Che cosa ci si deve attendere da un minimo solare in termini propagativi? Per prima cosa, il minimo dura in genere molto poco, un ciclo solare (il prossimo è il numero 24) in genere raggiunge il suo picco in pochi anni e poi impiega il resto degli undici in una più lenta discesa. La regola pratica è che molte macchie portano moltano attività solare e di conseguenza a un maggior numero di eventi geomagnetici intensi, come tempeste e così via. Attivo geomagnetismo significa a grandissime linee buona propagazione sulle frequenze elevate (aumento della MUF, la massima frequenza utlizzabile per un collegamento ionosferico). Viceversa, piccolo numero di macchie significa quiete geomagnetica, abbassamento della LUF (frequenza minima utilizzabile) e buona propagazione per le onde medie. Sono tutte regole non strettissime e soprattutto in quest'ultimo ciclo sono state contraddette da un forte geomagnetismo nella (teorica) fase calante del ciclo stesso. Fenomeni come l'E sporadico, invece, sembrano addirittura favoriti da una bassa attività solare. Con la ripresa del ciclo aumenteranno le chance di ascoltare meglio i programmi delle onde corte trasmessi a partire dai 25 metri, normalmente prodotti dalle emittenti internazionali. Le stazioni locali sono ormai quasi tutte concentrate nel segmento bande tropicali-31 metri, con qualche rara presenza più in alto. Difficile dire come andranno le cose tra i 2 e i 6 MHz, di solito le cose migliori si sentono quando la ionosfera sta subendo i primi effetti di una forte attività solare. I radioamatori invece attendono con buone aspettative i massimi solari, perché le due bande più frequentate, 15 e 20 metri, sono le più favorite.
On March 1 sunspot 944 was pointed straight at us. It was a small sunspot, followed a few days later by another small spot, 945. Sunspot 945 is visible in photos from March 5, just behind 944, but both spots seemed to disappear a day or two later, before they would have rotated off the visible solar disk. Now the Sun is blank, and the sunspot number is zero. The minimum non-zero sunspot number is 11, and lately sunspot numbers move from 0 to 11 to 23 or 24, and back to 11. No solar activity is expected for the next few days, so we will probably see at least several days with a zero sunspot number. Geomagnetic conditions are expected to remain quiet, at least until Monday or Tuesday, March 12-13. The USAF predicts a Planetary A index for March 9-15 of 5, 5, 7, 15, 20, 15 and 10. Geophysical Institute Prague predicts quiet conditions for March 9-10, quiet to unsettled March 11, active geomagnetic conditions for March 12, unsettled to active March 13, unsettled March 14 and quiet to unsettled March 15. A recurring solar wind stream is predicted for Monday, March 12, and should produce the expected geomagnetic instability. New predictions for the solar minimum are coming frequently of late. The monthly smoothed sunspot number forecast for the rest of 2007 from the NOAA Space Environment Center in the weekly Preliminary Report and Forecast has been adjusted again, the third time since the first of the year. The revised tables are on page 9 of issue 1635, and page 10 of issues 1640 and 1644 (the table in 1640 is mislabeled at the bottom of the page as 03 January when it is really 06 February). Currently they predict a solar minimum for right now, with a smoothed sunspot number of 6 for March and April 2007, then 7, 8, 10, 11, 13, 16, 18 and 21 for the remaining eight months of this year. As mentioned in past bulletins, these are smoothed sunspot numbers, averaged over a year. So the prediction of 6 for this month means that if the prediction is accurate, at the end of September 2007 you could take half the average of daily sunspot numbers for that month, add it to half the average of daily sunspot numbers for September 2006, add the total to the monthly averages for each month in between, divide by 12, and get 6 as the result. Currently we're seeing higher values, with an average daily sunspot number of 19 for last week, 19.6 the week before, 14.6 for the week prior to that, 6.3 for the previous week, and 28.7 for the week prior to that, which was February 1-7. You can see an explanation of the method for determining the 12 month smoothed sunspot number at, http://www.ngdc.noaa.gov/stp/IONO/sunspot.html. Jim Headrick, W3CP of Stanfield, Oregon sent in a different prediction from the Australian government. It has the solar minimum centered on September 2007, and you can see it at, http://tinyurl.com/2ymk92. Note that the NOAA version mentioned previously ends in December 2007, but the one Jim sent goes through 2008 and 2009 as well. By the way, I don't know how long Jim has been a ham, but he was born early in sunspot cycle 15, and I'm sure he hopes to see the new cycle 24 all the way through. See a page of all the 20th century sunspot cycles at, http://wm7d.net/hamradiosolar/historical.shtml.