• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 42
  • 4
  • 3
  • 2
  • 1
  • 1
  • 1
  • Tagged with
  • 63
  • 63
  • 12
  • 12
  • 12
  • 11
  • 11
  • 7
  • 7
  • 7
  • 6
  • 6
  • 6
  • 6
  • 6
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Evidence of solar oscillations in Rayleigh-scattered light.

Germain, Marvin Edward. January 1993 (has links)
A new instrument has been developed for making unimaged whole-disk observations of low-degree solar normal-mode oscillations. The apparatus, referred to as the sky monitor, does not track the solar disk, but instead measures the radiant flux scattered by the Earth's atmosphere at 1.6 and 0.5 μ. The expected diurnal flux variation was obtained from a detailed radiative transfer calculation. Data were acquired for 15.5 months during 1991-92. Seventy-two days of data were analyzed for evidence of solar p modes in the frequency range 1800-4776 μHz. Noise generated by the Earth's atmosphere was reduced by scaling the Fourier amplitude computed from the infrared signal and subtracting it from the Fourier amplitude computed from the visible signal. A superimposed frequency analysis was then performed which revealed ∼ 2 σ peaks within 0.3 μHz of symmetry-allowed locations, while no excess power was detected at the symmetry-forbidden frequencies. The probability of obtaining by chance the observed excess power density at symmetry-allowed frequencies and deficit of power density at symmetry-forbidden frequencies was computed to be 6.9 x 10⁻³. Correcting the frequencies for solar-cycle variations, the probability was reduced to 2.9 x 10⁻⁴. These results indicate that it is quite unlikely that the observed symmetry properties have occurred by chance, and support the hypothesis that solar normal-mode signals are manifested in the data. The amplitudes I'/Iₒ averaged over radial order n of the ℓ = 0 and ℓ = 2, m = 0 modes were found to be (7.54 ± 0.54) x 10⁻⁷ and (7.68 ± 0.56) x 10⁻⁷, respectively. These results are about a factor of two smaller than the amplitude of total irradiance oscillations measured from space. While the rotational splitting of the ℓ = 2 multiplet appears to be consistent with that reported by Hill (1985a), results for ℓ = 1 and ℓ = 3 are inconclusive.
2

The structure and orientation of large scale corotating structures in the heliosphere

Clack, Dorian January 2001 (has links)
No description available.
3

The equatorial ionospheric anomaly in East Asia from solar minimum to solar maximum /

Li, Yeuk-Yue, Tony. January 1993 (has links)
Thesis (M. Phil.)--University of Hong Kong, 1994. / Includes bibliographical references (leaves 135-145).
4

LONG-PERIOD SOLAR OSCILLATIONS: A SEISMOLOGICAL AND INTERCOMPARATIVE STUDY

Caudell, Thomas Preston January 1980 (has links)
This work deals with the subject of global solar oscillations. These oscillations are observed as fluctuations in the diameter of the sun. A diameter is determined by a mathematical solar edge definition at the Santa Catalina Laboratory for Experimental Relativity by Astrometry (SCLERA) instrument. The oscillations have periods ranging from a few minutes to several hours and have amplitudes measured in millionths of a solar radius. These small amplitudes are observable only due to the unique properties of the edge definition. The properties of the observed solar oscillations are determined from the data; their statistical significance and repeatability are then tested. The possibility of using the observed oscillations as a seismic tool for understanding the solar interior and its motions is explored.
5

A theoretical investigation of the effects of solar eclipses on the ionosphere

Walker, Anthony David Mortimer January 1962 (has links)
The behaviour of the ionosphere during a solar eclipse is of great interest because radiation from the sun is the cause of ionization in the upper atmosphere and it is useful to be able to conduct experiments where this radiation is cut off and restored in a known manner. Experimental results, especially those dealing with the F2 layer, have proved puzzling. Cusps which cannot be explained appear on the records obtained from ionosphere sounders and in the F2 region the electron density at a given height shows a maximum after the eclipse where one would expect it simply to rise to a steady value. An attempt is made in this thesis to explain some of the anomalies in terms of tilts in the ionospheric layers and minima of electron density or "valleys" between the ionospheric layers. The problem is attacked theoretically. Part I deals with the theoretical background to ionospheric physics in general and to this problem in particular. Standard methods of dealing with radio propagation in the ionosphere as well as some methods developed by the author are discussed. Part II deals directly with the effects of a solar eclipse on a theoretical ionosphere. Ionograms which would be obtained in the theoretical ionosphere are constructed. These are scaled by standard methods to show where errors may arise . It appears that tilts in the layers have only a small effect. The effect of the valley is, however, extremely important, giving rise to the apparent maximum of electron density in the F2 layer at a given height after the eclipse. This maximum does not in fact exist but arises from an error in the scaling method which ignores the possibility of a valley. Some records taken during the solar eclipse of 25 December, 1954 have been scaled. They support the conclusion reached theoretically.
6

OBSERVATIONS OF INDIVIDUAL SOLAR EIGENMODES: THEIR PROPERTIES AND IMPLICATIONS.

BOS, RANDALL JAY. January 1982 (has links)
This work analyzes data taken in 1979 using a modification of the solar detector at SCLERA (Santa Catalina Laboratory for Experimental Relativity) designed to enhance spatial properties of the previously observed solar oscillations. Unlike previous solar observations taken at SCLERA, where the data consisted of single solar diameter measurements, the 1979 data consisted of six recorded limb profiles. This has important ramifications for the amount of signal present in the data which was generated by the terrestrial atmosphere, for the origin of the observed solar oscillations in fluctuations of the solar limb darkening function, and, most importantly, for the spatial symmetry properties of the observed solar eigenfunctions. The data consisted of 18 days of observations averaging ten hours per day and covering a total of 41 days. A linked Fourier transform of all 18 days was done for signal generated from each limb profile, and combinations of these six Fourier transforms made to increase sensitivity to symmetric or antisymmetric properties of the observed solar eigenmodes. The following results were found: 1. The observed oscillations are manifestations of fluctuations in the solar limb darkening function. 2. Terrestrial atmospheric contributions to the observed signal are negligible; thus, the sun constitutes the only possible source of the signal. 3. Given a resolution element of 1/(41 days) or 0.28 μHz, the solar oscillations observed represent individual solar eigenstates. 4. The spatial properties of the eigenstates are consistent with their interpretation in terms of spherical harmonics defined with respect to the observed solar rotational axis. 5. The eigenstates are temporally coherent for > 2 days and, in selected samples, for > 41 days. 6. The observed spacing of groups of eigenmodes is shown to be indicative of solar rotational effects; this spacing implies that the core of the sun is rotating approximately six times faster than the observed surface rotational velocity.
7

NONLOCAL AND NONLINEAR EFFECTS ON SOLAR OSCILLATIONS (RADIATIVE DAMPING, LIMB DARKENING).

LOGAN, JERRY DAVID. January 1984 (has links)
This work investigates the response of the solar atmosphere to mechanical and thermal driving due to global solar oscillations. It was discovered that the coupling of thermal and mechanical modes was very important in reconciling theoretical predictions of the expected change in the solar limb due to solar oscillations and experimental observations of the variability in the solar limb darkening function undertaken at SCLERA (Santa Catalina Laboratory for Experimental Relativity). The coupling between the thermal and mechanical modes occur mainly due to the nonlocal nature of the radiation field. Previous theoretical calculations that used approximations for the radiative transfer that ignored the nonlocal nature of the radiation field predicted expected temperature perturbations (compared to the fluid displacement) that were much too small to be observed. Much larger ratios were found when the radiative transfer was treated properly. A particular solar oscillation can be influenced by the presence of a large number of other modes, if these modes can change the average properties of the medium. If the basic nonlinear equations are statistically averaged, the influence of the "mean field" can be investigated. This nonlinear effect can become important in the analysis for single modes in the upper photosphere.
8

Geomagnetic perturbations on stratospheric circulation in late winter and spring

Lu, Hua, Clilverd, Mark A., Seppälä, Annika, Hood, Lon L. 22 August 2008 (has links)
This study investigates if the descent of odd nitrogen, generated in the thermosphere and the upper mesosphere by energetic particle precipitation (EPP-NOx), has a detectable impact on stratospheric wind and temperature in late winter and spring presumably through the loss of ozone and reduction of absorption of solar UV. In both hemispheres, similar downward propagating geomagnetic signals in the extratropical stratosphere are found in spring for those years when no stratospheric sudden warming occurred in mid-winter. Anomalous easterly winds and warmer polar regions are found when the 4-month averaged winter Ap index (Ap) is high, and the signals become clearer when solar F10.7 is low. In May, significant geomagnetic signals are obtained in the Northern Hemisphere when the data are grouped according to the phase of the stratospheric equatorial QBO. The magnitudes of changes in spring stratospheric wind and temperatures associated with Ap signals are in the range of 10–20 m s−1 and 5–10 K, which are comparable with those of the 11-yr SC signals typically found in late winter. The spring Ap signals show the opposite sign to that expected due to in situ cooling effects caused by catalytic destruction of stratospheric ozone by descending EPP-NOx. Thus it is unlikely that the in situ chemical effect of descending EPP-NOx on stratospheric ozone would have a dominant influence on stratospheric circulation. Instead, we suggest that the detected Ap signals in the extratropical spring stratosphere may be an indirect consequence of geomagnetic and solar activity, dynamically induced by changes in wave ducting conditions.
9

Surface flux transport simulations of the photospheric magnetic field

Virtanen, I. (Iiro) 02 September 2019 (has links)
Abstract This thesis studies the long-term evolution of the photospheric magnetic field using surface flux transport simulations. The photospheric magnetic field and magnetic activity are tightly connected to space weather, and affect the whole heliosphere including the Earth. However, due to a lack of reliable observations our understanding of the long-term evolution of the photospheric magnetic field is still poor. Surface flux transport models, which are capable of simulating the evolution of the whole surface field from observations of solar activity, can be used to study the field in times when direct observations are not available. In this thesis we validate our surface flux transport model, optimize its parameters and test its sensitivity to uncertainties in parameter values and input data. We find a need to extend the model with a decay term to properly model the deep and long minimum between solar cycles 23 and 24, and simulate the photospheric magnetic field of cycles 21–24 using magnetographic observations as input. We also study consequences of hemispherically asymmetric activity, and show that activity in one hemisphere is enough to maintain polar fields in both hemispheres through cross-equatorial flow of magnetic flux. We develop a new method to reconstruct active regions from calcium K line and sunspot polarity observations. We show that this reconstruction is able to accurately capture the correct axial dipole moment of active regions. We study the axial dipole moments of observed active regions and find that a significant fraction of them have a sign opposite to the sign expected from Hale’s and Joy’s laws, proving that the new reconstruction method has an advantage over existing methods that rely on Hale’s and Joy’s laws to define polarities. We show one example of a long simulation covering solar cycles 15–21, demonstrating that using the active region reconstruction and surface flux transport model presented in this thesis it is possible to simulate the large-scale evolution of the photospheric magnetic field over the past century. / Original papers The original publications are not included in the electronic version of the dissertation. Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., Yeates, A., & Mursula, K. (2017). Reconstructing solar magnetic fields from historical observations. II. Testing the surface flux transport model. Astronomy & Astrophysics, 604, A8. https://doi.org/10.1051/0004-6361/201730415 http://jultika.oulu.fi/Record/nbnfi-fe2017103050356 Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., & Mursula, K. (2018). Reconstructing solar magnetic fields from historical observations. III. Activity in one hemisphere is sufficient to cause polar field reversals in both hemispheres. Astronomy & Astrophysics, 616, A134. https://doi.org/10.1051/0004-6361/201732323 http://jultika.oulu.fi/Record/nbnfi-fe201902205813 Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., Bertello, L., Yeates, A., & Mursula, K. (2019). Reconstructing solar magnetic fields from historical observations. IV. Testing the reconstruction method. Astronomy & Astrophysics, 627, A11. https://doi.org/10.1051/0004-6361/201935606 http://jultika.oulu.fi/Record/nbnfi-fe2019091828628 Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., & Mursula, K. (2019) Axial dipole moment of solar active regions in cycles 21-24. Manuscript.
10

The equatorial ionospheric anomaly in East Asia from solar minimun to solar maximum

李若愚, Li, Yeuk-Yue, Tony. January 1993 (has links)
published_or_final_version / Physics / Master / Master of Philosophy

Page generated in 0.0961 seconds