Start of Fajr in Northern Places

Assalamu 'alaykum dear brothers and sisters,

Is there a Zaydi position on the issue of when to determine fajr in the northern extremes where the sun does not descend to 18-degrees below the horizon during summer?

Different Sunni scholars have given different answers - some use fifteen degrees, some divide the night into parts, some say use the last time the sun did validly descend to 18 degrees below. I would like to follow the Zaydi position to the best of my ability.

With kind regards
wasalam fi iman Allah
Umm Yasmin

Jazakallahu khayran

Jazakumallahu khayran, thank you and to the Mufti!