The Experts below are selected from a list of 327 Experts worldwide ranked by ideXlab platform
Frank B. Mcdonald - One of the best experts on this subject based on the ideXlab platform.
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An asymmetric solar wind termination shock
Nature, 2008Co-Authors: E. C. Stone, A. C. Cummings, Frank B. Mcdonald, Bryant C. Heikkila, Nand Lal, William R. WebberAbstract:Voyager 2 crossed the solar wind termination shock at 83.7 au in the southern hemisphere, ~10 au closer to the Sun than found by Voyager 1 in the north. This asymmetry could indicate an asymmetric pressure from an interstellar magnetic field, from transient-induced shock motion, or from the solar wind dynamic pressure. Here we report that the intensity of 4–5 MeV protons accelerated by the shock near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in the shock at the two locations. (Companion papers report on the plasma, magnetic field, plasma-wave and lower energy particle observations at the shock.) Voyager 2 did not find the source of anomalous Cosmic Rays at the shock, suggesting that the source is elsewhere on the shock or in the heliosheath. The small intensity gradient of Galactic Cosmic Ray helium indicates that either the gradient is further out in the heliosheath or the local interstellar Galactic Cosmic Ray intensity is lower than expected.
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Galactic Cosmic Ray modulation: Effects of the solar wind termination shock and the heliosheath
Journal of Geophysical Research, 2004Co-Authors: R. A. Caballero-lopez, Harm Moraal, Frank B. McdonaldAbstract:[1] This paper studies the radial intensity profiles of Galactic Cosmic Ray protons (H) and α particles (He) during the solar minimum periods of 1987 (the so-called negative drift state) and 1977/1997 (both positive drift states). These intensities, as measured with the Pioneers 10/11, Voyagers 1/2, and IMP spacecraft, are examined with numerical solutions of the Cosmic Ray transport equation. Previous studies have shown that the Galactic Cosmic Ray intensities and radial gradients observed by the Voyagers during 1997 in the outer heliosphere were so low that they cannot be readily explained by a standard no-shock modulation model. Here we investigate whether acceleration at the solar wind termination shock and the transport of these particles through an extended heliosheath beyond this shock provide sufficient additional modulation to explain these low intensities. In this approach we take into account several different heliomagnetic field configurations. It is found that the acceleration at the termination shock and modulation in the heliosheath do not alleviate the problem but that a nonspherical shock and heliopause produce modulation features that may possibly explain the observations. These shock and heliosheath effects are discussed in detail.
W J Chaplin - One of the best experts on this subject based on the ideXlab platform.
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the behaviour of Galactic Cosmic Ray intensity during solar activity cycle 24
Solar Physics, 2019Co-Authors: Eddie Ross, W J ChaplinAbstract:We have studied long-term variations of Galactic Cosmic-Ray (GCR) intensity in relation to the sunspot number (SSN) during the most recent solar cycles. This study analyses the time lag between the GCR intensity and SSN, and hysteresis plots of the GCR count rate against SSN for Solar Cycles 20 - 23, to validate a methodology against previous results in the literature, before applying the method to provide a timely update on the behaviour of Cycle 24. Plots of SSN versus GCR show a clear difference between the odd- and even-numbered cycles. Linear and elliptical models have been fit to the data, with the linear fit and elliptical model proving the more suitable model for even- and odd-numbered solar-activity cycles, respectively, in agreement with previous literature. Through the application of these methods for Solar Cycle 24, it has been shown that Cycle 24 experienced a lag of two to four months between the GCR intensity and SSN, and this follows the trend of the preceding activity cycles, albeit with a slightly longer lag than previous even-numbered cycles. It has been shown through the hysteresis analysis that the linear fit is a better representative model for Cycle 24, as the ellipse model does not show a significant improvement, which is also in agreement with previous even-numbered cycles.
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the behaviour of Galactic Cosmic Ray intensity during solar activity cycle 24
arXiv: Solar and Stellar Astrophysics, 2018Co-Authors: Eddie Ross, W J ChaplinAbstract:We have studied long-term variations of Galactic Cosmic Ray (GCR) intensity in relation to the sunspot number (SSN) during the most recent solar cycles. This study analyses the time-lag between the GCR intensity and SSN, and hysteresis plots of the GCR count rate against SSN for solar activity cycles 20-23 to validate a methodology against previous results in the literature, before applying the method to provide a timely update on the behaviour of cycle 24. Cross-plots of SSN vs GCR show a clear difference between the odd-numbered and even-numbered cycles. Linear and elliptical models have been fit to the data with the linear fit and elliptical model proving the more suitable model for even-numbered and odd-numbered solar activity cycles respectively, in agreement with previous literature. Through the application of these methods for the 24th solar activity cycle, it has been shown that cycle 24 experienced a lag of 2-4 months and follows the trend of the preceding activity cycles albeit with a slightly longer lag than previous even-numbered cycles. It has been shown through the hysteresis analysis that the linear fit is a better representative model for cycle 24, as the ellipse model doesn't show a significant improvement, which is also in agreement with previous even-numbered cycles.
William R. Webber - One of the best experts on this subject based on the ideXlab platform.
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An asymmetric solar wind termination shock
Nature, 2008Co-Authors: E. C. Stone, A. C. Cummings, Frank B. Mcdonald, Bryant C. Heikkila, Nand Lal, William R. WebberAbstract:Voyager 2 crossed the solar wind termination shock at 83.7 au in the southern hemisphere, ~10 au closer to the Sun than found by Voyager 1 in the north. This asymmetry could indicate an asymmetric pressure from an interstellar magnetic field, from transient-induced shock motion, or from the solar wind dynamic pressure. Here we report that the intensity of 4–5 MeV protons accelerated by the shock near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in the shock at the two locations. (Companion papers report on the plasma, magnetic field, plasma-wave and lower energy particle observations at the shock.) Voyager 2 did not find the source of anomalous Cosmic Rays at the shock, suggesting that the source is elsewhere on the shock or in the heliosheath. The small intensity gradient of Galactic Cosmic Ray helium indicates that either the gradient is further out in the heliosheath or the local interstellar Galactic Cosmic Ray intensity is lower than expected.
Anna Wawrzynczak - One of the best experts on this subject based on the ideXlab platform.
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the connection of the interplanetary magnetic field turbulence and rigidity spectrum of forbush decrease of the Galactic Cosmic Ray intensity
Journal of Physics: Conference Series, 2015Co-Authors: Anna Wawrzynczak, M. V. AlaniaAbstract:We analyze the temporal changes in the rigidity spectrum of Forbush decrease (Fd) of the Galactic Cosmic Ray (GCR) intensity observed in November 2004. We compute the rigidity spectrum in two energy ranges based on the daily data from the worldwide network of neutron monitors and Nagoya ground muon telescope. We demonstrate that the changes in the rigidity spectrum of Fd are linked to the evolution/decay of the interplanetary magnetic field (IMF) turbulence during various phases of the Fd. We analyze the time-evolution of the state of the turbulence of the IMF in various frequency ranges during the Fd. Performed analysis show that the decrease of the exponent ν of the Power Spectral Density (PSD ∝ f−ν, where f is frequency) of the IMF turbulence with decreasing frequency lead to the soft rigidity spectrum of Fd for GCR particles with relatively higher energies.
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energy dependence of the rigidity spectrum of forbush decrease of Galactic Cosmic Ray intensity
Advances in Space Research, 2012Co-Authors: M. V. Alania, Anna WawrzynczakAbstract:Abstract We show that rigidity spectrum of Forbush decrease (Fd) of Galactic Cosmic Ray (GCR) intensity in September 9–23, 2005 clearly depends on energy. We calculated rigidity spectrum of the Fd based on the neutron monitors and Nagoya muon telescope channels’ data divided in three groups according to their cut off rigidities. We found that temporal changes of rigidity spectrum exponent γ are approximately similar for all cut off rigidity groups, but γ values are the larger the higher are cut off rigidities. We conclude that rigidity spectrum of Fd is hard for lower energy range and is soft for the higher energy range. We believe that an energy dependence of the power law rigidity spectrum of Fd is observed owing to the preferential convection–diffusion mechanism during Fd in September 9–23, 2005. It is a reflection of an influence of the temporal changes of the structure of the interplanetary magnetic field (IMF) turbulence in different range of frequency f during Fd. Particularly, a decisive role in formation of the character of the rigidity spectrum belongs to the changes of the exponent ν of the power spectral density (PSD) of the IMF turbulence (PSD ∝ f−ν). The exponent ν is greater for high frequency region of the IMF turbulence (responsible for scattering of low rigidity particles of GCR), than for low frequency region of the IMF turbulence (being responsible for scattering of higher rigidity particles). Also, we challenge to estimate an existence of slab/2D structure of solar wind turbulence during the Fd in September 9–23, 2005 based on the distribution of average turbulence energy among the IMF’s components.
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On the relationship of the 27-day variations of the solar wind velocity and Galactic Cosmic Ray intensity in minimum epoch of solar activity
Solar Physics, 2011Co-Authors: M. V. Alania, Renata Modzelewska, Anna WawrzynczakAbstract:We study the relationship of the 27-day variation of the Galactic Cosmic Ray intensity with similar changes of the solar wind velocity and the interplanetary magnetic field based on the experimental data for the Bartels rotation period 2379 of 23 November 2007-19 December 2007. We develop a three dimensional (3-D) model of the 27-day variation of Galactic Cosmic Ray intensity based on the heliolongitudinally dependent solar wind velocity. A consistent, divergence-free interplanetary magnetic field is derived by solving Maxwells equations with a heliolongitudinally dependent 27-day variation of the solar wind velocity reproducing in situ observations. We consider two types of 3-D models of the 27-day variation of Galactic Cosmic Ray intensity - (1) with a plane heliospheric neutral sheet, and (2)- with the sector structure of the interplanetary magnetic field. The theoretical calculation shows that the sector structure does not influence significantly on the 27-day variation of Galactic Cosmic Ray intensity as it was shown before based on the experimental data. Also a good agreement is found between the time profiles of the theoretically expected and experimentally obtained first harmonic waves of the 27-day variation of the Galactic Cosmic Ray intensity (correlation coefficient equals 0.98 0.02). The expected 27-day variation of the Galactic Cosmic Ray intensity is inversely correlated with the modulation parameter z (correlation coefficient equals -0.91 0.05) which is proportional to the product of the solar wind velocity V and the strength of the interplanetary magnetic field B (z VB). The high anticorrelation between these quantities indicates that the predictable 27-day variation of the Galactic Cosmic Ray intensity mainly is caused by this basic modulation effect.
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Modeling of the recurrent Forbush effect of the Galactic Cosmic Ray intensity and comparison with the experimental data
Advances in Space Research, 2008Co-Authors: Anna Wawrzynczak, M. V. AlaniaAbstract:Abstract We study temporal changes of the power law rigidity R spectrum δ D ( R ) D ( R ) ∝ R - γ of the four (2–18 September 1996, 30 September–15 October 1996, 18 March–4 April 2002, and 16 June–4 July 2003) recurrent Forbush effects of the Galactic Cosmic Ray intensity using neutron monitors experimental data and theoretical modeling. We show that the rigidity spectra for all (four) Forbush effects gradually are hardening (the exponent γ decreases) during the decreasing phases of the intensities and then steadily are softening (the exponent γ increases) during the recovery phases. A relationship between the rigidity spectrum exponent γ and the exponent ν of the power spectral density (PSD) of the components of the interplanetary magnetic field in the range of frequency 4 × 10−6 Hz ⩽ f ⩽ 10−5 Hz (PSD ∝ f−V) is established. During the Forbush effects the exponent ν increases and the exponent γ decreases. We assume that the relationship between the exponent γ and the exponent ν is observed owing to the dependence of the diffusion coefficient K on the Galactic Cosmic Ray particles rigidity R as, K ∝ Rα, where α = 2−ν. We develop models (stationary and nonstationary) of the recurrent Forbush effect and show that Parker’s anisotropic diffusion equation is successfully acceptable to describe the features of the recurrent Forbush effect of the Galactic Cosmic Ray intensity. The temporal changes of the rigidity spectrum exponent γ of the Forbush effect of Galactic Cosmic Ray intensity can be successfully used to estimate the temporal evolution of the interplanetary magnetic field turbulence (the exponent ν) for the short period (even less than 1 day), which is not achievable by the in situ measurements of the interplanetary magnetic field.
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Peculiarities of Galactic Cosmic Ray Forbush effects during October–November 2003
Advances in Space Research, 2005Co-Authors: Anna Wawrzynczak, M. V. AlaniaAbstract:Abstract Features of two successive Forbush effects of the Galactic Cosmic Ray intensity in October–November 2003 have been studied based on the neutron monitors data. The rigidity spectrum of the Galactic Cosmic Ray intensity in the course of the first Forbush effect (22–27 October) is gradually hardening, while the rigidity spectrum of the second Forbush effect (28 October–10 November) from the starting moment is very hard. As far, the energy range of the turbulence of the interplanetary magnetic field is in general responsible for the diffusion of Galactic Cosmic Ray particles of the energy 5–50 GeV (to which neutron monitors are sensitive), we postulate that the gradually hardening (from day to day) of the rigidity spectrum of the first Forbush effect is associated with the enhancement of the power spectral density in the energy range of the interplanetary magnetic field turbulence caused by the large scale irregularities generated due to the interaction of the extending high speed disturbances with the background solar wind. The very hard rigidity spectrum (from the starting moment) of the second Forbush effect is generally associated with the well established new structure of the energy range of the interplanetary magnetic field turbulence enriched by the already created large scale irregularities. The gradually softening of the rigidity spectrum during the recovery phase of the second Forbush effect confirms that the disturbed interplanetary magnetic field turbulence step by step returns to the initial state.
Eddie Ross - One of the best experts on this subject based on the ideXlab platform.
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the behaviour of Galactic Cosmic Ray intensity during solar activity cycle 24
Solar Physics, 2019Co-Authors: Eddie Ross, W J ChaplinAbstract:We have studied long-term variations of Galactic Cosmic-Ray (GCR) intensity in relation to the sunspot number (SSN) during the most recent solar cycles. This study analyses the time lag between the GCR intensity and SSN, and hysteresis plots of the GCR count rate against SSN for Solar Cycles 20 - 23, to validate a methodology against previous results in the literature, before applying the method to provide a timely update on the behaviour of Cycle 24. Plots of SSN versus GCR show a clear difference between the odd- and even-numbered cycles. Linear and elliptical models have been fit to the data, with the linear fit and elliptical model proving the more suitable model for even- and odd-numbered solar-activity cycles, respectively, in agreement with previous literature. Through the application of these methods for Solar Cycle 24, it has been shown that Cycle 24 experienced a lag of two to four months between the GCR intensity and SSN, and this follows the trend of the preceding activity cycles, albeit with a slightly longer lag than previous even-numbered cycles. It has been shown through the hysteresis analysis that the linear fit is a better representative model for Cycle 24, as the ellipse model does not show a significant improvement, which is also in agreement with previous even-numbered cycles.
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the behaviour of Galactic Cosmic Ray intensity during solar activity cycle 24
arXiv: Solar and Stellar Astrophysics, 2018Co-Authors: Eddie Ross, W J ChaplinAbstract:We have studied long-term variations of Galactic Cosmic Ray (GCR) intensity in relation to the sunspot number (SSN) during the most recent solar cycles. This study analyses the time-lag between the GCR intensity and SSN, and hysteresis plots of the GCR count rate against SSN for solar activity cycles 20-23 to validate a methodology against previous results in the literature, before applying the method to provide a timely update on the behaviour of cycle 24. Cross-plots of SSN vs GCR show a clear difference between the odd-numbered and even-numbered cycles. Linear and elliptical models have been fit to the data with the linear fit and elliptical model proving the more suitable model for even-numbered and odd-numbered solar activity cycles respectively, in agreement with previous literature. Through the application of these methods for the 24th solar activity cycle, it has been shown that cycle 24 experienced a lag of 2-4 months and follows the trend of the preceding activity cycles albeit with a slightly longer lag than previous even-numbered cycles. It has been shown through the hysteresis analysis that the linear fit is a better representative model for cycle 24, as the ellipse model doesn't show a significant improvement, which is also in agreement with previous even-numbered cycles.
