57 research outputs found
Oxygen ion energization by waves in the high altitude cusp and mantle
We present a comparative study of low frequency electric field spectral
densities and temperatures observed by the Cluster spacecraft in the high
altitude cusp/mantle region. We compare the relation between the O+
temperature and wave intensity at the oxygen gyrofrequency at each
measurement point and find a clear correlation. The trend of the correlation
agrees with the predictions by both an asymptotic mean-particle theory and a
test-particle approach. The perpendicular to parallel temperature ratio is
also consistent with the predictions of the asymptotic mean-particle theory.
At times the perpendicular temperature is significantly higher than predicted
by the models. A simple study of the evolution of the particle distributions
(conics) at these altitudes indicates that enhanced perpendicular
temperatures would be observed over many RE after heating ceases.
Therefore, sporadic intense heating is the likely explanation for cases with
high temperature and comparably low wave activity. We observe waves of
sufficient amplitude to explain the highest observed temperatures, while the
theory in general overestimates the temperature associated with the highest
observed wave activity, indicating that such high wave activity is very
sporadic
Localized practices and globalized futures: challenges for Alaska coastal community youth
An article from Maritime Studies (2015) 14:
O+ heating associated with strong activity in the high altitude cusp and mantle
We use the Cluster spacecraft to study three events with intense waves and energetic oxygen ions (O+) in the high altitude cusp and mantle. The ion energies considered are of the order 1000 eV and higher, observed above an altitude of 8 earth radii together with high wave power at the O+ gyrofrequency. We show that heating by waves can explain the observed high perpendicular energy of O+ ions, using a simple gyroresonance model and 25â45% of the observed wave spectral density at the gyrofrequency. This is in contrast to a recently published study where the wave intensity was too low to explain the observed high altitude ion energies. Long lasting cases (>10 min) of high perpendicular-to-parallel temperature ratios are sometimes associated with low wave activity, suggesting that high perpendicular-to-parallel temperature ratio is not a good indicator of local heating. Using multiple spacecraft, we show that the regions of enhanced wave activity are at least one order of magnitude larger than the gyroradius of the heated ions
Statistical evidence for O<sup>+</sup> energization and outflow caused by wave-particle interaction in the high altitude cusp and mantle
We present a statistical study of the low (<1 Hz) frequency electric and
magnetic field spectral densities observed by Cluster spacecraft in the high
altitude cusp and mantle region. At the O+ gyrofrequency (0.02â0.5 Hz)
for this region the electric field spectral density is on average 0.2â2.2 (mV mâ1)2 Hzâ1,
implying that resonant heating at the gyrofrequency can be
intense enough to explain the observed O+ energies of 20â1400 eV. The
relation between the electric and magnetic field spectral densities results
in a large span of phase velocities, from a few hundred km sâ1 up to a few
thousand km sâ1. In spite of the large span of phase velocity, the ratio between
the calculated local Alfvén velocity and the estimated phase velocity is
close to unity. We provide average values of a coefficient describing
diffusion in ion velocity space at different altitudes, which can be used in
studies of ion energization and outflow. The observed average waves can
explain the average O+ energies measured in the high altitude (8â15 RE)
cusp/mantle region of the terrestrial magnetosphere according to our
test particle calculations
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