Solar Wind and Alfvén Waves

 N0AX has shared a nice article on Solar Wind and Alfvén Waves on the Ham Radio Science Citizen Investigation (HamSCI) Google Group, which I found quite interesting. The article is from PHYS.ORG, titled "Solar Orbiter shows how solar wind gets a magnetic push". Solar wind is a stream of charged particles coming from the solar atmosphere called corona and flows past the Earth. When it is strong, it causes the colourful auroral displays, typically seen in the polar regions and on lower latitudes in case of powerful geomagnetic storms, which may also cause radio blackouts as we had in May 2024. The solar wind which has a speed of nearly 400 km/hour as it exits the Sun's corona, with a temperature over a million degrees Centigrade, it has been known to speed up to over 500 km/hour later even with temperature which is only less than one fifth of what it had while exiting the Sun's atmosphere. 


Two solar probes, the Parker Solar Probe by NASA and and Solar Orbiter by ESA found that this acceleration is due to large oscillations in solar magnetic field known as Alfvén Waves. Though the Parker Solar Probe and Solar Orbiter are at different distances from the Sun (former being much nearer to the Sun) and in quite different orbits, in February 2022, both were aligned along the stream of solar wind. That is how they could get sequential information on the solar wind and make this new observation possible. Incidentally, Parker Solar Probe was the first spacecraft to fly through the Sun's corona in 2021. The scientific journal article describing these findings has been published in the popular journal Science [1]. The researches noted that the same patch of solar wind passed by the two probes at a spacing of about 2 days. The magnetic energy lost by the plasma particles in transit were converted to heat and velocity so that speed increased and the cooling was less than that was expected otherwise.

Reference

  1. Yeimy J. Rivera et. al. In situ observations of large-amplitude Alfvén waves heating and accelerating the solar wind. Science. 2024; 385:962-966.

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