What is "ba'Galgal"?
Rashi: This is like k'Galgal. The sound of Your thunder was like a ball on the water, to confound Machaneh Mitzrayim.
Radak #1: There is a Patach under each Gimel. It is the name of a place with a Chirik under the first Gimel - "v'Hamam Gilgal Eglaso" (Yeshayah 28:28). Galgal is [a one of the nine spheres that houses Heavenly bodies in] Shamayim. Each of the nine is called Galgal, because it constantly rotates. Even though the thunder, lightning, hailstones and rain are not in Galgal, rather, in the air, the air is [imprecisely] called Galgal, Shamayim, Raki'a and Shechakim. It is called Shamayim due to the height of the air towards Shamayim. It is truly called Raki'a - "Yehi Raki'a b'Soch ha'Mayim" (Bereishis 1:6), and He called it Shamayim - "va'Yikra Elokim la'Raki'a Shamayim" (ibid., 8). It is called Shechakim - "Yizlu Shechakim" (Iyov 36:28), and it is called [here] Galgal.
Radak #2: Perhaps it is like "Gilgal Eglaso" (Yeshayah 28:28), i.e. the wheel of the wagon, like it says "va'Yasar Es Ofan Markevosav" (Shemos 14:25).
Malbim (19-20): This is the lowest sphere, in our Raki'a. It says "Ra'amcha", for it results from a higher thunder that came from You. The Mechilta says, from the sound of thunder above, the conduits below splashed
Do we find that lightning illuminated the entire land?
Radak: Yes - "va'Ya'er Es ha'Laylah 1 " (Shemos 14:20).
Seemingly, the verse says that the angel or cloud illuminated! Perhaps it illuminated via lightning. (PF)
How was the land agitated?
Radak: The Egyptians were agitated.
Malbim: It is the result of revelation of miracles and lightning above.
