Upper atmospheric long-term trends could be examined in the ion temperatures ($T_i$) at the ionospheric F-region altitudes by the close coupling between neutrals and ions. We have analyzed the $T_i$ data sets of Arecibo Observatory (AO) incoherent scatter radar (18\textdegree20’N, 66\textdegree45’W) from 1985 to 2019, to examine the long-term trends of the ion temperature as a function of height from $\sim$140 km to $\sim$677 km. For this, the responses of $T_i$ to solar and geomagnetic activities have been taken into account as forcings of the $T_i$ behavior as well the annual and semi-annual oscillations. By removing the known forcing that govern the Ti behavior by the difference between the $T_i$ data and a climatological model, our results indicate that the upper atmosphere/ionosphere over Arecibo is cooling over the 35 years studied. Around 350 km, our findings also show that the rate of cooling over Arecibo is lower than previously reported for high latitudes, suggesting a latitudinal dependency. These cooling trends are believed to be the result of increasing green house gases, but the observed cooling trends exceed the magnitude of the cooling expected from green house gases. We have made an attempt to find the additional driver for observed cooling trends by linking the these upper atmospheric trends to lower atmospheric weather phenomena. We found that gravity waves in the lower atmosphere associated with terrestrial weather phenomena might be contributing to the observed cooling trends in the upper atmosphere.

We employ in this work the first airglow dataset registered at the Remote Optical Facility (ROF) in Culebra, Puerto Rico, during the descending phase of the solar cycle #24. From November 4, 2015, to September 26, 2019, observations were carried out during 633 nights at ROF using a small all-sky imager, while events were identified in 225 of 499 nights classified as clear. A quantitative analysis of these and their dependency by geophysical parameters (solar and geomagnetic activities) are the main focus of this study. We introduce an original statistical methodology that examines the unique features of the dataset and minimizes the cross-contamination of individual modulators onto one another, avoiding bias in the results. Our findings include a primary peak of occurrence in the December solstice and a secondary peak in the June solstice. We observed a remarkable correlation in the occurrence rate of the with the geomagnetic activity. A notable modulation of the occurrence rate with the solar activity is also found, which includes periods of correlation and anti-correlation depending on the season. This modulation has an annual component that is ~33% and ~83% stronger than the semi-annual and terannual components, respectively. We discuss these findings based on the behavior of the thermospheric neutral winds derived from 30 years of Fabry-Perot interferometer observations. Our results, which are valid for low to moderate solar activity, point out circumstances that might explain differences in previous climatological studies of nighttime

Using ionograms from the Brazilian equatorial region of São Luís (SL, 2° S; 44° W, I =-3.8°), the relation between the uplift of the intermediated layers (ILs) at sunset and the prereversal enhancement of the zonal electric field (PRE) is investigated. The ILs were studied during periods of maximum (2003) and minimum solar activity (2009). The presence of the ILs during the PRE occurrence time was very low for both years. In 2003, six ILs’ events were observed, being four of them in the summer solstice and one in the March equinox. In 2009, only a single event was registered and occurred during the December month. The results show that depending on the height at which the ILs are located, their upward movement at sunset can be in some way related to the normal F layer rise at sunset due to the PRE. Additionally, the eastward prompt penetration electric fields (PPEF) during weak magnetic storms can also contribute to the IL’s rise. An interesting case of uplift of a sporadic-E layer from ~120 to 290 km of altitude probably due to the PPEF is investigated. The absence of answers of the F layer to the same disturbed electric field reveals the complex nature of the layers located in the ionosphere valley region.

Intermediate layers (ILs) are regions of enhanced electron density located in the ionospheric valley that extends from the peak altitude of the daytime E-region to the bottom side of the F-region. This work presents the daytime behavior of the ILs parameters (the virtual height - h’IL, and the top frequency - ftIL) for the deepest solar minimum of the last 500 years. In such a unique condition, this research reveals for the first time the ILs’ quiet state seasonal behavior as well as its responses to moderate changes in the geomagnetic activity. Among the finds, it is highlight the annual periodicity of the ftIL while the h’IL presents semiannual component. The results also show that even small variations of geomagnetic activity (quantified by the planetary Kp index) are able to modify the dynamics of the ILs parameters. For the first time, it was observed that during December solstice and September equinox, the h’IL/ftIL decrease/increase rapidly with the increase of geomagnetic activity at the beginning of the day. As the day progresses, smoothed rise in the h’IL is observed at the same time in which a considerable decrease in the ftIL occurs, except during June solstice when a different behavior is observed both in relation to the annual as the seasonal average values of the ftIL.

High frequency (HF) experiments inducing intensification of airglow emissions at 6300 Å; and 5577 Å (red and green lines, respectively) from the two lowest excited states of oxygen O(1D) and O(1S) has been studied since the early 1970s. The last generation Arecibo HF facility was commissioned in November 2015, and since then several campaigns have been conducted at AO. The current system consists of six transmitters, each connected to one of six dipole elements, and each capable of continuous wave (CW) operation at a nominal power of ~100 kW. Before the AO platform collapse on December 1, 2020, the HF transmitter system has two available frequencies, 5.125 MHz and 8.175 MHz, with 130 kHz and 100 kHz bandwidths, respectively. In this work we are analyzing the excitation of the red line airglow emission (3600 Å) by high-power radio waves at ~5.125MHz of 28 HF pulses of 5 minutes intercalated by 5 minutes of no HF interaction. The chosen periods were the pre-sunrise and post-sunset periods of June 5, 2016 (Figure 1). Coincidentally, a small geomagnetic storm occurred during these observations. The first experiment started along the initial phase of this disturbance and the second experiment at the end of the main phase (Figure 2). Up to now, our main findings are listed below: 1. Assuming that the modified red line comes from a narrow height range in the vicinity of the reflection height to a first approximation and considering that all of the excess emission comes from a single height (equation 1) (which corresponds to the height where the plasma frequency equals the transmitted frequency), it was detected that the lifetime of the O(1D) varies with altitude which a peak close to the red line emission altitudes (Figure 3). Θ_r=1/((T1+T2+T3+T4)) (1) Where T1 is the total Einstein transition probability of the O(1D) state, T2 is the N2 concentration at the altitude of reflection times its respective quenching coefficient (Q~5,0.10-11cm3s-1) as well as T3 the O2 concentration times its respective quenching coefficient (R~7,4.10-11cm3s-1). 2. Assuming a fixed lifetime for all altitudes, we detected variation of the N2 quenching coefficient O(1D) also varying with altitude. Such variation could be a miss determination of the N2 neutral concentration from the NRLMSISE-00 Atmosphere Model (Figure 4) (equation 1). 3. As a practical outcome, our study shows that the 5 minutes off is not sufficient for the excited region to return to the previous quiet condition. Our computations show that pulses of 3 minutes intercalated by 6 minutes off are the ones more appropriate (Figure 5).

Key Points: • Gravity waves impacts in the anomalous responses of the F layer over Brazil during a counter electrojet event; • Some signatures in ionograms suggest the gravity wave as responsible for the CEJ occurrence in this day. Abstract In this work, we report the ionospheric F-layer responses over the Brazilian equatorial sector to a counter electrojet (CEJ) event that occurred during the solar minimum period of June 2009. The data collected by the Digisonde over São Luis (2.33° S; 44° W; dip in 2009:-5.7°) showed a strong modification in the ionospheric F 2 layer trace, that in this case appeared to be “broken in half”. In this process, the first part of the F 2 layer (lower frequency) was thrown down whilst the upper part remained at higher altitudes. Such characteristics occurred simultaneously with an abrupt decrease in the strength of equatorial electrojet and with intensification in the auroral activity. The origin of this phenomenon seems to have a local nature and seems not to be connected to any magnetic disturbance since similar responses were not observed in other longitudinal and latitudinal sectors. Excluding this possibility, we assume that the strong changes observed in the F layer over São Luis had been caused probably by the gravity wave (GWs) propagation, as seen in the downward phase propagation of the altitude contours with time over São Luis and Fortaleza and the remarkable signatures in ionograms over both regions, such as the forking traces that are typically caused by GWs.