How Solar Wind Evolved? (Planetary Science)

It is fair to say that the Sun is the best studied star in the whole Universe: we can measure its rotation, magnetic activity, composition, size, irradiation, and wind properties with accuracies like no other star in the Universe. However, all this information just tells us about how the Sun looks like now. To understand the past, and future, evolution of the Sun, including its wind, magnetism, activity, rotation, and irradiation, astronomers rely on information from other “suns” in the Universe i.e. Stars that resemble the Sun and are at different evolutionary stages. In a broad sense, these other “suns” belong to the group of solar-like stars. In theory, connecting the observations of winds of different solar-like stars under one unique evolutionary sequence would tell us how the wind of our own star has evolved. In practice, however, this is not straightforward.

One reason for this is that detecting winds of solar-like stars is a very challenging task. Direct detection of winds of other Suns has not been possible and the Sun still remains the only star for which the wind has been probed in situ. Now, Vidotto in his paper overviewed some clever techniques (shown in table 1 below) that been developed to indirectly derive stellar wind properties, such as mass-loss rates and terminal velocity, which is the speed winds asymptotically reach at larger distances. Overall, these techniques have shown that the solar wind had a higher mass-loss rate for the past ∼3.5–4 billion years. Before that, from the time the Sun started its evolution in the main sequence until about 600–800 Myr, clues point in different directions.

© Vidotto et al.

Stellar evolution models predict that the Sun has evolved, since its premain sequence phase, from a spectral type K to its current G2 classification. It will leave the main sequence phase in another ∼ 4 Gyr, when it will become a red giant star, go through a planetary nebula phase, until it will finish its days as a white dwarf that will cool down indefinitely. The large variation in solar photospheric temperature, radius and luminosity during the Sun’s evolution is also accompanied by variations in the properties of the solar wind – this outflow of particles, embedded in the solar magnetic field, that propagates throughout the solar system. Although it is unlikely that the solar wind has been able to remove a significant amount of mass from the Sun, the solar wind has been able to remove significant amounts of angular momentum via magnetic field stresses. For this reason, the solar wind has played a fundamental role in the Sun’s evolution, as it regulates solar rotation.

Now, Vidotto suggested that one way to shed light on the solar wind evolution is to look at physical characteristics known to be intimately related to winds of solar-like stars: rotation, magnetism and activity. These three ‘ingredients’ are related to the evolution of the solar wind in a feedback loop that he summarised in Fig. 1.

Fig. 1 The big picture: evolution of winds of cool dwarf stars. As the star ages, its rotation and magnetism decrease, causing also a decrease in angular momentum removal. In the images, he highlighted some of the areas to which wind-rotation-magnetism interplay is relevant. Image reproduced with permission from Vidotto (2016a). © Vidotto et al.

In the sketch he show in Fig. 1, he exemplify the “life-cycle” of the wind of an isolated solar-like star. He started with the text bubble #1: winds of solar-like stars are magnetic in nature, and are thus able to carry away a significant amount of angular momentum from the star. Consequently, the star spins down as it evolves (#2). Because of this variation in surface rotation, there is a redistribution of internal angular momentum transport, which changes the interior properties of the star (#3). With a different internal structure, the dynamo that is operating inside the star changes, changing therefore the properties of the emerging magnetic fields (#4). With a new surface magnetism, the stellar wind also changes (back to text bubble #1). This cycle repeats itself during the entire main-sequence lifetime of solar-like stars. Therefore, as an isolated solar-like star ages, its rotation decreases along with its chromospheric–corona activity, and magnetism. These three parameters are key ingredients in stellar wind theories. As a consequence of their decrease with age, winds of main-sequence solar-like stars are also expected to decrease with time.

“The stellar wind carries away angular momentum, which leads stars to spin down. A decrease in rotation rate implies that the dynamo weakens, generating weaker magnetic fields. This affects the stellar wind and the amount of angular momentum it can carry away. Then, the loop restarts. As a result, rotation, magnetism and its proxy in the form of stellar activity decrease with time.”

— told Vidotto, author of the study.

“In this review, I focused on the solar wind evolution from the early main sequence until today. That does not mean that what lies ahead is less interesting! The Sun is about half-way through its main-sequence journey, and will have thus another ∼ 4 Gyr worth of evolution in a relatively calm way. When the hydrogen fuel in its core is finally exhausted, the Sun will evolve off the main sequence and become a red giant. The timescales of this phase and subsequent ones become shorter and thus the Sun will pass through these phases very quickly in comparison to the long evolution during its main sequence. Eventually, the Sun will end its life as a white dwarf and cool down indefinitely. Certainly, the solar wind will change radically. In the red giant phase alone, the solar wind will likely be cooler (≲ 10⁴ K), with lower velocities, but higher densities, leading to mass-loss rate that will be several magnitudes higher than today. Solar rotation will change considerably due to its radius expansion. Given that rotation also seems to be related to magnetism in evolved stars, the Sun’s magnetic field will change as well…”

— wrote Vidotto, author of the study

The evolution of the solar wind has applications that goes beyond understanding stellar rotational evolution – it affects planets and exoplanets, which at the end of the day are embedded in the outward-streaming plasma from their host stars. A strong solar wind in the past is often cited to explain the lack of a substantial atmosphere in Mars today – planetary atmospheric erosion can be enhanced under strong stellar wind conditions. Such strong stellar wind conditions are also faced by close-in exoplanets, even when orbiting old and less active stars. Additionally, the Earth’s magnetosphere is believed to have been smaller than it is today, as a result of a strong stellar wind. Similarly, the strong solar wind in the past was able to push further the boundary with the ISM – the heliosphere was likely larger in the past. A different solar wind condition and much more expanded heliosphere would then have likely reduced the amount of Galactic cosmic rays reaching Earth. Both of these (Earth’s magnetosphere and cosmic ray flux) are believed to be important factors for life formation and sustainability in this planet.

Finally he concluded that, understanding the physical processes heating and driving the present-day solar wind could provide a step further in modelling the solar wind at different ages. Missions like NASA’s Parker Solar Probe and ESA’s Solar Orbiter will obtain in situ measurements of the solar wind at unprecedented close heliospheric distances. These regions are where the bulk of the acceleration of the solar wind takes place, and thus these missions are expected to soon shed light on the physical mechanisms that accelerate the solar wind. As an added bonus, these close heliospheric distances coincide with the regions where many closein exoplanets are located – these missions will thus provide information about the environment at the orbits of close-in exoplanets, which will aid in further understanding how extreme wind conditions can affect exoplanets, and potentially, link this to how the stronger solar wind in the past interacted with the young solar system planets.

Reference: A. A. Vidotto, “The evolution of the solar wind”, ArXiv, pp. 1-88, 2021.

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