Winter 2008 Issue

PROPAGATION

The New Solar Cycle Has Begun!

By Tomas Hood, NW7US

This UV-wavelength image of the sun (left) and a map showing positive (white) and negative (black) magnetic polarities (right) illustrate an active region that sparked a lot of excitement in December 2007. The new high-latitude active region was magnetically reversed, marking it as a harbinger of a new solar cycle. If the active region turned into an actual sunspot, scientists would have declared the start of solar Cycle 24 during December. However, a sunspot group never developed with a reversed polarity during December. (Source: SOHO/NASA)

January 4, 2008 was a pretty exciting day for solar scientists, as well as amateur radio operators. When observers took a close look at the day’s images of the sun, they noticed that a small sunspot had developed with a much-anticipated feature—a reversed magnetic polarity. Such a reversal marks the start of the new solar cycle, the 24th recorded cycle.

The excitement actually started in December, when a magnetically reversed, highly active region appeared in the sun’s eastern limb. Because of its reversed polarity, scientists became hopeful that the region would develop into an actual sunspot. If it had, then scientists would have declared the official start of Cycle 24.

Sunspots have a complex magnetic structure. Typically, though, a sunspot will have at least one very clearly defined set of magnetic poles, north and south. At the start of a new solar cycle, the polarities of the new cycle’s sunspots are reversed from the polarities observed in sunspots belonging to the previous cycle. When the first sunspot arrives with a reversed magnetic structure, scientists declare the start of the new cycle. This occurred on January 4, 2008.

What is in store for 2008? Solar cycles take anywhere from two to five years to reach the point of maximum solar activity. The current consensus among most solar scientists places Cycle 24’s maximum sometime between 2011 and 2013. That means we have at least a year or two before we see major solar activity of the kind useful to VHF propagation. However, that does not mean that 2008 will be a disappointment for VHF DXers.

Space Weather and VHF Propagation

Since the sun’s magnetic field permeates the entire solar system and beyond (in a region called the heliosphere), it interacts with the Earth’s magnetic field (a field known as the magnetosphere). The sun’s huge magnetic field is called the Interplanetary Magnetic Field (IMF) and is a primary cause of space weather. Sprawling out away from the sun is a solar wind that rides the IMF.

As Earth orbits the sun, it dips in and out of the wavy current sheet of the IMF, known as the Parker Spiral. On one side the sun’s magnetic field it points north, or toward the sun. On the other side it points south, or away from the sun. The IMF’s orientation is indicated by the “Bz” index. When the Bz is negative, it indicates a southerly orientated IMF; when iy is positive, it indicates a northerly oriented IMF.

South-pointing solar magnetic fields tend to “magnetically reconnect” with Earth’s own magnetic field. This allows the solar wind, and the solar plasma that is riding the solar wind out away from the sun, to flow in and collect in a reservoir known as the boundary layer.

The energetic particles riding the solar wind can then penetrate the atmosphere and trigger geomagnetic storms, as well as aurora.
If the IMF is oriented northward, however, this magnetic reconnection does not take place. This instead creates a barrier against the solar wind and the plasma riding the IMF.
 

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