Particle usage of に in 南北に長い?


I was reading page 5 of my 上級へのとびら textbook, and in line 12, the following
sentence caught my eye:

> 日本は南北 **に** 長いので……

I cannot figure out why the に is used instead of が (南北が長い). Is に the 連用形 of なる
here?




Turns a noun 南北 into an adverbial expression, i.e. answers the question "how?"
Let's say ..."north-south-directionally".

And the expression then modifies the adjective "long".

In English it would translate to "long in north-south direction".

