Communications and Space Weather
Introduction
Most people are familiar with the effect of weather on their lives. Often this is relatively minor - determining what to wear and where to go; but on occasions, it is dramatic and costly as major events inflict severe damage and even loss of life.
Unseen and unknown to most people, there is another form of weather - space weather - which is of great importance to many modern technologies and the result of activity on the Sun. And like ordinary weather, space weather produces frequent small effects on human technology; and occasionally a dramatic event.
This article reviews some of the more interesting and important effects of space weather on communications. However, before we do this, we will need some background information about space weather and about its ultimate source - the Sun.
Background Information
The Sun-Earth Environment and Space Weather
The Sun-Earth environment is the region of space extending from the surface of the Sun out to, and including, the Earth's ionosphere and magnetic field. It is a harsh environment dominated by electromagnetic radiation and electrically charged particles from the Sun. It is subject to dramatic and violent change as events on the Sun, such as solar flares, blast streams of radiation and energetic particles towards the Earth.
Despite being far removed from everyday experience, the Sun-Earth environment has a surprisingly wide range of effects on many aspects of everyday life. Changes to conditions in the Sun-Earth environment are often called "space weather" and this can cause significant damage to technological systems, particularly to communications.
Space weather results from changes in the speed or density of the solar wind, the continuous flow of charged particles from the Sun past the Earth and into interplanetary space. This flow distorts the Earth's magnetic field, compressing it in the direction of the Sun and stretching it out in the anti-Sun direction. Fluctuations in the flow of solar wind cause variations in the strength and direction of the magnetic field measured near the surface of the Earth. Abrupt changes in this dynamic medium are called geomagnetic disturbances.
At the same time the Earth's ionosphere (the electrified layers of the upper atmosphere) can be severely disturbed by flows of charged particles in the region. This is important because the ionosphere acts as a "mirror", reflecting High Frequency (HF) signals and allowing cheap and convenient communication over long distances. HF is significant for many people including Defence, emergency services, broadcasters, and marine and aviation operators. Communications on other frequencies, from VLF to satellite, are also affected, making space weather and its prediction valuable to operations.
Many other phenomena are associated with space weather. Some of the more notable include: heating of the outer layers of the Earth's atmosphere altering the orbits of satellites and contributing to their early return to Earth; surge currents induced in power lines sometimes leading to the failure of power grids; currents in long pipelines leading to increased corrosion; and sightings of aurorae at more equatorial latitudes such as in mainland Australia.
The Solar Cycle
Space weather originates from the Sun and depends on the solar cycle. This cycle is typically 11 years in duration although cycles vary greatly both in amplitude and in length. The solar cycle is manifest in many properties of the Sun but is most evident in the appearance of sunspots on the solar disc. Sunspots are regions of stronger magnetic field which appear darker than the surrounding surface. At times, sunspots are rare and the Sun appears almost without blemish. This is known as solar minimum. Later sunspots become more common and it is normal for many groups of spots to be visible. The peak, when sunspots are most common, is called solar maximum.
The number of sunspots gives rise to the "sunspot number" which, when smoothed over a period of 12 months, is the traditional measure of the solar cycle. The peak sunspot number of historical solar cycles varies greatly. The largest cycles in recent years have been: Cycle 19 (peak sunspot number of 201 in 1957) was the largest cycle on record; Cycle 21 (peak sunspot number of 165 in 1979) the second largest; and Cycle 22 (peak of 159 in 1989) equal third largest.
Solar Features
Several solar features and events are connected with space weather. Firstly, solar flares are huge outbursts of energy seen on Earth at many wavelengths from visible light right through to the radio spectrum, and from space in X-ray observations. They are the outcome of the release of stored energy as the magnetic fields of sunspots become twisted and distorted due to the differential rotation of the Sun. If the complexity of the magnetic field is sufficiently large, the energy can be released in an explosive event - a solar flare. Along with the production of electromagnetic radiation, the flare can be associated with the ejection of clouds of charged particles into the solar wind. This process is called a coronal mass ejection and may occur with flares or with other types of events. The result of the material reaching the Earth is a geomagnetic/ionospheric storm.
Coronal holes, another type of solar feature connected with space weather, are extremely large regions in the solar corona - the outer atmosphere of the Sun. They are regions of reduced temperature and density and are the locations of magnetic field lines which are open into interplanetary space. Coronal holes contribute high speed streams to the solar wind which, if they reach the Earth, also produce space weather disturbances.
Space Weather and HF Communications
HF Communication Frequencies and the Solar Cycle
The ionosphere extends from a height of about 50 km up to over 500 km above the surface of the Earth. It is formed from the ionisation of atoms of air (i.e. electrons from removed from the atom) by incoming solar radiation. The chemistry of the atmosphere then determines the structure of the ionosphere which is generally divided into layers labelled D, E, F1 and F2 (in order of increasing height).
The F layer is of most importance for HF communications as it is present during day and night, it is located at the greatest height, and it reflects the highest frequencies in the HF band.
It is the Extreme Ultraviolet (EUV) radiation from the Sun which is responsible for forming and maintaining the ionosphere. This arises from the bright and hot regions which overlie sunspots. The number and size of sunspots varies with the solar cycle and so the properties of the ionosphere in turn exhibit a variation with the cycle.
At the low point of the solar cycle, EUV radiation from the Sun is weak and the density of charged particles in the F layer of the ionosphere is least. This means that only the lower frequency HF signals can be reflected. At the peak of the cycle, the EUV and the ionospheric density are both large and higher frequencies in the HF band can be reflected.
While the solar cycle is very important in determining HF frequencies, there are many other important factors. These include: the season; the time of day; the latitude; and the geometry of the circuit. Prediction of the best frequencies for HF circuits is performed by computer programs such as the ASWFC ASAPS program.
Solar Flares and HF Communication Fadeouts
Solar flares produce copious amounts of electromagnetic radiation, the X-ray component of which increases the ionisation of the ionospheric D layer. HF communication generally depends on the reflection of signals from the higher F layer and such signals must travel through the D layer at least twice. Increased ionisation, combined with the higher density of neutral particles, results in the absorption of the signal in the D layer during a major solar flare.
This effect is known as a Sudden Ionospheric Disturbance (SID). The SID is observed as an increased attenuation of HF signals particularly at the lower frequencies. This is often referred to as a SWF (short-wave fadeout), SSWF (sudden short-wave fadeout) or a GSWF (gradual short-wave fadeout). The fadeout follows closely the pattern of the solar flare, being observed at the same time as the flare. Fadeouts mostly have a rapid onset of a few minutes and a slower recovery lasting perhaps an hour (highly variable).
A property of SIDs is that they affect the lower frequencies in the HF band more than higher frequencies which may not be affected at all. The high frequencies are the last to be affected (if at all) and the first to recover.
An important feature of SIDs is that the ionospheric circuit is disturbed only when there is an ionospheric reflection point for the signal located in the sunlit hemisphere. No effect is observed if all the reflection points are located in the night hemisphere which is shadowed from the X-rays produced by the solar flare.
The intensity of flares at X-ray wavelengths is a good indicator of the chance of a significant fadeout. Hence, we can predict fadeouts if we can predict the occurrence of X-ray flares. At present, exact predictions of the timing and strength of flares are not possible. However, by observing the structure of sunspot regions we can predict intervals of time (perhaps lasting several days) during which flares and fadeouts are likely.
Being closely associated with solar flares, SIDs exhibit the same solar cycle distribution as do flares. SIDs are much more frequent near the peak of the cycle whilst they are relatively infrequent near solar minimum.
Ionospheric Disturbances and HF Communications
Electric currents caused by the arrival of charged particles alter the properties of the ionosphere, particularly the F-layer critical frequency (foF2) which determines the maximum usable frequency (MUF) that can be used on HF circuits. The response of the F-layer critical frequency is complicated as it depends on the time of the day, the season, the latitude, and the nature of the disturbance itself. In many cases, the critical frequency is enhanced early and then depressed later in the storm. These variations in ionospheric properties, particularly the depressions of critical frequencies, need to be anticipated by HF communicators.
It is often assumed that ionospheric disturbances occur whenever magnetic disturbances do. This is substantially correct but the relationship between them is complex and there is certainly no one-to-one relationship between a magnetic disturbance index, local or world-wide, and the level of ionospheric disturbance as measured by critical frequency depression, communications disruption or any other parameter. Quite severe magnetic disturbances can occur with little apparent ionospheric effect and vice-versa.
Periods of severe disturbance will affect more than the MUFs. Irregularities in the ionosphere result in signals travelling by more than one path and this can produce interference and consequent difficulties in communications.
Ionospheric disturbances have a similar variation during the solar cycle as do geomagnetic disturbances. In general, disturbances are more frequent at the high parts of the solar cycle. In some cycles, a second and sometimes larger disturbance peak occurs during the declining phase of the cycle.
Space Weather and VHF Propagation
Normally, signals in the VHF range (30 to 300 MHz) penetrate the ionosphere rather than being reflected. Hence, these frequencies are mostly used for line-of-sight communications. However, there are some circumstances under which VHF can be reflected back to Earth making long distance communications possible.
At the peak of especially strong solar cycles, VHF signals can in fact be reflected by the ionosphere. Examples of such cycles included the peak of Cycle 19 in 1957-58, of Cycle 21 in 1980, and Cycle 22 in 1990. At times during these peaks, the monthly sunspot number rises to extremely large values and the ionosphere reflects higher frequencies than is normally the case.
At peaks of a large solar cycle, VHF transmission in the lower part of the band is most likely for low latitude circuits around the local noon during the equinox periods of March and September.
VHF can also be reflected from clouds of increased ionisation in the E layer of the ionosphere. These phenomenon are known as sporadic E and the clouds are generally quite localised (around 100 km in size). Sporadic E occurs at a lower height than the F layer and this tends to limit the distance over which propagation is possible in a single hop. In some cases, multi hop transmission is possible to achieve longer distance transmission.
For low latitudes sporadic E appears throughout the year with the peak occurrence in the afternoon and evening. There is evidence of this peak coming later in the June/July period.
For mid latitudes sporadic E is most likely in summer over the local noon period and in the afternoon. This combination gives the largest chance of sporadic E for any latitude, season and time of day. Sporadic E is then considerably weaker away from the summer and the peak moves to the late afternoon. For the more southerly regions in this band, sporadic E is almost absent during the equinox periods of March and particularly September.
The reflection of VHF signals can also occur during aurorae - spectacular curtains of lights arising from charged particles originating from the Sun. The aurora is associated with increased ionisation in the E layer and it is from this that the signals can be reflected. Aurorae and large geomagnetic/ionospheric disturbances are associated and so VHF transmission by this means occurs at times when HF may be experiencing problems. Aurorae are also most commonly seen at polar latitudes at which location HF transmission is most likely to experience problems.
The ionised trails left by meteors as they burn up in the Earth's atmosphere is also a means by which VHF can be reflected. Meteors occur in "showers" - periods of a few days when meteors are most common arriving from a direction in space. Such showers are the remains of decayed comets and their dates are the same from year-to-year giving repeatable opportunities for VHF transmission.
Low band microwave communications (e.g. L band) and navigation systems (e.g. GPS) may be subject to degradation due to ionospheric scintillations, particularly in the tropical areas.
Space Weather and VLF, LF, and MF Propagation
Propagation of these frequencies is controlled by the lower regions of the ionosphere - mostly the D and the E layers although sometimes the F layer at night). Propagation is therefore subject to variations of the ionosphere some determined by the Sun and space weather.
Solar flares and their effects on the D region is one such influence. The intense flux of X-rays during a major flare increases the ionisation of the D region changing its ability to reflect in these bands. For the lowest frequencies - VLF - propagation of the signal occurs as the conducting Earth and the ionosphere act as a waveguide. Flares usually have a very rapid onset - measured in minutes - and this results in sharp changes in the amplitude and phase of signals as the waveguide height changes.
Solar Interference To Satellites
The geosynchronous orbit is now used by many satellites for routine telecommunication and broadcast purposes. Such satellites appear to be stationary as viewed from the ground, and are able to provide coverage over large areas. Although such communication is less subject to the vagaries of the ionosphere, it can be subject to interference from the Sun. Around the time of equinoxes each year the Sun, a wide band radio transmitter, passes behind a given geostationary satellite at some time of the day.
The level of interference that will be experienced depends upon a number of factors including the frequency of operation, the antenna beamwidth, the receiver bandwidth, the acceptable signal to noise ratio, and the level of solar activity at the time.
The exact time of year of solar interference varies around the equinoxes according to the latitude of the observing station. Interference of some intensity may be experienced up to about a week on either side of the date of maximum effect. At maximum, the interference may last up to 30 minutes, again depending upon the receiving antenna beamwidth. The time of day at which the interference will occur depends upon the relative position of the satellite. A satellite in the western sky will be subject to interference in the afternoon; but in the morning if in the eastern sky.
Satellite communications can also be affected in a dramatic manner by spacecraft charging. This results from the accumulation of electrical charge on a spacecraft as a result of the flow of solar wind past the craft. This can increase during a disturbance, sometimes resulting in disruption to the sensitive electronics on the craft. In the extreme, "phantom" commands resulting from charging effects can even result in the loss of the satellite!
ASWFC Services
ASWFC has provided services to communications (and other systems affected by space weather) for around 60 years. In doing so, ASWFC has developed a comprehensive range of products which are delivered by a various methods including the Internet.
ASWFC services depend on access to a wide range of Australian data. These include information from ionosondes, from solar observatories, and from magnetometers. But the Sun-Earth environment must be monitored continuously and so ASWFC is the Australian link to the International Space Environment Service, an organisation which co-ordinates the exchange of data. This link gives ASWFC access to significant information not available within Australia as well as an important role in scientific research which is vital in improving forecasting of the environment.
ASWFC also provides consultancy services which allow people to understand the effects of the changing ionosphere on their communications systems. ASWFC also advises its clients on all aspects of the Sun-Earth environment and its effects on a wide range of technology.
Space weather disturbances may arise at short notice after events such as solar flares, coronal mass ejections and short-lived coronal holes. The sporadic nature of these events sets a limit to the timescale over which they can be predicted. In the case of a flare, the delay time between the flare and the onset of a geomagnetic storm is typically 1-3 days. It is therefore necessary to monitor the Sun continuously and to assess the likely effects of any event. In Sydney, the Bureau of Meteorology's Space Weather Services operates the "Australian Space Forecast Centre" which receives observations from two solar observatories in Australia (located near Narrabri in NSW and near Exmouth in Western Australia) and from other international sources. The role of the Centre is to provide warnings of major activity and confirmations of events resulting from solar activity.
Material prepared by Richard Thompson
