Does anyone know the reason why you can't use 'that' in 2?
But you can......if...
2 The Aurora Borealis is a 'phenomenon', in that it is regarded by most people (other than blasé scientists), as a remarkable occurrence. .
2 The Aurora Borealis is a phenomenon in which curtains of faint and shimmering light are visible in the night sky.
So the question becomes, what's the difference between 'in that' and 'in which'?
Student or Learner