Spelling suggestions: "subject:"solar aactivity"" "subject:"solar a.ctivity""
41 |
A study of cosmic ray anisotropies in the heliosphere / Godfrey Sibusiso NkosiNkosi, Godfrey Sibusiso January 2006 (has links)
Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2007.
|
42 |
Modélisation MHD tridimensionnelle de tubes de flux coronaux utilisant l'assimilation des donnés 4D-VARBenslimane, Ali January 2008 (has links)
Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal
|
43 |
A study of cosmic ray anisotropies in the heliosphere / Godfrey Sibusiso NkosiNkosi, Godfrey Sibusiso January 2006 (has links)
The three-dimensional (3D) steady-state electron modulation model of Ferreira (2002),
based on Parker (1965) transport equation, is used to study the modulation of the 7 MeV
galactic and Jovian electron anisotropies in the inner heliosphere. The Jovian electrons
are produced in Jupiter's magnetosphere which is situated at ~ 5 AU in the ecliptic plane.
The propagation of these particles is mainly described by the diffusion tensor applicable
for the inner heliosphere. Some of the elements of the diffusion tensor are revisited in
order to establish what contribution they make to the three-dimensional anisotropy vector
and its components in the inner heliosphere. The 'drift' term is neglected since the focus
of this study is on low-energy electrons. The effects on the electron anisotropy of
different scenarios when changing the solar wind speed from minimum to maximum
activity is illustrated. The effects on both the galactic and Jovian electron anisotropy of
changing the polar perpendicular coefficient, in particular, are illustrated. It is shown that
the computed Jovian electron anisotropy dominates the galactic anisotropy close to the
Jovian electron source at ~5 AU, as expected, testifying to the validity of the3D-model.
For the latitudinal anisotropy, the polar perpendicular diffusion plays a dominant role for
Jovian electrons close to the source, with the polar gradient becoming the dominant factor
away from the electron source. Of all three anisotropy components, the azimuthal
anisotropy is dominant in the equatorial plane close to the source. It is found that there is
a large azimuthal gradient close to the source because the low-energy electrons tend to
follow the heliospheric magnetic field more closely than higher energy particles. The
transition of the solar wind speed from minimum to intermediate to maximum solar
activity condition was used to illustrate the modulation of the magnitude of the 7 MeV
total anisotropy vector along the Ulysses trajectory. It was found that during the two
encounters with the planet a maximum anisotropy of 38% was computed but with
different anisotropy-timepeaks as the approach to Jupiter was different. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2007.
|
44 |
A study of cosmic ray anisotropies in the heliosphere / Godfrey Sibusiso NkosiNkosi, Godfrey Sibusiso January 2006 (has links)
The three-dimensional (3D) steady-state electron modulation model of Ferreira (2002),
based on Parker (1965) transport equation, is used to study the modulation of the 7 MeV
galactic and Jovian electron anisotropies in the inner heliosphere. The Jovian electrons
are produced in Jupiter's magnetosphere which is situated at ~ 5 AU in the ecliptic plane.
The propagation of these particles is mainly described by the diffusion tensor applicable
for the inner heliosphere. Some of the elements of the diffusion tensor are revisited in
order to establish what contribution they make to the three-dimensional anisotropy vector
and its components in the inner heliosphere. The 'drift' term is neglected since the focus
of this study is on low-energy electrons. The effects on the electron anisotropy of
different scenarios when changing the solar wind speed from minimum to maximum
activity is illustrated. The effects on both the galactic and Jovian electron anisotropy of
changing the polar perpendicular coefficient, in particular, are illustrated. It is shown that
the computed Jovian electron anisotropy dominates the galactic anisotropy close to the
Jovian electron source at ~5 AU, as expected, testifying to the validity of the3D-model.
For the latitudinal anisotropy, the polar perpendicular diffusion plays a dominant role for
Jovian electrons close to the source, with the polar gradient becoming the dominant factor
away from the electron source. Of all three anisotropy components, the azimuthal
anisotropy is dominant in the equatorial plane close to the source. It is found that there is
a large azimuthal gradient close to the source because the low-energy electrons tend to
follow the heliospheric magnetic field more closely than higher energy particles. The
transition of the solar wind speed from minimum to intermediate to maximum solar
activity condition was used to illustrate the modulation of the magnitude of the 7 MeV
total anisotropy vector along the Ulysses trajectory. It was found that during the two
encounters with the planet a maximum anisotropy of 38% was computed but with
different anisotropy-timepeaks as the approach to Jupiter was different. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2007.
|
45 |
Análise do impacto do efeito ionosférico e cintilação ionosférica no Posicionamento Baseado em Redes e Por Ponto / Analysis of impact ionospheric effect and ionospheric scintillation in Network-Based Positioning And Point PositioningCaldeira, Mayara Cobacho Ortega [UNESP] 11 February 2016 (has links)
Submitted by Mayara Cobacho Ortega null (mayarac.ortega@gmail.com) on 2016-10-03T14:46:10Z
No. of bitstreams: 1
Dissertação_Mestrado_Mayara_Caldeira.pdf: 4776033 bytes, checksum: ecc26c1af2e4e6f23b65a5eecbd9ed03 (MD5) / Approved for entry into archive by Ana Paula Grisoto (grisotoana@reitoria.unesp.br) on 2016-10-04T18:11:06Z (GMT) No. of bitstreams: 1
caldeira_mco_me_prud.pdf: 4776033 bytes, checksum: ecc26c1af2e4e6f23b65a5eecbd9ed03 (MD5) / Made available in DSpace on 2016-10-04T18:11:06Z (GMT). No. of bitstreams: 1
caldeira_mco_me_prud.pdf: 4776033 bytes, checksum: ecc26c1af2e4e6f23b65a5eecbd9ed03 (MD5)
Previous issue date: 2016-02-11 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Visando usufruir do potencial dos sistemas de posicionamento global existentes, novos métodos de posicionamento têm surgido e outros vêm sendo aprimorados. Uma grande tendência nos últimos anos tem sido o uso de redes de estações GNSS de referência. Mas, tanto no uso de redes como nos demais métodos, um fator importante para melhorar a qualidade do posicionamento está relacionado com a modelagem atmosférica. Especial atenção deve ser dada aos erros que ocorrem devido à ionosfera, pois ela se tornou a principal fonte de erro no posicionamento GNSS, após desativação da técnica SA. Este erro é diretamente proporcional ao Conteúdo Total de Elétrons (TEC) e inversamente proporcional ao quadrado da frequência do sinal. O TEC e, consequentemente, o erro ionosférico variam no tempo e no espaço, e sofrem diversas influências, como: ciclo solar, época do ano, hora local, localização geográfica, atividade geomagnética, entre outros. Atualmente, o os erros proporcionados pela ionosfera podem ter seus efeitos minimizados a partir de arquivos IONEX ou por meio de modelagem ionosférica. Portanto, nesta pesquisa, foram utilizados dados das estações da RBMC em diferentes regiões do Brasil no período de baixa e alta densidade de elétrons do pico solar 24 para avaliar o desempenho dos mapas ionosféricos, no posicionamento baseado em redes, disponibilizados por diversos centros (CODE, ESA, JPL, UPC e IGS), bem como os fornecidos pelo projeto MIMOSA, e também os modelos de Grade (AGUIAR, 2010) e estimativa de TEC. Para tal fim, foi adotado um sistema computacional desenvolvido na FCT/UNESP, denominado VRS-Unesp, que emprega o conceito de Estação Virtual. De acordo com os resultados obtidos, nota-se que não há um único mapa fornecido pelos centros de análise do IGS que melhor se enquadra a realidade brasileira, além disso, o desempenho do mapa depende das condições ionosféricas e, principalmente, da localização da estação. Além disso, verificou-se que a acurácia obtida pelo IONEX do projeto MIMOSA, pelo modelo de grade e de Estimativa do TEC que utilizam dados regionais e possuem maior resolução espacial e temporal, foram os que apresentaram os melhores resultados. Por fim foi avaliada a correlação entre a acurácia do posicionamento por ponto e o índice de cintilação S4, já que a ionosfera pode não apenas degradar a acurácia do posicionamento GNSS como reduzir sua disponibilidade, pois existe uma alta dependência entre perdas do sinal e irregularidades ionosféricas. Como resultado, considerando a análise de espaço-frequência em relação ao tempo pelo método coerência wavelets para avaliação da correlação da série, nota-se uma correlação no périodo do equinócio superior a 70%. / In order to take advantage from global positioning systems, new positioning methods have emerged and others have been improved. An important tendency in recent years has been the use of GNSS reference stations networks. But, using networks or other positioning methods an important factor to improve the positioning quality is related to atmospheric modeling. Special attention should be given to errors that occur due to ionosphere, it became the largest error source in GNSS positioning after disabling SA technique. Ionosphere error depends on signal frequency and Total Electron Content (TEC) in the ionospheric layer. TEC and consequently the ionospheric error varies regularly in time and space and they are affected by different sources like: sunspot number (solar cycle), season, local time, geographic position, geomagnetic activity, and others. Currently, the errors provided by the ionosphere can be minimized using IONEX files or models. Therefore, in this research, the RBMC stations data were used in different regions of Brazil in the period of low and high electron density of the cycle solar 24 to evaluate the performance of the ionospheric maps, in network-based positioning, available from several centers, as CODE , ESA, JPL, UPC and IGS, as well as those provided by the MIMOSA project, and also the Grade Models (AGUIAR, 2010) and TEC Estimates. For this, a computer system developed in FCT / UNESP has been adopted, RSV-Unesp that uses the concept of Virtual Station. According to the results, we note that there is not single map of IGS analysis centers that best fits the Brazilian reality, moreover, the map performance depends on the ionospheric conditions and, primarily, the station location. Moreover, it was found that the accuracy obtained by IONEX the MIMOSA project, the Grade Model and TEC estimation using regional data and have higher spatial and temporal resolution, showed the best results. Finally we evaluated the correlation between the accuracy of point positioning and scintillation index S4, since the ionosphere can not only degrade the accuracy of GNSS positioning as well as reduce its availability, because there is a high dependency between signal loss and ionospheric irregularities. As result, considering the space-frequency analysis with respect to time by the wavelet coherence method for evaluation of the correlation of the series, there is a correlation in the period of higher equinox to 70%. / FAPESP: 2014/03858-9
|
46 |
An Investigation of magnetic storm effects on total electron content over South Africa for selected periods in solar cycles 23 and 24Van de Heyde, Valentino Patrick January 2012 (has links)
>Magister Scientiae - MSc / The development of regional ionospheric Total Electron Content (TEC) models has contributed to understanding the behavior of ionospheric parameters and the coupling of the ionosphere to space weather activities on both local and global scales. In the past several decades, the International Global Navigation Satellite Systems Service (GNSS) networks of dual frequency receiver data have been applied to develop global and regional models of ionospheric TEC. These models were mainly developed in the Northern Hemisphere where there are dense network of ground based GPS receivers for regional data coverage. Such efforts have been historically rare over the African region, and have only recently begun. This thesis reports the investigation of the effect of mid-latitude magnetic storms on TEC over South Africa for portions of Solar Cycles 23 and 24. The MAGIC package was used to estimate TEC over South Africa during Post Solar Maximum, Solar Minimum, and Post Solar Minimum periods. It is found that TEC is largely determined by the diurnal cycle of solar forcing and subsequent relaxation, but effects due to storms can be determined
|
47 |
Studium variability ionosféry / Study of ionospheric variabilityMošna, Zbyšek January 2014 (has links)
High variability of the ionosphere is connected to geomagnetic, solar, and neutral atmosphere wave activity. Results of scaling analysis of solar data (F10.7, SSN), geomagnetic indices (Dst, Kp, AE), and ionospheric critical frequencies (foF2) show similar structure of Kp, AE and foF2 at periods in the range from 4 to 32 days. Data structure depends on the location of ionospheric stations. Correlation coefficients between foF2 and geomagnetic and solar indices depend on length of time scale. We show that vertical coupling exists between neutral atmosphere activity and sporadic E layer area. This connection is located predominantly on periods corresponding to internal modes of planetary waves. Interplanetary magnetic field discontinuities (Coronal mass ejections, Magnetic clouds, Hight speed solar streams) affect strongly the ionosphere. Analysed events lead to lowering of foF2, increase in heights of the layer F2 and oscillations of hmF2 and foF2 on periods in the order of hours. Powered by TCPDF (www.tcpdf.org)
|
48 |
A comparative study of cosmic ray modulation models / Jan Louis RaathRaath, Jan Louis January 2015 (has links)
Until recently, numerical modulation models for the solar modulation of cosmic rays have
been based primarily on finite difference approaches; however, models based on the solution
of an appropriate set of stochastic differential equations have become increasingly
popular. This study utilises such a spatially three-dimensional and time-stationary model,
based on that of Strauss et al. (2011b). The remarkable numerical stability and powerful
illustrative capabilities of this model are utilised extensively and in a distinctly comparative
fashion to enable new insights into the processes of modulation. The model is
refined to provide for both the Smith-Bieber (Smith and Bieber, 1991) and Jokipii-Kota
(Jokipii and Kota, 1989) modifcations to the Parker heliospheric magnetic field (Parker,
1958) and the implications for modulation are investigated. During this investigation
it is conclusively illustrated that the Parker field is most conducive to drift dominated
modulation, while the Jokipii-Kota and Smith-Bieber modifcations are seen to induce
successively larger contributions from diffusive processes. A further refinement to the
model is the incorporation of a different profile for the heliospheric current sheet. This
profile is defined by its latitudinal extent given by Kota and Jokipii (1983), as opposed
to the profile given by Jokipii and Thomas (1981). An extensive investigation into current
sheet related matters is launched, illustrating the difference between these current
sheet geometries, the associated drift velocity fields and the effect on modulation. At
high levels of solar activity, such that the current sheet enters deep enough into the polar
regions, the profile of Kota and Jokipii (1983) is found to significantly reduce the effective
inward (outward) drifts of positively (negatively) charged particles during A > 0 polarity
cycles. The analogous effect is true for A < 0 polarity cycles and the overall effect is of
such an extent that the A > 0 and A < 0 solutions are found to coincide at the highest
levels of solar activity to form a closed loop. This is a result that has never before been
achieved without having to scale down the drift coefficient to zero at solar maximum,
as was done by e.g. Ndiitwani et al. (2005). Furthermore, it is found that the drift
velocity fields associated with these two current sheet profiles lead to significant differences
in modulation even at such low levels of solar activity where no difference in the
geometries of these profiles are yet in evidence. The model is finally applied to reproduce
four observed galactic proton spectra, selected from PAMELA measurements (Adriani
et al., 2013) during the atypical solar minimum of 2006 to 2009; a new proton local interstellar
spectrum was employed. The results are found to be in accordance with that
found by other authors and in particular Vos (2011), i.e. the diffusion was required to
consistently increase from 2006 to 2009 and, in addition, the rigidity dependence below ~
3 GV was required to change over this time so that the spectra became increasingly softer. / MSc (Space Physics), North-West University, Potchefstroom Campus, 2015
|
49 |
A comparative study of cosmic ray modulation models / Jan Louis RaathRaath, Jan Louis January 2015 (has links)
Until recently, numerical modulation models for the solar modulation of cosmic rays have
been based primarily on finite difference approaches; however, models based on the solution
of an appropriate set of stochastic differential equations have become increasingly
popular. This study utilises such a spatially three-dimensional and time-stationary model,
based on that of Strauss et al. (2011b). The remarkable numerical stability and powerful
illustrative capabilities of this model are utilised extensively and in a distinctly comparative
fashion to enable new insights into the processes of modulation. The model is
refined to provide for both the Smith-Bieber (Smith and Bieber, 1991) and Jokipii-Kota
(Jokipii and Kota, 1989) modifcations to the Parker heliospheric magnetic field (Parker,
1958) and the implications for modulation are investigated. During this investigation
it is conclusively illustrated that the Parker field is most conducive to drift dominated
modulation, while the Jokipii-Kota and Smith-Bieber modifcations are seen to induce
successively larger contributions from diffusive processes. A further refinement to the
model is the incorporation of a different profile for the heliospheric current sheet. This
profile is defined by its latitudinal extent given by Kota and Jokipii (1983), as opposed
to the profile given by Jokipii and Thomas (1981). An extensive investigation into current
sheet related matters is launched, illustrating the difference between these current
sheet geometries, the associated drift velocity fields and the effect on modulation. At
high levels of solar activity, such that the current sheet enters deep enough into the polar
regions, the profile of Kota and Jokipii (1983) is found to significantly reduce the effective
inward (outward) drifts of positively (negatively) charged particles during A > 0 polarity
cycles. The analogous effect is true for A < 0 polarity cycles and the overall effect is of
such an extent that the A > 0 and A < 0 solutions are found to coincide at the highest
levels of solar activity to form a closed loop. This is a result that has never before been
achieved without having to scale down the drift coefficient to zero at solar maximum,
as was done by e.g. Ndiitwani et al. (2005). Furthermore, it is found that the drift
velocity fields associated with these two current sheet profiles lead to significant differences
in modulation even at such low levels of solar activity where no difference in the
geometries of these profiles are yet in evidence. The model is finally applied to reproduce
four observed galactic proton spectra, selected from PAMELA measurements (Adriani
et al., 2013) during the atypical solar minimum of 2006 to 2009; a new proton local interstellar
spectrum was employed. The results are found to be in accordance with that
found by other authors and in particular Vos (2011), i.e. the diffusion was required to
consistently increase from 2006 to 2009 and, in addition, the rigidity dependence below ~
3 GV was required to change over this time so that the spectra became increasingly softer. / MSc (Space Physics), North-West University, Potchefstroom Campus, 2015
|
50 |
Modélisation de l'évolution du réseau magnétique au cours du cycle solaireThibault, Kim 01 1900 (has links)
Le réseau magnétique consiste en un ensemble de petites concentrations
de flux magnétique sur la photosphère solaire. Vu sa petite échelle de taille
et de flux, à la limite de
détection, son comportement n'est connu que depuis récemment.
Les interactions du réseau sont pourtant cruciales afin de comprendre
la dynamo et l'irradiance solaires, car beaucoup de caractérisques du réseau
dépendent de ces interactions. De plus, le réseau est la principale
contribution magnétique surfacique à l'irradiance solaire.
Les modèles existants du réseau ne tenaient jusqu'à maintenant pas compte
des interactions du réseau. Nous avons
tenté de combler cette lacune avec notre modèle.
Nos simulations impliquent une marche aléatoire en 2D de tubes de flux
magnétiques sur la photosphère solaire. Les tubes de flux sont injectés puis
soumis à des règles de déplacement et d'interaction.
L'injection se fait à deux échelles, respectivement la plus petite et la
plus grande observables: les tubes de flux élémentaires et les taches solaires.
Des processus de surface imitant ceux observés sont inclus, et consistent
en l'émergence, la
coalescence, l'annulation et la submergence de flux. La fragmentation des
concentrations n'est présente que pour les taches, sous forme de
désintégration libérant des tubes de flux.
Le modèle est appliqué au cycle solaire 21 (1976-1986, le mieux documenté
en termes de caractéristiques de taches solaires.
Il en résulte des réponses à deux questions importantes en physique solaire.
La première est: l'injection de flux magnétique à deux échelles très distinctes
peut-elle conduire à une distribution de flux en loi
de puissance comme on l'observe, si l'on inclut des processus de surface
qui retraitent le flux? Cette question est étroitement liée à
l'origine de la dynamo solaire, qui pourrait produire ladite
distribution. Nous trouvons qu'on peut effectivement produire une telle
distribution avec ce type d'injection et ce type de
processus de surface. Cela implique
que la distribution de flux observée ne peut servir à déterminer quel type
de dynamo opère dans le Soleil.
La deuxième question à laquelle nous avons apporté un élément de réponse
est celle
à savoir combien de temps il faut au réseau pour retrouver son état d'activité
de base. Cet état a été observé lors du minimum de Maunder en 1645-1715
et touche de près la
question de l'influence de l'activité solaire sur le climat terrestre. Le
récent minimum d'activité est considéré par certains comme ayant atteint
cet état. Nous trouvons plutôt que ça n'a pas été le cas. En effet, le
temps de relaxation du réseau que nous avons calculé est supérieur au temps
écoulé entre la fin du dernier cycle solaire d'activité et celui de l'amorce
du présent cycle. / The magnetic network is an ensemble of small magnetic flux concentrations
on the solar photosphere. Given its small scale in size and flux, at the
detection limit, its behavior has only been known since recently.
The network's interactions are crucial in understanding the solar dynamo
and the solar irradiance, as many network characteristics depend on
these interactions. The network is the main surface magnetic
contribution to the solar irradiance.
The extant models of the network so far did not consider interactions.
We have attempted to remedy this failing with our model.
Our simulations involve a random walk in 2D of magnetic flux tubes on the
solar photosphere. The flux tubes are injected, then undergo displacement
and interaction rules. Injection occurs on two scales, the smallest and
the largest observable respectively: elementary flux tubes and sunspots.
Surface processes are included which imitate the ones observed: emergence,
coalescence, cancellation and submergence of flux.
Fragmentation of concentrations
only happens for sunspots, as disintegration releasing flux tubes from the
spot. The model is applied to solar cycle 21 (1976-1986),
the best documented in terms of sunspot characteristics.
Two important questions in solar physics have been answered with this model.
The first pertains to whether flux injection at two very distinct flux scales
can lead to a flux distribution in the shape of a power law, as observed,
in the presence of surface mechanisms which reprocess the flux.
This question is tied to the origin of the solar dynamo, which could produce
(or not) this distribution. We find that it does produce the aforementioned
distribution. This implies that the observed flux distribution cannot be
used to constrain the type of dynamo operating in the Sun, because the
surface flows can equally well produce the observed flux distribution.
The second question is how long the network takes to return to its
baseline activity level during a prolonged activity minimum. This
state was observed during the Maunder minimum in 1645-1715 and bears
strongly on the relationship between solar activity and Earth climate.
The recent
activity minimum is considered by certain authors to have reached the
baseline state of solar activity. However, we find that this was not the
case. The network relaxation time we calculate is longer than the duration
of the last minimum.
|
Page generated in 0.0637 seconds