Low Earth orbit (LEO) radio occultation (RO) constellations can provide global electron density profiles (EDPs) to better specify and forecast the ionosphere-thermosphere (I-T) system. To inform future RO constellation design, this study uses comprehensive Observing System Simulation Experiments (OSSEs) to assess the ionospheric specification impact of assimilating synthetic EDPs into a coupled I-T model. These OSSEs use 10 different sets of RO constellation configurations containing 6 or 12 LEO satellites with base orbit parameter combinations of 520 km or 800 km altitude, and 24 degrees or 72 degrees inclination. The OSSEs are performed using the Ensemble Adjustment Kalman Filter implemented in the Data Assimilation Research Testbed and the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM). A different I-T model is used for the nature run, the Whole Atmosphere Model-Ionosphere Plasmasphere Electrodynamics (WAM-IPE), to simulate the period of interest is the St. Patrick’s Day storm on March 13-18, 2015. Errors from models and EDP retrieval are realistically accounted for in this study through distinct I-T models and by retrieving synthetic EDPs through an extension Abel inversion algorithm. OSSE assessment, using multiple metrics, finds that greater EDP spatial coverage leading to improved specification at altitudes 300 km and above, with the 520 km altitude constellations performing best due to yielding the highest observation counts. A potential performance limit is suggested with two 6-satellite constellations. Lastly, close examination of Abel inversion error impacts highlights major EDP limitations at altitudes below 200 km and dayside equatorial regions with large horizontal gradients and low electron density magnitudes.

The explosive eruption of the Hunga-Tonga volcano in the southwest Pacific at 0415UT on 15 January 2022 triggered gigantic atmospheric disturbances with surface air pressure wave propagating around the globe in Lamb mode. In space, concentric traveling ionosphere disturbances (CTIDs) are also observed as a manifestation of air pressure acoustic waves in New Zealand ~0500UT and Australia ~0630UT. As soon as the wave reached central Australia ~0800UT, CTIDs appeared simultaneously in the northern hemispheres through magnetic field line conjugate effect, which is much earlier than the arrival of the air pressure wave to Japan after 1100UT. Combining observations over Australia and Japan between 0800-1000UT, CTIDs with characteristics of phase velocities of 320-390 m/s are observed, matching with the dispersion relation of Lamb mode. The arrival of atmospheric Lamb wave to Japan later created in situ CTIDs showing the same Lamb mode characteristics as the earlier arriving CTIDs.

The Tonga volcano eruption of 15 January 2022 unleashed a variety of atmospheric perturbations, coinciding with the recovery phase of a geomagnetic storm. The ensuing thermospheric variations created rare display of extreme poleward-expanding conjugate plasma bubbles seen in the rate of total electron content index (ROTI) over 100-150°E. This is associated with fluctuations in FORMOSAT-7/COSMIC-2 (F7/C2) ion-density measurements and spread-F signatures in ionograms, reaching ~40°N geographic latitude. This was preceded by an unusually strong pre-reversal enhancement (PRE) in the global ionospheric specification (GIS) electron density profiles derived from F7/C2 observations. The GIS further revealed a decrease of equatorial ionization anomaly (EIA) crest density due to the storm impact. A sharp decrease of E-region conductivity by volcano-induced waves, combined with enhanced F-region wind over EIA with less ion-drag apparently intensified the PRE. The strong PRE and seed perturbations from the volcano-induced waves likely further triggered super plasma bubble activity.

Equatorial plasma bubbles (EPBs) are elongated plasma depletions that can occur in the nighttime ionospheric F region, causing scintillation in satellite navigation and communications signals. EPBs are believed to be Rayleigh-Taylor instabilities seeded by vertically propagating gravity waves. A necessary pre-condition for EPB formation is a threshold vertical ion drift from the E region, which is required to produce the vertical plasma gradients conducive to this instability. Factors affecting the variation of EPBs therefore include magnetic declination, the strength of the equatorial electojet, and the wind dynamo in the lower thermosphere controlling vertical plasma drifts. In most longitude zones, this results in elevated EPB occurrence rates during the equinoxes. The notable exception is over the central Pacific and African sectors, where EPB activity maximizes during solstice. \citet{tsunoda_jgr2015} hypothesized that the solstice maxima in these two sectors could be driven by a zonal wavenumber 2 atmospheric tide in the mesosphere and lower thermosphere. In this study, we find that the post-sunset electron density observed by FORMOSAT-3/COSMIC during the boreal summer from 2007 - 2012 does indeed exhibit a wave-2 zonal distribution, consistent with results expected from elevated vertical ion drift over the Central Pacific and African sectors. Numerical experiments are also carried out which found that forcing from the aforementioned tidal and stationary planetary wave (SPW) components produced wave-2 modulations on vertical ion drift, ion flux convergence, and midnight TEC. The relation between the vertical ion drift enhancements and the midnight TEC enhancements are consistent with the solstice maxima hypothesis.

This work explores the results of applying Square Euclidean and Correlation k-means cluster analysis on ion density irregularity profiles observed by the Advanced Ionospheric Probe (AIP) onboard the Formosat-5 (F-5) satellite from November 2017 to November 2020. The Square Euclidean cluster analysis yielded separate clusters each for ion density irregularities consistent with Equatorial Plasma Bubbles (EPBs) occurring over the southern low-latitude, northern low-latitude and equator. The Correlation k-means cluster analysis only yielded one cluster characterized by ion density irregularities consistent with EPBs occurring over the equator. Thus, this work suggests that a cluster analysis preferably a Square Euclidean Cluster analysis can be used to find and classify ion density irregularities consistent with EPBs. This work also shows that the F-5/AIP can perform multi-year observations of ion density irregularities at ~710 km altitude and at ~2230 pm local-time that are consistent with irregularities due to EPBs.