 However,  the aurora borealis  and aurora australis  sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere.
<meaning-changed>  However,  the aurora borealis  and aurora australis  sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere.
 However, the aurora borealis and aurora australis (southern lights)— sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere.