Listener's Guide

Terms Used In Shortwave Radio Propagation Forecasts

Terms Used In Shortwave Radio Propagation Forecasts
A IndexLinear index for measuring the disturbance level in the earth's magnetic field. The index is defined over a period of one day. An A Index can be defined for any location on earth and also for the entire globe. Levels of A index are often described as follows: A index less than 8 - quiet; A index from 8 up to and including 15 - unsettled; A index from 16 up to and including 24 - active; A index from 25 up to and including 35 - minor storm; A index from 36 upwards - major storm.
Ap IndexThe planetary index for measuring the strength of a disturbance in the earth's magnetic field. Defined over a period of one from a set of standard stations around the world.
AbsorptionThe loss of energy from a radio wave. Mostly occurs in the D region.
Absorption Limited FrequencyThis is the lowest frequency for reliable radio communications by the ionosphere. The ALF is significant only on daylight sectors of circuits.
ActiveWhen referring to the sun, the term means changing. Solar activity is the changing appearance of the sun.
Active RegionA region on the sun which is active. Usually an active region incorporates sunspots, plage and filaments. Active regions contain strong magnetic fields. Flares occur within active regions.
AttachmentThe collision of an electron with a neutral molecule or an atom which causes the formation of a negative ion. Later the negative charges disappear due to recombination between positive and negative ions. Attachment depends on the density of the oxygen atoms, the greater this density, the faster the ionisation will disappear.
AuroraExcitation of particles from the sun spiralling in the geomagnetic field near the poles resulting in the release of energy in different forms, including light.
Auroral OvalBand around each geomagnetic pole where aurora are most likely to occur.
ChromosphereColour sphere or layer of the sun's atmosphere between the photosphere and the corona. Appears as a red ring around the solar limb during a solar eclipse. Plage regions are visible in the chromosphere, usually overlying sunspot groups.
Central MeridianThe north-south meridian of the sun that passes through the centre of the disc as viewed from the earth.
CoreThe region of very high density and temperature located at the centre of the sun.
CoronaThe outer atmosphere of the sun with low density and high temperature. Visible as an extended bright region about the sun during solar eclipse.
Coronal HoleA low density region of the corona with relatively low temperature. Coronal holes are the sources of high speed solar wind streams.
Coronal Mass EjectionAn ejection of material from the sun into interplanetary space. If the material is directed towards the earth then the event is associated with a disturbance to the earth's magnetic field or ionosphere.
Critical FrequencyThe greatest frequency that can be reflected vertically from an ionospheric layer.
D regionThe lowest region of the ionosphere where most HF absorption occurs. Present during daylight hours only.
Daylight FadeoutWhen flares occur, increased absorption of a radio wave in the D region may make part of the HF spectrum is unusable. At times the whole HF spectrum is affected, but normally the lower frequencies are affected most. Only those circuits with daylight sectors can be affected.
DecileIf a set of numerical values is ordered from lowest to highest and divided into ten equal parts, then the values separating the ten parts are called deciles. Hence there are nine deciles which separate the ten sections. The upper and lower deciles are the 90% and 10% levels, respectively.
Differential Solar RotationRefers to the fact that the rotation of the sun varies with latitude on the sun, being generally faster closer to the equator.
Deviative AbsorptionAbsorption of a radio wave near the point of reflection.
DiskWhen referring to the sun, it is the visible hemisphere.
DiurnalThroughout the day, daily.
E LayerA solar controlled ionospheric region around 80-150 km. Present during daylight hours.
E Layer ScreeningA radio signal directed to the F layer is deflected and attenuated by the E layer. E layer screening occurs when the E layer MUF is greater than the operating frequency. The signal cannot reach the F layer and propagation is via multiple E layer hops. These modes are very heavily attenuated, especially when more than two hops occur, and effective communication is not possible.
Electromagnetic RadiationRadiation which has both electric and magnetic properties. Examples: microwaves, light, infra-red, ultraviolet, x rays, gamma, radio and television. Travels at 300 million m/s.
ElectronA light atomic particle with a fixed negative electric charge.
Electron DensityThe number of electrons in a unit volume, eg. 1 cubic centimetre.
Elevation AngleAngle between the horizontal and the direction of concern.
Equatorial AnomalyA depression in F layer frequencies at the geomagnetic equator relative to frequencies at low latitudes. A daytime phenomenon.
Equatorial ElectrojectThe equatorial electrojet is a thin electric current in the ionosphere over the dip equator around 100 to 115 km altitude normally flowing eastward. The electrojet may reverse its direction during geomagnetically disturbed conditions and magnetically quiet times. The reversals during quiet times have been related to lunar tides. There is a strong correlation between the electrojet and equatorial sporadic E.
EquinoxTime when the sun crosses the geographic equator. Day and night are of equal lengths. Occurs in March and September.
Extraordinary WaveA radio wave may split into two oppositely polarised components due to, in the case of ionospheric physics, the geomagnetic field. The ordinary wave obeys the laws of refraction and is unaffected by the magnetic field, the extraordinary wave however, is.
Extreme Ultraviolet RadiationElectromagnetic radiation at the high frequency end of the ultraviolet spectrum.
F RegionLocated above about 160 km. During the day, it often divides into two regions. The lower region is the F1 region and the upper region is called the F2 region. At night there is only an F region.
FilamentA cool, relatively dense region in the corona that appears dark against the background chromosphere. On the limb, filaments appear as prominences.
FlareAn explosion on the sun usually releasing large amounts of energy and particles, and usually occurring within an active region. Flares are more likely at solar maximum.
fminThe lowest frequency which is reflected from the ionosphere when an ionogram sounding is taken. Provides an indication of the amount of absorption occurring in the ionosphere.
foEThe critical frequency of the E layer. The maximum frequency which can be reflected from this layer.
foF2The critical frequency of the F2 layer. It is the maximum frequency which can be supported by the F2 layer when a wave is vertically incident upon the layer.
GammaUnit of charge of the earth's magnetic field. 1 gamma = 1 nanotesla = 10 gauss.
Geomagnetic ActivityNatural variations in the geomagnetic field classified into quiet, unsettled, active and storm conditions. Descriptive levels of activity such as these are determined by the A Index defined as follows: described as follows: A index less than 8 - quiet; A index from 8 up to and including 15 - unsettled; A index from 16 up to and including 24 - active; A index from 25 up to and including 35 - minor storm; A index from 36 upwards - major storm.
Geomagnetic Dip EquatorWhere the geomagnetic field is horizontal to the earth. Where the inclination is zero.
Ground WaveThe radio wave which propagates close to the earth's surface. Severe signal losses due to ground resistance limit the range of ground waves to about 100 km over land and 300 km over sea for the lowest HF frequencies. The ground waves for the higher HF frequencies cover much shorter distances.
h'F2The virtual height of the F2 layer. At night when the F2 and F1 layers merge to form the F layer, h'F is measured. Similar heights are obtained for the E and F1 layers.
H-alphaThe strongest spectral line in the solar spectrum. The wavelength is 656.3 nm. Energy of this wavelength is formed in the chromosphere and is responsive to solar flares. It is the spectral line used for world wide surveillance of solar flares.
Height ProfileThe variation of a parameter, eg. electron density with height.
Hop LengthThe distance a radio wave travels with one reflection from the ionosphere. It will depend on the antenna elevation angle and the height of the reflecting layer.
InterpolationTo estimate on the basis of some rule. Hence linear interpolation estimates the position of points from values of the respective function on either side of the point.
IonAn atom or molecule with one or more electrons removed (positive ion) or added (negative ion).
IonisationThe process of removal or attachment of an electron to an atom or molecule to form a positive or negative ion respectively.
IonogramRecord of the time delay (virtual height) of HF echoes from the ionosphere at varying frequencies. Recorded by an ionosonde.
IonosondeSwept frequency HF pulsed radar used to monitor the ionosphere. Pulses are transmitted vertically upwards and the ionosonde records the time delay of the returning echoes. Ionosondes normally sweep in frequency from about 1 to 20 MHz.
IonosphereThat part of the atmosphere that is ionised by the sun's radiation. Extends upwards from about 60 km. The free electrons in the ionosphere support radio wave propagation.
K IndexA three hourly index of geomagnetic activity relative to an assumed quiet day curve for the recording site. K index values range from 0 (very quiet) up to 9 (extremely disturbed).
Least Squares ApproximationA method where a polynomial approximation is chosen to relate to a given function in way that minimises the squares of the errors. Used for data smoothing and approximating in differentiation.
LimbThe edge of the solar disk.
Lower DecileThe smallest decile, one of nine, that has 90% of the ordered values above it.
LossIn the ionosphere it refers to the removal of free electrons from the ionosphere.
Lowest Usable FrequencyThe lowest frequency which allows an acceptable grade of HF service.
M Class FlareSolar flares which have a particular range of energy output of X-ray radiation. An M class flare will usually produce a shortwave fadeout in the daylight hemisphere of the earth. M class flares are less intense than X class flares but more intense than C class flares.
MagnetosphereThe outer magnetic field of the earth. The magnetosphere is buffeted by the solar wind.
Maximum Usable FrequencyThis is the highest frequency for reliable radio communications by the ionosphere. The median MUF is the highest frequency that will be usable at a particular hour for at least 50% of the days of the month.
MedianThe middle value when all values are ordered.
MeteorA body that enters the earth's atmosphere and becomes incandescent by friction. A 'shooting star'.
Mid Latitude TroughA band of decreased ionisation around 60 degrees magnetic latitude occurring during night hours. Seems to be related to the auroral oval since an equatorward expansion of the oval results in a decrease in width and movement of the trough towards the equator. Similarly a poleward movement of the oval sees a corresponding expansion and movement of the trough towards the pole. Possibly the result of plasma rising in this area. The region poleward is most likely replenished by ionisation due to precipitating particles.
Mixed ModeSometimes more than one mode of propagation is possible on an HF circuit. There will be small time delays between the different modes which may cause multipath fading. It may be possible to resolve the different modes by careful design of the antennas used.
ModeThe path followed by a radio wave between transmitter and receiver. The so called first propagation mode is the mode with the least number of hops for a circuit.
MoleculeSmallest part of an element or compound that exhibits the properties of the specific element or compound. A molecule is normally considered a group of atoms.
Multipath FadingSmall time delays can occur in radio signals travelling by a single mode (due to irregularities in the ionosphere) or by mixed modes. The superposition of these multiple echoes will degrade the quality of the received signal.
Non Derivative AbsorptionAbsorption of a radio wave as it transits its path. This mostly occurs in the D region of the ionosphere. Absorption of this kind is more significant than deviative absorption.
Obliquity FactorThe factor that relates the vertical critical frequency to the MUF for an HF circuit. It is denoted by M(D)layer, where D is the distance and layer if E, F1 or F2. This factor increases with increasing path length so longer paths will use higher frequencies.
Optimum Working FrequencyThis is the lower decile MUF. It is the frequency which is usable for at least 90% of the days of the month.
Ordinary WaveThe mode which follows the laws of refraction when propagating in a plasma containing a magnetic field. For the ordinary wave, the electric vector rotates in the opposite direction to that of an electron gyrating in the field when propagation is along the direction of the magnetic field. For propagation perpendicular to the magnetic field, the electric vector oscillates parallel to the magnetic field.
PhotonLight behaves as a wave in some circumstances and as a particle with energy in others. A particle of light is called a photon.
PhotoionisationThe production of positive ions and free electrons by the action of energetic radiation (EUV and X-rays) on atoms and molecules.
PhotosphereThe surface of the sun that we see. It lies beneath the corona and the chromosphere. Sunspots are seen in the photosphere.
PlageBright areas in the chromosphere overlying sunspots. The source of EUV radiation.
PlasmaA gas in which there are approximately equal numbers of positive ions and negative particles. There may also be many neutral particles, as is the case for the ionosphere.
Plasma FrequencyThe maximum frequency of internal oscillation of a plasma. The plasma frequency is proportional to the square root of the electron density.
PlasmapauseThe outer boundary of the plasmasphere. The plasmasphere resides in the magnetosphere and consists of ions and electrons - it may be considered an extension of the ionosphere.
Polar Cap AbsorptionThe ionisation of the D region over the polar latitudes by high energy solar protons causes radio blackouts for trans-polar circuits which can last for several days. PCAs are almost always preceded by a major solar flare on the visible hemisphere of the sun. The time between the flare event and the onset of the PCA ranges from a few minutes to several hours.
PolarisationIn an ionised medium in the presence of a magnetic field, a radio wave is split into two circularly polarised components, each propagating independently. In the ionosphere a radio wave is split by the earth's magnetic field into ordinary (o) and extra-ordinary (x) waves. The partitioning of the wave energy between the two depends on the angle the wave makes with the magnetic field. At low frequencies, the x-wave is heavily attenuated relative to the o-wave.
ProductionIn the ionosphere, it refers to the process of producing free electrons.
ProminencesFilaments seen on the limb of the sun.
Proton FlareA flare that liberates significant amounts of high energy protons. If this stream intercepts the earth. the protons cause a polar cap absorption (PCA).
Radio EmissionThe sun emits radio waves. The intensity of the radio emission from the sun often increases during solar flares (radio bursts) and above large sunspot groups (radio noise storms). In addition, the sun produces a background radio flux which depends on frequency and varies in the course of the solar cycle. Units for solar radio flux are 10-22 watts per square meter (also known as a solar flux unit or SFU)
Radio SeasonsShort-wave radio is split into broadcasting seasons. Traditionally, there are two major seasons, Winter and Summer, with two smaller ones centred on each vernal equinox. We DX'ers love the variable conditions an equinox can bring but to a Transmission Planner, a nightmare. All the stations try to get frequency allocations in all the bands so they can move to lower frequencies in Winter in a desperate attempt to be heard in the target country. Conditions during the compilation of this list have been so unreliable as to warrant mid-season changes. Like those for Derby County, they have had limited success. M represents March and April, J represents May, June, July and August, S represents September and October and D represents November, December, January and February.
RecombinationThe process of joining together of ions and free electrons to form neutral atoms or molecules.
Recurrent DisturbanceA disturbance (usually geomagnetic) which repeats at an interval of 27 days, the approximate rotation rate of some features on the sun.
ReflectionAlthough a radio wave is actually refracted in the ionosphere, it is often permissible to substitute a simple triangular ray path for the real ray path, as if the ray were reflected from a mirror. Thus radio waves are often referred to as being reflected from the ionosphere.
RefractionThe bending of a wave when it crosses a boundary between media due to a change in velocity of the wave. Until it reaches the ionosphere, a radio wave propagates in a straight line. Once in the ionosphere, it is refracted back towards the ground. The amount of refraction depends on the electron density of the ionosphere and the operating frequency.
Refractive IndexAn index to define the amount of refraction a wave will undergo when it enters a medium.
Reporting TimeHere in the UK, we have traditionally used Greenwich Mean Time. Around the world, this is referred to as Universal Time Co-ordinated or UTC, sometimes quoted on-air as just UT.
RetardationThe delay in propagation of a radio wave near the critical frequency caused by the slowing down of the wave by the ionosphere.
SIDSudden Ionospheric Disturbance. See Daylight Fadeout.
Signal-to-Noise-RatioThe ratio of the magnitude of a signal to that of noise.
Skip DistanceThe minimum distance from the transmitter for which a sky wave will return to earth when the operating frequency exceeds the vertical incidence critical frequency. Within the skip distance, only ground wave propagation may be possible. The only way to reduce the skip distance is to lower the operating frequency.
Sky WaveThe radio wave which propagates through the ionosphere. It is often called the ionospheric wave to distinguish it from the direct (line of sight) wave and the ground wave.
Smoothed Sunspot NumberAn average of monthly sunspot numbers centred on the month of concern. There are various formulas, however, the aim is to smooth discrete data points.
Solar ActivityAny change in the sun's appearance or behaviour. The sun's activity is described as being very low, low, moderate, high or very high.
Solar ControlThe term used to indicate that the behaviour of an ionospheric region is dominated by the sun.
Solar CycleSolar activity changes over a period of, on average, 11 years. At solar maximum, the solar activity is high and so too the EUV radiation output which affects the ionosphere. At solar minimum, the opposite is true.
Solar Flux UnitUnit of radio emission from the sun, usually given as 10-22 watts per square meter.
Solar MaximumThe time at which the solar cycle reaches its highest as defined by the 12-month smoothed value of the sunspot number.
Solar MinimumThe time at which the solar cycle reaches its lowest point as defined by the 12-month smoothed value of the sunspot number.
Solar WindThe outflow of solar material from the hot, unstable corona. The solar wind blows into interplanetary space with a speed of about 400 km/s (this can vary dramatically), carrying with it the magnetic fields that originate in the sun.
SolsticeThe times when the sun reaches its greatest declination away from the equator. The times of longest day and shortest night and vice versa. Occur in June and December.
Sporadic EA thin ionised layer in the E region that occurs irregularly.
Spread FIrregularities in the F region of the ionosphere which scatter radio signals causing a degradation in communications.
StormSevere departure from normal conditions in either the ionosphere or the earth's magnetic field.
Sudden CommencementA sudden impulse becomes a sudden commencement if the impulse is succeeded by a geomagnetic storm. In most cases, sudden commencements occur almost simultaneously around the world.
Sudden ImpulseAbrupt increase in the strength of the horizontal component of the earth's magnetic field.
SunspotsRelatively cool regions in the solar photosphere that appear dark. They contain intense magnetic fields which provide the energy for solar flares. Sunspots occur in groups. They underlie plage areas.
Sunspot NumberAn index of solar activity related to the number of sunspots and sunspot groups present on the sun.
Ten Centimetre FluxThe power output of the sun at a wavelength of 10.7 centimetres, frequently used in place of sunspot number as an indicator of solar activity.
T IndexAn indicator of the effect of solar activity on F region frequencies. The T index is an indicator of the critical frequency (foF2) of the ionosphere which determines the maximum usable frequencies for HF communication circuits. The higher the value of the T index the higher the value of the maximum usable frequency. The T Index has the same general scale of the sunspot number.
Total Electron ContentThe number of electrons along a wave path measured in electrons/square cm. The TEC is used in determining delay and directional changes of a wave in the ionosphere.
Travelling Ionospheric DisturbanceThese are probably a cause of focusing and defocusing of radio waves. The disturbance usually appears at higher frequencies, gradually affecting lower frequencies. Some are associated with magnetic storms and originate in the auroral zone. These may travel great distances. Others appear to be more localised, originating in the troposphere in local weather.
TroposphereThe lowest layer of the earth's atmosphere extending up to about 13 km in altitude.
Ultraviolet RadiationRadiation of immediately shorter wavelengths than visible light, between 5 and 400 nm. Extreme ultraviolet (EUV) and x-rays have shorter wavelengths again.
Universal TimeGreenwich Mean Time (GMT). Time referred to the zero meridian of longitude (through Greenwich, UK). 0000 UT is local midnight at Greenwich. For example, 10 am EST = 8 am WST = 9.30 am CST = 0000 UT.
Upper DecileThe largest decile, one of nine, that has 90% of the ordered values below it.
Upper Decile MUFThe frequency that will be usable at a particular hour for at least 10% of the month.
Virtual HeightThe apparent height of an ionospheric layer deduced from the time delay of a reflected radio pulse upon the assumption that it travelled at the speed of light over its entire path.
Winter AnomalyAt mid latitudes the F2 frequency is lower in summer than in winter, in spite of the larger solar zenith angle during summer.
X Class FlareFlares which have a particular range of energy output of X-ray radiation. X class flares are very energetic events which will definitely produce a shortwave fadeout in the daylight hemisphere of the earth.
X raysElectromagnetic waves of wavelength 0.001 to 1 nm. Emitted during solar flare activity and ionise the D region causing increased absorption of HF radio waves.
Zenith AngleThe angle between the overhead point for an observer and an object such as the sun. The solar zenith angle is zero if the sun is directly overhead and is 90 degrees when the sun is on the horizon.