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Yu Yi - One of the best experts on this subject based on the ideXlab platform.
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Forbush Decrease in lunar space environment observed by crater on the lro
Astrophysics and Space Science, 2019Co-Authors: Jongdae Sohn, S Y Oh, Yu YiAbstract:The Lunar Reconnaissance Orbiter (LRO) launched on 16 June 2009 has seven instruments, including the Cosmic Ray Telescope for the Effects of Radiation (CRaTER). The CRaTER instrument characterizes the global lunar radiation environment and its biological impact by measuring galactic and solar cosmic rays. A Forbush Decrease (FD) is a sudden Decrease in the intensity of galactic cosmic rays (GCRs) recorded at the ground neutron monitor. Using the cosmic ray (CR) data from CRaTER, we identified the lunar CR events with a similar FD profile to that on Earth. Thirty-two FDs were found in the lunar space environment between June, 2009 and December, 2017, by a one-to-one correspondence with the FDs recorded at the terrestrial South Pole neutron monitor and associated interplanetary coronal mass ejections (ICMEs). The minimum GCR intensity of the main phase in the lunar space environment showed a correlation with the maximum interplanetary magnetic field strength of the associated ICME and was double the terrestrial FD magnitude. Because the FDs recorded by CRaTER were identified in space not on the ground, ICME can form an effective magnetic barrier by surrounding the Moon without interactions. The lunar space environment shows the FDs unaffected by the Earth’s magnetosphere. In addition, the background intensity of GCR in the lunar space environment is anti-correlated with the solar activity.
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long term statistical analysis of the simultaneity of Forbush Decrease events at middle latitudes
Journal of Astronomy and Space Sciences, 2015Co-Authors: S Y Oh, Yu Yi, P A Evenson, Hwajin ChoiAbstract:Corresponding AuthorE-mail: euyiyu@cnu.ac.kr, ORCID: 0000-0001-9348-454XTel: +82-42-821-5468, Fax: +82-42-821-8891 This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http:// creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
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simultaneity of Forbush Decrease events observed at middle latitude neutron monitors
Journal of Geophysical Research, 2013Co-Authors: S Y Oh, Yu YiAbstract:[1] Ground neutron monitors (NMs) sometimes observe a sudden reduction in galactic cosmic ray intensity—the so-called Forbush Decrease (FD) event. Such events are mainly associated with interplanetary coronal mass ejections passing around the Earth and corotating interaction regions in the heliosphere. Some FD events are observed globally, either simultaneously or nonsimultaneously, at different NM stations in the case that the simultaneity is determined by the overlapping of the FD main phase, with the period of the cosmic ray intensity decreasing before returning to a steady state. Previous studies have identified two types of FD events with statistically significant differences in the distributions of the main phase onset time. It has been hypothesized that simultaneous FD events occur when a strong magnetic cloud passes by the Earth through the central part of the magnetic barrier, whereas nonsimultaneous events occur if a weaker magnetic cloud passes on the duskside of the magnetosphere. However, the previous statistical analyses were performed using only data from high geomagnetic latitude NM stations in the Northern Hemisphere. To address this shortcoming and to further test the above hypothesis, we repeated the analysis using data from NM stations located at middle latitudes (Jungfraujoch, Irkutsk, and Climax), employing cutoff rigidities 3–6 GV for the last solar maximum period (1998–2002), spanning the same time period as Oh et al. (2008) that employed high-latitude NM stations. The results of the present statistical analysis support the above hypothesis with high confidence levels.
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a simultaneous Forbush Decrease associated with an earthward coronal mass ejection observed by stereo
Solar Physics, 2012Co-Authors: S Y Oh, Yu YiAbstract:The intensity–time profile of Forbush Decrease (FD) events observed by neutron monitors (NMs) looks like that of a geomagnetic storm as defined by the Dst index. Oh, Yi, and Kim (J. Geophys. Res. 113, A01103, 2008) and Oh and Yi (J. Geophys. Res. 114, A11102, 2009) classified FD events based on the amount of overlap and simultaneity of their main phase in Universal Time (UT). Oh and Yi define an FD event as simultaneous if the main phases observed by NMs distributed evenly around the Earth overlap in UT, and nonsimultaneous if they overlap only in the local time of some stations. They suggested that the occurrence mechanisms of two types of FD events may be related to interplanetary (IP) magnetic structures such as IP shocks and magnetic clouds. In their model, the simultaneity of FD events depends on the strength and propagation direction of magnetic structures overtaking the Earth. Recently, the Solar Terrestrial Relations Observatory (STEREO) mission has been able to visualize the emergence and propagation direction of coronal mass ejections (CMEs) in three dimensions in the heliosphere; thus, it is now possible to test the suggested mechanisms. One simultaneous FD event observed on 18 February 2011 may have been caused by a CME heading directly toward the Earth, which was observed on 15 February 2011 by the STEREO mission. Therefore, the simultaneity of FD events is proven to be a useful analysis tool in understanding the geoeffectiveness of solar events such as interplanetary CMEs and IP shocks.
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statistical reality of globally nonsimultaneous Forbush Decrease events
Journal of Geophysical Research, 2009Co-Authors: S Y Oh, Yu YiAbstract:[1] The Forbush Decrease (FD) event, a sudden Decrease of galactic cosmic ray intensity measured by neutron monitor (NM), is known as a globally simultaneous phenomenon. However, some of them do not occur simultaneously in universal time. On the basis of the difference of main phase onset time distributions of two kinds of FD events, Oh et al. (2008) suggested that the global simultaneity of FD events depends on the solar wind physical parameters and propagation direction in three-dimensional heliosphere around the Earth. In order to get the hypothesis approved with a higher confidence level, the FD event data set is extended to 218 events with a longer period from 1971 to 2006 using Oulu, Inuvik, and Magadan station data. In addition, the probability of the same distribution of those two different classes of FD events is calculated in each NM station's view. All three NM stations confirm the probability of different distributions of FD main phase onset times of globally simultaneous and nonsimultaneous with a confidence level of 99%, compared with 94% of previous study. The statistics in this study may support the hypothesis that the simultaneous FD events occur when stronger magnetic barriers pass by the Earth through the central part of the magnetic barriers and in contrast that the nonsimultaneous FD events occur only if the less strong magnetic barriers pass the Earth on the dusk side of the magnetosphere.
M V Alania - 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: A 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, A 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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modeling and data analysis of a Forbush Decrease
Advances in Space Research, 2010Co-Authors: A Wawrzynczak, M V AlaniaAbstract:Abstract We study the Forbush Decrease of the galactic cosmic ray intensity observed in 9–25 September 2005 using the experimental data and a newly developed time-dependent three dimensional modeling. We analyze neutron monitors and muon telescopes, and the interplanetary magnetic field data. We demonstrate a clear relationship between the rigidity ( R ) spectrum exponent (γ) of the Forbush Decrease and the exponent ( ν ) of the power spectral density of the components of the interplanetary magnetic field in the frequency range of ∼ 10 −6 –10 −5 Hz. We confirm that an inclusion of the time-dependent changes of the exponent ν makes the newly developed nonstationary three dimensional model of the Forbush Decrease compatible with the experimental data. Also, we show that the changes of the rigidity spectrum exponent γ does not depend on the level of convection of the galactic cosmic rays stream by solar wind; depending on the changes of the exponent ν , i.e. on the state of the turbulence of the interplanetary magnetic field.
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Forbush Decrease of the galactic cosmic ray intensity experimental study and theoretical modeling
Astrophysics and Space Sciences Transactions, 2008Co-Authors: M V Alania, A WawrzynczakAbstract:We study the temporal changes of the power law rigidity spectrum of the Forbush Decrease (Fd) of the galac- tic cosmic ray (GCR) intensity. We show that the power law rigidity spectrum of Fd for the period of 6-20 Novem- ber 2004 found by neutron monitors and ground muon tele- scopes experimental data, gradually is hardening during the decreasing phase of the intensities and then steadily is soft- ening during the recovery phase. A relation of the rigidity spectrum exponent of the Fd of the GCR intensity with the exponent of the power spectral density (PSD) of the compo- nents of the interplanetary magnetic field (IMF) turbulence is established. We develop three dimensional (3-D) non sta- tionary model of the Fd and show that the results of the theo- retical modeling are in good agreement with the experimen- tal data. We suppose that temporal changes of the rigidity spectrum exponent of the Fd of GCR intensity can be used to calculate the exponent of the PSD of the IMF turbulence for the arbitrary period, which is not achievable by the in situ measurements of the IMF.
S Y Oh - One of the best experts on this subject based on the ideXlab platform.
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Forbush Decrease in lunar space environment observed by crater on the lro
Astrophysics and Space Science, 2019Co-Authors: Jongdae Sohn, S Y Oh, Yu YiAbstract:The Lunar Reconnaissance Orbiter (LRO) launched on 16 June 2009 has seven instruments, including the Cosmic Ray Telescope for the Effects of Radiation (CRaTER). The CRaTER instrument characterizes the global lunar radiation environment and its biological impact by measuring galactic and solar cosmic rays. A Forbush Decrease (FD) is a sudden Decrease in the intensity of galactic cosmic rays (GCRs) recorded at the ground neutron monitor. Using the cosmic ray (CR) data from CRaTER, we identified the lunar CR events with a similar FD profile to that on Earth. Thirty-two FDs were found in the lunar space environment between June, 2009 and December, 2017, by a one-to-one correspondence with the FDs recorded at the terrestrial South Pole neutron monitor and associated interplanetary coronal mass ejections (ICMEs). The minimum GCR intensity of the main phase in the lunar space environment showed a correlation with the maximum interplanetary magnetic field strength of the associated ICME and was double the terrestrial FD magnitude. Because the FDs recorded by CRaTER were identified in space not on the ground, ICME can form an effective magnetic barrier by surrounding the Moon without interactions. The lunar space environment shows the FDs unaffected by the Earth’s magnetosphere. In addition, the background intensity of GCR in the lunar space environment is anti-correlated with the solar activity.
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long term statistical analysis of the simultaneity of Forbush Decrease events at middle latitudes
Journal of Astronomy and Space Sciences, 2015Co-Authors: S Y Oh, Yu Yi, P A Evenson, Hwajin ChoiAbstract:Corresponding AuthorE-mail: euyiyu@cnu.ac.kr, ORCID: 0000-0001-9348-454XTel: +82-42-821-5468, Fax: +82-42-821-8891 This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http:// creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
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simultaneity of Forbush Decrease events observed at middle latitude neutron monitors
Journal of Geophysical Research, 2013Co-Authors: S Y Oh, Yu YiAbstract:[1] Ground neutron monitors (NMs) sometimes observe a sudden reduction in galactic cosmic ray intensity—the so-called Forbush Decrease (FD) event. Such events are mainly associated with interplanetary coronal mass ejections passing around the Earth and corotating interaction regions in the heliosphere. Some FD events are observed globally, either simultaneously or nonsimultaneously, at different NM stations in the case that the simultaneity is determined by the overlapping of the FD main phase, with the period of the cosmic ray intensity decreasing before returning to a steady state. Previous studies have identified two types of FD events with statistically significant differences in the distributions of the main phase onset time. It has been hypothesized that simultaneous FD events occur when a strong magnetic cloud passes by the Earth through the central part of the magnetic barrier, whereas nonsimultaneous events occur if a weaker magnetic cloud passes on the duskside of the magnetosphere. However, the previous statistical analyses were performed using only data from high geomagnetic latitude NM stations in the Northern Hemisphere. To address this shortcoming and to further test the above hypothesis, we repeated the analysis using data from NM stations located at middle latitudes (Jungfraujoch, Irkutsk, and Climax), employing cutoff rigidities 3–6 GV for the last solar maximum period (1998–2002), spanning the same time period as Oh et al. (2008) that employed high-latitude NM stations. The results of the present statistical analysis support the above hypothesis with high confidence levels.
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a simultaneous Forbush Decrease associated with an earthward coronal mass ejection observed by stereo
Solar Physics, 2012Co-Authors: S Y Oh, Yu YiAbstract:The intensity–time profile of Forbush Decrease (FD) events observed by neutron monitors (NMs) looks like that of a geomagnetic storm as defined by the Dst index. Oh, Yi, and Kim (J. Geophys. Res. 113, A01103, 2008) and Oh and Yi (J. Geophys. Res. 114, A11102, 2009) classified FD events based on the amount of overlap and simultaneity of their main phase in Universal Time (UT). Oh and Yi define an FD event as simultaneous if the main phases observed by NMs distributed evenly around the Earth overlap in UT, and nonsimultaneous if they overlap only in the local time of some stations. They suggested that the occurrence mechanisms of two types of FD events may be related to interplanetary (IP) magnetic structures such as IP shocks and magnetic clouds. In their model, the simultaneity of FD events depends on the strength and propagation direction of magnetic structures overtaking the Earth. Recently, the Solar Terrestrial Relations Observatory (STEREO) mission has been able to visualize the emergence and propagation direction of coronal mass ejections (CMEs) in three dimensions in the heliosphere; thus, it is now possible to test the suggested mechanisms. One simultaneous FD event observed on 18 February 2011 may have been caused by a CME heading directly toward the Earth, which was observed on 15 February 2011 by the STEREO mission. Therefore, the simultaneity of FD events is proven to be a useful analysis tool in understanding the geoeffectiveness of solar events such as interplanetary CMEs and IP shocks.
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statistical reality of globally nonsimultaneous Forbush Decrease events
Journal of Geophysical Research, 2009Co-Authors: S Y Oh, Yu YiAbstract:[1] The Forbush Decrease (FD) event, a sudden Decrease of galactic cosmic ray intensity measured by neutron monitor (NM), is known as a globally simultaneous phenomenon. However, some of them do not occur simultaneously in universal time. On the basis of the difference of main phase onset time distributions of two kinds of FD events, Oh et al. (2008) suggested that the global simultaneity of FD events depends on the solar wind physical parameters and propagation direction in three-dimensional heliosphere around the Earth. In order to get the hypothesis approved with a higher confidence level, the FD event data set is extended to 218 events with a longer period from 1971 to 2006 using Oulu, Inuvik, and Magadan station data. In addition, the probability of the same distribution of those two different classes of FD events is calculated in each NM station's view. All three NM stations confirm the probability of different distributions of FD main phase onset times of globally simultaneous and nonsimultaneous with a confidence level of 99%, compared with 94% of previous study. The statistics in this study may support the hypothesis that the simultaneous FD events occur when stronger magnetic barriers pass by the Earth through the central part of the magnetic barriers and in contrast that the nonsimultaneous FD events occur only if the less strong magnetic barriers pass the Earth on the dusk side of the magnetosphere.
A 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: A 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, A 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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modeling and data analysis of a Forbush Decrease
Advances in Space Research, 2010Co-Authors: A Wawrzynczak, M V AlaniaAbstract:Abstract We study the Forbush Decrease of the galactic cosmic ray intensity observed in 9–25 September 2005 using the experimental data and a newly developed time-dependent three dimensional modeling. We analyze neutron monitors and muon telescopes, and the interplanetary magnetic field data. We demonstrate a clear relationship between the rigidity ( R ) spectrum exponent (γ) of the Forbush Decrease and the exponent ( ν ) of the power spectral density of the components of the interplanetary magnetic field in the frequency range of ∼ 10 −6 –10 −5 Hz. We confirm that an inclusion of the time-dependent changes of the exponent ν makes the newly developed nonstationary three dimensional model of the Forbush Decrease compatible with the experimental data. Also, we show that the changes of the rigidity spectrum exponent γ does not depend on the level of convection of the galactic cosmic rays stream by solar wind; depending on the changes of the exponent ν , i.e. on the state of the turbulence of the interplanetary magnetic field.
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Forbush Decrease of the galactic cosmic ray intensity experimental study and theoretical modeling
Astrophysics and Space Sciences Transactions, 2008Co-Authors: M V Alania, A WawrzynczakAbstract:We study the temporal changes of the power law rigidity spectrum of the Forbush Decrease (Fd) of the galac- tic cosmic ray (GCR) intensity. We show that the power law rigidity spectrum of Fd for the period of 6-20 Novem- ber 2004 found by neutron monitors and ground muon tele- scopes experimental data, gradually is hardening during the decreasing phase of the intensities and then steadily is soft- ening during the recovery phase. A relation of the rigidity spectrum exponent of the Fd of the GCR intensity with the exponent of the power spectral density (PSD) of the compo- nents of the interplanetary magnetic field (IMF) turbulence is established. We develop three dimensional (3-D) non sta- tionary model of the Fd and show that the results of the theo- retical modeling are in good agreement with the experimen- tal data. We suppose that temporal changes of the rigidity spectrum exponent of the Fd of GCR intensity can be used to calculate the exponent of the PSD of the IMF turbulence for the arbitrary period, which is not achievable by the in situ measurements of the IMF.
K Nagashima - One of the best experts on this subject based on the ideXlab platform.
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interplanetary magnetic field collimated cosmic ray flow across magnetic shock from inside of Forbush Decrease observed as local time dependent precursory Decrease on the ground
Journal of Geophysical Research, 1994Co-Authors: K Nagashima, K Fujimoto, Isao MorishitaAbstract:In the previous papers (Nagashima et al., 1992, 1993) the authors pointed out the existence of the local-time-dependent precursory Decrease of cosmic rays in front of the shock wave of the interplanetary magnetic field (IMF) and interpreted it as being due to the IMF-collimated outward flow of the low-density cosmic rays across the shock from the inside of the Forbush Decrease. In those papers, however, the physical properties of the collimated flow such as the direction, the collimation angle, and the rigidity spectrum of the constituent cosmic rays were only estimated qualitatively owing to the lack of the simulation of the precursory Decrease produced by the flow. In the present paper these properties are quantitatively obtained by analyzing the hourly data of the precursory Decreases observed on January 25 and 26, 1968, at the worldwide neutron monitor stations. The obtained direction and collimation angle of the flow approximately coincide respectively with those expected from the IMF. This fact gives evidence for the reconfirmation of the existence of the IMF-guided collimated flow, responsible for the precursory Decrease.
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local time dependent pre imf shock Decrease of cosmic rays produced by their imf collimated outward flow across the shock from the inside of Forbush Decrease
Journal of geomagnetism and geoelectricity, 1993Co-Authors: K Nagashima, S Sakakibara, Kazuhiko FujimotoAbstract:In the previous paper (NAGASHIMA et al., 1992), the authors pointed out the existence of the local-time-dependent precursory Decrease of cosmic rays in front of the shock wave of the interplanetary magnetic field (IMF), and interpreted it as being due to their IMF-collimated outward flow across the shock from the inside of the Forbush Decrease (Fd). In the present paper, they report its typical example recently observed in front of the Fd on September 9, 1992, as it bears the following dominant characteristics. The precursory Decrease was not only exceedingly significant but also observed twice at the same station for two days running, which could not be clearly recognized in the previous analysis without taking the average of many events, and it was observed also with the underground muon telescope at Sakashita (depth: 80Hg/cm2), indicating its harder rigidity spectrum than the previous estimate. Based on these observations, the characteristics of the collimated flow responsible for the Decrease is discussed.
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local time dependent pre imf shock Decrease and post shock increase of cosmic rays produced respectively by their imf collimated outward and inward flows across the shock responsible for Forbush Decrease
Planetary and Space Science, 1992Co-Authors: K Nagashima, K Fujimoto, S Sakakibara, I Morishita, R TatsuokaAbstract:Abstract The cosmic-ray storm known as “Forbush Decrease” is produced generally as a result of the transient diffusion-convection of cosmic rays caused by the passage of the interplanetary magnetic field (IMF) shock wave. It is emphasized, however, that the storm is frequently accompanied by non-diffusion-convection-type phenomena. In the present paper, the authors show the existence of such phenomena, which are dependent on local time. These are: (1) the precursory Decrease of cosmic-ray intensity in front of the shock, which occurs in the morning (6–12 h), having nearly the same rigidity spectrum as that of the Forbush Decrease; and (2) the post-shock increase, which belongs to the daily variation in general, but bears the following anomalous characters; a steep peak as high as the pre-storm intensity level, an extremely soft rigidity spectrum and a phase of 18–24 h in space considerably later than the usual. It is concluded that the precursory Decrease is produced by the IMF-collimated outward flow of the low-density cosmic rays from the inside of the shock, and that the collimation is determined by the ratio between the ordered magnetic fields at the shock front and at the observation point. Inversely, the post-shock increase is produced by the IMF-collimated inward flow of the high-density cosmic rays from the outside. As an extreme case of the above phenomena, they also point out the existence of the IMF-guided square wave of cosmic-ray intensity with 24 h periodicity, which is produced as a result of the Earth's rotation in the unbalanced two-way flows along the magnetic lines of force connecting two separated regions occupied, respectively, by the high- and low-density cosmic rays. Finally, a serious influence of the precursory Decrease on the determination of the commencement of Forbush Decrease and also on the study of the precursory increase expected to appear in front of the shock wave, is discussed on the basis of definite examples.
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Localized pits and peaks in Forbush Decrease, associated with stratified structure of disturbed and undisturbed magnetic fields
Il Nuovo Cimento C, 1990Co-Authors: K Nagashima, K Fujimoto, S Sakakibara, R Tatsuoka, I MorishitaAbstract:Forbush Decrease (FD) is generally interpreted as a result of diffusion-convection of cosmic rays in a disturbed interplanetary magnetic field associated with the magnetohydrodynamic shock wave caused by solar flare. In this paper, we point out that a large number of FDs contain an isolated region or regions with pit-type time profile, in which cosmic rays are not in a diffusion-convection state but in a trapped state in undisturbed, uniform and strong magnetic field perpendicular to the solar wind. The trapped state is also characterized with a large ratio of the magnetic to ion thermal energy. The median duration time of the state is about 8 hours. About half of these states are associated with the northward (or southward) magnetic field, while the other half with the eastward (or westward) magnetic field. Flares responsible for the former state seem to be concentrated in an eastward region from about 30°W on the solar disk, while those for the latter state seem rather symmetric with respect to the centre of the solar disk. It is suggested that the trapped state is produced inside a magnetic tube of force which is not of a small scale such as that of the magnetic bubble pointed out by Klein and Burlaga, but of a large scale, having a horseshoe structure with its ends supposed to be connected to somewhere in an inner region near the Sun and with its cross-section supposed to be of a thin filament with radial and transverse dimensions of ≈0.1 a.u. and ≈1.1 a.u. at the Earth’s orbit. This belt-like tube of force is supposed to be produced on the solar surface or near the Sun and to be carried out by solar wind in a frozen state, trapping in itself low-density cosmic rays near the Sun. In addition to the pits, we point out also the existence of some peaks which are observed not only in the trapped region but also in a region of extremely disturbed magnetic field neighbouring in between two trapped regions. It is suggested that cosmic rays in the region of the latter type are supposed to be guided freely (or easily) from outer space through a path with similarly disturbed magnetic state, and therefore, they could maintain their density in the region always higher than in the neighbouring regions. Two kinds of cosmic-ray-guiding mechanism in the above can be regarded as being at opposite poles.