The 10.7 centimeter solar flux index (SFI) jumped to a Cycle 24 record of 262 on January 4, suggesting that Cycle 24 has not yet begun drawing to a close and may be approaching or at a “second peak.” The previous peak for the current cycle was 190 on September 24, 2011. As NASA’s Marshall Space Flight Center (MSFC) explains, the flux of the sun’s radio emissions at 10.7 centimeter (2.8 GHz) is another indicator of solar activity levels, since it tends to follow changes in the solar ultraviolet that influence Earth’s upper atmosphere and ionosphere. 

“Many models of the upper atmosphere use the 10.7 cm flux (F10.7) as input to determine atmospheric densities and satellite drag,” NASA/MSFC notes, adding that SFI “has been shown to follow the sunspot number quite closely.” The January 6 sunspot count was 225. The Cycle 24 sunspot count peaked at 282 on November 17, 2013, according to WM7D.net.

Canada’s Dominion Radio Astrophysical Observatory (DRAO) indicated an “official” flux of 262 at 2000 UTC on Saturday, January 3. The official figure for Sunday, January 5, was 217.5. ARRL solar observer Tad Cook, K7RA, who reports 10.7 centimeter flux numbers in his weekly “Solar Update” bulletins, suggests that they are not as valuable as sunspot numbers in predicting radio propagation. The solar flux was over the weekend was far higher what was anticipated, judging by predictions Cook reported in his January 3 “Solar Update” for this past weekend and the week ahead. NASA/MFSC indicates a 95 percent predicted flux for the month of January at 146.5, continuing at about the same level through the first half of the year.

The Daily DX says the SFI is expected to remain above 200 for the next week (220, 220, 215, 215, 215, 205 and 200 for Monday through Sunday). “The last time the SFI was above 200 was during Solar Cycle 23 in late October 2003, for about a week ranging from 210 to 298,” The Daily DX reported January 6. Editor Bernie McClenny, W3UR, cites the recollections of Frank Donovan, W3LPL, who recalled SFI numbers above 200 were common 12 years ago during the second peak of Cycle 23, from mid-October 2001 through mid February 2002. It was during that period that the last instance of “almost-daily transatlantic” F2 propagation took place on 6 meters.

As Ian Poole, G3YWX, explained solar flux in his article, “Understanding Solar Indices” in the September 2002 edition of QST, “[H]igh values generally indicate there is sufficient ionization to support long-distance communication at higher-than-normal frequencies.” He pointed out, though, that it can take a few days of high values for conditions to show improvement. “Typically values in excess of 200 will be measured during the peak of a sunspot cycle, with high values of up to 300 being experienced for shorter periods,” Poole wrote.