DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

energy for stars as early as 1925 by Sir Arthur
Eddington and Hans Bethe and others. The interaction
between two protons or a proton and
a heavier nucleus is a quantum mechanical process
in which barrier penetration must occur. As
a result, hydrogen burning occurs at a temperature
of 10 to 20 million K in stars of low to
high mass (0.085 to 100 solar masses). Lower
mass configurations do not reach this temperature
range, derive little or no energy from hydrogen
burning, and are called brown dwarfs. See
brown dwarf, CNO cycle, main sequence star,
proton-proton chain, red giant.
hydrograph A plot showing discharge vs.
time. Commonly used to illustrate flood events
on rivers or creeks.
hydrographic survey A term often used synonymously
with bathymetric survey. A survey
of the geometry of the seafloor. Typically involves
one system for establishment of horizontal
position and another for vertical position or
water depth. Traditionally performed from a
boat, but amphibious vehicles, sleds, and helicopters
have also been used.
hydrologic equation A water balance equation
often used in catchment hydrology based
on the law of mass conservation that states that
basin outputs are related to basin inputs plus or
minus changes in storage, and written in simplified
form as Q=P−ET ±S, where Q
is surface water runoff at the basin outlet, P is
precipitation input to the basin, ET are evapotranspiration
losses of water from the basin, and
S is the change in storage of water in the basin.
hydromagnetic Pertaining to the macroscopic
behavior of a magnetized electrically
conducting fluid or plasma. Hydromagnetic
phenomena are generally associated with large
length scales compared with the Larmor radii
and long time scales compared with the Larmor
periods of the particles that comprise the
fluid. Theoretical descriptions of such phenomena
may be based on magnetohydrodynamics
or kinetic theory. See magnetohydrodynamics,
Vlasov–Maxwell equations.
© 2001 by CRC Press LLC
hydromagnetic turbulence
hydromagnetics See magnetohydrodynamics.
hydromagnetic shock wave An abrupt transition
between two regions in a magnetized
plasma, analogous to the acoustic shock wave in
air. Hydromagnetic shock waves propagate relative
to the plasma, and may be regarded as the
short-wavelength limit of large-amplitude magnetoacoustic
waves. There are fast and slow
shock waves that correspond, respectively, to
the fast and slow magnetoacoustic wave modes;
a large-amplitude wave of either mode may
steepen to form a shock.
In a reference frame in which a shock is at
rest, the flow speed of the plasma upstream of
the shock is greater than the (fast or slow) magnetoacoustic
wave speed, and downstream this
inequality is reversed. As in the case of shocks
in an ordinary gas, plasma flowing through a
hydromagnetic shock undergoes heating and associated
entropy production. In ideal magnetohydrodynamics
a shock is infinitely thin; real
hydromagnetic shocks have a structured transition
region of finite thickness, comparable to the
ion Larmor radius in many situations.
Hydromagnetic shock waves are routinely
observed in situ in collisionless plasmas in
space. The high-speed solar wind passes
through a bow shock when it encounters a planetary
or cometary obstacle. From time to time
material ejected from the solar corona drives
transient shock waves into the heliosphere; if
such a shock encounters a planetary magnetic
field, it may produce magnetic storms and aurorae
in the planetary magnetosphere and upper
atmosphere. Shock waves can also be produced
internally in the solar wind as fast streams catch
up with slower streams; these shocks are frequently
observed from about 3 to about 10 astronomical
units from the sun. See hydromagnetic
wave.
hydromagnetic turbulence The turbulent,
irregular fluctuations that can arise in a magnetized
fluid or plasma. Just as turbulence arises at
high Reynolds number in an ordinary unmagnetized
fluid, hydromagnetic turbulence is associated
with high magnetic Reynolds number (the
ordinary Reynolds number with kinematic viscosity
replaced by electrical resistivity). Mag-
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