NASA's Parker Solar Probe is on its way to another close encounter with the sun, where investigators hope the spacecraft will solve a long-standing solar mystery: Why is the sun's outer atmosphere millions of degrees hot (Fahrenheit or Celsius), while the surface is only about 6,000 degrees Fahrenheit (3,300 degrees Celsius)?
https://www.space.com/parker-solar-probe-may-solve-sun-mystery.html