Normal‐ and Reversed‐Boomerang Stripes on Electron Pitch Angle Distributions: Solar Wind Dynamic Pressure Effect. Issue 2 (24th January 2022)

• Record Type: Journal Article
• Authors: Zhao, X. X.; Zong, Q.‐G.; Liu, J. J.; Yue, Chao; Zhou, X.‐Z.; Hao, Y. X.; Chen, X. R.; Klimushkin, D. Yu.; Rubtsov, A. V.; Blake, J. B.; Claudepierre, S. G.; Reeves, G. D.
• Is Part Of: Geophysical research letters. Volume 49:Issue 2(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-01-24
• Subjects: energetic electrons -- drift resonance -- off‐equatorial minima -- reversed boomerang stripes -- image‐dipole magnetic field -- solar wind dynamic pressure; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL096526
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: In the inner magnetosphere dominated by the dipole magnetic field, energetic electrons of 90° pitch angle drift faster than those of other pitch angles. Electron flux oscillations initiated by injections or drift resonances will be observed earlier at a 90° pitch angle, that is, 90°‐leading boomerang‐shaped stripes on pitch angle distributions(PADs) are expected. In this work, reversed‐boomerang stripes (90° observed last) are first reported from the observations of RBSP‐A at L‐shell ∼5.9 on 11 March 2016. The corresponding solar wind dynamic pressure is over 10 nPa, which suggests that strong compression on the magnetosphere changes the magnetic field and drift frequencies of energetic electrons. Test particles running in an image‐dipole magnetic field (off‐equatorial minima exists at dayside large L‐shell) reproduce the abnormal (90° slower) drift velocities at L‐shell >5.8 but are normal at L‐shell <4.5 dominated by a dipole field. Normal boomerang stripes on PADs are indeed observed by RBSP‐B at L‐shell ∼4.0, indicating the solar wind dynamic pressure effect. Plain Language Summary: The motions of energetically charged particles trapped in the outer radiation belt can be divided into three adiabatic motions: Gyro, bounce, and drift. Usually, the magnetic field within the 3.0 < L‐shell < 6.0 is close to a dipole magnetic field, where charged particles with pitch angles closer to 90° drift faster. Therefore, electron flux oscillations at a 90° pitch angle followed by other pitch angles symmetrically ("boomerang‐shaped" stripes) are often observed in injection/dropout echoes or drift resonance events. In this study, we present an observation where both normal‐(90° observed first) and reversed‐(90° last) boomerang stripes on electron pitch angle distributions are simultaneously observed by two Van Allen Probes on 11 March 2016. Besides, strong compression impacts the magnetosphere (the solar wind dynamic pressure is over 10 nPa). The reversed‐boomerang stripes suggest that the particles with 90° pitch angle drift more slowly, which disagrees with the dipole field. Test particle simulation in an image‐dipole magnetic field can reproduce the observed reversed‐boomerang feature at L‐shell >5.8, which suggests that the reversed‐boomerang stripes result from the strong compression on the magnetosphere. At L‐shell <4.5, normal boomerang stripes are observed since the magnetic field therein is less affected by the compression on the magnetosphere and still dominated by the dipole field. Key Points: Electron normal‐(90° first) and reversed‐(90° last) boomerang stripes on the pitch angle distributions (PADs) are simultaneously observed at different L‐shells The strong compression on the magnetosphere results in the dayside off‐equatorial minima, leading to reversed dispersions on electron PADs Test particle simulation in an image dipole model (off‐equatorial minima exists) reproduces the longer drift periods of 90° particles