What's the lowest altitude that an ion engine has been used for a significant orbital maneuver?

Question

I'm thinking about trying to calculate something for Where do ion propulsion's ions go? Do they remain in the solar system or shoot out into interstellar space?

Excluding low altitude tests of ion propulsion (Will the ISS have electric propulsion to maintain altitude? Is there enough power for it?), what is the lowest orbit or altitude where an ion engine has been used to generate a significant orbital maneuver?

Modern "all electric" GEO satellites do use electric propulsion to get to GEO as well as for station-keeping, but I am not sure if that orbit raising stars from circular LEO or the spacecraft is given a partial or full GTO boost first. How much time does it take to circularize a GTO orbit using ion propulsion?. Same with SMART-1: Going from LEO to lunar using only low-thrust ion propulsion - can it be done?

"Lowest orbit or altitude" is a little tricky, one case may have the lowest semimajor axis, another may have the lowest perigee, a third may have the lowest altitude at the time the engine was started. My primary interest is lowest altitude but other answers are interesting and may suffice if there's incomplete data.

Answer 1

JAXA's Tsubame satellite is planned to orbit to altitudes as low as 180 km, using an ion engine to reboost. It was launched in January 2018 into an initial orbit at ~480 km, its orbit will be gradually lowered over a period of 2 years.

The Super Low Altitude Test Satellite "TSUBAME" (SLATS) is the first Earth observation satellite to use a super low orbit.

SLATS will use the ion engine technology developed by JAXA in order to verify its technology for orbit control at super low altitudes. The test satellite will also collect technical data related to the atmosphere, which will be used in the design of future satellites. Furthermore, SLATS will photograph the Earth, and its technology will be evaluated for future Earth observation satellites.