What Is This?

Yes, I know it's an electric phenomenon of some sort, but what is the line going straight up? My best guess: A tethered balloon tied with a metal wire in the middle of a storm, acting as giant lightning rod.

Close, but not enough. It's a rocket trail, as this video shows:

A lightning rocket can have solid propellant that has cesium salts added which produces a conductive path when the exhaust gases are discharged from the rocket that is expelled to the static layer of a thundercloud. A conducting lightning rod which is grounded and positioned alongside the launch tube for the solid propellant is in communication with the conductive path to thereby control the time and location of a lightning strike from the thundercloud.

In a liquid expelled rocket a solution of calcium chloride is used to form a conductive path from a thundercloud. As the rocket flies to the thundercloud this liquid is expelled aft forming a column in the air of particles that are more electrically conductive than the surrounding air. In a similar fashion to the system employing a solid propellant as the conductive producer this conductive path conducts a lightning strike to ground to thereby control the time and location of a lightning strike from the thundercloud.

Here is a close up:

What Is This?

And here I was, hoping to find Thor at the bottom. [University of Florida and Wikipedia via PopSciThanks readers!]