Roderick A. Heelis

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.

We provide ionospheric storm timing from multiple topside satellites (four DMSP spacecraft) along with a vast database of ionospheric and thermospheric measurements, allowing comprehensive investigation of the timing and response of the ionosphere and thermosphere to the ionospheric storm associated with the geomagnetic storm of 26 September 2011. Such a comprehensive study of ionosphere storm timing and dynamics has not been reported previously. The positive phase of the ionospheric storm peaked near the end of the geomagnetic storm growth phase and continued several hours into the recovery phase before entering the negative phase. During the positive phase, neutral heating and vertical motion at high latitudes led to dramatic decreases in the O/N ₂ ratio that extended well into the mid‐latitudes in both hemispheres and lasted throughout the storm recovery phase. The ion temperature in the topside decreased during the positive phase and increased during the negative phase as a result of vertical ion velocities that extended into the mid‐latitudes during the positive phase, leading to adiabatic cooling and movement of the O ⁺ /H ⁺ transition height to higher altitudes, followed by vertically downward velocities and adiabatic heating. Density increases at the equator early in the positive phase are attributed to vertical drifts associated with the eastward zonal electric field driven by a penetration electron field while the merging of the Appleton peaks observed in the vTEC data is attributed to the reverse fountain effect associated with a change in the zonal electric field from eastward to westward and downward ion drifts.

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.

Continuous operation of the ICON mission during 2021 and 2022 allows a description of the thermal plasma number density, composition and temperature to be obtained at low and middle latitudes near 600 km altitude, during low and moderate solar activity levels. With appropriate averaging of the data over a 2‐month period, we describe the latitude, longitude and local time distributions of the ion number density, composition and temperature to reveal systematic differences at different seasons and different levels of solar activity. We find that near the equator the daytime ion temperature near 600 km is relatively insensitive to changes in solar activity, while at middle latitudes the ion temperature is higher at low solar activity levels than at moderate solar activity levels. At night, the topside ion temperature cools toward the neutral gas temperature and the ion temperature is lower at low solar activity than at moderate solar activity. This behavior is modified by seasonal variations, which show that the presence of interhemispheric flows induced by F‐region winds changes the latitude distribution of ion density, composition and temperature.

We report an extraordinary L‐band scintillation event detected in the American sector on the night of 23–24 March 2023. The event was detected using observations distributed from the magnetic equator to mid latitudes. The observations were made by ionospheric scintillation and total electron content (TEC) monitors deployed at the Jicamarca Radio Observatory (JRO, ∼−1° dip latitude), at the Costa Rica Institute of Technology (CRT, ∼20° dip latitude), and at The University of Texas at Dallas (UTD, ∼42° dip latitude). The observations show intense pre‐ and post‐midnight scintillations at JRO, a magnetic equatorial site where L‐band scintillation is typically weak and limited to pre‐midnight hours. The observations also show long‐lasting extremely intense L‐band scintillations detected by the CRT monitor. Additionally, the rare occurrence of intense mid‐latitude scintillation was detected by the UTD monitor around local midnight. Understanding of the ionospheric conditions leading to scintillation was assisted by TEC and rate of change of TEC index (ROTI) maps. The maps showed that the observed scintillation event was caused by equatorial plasma bubble (EPB)‐like ionospheric depletions reaching mid latitudes. TEC maps also showed the occurrence of an enhanced equatorial ionization anomaly throughout the night indicating the action of disturbance electric fields and creating conditions that favor the occurrence of severe scintillation. Additionally, the ROTI maps confirm the occurrence of pre‐ and post‐midnight EPBs that can explain the long duration of low latitude scintillation. The observations describe the spatio‐temporal variation and quantify the severity of the scintillation impact of EPB‐like disturbances reaching mid latitudes.

First results are presented from the conjugate maneuvers performed by NASA's Ionospheric Connection Explorer (ICON) spacecraft. During each several‐minute maneuver, ICON crosses the magnetic equator, measuring the plasma drift at the ∼600‐km apex of a magnetic field line and the neutral wind profiles (∼90–300 km altitude) along both ends of that field line. The analysis utilizes 149 pairs of maneuvers separated by ∼24 hr but at nearly the same location and local time. Principal component regression reveals that 39 ± 7% and 24 ± 9% of the day‐to‐day variance in the daytime vertical and zonal drift, respectively, is attributable to conjugate neutral winds. The remaining variance is likely driven by external potentials from non‐conjugate winds and geomagnetic activity (median Kp 2−). Zonal winds at 100–113 km and >120 km altitude are the primary drivers of conjugate vertical and zonal drift variance, respectively. These observations can test vertical‐coupling mechanisms in whole‐atmosphere models.

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.

Whitepaper #351 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research. Additional topics: planetary ionospheres / upper atmospheres; space-based missions/projects.

Measurements from the Ionospheric Connections Explorer satellite (ICON) form the basis of direct numerical forecast simulations of plasma convective instability in the postsunset equatorial F region ionosphere. ICON data are selected and used to initialize and force the simulations and then to test the results one orbit later when the satellite revisits the same longitude. Data from the IVM plasma density and drifts instrument and the MIGHTI red‐line thermospheric winds instrument are used to force the simulation. Data from IVM are also used to test for irregularities (electrically polarized plasma depletions). Fourteen datasets from late March 2022, were examined. The simulations correctly predicted the occurrence or non‐occurrence of irregularities 12 times while producing one false positive and one false negative. This demonstrates that the important telltales of instability are present in the ICON state variables and that the important mechanisms for irregularity formation are captured by the simulation code. Possible refinements to the forecast strategy are discussed.

Techniques developed in the past few years enable the derivation of high‐resolution regional ion convection and particle precipitation patterns from the Super Dual Auroral Radar Network (SuperDARN) and Time History of Events and Macroscale Interactions during Substorms All‐Sky Imager (ASI) observations, respectively. For the first time in this study, a global ionosphere‐thermosphere model (GITM) is driven by such high‐resolution patterns to simulate the I‐T response to the multi‐scale geomagnetic forcing during a real event. Specifically, GITM simulations have been conducted for the 26 March 2014 event with different ways to specify the high‐latitude forcing, including empirical models, high‐resolution SuperDARN convection patterns, and high‐resolution ASI particle precipitation maps. Multi‐scale ion convection forcing estimated from high‐resolution SuperDARN observations is found to have a very strong meso‐scale component. Multi‐scale convection forcing increases the regional Joule heating (integrated over the high‐resolution SuperDARN observation domain) by ∼30% on average, which is mostly contributed by the meso‐scale component. Meso‐scale electron precipitation derived from ASI measurements contributes on average about 30% to the total electron energy flux, and its impact on the I‐T system is comparable to the meso‐scale convection forcing estimated from SuperDARN observations. Both meso‐scale convection and precipitation forcing are found to enhance ionospheric and thermospheric disturbances with prominent structures and magnitudes of a few tens of meters per second in the horizontal neutral winds at 270 km and a few percent in the neutral density at 400 km through comparisons between simulations driven by the original and smoothed high‐resolution forcing patterns.