F7xSS orig. watchface: What's the meaning of this gauge?

Hi,

I tried out some of the nice original Garmin watchfaces on the watch, and there is only one gauge that I cannot read. Does somebody know its meaning or is there a documentation of this?

It's a gauge that shows something solar-current related, but it doesn't seem to correlate with the amount of Lux from the sun.

In the shadow it sometimes shows a higher "whatever" than in the direct sun. In direct sun, whereas the "normal sun gauge" from other watchfaces shows the max. solar input, this gauge shows variations I can't explain.

Note: After changing light conditions I waited about a minute or two to let the gauge react, but that either didn't help me to read it :-)

Here are some photos to explain (first two pictures taken in a room in the shadow, second two in direct sun):