A study of solar energetic particles depending on solar activities and source regions

다국어 초록 (Multilingual Abstract)

In this study, we have studied solar energetic particles (SEPs) depending on solar activities for the occurrence probability as well as the relationship between SEPs and solar activities. Also, we have investigated the SEP source regions using multiple spacecraft to understand its generation mechanism, especially for the relationship with EUV waves.

Firstly, we have examined the occurrence probability of solar proton events (SPEs) and their peak fluxes at Earth depending on three flare parameters (X-ray peak flux, longitude, and impulsive time). For this we used NOAA SPE list from 1976 to 2006, and their associated X-ray flare data. As a result, we selected 166 proton events that were associated with major flares; 85 events associated with X-class flares and 81 events associated with M-class flares. Especially the occurrence probability strongly depends on three parameters as follows. (1) We found that about only 3.5% (1.9% for M-class and 21.3% for X-class) of the flares are associated with the proton events. (2) It is also found that this fraction strongly depends on longitude; for example, the fraction for W30° < L ≤ W90° is about three times larger than that for E30° < L ≤ E90°. (3) We also note that the occurrence probability of SPEs associated with long duration (T ≥ 0.3 hours) flares is about 2 (X-class flare) to 7 (M-class flare) times larger than that with short duration (T < 0.3 hours) flares. (4) The largest difference is found between eastern short duration M-class flare group (0.3%) and western long duration X-class flare group (46.5%). In addition, the relationship between X-ray flare peak flux and proton peak flux as well as its correlation coefficient are strongly dependent on longitude and impulsive time.

Secondly, we have examined the occurrence probability of SPEs and their peak fluxes depending CME parameters, linear speed, angular width, and location. For this we used the NOAA SPE list and their associated CME data from 1997 to 2006. We found that the probability strongly depends on CME speed and angular width as follows. The highest association (36.1%) is found for the full halo CMEs with V ≥ 1500 km s-1 but the lowest association (0.9%) is found for the partial halo CMEs with 400 km s-1 ≤ V < 1000 km s-1. The SPE occurrence probabilities are different as much as 4.9 to 23 times according to CME speed and 1.6 to 6.5 times to angular width. The probabilities depending on CME speed and location increase from the eastern region to the western region and with speed. We have also examined the relationship between CME speed and SPE flux as well as its dependence on angular width (partial halo and full halo), longitude (east, center, and west) and direction parameter (< 0.4 and ≥ 0.4). Our results show that the relationships strongly depend on longitude as well as direction parameter.

Thirdly, we have made a statistical study of backside SPEs and frontside SPEs using the NOAA SPE list from 1997 to 2011. For this, we consider 88 events associated with coronal mass ejections (CMEs) and/or flares. In the case of that events have not enough locational information, we decided their source longitudes by searching for the strongest active region among possible source regions and considering solar rotation rate. From this analysis, we found that about 22% (19/88) of the all events originated from backside. Their maximum longitudes are around E120° for the eastern events and W180° for the western events, respectively. The maximum SPE peak flux (2360 pfu) for the backside group is about 13 times smaller than that of the frontside group (31700 pfu). The mean SPE peak fluxes of backside eastern (> E90°) and western (> W90°) events are 52 pfu and 468 pfu, which are much smaller than that (4517 pfu) of frontside central events (E30° ≤ L ≤ W30°). The average of CME speeds related to the east backside and the east frontside events are 1680 km s-1 and 1773 km s-1, which is a little larger than that of the others. There is a poor correlation (correlation coefficient = 0.41 and statistical significance = 91%) between CME speed and SPE peak flux for the backside group, which is contrast to that (correlation coefficient = 0.57 and statistical significantce = 100%) for the frontside group. Backside solar activities can generate a significant number of SPEs.

In last, we studied the source regions of 12 SEP events seen between 2010 August and 2012 January at STEREO-A, B and/or Earth (ACE/SOHO/GOES), when the two STEREO spacecraft were separated by about 180°. All events were associated with flares (C1 to X6) and fast CMEs and, except for one, accompanied by type II radio bursts. We have determined the arrival times of the SEPs at the three positions. Extreme ultraviolet (EUV) waves, observed in the 195Å and 193Å channels of STEREO and SDO, are tracked across the Sun to determine their arrival time at the photospheric source of open field lines connecting to the spacecraft. There is a good correlation between the EUV wave arrival times at the connecting footpoints and the SEP onset times. The delay time between electron onset and the EUV wave reaching the connecting footpoint is independent of distance from the flare site. The proton delay time increases with distance from the flare site. In three of the events secondary flare sites may have also contributed to the wide longitudinal spread of SEPs.

목차 (Table of Contents)

• Abstract i
• Table of Contents iv
• List of Figures vii
• List of Tables xiii
• Chapter 1 Introduction 1
• 1.1 Solar Energetic Particle 1
• 1.1.1 Acceleration Mechanisms of SEPs 3
• 1.1.2 Classifications of SEPs 6
• 1.2 Forecast of Solar Proton Event 10
• 1.3 Overview 12
• Chapter 2 Dependence of Solar Proton Events on Flare Parameters 14
• 2.1 Introduction 14
• 2.2 Data and Analysis 16
• 2.3 Results and Discussion 17
• 2.3.1 Proton Event Occurrence Probability 17
• 2.3.2 Relationship between Proton Peak Flux and Flare Peak Flux and Its Dependence on Longitude 21
• 2.3.3 Relationship between Proton Peak Flux and Flare Peak Flux and Its Dependence on Longitude and Impulsive Time 23
• 2.4 Summary and Conclusion 26
• Chapter 3 Dependence of Solar Proton Events on CME Parameters 28
• 3.1 Introduction 28
• 3.2 Data and Analysis 30
• 3.3 Results 31
• 3.3.1 Solar Proton Event Occurrence Probability 31
• 3.3.2 Relationship between CME Speed and SPE Flux on Angular width 34
• 3.3.3 Relationship between CME Speed and SPE Flux on Longitude and Direction Parameter 36
• 3.3.4 Relationship between CME Speed and SPE Flux on Radial CME Speed 39
• 3.4 Summary and Discussion 41
• Chapter 4 Statistical Study of Backside and Frontside Solar Proton Events 44
• 4.1 Introduction 44
• 4.2 Data and Analysis 45
• 4.3 Results 50
• 4.3.1 SPE Probability and Peak Flux 50
• 4.3.2 CME Speed and Angular Width 55
• 4.3.3 Intensity Rise Time 60
• 4.4 Summary 62
• Chapter 5 The Source Regions of Solar Energetic Particles Detected by Widely Separated Spacecraft 64
• 5.1 Introduction 64
• 5.2 Observations 66
• 5.3 Description of the Events 68
• 5.4 The Events from Single Source Regions 75
• 5.4.1 The Event of 2010 August 14 75
• 5.4.2 The Event of 2010 August 18 78
• 5.5 Events with Multiple Source Regions 80
• 5.5.1 The Events of 2011 September 21 and 22 80
• 5.5.2 The Event of 2011 November 3 85
• 5.5.3 The Event of 2011 November 26 88
• 5.6 SEP Flux Enhancements 91
• 5.6.1 Direct Magnetic Field Connections 91
• 5.6.2 EUV Wave Connection 93
• 5.6.3 Pre-Event Solar Activity 100
• 5.7 Summary 101
• 5.8 Appendix 102
• Chapter 6 Summary 115
• Bibliography 117
• Abstract (Korean) 122
• Acknowledgement (Korean) 125