It can even kill tons of animals/people at once[1]. Four legged animals are more vulnerable than humans, since the voltage across your heart depends on how far apart your feet are. Also, we wear shoes.
Imagine how spooky it would be to be struck by ground current without understanding... Whether or not you survive could depend entirely on what direction you're standing. One direction will mean your feet are at different voltages and current jumps into your nice conductive veins and arteries, all passing right through your heart. The other direction means the front and back of each foot is at different voltages, and you may not even feel it. Hundreds of people just drop dead of heart attacks around you.
Well, that would be the ground current of a lightning strike, the induced ground current of a solar event is much much lower, it only plays a role with long wires.
I'm not aware of anything like "voltage density", what do you mean by that?
The potential gradient is the thing that kills you. It can either be created by a lightning strike or by voltage induced into the ground as mentioned here.
Fun fact, voltage gradient across cell membranes is typically megavolts per meter. At scale that is millivolts per nanometer, but the field strength is the same either way. Membranes are badass.
Imagine how spooky it would be to be struck by ground current without understanding... Whether or not you survive could depend entirely on what direction you're standing. One direction will mean your feet are at different voltages and current jumps into your nice conductive veins and arteries, all passing right through your heart. The other direction means the front and back of each foot is at different voltages, and you may not even feel it. Hundreds of people just drop dead of heart attacks around you.
[1]: https://www.theverge.com/2016/8/29/12690402/lightning-strike...