126 research outputs found
Reply to comment by H. Hasegawa on "Evolution of Kelvin-Helmholtz activity on the dusk flank magnetopause"
No full textWe demonstrate, on experimental grounds, that the justifications for the comment by Hasegawa [2009], hereinafter
H09, on work done by Foullon et al. [2008], hereinafter F08, are not well founded
Fast magnetoacoustic waves in curved coronal loops. II, Tunneling modes
No full textAims. Fast magnetoacoustic waves in curved coronal loops are investigated and the role of lateral leakage in wave damping, which includes the mechanism of wave tunneling, is explored.
Methods. A coronal loop is modeled as a curved, magnetic slab in the zero plasma-β limit. In this model and for an arbitrary piece-wise continuous power law equilibrium density profile, the wave equation governing linear vertically polarised fast magnetoacoustic waves is solved
analytically. An associated dispersion relation is derived and the frequencies and eigenfunctions of the wave modes are characterised.
Results. For some equilibria, the waves are shown to be all damped due to lateral leakage. It is demonstrated that waves either leak straight out into the external medium or have to overcome an evanescent barrier, which is linked to wave tunneling. The wave solutions consist of alternating vertically polarised kink and sausage branches. Fast kink oscillations may have a non-zero density perturbation when
averaged across the loop. The calculated damping rate of fast magnetoacoustic kink oscillations is shown to be consistent with related numerical simulations and show that lateral leakage may explain the observed damping of (vertically polarised) fast magnetoacoustic kink oscillations
Coronal Alfvén speed determination : consistency between seismology using AIA/SDO transverse loop oscillations and magnetic extrapolation
No full textTwo transversely oscillating coronal loops are investigated in detail during a flare on the 6th September 2011 using data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory (SDO).We compare two independent methods to determine the Alfvén speed inside these loops. Through the period of oscillation and loop length information about the Alfvén speed inside each loop is deduced seismologically. This is compared with the Alfvén speed profiles deduced from magnetic extrapolation and spectral methods using AIA bandpass. We find that for both loops the two methods are consistent. Also, we find that the average Alfvén speed based on loop travel time is not necessarily a good measure to compare with the seismological result, which explains earlier reported discrepancies. Instead, the effect of density and magnetic stratification on the wave mode has to be taken into account. We discuss the implications of combining seismological, extrapolation and spectral methods in deducing the physical properties of coronal loops
Quasi-periodic pulsations in the gamma-ray emission of a solar flare
No full textQuasi-periodic pulsations (QPPs) of gamma-ray emission with a period of about 40 s are found in a single loop X-class solar flare on 2005 January 1 at photon energies up to 2-6 MeV with the SOlar Neutrons and Gamma-rays (SONG) experiment aboard the CORONAS-F mission. The oscillations are also found to be present in the microwave emission detected with the Nobeyama Radioheliograph, and in the hard X-ray and low energy gamma-ray channels of RHESSI. Periodogram and correlation analysis shows that the 40 s QPPs of microwave, hard X-ray, and gamma-ray emission are almost synchronous in all observation bands. Analysis of the spatial structure of hard X-ray and low energy (80-225 keV) gamma-ray QPP with RHESSI reveals synchronous while asymmetric QPP at both footpoints of the flaring loop. The difference between the averaged hard X-ray fluxes coming from the two footpoint sources is found to oscillate with a period of about 13 s for five cycles in the highest emission stage of the flare. The proposed mechanism generating the 40 s QPP is a triggering of magnetic reconnection by a kink oscillation in a nearby loop. The 13 s periodicity could be produced by the second harmonics of the sausage mode of the flaring loop
Multi-scale Structure Analyses of Magnetopause Kelvin-Helmholtz Waves: Applications of Four-spacecraft to MHD Simulations and Cluster and MMS Observations
No full textMagnetopause Kelvin-Helmholtz (KH) waves involve complex magnetic and flow structures that facilitate solar wind plasma transport. These plasma structures are resolved in 3-D with four-spacecraft forming a tetrahedral configuration. This thesis considers applications of the Magnetic Curvature Analysis (MCA) and vorticity analysis techniques to characterise in-situ KH waves. The techniques are applied onto 2.5-D magnetohydrodynamics (MHD) simulations of the waves on the dusk-side magnetopause using the varying (regular) tetrahedron size of the virtual probes to interpret four-spacecraft observations with a certain tetrahedron size. The main results of this analysis are (1) the characterisation of the KH vortex regions using the magnetic curvature and flow vorticity and (2) the dependence of the four-spacecraft measures on the spacecraft tetrahedron size. In particular, the negative vorticity, developed next to the positive vorticity of the vortex core, on the dusk-side magnetopause pertains to rolled-up vortex and reminisces of rolled-up vortex history. The dependence of the MCA properties on the tetrahedron size can be attributed to non-linear spatial variations of the magnetic structures that could be resolved by nested spacecraft tetrahedrons. Cluster and MMS are the four-spacecraft missions that are probing the Earth's magnetospheric environments in multi-scales. Both results are confirmed using Cluster observations, though other properties that can be linked to the excitation of the KH instability and non-linear KH development are also deduced. Applications of the techniques on small-scale structures observed by MMS are illustrated during KH events, featuring mid-latitude reconnection, a Flux Transfer Event, and a magnetic island. This thesis contributes to our understanding of multi-scale structures of the magnetopause KH waves that could shed light on how KH-associated mechanisms operate to allow solar wind plasma entries. For a comprehensive understanding of the KH phenomenon, a cross-scale coverage of KH observations would be desirable.Science and Technology Facilities Counci
Quasi-periodic modulation of solar and stellar flaring emission by magnetohydrodynamic oscillations in a nearby loop
No full textWe propose a new model for quasi-periodic modulation of solar and stellar flaring emission. Fast magnetoacoustic oscillations of a non-flaring loop can interact with a nearby flaring active region. This interaction occurs when part of the oscillation situated outside the loop reaches the regions of steep gradients in magnetic field within an active region and produces periodic variations of electric
current density. The modulation depth of these variations is a few orders of magnitude greater than the amplitude of the driving oscillation. The variations of the current can induce current-driven plasma micro-instabilities and thus anomalous resistivity. This can periodically trigger magnetic reconnection, and hence acceleration of charged particles, producing quasi-periodic pulsations of X-ray,
optical and radio emission at the arcade footpoints
Magnetic Helicity Flow in the Sun and Heliosphere
No full textMagnetic helicity, the measure of entanglement within a magnetic field, has the capability to further our knowledge of the magnetic fields which are ubiquitous across the physical universe. Discovered half a century ago by Lodewijk Woltjer in 1958, it was only given physical meaning by Keith Moffatt in 1969. Progress was initially slow due to the constraints on its calculation: it is assumed that the volume within which we wish to measure helicity does not have any magnetic field crossing its boundaries. But, in 1984, Mitchell Berger and George Field provided a resolution to this problem which allowed it to be applied to open astrophysical fields. From there, and particularly in the last two decades, interest in magnetic helicity has grown exponentially within the research community, resulting in this thesis. We will begin by providing a semi--formal introduction to the topic, in particular that of magnetohydrodynamics, which describes how a magnetic field and associated plasma co-interact. We provide a mathematical introduction to magnetic helicity, and demonstrate that unsolved problems remain in our understanding of the Sun's magnetic field that are associated with its magnetic helicity. With this knowledge in hand, we first tackle the topic of predicting the Solar Cycle, which has been an unachieved goal of the solar physics community for longer than we care to remember. We show that magnetic helicity, which is intrinsically linked to the emergence of sunspots, is a statistically stronger candidate for the predictor of activity than that of the polar field strength, which is the current 'best of the worst' of the known predictors. We then, for the first time, measure how much helicity is generated on the solar surface due to shear motions in a surface flux transport model, which is a method of modelling the magnetic field on the surface of the sun. We show that the results are not as obvious as we expect, and indeed that the flux of magnetic helicity within each hemisphere is carefully balanced between latitudes. We also provide an estimate of how much helicity is produced in a solar cycle, and correlate this with the dipole strength of that cycle. This is followed by the main result of the thesis: we demonstrate that helicity can be completely generalised for any physical system in terms of a two--point correlation, and fully described in terms of spatial scales and locality using wavelet analysis. In particular, we show that our generalised measure of helicity offers a physical meaning to this localisation. Our methods are demonstrated to have some notable advantages to that of Fourier analysis, which is shown to sometimes produce spurious results. Finally, we explore the hypothesis that the shape of a magnetic field domain can contribute to the magnetic helicity when using a toroidal--poloidal decomposition. Indeed, in some cases the asymmetry contains the entirety of the magnetic helicity, which we demonstrate numerically
Evolution of Kelvin-Helmholtz activity on the dusk flank magnetopause
No full textOur purpose is to characterize the evolution of the magnetopause Kelvin-Helmholtz
(KH) wave activity with changes in thickness of the adjacent boundary layer,
geomagnetic latitude and interplanetary magnetic field (IMF) orientation. As the IMF
turns northward, wave activity may be generated at the dayside before propagating
down the tail, where the boundary layer is expected to support longer wavelengths. We
use two-point observations on the dusk magnetopause at low latitudes, from Geotail on
the dayside and Cluster tailward of the dusk terminator. We quantify the wavelength,
power, wavefront steepness and propagation direction at Cluster. An estimate of the
thickness of the low-latitude boundary layer (LLBL) is obtained by correlating normal
distances to the magnetopause, derived from two empirical solar-wind-driven models,
with a systematic relationship (the "transition parameter") found between the electron
number density and temperature; the correlation factor is used to infer the temporal
evolution of the thickness of the locally sampled layer. We find that wavelengths are
controlled by the IMF clock angle, as expected when generated by the KH mechanism
at the dayside, although amplitudes, wavefront steepness and propagation directions are
more closely correlated with the layer thickness. A survey of parameter space provides
evidence of the contribution of the KH mechanism to the widening of the electron LLBL
Dual trigger of transverse oscillations in a prominence by EUV fast and slow coronal waves : SDO/AIA and STEREO/EUVI observations
No full textWe analyze flare-associated transverse oscillations in a quiescent solar prominence on 2010 September 8-9. Both the flaring active region and the prominence were located near the west limb, with a favorable configuration and viewing angle. The full-disk extreme ultraviolet (EUV) images of the Sun obtained with high spatial and temporal resolution by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory show flare-associated lateral oscillations of the prominence sheet. The STEREO-A spacecraft, 81.°5 ahead of the Sun-Earth line, provides an on-disk view of the flare-associated coronal disturbances. We derive the temporal profile of the lateral displacement of the prominence sheet by using the image cross-correlation technique. The displacement curve was de-trended and the residual oscillatory pattern was derived. We fit these oscillations with a damped cosine function with a variable period and find that the period is increasing. The initial oscillation period (P0) is ∼28.2 minutes and the damping time (τD) ∼ 44 minutes. We confirm the presence of fast and slow EUV wave components. Using STEREO-A observations, we derive a propagation speed of ∼250 km s–1 for the slow EUV wave by applying the time-slice technique to the running difference images. We propose that the prominence oscillations are excited by the fast EUV wave while the increase in oscillation period of the prominence is an apparent effect, related to a phase change due to the slow EUV wave acting as a secondary trigger. We discuss implications of the dual trigger effect for coronal prominence seismology and scaling law studies of damping mechanisms
Study of nonthermal continuum patches : wave propagation and plasmapause study
No full textNonthermal continuum (NTC) radiation is believed to be emitted at the plasmapause and near the magnetic equator. We present a particular type of NTC radiation, referred to as NTC patch, which appears over a wide frequency range and within a relatively short time interval. NTC patches are observed in all magnetospheric plasma environments of the Cluster 2 orbit and are shown to represent a quarter of the NTC events observed in 2003. A statistical analysis of the frequency pattern performed on the 2003 Cluster 2 Waves of High frequency and Sounder for Probing of Electron Density by Relaxation data indicates that the NTC patches can be divided into two classes: Those with banded emission in frequency are only observed close to the source region and are thus termed "plasmaspheric," while the others, nonbanded, are termed "outer magnetospheric." In an event on 26 September 2003, we localize the sources positions and study the expected propagation of each NTC frequency beam of a plasmaspheric patch. From the observations, we show that the sources are located very close to the satellite and to each other at positions projected on the XY GSE plane. Using a ray tracing code, we demonstrate that, close to the source regions, the satellite observes all frequency rays at the same time which overlap in the spectrogram making up the plasmaspheric patch. After the satellite crossing, the rays follow diverging paths and cannot therefore be observed further out by the same satellite simultaneously. Plasmaspheric patches are thus specific signatures of close and distorted source regions
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