The height within which some parameter, such as pressure or density, decreases by a factor 1/e in an isothermal atmosphere. The term is most often used in an ionospheric context, but is equally applicable to the neutral atmosphere. It is a measure of the effective “thickness” of an atmospheric layer.

It is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass.

Atmospheric constituents ionization caused by collisions with energetic particles, such as cosmic rays.

n aeronomy, the term used to design features that initiate Spread-F.

The term semidiurnal refers to an oscillation which has a period or cycle of approximately half of one day.

A shock wave, a type of propagating disturbance in a fluid, gas, or plasma medium. A shockwave is formed always that a travel at high, supersonic velocity from its point of origin.

A sidereal day lasts from when a distant star is on the meridian at a point on Earth until it is next on the meridian. A sidereal day lasts 23 hours and 56 minutes (of solar time), about 4 minutes less than a solar day.

The ratio of the magnitude of a signal to that of noise.

The distance between where the ground wave ends and the sky wave becomes detectable. The length of the skip distance is dependent on the operating frequency. Decreasing the operating frequency is likely to reduce the skip distance.

The 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 ground wave.

In the ionosphere the slab thickness τ is defined as a ratio of the total electron content (TEC) to the F-region peak electron density NmF2 and is a first order measure of the shape of the electron density profile.

The slow solar wind appears to originate from a region around the Sun's equatorial belt that is known as the "streamer belt". Coronal streamers extend outward from this region, carrying plasma from the interior along closed magnetic loops.

Gravity waves with periods of some minutes and wavelengths smaller than a hundred kilometer.

An average of monthly sunspot numbers centered on the month of concern. There are various formulas, however, the aim is to smooth discrete data points.

Transient perturbations of the solar atmosphere as measured by enhanced x-ray emission, typically associated with flares. Five standard terms are used to describe the activity observed or expected: Very low: x-ray events less than C-class. Low: C-class x-ray events. Moderate: isolated (one 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. Solar and Heliospheric Observatory (SOHO) - It is a spacecraft that was launched December 2, 1995 to study the Sun, and began normal operations in May 1996. Its scientific mission is currently the main source of near-real time solar data for space weather prediction.

It is the amount of energy received at the top of the Earth's atmosphere on a surface oriented perpendicular to the Sun’s rays at the mean distance of the Earth from the Sun. Satellite measurements show that the solar constant value is around of 1368 W/m2.

The term used to indicate that the behavior of an ionospheric region is dominated by the Sun.

High temperature region of the solar atmosphere, above the chromosphere; here, plasma is trapped by the local magnetic field or escapes into space if the magnetic field lines are open.

Solar activity changes over a period of, on average, 11 years. At solar maximum, solar activity is high and so too the EUV radiation output which affects the ionosphere. At solar minimum, the opposite is true. A solar cycle commences at solar minimum.

They are high-energy particles coming from the Sun, that consist of protons, electrons and heavy ions with energy ranging from a few tens of keV to GeV, the fastest particles can reach speed up to 80% of the speed of light.

Mass ejection from the Sun, related to a prominence.

It is a large explosion in the Sun's atmosphere that can release as much as 6E25 joules of energy. The term is also used to refer to similar phenomena in other stars, where the more accurate term stellar flare applies.

Unit of radio emission from the Sun, usually given as 1E-22 watts per square meter per Hertz.

It is a characteristic cellular brightness pattern visible on the solar surface, on scales from a fraction of an arc second to several arc seconds, corresponding to physical scales from a few hundred kilometers to several thousand kilometers.

The time at which the Sun reaches its highest activity as defined by the 12-month smoothed value of the sunspot number.

The time at which the Sun reaches its lowest point in solar activity as defined by the 12-month smoothed value of the sunspot number.

An area seen as a dark spot on the photosphere of the sun, with a mean diameter approaching a few thousand kilometers. Sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups. They appear dark because they are cooler than the surrounding photosphere.

Solar System is the planetary system in which we live. It is compound by the Sun, the eight planets, and other corps spinning around the Sun.

The 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.

Time when the angle between the orbital plans of the Sun and of the Earth is maximum, whose value is approximately 23.5 degrees. It occurs twice during year, in June and December. At solstice, the duration of the day (night) is maxima (minimum) in the Summer (Winter) hemisphere. South Atlantic Anomaly (SAA) - The region where Earth's inner van Allen radiation belt makes its closest approach to the planet's surface. The result is that, for a given altitude, the radiation intensity is higher over this region than elsewhere.

The term space weather refers to conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and that can affect human life and health.

Solar matter expelled permanently between the granules, in the form of proton and electron tongues.

A thin ionized layer in the E region that occurs irregularly. Unlike the E region, sporadic E is not solar controlled. Sporadic E may occur day or night.

SEE Sporadic E.

Irregularities in the F region of the ionosphere which scatter radio signals causing a degradation in communications.

They are large-scale electrical discharges which occur high above a thunderstorm cloud, or cumulonimbus, giving rise to a quite varied range of visual shapes. They are triggered by the discharges of positive lightning between the thundercloud and the ground. They normally are colored reddish-orange or greenish-blue, with hanging tendrils below, and arcing branches above, their location, and can be preceded by a reddish halo. They often occur in clusters, lying 80 km to 145 km above the Earth's surface.

It is defined as an aster that has enough mass to perform nuclear reactions in its nucleus and for this reason it has a huge temperature, high mass density, intense gravity, emits light and other radiation.

Electrical discharge that propagate in steps through the virgin air from the cloud to the ground, producing an ionized channel.

Severe departure from normal conditions in either the ionosphere or the geomagnetic field.

It is the transition level between the stratosphere and the mesosphere and is situated approximately at 50 kilometers above the Earth’s surface.

It is a layer above the troposphere, in which temperature increases with height. This level extends from 10 kilometers up to 50 kilometers and contains the major concentration of ozone of the atmosphere.

The point on a body at which the Sun is directly overhead at given point.

A 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.

Abrupt increase in the strength of the horizontal component of the geomagnetic field.

A rapid enhancement in the ionization of the dayside D layer of the ionosphere following a solar flare, resulting in disruption of radio communications. Strong X-ray emission from the flare causes increased ionization which absorbs radio waves, leading to signal fade-outs. Sudden ionospheric disturbances are most common around sunspot maximum, when there may be as many as 25 solar flares per day.

The instant at which the upper edge of the Sun appears above the horizon in the east.

It is the daily disappearance of the Sun below the horizon as a result of the Earth's rotation.

An index of solar activity related to the number of sunspots and sunspot groups present on the Sun.

Relatively 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.

Size scale referring generally to weather systems with horizontal dimensions of several hundred kilometers or more. Most high and low pressure areas seen on weather maps are synoptic-scale systems.