All solar and geomagnetic indicators declined again last week. Compared to the previous seven days, from March 31 through April 6 the average daily sunspot number slipped from 28.1 to 19.4. Average daily solar flux sank from 88.8 to 83.1, while average daily planetary A index went from 11.9 to 9.4. Average daily mid-latitude A index went to 7.6 from 8.6.
The latest prediction (from April 7) has solar flux at 92 and 90 on April 8-9, 95 on April 10-15, 78 on April 16-17, 80 on April 18-22, 78 on April 23, 80 on April 24-28, 82 on April 29 to May 1, 78 on May 2-5, 82 on May 6-7, 80 on May 8-12 and 78 on May 13-14. Solar flux then continues to dither between 78 and 80 over the remainder of the 45 day forecast. You can find daily updates of this forecast at ftp://ftp.swpc.noaa.gov/pub/forecasts/45DF/ .
Predicted planetary A index is 15 and 8 on April 8-9, 5 on April 10-12, then 12, 20, and 8 on April 13-15, 5 on April 16-20, 8 on April 21-22, then 5 and 12 on April 23-24, 10 on April 25-26, 8 on April 27, 5 on April 28-29, then 22, 8, 15 and 12 on April 30 through May 3, then 8 on May 4-5 and 5 on May 6-7.
The big factor in bringing the week’s average sunspot number down nearly 9 points was the fact that the daily sunspot number was 11 on March 31 through April 2. 11 is the lowest sunspot number we can possibly observe, unless there are zero sunspots, then the sunspot number is zero. Each sunspot group is counted as 10 points, and these are added to the total number of sunspots which count as one each, so a sunspot number of 11 is what you get with just one sunspot visible.
Spaceweather.com reported early Thursday that on April 7, Earth is expected to cross a fold in the Heliospheric Current Sheet, which could trigger unsettled geomagnetic conditions.
The Heliospheric Current Sheet separates regions of solar wind where the magnetic field points toward or away from the sun. See http://bit.ly/25MG4lW for a continuous animation of this effect from 2001 until 2009. See https://en.wikipedia.org/wiki/Heliospheric_current_sheet for more information.
Sure enough, early on April 8 at 0007 UTC the Australia’s Space Weather Services issued a geomagnetic warning:
“The geomagnetic conditions are currently at minor storm levels and are expected to remain at these levels for the next 6-12 hours. This is a combined effect of sustained strongly southwards IMF Bz (see http://bit.ly/1S6H68D ) starting from 07/1800 UTC but with stable, weak solar wind speeds (380 km/s). However, the solar winds are expected to gradually increase later today in response to a small recurrent southern hemisphere coronal hole moving into a geo-effective location on the solar disk. The aurora may be visible from as low as some parts of the state of Victoria, Australia, on the local night of 8 April.
Increased geomagnetic activity expected due to a coronal hole high speed wind stream from 08-09 April 2016.”
F.K. Janda, OK1HH reports:
Geomagnetic activity forecast for the period April 08-May 04, 2016
Geomagnetic field will be:
quiet on April 17-18, 21-22, 27-28, May 4
mostly quiet on April 19-20, 29, May 1, 3
quiet to unsettled on April 10, 14-16, 23-25, May 2
quiet to active on April 8-9, 13, and 26
active to disturbed on April 11-12, 30
Amplifications of the solar wind are expected on
April 9-10, (14,) 20, (24-25,) 28-30
Parenthesis means lower probability of activity enhancement.
For more information concerning radio propagation, see the
- ARRL Technical Information Service at http://arrl.org/propagation-of-rf-signals.
- For an explanation of the numbers used in this bulletin, see http://arrl.org/the-sun-the-earth-the-ionosphere.
- An archive of past propagation bulletins is at http://arrl.org/w1aw-bulletins-archive-propagation.
- More good information and tutorials on propagation are at http://k9la.us/.
My own archives of the NOAA/USAF daily 45 day forecast for solar flux and planetary A index are in downloadable spreadsheet format athttp://bit.ly/1VOqf9B and http://bit.ly/1DcpaC5 .
Click on “Download this file” to download the archive, and ignore the security warning about file format. Pop-up blockers may suppress the download.
Monthly propagation charts between four USA regions and twelve overseas locations are at http://arrl.org/propagation.
Instructions for starting or ending email distribution of ARRL bulletins are at http://arrl.org/bulletins.
Sunspot numbers for March 31 through April 6 were 11, 11, 11, 38, 23, 27, and 13, with a mean of 19.1. 10.7 cm flux was 81.7, 82.1, 81.5, 82.3, 83.4, 83.4, and 87.1, with a mean of 83.1. Estimated planetary A indices were 7, 3, 22, 15, 7, 5, and 7, with a mean of 9.4. Estimated mid-latitude A indices were 5, 2, 15, 13, 6, 5, and 7, with a mean of 7.6.