RadCom April 2024, Vol. 100, No. 4

66 April 2024 Regulars: Propagation Studies M aximum usable frequencies for HF transmissions are higher in winter than in summer at nearly the same solar activity levels. This seasonal variation is called the winter anomaly. Figure 1 shows maximum usable frequencies (MUF) from July 2022 to January, 2024. Vertical bars represent median MUF calculated using midday (1200UTC) ionosonde measurements in Dourbes, Belgium [1] , [2] . Compared with summer levels, MUF is elevated during the rest of the year, so the winter anomaly is also called the seasonal anomaly. The data are for the mid-points of single-hop 3000km transmissions, based on ionosonde measurements of critical frequency foF2. The magnitude of the winter anomaly is greater when solar activity in the 11- year cycle is high. The broken line in Figure 1 is a schematic plot of data from North America showing midday MUF during a previous solar cycle when solar activity was higher than it is now [3] . Monthly MUF levels in summer are similar to the data from Dourbes, while the anomaly, MUF(Winter)/MUF(Summer), is higher in the schematic. The magnitude of the winter anomaly varies around the world. Typically, it is highest in North America. Satellite measurements of peak electron density (Ne) of the F2-region of the ionosphere show geographic variations of the anomaly in more detail than data from widely-spaced ionosondes. MUF is proportional to (Ne) 1/2 . Figure 2 is a global map showing colour contours of Ne(Winter)/Ne(Summer) measured when solar activity is high [4] . The winter anomaly, MUF(Winter) > MUF (Summer), is apparent in regions where Ne(Winter)/Ne(Summer) is greater than 1.0. In blue areas there is no anomaly (Ne ratio = 1.0). In white areas the Ne ratio is slightly less than 1.0. The map shows that the highest levels of winter anomaly are concentrated at mid-latitudes of the northern hemisphere. Figure 3 shows data from Figure 2 averaged at mid-latitudes. The horizontal axis is longitude (-180° to +180°). The blue curve shows Ne(Winter)/Ne(Summer) averaged in the 40°N - 60°N latitude band. Winter anomaly is highest in North America and lowest in Eastern Europe. The red curve is for 40°S - 60°S. In comparison, Figure 4 shows lower average Ne(Winter)/Ne(Summer) at mid- latitudes when solar activity is low. The term ‘winter anomaly’ is also used for large increases in HF absorption that occur erratically during groups of days in winter. These disturbances in the D-region of the ionosphere are independent of the MUF anomaly in the F2- region. Both types of anomalies are caused by seasonal variations in the neutral atmosphere. You can follow MUF in Solar Cycle 25 with the online PROPquest application by Jim Bacon, G3YLA [5] . Select the foF2 tab to see MUF vs UTC time for several different transmission distances. The Archives tab compares MUF on different days. References [1] Lowell Global Ionosphere Radio Observatory Data Center. https://giro.uml.edu/ionoweb/ [2] Carl Luetzelschwab, K9LA, Problems with Ionosonde Data, November, 2019. https://k9la.us/Nov19_Problems_with_Ionosonde_Data.pdf Maximum Usable Frequencies in winter Peter DeNeef, AE7PD HamRadioAndVision@gmail.com FIGURE 4: Ne(Winter)/ Ne(Summer) during low solar activity. Blue line: average for 40°N - 60°N vs longitude. Red line: average for 40°S - 60°S (data reproduced from [4] under a Creative Commons licence). FIGURE 3: Ne(Winter)/ Ne(Summer) during high solar activity. Blue line: average for 40°N - 60°N vs longitude (-180° to +180°). Red line: average for 40°S - 60°S (data reproduced from [4] under a Creative Commons licence). FIGURE 2: Satellite measurements of the winter anomaly in terms of peak electron density (Ne) in the F2-region during high solar activity. Color contours on a global map show Ne(Winter)/ Ne(Summer) ranging from 1.0 to 3.0 (data reproduced from [4] under a Creative Commons licence). FIGURE 1: Ionosonde measurements of MUF (MHz) vs month (1-12). Vertical bars: monthly median MUF at 1200UTC at Dourbes, Belgium. Broken line: Schematic diagram of mid-day MUF in North America during a previous solar cycle (based on ionosonde data from [1]. Schematic data from [3]). [3] J.R. Kennedy, K6MIO/KH6, 50 MHz F2 Propagation Mechanisms, Proceedings of the 34th Conference of the Central States VHF Society, Winnipeg, 2000. https://www.uksmg.org/content/f2propagationmech.htm [4] Y.V. Yasyukevich, et al , Winter anomaly in NmF2 and TEC: when and where it can occur, J. Space Weather Space Climate, 8, A45, 2018. https://doi.org/10.1051/swsc/2018036 [5] Jim Bacon, G3YLA, PROPquest. https://www.propquest.co.uk/graphs.php