Question

There is no medium between earth and sun, except for a small layer of the atmosphere. What's that small layer of atmosphere?

Answer 1

It is atmosphere
Definition of atmosphere - the envelope of gases surrounding the earth or another planet,

The atmosphere is comprised of layers based on temperature. These layers are the troposphere, stratosphere, mesosphere and thermosphere. A further region at about 500 km above the Earth's surface is called the exosphere.