by several networks of GPS and GNSS receivers that operate in South and Central America and the Caribbean region are used to observe the morphology of the equatorial ionization anomaly (EIA), examine the evolution of plasma bubbles, and investigate the enhancement of L‐band scintillations that occurred on February 12 and 13, 2016. A few weak and short magnetic storms developed these days, and a minor sudden stratospheric warming (SSW) event was initiated a few days before. During these unusual conditions, TEC maps reported a split of the otherwise continuous crests of the EIA and the formation of a large‐scale (thousands of kilometers) almost‐circular structure. The western part of the southern crest faded, and a north‐south aligned segment developed near the center of the South American continent, joining the north and south crests of the EIA, forming an anomaly that resembled a closed loop on the eastern side of the continent. Concurrently with the anomaly events, several GPS stations reported increases in the L‐band scintillation index from 0.4 to values greater than 1. We analyzed TEC values from receivers between ±6° from the magnetic equator to identify and follow TEC depletions associated with plasma bubbles when they reach different stations. Although the magnetic activity was moderate (K_p = 3°), we believe that the anomaly redistribution and the scintillation enhancements are not related to a prompt penetration electric field but to enhancing the semidiurnal lunar tide propitiated by the onset of the minor SSW event. We found that depending on the lunar tide phase cycle, the neutral wind's meridional component can augment sub‐km scale irregularities and enhance L‐band scintillations through the wind gradient instability when U·n < 0 or the action of wind gradients (U) within the bubbles. Our observations imply that the SSW event enables prominent changes in the thermosphere wind system at F‐region altitudes.

Total Electron Content (TEC) and L-band scintillations measured by several networks of GPS and GNSS receivers that operate in South and Central America and the Caribbean region are used to observe the morphology of the equatorial ionization anomaly (EIA), examine the evolution of plasma bubbles, and investigate the enhancement of L-band scintillations that occurred on February 12 and 13, 2016. A few weak and short magnetic storms developed these days, and a minor Sudden Stratosphere Warming (SSW) event was initiated a few days before. During these unusual conditions, TEC maps reported a split of the otherwise continuous crests of the EIA and the formation of a large-scale (thousands of kilometers) almost-circular structure. The western part of the southern crest faded, and a north-south aligned segment developed near the center of the South American continent, joining the north and south crests of the EIA, forming an anomaly that resembled a closed loop on the eastern side of the continent. Concurrently with the anomaly events, several GPS stations reported increases in the L-band scintillation index from 0.4 to values greater than one. We analyzed TEC values from receivers between ±6° from the magnetic equator to identify and follow TEC depletions associated with plasma bubbles when they reach different stations. Although the magnetic activity was moderate (k _p =3 ⁰ ), we believe that the anomaly redistribution and the scintillation enhancements are not related to a prompt penetration electric field but to enhancing the semidiurnal lunar tide propitiated by the onset of the minor SSW event.

The high-latitude atmosphere is a dynamic region with processes that respond to forcing from the Sun, magnetosphere, neutral atmosphere, and ionosphere. Historically, the dominance of magnetosphere–ionosphere interactions has motivated upper atmospheric studies to use magnetic coordinates when examining magnetosphere–ionosphere–thermosphere coupling processes. However, there are significant differences between the dominant interactions within the polar cap, auroral oval, and equatorward of the auroral oval. Organising data relative to these boundaries has been shown to improve climatological and statistical studies, but the process of doing so is complicated by the shifting nature of the auroral oval and the difficulty in measuring its poleward and equatorward boundaries. This study presents a new set of open–closed magnetic field line boundaries (OCBs) obtained from Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) magnetic perturbation data. AMPERE observations of field-aligned currents (FACs) are used to determine the location of the boundary between the Region 1 (R1) and Region 2 (R2) FAC systems. This current boundary is thought to typically lie a few degrees equatorward of the OCB, making it a good candidate for obtaining OCB locations. The AMPERE R1–R2 boundaries are compared to the Defense Meteorological Satellite Program Special Sensor J (DMSP SSJ) electron energy flux boundaries to test this hypothesis and determine the best estimate of the systematic offset between the R1–R2 boundary and the OCB as a function of magnetic local time. These calibrated boundaries, as well as OCBs obtained from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) observations, are validated using simultaneous observations of the convection reversal boundary measured by DMSP. The validation shows that the OCBs from IMAGE and AMPERE may be used together in statistical studies, providing the basis of a long-term data set that can be used to separate observations originating inside and outside of the polar cap.

The spatial characteristics of mesoscale plasma perturbations in the high‐latitude ionosphere are important considerations for a more complete description of the energy deposition from the magnetosphere. In this study, ion drift and particle precipitation measurements from the Defense Meteorological Satellite Program F17 satellite during local summer seasons in 2012 are used to examine the flow perturbations and their closure paths and the particle precipitation associated with them. The closed circulation associated with a flow perturbation is identified from the potential distribution and can be described by a single‐cell or a two‐cell configuration with characteristic spatial scale sizes between 100 and 600 km and 200 and 1,200 km, respectively. Observations suggest that an asymmetry in space and flow speed is frequently seen with faster flows in a narrower region and slower return flows in wider adjacent regions. For a single‐cell configuration, the faster flows are preferentially sunward and in the same direction as the background convection, and the weaker return flows are preferentially duskward/dawnward of the potential maxima/minimum. For a two‐cell configuration, the faster flow is seen in the central region and also tends to follow the background convection with weaker return flows on the two sides. Except in the region poleward of the convection reversal boundary, 0.5–3 keV electron precipitation, displaying the same spatial asymmetry as the flow perturbation, is frequently seen in a single cell around a potential minimum on the dusk side.

The high latitude atmosphere is a dynamic region with processes that respond to forcing from the Sun, magnetosphere, neutral atmosphere, and ionosphere. Historically, the dominance of magnetosphere-ionosphere interactions has motivated upper atmospheric studies to use magnetic coordinates when examining magnetosphere-ionosphere-thermosphere coupling processes. However, there are significant differences between the dominant interactions within the polar cap, auroral oval, and equatorward of the auroral oval. Organising data relative to these boundaries has been shown to improve climatological and statistical studies, but the process of doing so is complicated by the shifting nature of the auroral oval and the difficulty in measuring its poleward and equatorward boundaries. This study presents a new set of open-closed magnetic field line boundaries (OCBs) obtained from Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) magnetic perturbation data. AMPERE observations of field aligned currents (FACs) are used to determine the location of the boundary between the Region 1 (R1) and Region 2 (R2) FAC systems. This current boundary is thought to typically lie a few degrees equatorward of the OCB, making it a good candidate for obtaining OCB locations. The AMPERE R1/R2 boundaries are compared to the Defense Meteorological Satellites Program Special Sensor J (DMSP SSJ) electron energy flux boundaries to test this hypothesis and determine the best estimate of the systematic offset between the R1/R2 boundary and the OCB as a function of magnetic local time. These calibrated boundaries, as well as OCBs obtained from Magnetopause-to-Aurora Global Exploration (IMAGE) observations, are validated using simultaneous observations of the convection reversal boundary measured by DMSP. The validation shows that the OCBs from IMAGE and AMPERE may be used together in statistical studies, providing the basis of a long-term data set that can be used to separate observations originating inside and outside of the polar cap.

Spatial and temporal characteristics of flow perturbations in the high‐latitude ionosphere are important considerations for energy deposition from the magnetosphere. In this study, we examine the temporal characteristics of plasma flow perturbations with spatial scales between 100 and 400 km from two consecutive Defense Meteorological Satellite Program (DMSP) passes that have about the same orbital plane and sample time spacing between a few seconds and 20 min during local summer seasons in 2007‐2015. The temporal characteristics of mesoscale flow perturbations are described by rise and saturation times for growth and decay derived from the changes in magnitude of perturbations and the time separation between consecutive samples. Observations suggest that the rise times for both growth and decay are shorter for small spatial scales (1–2 min, 100–200 km) and longer for large spatial scales (3–5 min, 200–400 km). The saturation time for decay is ~10 min for small scales and ~20 min for large scales. The growth saturation time is about 5–10 min for both scale sizes. These characteristic times for growth are always shorter than the decay times. If the difference in these characteristic times between growth and decay is produced by motion of a perturbation with the background flow through the observed volume, then a longitudinal scale size of 750 km or 1.5 hr of local time is implied.