Fundamentals of Ionospheric Propagation.
The ionosphere is that region of the earth's atmosphere in which free ions and electrons exist in sufficient abundance to affect the properties of electromagnetic waves that are propagated within and through it. Ions are produced in the atmosphere partly by cosmic rays but mostly by solar radiation.
The latter include ultraviolet light, x rays, and particle radiation (during storm periods). These radiations are selectively absorbed by the several gaseous constituents of the atmosphere, ion-electron pairs being produced in the process. For practical purposes, the ionosphere can usually be assumed to extend from about 50 to roughly 2000 km above the earth's surface. The structure of the ionosphere is highly variable, and this variability is imparted onto signals propagated via the ionosphere. The ionosphere is divided into three vertical regions - - D, E, and F -- which increase in altitude and in electron density.
The D region has an altitude range from 50 to 90 km. The electron density in the region has large diurnal variations highly dependent upon solar zenith angle. The electron density is maximum near local noon, is higher in summer than in winter, and is lowest at night.
The E region spans the altitude range from about 90 to 130 km. The maximum density occurs near 100 km, although this height varies with local time. The diurnal and seasonal variations of electron density are similar to those of the D region. Collisions between electrons and neutral particles, while important in the E region, are not as numerous as in the D region. The E region acts principally as a reflector of hf waves, particularly during daylight hours.
Embedded within the E region is the so-called sporadic-E layer. This layer is an anomalous ionization layer that assumes different forms -- irregular and patchy, smooth and disklike -- and has little direct bearing to solar radiation. The causes of sporadic-E ionization are not fully known. The properties of the sporadic-E layer vary substantially with location and are markedly different at equatorial, temperate, and high latitudes. "Short-skip" openings, sometimes on an otherwise dead band, are often a result of one-hop sporadic-E ionization. When sporadic-E ionization is sufficiently widespread, multi-hop propagation is possible.
The highest ionospheric region is termed the F region. The lower part of the F region, from 130 to 200 km, is termed the F1 region, and the part above 200 km is termed the F2 region.
The F2 region is the highest ionospheric region, usually has the highest electron density, and is the region of greatest value in long- distance hf ionospheric propagation. The region exhibits large variability in both time and space in response to neutral winds and electrodynamic drifts in the presence of the earth's magnetic field. The maximum electron density generally occurs well after noon, sometimes in the evening hours. The height of the maximum ranges from 250 to 350 km at midlatitudes to 350 to 500 km at equatorial latitudes. At midlatitudes, the height of the maximum electron density is higher at night than in the daytime. At equatorial latitudes, the opposite behavior occurs.
The F1 region, like the E region, is under strong solar control. It reaches a maximum ionization level about one hour after local noon. At night and during the winter the F1 and F2 regions merge and are termed simply "F region".
Electromagnetic waves are refracted when passing through an ionized medium, the refraction increasing with increased electron density and decreasing with increase of frequency. If the refraction is large enough, a wave reaching the ionosphere is bent back toward earth as though it had been reflected, thereby permitting reception of the wave at a large distance from the transmitter. The F2 layer is the most important in this regard because of its height and its high electron density. The maximum earth distance traversed in one F2-layer "hop" is about 4000 km. Round-the-world communication can occur via multiple hops.
If the frequency is too high, the wave is not refracted sufficiently to return to earth. The maximum frequency for which a wave will propagate between two points is called the maximum usable frequency (MUF). Frequencies higher than the existing MUF at any given time are not supported, no matter how much power is used. However, because of the large variability that exists in the electron density of the F2 region, predicted MUFs are not absolute limits, but are statistical in nature. The actual MUF at any given time may be higher or lower than the predicted MUF. Predicted MUFs are intended to be median values; i.e., the actual MUF will exceed the predicted MUF 50 percent of the time, and will be less than the predicted FMUF 50 percent of the time. he predicted frequency that will be supported only 10 percent of the time is a frequency higher than the predicted MUF called the highest probable frequency (HPF), but even higher frequencies are possible 10 percent of the time. The predicted frequency that will be supported 90 percent of the time is a frequency lower than the predicted MUF called the optimum traffic frequency (FOT). Curves of predicted HPF, MUF, and FOT for several paths appear each month in QST.
Signals on their way to or from the F2 layer must pass through the E region of the ionosphere. The E layer is also capable of "reflecting" hf signals, and if the E-layer MUF is too high, the signals to or from the F2 layer are blocked -- or cut off -- by the E layer. Signals at frequencies below the ECOF will not pass through the E layer. Signals can propagate between two points on earth via the E layer in the same manner as they do via the F2 layer, but the maximum earth distance traversed in one E-layer hop is only about 2000 km, so a significantly greater number of hops is usually required on DX paths.
The D region of the ionosphere must be traversed by signals on their way to and from the F2 or E layers. Electron densities in the D region are not large enough to cause hf signals to be returned to earth, but the high collision frequency between the electrons and neutral particles in the D region gives rise to absorption of signals passing through it. The reduction of signal strength can be substantial, particularly in daytime on the lower hf frequencies. Antenna installations that provide low radiation (take-off) angles can minimize the number of hops required between two stations, thereby reducing the number of passes through the D region and the amount of signal absorption.
Electron density in the ionosphere increases with increased solar activity. Therefore, MUFs and signal absorption both increase as solar activity increases. The Zurich smoothed mean sunspot number has been used extensively as an index of solar activity and the one with which propagation data has been correlated over the years. Therefore, most propagation prediction models require that the user specify the sunspot number to be used in making a prediction. The 2800-MHz (10.7-cm) solar noise flux is generally considered a more accurate measure of solar activity, but with a smaller base of propagation observations. Since the two indices are highly correlated, either index may be used.
Ionospheric propagation is susceptible to several kinds of short- term disturbance that are usually associated with solar flares. Depending upon the nature of the disturbance, they are called sudden ionospheric disturbances, polar cap absorption events, or ionospheric storms. These disturbances upset the electron configuration in the ionosphere, and consequently affect propagation. Propagation is also affected by changes in the earth's magnetic field. The magnetic field is constantly fluctuating, but the fluctuation occurs over much wider limits during magnetic storms that accompany ionospheric storms. Ionospheric and magnetic storms are also often accompanied by visible aurora.
Except for the tendency of these disturbances to recur in synchronism with the 27-day rotation period of the sun, they are difficult for the amateur (as well as the professional) to predict and to quantify. The severity of magnetic disturbances is indicated by A and K indices that are broadcast by WWV at 18 minutes past each hour. The A index is a daily measure of geomagnetic field activity on a scale of 0 to 400. The K index is a measure, for a 3-hour period, of variation or disturbance in the geomagnetic field on a scale of 0 to 9. In general, MUFs decrease and signal absorption increases as geomagnetic field activity increases, although MUFs sometimes increase in equatorial regions.
--CONDITIONS FOR THE PAST 24 HOURS
--FORECAST FOR THE NEXT 24 HOURS
The sun's electromagnetic spectrum is a continuum of radiation spanning not only infrared, visible, and ultraviolet wavelengths, but the radio portions, x-rays and beyond. Sensors on the Earth and in space continuously observe specific portions of the sun's energy spectrum to monitor their levels and give scientists indications of when significant events occur.
Solar emissions in this category are all electromagnetic in nature, that is, they move at the speed of light. Events detected on the sun in these wavelengths begin to affect the Earth's environment around 8 minutes after they occur.
In addition to electromagnetic radiation, the sun constantly ejects matter in the form of atomic and subatomic particles. Consisting typically of electrons, protons, and helium nuclei, this tenuous gas is accelerated to speeds in excess of the sun's gravitational escape velocity and thus moves outward into the solar system. The collective term for the gas and the particles making them up is the Solar Wind. The sun's approximately 27-day rotation period results in the clouds being slung outward in an expanding spiral pattern which, at the earth-sun distance, overtakes the earth from behind as it moves along in it's orbit. As the clouds encounter the earth, the geomagnetic field and the earth's atmosphere prevents the solar wind particles from striking the planet directly. Magnetic interactions between the clouds and the geomagnetic field cause the solar wind particles to flow around the field, forming a shell-like hollow with the earth at the center. The hollow, known as the earth's Magnetosphere, is actually distorted into a comet shape with the head of the comet always pointing directly into the solar wind and the tail directly away. In the absence of significant solar activity, the solar wind is uniform with a velocity of approximately 400 km/second. Under these conditions, the earth's magnetosphere maintains a fairly steady shape and orientation in space. When disturbances occur on the sun, some clouds of solar particles can be blasted away at tremendous velocities. As these higher speed solar particle clouds encounter the earth's magnetosphere, they perturb it, changing the intensity and direction of the earth's magnetic field. This is analogous to a weather vane in gusty wind; sudden higher speed gusts can strike it and cause it to move around. Moreover, changes in solar wind density and velocity can cause the Earthís surface and are referred to as a "sudden impulse" (SI). Geomagnetic activity, including solar particle-caused variations in the geomagnetic field are carefully monitored by instruments both on the Earth and in space. High levels of geomagnetic activity act indirectly to degrade the ability of the ionosphere to propagate HF radio signals. So they are of interest to users of that portion of the radio frequency spectrum. Like the electromagnetic radiation portions of the sun's output, geomagnetic activity comprises another family of interactions observed and reported by groups such as IPS and SESC.
The Geophysical Alert Broadcasts consist of three primary sections to describe the Solar-terrestrial environment: The most current information, then a summary of activity for the past 24 hours, and finally a forecast for the next 24 hours. The actual wording of each section of the broadcast is explained below with a brief description of what is being reported. Similar wording is also used in other broadcasts, so the WWV example is relevant to other reports too.
"Solar-terrestrial indices for (UTC Date) follow: Solar flux (number) and (estimated) Boulder A index (number) . Repeat, solar flux (number) and (estimated) Boulder A index (number) . The Boulder K index at (UTC time) on (UTC Date) was (number) repeat (number) ."
Since the final A index is not available until 0000 UTC, the word "estimated" is used for the 1800 and 2100 UTC announcements.
Solar Flux is a measurement of the intensity of solar radio emissions at a frequency of 2800 MHz made using a radio telescope located in Ottawa, Canada. Known also as the 10.7 cm flux (the wavelength of the radio signals at 2800 MHz), this solar radio emission has been shown to be proportional to sunspot activity. In addition, the level of the sun's ultraviolet and X-ray emissions is primarily responsible for causing ionization in the earth's upper atmosphere. It is these emissions which produce the ionized "layers" involved in propagating shortwave radio signals over long distances.
The solar flux number reported in the broadcast is in solar flux units (s. f. u.) and is recorded daily at Ottawa at 1700 UTC to be forwarded to the SESC. Solar flux readings range from a theoretical minimum of approximately 67 to actually-observed numbers greater than 300. Low solar flux numbers dominate during the lower portions of the 11-year sunspot cycle, rising as the cycle proceeds with the average solar flux a fairly reliable indicator of the cycle's long-term behavior. 1 s. f. u. = 10-22Watts/meter2 Hz = 104 jansky.
The A index is an averaged quantitative measure of geomagnetic activity derived from a series of physical measurements. Magnetometers measure differences between the current orientation of the magnetosphere and compare it to what it would be under "quiet" geomagnetic conditions. But there is more to understanding the meaning of the Boulder A index reported in the Geophysical Alert Broadcasts. The Boulder A index in the announcement is the 24 hour A index derived from the eight 3-hour K indices recorded at Boulder. The first estimate of the Boulder A index is at 1800 UTC. This estimate is made using the six observed Boulder K indices available at that time (0000 to 1800 UTC) and the SESC forecaster's best prediction for the remaining two K indices. To make those predictions, SESC forecasters examine present trends and other geomagnetic indicators. At 2100 UTC, the next observed Boulder K index is measured and the estimated A index is reevaluated and updated if necessary. At 0000 UTC, the eighth and last Boulder K index is measured and the actual Boulder A index is produced. For the 0000 UTC announcement and all subsequent announcements the word "estimated" is dropped and the actual Boulder A index is used. The underlying concept of the A index is to provide a longer-term picture of geomagnetic activity using measurements averaged either over some time frame or from a range of stations over the globe (or both). Numbers presented as A indices are the result of a several-step process: first, a magnetometer reading is taken to produce a K index for that station (see K INDEX below); the K index is adjusted for the station's geographical location to produce an a index (no typographical error here, it is a small case "a") for that 3-hour period; and finally a collection of a indices is averaged to produce an overall A index for the timeframe or region of interest.
A and a indices range in value from 0 to 400 and are derived from K-indices based on the table of equivalents shown in the APPENDIX.
The K index is the result of a 3-hourly magnetometer measurement comparing the current geomagnetic field orientation and intensity to what it would have been under geomagnetically "quiet" conditions. K index measurements are made at sites throughout the globe and each is carefully adjusted for the geomagnetic characteristics of its locality. The scale used is quasi logarithmic, increasing as the geomagnetic field becomes more disturbed. K indices range in value from 0 to 9. In the Geophysical Alert Broadcasts, the K index used is usually derived from magnetometer measurements made at the Table Mountain Observatory located just north of Boulder, Colorado. Every 3 hours new K indices are determined and the broadcasts are updated.
THE PAST 24 HOURS
"Solar-terrestrial conditions for the last 24 hours follow: Solar activity was (Very low, Low, Moderate, High, or Very high) , the geomagnetic field was (Quiet, Unsettled, Active, Minor storm, Major storm, Severe storm) ."
Solar activity is a measure of energy releases in the solar atmosphere, generally observed by X-ray detectors on earth-orbiting satellites. Somewhat different from longer-term Solar Flux measurements, Solar Activity data provide an overview of X-ray emissions that exceed prevailing levels. The five standard terms listed correspond to the following levels of enhanced X-ray emissions observed or predicted within a 24-hour period: Very Low - X-ray events less than C-class. Low - C-class x-ray events. Moderate - Isolated (1 to 4) M-class x-ray events. High - Several (5 or more) M-class x-ray events, or isolated (1 to 4) M5 or greater x-ray events. Very High - Several (5 or more) M5 or greater x-ray events.
The x-ray event classes listed correspond to a standardized method of classification based on the peak flux of the x-ray emissions as measured by detectors. Solar x-rays occupy a wide range of wavelengths with the portion used for flare classification from 0.1 through 0.8 nm. The classification scheme ranges in increasing x-ray peak flux from B-class events, through C- and M-class, to X-class events at the highest end (see APPENDIX).
In the Geophysical Alert Broadcasts, solar activity data provides an overview of x-ray emissions which might have effects on the quality of shortwave radio propagation. Large solar x-ray outbursts can produce sudden and extensive ionization in the lower regions of the earth's ionosphere which can rapidly increase shortwave signal absorption there. Occurring on the sun-facing side of the Earth, these sudden ionospheric disturbances are known as "shortwave fadeouts" and can degrade short wave communications for from minutes to hours. They are characterized by the initial disappearance of signals on lower frequencies with subsequent fading up the frequency spectrum over a short period (usually less than a hour). Daytime HF communication disruptions due to high solar activity are more common during the years surrounding the peak of the solar cycle. The sun rotates once approximately every 27 days, often carrying active regions on its surface to where they again face the Earth; periods of disruption can recur at about this interval as a result.
Rule of Thumb: The higher the solar activity, the better the conditions on the higher frequencies (i.e. 15, 17, 21, and 25 MHz). During a solar X-ray outburst, the lower frequencies are the first to suffer. Remember too that that signals crossing daylight paths will be the most affected. If you hear announcements on broadcast radio stations (e.g. Radio Netherlands) or via WWV/WWVH of such a solar disturbance try tuning to a HIGHER frequency. Higher frequencies are also the first to recover after a storm. Note that this is the opposite to disturbances indirectly caused by geomagnetic storms.
As an overall assessment of natural variations in the geomagnetic field, six standard terms are used in reporting geomagnetic activity. The terminology is based on the estimated A index for the 24-hour period directly preceding the time the broadcast was last updated:
Category - Range of A-index
Quiet - 0-7
Unsettled - 8-15
Active - 16-29
Minor storm - 30-49
Major storm - 50-99
Severe storm - 100-400
These standardized terms correspond to the range of a and A indices previously explained in the A INDEX section. Increasing geomagnetic activity corresponds to more and greater perturbations of the geomagnetic field as a result of variations in the solar wind and more energetic solar particle emissions. Using the earlier analogy, imagine the geomagnetic field to be like a weather vane in an increasingly violent windstorm. As the winds increase, the weather vane is continually buffeted by gusts and oscillates about the direction of the prevailing wind. Essentially, the reported geomagnetic activity category corresponds to how violently the geomagnetic field is being knocked about.
For shortwave radio spectrum users, high geomagnetic activity tends to degrade the quality of communications because geomagnetic field disturbances also diminish the capabilities of the ionosphere to propagate radio signals. In and near the auroral zone, absorption of radio energy in the ionosphereís D region (about 80 km high) can increase dramatically , especially in the lower portions of the HF band. Signals passing through these regions can become unusable. Geomagnetic disturbances in the middle latitudes can decrease the density of electrons in the ionosphere and thus the maximum radio frequency the region will propagate. Extended periods of geomagnetic activity known as geomagnetic storms can last for days. The impact on radio propagation during the storm depends on the level of solar flux and the severity of the geomagnetic field disturbance. During some geomagnetic storms, worldwide disruptions of the ionosphere are possible. Called ionospheric storms, short wave propagation via the ionosphereís F region (about 300 km high) can be affected. Here, middle latitude propagation can be diminished while propagation at low latitudes is improved. Ionospheric storms may or may not accompany geomagnetic activity, depending on the severity of the activity, its recent history, and the level of the solar flux.
Rule of thumb: Oversimplification is dangerous in the complex field of propagation. We know much less about the "radio weather" than ordinary weather. In general though, for long distance medium-wave listening, the A index should be under 14, and the solar activity low-moderate. If the A-index drops under 7 for a few days in a row (usually during sunspot minimum conditions) look out for really excellent intercontinental conditions (e.g. trans Atlantic reception).
During minor geomagnetic storms, signals from the equatorial regions of the world are least affected. On the 60 and 90 metre tropical bands you can expect interference from utility stations in Europe/North America/Australia to be lower. Sometimes, this means that weaker signals from the tropics can get through, albeit they may suffer fluttery fading. Signals on the higher frequencies fade out first during a geomagnetic storm. Signals that travel anywhere near the North or South Pole may disappear or suffer chronic fading.
FORECAST FOR THE
NEXT 24 HOURS
"The forecast for the next 24 hours follows: Solar activity will be (Very low, Low, Moderate, High, or Very high). The geomagnetic field will be (Quiet, Unsettled, Active, Minor storm, Major storm, Severe storm)."
The quantitative criteria for the solar activity forecast are identical to the "Conditions for the past 24 hours" portion of the broadcast as explained previously except that the forecaster is using all available measurement and trend information to make as informed a projection as possible.
Some of the key elements in making the forecast include the number and types of sunspots and other regions of interest on the sun's surface as well as what kinds of energetic events have occurred recently.
The same six standardized terms are used as in the "Conditions for the past 24 hours" portion of the broadcast with the forecast mainly based on current geomagnetic activity, recent events on the sun whose effects could influence geomagnetic conditions, and longer-term considerations such as the time of year and the state of the sunspot cycle.
a index. A 3-hourly "equivalent amplitude" of geomagnetic activity for a specific station or network of stations expressing the range of disturbance of the geomagnetic field. The a index is scaled from the 3-hourly K index according to the following table:
K 0 1 2 3 4
5 6 7 8 9
a 0 3 7 15 27 48 80 140 240 400
X-Ray flares from the Sun
Solar flares are rated according to the extent to which they emit x-rays in the 1 to 8 Angstrom band. Solar x-rays are classified into one of 5 different categories. These classes are categorized as follows:
Class A: X-ray emissions that are less than 10^-7 watts per
square meter (or Wm^-2).
Class B: X-ray emissions that range between 10^-7 and 10^-6 Wm^-2.
Class C: X-rays that range between 10^-6 and 10^-5 Wm^-2.
Class M: X-rays that range between 10^-5 and 10^-4 Wm^-2.
Class X: X-rays that reach or exceed 10^-4 Wm^-2.
You will notice that each of these classifications differs from each other neighboring class by a power of 10. In other words, x-rays behave according to a power-law. Class B flares are 10 times more powerful in x-rays than Class A flares. Similarly, class X-flares are 1,000 times more powerful than class B flares. Most solar flares never reach M-class levels. Those that do are considered minor energetic flares. Solar flares that reach or exceed 5.0 x 10^-5 watts per square meter (class M5.0 or larger) are considered major energetic M-class flares. These types of major flares are much less frequent than minor M-class flares, which can occur a handful of times each month during the more active years near the solar maximum. X-class solar flares are the rarest and often the most powerful of all types of solar flares. They can occur at any time during the solar cycle, but prefer periods near the solar maximum when sunspot regions are complex enough to generate the dynamics required to produce such powerful solar explosions.
Ranking of a flare based on its x-ray output. Flares are
classified according to the order of magnitude of the peak burst intensity (I) measured at
the earth in the 0.1 to 0.8 nm wavelength band as follows:
Class Peak, 0.1 to 0.8 nm band (Watts/square metre)
B I < 10-6
C 10-6 I < 10-5
M 10-5 I < 10-4
X I 10-4
A multiplier is used to indicate the level within each class.
M6 = 6 X 10-5 Watts/square metre
Flares are sometimes associated with what is known as a Type II spectral radio burst (or a sweep frequency event). These radio emissions are produced when a shock wave from the solar flare excites the lower coronal plasma as it propagates outward. Type II sweeps are often an indication of a coronal mass ejection and such events are fairly common with large X-class solar flares.
The x-rays from flares can be intense enough to have a considerable impact on ionospheric radio communications. Ionospherically propagated radio signals that travel through the sunlit hemisphere of the Earth can experience heavy absorption caused by the intense x-rays from the solar flare. In some cases, the absorption can be strong enough to completely blackout all radio communications between points more than 3,000 to 4,000 km up to frequencies as high as 10 MHz for a period of between 15 to 30 minutes. Minor absorption can maintain weaker than normal signal strengths for an additional 20 to 30 minutes.