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1.
Local helioseismology techniques yielding the temperature and flow-velocity distributions under a sunspot indicate an unambiguous sign for the horizontal gas-pressure difference between the spot and ambient medium at depths of 4 Mm and more. In the Parker sunspot model, the transverse equilibrium condition cannot be satisfied in these layers: a cluster of vertical, strongly compressed magnetic flux tubes in a plasma that is hotter than the ambient medium with flows that diverge sidewise cannot be in equilibrium. Equilibrium can be satisfied in the hot zone under the spot only if the magnetic flux tube expands sharply with depth, so that the mean magnetic-field strength decreases dramatically at depths exceeding 4 Mm. This corresponds to the “shallow” sunspot model that has been used to interpret long-period sunspot oscillations. 相似文献
2.
Space spectral analysis of zonal (u) and meridional (v) components of wind and time spectral analysis of kinetic energy of zonal waves at 850 hPa during monsoon 1991 (1st June
1991 to 31st August 1991) for the global belt between equator and 40°N are investigated. Space spectral analysis shows that
long waves (wavenumbers 1 and 2) dominate the energetics of Region 1 (equator to 20°N) while over Region 2 (20°N to 40°N)
the kinetic energy of short waves (wavenumbers 3 to 10) is more than kinetic energy of long waves. It has been found that
kinetic energy of long waves is dominated by zonal component while both (zonal and meridional) the components of wind have
almost equal contribution in the kinetic energy of short waves.
Temporal variations of kinetic energy of wavenumber 2 over Region 1 and Region 2 are almost identical. The correlation matrix
of different time series shows that (i) wavenumber 2 over Regions 1 and 2 might have the same energy source and (ii) there
is a possibility of an exchange of kinetic energy between wavenumber 1 over Region 1 and short waves over Region 2. Wave to
wave interactions indicate that short waves over Region 2 are the common source of kinetic energy to wavenumber 2 over Regions
1 and 2 and wavenumber 1 over Region 1. Time spectral analysis of kinetic energy of zonal waves indicates that wavenumber
1 is dominated by 30–45 day and bi-weekly oscillations while short waves are dominated by weekly and bi-weekly oscillations.
The correlation matrix, wave to wave interaction and time spectral analysis together suggest that short period oscillations
of kinetic energy of wavenumber 1 might be one of the factors causing dominant weekly (5–9 day) and bi-weekly (10–18 day)
oscillations in the kinetic energy of short waves. 相似文献
3.
4.
A comparative analysis of oscillatory spectra based on 66 time series for 14 active regions observed in 2001 shows that, although the chromospheric and photospheric oscillations in the Evershed flow zone possess many common features, there is no firm evidence that the direct and inverse flows have the same physical origin. The interactions between the various oscillation modes and stationary flows results in a complex pattern of wave motions in a sunspot. We studied the Doppler-velocity variations in the sunspot NOAA 0051 during its motion over the disk. The spatial-temporal distribution of the line-of-sight velocity in the chromospheric umbra displays a chevron structure, clearly indicating the presence of propagating waves. These waves move from the center of the umbra to outer regions with a phase speed of 45–60 km/s, a period of 2.8 min, and a measured Doppler speed of 2 km/s. The amplitude of these oscillations decreases abruptly at the boundary between the umbra and penumbra, and the observed waves are not directly related to propagating penumbral waves. Furthermore, the observed pattern of the photospheric velocities shows periodic motions (with a period of 5 min) directed from the inner boundary of the penumbra and superpenumbra toward the line of maximum Evershed velocity. 相似文献
5.
The properties of Doppler-velocity oscillations in solar faculae are analyzed at the photospheric level (based on Fe I 6569
? and Fe I 8536 ? lines) and chromospheric level (based on Hα and Ca II 8542 ? lines) to search for upward propagating waves.
The similarity of the averaged power spectra at 2.5–4 mHz is not found to be convincing proof of the presence of unidirectional
wave-energy transfer from the photosphere to the chromosphere. Phase relations between the photospheric and chromospheric
oscillations that are indicative of either upward or downward propagating waves are obtained for various areas in many faculae.
This suggests that the wave energy of the five-minute oscillations returns to the photosphere, at least partially. The derived
properties suggest that the role of faculae in the transfer of the five-minute oscillations to the chromosphere and overlying
layers is not as obvious as could be expected. The relatively typical presence of low-frequency (0.5–2 mHz) oscillations in
faculae and their possible important role in this energy transfer are noted. 相似文献
6.
Trapped waves of the 27 November 1945 Makran tsunami: observations and numerical modeling 总被引:1,自引:1,他引:0
S. Neetu I. Suresh R. Shankar B. Nagarajan R. Sharma S. S. C. Shenoi A. S. Unnikrishnan D. Sundar 《Natural Hazards》2011,59(3):1609-1618
The 27 November 1945 earthquake in the Makran Subduction Zone triggered a destructive tsunami that has left important problems
unresolved. According to the available reports, high waves persisted along the Makran coast and at Karachi for several hours
after the arrival of the first wave. Long-duration sea-level oscillations were also reported from Port Victoria, Seychelles.
On the other hand, only one high wave was reported from Mumbai. Tide-gauge records of the tsunami from Karachi and Mumbai
confirm these reports. While the data from Mumbai shows a single high wave, Karachi data shows that high waves persisted for
more than 7 h, with maximum wave height occurring 2.8 h after the arrival of the first wave. In this paper, we analyze the
cause of these persistent high waves using a numerical model. The simulation reproduces the observed features reasonably well,
particularly the persistent high waves at Karachi and the single high wave at Mumbai. It further reveals that the persistent
high waves along the Makran coast and at Karachi were the result of trapping of the tsunami-wave energy on the continental
shelf off the Makran coast and that these coastally-trapped edge waves were trapped in the along-shore direction within a
∼300-km stretch of the continental shelf. Sensitivity experiments establish that this along-shore trapping of the tsunami
energy is due to variations in the shelf width. In addition, the model simulation indicates that the reported long duration
of sea-level oscillations at Port Victoria were mainly due to trapping of the tsunami energy over the large shallow region
surrounding the Seychelles archipelago. 相似文献
7.
I. B. Abbasov 《Doklady Earth Sciences》2009,429(2):1605-1607
This report considers the problem of the propagation of nonlinear surface gravity waves over shallow waters with regard to
dispersion. By means of the semigraphical method proposed, the transformation of the profile of nonlinear surface gravity
waves is described. The profiles of surface gravity waves are presented for the stages of crest sharpening and front steepening,
as well as the wave train disintegration caused by dispersion. 相似文献
8.
L. V. Didkovskii 《Astronomy Reports》2000,44(12):841-847
Short-period (1–60 min) variations in the coordinates of the centers of gravity of isolated sunspots are analyzed. The sunspot coordinated were determined using two sets of observational data—magnetograms and intensities—obtained by SOHO (MDI) on December 6, 1998, from 01:00 to 21:57 UT with temporal resolution 60 s and spatial resolution 0.6″/pixel. A slow drift in the sunspot coordinates was removed using a low-frequency filter with a 61-min integration window. The guiding errors (RMS~0.014″) were determined by analyzing correlated motions in pairs of sunspots, and were removed from the time series before determining the sunspot proper motions. Based on the calculated power spectra for the sunspot proper motions, two period intervals containing appreciable power were identified. One coincides with the well-known 5-min acoustic solar oscillations. The concentration of power in this interval is greater for the coordinate variations derived the magnetograms than those derived from the intensities; the harmonic amplitude for some peaks reaches ~±30 km. The other spectral interval corresponds to periods exceeding 30 min. Overall, the rms short-period variations in the sunspot proper motions are 9.9±2.2 and 16.7±7.6 km (0.014″±0.003″ and 0.024″±0.010″) for the magnetogram and intensity data, respectively. 相似文献
9.
Information on reflected surface gravity waves from the shoreline is required for understanding the coastal hydrodynamics. We have quantified the reflected swells (frequency band 0.045–0.12 Hz) from the west and east coast of India based on the spectral wave data derived from the directional waverider buoys. Reflection coefficient, ratio of the reflected and incident spectral energy, was used to quantify the reflected waves. Influence of the seasons, cyclone, relative depth, land/sea breeze, tides and tidal current on the reflected waves were examined. For the locations off the west coast of India, seasons have large impact on the reflection coefficient and were relatively less during the monsoon season due to the increase in incident wave energy. Locations off the east coast of India show almost the same reflection coefficient throughout the year and have no significant seasonal variations. The reflection coefficient off Puducherry was higher than that for other locations due to the low incident wave energy. The reflection coefficient was low during the cyclone period, but the reflected energy during cyclone was higher than that during the normal condition due to the high incident wave energy. High-energy reflected waves show large variation with tide due to the trapping and dissipation of reflected wave by bottom friction and this effect cause low reflection in deep water location than shallow water location. The reflection coefficient decreases with increase in relative depth off west coast of India. 相似文献
10.
P. D. Kovalev G. V. Shevchenko D. P. Kovalev A. G. Chernov D. Ye. Zolotukhin 《Russian Journal of Pacific Geology》2009,3(5):443-449
The November 16, 2006, Simushir and August 2, 2007, Nevelsk tsunami records obtained by bottom pressure gauges in Kholmsk
Bay are analyzed. The dominant role of the zero mode of eigen oscillations in the bay during the the wave field formation
is shown: in the initial record interval (for the remote tsunami source) and 1.5 h after the first wave (for the nearly tsunami).
Numerical modeling showed that the longer waves propagated toward Kholmsk in the case of the Nevelsk earthquake and they did
not generate eigen oscillations of the bay. These oscillations were generated 1.5 h later when the shorter waves reflected
from the Primorye coast arrived. 相似文献
11.
Based on numerous high-resolution seismic profiles, sediment waves and their distribution, morphological characteristics, internal structure, and potential origins were revealed in the eastern waters of Taiwan. The sediment waves are located at the junction between the Taitung Canyon and other canyons in the slope. The wave length and the wave height of a single waveform ranged from 0.8 to 7.2 km and from 18 to 75 m, respectively (NE-SW direction). Sediment waves, located inside the bend of the Taitung Canyon, were characterized by an upward migration and showed mass transport deposits (MTDs) at the bottom, while the inner curve of the bend was subdivided into lower and upper wavy transition units. The sediment waves on the outer curve of the bend were characterized by vertical accumulation, and there was no mass flow deposit at the bottom. According to the geometry of the sediment waves, the calculated flow thicknesses across the entire wave field ranged from 196 to 356 m, and the current velocity ranged from 15 to 21 cm/s. The morphological characteristics, the internal structure, and the distribution of sediment waves, as well as the numerical calculations, evidenced that these sediment waves had formed by turbidity currents. The development of the sediment wave field in eastern Taiwan was found to be similar to that in southwestern Taiwan. It was the sedimentary response of the tectonic movement between 3 and ~1 Ma which created the sedimentary systems where gravity flow processes predominated. Turbidity current sediments settled in the place of less topographical constraints or overflowed in the bend section of the Taitung Canyon, which resulted in the formation of sediment wave fields. 相似文献
12.
Mixing in the surface layers in association with internal waves during winter in the northwestern Bay of Bengal 总被引:1,自引:1,他引:0
The upper ocean has complex and variable temperature stratification, and the surface layers in the northwest Bay of Bengal
in winter indicate the presence of transient thermal inversions that wane with the advancement of the season. During winter,
the sea surface loses heat and the surface waters of the coastal regions of the east coast of India are fairly stratified
with the residual freshwater atop from the preceding southwest monsoonal discharge. The vertical stability favors the formation
and sustenance of temperature inversions. To investigate the mechanism and the influence of ubiquitous internal waves that
thrive on stability, a three-dimensional Princeton Ocean Model is configured for the east coast of India and is applied to
study the process in the surface layers in association with the internal waves. The model domain constitutes a variable curvilinear
grid, and the input fields comprise bathymetry, initial temperature and salinity, wind stress, air-sea heat fluxes and tidal
forcing at the open boundaries. The numerical experiments demonstrate that vertical stability alone cannot cause, support
or augment the internal wave oscillations, if the stratification is attributed to salinity only. Internal waves may therefore
be perceived in stable layers, essentially from temperature-induced stratification. Despite stratification and enough vertical
density gradient in the upper ocean, the conditions may not suit for the occurrence of internal waves due to thermal diffusive
processes that overpower the salinity gradients. The vertical spreading of heat due to double diffusion is believed to be
transparent to tidal forcing as the generation of internal waves is subdued even under density stratification. The model simulations
indicate that the horizontal convergence/divergence motions, required for the manifestation of internal waves at the surface
are inhibited in the presence of temperature inversion. The available SAR imageries in winter endorse the model simulations
to this effect. 相似文献
13.
Hilkka Pellikka Jenni Rauhala Kimmo K. Kahma Tapani Stipa Hanna Boman Antti Kangas 《Natural Hazards》2014,74(1):197-215
We present four case studies of exceptional wave events of meteorological origin, observed on the Finnish coast in the summers of 2010 and 2011. Eyewitnesses report unusually rapid and strong sea-level variations (up to 1 m in 5–15 min) and strong oscillating currents during these events. High-resolution sea-level measurements confirm the eyewitness observations, but the oscillations recorded by tide gauges mostly have a considerably smaller amplitude. The oscillations coincide with sudden jumps in surface air pressure at coastal observation stations, related to the passage of squall lines or gust fronts. These fronts propagate above the sea at a resonant speed, allowing efficient energy transfer between the atmospheric disturbance and the sea wave that it generates. Thus, we interpret the observed sea-level oscillations as small meteotsunamis, long tsunami-like waves generated by meteorological processes and resonance effects. 相似文献
14.
We examine plasma heating due to the dissipation of acoustic waves excited in coronal magnetic loops by parametric resonance
with the five-minute oscillations in the velocity of the photospheric convection. The energy of acoustic waves excited in
the coronal magnetic loop, rate of dissipation of acoustic waves, and rate of heating of the coronal plasma are determined.
The maximum temperature predicted for the apex of the loop is calculated as a function of the velocity of photospheric oscillations,
length of the loop, and electric current in the loop. It is shown that the mechanism proposed can explain the origin of quasi-stationary
X-ray loops with temperatures of 3–6 MK. The lengths of these loops are resonant for acoustic waves excited by the five-minute
photospheric oscillations. The use of the proposed mechanism to explain heating of the X-ray loops expected to be on stars
of late spectral types is discussed. 相似文献
15.
16.
R. Raghavarao R. Suhasini R. Sridharan B. V. Krishnamurthy O. P. Nagpal 《Journal of Earth System Science》1990,99(3):413-423
The equatorial wave campaign-II which formed a part of the Indian Middle Atmosphere Programme (IMAP), was conducted from SHAR
(13.7°N, 80.2°E) from 15 January to 28 February 1986. Winds were measured from ground to 60 km by means of high altitude balloon
and a meteorological rocket (RH-200), once everyday, for 45 days. The frequencies of the oscillations in the deviations of
the east-west component of the winds from its mean at each height with one kilometer interval were obtained by the maximum
entropy (ME) method and phases/amplitudes of these frequencies were determined by the least squares technique on the wind
variation time series. The ME method has the inherent advantage of providing periodicities up to 1.5 times the data length.
The height structure of the long period waves of > 23 day periodicities that have larger amplitudes nearly by a factor of
2 as compared to the medium (9 to 22 day) or shorter period (4 to 8 day) ones, reveal two height regions of enhanced amplitudes,
one in the troposphere and another in the upper stratosphere/lower mesosphere, that too, mostly in the regions of positive
(westerly increasing or easterly decreasing with height) wind shears. The waves are seen to be inhibited in the negative wind
shear regions. From the abrupt changes in the altitude variation of phase, the possible source region has been identified.
The vertical wavelengths have been estimated to be 34 km and 19 km in the troposphere and lower stratosphere respectively
and 8 km in the upper stratosphere and lower mesosphere. Around 56 km the wave amplitude is reduced to 1/4 of its value below,
while the vertical shear strength in the mean wind doubled up. The tropospheric waves are suggested to be Rossby waves of
extratropical origin penetrating to tropical latitudes. The stratospheric/mesospheric waves however appear to emanate from
a source around the stratopause. 相似文献
17.
N. I. Kobanov D. Yu. Kolobov A. A. Sklyar S. A. Chupin V. A. Pulyaev 《Astronomy Reports》2009,53(10):957-967
Recognizing that waves play an important role in energy-exchange processes between layers of the solar atmosphere, and that
the characteristics of propagating waves are determined by the physical conditions of the medium, and, most importantly, the
magnetic-field configuration, we have carried out a comparative analysis of the properties of oscillations in solar structures
with various magnetic-field topologies: sunspots, faculae, and coronal holes. Simultaneous measurements of the Doppler velocities
and intensities at the photospheric and chromospheric levels were accompanied by episodic measurements of the longitudinal
magnetic field. In the chromosphere, spot umbrae dominate the three-minute oscillations, while lower-frequency modes are also
observed in the penumbrae and at the outer parts of spots. Clear signs of propagating waves have been observed at the bases
of coronal holes and in faculae only at frequencies close to 3 mHz. 相似文献
18.
Sanjeev Anand Sahu Soniya Chaudhary Pradeep Kumar Saroj Amares Chattopadhyay 《Arabian Journal of Geosciences》2017,10(5):120
The study of surface waves (Rayleigh wave) finds their virtuous applications in a numerous geological and geophysical fields including water, oil, gas, and other subsurface geological probing and exploration. The present paper efforts to investigate the influence of initial stress, Earth magnetism, and gravity on propagation of Rayleigh waves. Considered model is consist of a liquid layer lying over a magnetoelastic orthotropic half-space under self-weight and initial stress. Method of separation of variable is used to solve the equation of motion. Solutions of governing equations are obtained in terms of displacement. Frequency relation for Rayleigh wave has been obtained and matched with classical Rayleigh wave equation. In addition to classical case, some existing results have been deduced as particular case of the present study. Obtained results have been shown through numerical illustrations. It is found that the considered parameters (initial stress, Earth magnetism, and gravity) have prominent effect on phase velocity of Rayleigh wave. Graphical representations have been made to exhibit the velocity profile of Rayleigh waves for different cases with the help of MATHEMATICA. The present study may be useful for seismologists and engineers who are concern with applications of wave propagation in magnetoelastic orthotropic medium. 相似文献
19.
基于多相孔隙介质弹性理论,给出了非饱和土中不同弹性波的传播方程。根据分界面上的边界条件,建立了各势函数波幅值之间的关系式,讨论了入射剪切波在不同饱和度土层分界面上的反射与透射问题。在无限空间非饱和土体中存在3种压缩波和1种剪切波,因此,当剪切波传播到不同饱和度的非饱和土层分界面上将分别在上、下土层激发产生4种反射波和4种透射波。推导出不同反射波和透射波的振幅比例系数和能量比例系数的理论表达式,并且在此基础上进行数值分析。在数值算例中分别研究了各反射波与透射波的能量比例系数(即能量反射率和能量透射率)受入射频率、入射角度以及上、下土层土体饱和度变化的影响情况。计算结果表明:各能量反射率和能量透射率不仅与入射角和入射频率有关,而且其受上、下土层饱和度变化的影响也同样不能忽视。 相似文献