QST de W1AW
Propagation Forecast Bulletin 12 ARLP012
From Tad Cook, K7VVV
Seattle, WA March 22, 2002
To all radio amateurs
SB PROP ARL ARLP012
ARLP012 Propagation de K7VVV
Average daily solar flux and sunspot numbers for the past week were almost identical to the previous week's. Average sunspot numbers were less then two points lower, and average solar flux rose less than one point. Geomagnetic activity was essentially the same, with the average estimated planetary A index less than one point higher this week.
On March 19 the interplanetary magnetic field moved south, leaving Earth vulnerable to solar wind. The planetary K index rose to five over two three-hour periods, but during other periods through the day K indices were very low. This resulted in a planetary A index for that day of only 17, which indicates unsettled to active conditions. If the K index were five through all eight three-hour periods, the A index would be around 50, indicating a geomagnetic storm. An A index of 17 for the day corresponds to an average K index of a little above three.
The K index is a quasi-logarithmic index over three hours of geomagnetic activity from a particular observatory. For instance, on WWV at 18 minutes after the hour (you can also hear this report anytime by calling 303-497-3235) you would hear the Boulder K index from Colorado, although they have switched to a new reporting scale that uses the mid-latitude K index.
The planetary K-index, or Kp, is a mean of the values taken from 13 observatories that are between 44º and 60º north or south latitude. The planetary A index, or Ap, is derived from the average of the Kp values for the day. If the Kp average is one, this corresponds to an Ap for the day of four. Kp of two equals Ap of seven, Kp of three equals Ap of 15, Kp of four equals Ap of 27, Kp of five equals Ap of 48, Kp of six equals Ap of 80, and the scale Continues up to a Kp of nine. The Boulder, or any other local A index is derived the same way. For a thorough explanation of the scale, see NOAA's Geomagnetic kp and ap Indices Web page and NOAA's The K-index Web page.
At 1330 UTC on March 20 a coronal mass ejection that had left the sun on March 18 near sunspot 9866 passed Earth. It triggered some geomagnetic activity, but only for a short period when both the planetary K index reached four and the mid-latitude K index reached five. The rest of the day had very low K index numbers.
N4KG sent a note about the equinox and the effect on propagation. He writes, "To my mind, the major benefit of the equinox period is improved polar path propagation on the high bands (10-17 meters) from three weeks before to three weeks after the equinox." He said that 9N7RB (Nepal) was coming in during North America's local morning and evening hours on 10 meters, shortly after sunrise and after sunset, wrapping around the poles from both sides.
K0ZN is wondering when the next solar minimum will be. According to a recent NOAA Preliminary Report and Forecast of Geophysical Data, their prediction shows it to be some time between September 2006 and April 2007. Still a way off, and right now we are enjoying the peak of the current cycle. You can read these NOAA reports on the Web, and the one showing the latest solar cycle projection is the March 5 issue. You can also see some nice historic charts of this and previous solar cycles on WM7D's Solar Resources Page, Solar Cycle Graphs and Historical Solar Charts.
KN9P and several others wrote to ask about the new, unfamiliar propagation reports on WWV. NOAA is using some new scales. An article, "New Scales Help Public, Technicians Understand Space Weather," has been posted on the NOAA Web site. The scales are explained on the NOAA Space Weather Scales page.