This study develops a new Bubble Index to quantify the intensity of 2‐D postsunset equatorial plasma bubbles (EPBs) in the American/Atlantic sector, using Global‐scale Observations of the Limb and Disk (GOLD) nighttime data. A climatology and day‐to‐day variability analysis of EPBs is conducted based on the newly‐derived Bubble Index with the following results: (a) EPBs show considerable seasonal and solar activity dependence, with stronger (weaker) intensity around December (June) solstice and high (low) solar activity years. (b) EPBs exhibit opposite geomagnetic activity dependencies during different storm phases: EPBs are intensified concurrently with an increasing Kp, but are suppressed with high Kp occurring 3–6 hr earlier. (c) For the first time, we found that EPBs' day‐to‐day variation exhibited quasi‐3‐day and quasi‐6‐day periods. A coordinated analysis of Ionospheric Connection Explorer (ICON) winds and ionosonde data suggests that this multi‐day periodicity was related to the planetary wave modulation through the wind‐driven dynamo.

We describe a new class of nonthermal plasma density irregularities observed in the postmidnight topside equatorial ionosphere under low solar flux conditions. They are distinct from irregularities associated with equatorial spread F (ESF) in terms of their morphology and because they exhibit strong spectral sidebands at the lower‐hybrid frequency. The coherent echoes were observed in a series high‐altitude radar experiments performed at Jicamarca utilizing long‐ and coded double‐pulse modes and a dual‐beam mode. The coded double‐pulse mode was used to measure the low‐frequency characteristics of the echoes with fine range resolution. Doppler shifts of the main backscatter line were observed to fall between ±150 m/s. The long‐pulse mode was employed for high‐frequency spectral analysis which revealed the presence of strong spectral sidelobes at the lower‐hybrid frequency. A dual‐beam mode was used to investigate the horizontal structure of the echoes. Zonal drift speeds of 50–70 m/s were inferred with this mode, and longitudinal dimensions of approximately 270 km were estimated. The study summarizes with a discussion of different mechanisms that may be responsible for the phenomenon and the lower‐hybrid sidebands in particular.

High altitude echoes are topside or lower plasmaspheric coherent echoes that have been observed at altitudes between 1,000 and 2,200 km above Jicamarca and occur mainly in the postmidnight and predawn sectors (Derghazarian et al., 2021;Hysell et al., 2019). Their morphological and spectral characteristics are different from ESF. They are characterized by thin layers rather than convective plumes, and they exhibit spectral sidebands at the lower hybrid frequency.Two processes thought to generate lower hybrid waves in the ionosphere, the lower hybrid drift instability (LHDI), and linear mode conversion of VLF whistlers via density depletions found in lower hybrid solitary structures (LHSS), were described in Derghazarian et al. (2021) as potential sources of lower hybrid waves in high altitude echoes. The LHDI in particular was shown to require the assistance of a cross-coupling mechanism for the instability to be able to excite lower hybrid frequencies at the 3 m wavelengths measured by the Jicamarca radar. Experimental evidence favoring one mechanism over the another is not definitive, however.One argument favoring the LHDI is that it has shown promise as a potential source of low frequency irregularities in the equatorial F region. Kelley and McClure (1981) and Huba and Ossakow (1981a) argue that LHDI is the most likely explanation for 36 and 11 cm irregularities seen by the ALTAIR (Huba et al., 1978;Tsunoda, 1980) and TRADEX radars (Tsunoda, 1980), respectively, in conjunction with equatorial spread F (ESF). Apart from these studies, the theoretical treatments and reported measurements of the LHDI in the literature generally concern very low density, high temperature space plasmas such as at the magnetopause (Bale et al., 2002;Vaivads et al., 2004), in the magnetotail (Huba et al., 1978;Norgren et al., 2012), and at the plasmapause boundary (Mishin, 2013), but little is known about its prevalence in the equatorial inner plasmasphere.The range of density gradient scale lengths (L n = (∇n/n) −1 ) present in the background plasma is a key factor that determines the frequencies produced by the LHDI. Experimental measurements of L n have been performed in the O + dominated F-region examined by Huba (1981) and Kelley et al. (1982) but such measurements are lacking in the inner plasmasphere.If these gradients can become as steep as approximately half the proton gyroradius (L n ≈ 1.5 m) under some circumstances, along with the appropriate background plasma parameters, the LHDI can produce waves at the lower hybrid frequency with growth rates close to the maximum growth rate at the altitudes inhabited by the echoes. We argue in the text below that only for a narrow frequency band around the lower hybrid frequency, with waves that are of sufficient amplitude, and with wavenumbers that are not excessively large can the LHDI

We present measurements of the equatorial topside ionosphere above Jicamarca made during extremely low solar flux conditions during the deep solar minimum of 2019-2020. Measurements were made in October, 2019, February, 2020, and September, 2020. The main features observed are a large and extended decrease in noontime temperatures unlike that seen in studies at moderate solar flux levels, predawn ionospheric heating as early as 0300 LT, large day-to-day variability in the O + /H + transition height, and negligible helium ion concentration at all altitudes. Data from the Ion Velocity Meter (IVM) instrument onboard the Ionospheric Connection Explorer (ICON) and the Topside Ionospheric Plasma Monitor (SSIES) onboard the Defense Meteorological Satellite Program (DMSP) satellites are used to assess agreement with ISR data and assist with the analysis of the predawn heating phenomena. We also analyze the data in light of the SAMI2-PE model which shows less agreement with the data than at higher solar flux. The main areas of discrepancy with the data are outlined, such as the absence of significant predawn heating, less pronounced decreases in noontime temperatures, and much higher O + fractions at high altitudes, particularly in September. Finally, a sensitivity analysis of the model to various forcing agents such as neutral winds, plasma drifts, solar flux, and heat flow is performed. A discussion is presented on bridging the discrepancies in future model runs. Novel techniques of clutter removal and noise power bias correction are introduced and described in the appendices.