AE index: A geomagnetic index describing the auroral electrojet.
ap index: A mean, 3-hourly "equivalent amplitude" of magnetic
activity based on K index data from a planetary of 11 Northern and 2 Southern
Hemisphere magnetic observatories between the geomagnetic latitudes of 46 and
63 degrees. ap values are given in units of 2 nT.
Ap index: A daily index determined from eight ap index values.
Aurora: A sporadic, faint visual phenomena associated with
geomagnetic activity that occurs mainly in the high-latitude night sky.
Auroras occur within a band of latitudes known as the auroral oval, the
location of which is dependent on geomagnetic activity. Auroras are a result
of collisions between atmospheric gases and precipitating charged particles
(mostly electrons) guided by the geomagnetic field from the magnetotail. Each
gas (oxygen and nitrogen molecules and atoms) gives out its own particular
color when bombarded, and atmospheric composition varies with altitude. The
auroral altitude range is 80 to 1000 km, but typical auroras are 100 to 250 km
above the ground; the color of the typical aurora is yellow-green, from a
specific transitions of atomic oxygen. Auroral light from lower levels in the
atmosphere is dominated by blue and red bands from spectral line of atomic
oxygen. The patterns and forms of the aurora include quiescent arcs, rapidly
moving rays and curtains, patches, and veils.
Auroral electrojet: A current that flows in the ionosphere in the
Bartel' rotation number: The serial number assigned to 27-day
rotation periods of solar and geophysical parameters. Rotation 1 in this
sequence was assigned arbitrarily by Bartels to begin in January 1833, and the
count has continued by 27-day intervals to present. (For example, rotation
2195 began on April 17, 1994). The 27-day period was selected empirically from
the observed recurrence of geomagnetic activity attributed to corotating
features on the sun.
Bow shock: A collisional shock wave in front of the magnetosphere
arising from the interaction of the supersonic solar wind with earth's
Coronal hole: An extended region in the corona, exceptionally low
in density and associated with unipolar magnetic photospheric regions having
"open" magnetic field topology. Coronal holes are largest and most stable at
or near the solar poles, and a source of high-speed solar wind. Coronal holes
are visible in several wavelengths but most notably in solar x-rays.
Coronal mass ejection: An observable change in coronal structure
that (1) occurs on a time scale between a few minutes and several hours and
(2) involves the appearance of a new, discrete, bright white-light feature in
the coronograph field of view. They are associated with the large-scale,
closed magnetic structures in the corona. At times of coronal mass ejections
large quantities of material (10^15 - 10^16 g) are sporadically ejected from
the Sun into the interplanetary space. The speed of the leading edge of the
coronal mass ejection may vary from 50 km/s to 1200 km/s. Average speed is
about 400 km/s. The average heliocentric width is about 45 degrees. The
largest geomagnetic storms are caused by coronal mass ejections.
Coronal neutral line: The line in the corona that separates solar
magnetic fields of opposite polarity. It is calculated from solar observations
of the photospheric magnetic field. Extension of the neutral line radially
outward by the solar wind defines the current sheet in the heliosphere.
Coronal streamer: A feature of the white light corona that looks
like a ray extending away from the sun out to about one solar radius, having
an arch-like base.
Dst index: A measure of variation in the geomagnetic field due to
the equatorial ring current. It is computed from the H-components at
approximately four near-equatorial stations at hourly intervals. At a given
time, the Dst index is the average of variation over all longitudes; the
reference level is set so that Dst is statistically zero on internationally
designated quiet days. An index of -50 nT or deeper indicates a storm-level
disturbance, and an index of -200 nT or deeper is associated with
Geomagnetic activity: Natural variations in the geomagnetic field
classified into quiet, unsettled, active, and geomagnetic storm levels.
Geomagnetic field: The magnetic field in and around earth. The
intensity of the magnetic field at the earth's surface is approximately 31 000
nT (0.31 gauss) at the equator and 62 000 nT (0.62 gauss) at the north pole.
The geomagnetic field is dynamic and undergoes continual slow secular changes
as well as short-term disturbances (geomagnetic activity). The magnetic field
can be approximated by a centered dipole field, with the axis of the dipole
inclined to the earth's rotational axis by about 11.5 degrees.
Geomagnetically induced currents: According to Faraday's law of
induction, a temporal change of a magnetic field is always accompanied by an
electric field. Hence an electric field is associated with geomagnetic
activity. The geomagnetic variation and the geoelectric field observed at the
earth's surface depend primarily on ionospheric-magnetospheric currents, and
secondarily on currents and charges induced in earth. A part of the earth
currents can flow into man-made conductors, like power transmission systems,
pipelines, telecommunication cables, and railroads. Such currents are called
geomagnetically induced currents (GICs).
Geomagnetic storm: A worldwide disturbance of the earth's magnetic
field, distinct from regular diurnal variations.
operational definition: A storm occurs when the Ap > 29, a
minor storm when 29 < Ap < 50, a major storm when 50 <= Ap < 100
and a severe storm when Ap >= 100. A physical definition: An interval of time when a sufficiently intense and long-lasting
interplanetary convection electric field leads, through a substantial
energization in the magnetosphere-ionosphere system, to an intensified ring
current sufficiently strong to exceed some key threshold of quantifying storm
time Dst index.
Heliospheric current sheet: A current layer that separates adjacent
interplanetary magnetic field regions with opposite magnetic polarity.
K index: A 3-hourly quasi-logarithmic local index of geomagnetic
activity relative to an assumed quiet-day curve for the recording site, range
is from 0 to 9. The K index measures the deviation of the most disturbed
Kp index: A 3-hourly planetary index of geomagnetic activity
calculated by the Institut fur Geophysik der Gottingen Universitat, Germany,
from the K indices observed at 13 stations primarily in the Northern
Magnetopause: The boundary surface between the solar wind and the
magnetosphere, where the pressure of the magnetic field of the object
effectively equals the dynamic pressure of the solar wind.
Magnetosheath: The region between the bow shock and the
magnetopause, characterized by very turbulent plasma.
Magnetosphere: The magnetic cavity surrounding a magnetized body,
carved out of the passing solar wind by virtue of the magnetic field, which
prevents, or at least impedes, the direct entry of the solar wind plasma into
Magnetotail: The extension of the magnetosphere in the antisunward
direction as a result of the interaction with solar wind. In the inner
magnetotail, the field lines maintain a roughly dipolar configuration. But at
greater distances in the antisunward direction, the field lines are stretched
into northern and southern lobes, separated by a plasmasheet. There is
observational evidence for traces of earth's magnetotail as far as 1000 earth
Plasma: A plasma is a quasineutral gas of charged and neutral
particles which exhibit collective behavior. A plasma must satisfy three
conditions: (1) the Debye length must be much less than a characteristic
distance of the system, (2) the number of particles in a Debye sphere must be
much larger than one and (3) the plasma frequency times the mean time between
collisions with neutral atoms must be larger than one. Plasmas behave
sometimes like fluids and sometimes like a collection of individual particles.
Radiation belts: Regions of the magnetosphere roughly 1.2 to 6
earth radii above the equator in which charged particles are stably trapped by
closed geomagnetic field lines. There are two belts. The inner belt is part of
the plasmasphere and corotates with earth. The outer belt extends on to the
magnetopause on the sunward side (10 earth radii under normal quiet
conditions) and to about 6 earth radii on the nightside. The radiation belts
are often called the "Van Allen radiation belt" because the were discovered in
1968 by J.A. Van Allen.
Reconnection: A plasma process by which differently directed field
lines link up, allowing topological changes of the magnetic field to occur,
determining patterns of plasma flow, and resulting in conversion of magnetic
energy to kinetic and thermal energy of plasma. Reconnection is invoked to
explain the energization and acceleration of the plasmas that are observed in
coronal mass ejections, magnetic substorms, and elsewhere in the solar system.
Ring current: In the magnetosphere, a region of current that flows
in a disk-shaped region near the geomagnetic equator in the outer of the Van
Allen radiation belts. The current is produced by the gradient and curvature
drift of trapped charged particles. The ring current is greatly augmented
during magnetic storms because of the hot plasma injected from the
Solar wind: The outward flow of solar particles and magnetic fields
from sun. Typically at 1 AU, solar wind velocities are near 450 km/s and
proton and electron densities are near 5 cm^-3. The total intensity of the
interplanetary magnetic field is nominally 5 nT. The fast solar wind
originates from coronal holes and the slow wind is assumed to originate from
regions near the coronal neutral line.
Sector boundary: Area between sectors, which are large-scale
features distinguished by the predominant direction of the interplanetary
magnetic field, toward the sun (a negative sector), or away from the sun (a
positive sector). The sector boundary separating fields of opposite polarity
is normally narrow, passing the earth within minutes to hours as opposed to
the week or so needed for passage of a typical sector.
Substorm: A geomagnetic perturbation lasting one to two hours,
which tends to occur during local post-midnight nighttime. A substorm
corresponds to an injections of charged particles from the magnetotail into
the auroral oval.