5,626 research outputs found
Relationships between various characterisations of wave tails
One can define several properties of wave equations that correspond to the
absence of tails in their solutions, the most common one by far being Huygens'
principle. Not all of these definitions are equivalent, although they are
sometimes assumed to be. We analyse this issue in detail for linear scalar
waves, establishing some relationships between the various properties. Huygens'
principle is almost always equivalent to the characteristic propagation
property, and in two spacetime dimensions the latter is equivalent to the
zeroth order progressing wave propagation property. Higher order progressing
waves in general do have tails, and do not seem to admit a simple physical
characterisation, but they are nevertheless useful because of their close
association with exactly solvable two-dimensional equations.Comment: Plain TeX, 26 page
Chromospheric Velocities of a C-class Flare
We use high spatial and temporal resolution observations from the Swedish
Solar Telescope to study the chromospheric velocities of a C-class flare
originating from active region NOAA 10969. A time-distance analysis is employed
to estimate directional velocity components in H-alpha and Ca II K image
sequences. Also, imaging spectroscopy has allowed us to determine flare-induced
line-of-sight velocities. A wavelet analysis is used to analyse the periodic
nature of associated flare bursts. Time-distance analysis reveals velocities as
high as 64 km/s along the flare ribbon and 15 km/s perpendicular to it. The
velocities are very similar in both the H-alpha and Ca II K time series.
Line-of-sight H-alpha velocities are red-shifted with values up to 17 km/s. The
high spatial and temporal resolution of the observations have allowed us to
detect velocities significantly higher than those found in earlier studies.
Flare bursts with a periodicity of approximately 60 s are also detected. These
bursts are similar to the quasi-periodic oscillations observed at hard X-ray
and radio wavelength data. Some of the highest velocities detected in the solar
atmosphere are presented. Line-of-sight velocity maps show considerable mixing
of both the magnitude and direction of velocities along the flare path. A
change in direction of the velocities at the flare kernel has also been
detected which may be a signature of chromospheric evaporation.Comment: Accepted for publication in Astronomy and Astrophysics, 5 figure
The application of a Trous wave filtering and Monte Carlo analysis on SECIS 2001 solar eclipse observations
8000 images of the Solar corona were captured during the June 2001 total
Solar eclipse. New software for the alignment of the images and an automated
technique for detecting intensity oscillations using multi scale wavelet
analysis were developed. Large areas of the images covered by the Moon and the
upper corona were scanned for oscillations and the statistical properties of
the atmospheric effects were determined. The a Trous wavelet transform was used
for noise reduction and Monte Carlo analysis as a significance test of the
detections. The effectiveness of those techniques is discussed in detail.Comment: 17 pages, 8 figures, accepted by Solar Physics Journal for
publication in Topical Issue: "Frontiers in Solar Image Processing
Calibration of centre-of-mass energies at LEP 2 for a precise measurement of the W boson mass
The determination of the centre-of-mass energies for all LEP 2 running is
presented. Accurate knowledge of these energies is of primary importance to set
the absolute energy scale for the measurement of the W boson mass. The beam
energy between 80 and 104 GeV is derived from continuous measurements of the
magnetic bending field by 16 NMR probes situated in a number of the LEP
dipoles. The relationship between the fields measured by the probes and the
beam energy is defined in the NMR model, which is calibrated against precise
measurements of the average beam energy between 41 and 61 GeV made using the
resonant depolarisation technique. The validity of the NMR model is verified by
three independent methods: the flux-loop, which is sensitive to the bending
field of all the dipoles of LEP; the spectrometer, which determines the energy
through measurements of the deflection of the beam in a magnet of known
integrated field; and an analysis of the variation of the synchrotron tune with
the total RF voltage. To obtain the centre-of-mass energies, corrections are
then applied to account for sources of bending field external to the dipoles,
and variations in the local beam energy at each interaction point. The relative
error on the centre-of-mass energy determination for the majority of LEP 2
running is 1.2 x 10^{-4}, which is sufficiently precise so as not to introduce
a dominant uncertainty on the W mass measurement.Comment: 79 pages, 45 figures, submitted to EPJ
Evaluation of the LEP Centre-of-Mass Energy Above the W-Pair Production Threshold
Knowledge of the centre-of-mass energy at LEP2 is of primary importance to
set the absolute energy scale for the measurement of the W-boson mass. The beam
energy above 80 GeV is derived from continuous measurements of the magnetic
bending field by 16 NMR probes situated in a number of the LEP dipoles. The
relationship between the fields measured by the probes and the beam energy is
calibrated against precise measurements of the average beam energy between 41
and 55 GeV made using the resonant depolarisation technique. The linearity of
the relationship is tested by comparing the fields measured by the probes with
the total bending field measured by a flux loop. This test results in the
largest contribution to the systematic uncertainty. Several further corrections
are applied to derive the the centre-of-mass energies at each interaction
point. In addition the centre-of-mass energy spread is evaluated. The beam
energy has been determined with a precision of 25 MeV for the data taken in
1997, corresponding to a relative precision of 2.7x10^{-4}. This is small in
comparison to the present uncertainty on the W mass measurement at LEP.
However, the ultimate statistical precision on the W mass with the full LEP2
data sample should be around 25 MeV, and a smaller uncertainty on the beam
energy is desirable. Prospects for improvements are outlined.Comment: 24 pages, 10 figures, Latex, epsfig; replaced by version accepted by
European Physical Journal
Observed Effect of Magnetic Fields on the Propagation of Magnetoacoustic Waves in the Lower Solar Atmosphere
We study Hinode/SOT-FG observations of intensity fluctuations in Ca II H-line
and G-band image sequences and their relation to simultaneous and co-spatial
magnetic field measurements. We explore the G-band and H-line intensity
oscillation spectra both separately and comparatively via their relative phase
differences, time delays and cross-coherences. In the non-magnetic situations,
both sets of fluctuations show strong oscillatory power in the 3 - 7 mHz band
centered at 4.5 mHz, but this is suppressed as magnetic field increases. A
relative phase analysis gives a time delay of H-line after G-band of 20\pm1 s
in non-magnetic situations implying a mean effective height difference of 140
km. The maximum coherence is at 4 - 7 mHz. Under strong magnetic influence the
measured delay time shrinks to 11 s with the peak coherence near 4 mHz. A
second coherence maximum appears between 7.5 - 10 mHz. Investigation of the
locations of this doubled-frequency coherence locates it in diffuse rings
outside photospheric magnetic structures. Some possible interpretations of
these results are offered.Comment: 19 pages, 6 figure
Twisting Flux Tubes as a cause of Micro-Flaring Activity
High-cadence optical observations of an H-alpha blue-wing bright point near
solar AR NOAA 10794 are presented. The data were obtained with the Dunn Solar
Telescope at the National Solar Observatory/Sacramento Peak using a newly
developed camera system, the Rapid Dual Imager. Wavelet analysis is undertaken
to search for intensity-related oscillatory signatures, and periodicities
ranging from 15 to 370 s are found with significance levels exceeding 95%.
During two separate microflaring events, oscillation sites surrounding the
bright point are observed to twist. We relate the twisting of the oscillation
sites to the twisting of physical flux tubes, thus giving rise to reconnection
phenomena. We derive an average twist velocity of 8.1 km/s and detect a peak in
the emitted flux between twist angles of 180 and 230 degrees.Comment: 8 pages, 10 figure
The complex behaviour of the microquasar GRS 1915+105 in the rho class observed with BeppoSAX. I: Timing analysis
GRS 1915+105 was observed by BeppoSAX for about 10 days in October 2000. For
about 80% of the time, the source was in the variability class ,
characterised by a series of recurrent bursts. We describe the results of the
timing analysis performed on the MECS (1.6--10 keV) and PDS (15--100 keV) data.
The X-ray count rate from \grss showed an increasing trend with different
characteristics in the various energy bands. Fourier and wavelet analyses
detect a variation in the recurrence time of the bursts, from 45--50 s to about
75 s, which appear well correlated with the count rate. From the power
distribution of peaks in Fourier periodograms and wavelet spectra, we
distinguished between the {\it regular} and {\it irregular} variability modes
of the class, which are related to variations in the count rate in the
3--10 keV range. We identified two components in the burst structure: the slow
leading trail, and the pulse, superimposed on a rather stable level. We found
that the change in the recurrence time of the regular mode is caused by the
slow leading trails, while the duration of the pulse phase remains far more
stable. The evolution in the mean count rates shows that the time behaviour of
both the leading trail and the baseline level are very similar to those
observed in the 1.6--3 and 15--100 keV ranges, while that of the pulse follows
the peak number. These differences in the time behaviour and count rates at
different energies indicate that the process responsible for the pulses must
produce the strongest emission between 3 and 10 keV, while that associated with
both the leading trail and the baseline dominates at lower and higher energiesComment: Astronomy and Astrophysics, in pres
High-Frequency Oscillations in a Solar Active Region observed with the Rapid Dual Imager
High-cadence, synchronized, multiwavelength optical observations of a solar
active region (NOAA 10794) are presented. The data were obtained with the Dunn
Solar Telescope at the National Solar Observatory/Sacramento Peak using a newly
developed camera system : the Rapid Dual Imager. Wavelet analysis is undertaken
to search for intensity related oscillatory signatures, and periodicities
ranging from 20 to 370 s are found with significance levels exceeding 95%.
Observations in the H-alpha blue wing show more penumbral oscillatory phenomena
when compared to simultaneous G-band observations. The H-alpha oscillations are
interpreted as the signatures of plasma motions with a mean velocity of 20
km/s. The strong oscillatory power over H-alpha blue-wing and G-band penumbral
bright grains is an indication of the Evershed flow with frequencies higher
than previously reported.Comment: 9 pages, 9 figure
Seismic and spectroscopic characterization of the solar-like pulsating CoRoT target HD 49385
The star HD 49385 is the first G-type solar-like pulsator observed in the
seismology field of the space telescope CoRoT. The satellite collected 137 days
of high-precision photometric data on this star, confirming that it presents
solar-like oscillations. HD 49385 was also observed in spectroscopy with the
NARVAL spectrograph in January 2009. Our goal is to characterize HD 49385 using
both spectroscopic and seismic data. The fundamental stellar parameters of HD
49385 are derived with the semi-automatic software VWA, and the projected
rotational velocity is estimated by fitting synthetic profiles to isolated
lines in the observed spectrum. A maximum likelihood estimation is used to
determine the parameters of the observed p modes. We perform a global fit, in
which modes are fitted simultaneously over nine radial orders, with degrees
ranging from l=0 to l=3 (36 individual modes). Precise estimates of the
atmospheric parameters (Teff, [M/H], log g) and of the vsini of HD 49385 are
obtained. The seismic analysis of the star leads to a clear identification of
the modes for degrees l=0,1,2. Around the maximum of the signal (nu=1013
microHz), some peaks are found significant and compatible with the expected
characteristics of l=3 modes. Our fit yields robust estimates of the
frequencies, linewidths and amplitudes of the modes. We find amplitudes of
about 5.6 +/- 0.8 ppm for radial modes at the maximum of the signal. The
lifetimes of the modes range from one day (at high frequency) to a bit more
than two days (at low frequency). Significant peaks are found outside the
identified ridges and are fitted. They are attributed to mixed modes.Comment: 13 pages, 14 figures, accepted in A&
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