Gilda González

Several indexes have been used to study the variability of foF2. Using the quartiles, the probability that the foF2 value falling between lower quartile and median is 25% and the probability that the foF2 value falling between median and upper quartile is 25%. In addition, the median and quartiles are less affected by magnetic storms. Since the data behaves as a non-normal distribution, the following variability parameters are used for the analysis:
• The monthly medians m, the upper quartile Qup and lower quartile Qlo
• the interquartile range Qr = Qup - Qlo
• and the variability indexes Cup = Qup / m, Clo = Qlo / m and Cr = Cup - Clo.
For example, if Cup = 1.20 means that Qup is 20% greater than median and, Clo = 0.80 means that Qlo is 20% lower than median. The variability is low when Cup and Clo are close to 1 and is high when the indexes are far from 1.
The data used in this study are the ionospheric parameters foF2 and hmF2 at Tucumán, obtained with the “Advanced Ionospheric Sounder” (AIS). This ionosonde sweeps between 1 and 25 MHz in 30 seconds every 5 minutes.
The median is calculated per hour for each month analyzed, only those hours for which there are more than 10 measurements in the month are considered. The data were arranged in two data sets to analyze the ionospheric variations according to the solar activity. The low solar activity (LSA) data set using the 2009 (Rzmedium = 4.8) data and the high solar activity (HSA) data set using 2016 (Rzmedium = 39.8) data.
The outcomes are:
• A clear solar activity dependence is observed in foF2 and hmF2, both parameters increase with increase in solar activity, except during December when foF2 data under low solar activity and high solar activity are very similar.
• The variability of foF2 is higher at low solar activity. This is not observed in hmF2, that present similar values under both periods.
• In general, the variability of foF2 is larger at night than during the day, this behavior is more pronounced during the high solar activity period. The variability of hmF2 is similar during all day, with a small increase between 8 and 10 LT under high solar activity.
• The semiannual anomaly is present irrespective of solar activity.
• In general, the variability of foF2 is higher than that of hmF2.
• Significant planetary wave spectral peaks at about 2 and 5 days are observed for high and low solar activity.
A possible reason of the larger variability under low solar activity is that electron density of the F2 layer is maximum during high solar activity, it is saturated and, therefore, the ionization is maximum, so the electron density is more stable than during low solar activity and the variability is smaller. The same happens during daytime, where due to the presence of the sun, the ionization is higher than during the nighttime, and there is less variability.
When comparing the observed values with the IRI-2016 model, it is observed that, IRI modeled values followed well the general trend for seasons under both high and low solar activity period, foF2 is higher in autumn and spring than in winter. The highest discrepancy occurs during daytime, where in general IRI overestimates foF2. During post-sunset period IRI underestimates foF2 and it shows a better agreement during nighttime. On the other hand, IRI modeled values are higher under high solar activity than during low solar activity, it coincides with the experimental observations.