The gaseous materials extend to several hundred kilometers in altitude, though there is no well defined boundary for the upper limit of the atmosphere. The tops of these layers are known as the tropopause , stratopause , mesopause and thermopause , respectively. A layer of aerosol particles normally exists near to the earth surface. The aerosol concentration decreases nearly exponentially with height, with a characteristic height of about 2 km. The temperature at the lower 20 km of the stratosphere is approximately constant, after which the temperature increases with height, up to an altitude of about 50 km.
Ozone exists mainly at the stratopause.
The temperature decreases in this layer from an altitude of about 50 km to 85 km. The gases exist mainly in the form of thin plasma, i.e. they are ionized due to bombardment by solar ultraviolet radiation and energetic cosmic rays. The term upper atmosphere usually refers to the region of the atmosphere above the troposphere. Many remote sensing satellites follow the near polar sun-synchronous orbits at a height around 800 km, which is well above the thermopause. They are gases present in nearly constant concentration, with little spatial variation.