what caused this increase of the Kp in 1999

[farm24]

i was looking at the events that have happened this day in history and i found that on this day in 1999 the Kp reached a level of 8 (see photo) but i cant see any flares or anything that would have caused this so why did it happen?

[Vancanneyt Sander]

If you browse our archive, visit the activity report where you can find a brief summary of the events that happened that day:

and if you take a look at the solar wind  and IMF parameters, also available in our archive, you’ll see there was a very long period of negative Bz up to -20nT which makes a Kp8 possible

[Marcel de Bont]

Could be this C8 solar flare but the report doesn't mention a CME. https://www.spaceweatherlive.com/en/archive/1999/09/18/rsga.html 

Search trough the reports or try to find old LASCO imagery if you want to dig deeper.