hfradio.org - Propagation (page 2)

Space Weather, Propagation, and so on... General Information Worth Reading

(by Tomas David Hood, NW7US - copyright, 2002, all rights reserved)

At the time of writing this section of the webpage, the Solar Wind is at 736.0 km/s at 1.6 protons/cm3 - under typical conditions during the Solar Cycle Maximum years, the average Solar Wind is around 400 km/s. So, today's speed is a lot higher than "normal." What does this mean?

First, let me tell you a few things about the Sun.

1. 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. But, using a certain type of instrument called a coronagraph, we can see the corona. 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 then can be seen, like coronal holes, solar flares, and popping bubbles called coronal mass ejections.

Let's first talk about coronal holes. 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.

So, what is a sunspot? Sunspots are magnetic regions on the Sun with magnetic field strengths thousands of times stronger than the Earth's magnetic field. Remember, plasma flows in the magnetic field lines of the sun. Sunspots appear as dark spots on the surface of the Sun. Temperatures in the dark centers of sunspots drop to about 3700 K (compared to 5700 K for the surrounding photosphere). This difference in temperatures makes the spots appear darker than elsewhere. Sunspots typically last for several days, although very large ones may live for several weeks. They are seen to rotate around the sun, since they are on the surface, and the sun rotates fully every 27.5 days.

Sunspots usually come in groups with two sets of spots. One set will have positive or north magnetic field while the other set will have negative or south magnetic field. The field is strongest in the darker parts of the sunspots (called the "umbra"). The field is weaker and more horizontal in the lighter part (the "penumbra").

Galilea Galileo made the first European observations of Sunspots in 1610. The Chinese and many other early civilizations have record of sunspots. Daily observations were started at the Zurich Observatory in 1749. By 1849 continuous observations were obtained. See this chart of all the solar cycles.

The sunspot number is calculated by first counting the number of sunspot groups and then the number of individual sunspots. The "sunspot number" is then given by the sum of the number of individual sunspots and ten times the number of groups. Since most sunspot groups have, on average, about ten spots, this formula for counting sunspots gives reliable numbers even when the observing conditions are less than ideal and small spots are hard to see. Monthly averages (updated monthly) of the sunspot numbers show that the number of sunspots visible on the sun waxes and wanes with an approximate 11-year cycle.

And, what is a solar flare? Solar flares occur near sunspots, usually along the dividing line (neutral line) between the two sets of spots, or areas of oppositely directed magnetic fields. These flares, tremendous explosions, heat material to many millions of degrees and release as much energy as a billion megatons of TNT and release many forms of energy. Electro-magnetic energy (Gamma rays and X-rays) are what affect ionospheric conditions within moments of a flare, and energetic particles (protons and electrons) ride the solar wind, to impact our magnetoshere. Flares are characterized by their brightness in X-rays (X-Ray flux). The biggest flares are X-Class flares. M-Class flares have a tenth the energy and C-Class flares have a tenth of the X-ray flux seen in M-Class flares.

2. Solar Wind. Space is not a vacuum. At least not in our solar system. The sun's atmosphere actually extends very far out from the sun. Space in our system is filled with plasma. The temperature of the corona is so high that the Sun's gravity cannot hold on to it. The solar wind streams off of the Sun in all directions at speeds of about 400 km/s (about 1 million miles per hour). (So when you see the solar wind speed around 400 km/s, you know that things are "normal" and our solar/geophysical "weather" should be normal, for the most part). The solar wind changes speed and carries with it magnetic clouds, interacting regions where high speed wind catches up with slow speed wind. The solar wind speed is high (800 km/s) over coronal holes and low (300 km/s) over streamers. These high and low speed streams interact with each other and alternately pass by the Earth as the Sun rotates. These wind speed variations buffet the Earth's magnetic field and can produce storms in the Earth's magnetosphere. Many Coronal Mass Ejections combine with the solar wind and cause shock waves which, if directed to the Earth, can ignite the Aurora and major ionospheric / geomagnetic storms.

3. The Earth's magnetosphere - put up the force fields, Captain! The Earth has a magnetic field with north and south poles which is enclosed in a region surrounding the Earth called the magnetosphere. As the Earth rotates, its hot core generates strong electric currents that produce the magnetic field which reaches 36,000 miles into space. The magnetosphere prevents most of the particles from the sun, carried in solar wind, from impacting the Earth. The solar wind distorts the shape of the magnetosphere by compressing it at the front and causing a long tail to form on the side away from the Sun. This long tail is called the magnetotail.

4. What does this all mean to me? Radio wave propagation is directly tied to the Ionosphere. The more ionization occurring in the F-layers, the higher the frequencies which refract back toward the Earth. The highest frequency that will refract back from the Ionosphere over a selected point-to-point path is known as the Maximum Usable Frequency, or MUF. The other layers of the Ionosphere can block our transmissions, and the earth's geomagnetic field has an impact, as well. The Lowest Usable Frequency, or LUF, is the lowest frequency that can propagate via the Ionosphere over a particular point-to-point path. When the LUF increases to or above the MUF, then communications are next to impossible over the given path.

The Sunspots give us a general correlation to the sun's activity - the overall energy that radiates out from the sun and causes the ionization of the Ionosphere. But, we have found that sunspot numbers are not that accurate of a way to gauge this influence. We have adopted the measurement of the 10.7cm Wavelength Flux. But, even this does not always capture the direct impact of the solar energy on the Ionosphere.

With an increase of solar sunspot counts during the Solar Cycle Maximum, an increase in solar flares, and coronal holes also happens. When those X-rays from these increased number of Flares pass by Earth, they, in part, increase the ionization of the D Layer. When this happens, we experience degradation of radio signal strength. The D Layer begins to absorb the radio waves, causing Radio Blackouts, as the LUF is raised to the MUF.

Let's look at the relationship between coronal material and magnetic fields. The Corona is so hot that the gases in it loose some of their electrons in the powerful collisions between atoms. This plasma is a mixture of positively-charged ions and negatively-charged electrons. Take a look at a Neon light. You are looking at plasma. Because plasmas are electrically conductive, they can steer magnetic fields. And they are steered by magnetic fields. CMEs drag a piece of the Sun's magnetic field with it. These loops of magnetic force are stretched and dragged into interplanetary space by the inertia of the expanding plasma. When these magnetic forces impact the Earth they are either diverted by or combined with Earth's magnetic field.

The speed of a CME ranges from less than 50 to about 2000 kilometers per second. As the CME moves outward from the Sun, it generates a shock wave that can accelerate particles in interplanetary space to high energies. When a CME or its shock wave passes the Earth, geomagnetic storms are triggered. The majority of large and major geomagnetic storms are generated by the encounter with both the interplanetary shock and the CME that drives it. Their ability to disturb the Earth's magnetosphere is a function of their speed, the strength of their magnetic field, and the presence of a strong southward magnetic field component.

The Earth's magnetosphere is formed from two essential ingredients, the Earth's magnetic field (which has much the same form as that of a bar magnet, and is from pole-to-pole), and the solar wind. When the CME combines with the Earth's magnetic field, it alters the shape and intensity of this shield around the Earth. The Ionosphere is affected by these changes, either by an increase of ionization, or a decrease or even a depletion of ionization. Depressions in ionospheric density cause major communications problems because radio frequencies that previously had been refracting off the ionosphere now punch through. The MUF can be decreased by a factor of two during an ionospheric storm event. Storm effects are more pronounced at high latitudes.

When a CME is directed toward Earth and arrives after the two- to three-day journey, the interaction between the CME, solar wind, and the magnetosphere and ionosphere causes Aurora. Propagation off of Aurora is an exciting activity. At the same time, we bemoan the loss of communication caused by the degraded ionization of the Ionosphere. But hang in there! While CMEs affect us year-round, they are not as common as solar flares and solar wind.

Overview of Solar Terrestrial Indices

To start with, check out this fine page: The Basics of Radio Wave Propagation. It is a bit of reading. But easy to understand and utilize in our daily activity on radio frequencies. It is part of AE4TM's really excellent propagation research page.

Also worth seeing if you are new to this science: Propagation Primer

NEW! - Join us in the NEW on-line propagation forum. Talk about current solar conditions, ask questions about the science of propagation, and so forth. These forums cover propagation modes like Aurora, Sporadic-E, and others. We need your participation to get things going. Please take some time, and join the HFRadio.org On-line Community Forums. Thanks!

The Space Environment Center, (SEC), provides you with a glossary of space environment terms.

One of the critical resources at the disposal of Amateur Radio Operators is the observed data of the solar, geomagnetic, and ionospheric conditions, events, and related phenomena. This page is your starting point to the wealth of information you need to plan your radio activity.

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