Magnetic perturbations to the acoustic modes of roAp stars     

Margarida S. Cunha  Douglas Gough 《Monthly notices of the Royal Astronomical Society》2000,319(4):1020-1038

We study the effect of a large-scale surface magnetic field on the non-radial acoustic modes of roAp stars. Special attention is given to the use of a variational principle which is used for determining the shifts in the frequencies with relative ease, enabling us to avoid having to calculate the perturbed eigenfunctions. With knowledge of the frequency shifts we then estimate the eigenfunctions in a simpler, albeit approximate way. The results indicate frequency shifts of the order of few μHz, which depend on the order, degree and azimuthal order of the mode. The loss of energy through Alfvén waves is also estimated from the imaginary parts of the frequency shifts. The results indicate that the loss is particularly high near specific frequencies. This might indicate the presence of a selection effect, which could make some modes more likely to be excited than others. However, our results do not explain why the modes observed appear always to be aligned with the axis of the magnetic field. Finally, the estimated perturbed eigenfunctions contain strong components of spherical harmonics that differ from those of the original unperturbed modes.  相似文献   

Is the Sun a Magnet?     

D. O. Gough 《Solar physics》2017,292(5):70

It has been argued (Gough and McIntyre in Nature 394, 755, 1998) that the only way for the radiative interior of the Sun to be rotating uniformly in the face of the differentially rotating convection zone is for it to be pervaded by a large-scale magnetic field, a field which is responsible also for the thinness of the tachocline. It is most likely that this field is the predominantly dipolar residual component of a tangled primordial field that was present in the interstellar medium from which the Sun condensed (Braithwaite and Spruit in Nature 431, 819, 2004), and that advection by the meridional flow in the tachocline has caused the dipole axis to be inclined from the axis of rotation by about \(60^{\circ}\) (Gough in Geophys. Astrophys. Fluid Dyn., 106, 429, 2012). It is suggested here that, notwithstanding its turbulent passage through the convection zone, a vestige of that field is transmitted by the solar wind to Earth, where it modulates the geomagnetic field in a periodic way. The field variation reflects the inner rotation of the Sun, and, unlike turbulent-dynamo-generated fields, must maintain phase. I report here a new look at an earlier analysis of the geomagnetic field by Svalgaard and Wilcox (Solar Phys. 41, 461, 1975), which reveals evidence for appropriate phase coherence, thereby adding support to the tachocline theory.  相似文献   

Inverting helioseismic data     

Douglas Gough 《Solar physics》1985,100(1-2):65-99

Methods by which the observed frequencies of solar oscillations can be, and in some cases have been used to infer the internal structure of the Sun are discussed. Attention is confined to so-called inverse methods that identify and extract the information that is actually contained in the data. Because only a finite quantity of data can ever be acquired, the functions describing the interior stratification of the Sun can never be established completely without the acceptance of certain assumptions. Nevertheless, the assumptions that are required are simple to understand, and the results do not depend on the complicated and uncertain theory of stellar evolution which has traditionally been used to construct solar models. First results of the inversions have given us an estimate of the sound speed and the angular velocity throughout much of the solar interior. These estimates have already stimulated speculation which hopefully will encourage further theoretical and observational research that will improve our understanding of the Sun.  相似文献   

Sensitivity of solar eigenfrequencies to the age of the sun     

D. O. Gough  E. Novotny 《Solar physics》1990,128(1):143-160

The increasing central concentration of the Sun with age modifies the acoustic eigenfrequencies. In particular, the frequency separation d _l =3(2l+3)^–1v _n,l –v _n–1,l+2 for modes with l + 1/2 n decreases as nuclear reactions augment the molecular-weight gradient in the energy-generating core. If, for example, the Sun were older than is generally believed, one might therefore expect d _l to be smaller than current theoretical predictions. On the other hand, to ensure that the luminosity is consistent with observations, the presumed initial hydrogen abundance would need to be enhanced, thereby reducing the resultant molecular-weight gradient. Thus there is some degree of cancellation of the two major factors that determine d _l .Various authors have either reported directly on the sensitivity of d _l , or have provided the information from which it can be calculated. We have added our own computations. There is broad agreement amongst the results: d _l diminishes with the presumed age of the Sun at the rate of about 1 Hz per Gy for l = 0; the magnitude of the rate appears to decline with increasing l.  相似文献   

Trends in the seasonal length and opening dates of a winter road in the western James Bay region,Ontario, Canada     

Hori  Yukari  Gough  William A.  Butler  Ken  Tsuji  Leonard J.S. 《Theoretical and Applied Climatology》2017,129(3-4):1309-1320

Theoretical and Applied Climatology - In northern Canada, winter roads are essential for communities. The duration of the winter road season depends on particular meteorological conditions. In this...  相似文献   

STRUCTURE AND ROTATION OF THE SOLAR INTERIOR: INITIAL RESULTS FROM THE MDI MEDIUM-L PROGRAM     

Kosovichev  A. G.  Schou  J.  Scherrer  P. H.  Bogart  R. S.  Bush  R. I.  Hoeksema  J. T.  Aloise  J.  Bacon  L.  Burnette  A.  De Forest  C.  Giles  P. M.  Leibrand  K.  Nigam  R.  Rubin  M.  Scott  K.  Williams  S. D.  Basu  Sarbani  Christensen-dalsgaard  J.  DÄppen  W.  Duvall  T. L.  Howe  R.  Thompson  M. J.  Gough  D. O.  Sekii  T.  Toomre  J.  Tarbell  T. D.  Title  A. M.  Mathur  D.  Morrison  M.  Saba  J. L. R.  Wolfson  C. J.  Zayer  I.  Milford  P. N. 《Solar physics》1997,170(1):43-61

The medium-l program of the Michelson Doppler Imager instrument on board SOHO provides continuous observations of oscillation modes of angular degree, l, from 0 to 300. The data for the program are partly processed on board because only about 3% of MDI observations can be transmitted continuously to the ground. The on-board data processing, the main component of which is Gaussian-weighted binning, has been optimized to reduce the negative influence of spatial aliasing of the high-degree oscillation modes. The data processing is completed in a data analysis pipeline at the SOI Stanford Support Center to determine the mean multiplet frequencies and splitting coefficients. The initial results show that the noise in the medium-l oscillation power spectrum is substantially lower than in ground-based measurements. This enables us to detect lower amplitude modes and, thus, to extend the range of measured mode frequencies. This is important for inferring the Sun's internal structure and rotation. The MDI observations also reveal the asymmetry of oscillation spectral lines. The line asymmetries agree with the theory of mode excitation by acoustic sources localized in the upper convective boundary layer. The sound-speed profile inferred from the mean frequencies gives evidence for a sharp variation at the edge of the energy-generating core. The results also confirm the previous finding by the GONG (Gough et al., 1996) that, in a thin layer just beneath the convection zone, helium appears to be less abundant than predicted by theory. Inverting the multiplet frequency splittings from MDI, we detect significant rotational shear in this thin layer. This layer is likely to be the place where the solar dynamo operates. In order to understand how the Sun works, it is extremely important to observe the evolution of this transition layer throughout the 11-year activity cycle.  相似文献   

Climate context of the cold summer of 2014 in Toronto,ON, Canada     

William A. Gough  Srishtee Sokappadu 《Theoretical and Applied Climatology》2016,126(1-2):183-189

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Air mass distribution and the heterogeneity of the climate change signal in the Hudson Bay/Foxe Basin region,Canada     

Andrew Leung  William Gough 《Theoretical and Applied Climatology》2016,125(3-4):583-592

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Travelling and hunting in a changing Arctic: assessing Inuit vulnerability to sea ice change in Igloolik, Nunavut     

Gita J. Laidler  James D. Ford  William A. Gough  Theo Ikummaq  Alexandre S. Gagnon  Slawomir Kowal  Kevin Qrunnut  Celina Irngaut 《Climatic change》2009,94(3-4):363-397

The observations of community members and instrumental records indicate changes in sea ice around the Inuit community of Igloolik, in the Canadian territory of Nunavut. This paper characterizes local vulnerability to these changes, identifying who is vulnerable, to what stresses, and why, focusing on local and regional use of sea ice for the harvesting of renewable resources and travel. This analysis is coupled with instrumental and sea ice data to evaluate changing temperature/wind/sea ice trends over time, to complement local observations. We demonstrate the relationships between changing sea ice conditions/dynamics and harvesting activities (i.e. dangers and accessibility), with specific emphasis on ringed seal and walrus seasonal hunting, to illustrate current sea ice exposures that hunters are facing. Community members are adapting to such changes, as they have done for generations. However, current adaptive capacity is both enabled, and constrained, by social, cultural, and economic factors that manifest within the modern northern Hamlet. Enabling factors include the ability of hunters to manage or share the risks associated with sea ice travel, as well as through their flexibility in resource use, as facilitated by sophisticated local knowledge and land/navigational skills. Constraining factors include the erosion of land-based knowledge and skills, altered sharing networks, as well as financial and temporal limitations on travel/harvesting. The differential ability of community members to balance enabling and constraining factors, in relation to current exposures, comprises their level of vulnerability to sea ice change.  相似文献   

The influence of sea-surface temperatures on Eastern North Pacific tropical cyclone activity     

Trisha U. Ralph  William A. Gough 《Theoretical and Applied Climatology》2009,95(3-4):257-264

The influence of sea-surface temperatures on six measures of tropical cyclone activity in the Eastern North Pacific is examined using historical sea-surface temperature and tropical cyclone data spanning from 1971 to 2002. Relationships are evaluated using methods of trend analysis, extreme year analysis, and bivariate correlation. Results suggest that in order to understand the climatological factors affecting topical cyclone activity in the Eastern North Pacific, the main development region must be divided into two sub-regions of development to the east and west of 112°W longitude. Increasing trends of sea-surface temperature are not accompanied by increasing trends in tropical cyclone activity. In the western development region, sea-surface temperatures are significantly correlated with all measures of tropical cyclone activity during extreme years. In this region, sea-surface temperatures are on average below the threshold for tropical cyclone development. In the Eastern development region, the only significant correlation with sea-surface temperatures is for the more intense measures of hurricane activity. In this region, sea-surface temperatures are on average above the threshold for cyclone formation. This leads to the hypothesis that the proximity to the cyclone formation temperature threshold in the WDR enhances the sensitivity of tropical cyclone activity to SSTs. This may have application to other tropical cyclone basins such as the North Atlantic.  相似文献