This is from an email by Paul Harden NA5N of the National Radio Astronomy Observatory VLA/VLBA in Socorro, New Mexico. Paul is well known for his knowledge of how the Sun effects our hobby and if you do a search of the Internet you will find many contributions by Paul on a variety of subjects. Anyway, I thought some of you may find this note interesting.
Solar Minimum Here?
It appears we are at, or nearly at, the solar minimum. Look at the solar flux chart at:
http://www.dxlc.com/solar
The solar flux varies over the 28-day rotation of the sun, going from some minimum value to a maximum. During more active days, this 28-day cycle produces variations in the solar flux running, from say, 90 to 150, making more a less a 28-day sinewave when plotted. For the past two solar rotations, the 28-day variation in the solar flux has been between about 70 and 75, or a variation of only 5 counts. Statistically, this is a virtual “flat line.”
The solar flux average over the past 90 days is 75; for the past several days, it has been 69 with the forecast for the next few days at 70. Thus, solar flux is varying only by a few counts. Solar flux in the 60-70 range, with almost no 28-day variation, and the complete absense of sunspots, are indications we may have hit the solar minimum.
However, we won’t know for 2-3 more months. In fact, there is no way of knowing when we are at the actual solar minimum until AFTER it happens.
This is because the solar flux can do one of three things over the next few months.
1) It can remain flat, pretty much as it is right now, if this is the minimum.
2) Solar flux can actually drop a bit more, if the minimum is yet to come.
3) Solar flux can begin to rise, indicating we’re coming out of the minimum.
Only historical data will tell us when the minimum actually occured.
There are several models of the solar minimum. These models show the solar minimum occuring around now, some say this summer (June-July), others later this autumn. Present indications suggest we are either at the solar minimum right now, or very close to it.
Not that it makes much difference. Even if the solar flux begins to rise a bit over the next few months, from our present 70 to 75, then 80, it will hardly make a difference in the propagation we’ve been experiencing. For the good old days, with the 28-day solar flux varying between 100 and 150, with solar activity shooting it above 200, is still a few years away.
More specific to ham radio, the solar minimum represents the minimal amount of ionizing radiation the sun delivers to the earth. As a result, our E and F layers are not being well ionized right now, and thus not very reflective.
And, once the sun goes away at local sundown, recombination of the free electrons is very swift. This is why 20M shuts down pretty much right at dusk, compared to staying open into the wee hours of the night when the solar flux is >100, or around-the-clock at the solar maximum. When the ionizing radiation from the sun stops at sundown, the MUF drops to around
10-12 MHz right now, shutting down 20M. During the more active sun, the MUF seldoms drops below 18MHz - allowing 20M to be open 24 hours a day.
Ionizing radiation from the sun, over the past two solar cycles (22 years), has been measured by the x-ray radiation received on earth from the sun.
Or, near the earth. Since x-rays do not normally penetrate our ionosphere, x-ray radiation is measured by sensors on satellites in orbit above our ionosphere. This is shown at:
http://www.sec.noaa.gov/today.html
You’ll notice the x-ray radiation from the sun is virtually flat-lined right now as well, below the “A” level. For comparison, it takes a “D” class, or “D” level flare to effect HF propagation. Right now, solar radiation is way down in the noise ... around 10^-9 ... or a mere nano-watt per square meter.
Prior to satellites, the x-ray radiation from the sun was measured by sensors on high altitude balloons or small rockets. Of course, this provided only a few minutes of data for each rocket launch, and a few hours of data on a balloon platform. It has only been over the past two solar cycles that almost continuous real-time x-ray data has become available.
Even more astounding is how quickly that data became available not only to scientists, but to us via the internet. In the 1970s-80s, when I first started working at the VLA observatories, our professional astronomers would have killed to have access to the data that is available to us all today courtesy the internet.
What about the solar wind? You can see what the solar wind and density has been doing lately at:
The escape velocity from the sun is generally believed to be around 320-350 km/sec. Notice that is what the solar wind has been for some time ...
barely enough to escape the sun. Another indication of the minimal solar activity. The spikes in solar DENSITY are due to the earth flying through coronal hole streams, not changes in the solar wind SPEED.
This minimal solar wind is good news for the lower bands, particularly 80M and 40M. Solar wind buffeting the earth is what causes noise on the lower bands. Sure, these bands are noisy. They’re always noisy. But, not all of that noise is due to the sun. Right now, those low sodium vapor lights at your nearby Wal-Mart parking lot is contributing more noise than the sun!
At my QTH in Socorro, NM, the largest source of 40M noise is the Burger King sign. Too bad there’s not a plot for that :-) In other words, what you are hearing on 80/40M right now is about as good as it gets. When the sun wakes up, it will only get worse.
Whether we are legally at the solar minimum or not, 20M is the daytime band, and 40M the night time band. And, if lucky, sporaidic daytime openings on 15M and 10M. The solar flux has little effect on 30M, 40M, 80M or 160M.
And, that’s just how it’s going to be for the next year or two.
If you’re a purist, keep your eye on the solar flux plots. It could be we’re seeing the solar minimum right now, or it may be another month or two away. Regardless, we’re close enough to it we can taste it ... or more precisely, hear it - from the lack of significant HF propagation. I would expect an official announcement from NOAA in a couple of months.
The good news ... this is as low as it gets. We’ve hit rock bottom.
It will start the upward swing into the new solar cycle (#24) shortly, though it will be awhile before we notice any significant improvement.
72, Paul NA5N
