A Statistical Study of Pancake Pitch Angle Distribution of Low‐Energy Protons and Their Correlation With Magnetosonic Waves

• Published in: Journal of geophysical research. Space physics 2022-05, Vol.127 (5), p.n/a
• Main Authors: Wu, Z., Teng, S., Ma, Q., Tao, X.
• Format: Article
• Language: English
• Subjects: Correlation; Density; Diffusion coefficient; diffusion coefficients; Earth magnetosphere; Energy; Energy distribution; Geomagnetic activity; Geomagnetism; Heating; Helium; low energy proton; magnetosonic waves; Oxygen; Particle dynamics; Pitch (inclination); pitch angle distribution; Probes; Proton diffusion; Proton energy; Protons; Waves
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2021JA030174

We performed a statistical study of the proton pancake pitch angle distributions (PADs) at the energy from 10 to 300 eV with the Helium Oxygen Proton Electron instrument on board the Van Allen Probes. The occurrence rate of low energy proton pancake PADs and their dependence on the energy, L shell, magnetic local time (MLT) and AL* index are analyzed. The results indicate that: (a) the occurrence rate of pancake PADs decreases as the proton energy increases; (b) under active geomagnetic activities (AL*  −100 nT), the occurrence rate of pancake PADs has little dependence on MLT. We further discuss the contribution of magnetosonic (MS) waves on the formation of proton pancake PADs at low energy. The strong correlation between the occurrence of pancake PADs and the appearance of MS waves indicates that MS waves contribute to the formation of low energy proton pancake PADs. Moreover, the proton diffusion coefficients caused by MS waves suggest that MS waves can cause the proton heating at low energies in high density regions. Plain Language Summary Studying the characteristics of the pitch angle distributions (PAD) of different particles is crucial to understand particle dynamics in the Earth's magnetosphere. However, previous studies mostly focused on high‐energy electrons and paid little attention to low‐energy protons. The twin Van Allen Probes with advanced instrument provide us a great opportunity to investigate low‐energy proton in detail. In our research, we adopt an automatic method to find out the pancake PAD of low energy protons. We then analyze the occurrence rate of low‐energy proton pancake PAD under different conditions. The results show that low‐energy proton pancake PAD is more likely to appear in the dayside region under strong geomagnetic activities. Moreover, we study the relationship between proton pancake PADs and magnetosonic (MS) waves. It is found that MS wave can cause the proton heating at low energies in high density regions, which may affect the occurrence rate of proton pancake PADs. Key Points The occurrence rate of proton pancake pitch angle distribution (PAD) at the energy below 300 eV increases with the decrease of energy The occurrence rate of proton pancake distribution is higher when magnetosonic (MS) waves occur than when MS waves do not appear MS waves can heat the low energies (