Our Sun has again become very quiet. We saw 13 days with no sunspots, April 15-27, then new sunspot group 1063 appeared April 28, and on April 29 it was gone again. Although the sunspot number for April 29 is 0, early on April 30 I can still see group 1063 in a magnetogram, so perhaps it rises again. We watch the STEREO mission, looking for bright hints of activity, but lately they just seem to be magnetically complex areas that don’t turn into sunspots.

With no sunspot expected for today, the last day of April, this should be the second monthly decline in a row for monthly sunspot averages. Monthly averages of daily sunspot numbers for February, March and April are 30.5, 25.2 and 10.8. Sunspot numbers for April 22-28 were 0, 0, 0, 0, 0, 0 and 12, with a mean of 1.7. The 10.7 cm flux was 76.1, 75, 74.2, 75.4, 76, 74.8 and 76.1, with a mean of 75.4. The estimated planetary A indices were 6, 13, 8, 3, 2, 4 and 4, with a mean of 5.7. The estimated mid-latitude A indices were 4, 7, 8, 2, 0, 3 and 2, with a mean of 3.7.

USAF/NOAA predicts solar flux of 76 for April 30-May 6, rising to 80 on May 7-8, 78 May 9 and 75 after that. They also predict higher geomagnetic activity for May 4, with planetary A index from April 30-May 7 at 5, 5, 5, 8, 18, 10, 5 and 5. Geophysical Institute Prague says April 30-May 2 should be quiet, quiet to unsettled May 3, unsettled May 4-5 and quiet May 6.

This week, many comments came in regarding the volcanic ash cloud in Europe and possible effects to VHF propagation, perhaps due to lack of aircraft in the sky.

Brett Graham, VR2BG, wrote: “No aircraft in the air removes a lot of reflective surface, though ‘aircraft bounce’ isn’t generally thought of at lower frequencies. This is a bit like an equation with several variables, somewhat difficult to solve. Trails of meteors somehow change due to presence of ash in a way that seemingly affects their reflectivity at higher frequencies, but not so noticeable at lower frequencies. Did the ash cloud reach that sort of altitude? If the trails are in the E-layer region, then it is about 90-150 km up. Maybe just a little ash gets that high up and could have an effect. Though aircraft do provide more reflective surface at higher frequencies, they will move into and out of random points in the sky, potentially including area where reflectivity is needed for a particular path. The big aircraft usually move quite a bit faster than the wind, which is what moves meteor trails around and causes the Doppler shift seen when working meteor scatter. Of course, Doppler is a function of frequency, but perhaps could help reduce the rather big variable of a lot less metal up in the sky. Ah, the black magic of RF -- will we ever figure it all out?” Brett also sent along this instructive link.

Bill Echols, NI5F, wonders “Will the strongly reduced amount of air traffic over Europe affect the ability to complete MS QSO in as rapid a time as usual? In other words, does it have to be ash attenuation making the difference?”

Budd Hippisley, W2RU, wrote: “How about the total absence of aluminum aircraft in the European air space?” in response to the assertion that volcanic ash was attenuating signals.

Likewise, Don Kerns, AE6RF, wrote: “Degraded propagation in the 4 meter band during the time the ash was active might have been due to the stark decrease in air traffic. Four meters should ‘bounce’ off of airliners pretty well.”

Dan Zimmerman, N3OX, provided some information on airplane scatter and wrote: “I don’t know if the procedures in meteor scatter operation might rule out airplane scatter, but it seems like it might be the simplest explanation. I get a lot of what I assume is airplane scatter around here -- pretty useful in contests, because I have low power and low antennas. I think that all the Doppler shifted traces in this spectrogram of WA1ZMS/B on 2 meters are airplanes.”

J.D. Erskine, VA7OTC, wrote: “While attenuation of the signal from the one mentioned satellite might provide an independent space signal -- signals from outside the atmosphere, for comparison with terrestrial VHF signals -- two events occurred during this period. As well as the presence of volcanic ash, there was an absence of aircraft, a significant one at that. Might that have as much to do with the attenuation, real or apparent, of longer distance reception of lower frequency VHF signals?”

Gene Zimmerman, W3ZZ, writes the monthly “World Above 50 MHz” column in QST, and cc’d us on an e-mail to LA4LN, mentioned in last week’s bulletin: “Some have speculated that the volcanic ash might affect VHF propagation both in terms of MS (meteor scatter) and in terms of Es. While random MS conditions are reasonable in April, Es is usually quite poor. Yours is the first report I have seen that indicates that MS was significantly poorer in regions covered by the ash cloud. You have indicated why a mechanism to explain any effect of volcanic ash would be difficult to find. This volcano has driven ash to only ~6 km heights. MS reflections occur at ~60 km or 10× higher. As you accurately point out, 4 meter signals are much longer wavelengths than the diameter of the ash particles. I see no mechanism by which the ash cloud could affect gas ionized by meteor burns in the E layer where all the refraction is taking place. It is also difficult, but less so to postulate, that the fairly dense ash particle cloud could scatter the signals refracted from meteor trails such that FSK441 would detect them but not be able to decode them; however, you indicate that many fewer signals were even detected and not that these were more difficult to decode.”

In relation to reflections from aircraft, John Sahr, WB7NWP, built a passive radar system that correlates FM broadcast signals with their reflections from aircraft and other objects in the sky. Read about it here.

Chip Margelli, K7JA, writes from Garden Grove, California (DM03xs): “I worked Fred, KH7Y, in Hawaii around 2010 UTC on April 24 on 50.110 MHz. His signal came up to over S9, and this is rather early for us to work Hawaii on 6 meters. I hope it is a good omen.”

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 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 at k7ra@arrl.net.