What are the differences between those two sentences? (In meaning connotations, anything.)
Around the core is a mixed zone of ...
Around the core there is a mixed zone of ...
Are there any contexts in which it is compulsory to use "there is/are"?
I'm not a teacher
But you'd say There's a cat in the garden rather than In the garden is a cat.* Btw, Polish learners often forget about 'there is' because we translate literally from Polish: W ogrodzie jest kot.
Now I wonder myself why it is okay to say Around the core is a mixed zone of ... but not so much In the garden is a cat.*
For things that are permanent, it's less common to use "There is".
"Under the earth's crust is the mantle", not "There is a mantle under the earth's crust"
"Inside the albumenous white of the egg is the yolk." more common than "There is a yolk inside the albuinous white of an egg."
For temporary things, "There is ..." is more common.
"In the garden, there is a cat."