While most mariners tend to pay plenty of attention to the state of the weather in the ocean of air above their heads, few probably give much thought to the weather in space. This is understandable since this isn’t weather in the standard senseno warm fronts or hurricanes. Space weather concerns the streams of charged particles and radiation flying off the sun. These various flows of particles, electromagnetic radiation, and magnetic fields (known as solar wind, solar flares, and coronal mass ejections) can affect radio communications and even bring satellites falling to Earth.
Space weather is of particular importance this year and next as the sun nears the most active stage of its 11-year sunspot cycle. Sunspots are cooler (relatively speaking) areas of the sun’s disk that appear darker than the surrounding photosphere (the bright surface of the sun that we see). The spots are formed when lines of force from the sun’s magnetic field become twisted and interact with the sun’s surface. An increase in the numbers of spots coincides with a more active sun. When the sun is in an active stage it produces increased numbers of solar flares and coronal mass ejections. Solar flares are huge explosions of particles (including highly energetic protons) and electromagnetic radiation that flow out from the sun into interplanetary space. Coronal Mass Ejections (CME) are vast sheets of hot gases entwined with magnetic field lines that are thrown out from the sun and travel across the solar system like a wave.
Not all solar flares or CMEs are directed toward the Earth; often the energy from these events surges off into other sectors of the solar system. There are cases, however, when the Earth is directly in the field of fire. Luckily, the Earth has a protective shield against such attacks: its geomagnetic field. Like a huge bar magnet, the Earth produces a magnetic field that not only directs the compass card on your boat but also acts as a barrier to most of the radiation streaming in from space.
When the charged particles from solar events like flares and CMEs impact the magnetic field they cause something called a geomagnetic storm. The interaction between the particles and the Earth’s magnetic field causes rapid variations in the field’s direction and intensity. When hit with high-powered solar energy, the outer edge of the magnetic field, which normally extends 40,000 miles out into space, can be compressed down to 26,000 miles, close to the altitude of geosynchronous satellites. While most of the energy from solar events is absorbed by the magnetic field, some of the energy does leak through the magnetic net to directly energize the electrically charged upper atmosphere called the ionosphere. How does all this heightened solar activity affect mariners? The most obvious fallout from this solar blast is the added activity pumped into the ionosphere that directly affects high frequency (HF) radio propagation. A more energetic ionosphere is denser and lower in altitude. This means more radio frequency interference at certain frequencies plus unusual and unreliable propagation paths. “The ionosphere gets less predictable,” said Gordon West, HF radio expert and marine writer. “We see that 4 and 8 MHz has a harder time getting through in the daytime. They are absorbed by the ionosphere. And, during peaks of solar activity, the lower frequencies are noisier.” On the other hand, it’s not all bad. While 4 and 8 MHz may be less effective during the day, they are more effective at night. For long-range connections during the day, the higher HF frequencies (from 12 up to 22 MHz) are more effective. “The higher bands are more usable,” said Don Melcher, long-time ham radio operator and owner of HF Radio on Board in Alameda, Calif. “You can talk over greater distances with greater signal strength.” And all frequencies in the HF band are more effective for nighttime communications. The added energy from the sun pumps up the ionosphere to a higher level during the day. This means there is more residual charge and the ionosphere stays strong all night.
HF radio users who’d like an idea of when solar activity will change propagation conditions can listen to WWV (at 2.5, 5, 10, 15 MHz) from Ft. Collins, Co. This station broadcasts time ticks for setting clocks to Greenwich Mean Time, but it also broadcasts, at 18 minutes after the hour, forecasts of imminent solar activity.
Systems that use higher frequencies, like VHF marine radio and satellite communications systems, aren’t greatly affected by increased ionospheric activity. These frequencies are not reflected by the ionosphere, so there is little change in their performance.
One system that can be affected by a more energetic ionosphere is GPS. Ultrahigh frequency (UHF) signals from GPS satellites are slowed and bent somewhat when they travel through the ionosphere. This effect is well known, and GPS receivers can take this into account using a general ionospheric propagation model. However, when the ionosphere is more active near the time of solar maximum, UHF signals are more highly affected. “The ionosphere becomes more turbulent and irregular,” said Joe Kunches, lead forecaster at NOAA’s space environmental center in Boulder, Colo. “During solar maximum the more active ionosphere acts like a big resistor, slowing down GPS signals. But there are many levels of different densities, so the signals speed up and then slow down and speed up.” This effect is called scintillation, and it can give GPS receivers problems both with locking onto a signal and with fix accuracy, although this is a small error of perhaps nine to 10 meters that is usually masked by selective availability. This variability in the ionosphere can also have an effect on differential GPS (DGPS) corrections. When you are close to a DGPS beacon, the corrections will still be accurate. Farther away from a DGPS beacon, however, a GPS unit will be receiving signals through a different mix of ionosphere levels, and so the corrections may not be quite so accurate.
In addition to making the ionosphere more electrically active, the added solar energy absorbed by the ionosphere causes it to expand and extend farther into space. When this happens, low Earth orbit (LEO) and polar Earth orbit (POE) satellites in orbits up to about 500 miles can experience greater atmospheric drag. This can rob them of orbital speed, which leads to orbital decay. As they fall closer to Earth they experience more drag and slow still further, causing them to fall lower, etc. This cycle can continue until the satellite burns up in the atmosphere. For example, the U.S. space station SkyLab, which fell to Earth in July 1979, was the victim of this type of increased drag.
One type of POEs satellites used by mariners either directly or indirectly are NOAA’s television and infrared observational satellites (TIROS). These weather satellites are in 450- to 470-nm-high orbits. Luckily, this is high enough to prevent major atmospheric drag problems. “We’re actually a little higher than some polar orbiters,” said Tim Walsh, chief of the engineering branch at NOAA’s satellite operations control center. “We have seen problems with attitude disturbances [changes in satellite orientation] in the past. But generally we haven’t had any loss of altitude.”
Geostationary satellites, in orbits 23,000 miles up, are in little danger of added atmospheric drag. However, GEO spacecraft can be affected in other ways. One of these involves critical electronic components like microchips being struck by high-energy particles. These can cause physical damage to chips, destroying microcircuits and obliterating data. These “bit hits” (technically called “single-event upsets” in satellite engineering jargon) can disrupt software instructions in the satellite’s memory and affect performance. GEO satellites can also be subjected to differential electrostatic surface charging. This occurs when solar energy causes an electrical charge to build up on the sun side of a satellite. This build-up can arc discharge to the other parts of satellite, damaging sensitive circuits. “On GOES 8 we found that one of the static discharge paths was through the computer,” said Walsh. That meant that electrical charges could flow through the computer circuits and cause damage. “We fixed that problem with GOES 9.”
When geomagnetic storms are particularly strong, they can turn the Earth into a gigantic generator and induce electrical current flows along conductors found on the Earth’s surface. For example, electrical currents can be induced in power lines and pipelines like the Alaska Pipeline. In fact, on March 13, 1989, currents induced by a large geomagnetic storm caused circuit breakers to trip and cut power to millions of people in Montreal, Quebec.
Given the effect increased solar activity has had in the past, we can expect some disruptions as we reach the high point in solar activityin the year 2000. Solar maximum should occur just as the Y2K bug gets into full swing. Sounds like a good time to be off voyaging!
