6,603 research outputs found
Monochromatic knots and other unusual electromagnetic disturbances: light localised in 3D
We introduce and examine a collection of unusual electromagnetic disturbances. Each of these is an exact, monochromatic solution of Maxwell's equations in free space with looped electric and magnetic field lines of finite extent and a localised appearance in all three spatial dimensions. Included are the first explicit examples of monochromatic electromagnetic knots. We also consider the generation of our unusual electromagnetic disturbances in the laboratory, at both low and high frequencies, and highlight possible directions for future research, including the use of unusual electromagnetic disturbances as the basis of a new form of three-dimensional display
Two Days of My Six Year War: 9–10 July 1944
Early in July I was called back to Battery H.Q. and briefed on Operation “Charnwood,” after which I went back to the troop and told the sergeants who in turn gave the information to the crews so that everyone knew what was intended and could carry on should he end up as senior man. The gist of the briefing was that an attack was to start by the English on our left at 0700 hours and would proceed south through Buron, Gruchy and Authie just north of Carpiquet airport by mid-morning, after which the Canadian 9th Brigade on the left and the 7th Brigade on the right would move east through their ground and drive to Caen. General FU with whom the division was very familiar and who had a large part in our training in England, was very much in evidence. The English advance was held up and 9th Brigade, according to my notes, moved in and took the villages and 7th Brigade went east to the Abbaye Ardenne and Caen.
My recollection of one 48-hour period with the most action of any is as follows
Understanding solar cycle variability
The level of solar magnetic activity, as exemplified by the number of
sunspots and by energetic events in the corona, varies on a wide range of time
scales. Most prominent is the 11-year solar cycle, which is significantly
modulated on longer time scales. Drawing from dynamo theory together with
empirical results of past solar activity and of similar phenomena on solar-like
stars, we show that the variability of the solar cycle can be essentially
understood in terms of a weakly nonlinear limit cycle affected by random noise.
In contrast to ad-hoc `toy models' for the solar cycle, this leads to a generic
normal-form model, whose parameters are all constrained by observations. The
model reproduces the characteristics of the variable solar activity on time
scales between decades and millennia, including the occurrence and statistics
of extended periods of very low activity (grand minima). Comparison with
results obtained with a Babcock-Leighton-type dynamo model confirms the
validity of the normal-mode approach.Comment: ApJ, accepte
The crucial role of surface magnetic fields for the solar dynamo
Sunspots and the plethora of other phenomena occuring in the course of the
11-year cycle of solar activity are a consequence of the emergence of magnetic
flux at the solar surface. The observed orientations of bipolar sunspot groups
imply that they originate from toroidal (azimuthally orientated) magnetic flux
in the convective envelope of the Sun. We show that the net toroidal magnetic
flux generated by differential rotation within a hemisphere of the convection
zone is determined by the emerged magnetic flux at the solar surface and thus
can be calculated from the observed magnetic field distribution. The main
source of the toroidal flux is the roughly dipolar surface magnetic field at
the polar caps, which peaks around the minima of the activity cycle.Comment: This manuscript has been accepted for publication in Science. This
version has not undergone final editing. Please refer to the complete version
of record at http://www.sciencemag.org/. The manuscript may not be reproduced
or used in any manner that does not fall within the fair use provisions of
the Copyright Act without the prior, written permission of AAA
Optical activity in the scattering of structured light
We observe that optical activity in light scattering can be probed using types of illuminating light other than single plane (or quasi plane) waves and that this introduces new possibilities for the study of molecules and atoms. We demonstrate this explicitly for natural Rayleigh optical activity which, we suggest, could be exploited as a new form of spectroscopy for chiral molecules through the use of illuminating light comprised of two plane waves that are counter propagating
Energy conservation and the constitutive relations in chiral and non-reciprocal media
We consider the possibility that the chirality parameters and the non-reciprocity parameters appearing in the constitutive relations for the displacement and magnetic induction fields in a bi-isotropic medium might not be equal and thereby shed light on the physical significance of the fact that they are the same. We find, in particular, that they must be equal in order to retain the local conservation of energy
Inflows towards active regions and the modulation of the solar cycle: a parameter study
Aims: We aim to investigate how converging flows towards active regions
affect the surface transport of magnetic flux, as well as their impact on the
generation of the Sun's poloidal field. The inflows constitute a potential
non-linear mechanism for the saturation of the global dynamo and may contribute
to the modulation of the solar cycle in the Babcock-Leighton framework.
Methods: We build a surface flux transport code incorporating a parametrized
model of the inflows and run simulations spanning several cycles. We carry out
a parameter study to assess how the strength and extension of the inflows
affect the build-up of the global dipole field. We also perform simulations
with different levels of activity to investigate the potential role of the
inflows in the saturation of the global dynamo.
Results: We find that the interaction of neighbouring active regions can lead
to the occasional formation of single-polarity magnetic flux clumps
inconsistent with observations. We propose the darkening caused by pores in
areas of high magnetic field strength as a plausible mechanism preventing this
flux-clumping. We find that inflows decrease the amplitude of the axial dipole
moment by a , relative to a no-inflows scenario. Stronger (weaker)
inflows lead to larger (smaller) reductions of the axial dipole moment. The
relative amplitude of the generated axial dipole is about larger after
very weak cycles than after very strong cycles. This supports the inflows as a
non-linear mechanism capable of saturating the global dynamo and contributing
to the modulation of the solar cycle within the Babcock-Leighton framework
Surface flux transport simulations: Effect of inflows toward active regions and random velocities on the evolution of the Sun's large-scale magnetic field
Aims: We aim to determine the effect of converging flows on the evolution of
a bipolar magnetic region (BMR), and to investigate the role of these inflows
in the generation of poloidal flux. We also discuss whether the flux dispersal
due to turbulent flows can be described as a diffusion process.
Methods: We developed a simple surface flux transport model based on
point-like magnetic concentrations. We tracked the tilt angle, the magnetic
flux and the axial dipole moment of a BMR in simulations with and without
inflows and compared the results. To test the diffusion approximation,
simulations of random walk dispersal of magnetic features were compared against
the predictions of the diffusion treatment.
Results: We confirm the validity of the diffusion approximation to describe
flux dispersal on large scales. We find that the inflows enhance flux
cancellation, but at the same time affect the latitudinal separation of the
polarities of the bipolar region. In most cases the latitudinal separation is
limited by the inflows, resulting in a reduction of the axial dipole moment of
the BMR. However, when the initial tilt angle of the BMR is small, the inflows
produce an increase in latitudinal separation that leads to an increase in the
axial dipole moment in spite of the enhanced flux destruction. This can give
rise to a tilt of the BMR even when the BMR was originally aligned parallel to
the equator
Reply to comment on `Energy conservation and the constitutive relations in chiral and non-reciprocal media'
We respond to the comment by Mackay and Lakhtakia on our paper. These authors have missed the simple point that our chirality and non-reciprocity parameters are real. The 'inconsistency' claimed by them emerges from their incorrect attempt to apply our results instead to complex chirality and non-reciprocity parameters
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