Reduction Of Magnetic Field Strength Research Articles

Longitudinal modulation of electron and mass densities at middle and auroral latitudes: Effect of geomagnetic field strength

AbstractIn this study, the effect of a reduction in magnetic field strength on the longitudinal modulation of electron density (ΔNe) and mass density (Δρ) is investigated. We study conditions during the September equinox and June and December solstices at middle and auroral latitudes in both Northern and Southern Hemispheres. The Global Ionosphere‐Thermosphere Model was employed for the theoretical study. Simulations were performed with a realistic International Geomagnetic Reference Field (IGRF) model and an IGRF model reduced in strength by 50%. The reduction in field strength increased the longitudinal variations in ΔNe by 2–34% in the nighttime sector but reduced the variation by 7–20% in the daytime sector at midlatitudes in both hemispheres. Conversely, at auroral latitudes ΔNe was reduced by 2–32% at all local times. The changes during equinoxes are larger than those for the solstices. The differences with local time in the response of ΔNe to a reduction in magnetic field strength were mainly due to changes in the oxygen/molecular nitrogen (O/N2) ratio in the Northern Hemisphere, whereas the differences were due to the combined effects of vertical winds and changes in the O/N2 ratio in the Southern Hemisphere. Both vertical winds and horizontal convection eliminated ΔNe at high latitudes. Δρ got enhanced at both middle and auroral latitudes, which was related to the enhanced Joule heating effect when the magnetic field was reduced.

Reconstruction of propagating Kelvin–Helmholtz vortices at Mercury's magnetopause

A series of quasi-periodic magnetopause crossings were recorded by the MESSENGER spacecraft during its third flyby of Mercury on 29 September 2009, likely caused by a train of propagating Kelvin–Helmholtz (KH) vortices. We here revisit the observations to study the internal structure of the waves. Exploiting MESSENGER's rapid traversal of the magnetopause, we show that the observations permit a reconstruction of the structure of a rolled-up KH vortex directly from the spacecraft's magnetic field measurements. The derived geometry is consistent with all large-scale fluctuations in the magnetic field data, establishes the non-linear nature of the waves, and shows their vortex-like structure. In several of the wave passages, a reduction in magnetic field strength is observed in the middle of the wave, which is characteristic of rolled-up vortices and is related to the increase in magnetic pressure required to balance the centrifugal force on the plasma in the outer regions of a vortex, previously reported in computer simulations. As the KH wave starts to roll up, the reconstructed geometry suggests that the vortices develop two gradual transition regions in the magnetic field, possibly related to the mixing of magnetosheath and magnetospheric plasma, situated at the leading edges from the perspectives of both the magnetosphere and the magnetosheath.

Shielding Performance of Open-Type Magnetic Shielding Box Structure

The reduction of magnetic field strength at the center of a shielding box located in a uniform magnetic field has been reported in many studies. The space in the box was completely enclosed by magnetic materials to effectively reduce penetration of outside magnetic fields into the box. This conventional sealed box method is thought to be complicated for practical use. In contrast, this study proposes the magnetic shielding box to be a ldquolattice frame structurerdquo using the open-type magnetic shielding method. Each lattice contains the same structure in which closed-loop frames align to form a box with gaps. Small, medium, and large lattice boxes are put together with the center axes orthogonal to each other. This study reports magnetic shielding performance for the open-type magnetic shielding boxes of single, double, and triple lattice frames in a uniform magnetic field with some experimental comparison to the conventional sealed box method.

CURRENT-CARRYING CAPACITY OF SUPERCONDUCTING Nb–Zr SOLENOIDS

Explanation of the discrepancy between Nb -- Zr short sample tests and solenoid performance and prediction of the current carrying capacity of Nb -- Zr solenoids are based on the mechanism of induced persistent-eddy currents. Study of residual fields following reduction of magnetic field strength to zero leads to assumptions permitting calculations of circulating current magnitudes. Data are presented graphically, and generation of the same field using less material is found to result from spreading out layers or turns. (D.C.W.)