DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

stems from the heated plasma at the flare site,
and does not indicate the presence of accelerated
particles. Microwave and radio emission
can also continue, evidence for energetic electrons
trapped in closed magnetic loops.
1. Soft X-rays and Hα originate as thermal
emission in a plasma with temperatures of about
10 7 K. Most of the emission is continuum, lines
of highly ionized O, Ca, and Fe are observed as
well.
2. Hard X-rays are photons with energies
between a few tens of keV and a few hundred
keV generated as bremsstrahlung of electrons
with slightly higher energies. Only a very small
amount of the total electron energy, about 1 out
of 10 5 , is converted into hard X-rays.
3. Microwaves are generated by the same
electron population that also generates the hard
X-rays as can be deduced from the similarities,
in particular multiple spikes, in both intensity
time profiles. Microwave emission is gyrosynchrotron
emission of accelerated electrons.
4. γ -rays are the best indicators for the presence
of energetic particles. The spectrum can
be divided into three parts: (a) bremsstrahlung
of electrons and, to a lesser extent, the Doppler
broadening of closely neighbored γ -ray lines
leads to a γ -ray continuum. (b) Nuclear radiation
of excited CNO-nuclei leads to aγ -ray line
spectrum in the MeV range. The most important
lines are the 2.23 MeV line due to neutron capture
in the photosphere and the 4.43 MeV line
from the relaxation of excited 12 C nuclei. These
reactions require particle energies of some 10
MeV/nucl, thus the γ -ray line emission indicates
the presence of very energetic nuclei at
the flare site. (c) Decaying pions lead to γ -ray
continuum emission above 25 MeV.
5. Radio emission results from electron
streams in the corona, exciting Langmuir oscillations.
Frequencies are in the meter range (see
metric radio emission). According to their frequency
drift, radio bursts are classified as type I
to type V metric bursts. See type i radio burst, i
= I ... V.
flare star A star whose brightness increases
detectably for a few hours at irregular intervals
due to flare analogous to those in the sun (solar
flares), but much brighter. They are a signature
of a star that is rotating rapidly and has a strong
© 2001 by CRC Press LLC
flattening of the Earth
magnetic field, either because it is young or because
it is in a close binary system. The most
conspicuous flare stars are of spectral type MV,
both because the stars are intrinsically faint and
because the convection zones are deep, producing
stronger fields and brighter flares.
flaring angle On the magnetopause at a given
point P on it — the angle χ (sometimes also denoted
α) between the magnetopause surface and
the flow direction of the distant solar wind (or
else, 90 ◦ − χ is the angle between flow of the
distant solar wind and the normal to the magnetopause
at P). By the Newtonian approximation,
the perpendicular pressure of the solar wind on
the magnetopause at P equals p sin χ, where p
is the dynamic pressure of the solar wind.
flat field, sky flat, dome flat A CCD’s pixelto-pixel
variation in sensitivity is called the flat
field function. To make accurate measurements
of the number of counts from objects on an image,
this variation (which is a multiplicative factor)
must be removed. This is done by making a
flat field image, and dividing this image into all
the data images.
There are a number of methods for obtaining
a flat field image, and they are very dependent
on the instrument, type of data, and filters used.
Different flat field images must be obtained for
each filter, as filters can modify the illumination
across a CCD, and narrowband filters can produce
interference fringes. A common method is
to get a number of exposures of a blank screen in
the telescope dome and average them together;
these are called dome flats. One may also get images
of the twilight sky, or combine many data
images. In these latter cases, one must be sure
that no actual objects (stars, etc.) appear on the
flats. If these sky flats are not of the same point
in the sky, it is possible to combine them with a
median filter and obtain a good flat field frame.
Each data image is divided by an average flat
field image of the same filter, etc.
flattening of the Earth Because of its rotation
the polar radius of the Earth is smaller than
the equatorial radius by about 21 km. This leads
to polar flattening and an equatorial bulge.
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