48 research outputs found
Enhancement of electric and magnetic wave fields at density gradients
We use Freja satellite data to investigate irregular small-scale density variations. The observations are made in the auroral region at about 1000-1700 km. The density variations are a few percent, and the structures are found to be spatial down to a scale length of a few ion gyroradii. Irregular density variations are often found in an environment of whistler mode/lower hybrid waves and we show that at the density gradients both the electric and magnetic wave fields are enhanced
A statistical study of ion energization at 1700 km in the auroral region
We present a
comprehensive overview of several potentially relevant causes for the oxygen
energization in the auroral region. Data from the Freja satellite near 1700 km
altitude are used for an unconditional statistical investigation. The data are
obtained in the Northern Hemisphere during 21 months in the declining phase of
the solar cycle. The importance of various wave types for the ion energization
is statistically studied. We also investigate the correlation of ion heating
with precipitating protons, accelerated auroral electrons, suprathermal
electron bursts, the electron density variations, Kp index and solar
illumination of the nearest conjugate ionosphere. We find that sufficiently
strong broad-band ELF waves, electromagnetic ion cyclotron waves, and waves
around the lower hybrid frequency are foremost associated with the ion heating.
However, magnetosonic waves, with a sharp, lower frequency cutoff just below
the proton gyrofrequency, are not found to contribute to the ion heating. In
the absence of the first three wave emissions, transversely energized ions are
rare. These wave types are approximately equally efficient in heating the ions,
but we find that the main source for the heating is broadband ELF waves, since
they are most common in the auroral region. We have also observed that the
conditions for ion heating are more favourable for smaller ratios of the
spectral densities SE /SB of the broadband ELF waves at the oxygen
gyrofrequency.Key words. Ionosphere (auroral
ionosphere; wave propogation) Magnetospheric physics (electric fields
Azimuthal velocity shear within an Earthward fast flow – further evidence for magnetotail untwisting?
It is well known that nonzero IMF By conditions
lead to a twisted magnetotail configuration. The plasma sheet
is rotated around its axis and tail magnetic field lines are
twisted, which causes an azimuthal displacement of their
ionospheric footprints. According to the untwisting hypothesis,
the untwisting of twisted field lines is suggested to influence
the azimuthal direction of convective fast flows in the
nightside geospace. However, there is a lack of in situ magnetospheric
observations, which show actual signatures of
the possible untwisting process. In this paper, we report detailed
Cluster observations of an azimuthal flow shear across
the neutral sheet associated with an earthward fast flow on
5 September 2001. The observations show a flow shear velocity
pattern with a V⊥y sign change, near the neutral sheet (Bx ∼ 0) within a fast flow during the neutral sheet flapping
motion over the spacecraft. Firstly, this implies that convective
fast flows may not generally be unidirectional across the
neutral sheet, but may have a more complex structure. Secondly,
in this event tail By and the flow shear are as expected by the untwisting hypothesis. The analysis of the flow shear
reveals a linear dependence between the Bx and V⊥y close to
the neutral sheet and suggests that Cluster crossed the neutral
sheet in the dawnward part of the fast flow channel. The magnetospheric
observations are supported by the semi-empirical T96 and TF04 models. Furthermore, the ionospheric SuperDARN
convection maps support the satellite observations
proposing that the azimuthal component of the magnetospheric
flows is enforced by a magnetic field untwisting. In
summary, the observations give strong supportive evidence to the tail untwisting hypothesis. However, the T96 ionospheric
mapping demonstrates the limitations of the model in mapping
from a twisted tail
Disruption of Cell-to-Cell Signaling Does Not Abolish the Antagonism of Phaeobacter gallaeciensis toward the Fish Pathogen Vibrio anguillarum in Algal Systems
Quorum sensing (QS) regulates Phaeobacter gallaeciensis antagonism in broth systems; however, we demonstrate here that QS is not important for antagonism in algal cultures. QS mutants reduced Vibrio anguillarum to the same extent as the wild type. Consequently, a combination of probiotic Phaeobacter and QS inhibitors is a feasible strategy for aquaculture disease control