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The Radial Dependence of Proton-scale Magnetic Spectral Break in Slow Solar Wind during PSP Encounter 2

AuthorDuan, Die; Bowen, Trevor; Chen, Christopher; Mallet, Alfred; He, Jiansen; Bale, Stuart; Vech, Daniel; Kasper, J.; Pulupa, Marc; Bonnell, John; Case, Anthony; de Wit, Thierry; Goetz, Keith; Harvey, Peter; Korreck, Kelly; Larson, Davin; Livi, Roberto; MacDowall, Robert; Malaspina, David; Stevens, Michael; Whittlesey, Phyllis;
KeywordsParker Data Used; parker solar probe; Physics - Plasma Physics; Physics - Space Physics; Solar Probe Plus

Magnetic field fluctuations in the solar wind are commonly observed to follow a power-law spectrum. Near proton-kinetic scales, a spectral break occurs that is commonly interpreted as a transition to kinetic turbulence. However, this transition is not yet entirely understood. By studying the scaling of the break with various plasma properties, it may be possible to constrain the processes leading to the onset of kinetic turbulence. Using data from the Parker Solar Probe, we measure the proton-scale break over a range of heliocentric distances, enabling a measurement of the transition from inertial to kinetic-scale turbulence under various plasma conditions. We find that the break frequency fb increases as the heliocentric distance r decreases in the slow solar wind following a power law of fb ̃ r-1.11. We also compare this to the characteristic plasma ion scales to relate the break to the possible physical mechanisms occurring at this scale. The ratio fb/fc (fc for Doppler-shifted ion cyclotron resonance scale) is close to unity and almost independent of plasma βp. While fb/fρ (fρ for Doppler-shifted proton thermal gyroradius) increases with βp approaching to unity at larger βp, fb/fd (fd for Doppler-shifted proton inertial length) decreases with βp from unity at small βp. Due to the large comparable Alfv\ en and solar wind speeds, we analyze these results using both the standard and modified Taylor hypotheses, demonstrating the robust statistical results.

Year of Publication2020
JournalThe Astrophysical Journal Supplement Series
Number of Pages55
Date Published02/2020