ABSTRACT
We
have studied the variability of Cosmic rays flux during solar quiet days at mid
and high latitudes. By using the five (5) quietest days for each month, the
monthly mean diurnal variation of cosmic ray anisotropy have been derived for
the period 1981-2007, which covers part of cycles 21, 22 and 23. These
quiet days are days during which the sun is relatively magnetically quiet,
leading to less anisotropic behavior in the diurnal flux of cosmic rays
measured on the earth’s surface. Four stations (Rome, Oulu, Inuvik and Thule)
were used in this study to understand the important features of the high
latitude and mid-latitude diurnal wave, and how solar and geomagnetic activity
may be influencing the wave characteristics. Cosmic ray wave characteristics
were obtained by discrete Fourier transform (DFT). The mean, diurnal
amplitude, phase and dispersion for each month’s diurnal wave were calculated
and profiled. There was clear indication that the terrestrial effect on the
variability of the monthly mean was more associated with geomagnetic activity
rather than rigidity of the cosmic rays. Correlation of the time series
of these wave characteristics with solar and geomagnetic activity index showed
better association with solar activity.
TABLE OF CONTENTS
Title Page i
Approval page ii
Certification iii
Dedication iv
Acknowledgement v
Abstract vi
Table of Contents vii
List of Figures ix
List of Tables x
CHAPTER ONE: INTRODUCTION
1.1 INTRODUCTION 1
1.2 DISCOVERY AND EARLY RESEARCH 1
1.3 COSMIC RAY COMPOSITION 2
1.4 COSMIC RAYS IN THE SOLAR SYSTEM 2
1.5 COSMIC RAYS ON EARTH 3
1.6 COSMIC RAY VARIABILITY 5
1.7 SOLAR QUIET DAYS AND SOLAR ACTIVITY CYCLE 8
1.8 EFFECTS OF COSMIC RAYS 9
1.9 PURPOSE OF STUDY 11
CHAPTER TWO: LITERATURE REVIEW
2.1 INTRODUCTION 12
2.2 SOLAR MODULATION OF COSMIC RAYS 12
2.3 GEOMAGNETIC FIELD MODULATION OF COSMIC RAYS 13
2.4 COSMIC RAY VARIABILITY AND PERIODICITY 14
CHAPTER THREE: THEORY, METHOD OF DATA ANALYSIS AND RESULTS
3.1 SOURCE OF DATA 16
3.2 THEORY AND METHOD OF DATA ANALYSIS 17
3.3 RESULTS 22
3.3.1 TIME SERIES PROFILES OF MONTHLY MEAN, AMPLITUDE AND DISPERSION 22
3.3.2 PHASE-TIME HISTOGRAMS 26
3.3.3 CORRELATION RESULTS 32
CHAPTER FOUR: DISCUSSION OF RESULTS
4.1 INTRODUCTION 33
4.2 TIME SERIES OF MONTHLY MEAN AND AMPLITUDE OF DIURNAL
WAVE OF COSMIC RAYS 33
4.3 DISPERSION PROFILE 35
4.4 PHASE-TIME VARIABILITY OF COSMIC RAY DIURNAL WAVE 35
CHAPTER FIVE: CONCLUSIONS AND RECOMMENDATION
5.1 CONCLUSION 37
5.2 RECOMMENDATION 38
REFERENCES
APPENDIXES
LIST OF FIGURES
Figure 1: Air shower array created when cosmic rays impinge on the atmosphere 4
Figure 2: Solar wind modulation of cosmic rays 6
Figure 3: Organogram of geomagnetic temporal variations. 8
Figure 4: Monthly SQ diurnal variation with the fit of the first harmonic for Rome station 19
Figure 5: Monthly SQ diurnal variation with the fit of the first harmonic for Oulu station 19
Figure 6: Monthly SQ diurnal variation with the fit of the first harmonic for Inuvik station 20
Figure 7: Monthly SQ diurnal variation with the fit of the first harmonic for Thule station 20
Figure 8: Time series of the monthly mean count rates for the four stations 23
Figure 9: Time series of the monthly diurnal amplitude for the four stations 24
Figure 10: Time series of the dispersion for the four stations 25
Figure 11: Phase-time histograms for all stations 26
Figure 12: Phase-time histograms for Rome station 28
Figure 13: Phase-time histograms for Oulu station 29
Figure 14: Phase-time histograms for Inuvik station 30
Figure 15: Phase-time histograms for Thule station 31
LIST OF TABLES
TABLE 1: Characteristics of the four neutron monitor stations used in our study. 16
TABLE 2: Inter-station monthly mean count rates correlations 32
TABLE 3: Inter-station amplitude correlations 32
TABLE 4: Inter-station dispersion correlations 32
TABLE 5: Geomagnetic activity index (aa) and sunspot number correlation (and partial correlation) with the monthly mean count rates of all four stations 32
TABLE 6: Geomagnetic activity index (aa) and sunspot number correlation (and partial correlation) with the amplitude of all four stations 32
TABLE 7: Stations phase-time frequency records around solar minimum and maximum periods 36
CHAPTER ONE
1.1 INTRODUCTION
Cosmic rays are high-energy charged particles originating mainly from the outer space. They travel at nearly the speed of light and strike the earth from all directions. Most cosmic rays are nuclei of atoms ranging from the lightest to the heaviest elements in the periodic table. Cosmic rays also include high energy electrons, positrons and other subatomic particles. There are broadly three types of cosmic rays; solar cosmic rays (SCRs), galactic cosmic rays (GCRs) and anomalous cosmic rays (ACRs). SCRs originate from the sun and have energy within the range (kilo-electron volt) KeV < 10 to 100 MeV (Mega-electron volt) occasionally reaching 1GeV (Giga-electron volt). GCRs originate from supernovae explosions. They have energy within the range 100MeV < E < 10GeV. ACRs originate from neutral interstellar atoms that have been ionized by solar UV radiation after entering the heliosphere. They have energies of 107 – 108 eV. Cosmic rays attract great interest due to the damage they inflict on electronics and life outside the protection of an atmosphere and a magnetic field, they also provide important channels for astrophysical information.
1.2 DISCOVERY AND EARLY RESEARCH
Cosmic rays were discovered in 1912 by Victor Hess when he found that an electroscope discharged more rapidly as he ascended in a balloon. He attributed this to a source of radiation entering the atmosphere from above and in 1936 was awarded a noble prize in physics for his discovery. For some time it was believed that the radiation was electromagnetic in nature (hence the name cosmic “ray”) and some textbooks still incorrectly include cosmic rays as part of the electromagnetic spectrum. However, during the 1930’s it was found that cosmic rays must be electrically charged because they are affected by the earth’s magnetic field.
From the 1930s to the 1950s before man-made particle accelerators reached very high energies, cosmic rays served as source of particles for high energy physics investigations which led to the discovery of subatomic particles that included the positron and muon. Some of these applications have continued ever with the dawn of space age. The main focus of cosmic ray research has been directed towards astrophysical investigations of where cosmic rays originate, how they get accelerated to such high velocities, what role they play in the dynamics of the galaxy and what their composition tells us about matter from outside the solar system. To measure cosmic rays count rate before they have been slowed down and broken up by the atmosphere, research is carried out by instruments mounted on spacecraft and high altitude balloons, using particle detectors similar to those used in nuclear and high energy physics experiments.
1.3 COSMIC RAY COMPOSITION
Cosmic rays include essentially all of the elements in the periodic table; about 89% of the nuclei are hydrogen (protons), 10% helium and about 1% other heavier elements. The common heavier elements (such as carbon, oxygen, magnesium, silicon and iron) are present in similar relative abundances as in the solar system but there are important differences in elemental and isotopic composition that provide information on the origin and history of galactic cosmic rays. For example, there is a significant overabundance of the rare elements Li, Be and B produced when heavier cosmic rays such as carbon, nitrogen and oxygen disintegrate into lighter nuclei during collisions with the interstellar gas. The isotope 22Ne is also overabundant, showing that the nucleosynthesis of cosmic rays and solar system material differ.
1.4 COSMIC RAYS IN THE SOLAR SYSTEM
Just as cosmic rays are deflected by the magnetic field in interstellar space, they are also affected by the interplanetary magnetic field embedded in the solar wind (the plasma of ions and electrons blowing from the solar corona at about 400km/sec) and therefore have difficulty reaching the inner solar system. Spacecrafts (e.g. voyager 1 and 2) venturing out towards the boundary of the solar system has found that the intensity of galactic cosmic rays increases with distance from the sun. As solar activity varies over the 11-year solar cycle the intensity of cosmic rays at Earth also varies in anti-correlation with the sunspot number.
The sun is also a sporadic source of cosmic ray nuclei and electrons that are accelerated by shock waves traveling through the corona as solar flares thereby releasing magnetic energy; during such occurrences, the intensity of energetic particles in space can increase by a factor of 102 to 106. Such solar particle events are much more frequent during the active phase of the solar cycle. The maximum energy reached in solar particle events is typically 10 to 100MeV, occasionally reaching 1GeV (approximately once a year) to 10GeV (approximately once a decade). Solar energetic particles can be used to measure the elemental and isotopic composition of the sun, thereby complementing spectroscopic studies of solar material.
1.5 COSMIC RAYS ON EARTH
When high energy cosmic rays undergo collisions with atoms of the upper atmosphere, they produce a cascade of “secondary” particles that shower down through the atmosphere to the earth’s surface. Secondary cosmic rays include pions (which quickly decay to produce muons, neutrinos and gamma rays), as well as electrons and positrons produced by muon decay and gamma ray interaction with atmospheric atoms. The number of particles reaching the Earth’s surface is directly related to the energy of the cosmic ray that strikes the upper atmosphere as low energy cosmic rays are blocked off by the atmosphere. Cosmic rays with energies beyond 1014eV are studied with large “air shower” arrays of detectors distributed over many square kilometres that sample the particles produced.
