DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

esidual circulation
of the water in a basin (e.g., the Mediterranean)
or region to be replaced. For systems at steady
state, residence time can be easily calculated for
a reservoirS because the inflow (i) and outflow
(q) rates are identical: TR = S q = S i .
residual circulation The time-averaged circulation
in a water body, after averaging over a
long period (greater than the wind wave period
and tidal period).
resolution A measure of the minimum angular
separation between two sources at which
theycanunambiguouslybedistinguishedasseparate.
resonance In planetary dynamics, an orbital
condition in which one object is periodically
subjected to the gravitational perturbations
caused by another object. The two bodies are
usually in orbit around a third, more massive
object and their orbital periods are some whole
number ratios of each other. For example, Io and
Europa are in a resonance as they orbit around
Jupiter: Io orbits twice for every orbit of Europa.
Europa and Ganymede are also in a resonance,
with Europa orbiting twice for every orbit of
Ganymede. In the case of Io and Europa, the
resonances cause tidal heating to be a major internal
heat source for both objects. Resonances
can also create gaps, such as the Kirkwood Gaps
within the asteroid belt (when asteroids are in
resonance with Jupiter) and several of the gaps
withinSaturn’srings(causedbyresonanceswith
some of Saturn’s moons).
resonance scattering A magnetohydrodynamical
phenomenon: the scattering of particles
by waves can be described as a random
walk process if the individual interactions lead
to small changes in pitch-angle only. Thus, a
reversal of the particle’s direction of propagation
requires a large number of such small-angle
scatters. If, however, the particle motion is in
resonance with the wave, the scattering is more
efficient because the small-angle scatterings all
work together in one direction instead of mostly
cancelling each other. Thus, pitch-angle scattering
will mainly occur at the magnetic field
fluctuations with wavelength in resonance with
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the particle motion parallel to the field:
k = ωc
v
= ωc
µv
with k being the wave number of the waves
leading to the particle scattering,v the particle
speed parallel to the magnetic field, and µ the
cosine of the particle’s pitch-angle.
Resonance scattering.
From the resonance condition we can see that
for a given particle speed particles resonate with
different waves, depending on their pitch-angle.
Since the amount of scattering a particle experiences
basically depends on the power density
f(k) of the waves at the resonance frequency,
scattering is different for particles with different
pitch-angles although their energy might be the
same. Therefore, the pitch-angle diffusion coefficientκ(µ)
depends on pitch-angle thoughµ.
See slab model.
resonant absorption In solar physics, in a
closed magnetic loop, resonant frequencies appear
at multiples of vA/2L, where vA = B/
4πmpn is the Alfvén speed andL is the coronal
length of the loop. The resonances occur because
of reflections off the transition region parts
of the loop. Resonant absorption occurs when
the frequency of a loop oscillation matches the
local Alfvén frequency. This creates a resonant
layer in which there is a continuous accumulation
of energy and, consequently, may result in
the heating of the coronal plasma. See Alfvén
speed.
resonant damping and instability In a collisionless
plasma, wave damping or growth associated
with the interaction between a wave
and particles moving with a velocity such that
the wave frequency is (approximately) Dopplershifted
to zero, or to a multiple of the particle’s
Larmor frequency. Thus, in a magnetized