Subject: Re: [DX] 20/17 Meters From: Tomas To: [email protected]
Sent: Saturday, October 05, 2002 1:42 PM Subject: Re: [DX] 20/17 Meters

OR: What Happen To The Bands
By Tomas Hood, NW7US of Brinnon, Washington

Hi, WA1TJB wrote: "I have been on 20 and 17 meters late in the evening for the past couple of nights and the bands seems like it's almost dead. I have a brand new IC-746 pro and I'm wondering if it's the band or my radio? Have there been any geomagnetic disturbances in the past few days?"

The propagation of radio signals over the last week has been fair to poor, as a result of a continual solar wind with just the right magnetic orientation to cause geomagnetic storminess. This solar wind has mostly originated in coronol hole "mass ejections." A lot of folks might think that these conditions were a result of high solar flare activity. But that is not really the cause. The CME - coronal mass ejection - activity has been very active.

The atmosphere above the Sun's surface is called the "corona," under which is the chromosphere and the photosphere. On the photosphere exist several types of features. Sunspots are the most obvious. Using a certain type of instrument called a coronagraph, we can see the corona, and other features like the CME. The coronagraph is a man-made eclipse aboard a space vehicle (satellite), that allows us to see the pearly white crown surrounding the Sun. Features seen using these eclipes include the coronal holes, solar flares, and popping bubbles called coronal mass ejections.

Coronal holes are regions where the corona is dark. It is not a real "hole" as in a dip in some surface. The corona is not part of the sun's surface. The corona, again, is part of the sun's atmosphere (like our troposphere, stratosphere, and so on). These features were discovered when X-ray telescopes were first flown above the earth's atmosphere to reveal the structure of the corona across the solar disc. Coronal holes are associated with "open" magnetic field lines and are often found at the Sun's poles. A coronal hole simply means an area where a break-down in the magnetic fields in the solar corona have occurred. It is not part of the sun "burning out" or anything close to this. It is a normal part of the way the sun's corona acts. Often, high-speed solar wind is known to originate in coronal holes. This escape of solar plasma and energy streams outward away from the sun. When this outward stream, or solar wind, is directed toward Earth, we see an increase in the Solar Wind speed and intensity. More on this in a bit.

When a bubble of plasma (formed by the strong magnetic fields of the sun) originating in the break-down of the corona (the coronal hole) bursts, and spews outward away from the sun the huge cloud of plasma, we call it a "Coronal Mass Ejection." It once was thought that Coronal Mass Ejections were initiated by solar flares. Although flares accompany some CMEs, it is now known that most CMEs are not associated with flares. CMEs can occur at any time during the solar cycle, but their occurrence rate increases with increasing solar activity and peaks around solar maximum. Since the Sun completes a full rotation every 27 to 28 days, the same CMEs may recur every month. The exact processes involved in the release of CMEs are not known, but we do know a lot about how they affect the Earth.

Anyway, the result of a well-placed CME is a bombardment of plasma into our magnetosphere (the magnetic force field that in part protects us from lethal doses of solar energy), as well as an increase in the density, power level, and speed of the solar wind.

When the solar wind, which contains magnetic field lines, reaches the magnetoshere, one of two thing can happen. If the magnetic lines in the solar wind are orientated just right, or in a southerly orientation, they will combine in a way that nullifies the magnetosphere at that point, causing a "window" to open, allowing for solar plasma to enter into our atmosphere. If the magnetic lines in the solar wind are NOT orientated this way, then they will combine with the magnetosphere in a way that enhances the magnetospere in a way that strengthens the force field. So, when plasma makes it through, the geomagnetic fields as well as the ionosphere become highly disturbed. When the plasma and radiation is blocked, we have more quiet geomagnetic conditions.

A solar flare, on the other hand, will impact us in two ways (where the CME only imacts us in one way: The shock wave of the plasma cloud and the increase of the solar wind); the immediate x-ray and ultraviolet radiation, which directly causes increased ionization of the D- and E-layers, and therefore an increase in the absorption of radio signals, and the later (two to three days out) impact of the solar plasma and increased solar wind speed, much as is seen from a CME.

In the last week, we have had more CME activity than solar flare activity. Solar flares have been minor in stength, so radio blackouts (the result of the immediate arrival of x-ray and ultraviolet radiation from a solar flare) have been infrequent. But, CME activity has been ongoing and moderate in strength, with a predominant southern component of the solar wind's magnetic orientation.

Expect a few more days of this, at least. You may see these current conditions at http://prop.hfradio.org/ 73 de Tomas, NW7US // AAR0JA