Question: Joining unbalanced lists

If I have several sets of listed data of different lengths,

a1:=[1,2]
a2:=[5,3,4,5]
a3:=[4,8,4,5,3,2,3,4,5]
a4:=[2,2,5,4,7,8]
...

How do I join the 1st positions into one variable and the 2nd positions into another and so on?

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