(See image) At what distance does the temperature of the Corona decline?

[smithpa9]

My astronomy textbook includes this graph of the temperature of the Sun's Corona versus the distance above the photosphere. It appears to go up from 4,500 K to over 1 million K at 20,000 km above the photosphere. But at what point does it come back down? Or does it not? Does that mean the temperature of the interplanetary space in our solar system is really over 1 million degrees K? (Source: Astronomy Today, 9th edition, by Chiasson and McMillan. Pearson Publishing, 2017).

 

[anuttarasammyak]

I, a layman of astronomy, think there seems no mechanism to decelerate speed of ions in corona gas, so temperature is maintained. I read in Wiki Solar Wind that solar wind originated from corona gas I think, has much higher temperature. There should be mechanism of accelerating solar gas by magnetic field.

 

[davenn]

But at what point does it come back down?

it drops again quickly with distance from the sun

Does that mean the temperature of the interplanetary space in our solar system is really over 1 million degrees K?

Of course not, else everything in interplanetary space would be toasted to a crisp, including everything in orbit around the Earth.
Even in the near proximity of the Sun, the Corona is very tenuous, (thin) it's about 0.0000000001 times that of the Earth's sea-level
atmosphere, says one report from NASA (https://imagine.gsfc.nasa.gov/science/objects/sun1.html)

The temperature of an object in space in sunlight is 394 K. Keep in mind that the majority of "heat" transferred from the Sun
is radiative, that is IR radiation. The solar wind is just too thin to transfer heat by conduction or convection.

The temperature in space in the shadow of a planet/moon is approx. 2.73 K (-270.42 Celsius), just above abs. zero. That being the
temperature of the CMB ( Cosmic Microwave Background).Dave