Will external high electric field strength erase flash memory?
Flash memory uses floating gate MOSFETs to store a charge.

The amount of charge stored is the actual information.

Let's compare your 1MV/m to the field strength which is used to program such a memory cell.

First question, how thick is the oxide? I found this article where they mention the oxide thickness to be typically between 15 to 20 nm. Let's use 20 nm.

The programming voltage is often about 10 V, this is very process dependent but let's just use 10 V.

Then we get a field strength in the oxide of: 10 V / 20 nm = 500 MV/m

So that's a 500 times higher field strength than your 1 MV/m

Of course this is a value that ensures reliable programming of the cells, I mean, it could already work at 100 MV/m for example but that could be unreliable.

So in my view, 1 MV/m would by far not be enough to have any effect.