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thermosphere
The outermost layer of the earth's atmosphere is the exosphere (about 500 to 200 km altitude). It is practically a vacuum. When listing only the four major layers, the exosphere is sometimes considered part of the thermosphere. (see related questions)
ionosphere
the exosphere is a layer in space and generally is not taken as one of the atmospheric layers whereas the ionosphere is a layer containing highly charged anions and is an upper part of the thermosphere.
Just a guess but maybe the exosphere!! But also it could be the Thermosphere, which is the Ionosphere and Exosphere combined.
The layer of the atmosphere that contains the ionosphere and exosphere is called the thermosphere. The exosphere is at the top and the ionosphere is at the bottom.
The thermosphere is divided into ionosphere and exosphere
thermosphere
The most upper layer of the atmosphere is the Exosphere. novanet> mesophere
this is just my hyothesis- but maybe either 1.) ionosphere 2.) exosphere 3.) thermosphere
ionosphere