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
Jia Huang
J. C. Kasper
L. A. Fisk
Davin E. Larson
Michael D. McManus
C. H. K. Chen
Mihailo M. Martinović
K. G. Klein
Luke Thomas
Mingzhe Liu
Bennett A. Maruca
Lingling Zhao
Yu Chen
Qiang Hu
Lan K. Jian
J. L. Verniero
Marco Velli
Roberto Livi
P. Whittlesey
Ali Rahmati
摘要： Switchbacks are rapid magnetic field reversals that last from seconds to hours. Current Parker Solar Probe (PSP) observations pose many open questions in regards to the nature of switchbacks. For example, are they stable as they propagate through the inner heliosphere, and how are they formed? In this work, we aim to investigate the structure and origin of switchbacks. In order to study the stability of switchbacks, we suppose the small scale current sheets therein may work to braid and stabilize the switchbacks. Thus, we use the partial variance of increments method to identify the small scale current sheets, and then compare their distributions in switchbacks. With more than one thousand switchbacks identified with PSP observations in seven encounters, we find many more current sheets inside than outside switchbacks, indicating that these micro-structures should work to stabilize the S-shape structures of switchbacks. Additionally, with the helium measurements, we study the variations of helium abundance ratios and alpha-proton differential speeds to trace switchbacks to their origins. We find both helium-rich and helium-poor populations in switchbacks, implying the switchbacks could originate from both closed and open magnetic field regions in the Sun. Moreover, we observe that the alpha-proton differential speeds also show complex variations as compared to the local Alfv\'en speed. The joint distributions of both parameters show that low helium abundance together with low differential speed is the dominant state in switchbacks. The presence of small scale current sheets in switchbacks along with the helium features are in line with the hypothesis that switchbacks could originate from the Sun via interchange reconnection process. However, other formation mechanisms are not excluded.
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.
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