Solar Cycle 25 Gave Us A Wild Week

Solar cycle 25 set some new 20 year high points again this week, and some that go back quite a way. Depending on how you look at it.

Let me do a small preliminary review because there's a lot of details here that matter. There are a few main ways solar activity is summarized and different ways of stating all of them. The most direct (and some judgement is still required) is the sunspot number. Sunspot numbers are never one digit; if there was one little spot visible on the entire side facing us, the sunspot number would 11. The number isn't obtained by taking an image of the sun and counting all the dark spots.  It's a two digit number where the first is ten times the number of spot groups and the second is the number of spots in all of those groups.  That's why the single dark spot gets a sunspot number of 11: one group, one spot.  In general the number is k*(10*G+S). (where k is a coefficient for each observatory that helps adjust for differences in their capabilities). 

Once those numbers are taken, they're combined in plain, numerical averages and a Smoothed Sunspot Number or SSN. That's also covered in that previous link. 

The next main way solar activity is measured is the amount of energy the sun radiates in the radio spectrum. The most common of these is the 10.7 cm (radio wavelength) solar flux, usually referred to as the SFI. The wavelength is a frequency of 2800 MHz, and it's a direct measurement of the amount of power received by tuning an antenna and radio to 2800 MHz and measuring the energy received. Because that varies a lot, there are different adjustments to and averages used for it, as well.

Another of the ways the sun is monitored is the amount of disturbance of the Earth's magnetic field - the Geomagnetic Indices. There two frequently cited numbers, the A and K indices, that often look quite different because they measure different effects. Of the two, the most commonly used value of the K index, is the Planetary K Index and written K_P. I've used the K_P much more often than the A index and K_P appears to be the most widely used index. 

The big deal is that the solar flux on Thursday and Friday went to and past 300.  This is a level that the solar flux hasn't reached since 1991.

Date           Time         Julian day      Observed Flux
2024-08-08     17:00:00     2460531.197     300.1     
2024-08-08     20:00:00     2460531.322     336.0     
2024-08-08     23:00:00     2460531.447     304.7     
2024-08-09     17:00:00     2460532.197     305.1     
2024-08-09     20:00:00     2460532.322     305.5     
2024-08-09     23:00:00     2460532.447     316.1

Two days in 1991 - 34 years ago - were the last time the SFI went over 300 (different source so different format data, of course):

Feb 22, 1991 302.6
Feb 22, 1991 313.1
Feb 23, 1991 311.5

On Friday (August 9), the SpaceWeather Prediction Center of NOAA noted the milestone in a different way - Sunspot Number instead of Solar Flux. In the top left, they say the "The estimated sunspot number (SSN) for August 8th..." which might be a little confusing or misleading. SSN is not an acronym for Sun Spot Number; it's for Smoothed Sunspot Number - a smoothing math operation, which I've assumed is averaging.  

While the SFI was up to 336, the K_P was low, and the HF bands were better than VHF.  The buzz lately on some of the VHF groups I hang out around is that this fall might be the best year for F layer (highest layer, so the longest propagation distances) in the same kind of time frame. Best in 35 years? Better than 35 years ago? A lot of guys are looking at their stations and saying, "what can I do to make it better for that?"  

Temper any excitement about wonderful propagation on the ham bands with the fact that it is, after all, solar maximum, and that means the sun spits out flares regularly. Right now, for example, the widely used sources are saying SFI is 282 (still outrageously good) and K index is over 4.5. We're having a G2 (Moderate) class geomagnetic storm.