分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We investigate dispersive and kinetic effects on the evolution of a two-dimensional kinked Alfv\'en wave packet by comparing results from MHD, Hall-MHD and hybrid simulations of a low-$\beta$ plasma. We find that the Hall term determines the overall evolution of the wave packet over a characteristic time $\tau^*=\tau_a\ell/d_i$ in both fluid and hybrid models. Dispersion of the wave packet leads to the conversion of the wave energy into internal plasma energy. When kinetic protons are considered, the proton internal energy increase has contributions from both plasma compressions and phase space mixing. The latter occurs in the direction parallel to the guiding mean magnetic field, due to protons resonating at the Alfv\'en speed with a compressible mode forced by the wave packet. Implications of our results for switchbacks observations and solar wind energetics are discussed.
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
Zesen Huang
Nikos Sioulas
Chen Shi
Marco Velli
Trevor Bowen
Nooshin Davis
B. D. G. Chandran
Ning Kang
Xiaofei Shi
Jia Huang
Stuart D. Bale
J. C. Kasper
Davin E. Larson
Roberto Livi
P. L. Whittlesey
Ali Rahmati
Kristoff Paulson
M. Stevens
A. W. Case
Thierry Dudok de Wit
摘要： The trace magnetic power spectrum in the solar wind is known to be characterized by a double power law at scales much larger than the proton gyro-radius, with flatter spectral exponents close to -1 found at the lower frequencies below an inertial range with indices closer to $[-1.5,-1.6]$. The origin of the $1/f$ range is still under debate. In this study, we selected 109 magnetically incompressible solar wind intervals ($\delta |\boldsymbol B|/|\boldsymbol B| \ll 1$) from Parker Solar Probe encounters 1 to 13 which display such double power laws, with the aim of understanding the statistics and radial evolution of the low frequency power spectral exponents from Alfv\'en point up to 0.3 AU. New observations from closer to the sun show that in the low frequency range solar wind turbulence can display spectra much shallower than $1/f$, evolving asymptotically to $1/f$ as advection time increases, indicating a dynamic origin for the $1/f$ range formation. We discuss the implications of this result on the Matteini et al. (2018) conjecture for the $1/f$ origin as well as example spectra displaying a triple power law consistent with the model proposed by Chandran et al. (2018), supporting the dynamic role of parametric decay in the young solar wind. Our results provide new constraints on the origin of the $1/f$ spectrum and further show the possibility of the coexistence of multiple formation mechanisms.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Chen Shi
Olga Panasenco
Marco Velli
Anna Tenerani
Jaye L. Verniero
Nikos Sioulas
Zesen Huang
A. Brosius
Stuart D. Bale
Kristopher Klein
Justin Kasper
Thierry Dudok de Wit
Keith Goetz
Peter R. Harvey
Robert J. MacDowall
David M. Malaspina
Marc Pulupa
Davin Larson
Roberto Livi
Anthony Case
摘要： Parker Solar Probe (PSP) has shown that the solar wind in the inner heliosphere is characterized by the quasi omni-presence of magnetic switchbacks ("switchback" hereinafter), local backward-bends of magnetic field lines. Switchbacks also tend to come in patches, with a large-scale modulation that appears to have a spatial scale size comparable to supergranulation on the Sun. Here we inspect data from the first ten encounters of PSP focusing on different time intervals when clear switchback patches were observed by PSP. We show that the switchbacks modulation, on a timescale of several hours, seems to be independent of whether PSP is near perihelion, when it rapidly traverses large swaths of longitude remaining at the same heliocentric distance, or near the radial-scan part of its orbit, when PSP hovers over the same longitude on the Sun while rapidly moving radially inwards or outwards. This implies that switchback patches must also have an intrinsically temporal modulation most probably originating at the Sun. Between two consecutive patches, the magnetic field is usually very quiescent with weak fluctuations. We compare various parameters between the quiescent intervals and the switchback intervals. The results show that the quiescent intervals are typically less Alfv\'enic than switchback intervals, and the magnetic power spectrum is usually shallower in quiescent intervals. We propose that the temporal modulation of switchback patches may be related to the "breathing" of emerging flux that appears in images as the formation of "bubbles" below prominences in the Hinode/SOT observations.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The conservation of wave action in moving plasmas has been well-known for over half a century. However, wave action is not conserved when multiple wave modes propagate and coexist close to degeneration condition (Sound speed equals Alfv\'en speed, i.e. plasma $\beta \sim 1$). Here we show that the violation of conservation is due to wave mode conversion, and that the total wave action summed over interacting modes is still conserved. Though the result is general, we focus on MHD waves and identify three distinctive mode conversion mechanisms, i.e. degeneracy, linear mode conversion, and resonance, and provide an intuitive physical picture for the mode conversion processes. We use 1D MHD simulations with the Expanding Box Model to simulate the nonlinear evolution of monochromatic MHD waves in the expanding solar wind. Simulation results validate the theory; total wave action therefore remains an interesting diagnostic for studies of waves and turbulence in the solar wind.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Chen Shi
Marco Velli
Roberto Lionello
Nikos Sioulas
Zesen Huang
Jasper S. Halekas
Anna Tenerani
Victor Réville
Jean-Baptiste Dakeyo
Milan Maksimović
Stuart D. Bale
摘要： The heating and acceleration of the solar wind remains one of the fundamental unsolved problems in heliophysics. It is usually observed that the proton temperature $T_i$ is highly correlated with the solar wind speed $V_{SW}$, while the electron temperature $T_e$ shows anti-correlation or no clear correlation with the solar wind speed. Here we inspect both Parker Solar Probe (PSP) and WIND data and compare the observations with simulation results. PSP observations below 30 solar radii clearly show a positive correlation between proton temperature and wind speed and a negative correlation between electron temperature and wind speed. One year (2019) of WIND data confirm that proton temperature is positively correlated with solar wind speed, but the electron temperature increases with the solar wind speed for slow wind while it decreases with the solar wind speed for fast wind. Using a one-dimensional Alfv\'en-wave-driven solar wind model with different proton and electron temperatures, we for the first time find that if most of the dissipated Alfv\'en wave energy heats the ions instead of electrons, a positive $T_i-V_{SW}$ correlation and a negative $T_e-V_{SW}$ correlation arise naturally. If the electrons gain a small but finite portion of the dissipated wave energy, the $T_e-V_{SW}$ correlation evolves with radial distance to the Sun such that the negative correlation gradually turns positive. The model results show that Alfv\'en waves are one of the possible explanations of the observed evolution of proton and electron temperatures in the solar wind.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Nooshin Davis
B. D. G. Chandran
T. A. Bowen
S. T. Badman
T. Dudok de Wit
C. H. K. Chen
S. D. Bale
Zesen Huang
Nikos Sioulas
Marco Velli
摘要： The power spectrum of magnetic-field fluctuations in the fast solar wind
($V_{\rm SW}> 500 \mbox{ km} \mbox{ s}^{-1}$) at magnetohydrodynamic (MHD)
scales is characterized by two different power laws on either side of a break
frequency $f_{\rm b}$. The low-frequency range at frequencies $f$ smaller than
$f_{\rm b}$ is often viewed as the energy reservoir that feeds the turbulent
cascade at $f>f_{\rm b}$. At heliocentric distances $r$ exceeding $60$ solar
radii ($R_{\rm s}$), the power spectrum often has a $1/f$ scaling at $f
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