DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

gradual flare
gradual flare Solar flare with long-lasting
electromagnetic radiation, originating high in
the corona. In more detail, the properties of
gradual flares are: (a) the soft X-ray emission
lasts for more than 1 h, (b) the decay constant of
the soft X-ray emission is larger than 10 min,
(c) the duration in hard X-rays is at least 10
min, (d) the duration in microwaves is at least
5 min, (e) gradual flares are always accompanied
by metric type II and metric type IV bursts,
metric type III bursts are observed in half of
the gradual flares, (f) the height in the corona is
about 5·10 4 km, (g) the flare volume is between
10 28 cm 3 and 10 29 cm 3 , (h) the energy density
is low, (i) the size in Hα is large, and (j) gradual
flares are always accompanied by coronal mass
ejections.
If a gradual flare gives rise to an energetic
particle event observed in interplanetary space,
this event has properties different from particle
events caused by impulsive flares. Many
of these differences can be attributed to the interplanetary
shock caused by the coronal mass
ejection in gradual flares. In detail, the properties
of a particle event originating in a gradual
flare are: (a) the event is proton-rich with H/He
being about 100, (b) the 3 He/ 4 He ratio is about
0.0005 which is the ratio observed in the solar
wind, (c) the Fe/O ratio is 0.155, which is also
close to the ratio in the solar wind, (d) the charge
state of iron is about 14, which suggests rather
low temperatures in the acceleration region and
therefore acceleration out of the corona or the solar
wind, (e) the particle event in interplanetary
space at the orbit of Earth lasts for some days,
(f) particles can be observed over a longitude
cone of about 180 ◦ , and (g) an interplanetary
shock is observed. Occurrence of such events is
about 10 per year.
The distinct feature of the gradual flare is the
coronal mass ejection and consequently the interplanetary
shock. Some instability below a
filament causes reconnection at the anchoring
field lines which leads to heating and the generation
of electromagnetic radiation (the flare) and
to a disconnection and expulsion of the filament
(the coronal mass ejection).
gradually varied flow A description of flow
in an open channel (such as a canal or river)
which assumes that the rate of change of depth
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is small (i.e., h/L is small, where h is the
change in depth, and L is the distance over which
the depth changes).
Grad–Vasyliunas theorem An equation
used in the study of guiding center plasmas in
the Earth’s magnetic field, an equivalent formulation
of force balance. It assumes that under
the influence of convection electric fields, the
plasma of each flux tube moves together, from
one tube to another, that its particle motion has
no preferred direction (isotropy), that a scalar
(rather than tensor) pressure p can be used, and
that Euler potentials (α, β) of the magnetic field
are given. Under those assumptions, it relates
the field aligned current density j|| with the distributions
of pressure p(α, β) and of specific volume
V(α, β), as j|| =|∇V(α,β)×∇p(α, β)|.
Since the above assumptions are best met in the
Earth’s plasma sheet, it has been widely used to
deal with plasma convection there. Also known
as Vasyliunas theorem.
grain-boundary migration To help minimize
the energy of the solid materials in response
to changing P-T conditions, the atoms
forming the contacts between individual grains
self rearrange to a more stable configuration. To
a large extent this is achieved by the process of
grain-boundary migration. This involves movement
at a high angle to the plane of the grain
boundary. Such a process usually produces
regular interfaces and a polygonal aggregate of
grain.
grain-boundary sliding Movement of grains
is limited within the plane of the grain boundary.
Grain-boundary sliding can be envisaged
in terms of the physical movement of individual
grains past each other under an applied shear
stress. Grain-boundary sliding is recognized as
one of the most important deformation mechanisms
in fine-grained polycrystalline aggregates,
and is considered one of the main deformation
mechanisms responsible for superplasticity
of polycrystalline materials.
grain chemistry: dense interstellar In
dense interstellar media (i.e., visual extinction
Av ≥ 5 magnitudes, or nH ≥ 10 4 cm −3 ), where
the radiation field is attenuated and the average