MEGNO and non-gravitational forces

[fhlobik]

Hello,

we are investigating Lyapunov times of meteoroids affected by radiation forces (from REBOUNDx) in the Solar System. The times we are getting are quite short (few years to low hundreds of years - depending on the value of beta of the meteoroids). One possible explanation we are considering would be close encounters with Jupiter.

However, just to be sure, do the MEGNO and LCN calculations in REBOUND take non-gravitational forces from REBOUNDx into account?

Thank you.
Filip

[dtamayo]

No they don’t. That would definitely be a valuable feature, but would require some thought and work for how to add in in a general way for different effects

[hannorein]

Good question! Dan is right, currently the REBOUNDx forces do not apply to variational particles (and hence MEGNO/LCN). It would be a lot of extra work to implement this. We'd basically need to calculate the gradient of all the additional forces. It might be relatively easy to implement for GR. But I suspect the radiation forces are a lot harder.

As a temporary workaround, you could implement your own LCN calculation by using shadow particles. Just initialize a swarm of particles on very similar orbits, then measure the divergence as the integration progresses.

[fhlobik]

Alright, thanks for the answers. I'll look into the shadow particles implementation.

[hannorein]

I'll keep this open. I think we should at least add a warning message.