Comment on angeo-2021-70 Anonymous Referee # 2 Referee comment on " Time and Altitude spread F echoes distribution over the Christmas Island VHF radar

From the 10 years of data on radar echoes observed at Christmas Island, the authors studied temporal variation of occurrence of the F-layer irregularity, diurnal, seasonal and annual variations. The data and the statistical analyses are interesting and worth to publish. However, the authors did not try to explain, quantitatively or qualitatively, why it occurred. The authors mention that during the solar minimum condition the irregularity concentrates around local midnight. Readers expect further physical explanation for these observational results.


Introduction
The contemporaneous understanding of the formation of F-region plasma irregularities depends mainly on the Rayleigh-Taylor (RT) instability process, due to its appearance at the bottomside of the F-region, then becoming unstable to finally generate plasma bubbles. These recently formed plasma bubbles evolve in nonlinear process then extend into high altitudes into the F-region. The small-scale (centimeter to a few tens of meters) irregularities formed in this process are the responsibles for radar backscatter, which can be observed as structures in the RTI image of the radar. The pioneering ionospheric radar work of Woodman and LaHoz (1976) attributed the term "plumes" to describe radar echoes reaching the topside ionosphere. They observed an slope in the formation of the plumes, then explained using numerical simulation by Ossakow (1981) and Zalesak et al. (1982).
The RT instability (and ESF) is controlled by a number of parameters including the prereversal enhancement (PRE) of the zonal equatorial electric field, zonal and meridional neutral winds, longitudinal conductivity gradients, flux tube integrated conductivities, and, possibly, variations in initial (or seed) perturbations (Abdu, 2001;Fejer et al., 1999). It has been noted that ESF bubbles at pre-midnight and post-midnight hours could be driven by different mechanisms (Dao et al., 2011;Yizengaw et al., 2013). The mechanisms that should control the appearance or suppression of equatorial plasma irregularities are different for the pre-and post midnight periods, due to the ambient conditions that prevail along night. Yizengaw et al. (2009) shown that h'F presents a peak at post-midnight hours that indicate the existence of some electrodynamic force that drives the F layer upward creating conditions for irregularities development.
The effects of solar and geomagnetic activities on spread-F vary with latitude and longitude. Cueva et al. (2013) examined data from three equatorial stations along solar minimum and maximum conditions. Their results showed that there was an incread in the spread-F occurrence rate with solar flux. Although many researchers have discussed the characteristics of spread-F irregularities at equatorial and low latitudes, some issues are needed of better understanding in their spatial and temporal variability of spread-F and plasma bubbles. So, the analysis of large data were performed in this work covering high and low solar conditions with spread F echoes observations over the Central Pacific region using the VHF radar installed in Christmas Island. In this study we present results from data analysis of echoes distribution using the 50-MHz

Data Measurements
The Christmas Island VHF radar provides data of meter-scale F-region irregularities routinely, being initially operated by SRI International (2002-2007  Lack of data are also presented as black space in the figure, mainly for 2014 (Mach equinox and June solstice).

Data Analysis
Our interest focus in the local occurrence of F-region echoes (5-meter-scale irregularities) as one of the most interesting and challenging phenomenon for space weather and climatological models. The physical mechanism responsible for this phenomenon are complex and not fully understood. So, we had organized our data attempting to present the difference in seasonal and solar flux conditions as a function of time and height of irregularities observed in the VHF-Radar. For this study, we limit our focus to quiet-time irregularities.
Is well known that geomagnetic activities directly cause perturbations in the zonal electric field affecting the growth and development of ionospheric irregularities. These influences can be related to the eastward PPEF ("prompt penetration electric field") which behaves increasing the amplitude scintillation in VHF or the westward ionospheric disturbance dynamo electric fields which act suppressing the occurrence of irregularities (Singer et al., 1994;Wang et al., 2008). In 3 65 70 75 80 85 sequence, to classify the data with low geomagnetic conditions, we used the 3-hour Planetary K index (Kp). Each measurement was tagged with the value of Kp for the time, of the measurement, plus the previous 3 Kp values. We limited our study to quiet geomagnetic conditions, to be those when none of the three Kp indexes exceeded 3.
Then we sort the seasons for our measurements: Spring Equinox, Summer Solstice, Fall Equinox, and Winter Solstice using 91 days of data centered on each day 21 of March, June, September, and December, respectively. We used the quiet time radar echoes, for each season, to obtain the occurrence rate of echoes. We establish that a good representation of irregularity occurrence is given by echoes distribution above 0 dB divided by the total number of observations. Our criterion is a good commitment among being able to identify the occurrence of spread F echoes and to eliminate the effects of non-geophysical echoes.
The sample rate of the VHF radar is estimated for every 15-minute intervals starting at 18:00 LT, right before sunset until 05:00 LT near sunrise. To construct maps of irregularity occurrence rate in function of height and local time we had computed for every 15-km height intervals starting at 200 km up to 1000 km altitude.

Results and Discussions
During solar maximum the spread F events occur near the time when upward drift is large which is promptly after local sunset (Fejer et al., 1999), while during solar minimum when the upward drift is usually short, the spread F exists throughout the whole night, and upward and downward ionospheric conditions may play a role in the morphology of irregularities. Stoneback et al. (2011) showed the role of vertical drift during the extended solar minimum and how it vary from sunset until postmidnight period. These previous work observations increase the need of further study of climatology of echoes evolution in time and altitude.
In the way of understanding this climatology of spread F evolution along seasonality and solar activity we analyzed radar echoes occurrence as function of time and altitude along solar maximum and extended solar minimum period, since the evening vertical drifts and layers heights increase noticeably with solar activity, and along nighttime. Comparing the peak of altitude echoes along season, we can observe higher occurrence rates of all years over June solstice and September equinox than March equinox and December solstice seasons. This observation match with previous observation by Cueva et al. (2013), that shown the peak occurrence of equatorial spread F for this region being around July-August months. The higher occurrence of echoes in altitude is compared with the density profiles provided by Digisondes.
When observing all years data we conclude that the peak echoes altitude was higher in altitude in June equinox than September solstice, even when its occurrence was the opposite. For the year 2003 and 2012 (high solar flux period) we can mention that peak altitude distribution is the highest, nevertheless present minor percentage of occurrence than solar minimum years (2006 to 2008). The minimum occurrence of peak altitude occurs in March solstice, which is the period of scarce spread F echoes over Christmas Island region.
The altitude distribution of echoes above 350 km also presents same behavior as below this threshold. During solar maximum period radar echoes have less occurrence than solar minimum echoes. During September equinox higher plumes are frequently observed than in other periods, which agrees with results presented by Cueva et al.(2013).

On the time variability of echo occurrence rates
Time variation in the occurrence rates of F-region echoes for the period in study is shown in Figure 3, lower panels, also separated by seasons (March equinox, June Solstice, September equinox and December solstice, from left to right respectively). The vertical dashed lines represent local sunset and local midnight. As we can observe the percentage of occurrence of echoes presents a solar flux dependence. During solar maximum radar echoes are confined to a few hours after sunset, on the other hand during solar minimum echoes are more broaden out in time and can arise late in the evening after sunset and more closely to midnight hours. As we get closer to solar minimum period the amplitude of echoes occurrence increases due to high probability to occur echoes along all night. This can be observed during years 2006 to 2008 with more amplitude than echoes observed during solar maximum, similar finding was mentioned by Niranjan et al. (2003) when analyzed spread F data from 1997-2000 period, also by Burke et al. (2004) and Dao et al. (2011)  Seasonal dependence of echoes along solar cycle is also observed. September solstice has more conditions to develop irregularities over the region as explained before, as well higher echoes occurrence either for solar minimum and maximum periods. For March equinox and December solstice we have less probability of echoes occurrence as can be seen in the Figure 3, moreover amplitude of echoes occurrence is always lower in solar maximum than in solar minimum.

Conclusions
The variability over seasonality observed in the amplitude of peak echoes occurrence, either for altitude or time, is suitable for the seasonal spread F occurrence over the Pacific region. Post-midnight events were observed along the solar cycle, decreasing from solar minimum to solar maximum. For the extended solar minimum period, during June solstice and December solstice months, we observed post-midnight echoes similar as previously reported by Otsuka et al. (2012) during 2005 to 2009 period. September equinox also presents post-midnight events for solar minimum period.
These findings are summarized in Figure 4, on top panel is the time peak variation along the period studied separated by seasonality. We can clearly observe the time dependence of echoes with solar cycle, being close to PRE hours in high solar activity and around midnight hours in solar minimum conditions. December solstice during high solar conditions is not following this trend, and further study must be necessary in this point.
Bottom panel on Figure 4 shows altitude peak variation along solar cycle, also separated by seasonality. Altitude parameter seems to follow a very good trend, being higher altitudes for solar maximum conditions and lower altitudes for solar minimum conditions. Again December solstice doesn't match very well with this trend. The altitude parameter is an important parameter due to the key process in the generation mechanism for ionospheric irregularities.
So, for this longitudinal sector we can conclude that during solar maximum conditions we can expect echoes occurring short time after local sunset, and the altitude peak of occurrence around the range of 330 km to 390 km. During solar minimum conditions we can expect echoes around local midnight and around 300 km altitude mainly.