分类： 天文学 >> 天文学 提交时间： 2023-02-19
Trevor A. Bowen
Benjmin D. G. Chandran
Jonathan Squire
Stuart D. Bale
Die Duan
Kristopher G. Klein
Davin Larson
Alfred Mallet
Michael D. McManus
Romain Meyrand
Jaye L. Verniero
Lloyd D. Woodham
摘要： The dissipation of magnetized turbulence is an important paradigm for describing heating and energy transfer in astrophysical environments such as the solar corona and wind; however, the specific collisionless processes behind dissipation and heating remain relatively unconstrained by measurements. Remote sensing observations have suggested the presence of strong temperature anisotropy in the solar corona consistent with cyclotron resonant heating. In the solar wind, in situ magnetic field measurements reveal the presence of cyclotron waves, while measured ion velocity distribution functions have hinted at the active presence of cyclotron resonance. Here, we present Parker Solar Probe observations that connect the presence of ion-cyclotron waves directly to signatures of resonant damping in observed proton-velocity distributions. We show that the observed cyclotron wave population coincides with both flattening in the phase space distribution predicted by resonant quasilinear diffusion and steepening in the turbulent spectra at the ion-cyclotron resonant scale. In measured velocity distribution functions where cyclotron resonant flattening is weaker, the distributions are nearly uniformly subject to ion-cyclotron wave damping rather than emission, indicating that the distributions can damp the observed wave population. These results are consistent with active cyclotron heating in the solar wind.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Nikos Sioulas
Zesen Huang
Chen Shi
Marco Velli
Anna Tenerani
Loukas Vlahos
Trevor A. Bowen
Stuart D. Bale
J. W. Bonnell
P. R. Harvey
Davin Larson
arc Pulupa
Roberto Livi
L. D. Woodham
T. S. Horbury
Michael L. Stevens
T. Dudok de Wit
R. J. MacDowall
David M. Malaspina
K. Goetz
摘要： Parker Solar Probe and Solar Orbiter data are used to investigate the radial evolution of magnetic turbulence between $0.06 ~ \lesssim R ~\lesssim 1$ au. The spectrum is studied as a function of scale, normalized to the ion inertial scale $d_{i}$. In the vicinity of the Sun, the inertial range is limited to a narrow range of scales and exhibits a power-law exponent of, $\alpha_{B} = -3/2$, independent of plasma parameters. The inertial range grows with distance, progressively extending to larger spatial scales, while steepening towards a $\alpha_{B} =-5/3$ scaling. It is observed that spectra for intervals with large magnetic energy excesses and low Alfv\'enic content steepen significantly with distance, in contrast to highly Alfv\'enic intervals that retain their near-Sun scaling. The occurrence of steeper spectra in slower wind streams may be attributed to the observed positive correlation between solar wind speed and Alfv\'enicity.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Die Duan
Jiansen He
Trevor A. Bowen
Lloyd D. Woodham
Tieyan Wang
Christopher H. K. Chen
Alfred Mallet
Stuart D. Bale
摘要： The anisotropy of solar wind turbulence is a critical issue in understanding the physics of energy transfer between scales and energy conversion between fields and particles in the heliosphere. Using the measurement of \emph{Parker Solar Probe} (\emph{PSP}), we present an observation of the anisotropy at kinetic scales in the slow, Alfv\'enic, solar wind in the inner heliosphere. \textbf{The magnetic compressibility behaves as expected for kinetic Alfv\'enic turbulence below the ion scale.} A steepened transition range is found between the inertial and kinetic ranges in all directions with respect to the local background magnetic field direction. The anisotropy of $k_\perp \gg k_\parallel$ is found evident in both transition and kinetic ranges, with the power anisotropy $P_\perp/P_\parallel > 10$ in the kinetic range leading over that in the transition range and being stronger than that at 1 au. The spectral index varies from $\alpha_{t\parallel}=-5.7\pm 1.0$ to $\alpha_{t\perp}=-3.7\pm 0.3$ in the transition range and $\alpha_{k\parallel}=-3.12\pm 0.22$ to $\alpha_{k\perp}=-2.57\pm 0.09$ in the kinetic range. The corresponding wavevector anisotropy has the scaling of $k_\parallel \sim k_\perp^{0.71\pm 0.17}$ in the transition range, and changes to $k_\parallel \sim k_\perp^{0.38\pm 0.09}$ in the kinetic range, consistent with the kinetic Alfv\'enic turbulence at sub-ion scales.
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