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Onset detection | Bays | FAC Location | Validity

(4) Are the results meaningful?

Whilst we do not claim any great accuracy for this method, it is capable of giving a good indication of the presence of one of the Field Aligned Currents within the field of view or whether the onset was to the east or west of SAMNET. This can be useful in providing the general context for measurements with other instruments e.g. radars and spacecraft.

The user should bear in mind however, that the substorm location code will return results for any random piece of data. Although the majority of the automated Pi2 detections are substorm onset related, the response of the SAMNET magnetometers depend on many factors such as distance from the onset region, the magnitude of the currents involved and the existence of other current systems (e.g., during a storm).

In order to assess the validity of your results you should bear in mind the following points.

  1. If the bay amplitudes are not significantly larger than the Pi2 amplitudes then we are dealing either with a distant or a weak susbtorm (also possibly a 'pseudo-onset'). The H and D component deflections will be dominated by pre-existing gradients and the Pi2 pulsation itself.
  2. Comparison of the results from the 2 chains of stations at different latitudes (the 'red' lower latitude chain of YOR, GML, NUR, KVI, BOR and the 'blue' higher latitude chain of FAR, NOR, OUL) gives some indication of the robustness of the result. For instance in the previous example the red and blue chains agree in the major features. The higher latitude chain however may sense more of the electrojet current and so the results from the 2 chains can be inconsistent. In such cases the lower latitude chain is more likely to give a reasonable result.
  3. Check that the Pi2 onset time and the start of any significant bays are approximately coincident. If there is a difference either way then the gradients detected as the first 4 minutes of the bay may be in error. This can be seen in the output from the detection code which can then be run again with a new start time.
  4. For best results at least 3 of the lower latitude stations (YOR, GML, KVI, NUR, BOR) should be available. If yesterday's data froma any site is missing attempts are made to retrieve it on successive days. Where possible the data from the transcribed 5 second database is used instead of the incoming unchecked data as this speeds up the processing.

A more accurate substorm location method was developed by the Magnetospheric Physics Group at the University of York (the former operators of SAMNET) and is described in Cramoysan et al., 1995. This model is now available at Lancaster for use on a collaborative research basis. For further details contact us.

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