What is the flash of light from Minas Tirith?

Question

In LOTR The Return of the King Theoden briefly loses hope before leading the charge against the siege of Minas Tirith. However, when a "flash, as if lightning had sprung from the earth beneath the City." is seen he regains his strength and leads the charge. What is this flash and who or what made it?

"Then suddenly Merry felt it at last, beyond doubt: a change. Wind was in his face! Light was glimmering. Far, far away, in the South the clouds could be dimly seen as remote grey shapes, rolling up, drifting: morning lay beyond them.

But at that same moment there was a flash, as if lightning had sprung from the earth beneath the City. For a searing second it stood dazzling far off in black and white, its topmost tower like a glittering needle: and then as the darkness closed again there came rolling over the fields a great boom."

_{The Lord of the Rings Book Five, Chapter 5: The Ride of the Rohirrim}

In the previous chapter Gandalf faces off against the Lord of the Nazgul at the gates but the only light I can see from that is the flaming of the Nazgul's sword. But there is no sound associated with that for the "boom" that is heard and I doubt it would be bright enough to reach the Rohirrim.

"In rode the Lord of the Nazgûl. A great black shape against the fires beyond he loomed up, grown to a vast menace of despair. In rode the Lord of the Nazgûl, under the archway that no enemy ever yet had passed, and all fled before his face.

All save one. There waiting, silent and still in the space before the Gate, sat Gandalf upon Shadowfax: Shadowfax who alone among the free horses of the earth endured the terror, unmoving, steadfast as a graven image in Rath Dínen.

‘You cannot enter here,’ said Gandalf, and the huge shadow halted. ‘Go back to the abyss prepared for you! Go back! Fall into the nothingness that awaits you and your Master. Go!’

The Black Rider flung back his hood, and behold! he had a kingly crown; and yet upon no head visible was it set. The red fires shone between it and the mantled shoulders vast and dark. From a mouth unseen there came a deadly laughter.

‘Old fool!’ he said. ‘Old fool! This is my hour. Do you not know Death when you see it? Die now and curse in vain!’ And with that he lifted high his sword and flames ran down the blade.

Gandalf did not move. And in that very moment, away behind in some courtyard of the City, a cock crowed. Shrill and clear he crowed, recking nothing of wizardry or war, welcoming only the morning that in the sky far above the shadows of death was coming with the dawn.

And as if in answer there came from far away another note. Horns, horns, horns. In dark Mindolluin’s sides they dimly echoed. Great horns of the North wildly blowing. Rohan had come at last."

_{The Lord of the Rings Book Five, Chapter 4: The Siege of Gondor}

Answer 1

The flash is from Grond breaching the gate. The implication is that it did so with the assistance of an enchantment employed by the Witch King.

The drums rolled and rattled. With a vast rush Grond was hurled forward by huge hands. It reached the Gate. It swung. A deep boom rumbled through the City like thunder running in the clouds. But the doors of iron and posts of steel withstood the stroke.

Then the Black Captain rose in his stirrups and cried aloud in a dreadful voice, speaking in some forgotten tongue words of power and terror to rend both heart and stone.

Thrice he cried. Thrice the great ram boomed. And suddenly upon the last stroke the Gate of Gondor broke. As if stricken by some blasting spell it burst asunder: there was a flash of searing lightning, and the doors tumbled in riven fragments to the ground.