Until I became interested in amateur radio, I thought magnetic storms were simply the invention
of science fiction writers, no less fictitious than phasers, photon torpedoes and warp drive
engines. Even when I studied for my amateur radio license, I didn't realize the true
significance of these events and their effect on radio communications. It wasn't until the DX
bug bit me that I started listening to the WWV forecasts. After all, a true blue DXer has to be
able to quote "the numbers" at a moment's notice. Throwing in a few phrases like "polar cap
absorption" and "major geomagnetic storm" does wonders for one's stature on the local DX spotting
repeater. The trouble was, I began to worry that someone would ask me what all these terms meant!
So, I spent some time researching the literature and talking to people whose background was in
world of geophysics.
The Earth's Magnetic Field The earth has a magnetic field that closely resembles that of a dipole magnet. This type of magnetic field has been demonstrated many times with the classic high school experiment using a small bar magnet and iron filings. In this experiment, a piece of paper is placed over the magnet. Iron filings are sprinkled on the paper. The filings form the pattern of the magnetic field and illustrate the lines of magnetic flux or force. With a bar magnet the lines force move outward from ends and connect at the centre. In the case of the earth, the magnetic field resembles a spherical shell with lines of force propagating outward from the poles and connecting over the equator. This is a theoretical model of the earth's magnetic field and thus is not perfect. There are some internal magnetic anomalies that generate slight distortions in the field, but this visualization is close enough. Scientists refer to the earth's magnetic field as the magnetosphere.
If the earth were the only object in the solar system, the magnetosphere would indeed exist as a
spherical shell described above. However, the sun has a profound affect on its shape. The sun
is constantly emitting energy in the form of cosmic rays; atomic particles etc. collectively
called the solar wind. This wind exerts pressure on all objects in the solar system, including
the earth. The pressure from the solar wind distorts the earth's magnetic field from an ideal
sphere and causes it to take on a teardrop or comet-like appearance. Like a comet, the
magnetosphere has a head and tail. The head of this magnetic field surrounds the earth at a
distance of about 10 times the earth's radius. The outer boundary of the magnetosphere is called
the magnetopause. When conditions are quiet (no significant solar flares or coronal holes), the
pressure from the solar wind is fairly constant and the shape of the earth's magnetosphere
remains about the same.
Fluctuations in the Magnetosphere
As I mentioned above, constant solar wind pressure results in a stable magnetosphere. The solar wind does not always remain steady. It is usually gusting and fluctuating to some degree in response to solar activity. Like all magnetic fields, the magnetosphere has flexible spring-like properties. If the solar wind pressure increases, the magnetosphere compresses inward toward earth. When the solar wind pressure decreases, it rebounds and expands outward. When the solar wind causes the magnetosphere to compress and expand, we have a moving magnetic field. This field is moving through the ionosphere, which is composed of ionized gases due to solar ultraviolet radiation. Basic electric theory tells us a moving magnetic field (the magnetosphere ) within a conductor (the ionosphere) will induce a current. It is these currents induced in the ionosphere, particularly in the auroral zones, that have the greatest affect on radio propagation and other phenomena associated with geomagnetic storms.
Short-lived fluctuations in the magnetosphere are called geomagnetic substorms. They usually
last from 30 minutes to several hours and are most prevalent in the polar and auroral zones.
Small numbers of geomagnetic substorms do not usually present a serious problem to radio
How Geomagnetic Storms Progress
When numerous geomagnetic substorms occur over a short time (within a day or two), the event is called a geomagnetic storm. Great magnetic storms last many days, but most occur over a period of 24 to 48 hours.
Geomagnetic storms usually progress through three phases. First, a shock wave is generated in the solar wind by a flare. This wave slams into the earth's magnetosphere and produces a large magnetic impulse. This impulse is called a sudden storm commencement (SSC) or sudden commencement (SC). It causes an instantaneous compression and distortion of the earth's magnetosphere. Once the initial shock wave has passed the solar wind returns to normal pressure and the magnetosphere recovers. Over the next several hours, the magnetic field remains fairly stable with only minor fluctuations. It is important to note that not all geomagnetic storms begin with a SSC. SSC impulses are caused by flares, which are huge solar explosions. Some storms begin with the main phase, described below. Such storms are usually caused by coronal holes that can eject solar material without the violent explosions associated with flares.
About three to six hours after the SSC, the main phase of the storm begins. At this time, particles that have been ejected from the solar flare or coronal hole begin to arrive at earth. The solar wind pressure begins to change rapidly. The magnetosphere responds with wild fluctuations. Each fluctuation (substorm) induces heavy currents in the ionosphere, particularly within the auroral electrojet. The auroral electrojet is an oval shaped core of strong electrical current that travels through the ionosphere within the auroral zones. The earth takes from one to several days to pass through the cloud of solar debris.
The third stage of a geomagnetic storm is the recovery phase. This can take several days
following the main phase, depending on the intensity of the storm. The earth's magnetic field
slowly returns to normal. During this phase there are usually some periods of substorming, but
the overall activity decreases. After a few days, and provided there are no further flares or
coronal ejections, the geomagnetic field returns to normal.
Some Effects of a Major Geomagnetic Storm
All magnetic storms produce terrestrial effects to some degree. Most are only of concern to those involved in HF communications, be it commercial, military or amateur. It is only the great storms that have a direct effect on the general public. In the past 50 years there have been seven major magnetic storms that have caused the geomagnetic Ap index to exceed 250. The largest ever recorded occurred in March 1989, just three months before the peak of solar cycle 22 . This is likely a coincidence because comparison of the number of days with high magnetic activity against annual sunspot numbers show that peaks in magnetic storming do not necessarily occur during the years of maximum solar activity.
The 1989 storm was the result of a large and complex sunspot group identified as region 5395. This region produced 11 X-class and 48 M-class X-ray flares between March 6-19. As I mentioned above, the magnetopause is typically located at about 10 times the earth's radius. On March 13, 1989, the solar wind pressure pushed the magnetopause to an estimated distance of 4.7 times the earth's radius, more than a factor of two compression in linear size. To put this in prospective , it was estimated the energy required to do so was equal to one-sixth the average daily U.S. electrical consumption in 1987. The Boulder Ap index rose to 279 on March 13.
During the geomagnetic storm of March 1989 I recall being onboard an oceanographic research vessel in the North Atlantic. It was a daily occurrence to see the radio operator slowly shaking his head and saying that in his 35 years experience he'd never seen anything like it. We were virtually without HF communications for days at a time.
There were a number of other consequences of this storm. Numerous LORAN navigation problems occurred, especially on March 6 and 13. This was exacerbated because land stations could not use HF radio to report the problems to users. The MARS HF service from was out world-wide while 144-148 MHz transceivers, normally used for short-range communications, were receiving powerful signals from remote locations. California Highway Patrol messages were overpowering local transmissions in Minnesota.
The Hydro-Quebec Power Company experienced a massive failure that left most of Quebec without power for up to nine hours. The magnetic storm induced a huge current in the power lines of the James Bay generating station. As a result, overcurrent protection tripped circuit breakers province-wide. In central Sweden there was a simultaneous power loss on six different 130 KV power distribution systems at about the same time as the Hydro-Quebec failure.
Brilliant aurora was seen across the U.S. on the nights of March 12, 13 and 14. Backpackers in
the remote mountains of North Carolina reported seeing static red aurora for about four to six
hours. Red auroras were reported from the Florida Keys, Grand Cayman Island and Cancun, Mexico.
I have described how fluctuations in the solar wind generate geomagnetic storms. I've also described some of the effects of the March 1989 magnetic storm as an example of how severe the terrestrial impact can be. As hams, we will see their effect on the amateur radio service. Most magnetic storms we encounter will not have severe consequences like those described above. Nonetheless, there will be times when we will be trying to diagnose transceiver problems or looking for faulty coax because we are hearing little or nothing. Before dismantling the rig, check the WWV numbers!
Allen, J.H., H. Sauer, L. Frank and P. Reiff, Effects of the March 1989 Solar Activity, EOS,70, 1478, 1989
Tsurutani, B.T., Gonzalez, W. D., Tang, F., and Yen, T.L., Great Magnetic Storms, Geophysical Research Letters, Vol. 19, No.1, pages 73-76, Jan. 3, 1992
Dunphy, P.M., Solar Terrestrial Indicies and HF Radio Propagation, QST Canada, Jan. 1992, page 3-4