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Register a new userThe evolution of the solar wind
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A. A. Vidotto
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How has the solar wind evolved to reach what it is today? In this review, I discuss the long-term evolution of the solar wind, including the evolution of observed properties that are intimately linked to the solar wind: rotation, magnetism and activity. Given that we cannot access data from the solar wind 4 billion years ago, this review relies on stellar data, in an effort to better place the Sun and the solar wind in a stellar context. I overview some clever detection methods of winds of solar-like stars, and derive from these an observed evolutionary sequence of solar wind mass-loss rates. I then link these observational properties (including, rotation, magnetism and activity) with stellar wind models. I conclude this review then by discussing implications of the evolution of the solar wind on the evolving Earth and other solar system planets. I argue that studying exoplanetary systems could open up new avenues for progress to be made in our understanding of the evolution of the solar wind.
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Radio waves are strongly scattered in the solar wind, so that their apparent sources seem to be considerably larger and shifted than the actual ones. Since the scattering depends on the spectrum of density turbulence, better understanding of the radio wave propagation provides indirect information on the relative density fluctuations $epsilon=langledelta nrangle/langle nrangle$ at the effective turbulence scale length. Here, we have analyzed 30 type III bursts detected by Parker Solar Probe (PSP). For the first time, we have retrieved type III burst decay times $tau_{rm{d}}$ between 1 MHz and 10 MHz thanks to an unparalleled temporal resolution of PSP. We observed a significant deviation in a power-law slope for frequencies above 1 MHz when compared to previous measurements below 1 MHz by the twin-spacecraft Solar TErrestrial RElations Observatory (STEREO) mission. We note that altitudes of radio bursts generated at 1 MHz roughly coincide with an expected location of the Alfv{e}n point, where the solar wind becomes super-Alfv{e}nic. By comparing PSP observations and Monte Carlo simulations, we predict relative density fluctuations $epsilon$ at the effective turbulence scale length at radial distances between 2.5$R_odot$ and 14$R_odot$ to range from $0.22$ and $0.09$. Finally, we calculated relative density fluctuations $epsilon$ measured in situ by PSP at a radial distance from the Sun of $35.7$~$R_odot$ during the perihelion #1, and the perihelion #2 to be $0.07$ and $0.06$, respectively. It is in a very good agreement with previous STEREO predictions ($epsilon=0.06-0.07$) obtained by remote measurements of radio sources generated at this radial distance.
Over 54 years of hourly mean value of solar wind velocity from 27 Nov. 1963 to 31 Dec. 2017 are used to investigate characteristics of the rotation period of solar wind through auto-correlation analysis. Solar wind of high velocity is found to rotate faster than low-velocity wind, while its rotation rate increases with velocity increasing, but in contrast for solar wind of low velocity, its rotation rate decreases with velocity increasing. Our analysis shows that solar wind of a higher velocity statistically possesses a faster rotation rate for the entire solar wind. The yearly rotation rate of solar wind velocity does not follow the Schwable cycle, but it is significantly negatively correlated to yearly sunspot number when it leads by 3 years. Physical explanations are proposed to these findings.
We investigate the anisotropy of Alfvenic turbulence in the inertial range of slow solar wind and in both driven and decaying reduced magnetohydrodynamic simulations. A direct comparison is made by measuring the anisotropic second-order structure functions in both data sets. In the solar wind, the perpendicular spectral index of the magnetic field is close to -5/3. In the forced simulation, it is close to -5/3 for the velocity and -3/2 for the magnetic field. In the decaying simulation, it is -5/3 for both fields. The spectral index becomes steeper at small angles to the local magnetic field direction in all cases. We also show that when using the global rather than local mean field, the anisotropic scaling of the simulations cannot always be properly measured.
The release of density structures at the tip of the coronal helmet streamers, likely as a consequence of magnetic reconnection, contributes to the mass flux of the slow solar wind. In situ measurements in the vicinity of the heliospheric plasma sheet of the magnetic field, protons and suprathermal electrons reveal details of the processes at play during the formation of density structures near the Sun. In a previous article, we exploited remote-sensing observations to derive a 3-D picture of the dynamic evolution of a streamer. We found evidence of the recurrent and continual release of dense blobs from the tip of the streamers. In the present paper, we interpret in situ measurements of the slow solar wind during solar maximum. Through both case and statistical analysis, we show that in situ signatures (magnetic field magnitude, smoothness and rotation, proton density and suprathermal electrons, in the first place) are consistent with the helmet streamers producing, in alternation, high-density regions (mostly disconnected) separated by magnetic flux ropes (mostly connected to the Sun). This sequence of emission of dense blobs and flux ropes also seems repeated at smaller scales inside each of the high-density regions. These properties are further confirmed with in situ measurements much closer to the Sun using Helios observations. We conclude on a model for the formation of dense blobs and flux ropes that explains both the in situ measurements and the remote-sensing observations presented in our previous studies.
Solar flare accelerated electron beams propagating away from the Sun can interact with the turbulent interplanetary media, producing plasma waves and type III radio emission. These electron beams are detected near the Earth with a double power-law energy spectrum. We simulate electron beam propagation from the Sun to the Earth in the weak turbulent regime taking into account the self-consistent generation of plasma waves and subsequent wave interaction with density fluctuations from low frequency MHD turbulence. The rate at which plasma waves are induced by an unstable electron beam is reduced by background density fluctuations, most acutely when fluctuations have large amplitudes or small wavelengths. This suppression of plasma waves alters the wave distribution which changes the electron beam transport. Assuming a 5/3 Kolmogorov-type power density spectrum of fluctuations often observed near the Earth, we investigate the corresponding energy spectrum of the electron beam after it has propagated 1 AU. We find a direct correlation between the spectrum of the double power-law below the break energy and the turbulent intensity of the background plasma. For an initial spectral index of 3.5, we find a range of spectra below the break energy between 1.6-2.1, with higher levels of turbulence corresponding to higher spectral indices.
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