The formation of layers at mid-latitudes has been related to neutral winds activity at altitudes below 130km in the Mesosphere and Lower Thermosphere (MLT). Recent SAMI3 simulations by Krall et al. (2020) of ionospheric metallic layers at Arecibo suggest that forces induced by the meridional winds cause low altitude layers near 100 km. However, the classic mechanism, originally proposed by Whitehead (1961), correctly states that zonal wind shear has a bigger effect than meridional wind shear in the lower E region. Haldoupis and Shalimov (2021), referring to observations of ionosonde-based sporadic E statistics and radio occultation sporadic E measurements using low Earth orbiting satellites, support the idea that zonal winds dominate layer formation at these altitudes, apparently disputing the findings of Krall et al. (2020). Perhaps the latest technique to continuously measure mid-latitude MLT daytime neutral winds was developed by Hysell et al. (2014). That technique used a unique configuration of the Arecibo radar dual-beam. Unfortunately, since Arecibo lost the capability of the dual-beam in 2017 (when one antenna was destroyed by Hurricane Maria), there are only few valuable data sets that can help elucidate the origin of the lower altitude layers at Arecibo. We present Arecibo neutral wind data correlated with lower altitude layers. While not disputing current theory, we find that, near 100 km, meridional neutral wind shear can be much stronger than zonal wind shear when a layer is present, with the meridional shear correctly positioned to support the layer. We also present a complete analysis of the vertical ion drift, including declination, where the meridional winds become more important and with a reversed mechanism for altitudes below 115km for Arecibo conditions. References: Haldoupis et al. (2021), https://doi.org/10.1016/j.jastp.2021.105537 Hysell et al. (2014), http://doi.org/10.1002/2013JA019621 Krall et al. (2020), https://doi.org/10.1029/2019JA027297

The Arecibo Observatory (AO) could modify the ionosphere using high frequency (HF) waves. During the HF experiments, the incoherent scatter radar (ISR) was used to study the behavior of the ion, plasma, and gyro lines with 150m height resolution. One year ago, the AO platform collapsed and put a pause for new experiments. However, the archived ISR data can answer open questions like the electron heating evolution in the interaction region. This paper presents a new methodology to estimate the electron temperature (Te) at the resonance altitude based on the physics of the HF wave-plasma interaction. Estimating Te inside regions where the ionospheric plasma interacts with the powerful HF ground waves is a challenge. Standard ISR techniques to assess the temperatures using the ion line are based on Maxwellian approximations. However, the irregularities generated by HF experiments induce non-Maxwellian behaviors. Therefore, a new approach is proposed using the ion-acoustic phase velocity (C_ia) of the ion-acoustic waves generated during the HF experiments. The ion acoustic velocity can be derived from the ISR enhanced plasma line (HFPL) produced during active experiments. The HFPL is mainly attributed to the HF wave decaying into a cascade of Langmuir and ion-acoustic waves, known as Parametric Decay Instability (PDI). The ion-acoustic waves travel at speed: C_ia=λf_ia, where f_ia is the ion-acoustic frequency, and λ is the radar (Bragg backscatter) wavelength. The PDI signature is characterized at the HFPL by cascaded lines spaced in frequency by multiples of f_ia. After measuring f_ia, Te is obtained using f_ia=1/λ √(k_B (Te+2Ti)/m_i), where Ti and mi are the ion temperature and mass. Estimates for one particular experiment on June 12, 2019 show that Te is usually higher at the top of the layer and the beginning of every HF pulse. For example, at 280s after 16:43:00LT, it reached a value near 3500 K, when the temperature outside of the interaction region was below 1600 K.

The Naval Research Laboratory (NRL) Sami3 is Also a Model of the Ionosphere (SAMI3) ionosphere/plasmasphere code is used to examine the physics of metallic layers at altitudes from 80 to 160 km. Results are presented near the simulated location of the Arecibo observatory (18N, 66W). We find that simulations, using winds from the empirical horizontal wind model (HWM14), produce layers consistent with those observed at Arecibo. Specifically, we find upper semidiurnal and lower diurnal traces similar to those identified in previous observational surveys. While metallic layers are shaped by meridional winds, zonal winds, and electric fields, much of the observed structure is found if only meridional wind forces are included in the model. Stratification below 110 km, where the ions are very weakly magnetized, is supported mainly by meridional wind shear.