DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

ionospheric regions
subsequently refracted towards the vertical as it
leaves the ionosphere. The amount of refraction
depends on the electron density of the ionosphere
and the operating frequency. Although
oblique radio waves are actually refracted in the
ionosphere, it is often referred to as being reflected
from the ionosphere. The latter is common
in connection with simple mirror models
of the ionosphere used for estimating HF system
parameters (e.g., ASAPS, IONCAP) or when referring
to ionospheric propagation modes. See
ionospheric radio propagation path.
ionospheric regions The ionized part of the
Earth’s upper atmosphere above about 70 km is
known as the ionosphere consisting of overlapping
ionized layers. These layers are produced
by the action of solar electromagnetic radiation
(ultraviolet and X-rays) and cosmic rays on the
neutral particles. As these regions are ionized,
the electrons affect the radio frequency propagation
between any points on Earth or from
outer space to the Earth. The number density of
positive ions equals the electron density to keep
charge neutrality at all heights. The layers have
an altitude of maximum, above and below which
the ionization drops off. The altitude profiles of
ionization are functions of solar activity, time of
day, latitude, season, and extra-terrestial events
such as magnetospheric storms, solar wind, interplanetary
magnetic field and energetic particles,
and solar flares to name a few.
The D region is situated normally around
85 km and present only during the day. The
E region peak is situated around 110 km. The D
and E region consist mainly of NO + ,O +
2 ions
and electrons. The F region consists of two regions,
F1 and F2. The F1 layer is situated at
a height of about 180 to 200 km and is absent
at night. The F2 region peaks around 250 to
300 km. The F region consists mainly of O + ,
NO + ,N + and electrons. The region above F2 is
called the topside ionosphere. As we go higher
in the topside ionosphere, we encounter the region
called “heliosphere” above about 400 km,
where helium ions are predominant. The region
above about 600 km is called as “protonsphere”,
where atomic hydrogen ions are predominant.
At higher latitudes, H + and He + may escape
along field lines as “polar wind” into the magnetosphere.
At low and middle latitudes, the
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ions are trapped along inclined magnetic lines of
force and do not escape. This region is called the
“plasmasphere”. The outer edge of the plasmasphere
where the magnetic line of force is open
to the magnetosphere is called “plasmapause”
and is located along the line of force mapping
down to about 60 ◦ magnetic latitude.
ionospheric sounder Usually, a radio transmitter
on the ground probing the electron density
distribution of ionosphere below the density
maximum. It is based on the tendency of a
plasma to reflect radio signals below its plasma
frequency. The sounder sends out a series of signals
of increasing frequency, and measures the
delay at which their echoes return: each delay
gives the lowest altitude at which the electron
density sufficed to reflect the signal.
Based on ionospheric soundings, the socalled
D, E, and F layers of the ionosphere were
identified long ago, the latter resolved into the
F1 and F2 layers. Above the F-layer the ion
density slowly decreases again, and its density
profile there was first probed in 1962 by the orbiting
ionospheric sounder aboard the Canadian
satellite Alouette 1. That plasma ends at the
plasmapause.
ionospheric sounding The process of obtaining
ionospheric information using a radio sounding
technique. This usually refers to operating
an ionosonde.
ionospheric storm A global disturbance in
the F region of the ionosphere, which occurs
in connection with geomagnetic activity and especially
a geomagnetic storm. The high latitude
atmosphere is heated during geomagnetic
activity, causing neutral winds to blow towards
the equator. These winds substantially alter the
chemistry of the atmosphere and thereby alter
the global ionosphere. During the first few hours
of a storm, the positive phase, the daytime Fregion
ionization can increase, sometimes significantly.
It is generally attributed to the action
of winds forcing ionization up the magnetic
field lines and is most apparent in the middle
latitudes. Generally, this phase ceases after
sunset and is often followed by the negative
phase, when a decrease in F-region ionization
occurs and sometimes lasts a few days. Individ-