I've read an article in nature and found technical words "B mode signal" and "E mode signal". I don't understand what they are. In a website, they say it's like the electromagnetic field line but I'm still not clear. I also don't understand what k means in " the linear greyscale shows the E-mode lensing convergence field k(Kappa)". I think it's also the technial word. The article is about the dark matter maps.