It is situated at around 90 km up to 120 km above the surface of the Earth. Ionization is due to soft X-ray (1-10 nm) and far ultraviolet solar radiation ionization of molecular oxygen. Normally this layer can only reflect radio waves having frequencies lower than about 10 MHz , however during intense Sporadic E events it can reflect frequencies as high as 250 MHz . At night the E layer almost disappears because the primary source of ionization is not present. Top

When a radio wave directed for F region communications is instead refracted from the E region. This is likely to occur when the take-off angles of the F mode and the E mode are similar and the operating frequency is below the E region MUF for the E mode. The radio wave is then likely to propagate via multiple E layer hops. The E modes can be heavily attenuated, especially when more than two hops occur. Top

A solar controlled ionospheric region around 90-140 km capable of refracting HF radio waves. This region is highly conductive and is only significant for HF communications during daylight hours. Top

In aeronomy terms, It is the reflected electromagnetic wave by ionized atmosphere. Top

It is the apparent path that the Sun traces out in the sky during the year, appearing to move eastwards on an imaginary spherical surface, the celestial sphere, relative to the (almost) fixed stars. More accurately, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun. Top

A small disturbance of wind in a large wind flow which can produce turbulence. Top

Probability, per time unit, of occurrence of a transition from excited state to another state of lower energy in an atom or molecule. Top

Means the flow of charged particles (quantitatively it is a rate of change of charge per unit time and its unit, in SI, 1 Ampère.equal to Coulomb/second). The current is positive in the in the same direction of the ions. Top

Radiation which has both electric and magnetic properties. Oscillations in the electric and magnetic fields which propagate in space and time. Examples: microwaves, light, infra-red, ultraviolet, x-rays, gamma, radio and television. In vacuum it travels with the speed of light (at 3E8 meters per second). Top

Ensemble composed by all possible frequencies which an electromagnetic field can oscillate. The electromagnetic spectrum extends from low frequencies (radio waves) to high frequencies (gamma rays), passing through microwaves, infrared, visible, ultra-violet, x-rays. Top

An atomic particle with a fixed negative electric charge. One of the elementary particle. Top

The number of electrons in a volume unit, e.g., 1 cm-3. Top

It is a unit of energy equivalent to approximately 1.602E-19 Joules. An electron volt is the amount of kinetic energy gained by a single electron when accelerated by an electric potential difference of one volt, in vacuum. Top

Angle between the horizontal and the direction of concern. Top

Elliptic orbit is a Kepler orbit with the eccentricity greater than 0 and less than 1. Top

It is a frivolous acronym for Emissions of Light and Very Low Frequency Perturbations from Electromagnetic Pulse Sources. This refers to the process by which the light is generated; the excitation of nitrogen molecules due to electron collisions. Appear as a dim, flattened, expanding glow around 400 km in diameter that lasts for, typically, just one millisecond. They occur in the ionosphere 100 km above the ground over thunderstorms. Top

Area on the Sun where new magnetic flux is erupting. Top

It is the emission of photons by a atom or molecule at determined wavelength. Top

Ration between the intensity of the radiation emitted by a body at determined temperature wavelength and the intensity of the radiation emitted by a black body at same wavelength and at same temperature. Top

(plural - Ephemerides) - Ephemeris data consisting of health and exact location data that GPS receivers then use (together with the signal's elapsed travel time to the receiver) to calculate their own location on Earth using trilateration. Top

A depression in F region frequencies at the geomagnetic equator relative to frequencies at low latitudes. A daytime phenomenon. Top

A thin electric current in the ionosphere over the magnetic equator around 100 to 115 km altitude normally flowing eastward during the day. 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. Top

Anomaly that creates two ionizations crests in the diurnal sector, 18 degrees approximately on each side of the geomagnetic equator. These crests are formed, specially, through the interaction of the electric field and the magnetic field and by thermospheric winds. The configuration of these peaks depends on the local time, solar cycle, day of the year, direction of the winds, etc. Top

It is an orbit which is contained in the equatorial plane. Top

Time when the angle between the orbital plans of the Sun and of the Earth is zero. It occurs twice during year, in March and September. Day and night are of equal lengths during the equinox. Top

Electric current system produced due the action of the atmospheric tides wind on the charged particles in the E-region. Top

Energy unit in the British CGS (centimeter-gram-second system) equal to 1 gram(square centimeter)/(square second) (In the SI 1erg equal to 1E-7 Joule). Top

Solar activity levels with at least one radio event (10cm) and several chromospheric events per day (Class C Flares). Top

See Extreme Ultraviolet Radiation. Top

An evanescent wave is a nearfield standing wave with an intensity that exhibits exponential decay with distance from the boundary at which the wave was formed. They are formed at the boundary between two "media" with different properties in respect of wave motion, and are most intense within one-third of a wavelength from the surface of formation. Top

The phase change from liquid to gas.

The level in the atmosphere above which the mean free path of particles is longer than a scale height. Top

The exosphere is the outer layer of the atmosphere and is a transitional zone between Earth’s atmosphere and interplanetary space. This layer begins around 600 km height and contains light particles. Particles traveling with velocity greater than the escape velocity can escape to the space. Top

The temperatures on the exosphere are very high (~1800K) depending on the solar flux, cosmic rays and other factors. Top

The highest radio frequency band from 30 to 300 gigahertz. Top

A radio wave may split into two oppositely polarized components due to, in the case of the ionosphere, the geomagnetic field. The ordinary wave obeys the laws of refraction and is unaffected by the magnetic field. The extraordinary wave travels a different path through the ionosphere. Top

It is part of the electromagnetic spectrum from 120 nanometers down to 10 nanometers, naturally generated by solar corona, and the most highly absorbed component of the spectrum in the Earth’s atmosphere. Top

Electromagnetic radiation with frequency higher than the frequencies of the ultraviolet spectrum and less than the x-ray frequencies range. Top

The radio frequency band from 3 to 30 Hertz. Top