I couldn’t find what coordinate system the wind speed is supposed to be in. I assume it is heliocentric. Could anyone confirm? Thanks!

The wind speed is numerically equivalent to vector magnitude, i.e. a coordinate free scalar.

Thanks for your answer. I understand it’s a scalar, but I disagree that it’s coordinate-free. Speed only makes sense with respect to a reference frame. This wouldn’t be an issue if the orbit of the Earth around the Sun had zero eccentricity, because then the problem would be spherically symmetric and wind speed in geocentric coordinates would be time-independent. However, this is not the case: the orbit is an ellipse, which means the Earth accelerates and decelerates with respect to the solar wind every year.

I think I’m simply going to look for a periodic component in the signal with period ~1 year and if nothing’s there I’ll assume heliocentric coordinates since, again, those make more sense in this context.