The Variability of Energetic Storm Particle Heavy Ion Properties During Solar Cycles 23 and 24

This dissertation studies energetic Helium (He), Oxygen (O), and Iron (Fe) ion energetic storm particle (ESP) events observed by the Advanced Composition Explorer (ACE) and the Solar TEr-restrial RElations Observatory (STEREO) NASA missions to determine variations in the properties of ESP events during solar cycles (SCs) 23 and 24. ACE is located at the Sun-Earth L1 Lagrange point, whereas, STEREO consists of two spacecraft at 1 au, one orbiting ahead of Earth (STEREO-A) and the other behind it (STEREO-B), each drifting apart from Earth in opposite directions at about 22◦ per year. The configuration of the three spacecraft provides different perspectives for studying ESP events at 1 au. We used data from instruments onboard both missions. The solar wind plasma instrument (PLASTIC), the magnetometer (MAG), and the Suprathermal Ion Telescope (SIT) on STEREO survey the interplanetary (IP) shock properties, the solar wind conditions, and the ∼ 0.1 to 3 MeV/nucleon energetic He, O, and Fe ion intensities of ESP observations between 2010 January and 2019 December. The solar wind plasma instrument (SWEPAM), the magnetometer (MAG), the Ultra-Low Energy Isotope Spectrometer (ULEIS), and the Solar Isotope Spectrometer (SIS) on ACE are used to survey the interplanetary (IP) shock properties, the solar wind conditions, and ∼ 0.1 to 35 MeV/nucleon energetic He, ∼ 0.1 to 76 MeV/nucleon O, and ∼ 0.034 to 31 MeV/nucleon Fe ion observations during ESP events identified between 1997 August and 2019 December. Chapter 2 describes these instruments in more detail. In Chapter 3 we show that the probability of observing a heavy-ion ESP event is highest when the observing spacecraft is at longitudes near the driving-coronal mass ejection (CME) solar source. ESP events driven by high-Near Sun speed (>1300 km/s) CMEs have a larger angular extent compared to events driven by low-Near Sun speed (<1300 km/s) CMEs. The heavy-ion peak intensities and spectral indices for fast events (near-Sun CME speed > 1300 km/s) show a clear organization along the CME shock at 1 au. The peak intensities are largest and spectral indices are smallest, near 0◦ longitude. The peak intensities show up to a 2 - 3 order of magnitude decrease along the shock flanks while the spectral indices are up by a factor of 2 - 3 larger, i.e., softer at the flanks. The slow ESP events (near-Sun CME speed < 1300 km/s), however, do not show a clear organization. In Chapter 4 we show that during the stronger SC 23, there were more heavy-ion ESP events observed at ACE than during the weaker SC 24. The near-Sun and the average transit speed of CMEs associated with heavy ion ESP events are on average faster for SC 23 than for SC 24. However, the SC 24 CMEs decelerated less rapidly on their transit between the Sun and 1 au, likely due to lower ambient pressure in the inner heliosphere, which combined with a reduction of the Alfven speed during SC 24, may have resulted in shocks that are as efficient at accelerating energetic particles below ∼2 MeV/nucleon between these two solar cycles. The heavy-ion average intensities show similar mean values for ESP events in both SCs; however, the number of SC 24 events decreases faster with increasing energy per nucleon, and these events are less likely to accelerate heavy ions to energies above 10 MeV/nucleon. Chapter 5 explores the east-west asymmetry, with respect to the source flare location, of the intensity of solar energetic particle (SEP) and ESP events. According to the models, eastern energetic particle events tend to be more intense than western events. We show that the intensities of ESP events observed at ACE during SCs 23 and 24 seem to agree with the models. It is also shown that the strength of the SC will affect the ESP intensity asymmetry, where the weaker SC 24 ESP events are more asymmetric than the stronger SC 23 ESP events. Chapter 6 presents the summary of the dissertation results. In Appendix A, we introduce the Procrustean bed effect for light.