The K7RA Solar Update

SEATTLE, WA, Jul 15, 2005--The big patch of sunspots that energized activity less than two weeks ago has drifted around the edge of the sun. The result has been falling daily sunspot numbers and solar flux. The average daily sunspot number dropped a little more than 63 points to 91.3. At the same time, geomagnetic disturbances increased, so ideal conditions with high sunspot activity and quiet geomagnetic conditions are reversing.

On July 10 a coronal mass ejection (CME) from a day earlier hit Earth, and caused a geomagnetic storm. The same day it hit, the planetary A index jumped to 47, and another CME began its journey from the sun. As a result, the planetary A index went back up--this time to 48 on July 12. All this as sunspot numbers and solar flux dropped.

Solar activity is currently increasing, but only from some sunspots that are drifting from view. They may deliver an indirect hit to Earth in the next couple of days. The interplanetary magnetic field, or IMF, is pointing south, which means Earth is vulnerable. Currently the planetary A indices for Friday through Monday, July 15-18, are predicted to be 25, 25, 20 and 12.

Sunspot numbers and solar flux should reach a short-term minimum around July 16-19, and another maximum around August 2-5. This is based on the recent peak in activity and the fact that the sun rotates relative to Earth about once every 27-28 days.

This time of year West Coast stations often see good propagation in the evening to the west and southwest, to Hawaii and down toward Australia, where it's now winter. Twenty meters can be open all night, with 15 and 17 meters showing good propagation through the evening. The East Coast of North America will see good 20 meter propagation into the evening toward Europe, with the band often staying open four or five hours later than it will about a month from now.

For more information concerning radio propagation and an explanation of the numbers used in this bulletin visit the ARRL Technical Information Service Propagation page. An archive of past bulletins also is available.

Sunspot numbers for July 7 through 13 were 149, 111, 126, 78, 68, 52 and 55, with a mean of 91.3. The 10.7 cm flux was 124.9, 110.4, 106.6, 101.8, 93.3, 95.3 and 91.7, with a mean of 103.4. Estimated planetary A indices were 8, 5, 19, 47, 23, 48 and 30 with a mean of 25.7. Estimated mid-latitude A indices were 6, 5, 13, 28, 14, 17 and 20, with a mean of 14.7.

Amateur solar observer Tad Cook, K7RA, 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 in The ARRL Letter. Readers may contact the author via e-mail.

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