I would guess the negative indicates that the field has reduced strength there.
e.g. - if you look at the wiki reference  "The Bangui Magnetic Anomaly Revisited" they clearly say no with what seems like an air tight explanation.
I know wiki isn't a source of truth, but academia doesn't seem to be stepping up with solutions. Someone needs to say the meteor strike theory is very unlikely and needs more updated evidence or modelling to still be taken seriously.
"The thing's hollow - it goes on forever - and - oh my God! - it's full of stars!"
Are those links automatically generated or are they added manually by editors?
Added in the 12:39 edit and removed in the 14:41 (current, newest) edit