Breakpoint analysis

Once a new breakpoint has been detected (see Breakpoints detection), the next step is to decide what type of breakpoint it is. It can be:

• A quiet breakpoint (due to normal variations of the solar zenith angle throughout the day)

• The beginning or the onset of a flare

• A breakpoint with a flat slope at the maximum of a flare

• A breakpoint in the decay part of a flare

• A breakpoint after a flare, when we go back to quiet times

The decision algorithm which is used to analyse breakpoint is presented in Breakpoint analysis algorithm

Breakpoint analysis algorithm

This is done internally by the vlf4ions.analyse_change_points.analyse_breakpoint() function. The main quantity that will determine which kind of breakpoint has just been detection is the detection_threshold (which is an attribute of the station class, see Station), which is dependent on the station and the receiver and needs to be computed beforehand. Below is an explanation of how to compute it.

Detection threshold

The detection threshold (detection_threshold) is the slope above which we consider that there is a flare detection. To find it, the algorithm can be run on multiple days (e.g. one year of past data), and all the slopes detection in daytime, in quiet time (i.e. when GOES flux is below 3e-6 W/m^2^ for example) can be found.

Arbitrarily, we have decided to take the detection_threshold to be the 75^th^ quantile of all the slopes found previously, but this threshold can be decided by the user. A high detection threshold will limit the false positive detection, but a lower threshold will ensure a maximum of true detection. It should also be noted that for a single station, most ‘false positive’ detections are due to error in the phase unwrap (with slopes which can reach several thousands degrees per hour) and thus increasing the threshold will not solve this problem.

A rougth outline of how we can compute this is provided in the Notebook .