What algorithm can be used to divide a time series Hi everyone

I have a data as on the image. I would like to derive a line that separates the data (like the red line), but not sure what algorithm can be used.

I thought about k-means in 1-d data, but then the high points (i.e. y-value ~0.01 are not really in the same cluster with the data point of 0.002).

Thanks a lot.

P/S: In fact, the values here are reconstructed MSE from h2o Autoencoder, and now I want to find an automatic way to determine the threshold, to say a point is an anomaly or not.
First of all, there is nothing wrong with using a threshold.

If you can't manually choose the threshold, use standard statistics. For example trim the largest 10% of points. Then compute the mean, standard deviation, and use 2*stddev as threshold.