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