DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

ock debris carried on or within a glacier. It results
from the convergence of lateral moraines
where two glaciers meet. A terminal moraine
is a sinuous ridge of unsorted material pushed
ahead of an advancing glacier and deposited
when the glacier begins to recede.
Moreton wave The chromospheric component,
seen in Hα radiation, of a solar flareinduced
wave which propagates away from the
flaresite. AMoretonwavetravelswitharoughly
constant velocity of ∼ 1000 km s −1 and is attributed
to MHD fast mode shocks generated in
the impulsive phase of the flare.
morning cloud (Mars) Morning clouds of
Mars appear in the early morning and disappear
in midday. They reappear the next morning at
the same place. They tend to appear in the equatorial
region and mid-latitudes of the northern
hemisphere in spring to summer of the northern
hemisphere. In late spring to mid-summer of the
northern hemisphere morning clouds are seen
everywhere in low latitudes. They do not disappear
even in midday and shift to evening clouds,
so that the equatorial region is surrounded with
a cloud belt. A notable morning cloud is the one
that appears in an area centered at 10 N and 120
W, surrounded by gigantic volcanos of Olympus
Mons and Tharsis Montes. The altitude of the
cloud is lower than the tops of volcanos. The
vertical optical depth of the cloud is about 2 in
the early morning. A large morning cloud extends
from low latitudes of the southern hemisphere
to mid-latitudes of the northern hemisphere
in the most active period. See evening
cloud, afternoon cloud.
morphodynamics A term used to describe
the changing morphology, or form and structure,
of a coastal area.
morphology-density relationship The observational
fact that more elliptical galaxies are
found near the center of a cluster of galaxies
where the number density of galaxies is higher
while more spiral galaxies are observed in the
outskirts of the cluster where the number density
is lower (Dressler, 1980). The relative abundance
of ellipticals, lenticulars, and spirals is
now known to vary smoothly over the range
© 2001 by CRC Press LLC
multipath fading
from 10 4 galaxies per Mpc 3 near the cluster center
to 10 −2 galaxies per Mpc 3 near the cluster
boundary. It is not completely understood if the
segregation of galaxy types is primordial, but is
widely believed to be the result of evolutionary
effects subsequent to the formation of the galaxies.
mountain climate Climate of relatively high
elevations. Typically populated by northern (or
arctic) biome. Conditions include short growing
season (in nontropical mountains), decreased
nighttime temperature, reduced pressure and
oxygen availability, substantial rainfall, and increased
ultraviolet solar radiation.
moving magnetic features Small-scale regions
of magnetic flux which migrate across the
solar surface. When associated with the growth
of sunspots these flux regions emerge and approach
sunspots with speeds of 0.25-1kms −1 .
M star Star of spectral type M. Betelgeuse
and Antares are M stars.
MUF fading See skip fading.
multicell storm Storm with multicells. Multicell
storms come in a wide variety of shapes,
sizes, and intensities and most of them move
systematically toward the right of the environmental
winds in the middle troposphere. However,
the storm propagates toward the right in
discrete jumps as individual cells form and dissipate.
See supercell storm.
multifingered time The concept that, in general
relativity, a time function may be defined
which is very general, and in particular does not
correspond to uniform proper time advance into
the future. See proper time.
multipath fading Multipath, for any propagation
link, is the condition where more than
one ray can pass between the transmitter and the
receiver. Multipath, on an ionospheric propagation
path, is a common situation arising from the
different paths a ray can follow through the ionosphere.
The most obvious source of multipath
occurs when several different modes are excited
(e.g., the 2E-mode, 1F-mode, and 2F-mode may
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