1.2. The solar wind

By 1950, observations of the charge states of heavier elements like O, Si, Fe, Mg had indicated that the characteristic temperature of the solar corona was around ${10}^6$ K, which is around 200 times higher than the photospheric temperature \citep{Grotrian39, Alfven_corona, Edlen_corona} .
This rapid increase in temperature occurs in the transition region, spanning only a few hundred kilometers. The natural question that arises is this: How is the solar corona heated to millions of Kelvins, given that the photosphere has a temperature of only around a few thousand Kelvins?

Due to the high temperature of the solar corona, the gravity of the Sun is not enough to hold the plasma in place. This leads to the coronal plasma being ejected into space at high velocities. This continuous flow of charged particles emitted from the solar corona is known as the solar wind.
Despite this being a well established fact now due to in situ spacecraft observations, the idea of the presence of a solar wind was not widely accepted until very recently.