DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

impact velocities A factor in crater formation
that determines crater size and is controlled
by the thickness of the atmosphere around the
planet (assuming cosmic velocities are constant
throughout the solar system for the same sized
objects). Thus, the moon has a higher impact
velocity than Mars. For example, a typical
asteroid may hit Mars at 10 km/sec and the
moon at 14 km/sec, causing the crater formed
on Mars to be smaller by 0.66. By recognizing
that smaller craters are produced on Mars, sizefrequency
curves developed there can be displaced
to make them directly comparable with
those on the moon, etc. This is also applicable
to the other solar system planets.
impermeable Does not allow penetration by
a fluid.
impulsive flare Solar flares with short duration
electromagnetic radiation occurring low in
the corona in compact volumina. In contrast
to gradual flares, impulsive flares are not accompanied
by coronal mass ejections. In more
detail, the properties of an impulsive flare are:
(a) the soft X-ray emission lasts less than one
hour, (b) the decay constant of the soft X-ray
emission is less than 10 minutes, (c) the duration
of the hard X-ray emission is less than
10 minutes, (d) the duration of the microwave
emission is less than 5 minutes, (e) impulsive
flares are always accompanied by metric type
III bursts, about 75% of them are also accompanied
by metric type II bursts, but metric type IV
emission is observed only rarely, (f) the height in
the corona is less than 10 4 km, (g) the flare occupies
a volume between 10 26 cm 3 and 10 27 cm 3 ,
(h) the energy density is high, (i) the size in Hα
is small, and (j) impulsive flares are rarely accompanied
by coronal mass ejections. If coronal
mass ejections are observed in impulsive
flares, they tend to be small and slow with speeds
well below the combined solar wind and Alfven
speed which is required to drive a shock.
If an impulsive flare gives rise to a particle
event in interplanetary space, this event has
properties different from the ones of particle
events caused by gradual flares. These properties
include: (a) the event is electron-rich and the
H/He ratio is about 10, (b) the 3 He/ 4 He ratio is of
the order of 1, which is enhanced by a factor of
© 2001 by CRC Press LLC
impulsive flare
2000 compared to abundances in the corona and
in the solar wind, (c) the Fe/O ratio is about 1.2,
an enhancement by a factor of 8 compared to the
corona or the solar wind, (d) the charge states of
iron are about 20, indicative for the acceleration
out of a very hot environment with temperatures
of about 10 million K, (e) the particles event lasts
(at the orbit of Earth) for some hours up to about
1 day, (f) particles can be observed only in a relatively
narrow cone of ±30 ◦ around the source
region, and (g) there is no interplanetary shock
accompanying the event. Event rates are up to
about 1000 per year during solar maximum.
In impulsive flares, particles are accelerated
in a hot but confined volume low in the corona.
The enrichment in 3 He and in heavier elements
such as Fe can be explained by the process of selective
heating: Particles are accelerated inside a
close loop, giving rise to electromagnetic emission
(the flare). The loop is very stable, preventing
particles from escaping into interplanetary
space. As the particles bounce back and forth
in the closed loop, they excite electromagnetic
waves which can propagate into all directions,
in particular also perpendicular to the magnetic
field lines. When these waves interact with the
ambient plasma, they can accelerate particles.
If these “secondary” particles are accelerated on
open field lines, they can escape into interplanetary
space. Because the acceleration requires
particles and waves to be in resonance, different
particles are accelerated by different types
of waves. If a particle species is common in the
corona, such as H and 4 He, the corresponding
waves are absorbed more or less immediately;
thus, these particles are predominately accelerated
inside the closed loop and therefore do not
escape into interplanetary space. Other waves,
however, can travel larger distances before being
absorbed by the minor constituents and therefore
are more likely to accelerate these species
on open field lines. Since the escaping particle
component is selectively enriched in these
minor species, the acceleration process is called
selective heating.
impulsive flare Flares displaying impulsive
spikes or bursts in their hard X-ray time profiles.
These flares are generally confined in long
(10 4 km) sheared loops and are considered as
non-thermal X-ray sources.
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