DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

first-order wave theory
the fields are correlated, whereas no correlation
exists between fields belonging to different domains.
After its creation, a bubble will expand
at the speed of light surrounded by a “sea” of
false vacuum domains. As opposed to secondorder
phase transitions, here the nucleation process
is extremely inhomogeneous and φ(x) is
an abruptly changing function of time. See also
Ginzburgtemperature, GeV,Kibblemechanism,
QCD, spontaneous symmetry breaking.
first-order wave theory Also known as linear
wave theory or Airy wave theory. The
derivationinvolvesassumptionthattermsofsecond
and higher order are negligible.
fjord (also fiord). A long, narrow, typically
deep inlet, connected to a sea. Generally found
in mountainous regions at high latitudes (Norway,
Alaska, New Zealand).
FK Comae stars A rapidly rotating, red giant.
Because conservation of angular momentum
during the expansion of a star from the main
sequence to red giant stage should slow its surface
rotation, the phenomenon must arise from
the interaction between binary stars, possibly
from the mergers of W Ursa Majoris binaries.
flare A pronounced, transient increase in the
sun’s global output related to sunspots and solar
activity. Flares are generally manifested by
a rapid (≈ 1 min) brightening in the red hydrogen
line, emitted in the sun’s chromosphere,
and a very sudden increase (a few seconds) in
X-ray emission from that region. Energies up to
10 25 J can be released over a time period of some
minutes though much shorter events are known.
White-light flares, that is a brightening of the
sun in visible light, are rare because even for the
largest flares the brightness is less than 1% of the
total luminosity of the photosphere. In certain
frequency ranges, e.g., at the wings of the black
body, the intensity of the electromagnetic radiation
can increase by orders of magnitude. Although
the flare is defined as an outburst in electromagnetic
radiation, large flares are associated
with large fluxes of energetic ions from the sun,
at times at energies of 1 GeV and more (see energetic
particles) and the eruption of filaments,
coronal mass ejections (see coronal mass ejec-
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tions). Depending on the time-scales and the occurrence
of coronal mass ejections, flares can be
classified as impulsive or gradual. The energy
for these rapid events is taken from reconnection
in the magnetic fields of the sunspot region near
which the flare occurs. Particle events in interplanetary
space caused by these different kinds
of flares also show distinct properties. See gradual
flare, impulsive flare.
Flares can be classified according to the size
of the Hα emitting region (cf. table), the peak
intensity in X-ray emission (SXR class), or the
radio flux at 5000 MHz (given in solar flux units
sfu).
Flare Classification
Class Area∗ SXR Radio
(Hα) Class Flux
S ≤ 200 C2 5
1 200–500 M3 30
2 500–1200 X1 300
3 1200–2400 X5 3000
3+ >2400 X9 30000
∗ In millionth of sun’s area with 1 millionth
equal to 6.08 · 106 km2 .
flare electromagnetic radiation The electromagnetic
radiation released in a solar flare
shows typical time profiles in different frequency
ranges. In a large flare, the flare emission
can be divided into three phases: (a) a precursor
(also called preflare phase) lasting for some
minutes to some 10 minutes, visible as a weak
brightening of the flare region in Hα and soft
X-rays, indicating the heating of the flare site.
(b) The impulsive or flash phase in which electromagnetic
radiation is emitted over the entire
frequency range from γ -rays over X-rays, UV,
the visible, IR up to radio waves. In this phase,
the hardest part of the electromagnetic emission
is most abundant, which indicates the acceleration
of particles in addition to just a heating
of the flare site. The impulsive phase can be
observed in most flares, lasting for some minutes.
In larger flares it can be followed by (c) a
gradual phase lasting for some 10 minutes to
some hours during which the emission mainly
occurs in Hα and soft X-rays. This emission