The average daily sunspot numbers this week plummeted 45 points to 69, while the average daily solar flux was off over 26 points to 113.1. Sunspot numbers for March 10-16 were 88, 105, 78, 64, 51, 47 and 50, with a mean of 69. The 10.7 cm flux was 131.3, 123.1, 120.7, 112.9, 107.4, 101.5 and 95, with a mean of 113.1. The estimated planetary A indices were 20, 40, 13, 6, 6, 0 and 0, with a mean of 12.1. The estimated mid-latitude A indices were 14, 18, 10, 5, 2, 0, and 1 with a mean of 7.1.

In a brief report for this week’s ARRL Letter, we said that average flux was off nearly 25 points to 114.5. The average changed because the observatory in Penticton reported the solar flux for Wednesday, March 16 as 104.9, but later it was adjusted down to 95 by NOAA’s Space Weather Prediction Center. This happens sometimes when energy from a Coronal Mass Ejection overwhelms the 2.8 GHz (10.7 cm wavelength) receiver at the Dominion Radio Astrophysical Observatory in Penticton, British Columbia. A good example was on March 7, 2011 when the noon solar flux reading was 938.6. This was an obvious outlier, caused by a solar flare out of sunspot group 1166 that was positioned near optimal geoeffectiveness. As a result, the official number was adjusted down to 122. But Wednesday’s number didn’t seem so obviously out of line, and the timing didn’t seem to line up with the only event I knew of, an eruption from sunspot group 1169, which was nearly over the solar horizon.

When I saw an image showing the CME on the western limb with a time stamp of 2048, I assumed this meant the event occurred 48 minutes after the 2000 flux reading. But it turned out to be a long duration event.

Rob Steenburgh, KA8JBY, works at the Space Weather Prediction Center in Boulder, and looked into this for us. He checked with the forecasters, who wrote: “The Penticton noon flux did come at 105, but was flare-enhanced. We estimated a value of 95 based on the morning flux. The long duration event (LDE) began at 16/1752, max at 16/2034 and end at 16/2324. It peaked at C3.7 from Region 1169 (N18W75). The Penticton noon reading is taken between the hours of 1900-2000, so it was right in the middle of the LDE. It was an extremely impressive looking event on GOES-15 SXI.”

So I didn’t realize that the noon reading was actually taken over the course of the hour before noon, not just a snapshot right at local noon (2000). Rob clarified this and said that there are four readings taken over the hour. One is ignored (the outlier, or the one most different from the other three), while the rest are averaged to produce the noon solar flux. So that estimate for Wednesday was 95 and the measured value for Thursday was 90.1, which seems consistent with the declining sunspot numbers for Wednesday and Thursday -- 50 and 45 -- and the shrinkage of total sunspot area, 300 and 170 millionths of a solar hemisphere.

Last Friday, March 11, the planetary A index reached 40 due to a powerful X1.5 class solar flare at 2323 on March 9. This is another definite indication that Solar Cycle 24 is ramping up. After four years of no X class flares, there have been two in the last month.

The past few days have seen very quiet geomagnetic conditions, with the K index in most locations at 0. The latest forecast shows quiet conditions with planetary A index around 5 until March 27-30, when the predicted planetary A index is expected to rise to 7, 7, 19 and 7. Geophysical Institute Prague predicts quiet conditions March 18-19, unsettled March 20, quiet to unsettled March 21, unsettled March 22, quiet to unsettled March 23 and quiet on March 24. The predicted solar flux for March 18-25 is 85, 80, 85, 90, 95, 100, 105, 110 and the 85 for March 26-29.

This Sunday is the spring equinox in the northern hemisphere, a welcome sign for DXers everywhere, when both the northern and southern hemispheres are bathed in an equal amount of solar radiation.

In the bulletin preview yesterday, we promised to explain how the geomagnetic A index for the day is calculated using the eight K index values throughout the day. Here is how it works:

The K index value is not linear; it is based on magnetic measurements in units called nano-Teslas. K index of 0 corresponds to a three-hour measurement of 0-5 nT, 1 is >5 nT, K of 2 is >10, 3 is >20, 4 >40, 5 >70, 6 >120, 7 >200, 8 >339 and 9 is >500 nT. The idea of the A index is to take an average of the K index to express a value for the whole day, but because the K index is not a linear scale, so it is not realistic to just average the eight K index measurements. Instead, we convert each K index into a linear scale -- called the A index -- then average those and that yields the A index value for the day, which is always expressed in whole numbers. The equivalent A values for each K value are, K of 1 is 3, 2 is 7, 3 is 15, 4 is 27, 5 is 48, 6 is 80, 7 is 140, 8 is 240 and 9 is 400.

So looking here, note that the high-latitude A index (measured at the University of Alaska at Fairbanks) on March 11, 2011 is 56, a high value. The eight K index values are 4, 4, 5, 5, 5, 7, 6 and 4. Those convert to A values of 27, 27, 48, 48, 48, 140, 80 and 27. The sum of those is 445. Divide by eight and the average is 55.625, which rounds out to 56, the A index for the day.

Bert Ingalls, KH6HI, operates a 6 meter beacon in Hawaii on the island of Oahu. The KH6HI/B beacon is on 50.0645 MHz and runs 20 W into a pair of horizontally polarized loop antennas stacked vertically at about 2/3 wavelength, or 12 feet apart. It is at a commercial broadcast site in the Waianae mountain range 2500 feet, with an unobstructed shot to the horizon in all directions.

On Saturday, March 12, LW3EX in Buenos Aires reported hearing the beacon at 2349 with an RST of 539. The distance between grid squares BL01 and GF05 is about 7600 miles. Bert wrote: “I believe the following report is the first TE/F2 to South America from Hawaii for Solar Cycle 24. It occurred at 1:49 PM local time, which is typical for TE/F2 in that direction during previous solar cycles. I would expect that propagation to ZL/VK and other South Pacific islands will begin to occur as well, providing the solar flux continues to remain at current or higher levels.”

Bob Forsman, WK5X, of Stuarts Draft, Virginia, wrote about the recent report of possibly very long distance Argentina to North Carolina 2 meter propagation, and notes that this type of report isn’t unprecedented. He found a couple of reports from 1979 and 1981 in the March 2010 “World Above 50 MHz” column in QST, and included these with some of his own comments.

“The December 1981 “World Above 50 MHz” column details a September 23, 1981 (evening) occurrence of 2 meter propagation between CP5CL (Cochabamba, Bolivia) and the WB5QFM 2 meter repeater near Arcadia, Louisiana. It is interesting that this path, along with last week’s path, both occurred during an equinoctial (but opposite) period. The South American station was full quieting into the Louisiana repeater for 30 minutes, and then dropped to about 3 × 5 for a couple of hours. LU1DMA reports a QSO on 2 meter SSB with Steve, WB9YWN (now AF9X), on March 1, 1979 (last week’s report was from March 2). After tracking down AF9X, it was determined that Steve was likely operating either maritime mobile from the vicinity of Bermuda, or fixed from Charleston, South Carolina. These paths are not dissimilar to the path reported last week.”

All times listed are UTC, unless otherwise noted.

Amateur solar observer Tad Cook, K7RA, of Seattle, Washington, provides this weekly report on solar conditions and propagation. This report also is available via W1AW every Friday, and an abbreviated version appears each Thursday in The ARRL Letter. You can find a guide to articles and programs concerning propagation here. Check here and here for a detailed explanation of the numbers used in this bulletin. An archive of past propagation bulletins can be found here. You can find monthly propagation charts between four USA regions and 12 overseas locations here. Readers may contact the author via e-mail.