Volume 654, October 2021
|Number of page(s)
|The Sun and the Heliosphere
|19 October 2021
Solar wind Alfvénicity during solar cycle 23 and 24
Perspective for future observations with Parker Solar Probe and Solar Orbiter
National Institute for Astrophysics (INAF) – Institute for Space Astrophysics and Planetology (IAPS), Via Fosso del Cavaliere, 100, 00133 Rome, Italy
2 Department of Physics, Helwan University, Helwan, Egypt
3 Department of Mathematics, Physics and Computer Science, Aberystwyth University, Ceredigion, Cymru SY23 3BZ, UK
4 Italian Space Agency (ASI), Via del Politecnico snc, 00133 Rome, Italy
5 National Institute for Astrophysics (INAF) – Astrophysical Observatory of Turin, Via Osservatorio, 10025 Pino Torinese, Turin, Italy
6 Department of Climate and Space Sciences and Engineering, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109-2143, USA
Accepted: 22 July 2021
Context. Alfvénic fluctuations are ubiquitous features observed in solar wind, especially in the inner heliosphere. However, strong Alfvénic fluctuations are recovered in the near-Earth solar wind too, mainly in fast streams, but also in some cases in slow wind intervals, as highlighted in recent studies.
Aims. The present study focuses on a statistical comparison between different phases of solar cycles 23 and 24 with regard to the Alfvénic content of solar wind fluctuations. Particular attention is devoted to the Alfvénic slow solar wind, in relation to the solar wind composition and other parameters.
Methods. Two-dimensional histograms of the solar wind speed versus the normalized cross-helicity have been used to feature the Alfvénic character of solar wind turbulence on each phase of the solar cycles considered. Moreover, we characterize the different phases of solar cycles by also using composition data. Finally, case studies are discussed to better highlight the similarities and differences between the two solar maxima, which more clearly show a predominance of Alfvénic slow solar wind.
Results. The statistical analysis highlights similarities between two solar cycles and confirms that the Alfvénic slow wind is more frequently observed during the maximum of solar activity. The two representative time intervals, containing samples of this solar wind regime, show similar characteristics, with a particular reference to the spectral analysis.
Conclusions. This study has important implications for future observations by Parker Solar Probe and Solar Orbiter, devoted to the study of the inner heliosphere inside Mercury’s orbit. In fact, both missions will operate up to the maximum of solar cycle 25 which is fast approaching. These unprecedented measurements will then provide insights into the origin and evolution of the Alfvénic solar wind close to the region where it is generated and accelerated.
Key words: plasmas / Sun: heliosphere / solar wind / turbulence / methods: data analysis / interplanetary medium
© ESO 2021
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