Mid-Latitude Ionospheric Behavior During Descendant Phase of Solar Cycle 24

ACADEMIA Letters Mid-Latitude Ionospheric Behavior During Descendant Phase of Solar Cycle 24 Eduardo Araujo-Pradere, Miami Dade College As it is well known, the ionosphere behavior follows a diurnal cycle as well as a seasonal one, and the physical processes supporting these cycles are well understood. However, it is less comprehended the mechanisms behind the ionospheric cycle following the 11- year solar cycle. Our work is focused on an initial description of the ionospheric behavior during the descendant phase of solar cycle 24, and it is part of a broader project targeting a detailed comparison of the last two solar minima. For this project, we have identified, downloaded and analyzed data from five mid-latitude ionospheric stations distributed globally. In order to describe the average ionospheric behavior, we calculated a “mean day” by averaging the ionospheric parameters for each hour for the whole month. We then ran basic statistics to have an idea of the data variability around each mean and proceed to graph the mean days using the standard deviation as the error bar at each point. Our results show that although there is a clear descending solar activity, the ionospheric parameters remain fairly leveled, with only average trends in the South Hemisphere of - 0.35% for foF2 and -0.745% for hmF2, and respectively -0.130% and -3.49% for the North Hemisphere. These results could be explained by a consistent amount of ionospheric plasma being produced, but at lower heights during the descendant phased of the solar cycle, a clear indication of a contracted atmosphere. Introduction Since the ionosphere is the direct product of the absorption of the solar energy in the upper atmosphere, it is expected to follow the solar behavior in some way. Of special interest is the Academia Letters, July 2021 ©2021 by the author — Open Access — Distributed under CC BY 4.0 Corresponding Author: Eduardo Araujo-Pradere, earaujop@mdc.edu Citation: Araujo-Pradere, E. (2021). Mid-Latitude Ionospheric Behavior During Descendant Phase of Solar Cycle 24. Academia Letters, Article 1799. https://doi.org/10.20935/AL1799. 1 current lowering trend of the solar activity, and how the earth system responds. Many studies have focused their attention on the ionospheric response to solar activity with emphasis on the most recent solar minima (see, for example, Araujo-Pradere et al., 2005, Emmert, et al., 2010, Araujo-Pradere et al., 2011). In this study, we target the most recent solar descendant phase, as shown in Figure 1, obtained from the National Oceanic and Atmospheric Administration’s Space Weather Prediction Center (NOAA’s SWPC). Here it is obvious that not just the maxima values show a decreasing trend, the same is observed for the last three solar minima, con values that are not just lower, but flatter (less variability) and extending for longer periods of time. The specific period of study is enclosed by the black frame in Figure 1. Figure 1: Solar Cycle Sunspot Number, last three solar minima. Period of study appears enclosed by the black frame. In our effort to understand the ionospheric response to the decreasing phase of cycle 24, Academia Letters, July 2021 ©2021 by the author — Open Access — Distributed under CC BY 4.0 Corresponding Author: Eduardo Araujo-Pradere, earaujop@mdc.edu Citation: Araujo-Pradere, E. (2021). Mid-Latitude Ionospheric Behavior During Descendant Phase of Solar Cycle 24. Academia Letters, Article 1799. https://doi.org/10.20935/AL1799. 2 the most recent trend conducing to the latest minimum, we have selected ionospheric data running from January 2014 (solar cycle 24 maximum) to late 2020 (solar minimum between cycles 24 and 25), encompassing the period of interest. Although a recent announcement from NOAA’s SWPC located the solar minimum between Solar Cycle 24 and 25 - the period when the sun is least active - in December 2019, we used data for all 2020. Ionospheric Parameters Figure 2: Graphical description of the parameters used in the study. Academia Letters, July 2021 ©2021 by the author — Open Access — Distributed under CC BY 4.0 Corresponding Author: Eduardo Araujo-Pradere, earaujop@mdc.edu Citation: Araujo-Pradere, E. (2021). Mid-Latitude Ionospheric Behavior During Descendant Phase of Solar Cycle 24. Academia Letters, Article 1799. https://doi.org/10.20935/AL1799. 3 This breve description of the ionospheric parameters of interest has been adapted from Araujo Pradere et al., 2011. There are many parameters that describe the ionospheric structure, dynamics, and composition. We have focused our analysis on the critical frequency of the F region (foF2), directly obtained from the maximum concentration of the F region (NmF2), normally expressed as number of particles per cubic centimeter, and the height of this peak of concentration (in Km). Figure 2 shows sketches of both parameters. NmF2, the maximum concentration of the F region, is a direct measure from an ionosonde. The ionosonde transmitter sweeps the high frequency range, transmitting short pulses. These pulses are reflected at various layers of the ionosphere, at heights of 100–400 km. The maximum frequency reflected is known as the F region critical frequency (foF2). The relation between the plasma frequency foF2 and the electron density NmF2 at the F peak can be expressed as NmF2 = foF2/80.3 (for details see Budden, 1985), where NmF2 is expressed in electron/m3 and foF2 in Hertz. In order to cover different seasonal, longitudinal and latitudinal dependences, several stations were selected across the planet, including three stations in the north hemisphere and two in the south hemisphere, all localized in mid- to mid-high geomagnetic latitudesso to avoid the complex dynamics of high- and low- geomagnetic latitudes. They are shown in Figure 3. These stations are well established, with a long history of high-quality data, and efficient methods to verified and preserve the measurements. This was an important factor considered in the identification of the stations for the study. Method A massive amount of data corresponding to the descendant phase of the solar cycle 24 was identified, downloaded and quality checked. Ionospheric data was mainly obtained from the Global Ionosphere Radio Observatory (GIRO, http://giro.uml.edu). The quality assurance process was quite stringent, with all missing values flagged, substituted by interpolated values when possible (and flagged accordingly), with time stamp verified and corrected when needed, and with missing batch of data identified and downloaded from other sources (specifically from NOAA’s National Geophysical Data Center, NGDC, https://www.ngdc.noaa.gov/ stp/iono/ ionohome.html). Once confident in the quality of the data, the method of “mean day” (adapted from Araujo Pradere, et al., 2005) was applied to the data. This method consists in the calculation of the monthly average for each hour, and the construction of an average day for the month. As a result, we were working with only twelve “mean days” per year, which makes the data handling a lot lighter. Academia Letters, July 2021 ©2021 by the author — Open Access — Distributed under CC BY 4.0 Corresponding Author: Eduardo Araujo-Pradere, earaujop@mdc.edu Citation: Araujo-Pradere, E. (2021). Mid-Latitude Ionospheric Behavior During Descendant Phase of Solar Cycle 24. Academia Letters, Article 1799. https://doi.org/10.20935/AL1799. 4 Figure 3: Ionospheric stations selected. Next, a basic statistical analysis was applied to the data to obtain the average values, and the root mean square error (RMSE) to describe the dispersion around the mean. Data was plotted, and trends calculated. There were two main grouping of the data, first data was plotted for the whole period of interest and the rends determined, and second, the same month for all years considered were plotted together to observe the ionospheric trend from max to min solar activity. Finally, months with higher and lower dispersion were identified. Results Given the significant output of our analysis, just a sample will be shown in this section. With this in mind, we selected two stations, Pt. Arguello, a north-hemisphere, mid-latitude station in the east coast of USA, and Hermanus, a south-hemisphere, mid-latitude station in the southern coast of the Western Cape province of South Africa. We will focus our discussion on the periods with the higher and lowest foF2 dispersion for each of the stations and the full foF2 data in order to remark our findings. The main results for the critical frequency foF2 measured in Pt. Arguello, a north-hemisphere, mid-latitude station in the east coast of USA, are shown in Figure 4. The same results are presented for Hermanus in Figure 5. These figures are divided in three panels: panel (a) at the Academia Letters, July 2021 ©2021 by the author — Open Access — Distributed under CC BY 4.0 Corresponding Author: Eduardo Araujo-Pradere, earaujop@mdc.edu Citation: Araujo-Pradere, E. (2021). Mid-Latitude Ionospheric Behavior During Descendant Phase of Solar Cycle 24. Academia Letters, Article 1799. https://doi.org/10.20935/AL1799. 5 top right shows the “mean days” for the month with the higher dispersion, while panel (b), top left, presents the period with lower dispersion. The bottom panel (c) displays the whole set of data and the trend calculated for the full descendant phase of solar activity. It is interesting to notice that the months of higher and lower dispersion occurs at the same time in both hemispheres, which indicates that there is no seasonal dependence underlying in these results. Although there is a significant difference between these two plots for both stations, it is observed that the lowest values consistently correspond to 2019/2020, the deepest part of the latest solar minimum. Figure 4: Results for ionospheric station Pt. Arguello, North Hemisphere. Academia Letters, July 2021 ©2021 by the author — Open Access — Distributed under CC BY 4.0 Corresponding Author: Eduardo Araujo-Pradere, earaujop@mdc.edu Citation: Araujo-Pradere, E. (2021). Mid-Latitude Ionospheric Behavior During Descendant Phase of Solar Cycle 24. Academia Letters, Article 1799. https://doi.org/10.20935/AL1799. 6 Figure 5: Results for ionospheric station Hermanus, South Hemisphere. As expected, the highest dispersion of the data happens during daytime values (the higher values in each of the top plots), when there is a more direct solar control of the ionosphere, in contrast, nocturnal values are less dispersed because they are controlled mostly by transport and recombination processes). The bottom panels for both figures are slightly different. While the foF2 data for Hermanus shows no significant change in the variability and an almost non-existent trend (- 0.02%), the same plot for Pt. Arguello shows a significant attenuation of the variability of the data and a trend of negative 0.13%, an order of magnitude higher. Our trend analysis shows that the ionospheric trend, although consistently negative, is much less significant that the solar trend during this descendant phase of the solar activity. Of interest is the difference between the trends for foF2 and for hmF2, and the differences between the hemispheres. While the ionosphere concentration remained roughly constant, with just a marginal decreasing trend, the trend of the altitude of the peak of concentration of the ionosphere (hmF2) to lowering values is consistently higher, with values one and even two orders of magnitude higher. In contrast, trend values for both parameters are more pronounced for foF2 for the South Hemisphere while trends for hmF2 are deeper for the North Hemisphere. Academia Letters, July 2021 ©2021 by the author — Open Access — Distributed under CC BY 4.0 Corresponding Author: Eduardo Araujo-Pradere, earaujop@mdc.edu Citation: Araujo-Pradere, E. (2021). Mid-Latitude Ionospheric Behavior During Descendant Phase of Solar Cycle 24. Academia Letters, Article 1799. https://doi.org/10.20935/AL1799. 7 Conclusions All stations selected for this study show a consistent decreasing of the ionosphere concentration and height. This trend corresponds with the solar activity descending phase, as expected. However, the concentration (foF2) trends are significantly smaller that height (hmF2) trends, which could be explained by a consistent amount of ionospheric plasma being produced, but at lower heights during the descendant phased of the solar cycle, a clear indication of a contracted atmosphere. There is also a hemispheric dependence yet to be explained. Acknowledgements This original research was supported, in part, by U.S. Department of Education grant award P031C160161 (STEM SPACE). References Araujo-Pradere, E.A., T.J. Fuller-Rowell, M.V. Codrescu (2005), Characteristics of the ionospheric variability as a function of season, latitude, local time, and geomagnetic activity, Radio Sci., 40, RS5009, doi:10.1029/2004RS003179. Araujo-Pradere, E.A., R. Redmon, M. Fedrizzi, R. Viereck, and T. J. Fuller-Rowell (2011), Some characteristics of the ionospheric behavior during solar cycle 23/24 minimum, Solar Phys., The Sun–Earth Connection near Solar Minimum, edited by M. M. Bisi, B. Emery, and B. J. Thompson, doi:10.1007/s11207-011-9728-3, Springer. Budden, K. G. (19850 The Propagation of Radio Waves, Cambridge University Press, New York, 669 pp. Emmert, J. T., J. L. Lean, and J. M. Picone (2010), Record-low thermospheric density during the 2008 solar minimum, Geophys. Res. Lett., 37, L12102, doi:10.1029/2010GL043671 Academia Letters, July 2021 ©2021 by the author — Open Access — Distributed under CC BY 4.0 Corresponding Author: Eduardo Araujo-Pradere, earaujop@mdc.edu Citation: Araujo-Pradere, E. (2021). Mid-Latitude Ionospheric Behavior During Descendant Phase of Solar Cycle 24. Academia Letters, Article 1799. https://doi.org/10.20935/AL1799. 8