A strong geomagnetic storm on Monday and Tuesday -- April 5-6 -- was the biggest since 2006, at least in terms of high planetary A index. On those days, the mid-latitude A index was 28 and 22 and the planetary A index was 49 and 46. This was caused by a mighty solar wind stream. A search for similar numbers over the past few years yields nothing. In 2009, the highest planetary A index was 19 on August 30, and in 2008 it was 37 on October 11. This is indicative of how quiet space weather has become in the past few years. In 2007, the planetary A index reached 30 on April 1 -- we have to go back to 2006 to find any geomagnetic activity as strong, when the planetary A index reached 63 and 104 on December 14-15.

It seems counter-intuitive, but the average daily sunspot numbers rose by 3.5 points this week to 32.4, while average daily solar flux dropped over 7 points to 77.8. Sunspot numbers for April 1-7 were 25, 28, 27, 40, 41, 41 and 25, with a mean of 32.4. The 10.7 cm flux was 79.2, 76.2, 77.4, 78.7, 79.3, 77.5 and 76.3, with a mean of 77.8. The estimated planetary A indices were 12, 12, 8, 13, 49, 46 and 21, with a mean of 23. The estimated mid-latitude A indices were 9, 10, 5, 11, 28, 22 and 15, with a mean of 14.3.

NOAA and USAF predict solar flux at 75 for April 9-15, rising to 78 on April 16. This is substantially different from Wednesday’s prediction, which had solar flux rising to 80 by April 15 --now they don’t see a rise to 80 again until April 23. The same forecast predicts planetary A index for April 9-15 at 8, 5, 5, 12, 10, 5 and 5. Geophysical Institute Prague predicts unsettled conditions for April 9, quiet April 10-12, April 13 unsettled to active, April 14 active and April 15 unsettled to active.

Big geomagnetic activity is often good for VHF propagation, and this week we heard from Ed Richardson, VE4EAR, of Winnipeg, Manitoba, about his experiences on 6 meters. Ed wrote, “Just to let you know, this week’s increase in geomagnetic activity was sufficient for several auroral contacts on 6 meters. On April 6 between 2:30 and 3:30, I enjoyed contacts with stations in Ontario, Quebec and Minnesota, and as well as VE8NSD up in the Northwest Territories. All stations were worked with the beam pointed north-northeast and strengths varied from 52 to 57. There were reports of stations in Minnesota working Alaska via auroral E skip.” Go here and click on the antenna image to see what’s on Ed’s tower. Also note the number of lookups listed below his address.

Carl Zelich, AA4MI, of Chuluota, Florida, sent images of logbook pages, showing many worldwide contacts during recent good conditions. He comments on changing conditions this week: “HF band openings have been opening later and closing sooner. This was true of 40, 10 and 20 meter bands. The 15 meter band has been very quiet. My log shows only one Indian Ocean QSO. This is in contrast to 5-6 Indian Ocean QSOs during the prior week. Only one African QSO; as a note, the African continent has been generally quiet with a lack of stations. Eastern Europe and Caribbean activity had strong signals, while Western Europe had weak signal and a scarcity of stations. Overall conditions were fading in and out with a wide swing in signal strength over a short time period. One point of note: a 6 meter QSO from Central Florida to Michigan lasted for 40 minutes! Can spring be far behind?”

Last week I was across the lake in Redmond, and stopped in to Northwest Research Associates to chat with Jim Secan, who produces an indicator called Effective Sunspot Numbers. These numbers are derived from data produced by ionospheric soundings. A signal beamed upward is swept through the HF spectrum and the reflected signal is measured to determine the optimum frequencies and signal strengths bounced from the sky overhead. The Effective Sunspot Number is not a count of sunspots, but an indicator of actual HF radio conditions. Check the NWRA pages here, here and here.

We’ve mentioned in the past that propagation prediction programs such as W6ELprop are modeled on using the predicted smoothed sunspot number as the data to drive propagation analysis, but that another alternative was to use the average of several days of sunspot numbers for a more up-to-date forecast. Jim feels that the best results are obtained by averaging the latest seven days of the 10.7 cm solar flux, from here or here for more detailed and timely data directly from the Penticton Observatory. The local noon reading (2000 UTC) is the official daily number, and the numbers you want are from the fluxobsflux column. This is the same solar flux data presented in this propagation bulletin.

Jim is not an Amateur Radio operator, but has done considerable research on solar indices and propagation for the federal government and military. His opinion on using solar flux data instead of sunspots is not shared by most of the Amateur Radio operators who I consider most knowledgeable about propagation. Jim says his studies show that recent solar flux numbers give a more realistic view of actual HF propagation when used with the propagation prediction programs.

Red Haines, WO0W (that third character in his call sign is a zero), sent along some thoughts and recent observations on April 7: “Following a couple days of low critical frequencies, during which the Boulder f0F2 never reached 4.0 MHz, the f0F2 reached 8.9 MHz at 2000 today. A quick scan of my records shows that is a high for the year for Boulder. A more detailed scan will probably show that is a high for a long time, possibly a couple years. Six of the 15 meter and 3 of the 10 meter DX beacons recorded on BeaconSee between 2130 and 2200, compared to none yesterday. The major difference in solar activity appears in the hard X-ray flux detected by GOES 14. It increased by an order of magnitude, from around 2 × 10-9 W per square meter for a few days to 2 × 10-8 W per square meter. Other solar activity indicators show little change. Soft X-ray remains unchanged at about 4 × 10-8 for most of that time and the sunspot number, April 4-7 was 40, 41, 41 and 41. The number for today is not yet available, but the MDI Magnetogram image shown on the SOHO site shows the same groups, moved slightly. Geomagnetic activity is decreasing, from G2 to G1 in that period”.

The foF2 data that Red mentioned is available from, and for Boulder Colorado, Wallops Island, Virginia and Millstone Hill in Westford, Massachusetts. Note there is data going back a bit further here. For Millstone Hill, from this list you can go here for February data, and you can change the 201002 in the URL to 201003 for March or 201004 for April.

All times listed are UTC.

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 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.