But to which of the angels said he at any time, Sit on my right hand, until I make thine enemies thy footstool? Heb 1:13 The angels were not eligible for the position that Christ Jesus has attained because they have their own spirit - not the Holy Spirit.

Are they not all ministering spirits, sent forth to minister for them who shall be heirs of salvation? Heb 1:14 Christ and those who receive Him are born of His Holy Spirit and are therefore above the angels.

But we know that Satan (archangel) rebelled against the Lord Isa 14:12.    We know that other angels followed Satan in his rebellion Jud 1:6 .   

Do angels appear on earth?   Yes Heb 13:2.   
Only good angels?   Heb 13:2 Rev 8:6 Rev 9:11 indicate there are fallen angels heavily involved in the events occurring at the end of the Great Tribulation.   
I suspect that the alien sightings are evil angels.   

Satan will be cast down to earth with his angels Rev 12:7-9.    They will manifest on earth at that time 2Th 2:9.   

In most cases where angels are spoken of in the Bible the term Angel Of The Lord or His Angel is used to make it clear who they represent.