The term 'sphere starter' refers to four different layers of the Earth's atmosphere: ATMO (4 letters), HEMI (4 letters), IONO (4 letters), and STRATO (6 letters). ATMO stands for the troposphere, which is the lowest layer of the atmosphere and is closest to the Earth's surface. HEMI stands for the mesosphere, which is the layer of the atmosphere that lies between the stratosphere and the thermosphere. IONO stands for the ionosphere, which is a region of the upper atmosphere that is ionized by solar radiation. Finally, STRATO stands for the stratosphere, which is the second-lowest layer of the atmosphere and is located above the troposphere. Together, these four layers form the Earth's atmosphere and are referred to as the 'sphere starter'.