Ionospheric day-to-day variability is ubiquitous, even under undisturbed geomagnetic and solar conditions. In this paper, quiet-time day-to-day variability of equatorial vertical E × B drift is investigated using observations from ROCSAT-1 satellite and the Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (WACCM-X) v2.1 simulations. Both observations and model simulations illustrate that the day-to-day variability reaches the maximum at dawn, and the variability of dawn drift is largest around June solstice at~90-180°W. However, there are significant challenges to reproduce the observed magnitude of the variability and the longitude distributions at other seasons. Using a standalone electro-dynamo model, we find that the day-to-day variability of neutral winds in the E-region (≤~130 km) is the primary driver of the day-to-day variability of dawn drift. Ionospheric conductivity modulates the drift variability responses to the E-region wind variability, thereby determining its strength as well as its seasonal and longitudinal variations. Further, the day-to-day variability of dawn drift induced by individual tidal components of winds in June are examined: DW1, SW2, D0, and SW1 are the most important contributors.Plain Language Summary The ionosphere is different from one day to the next, even under geomagnetic and solar quiet condition. The vertical E × B drift at the geomagnetic equator is a key parameter that influences the state of the ionosphere and atmosphere. In this paper, we study the quiet-time day-to-day variability of the equatorial vertical E × B drift by ROCSAT-1 observations and the Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (WACCM-X) v2.1 simulations. Both observations and WACCM-X show that day-to-day variability is large at dawn and dusk and it also changes with season and longitude. To better study the variability, we carried out numerical experiments with a new standalone electro-dynamo model. We found that wind variability below~130 km is the main contributor, and winds above~130 km plays a secondary role. The ionospheric conductivity mediates the drift variability response to the wind variability and thus affects its strength and seasonal and longitudinal variations. Further, we examine the variability of dawn drifts generated by different tidal components in June and find that DW1, SW2, D0, and SW1 are the most important ones. This work emphasizes the importance of lower atmospheric variability in studying and predicting the day-to-day variations of ionosphere and space environment. Key Points:• We quantify the quiet-time day-to-day variability of equatorial vertical drift as a function of local time, longitude, and season • Both observations and models show the largest variability occurring at LT 06, which is mainly driven by the variability of E-region winds • The variability of dawn drift induced by tidal components are examined,

A dramatic thermospheric temperature enhancement and inversion layer (TTEIL) was observed by the Fe Boltzmann lidar at McMurdo, Antarctica during a geomagnetic storm (Chu et al. 2011,

Radar echoes from the daytime lower F region near the magnetic equator, so-called 150-km echoes, have been puzzling researchers for decades. Neither the mechanisms that generate the enhanced backscatter at very high frequencies (typically 30-50 MHz), the sharp lower cutoff height, the intricate layering with multiple echo layers separated by narrow gaps, nor the modulation of the echoes by short-period gravity waves is well understood. Here we focus on the diurnal variation of the echo layers-specifically, certain wide gaps in the vertical structure-which apparently descend in the morning, reach their lowest altitude near local noon, and ascend in the afternoon, sometimes described as necklace structure based on the appearance of the layers in range-time-intensity diagrams. Analyzing high-resolution data obtained with the Jicamarca radar between 2005 and 2017, spanning more than one solar cycle, we find that (a) wide gaps and narrow lines occur in vertically stacked, systematically repeating pattern; (b) the gap heights vary with season and solar cycle; and (c) the gap heights can be associated with specific contours of plasma frequencies or electron densities. The last two findings are supported by simultaneous observations of VIPIR ionosonde reflection heights and by comparison of gap heights with electron density contours obtained with the WACCM-X 2.0 global model. Finally, the wide gaps appear to coincide with the double resonance condition, where the upper hybrid frequency equals integer multiples of the electron gyrofrequency. This may explain why field-aligned plasma irregularities are suppressed and enhanced radar backscatter is not observed inside the gaps.

Analyzing Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observations from 2003 to 2018, the interannual variability of 2–5d eastward propagating planetary waves is found to correlate positively with zonal‐mean zonal winds averaged over 67.5°±10°S but negatively with the quasi‐biennial oscillation (QBO) index in austral winter. The composite‐mean wave amplitudes are ~20% larger in QBOe than in QBOw. On statistical average, the poleward flank strengthening and the equatorward flank weakening of polar night jet (PNJ) during QBOe form a dipole‐cell pattern. In contrast, only a single negative cell is seen in the Northern Hemisphere zonal‐mean zonal winds (January) previously explained by the Holton‐Tan theory. Such difference implies an interhemispheric asymmetry and other processes needed to explain the additional positive cell in Antarctica. Mechanistic modeling illustrates that the stronger PNJ generates eastward propagating planetary waves with larger growth rates (stronger waves) in QBOe than QBOw, explaining the QBO‐like signal in the Antarctic planetary waves.

An interplanetary shock can abruptly compress the magnetosphere, excite magnetospheric waves and field‐aligned currents, and cause a ground magnetic response known as a sudden commencement (SC). However, the transient (<∼1 min) response of the ionosphere‐thermosphere system during an SC has been little studied due to limited temporal resolution in previous investigations. Here, we report observations of a global reversal of ionospheric vertical plasma motion during an SC on 24 October 2011 using ∼6 s resolution Super Dual Auroral Radar Network ground scatter data. The dayside ionosphere suddenly moved downward during the magnetospheric compression due to the SC, lasting for only ∼1 min before moving upward. By contrast, the post‐midnight ionosphere briefly moved upward then moved downward during the SC. Simulations with a coupled geospace model suggest that the reversed trueE⃗×B⃗ $\vec{E}\times \vec{B}$ vertical drift is caused by a global reversal of ionospheric zonal electric field induced by magnetospheric compression during the SC.