This study investigates the phenomenon of Low Amplitude Anisotropic Wave Train Events (LAWEs) in cosmic ray intensity using data from the ground-based Deep River neutron monitor for the period 1991-94. The analysis reveals that the amplitude of diurnal anisotropy during LAWE events consistently remains low and statistically constant compared to the quiet day annual average amplitude for the majority of events. Furthermore, the timing of the maximum diurnal anisotropy for LAWE events shifts significantly towards earlier hours, aligning with the co-rotational direction and remaining consistent with the direction of quiet day annual average anisotropy for most events.
Conversely, the amplitude of semi/tri-diurnal anisotropy remains statistically high and unchanged. However, the phase shift occurs towards later hours compared to the quiet day annual average values for the majority of LAWEs. Analysis of diurnal anisotropy vectors reveals a shift towards earlier hours in 50% of events and a shift towards later hours in the remaining 50%, relative to the average vector for the entire period. Additionally, it is observed that the amplitude of these vectors significantly increases with the shift towards later hours.
The study suggests that high-speed solar wind streams do not play a significant role in causing LAWE events on a short-term basis, but they may contribute to these events on a long-term basis. The occurrence of LAWE is found to be dominant when the polarity of Bx and Bz remains positive, and the polarity of By remains negative — a phenomenon not reported previously. Furthermore, the amplitude of the first harmonic exhibits a good anti-correlation with solar wind velocity, while the direction of the first and third harmonics shows nearly good anti-correlation. The direction of the second harmonic shows nearly good anti-correlation with interplanetary magnetic field strength.
Table of Contents
1. Introduction
2. Data Sources and Analysis
2.1 Trend Correction
3. Results and discussion
4. Conclusions
5. Acknowledgements
6. References
7. Captions to Figures
Research Objectives and Topics
This study aims to investigate the physical mechanisms and interplanetary configurations responsible for an unusual class of low amplitude anisotropic wave train events (LAWEs) in cosmic ray intensity observed between 1991 and 1994, utilizing ground-based neutron monitor data to analyze amplitude and phase variations.
- Analysis of cosmic ray diurnal, semi-diurnal, and tri-diurnal anisotropy variations.
- Examination of the solar wind plasma (SWP) and interplanetary magnetic field (IMF) parameters in relation to LAWEs.
- Application of the superposed epoch analysis method to identify trends in interplanetary magnetic field polarity.
- Investigation of the relationship between solar wind velocity and harmonic amplitudes of cosmic ray intensity.
Excerpt from the Book
Introduction
Daily variation in cosmic ray intensity arises from spatial anisotropies in interplanetary space. Ground-based detectors record these once everyday as their 'asymptotic cone of acceptance' sweeps through the direction containing the spatial anisotropy. The asymptotic cone of acceptance of a detector is the solid angle that contains all the asymptotic directions of approach of particles of various energies, which make a significant contribution to the counting rate of the detector.
The solar diurnal variation of the cosmic ray intensity was interpreted initially on the basis of an outward radial convection and an inward diffusion along the interplanetary magnetic field (IMF). The balance between the convection and diffusion generates an energy independent anisotropic flow of cosmic ray particles from the 18-hour co-rotational direction. Ananth et al. (1974) on their study of diurnal anisotropy on day to day concluded that on an average basis the diurnal anisotropy of cosmic radiation is completely understood as a superposition of simple convection and field aligned diffusion. Cosmic ray intensity observed on the ground is subject to the solar semi-diurnal variation of extraterrestrial origin. The variation is due to the second order anisotropy produced by the diffusion-convection of cosmic rays in interplanetary space (Quenby and Lietti, 1968; Munakata and Nagashima, 1986).
Summary of Chapters
Introduction: Provides the theoretical background of cosmic ray variation, explains the convective-diffusive model, and states the motivation for studying low amplitude anisotropic wave train events.
Data Sources and Analysis: Details the usage of Deep River neutron monitor data, selection criteria for LAWEs, and the mathematical implementation of trend correction and Fourier Techniques.
Results and discussion: Presents the findings regarding amplitude and phase shifts, solar wind velocity correlations, and the specific IMF polarity conditions associated with LAWE occurrences.
Conclusions: Summarizes the eight primary findings derived from the analysis, including the role of IMF polarity and the lack of correlation with high-speed solar wind streams on a short-term basis.
Acknowledgements: Credits the various individuals and institutions, such as the Tropical Forest Research Institute and NSSDC, that provided data and support.
References: Lists the academic literature and scientific sources cited throughout the study.
Captions to Figures: Describes the content and graphical parameters of the nine figures presented in the research.
Keywords
cosmic ray, diurnal, semi-diurnal, anisotropy, solar wind, interplanetary magnetic field, LAWE, convection, diffusion, harmonic analysis, IMF, neutron monitor, solar modulation, polarity, amplitude
Frequently Asked Questions
What is the primary focus of this research?
The work investigates the characteristics of low amplitude anisotropic wave train events (LAWEs) in cosmic ray intensity during the period 1991-1994.
What are the central thematic fields?
The study centers on cosmic ray physics, specifically analyzing the diurnal, semi-diurnal, and tri-diurnal anisotropies in relation to solar wind and interplanetary magnetic field parameters.
What is the main research objective?
The goal is to determine the physical mechanisms and specific interplanetary configurations that trigger low amplitude anisotropic wave trains.
Which scientific methods are employed?
The authors use harmonic analysis based on the Fourier Technique, superposed epoch analysis, and investigations of IMF and solar wind plasma data retrieved from the Deep River neutron monitor.
What does the main body of the work cover?
It provides an introduction to cosmic ray variations, methodology for data correction, a detailed graphical discussion of amplitude and phase changes, and correlation analyses with IMF and solar wind components.
What are the primary keywords characterizing this study?
Key terms include cosmic ray, anisotropy, diurnal variation, solar wind, interplanetary magnetic field, and LAWE.
How is the occurrence of LAWE influenced by IMF polarity?
The study finds that LAWE occurrences are dominant when the polarity of Bx and Bz remains positive while the polarity of By remains negative.
Do high-speed solar wind streams cause LAWEs?
The evidence suggests that high-speed solar wind streams do not play a significant role in causing these events on a short-term basis.
How is trend correction applied?
The data is corrected for secular variations by applying a trend correction to the hourly ordinates, accounting for the difference between the 0th and 24th-hour readings.
- Citation du texte
- Rajesh Kumar Mishra (Auteur), Rekha Agarwal (Auteur), 2024, Harmonics of Low Amplitude Anisotropic Wave Train Events. Insights from Deep River Neutron Monitor Data (1991-94), Munich, GRIN Verlag, https://www.grin.com/document/1437594
