Afterglow system, part of the glowplug setup. Intermittently fires off the glowplugs (for short bursts) to get full combustion happening sooner. Mine always seemed a little random as to when it actually chose to "afterglow", but I always figured "betsy knows best"?

Anyway, as long as its during the warmup period, its completely normal.

1991 HZJ77, 1999 KZJ90, 1999 UZJ100, 1992 M101CDN2
https://www.thegearheadproject.com/category/jdm-journeys/