The line is detonation cording. It is there to shatter the canopy as part of the ejection sequence. It can be seen on many older combat jets. It is a less common system now as many aircraft use explosive bolts to jettison the entire canopy structure instead.

In the case of a helicopter this is safer however. Because a helicopter is more likely to have little or no forward motion a canopy that has been jettisoned would be an impact hazard for ejecting crew.

F in chat for Goose. o7