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Magnetometer Observations

Magnetometer Observations

Magnetometers are very sensitive instruments, capable of detecting changes of within a few nT. Interpreting these changes requires knowledge of your instrument and a clear understanding of the factors that affect the magnetometer output.

A typical quiet day will show a diurnal effect that is caused by the magnetosphere being compressed during the daytime phase of the terrestrial day and then stretched during the night.

This effect is demonstrated by this NASA graphic.

The magnetometer will detect events on the Sun that affect the Solar Wind and hence changes in the local magnetic field.

The two major contributors to this affect are Coronal Holes and Coronal Mass Ejections.

Coronal Holes

Coronal Holes cause a slow but distinctive response to changes in the magnetosphere. As Coronal Holes may form and dissipate over several days, this can result in a period of quite unsettled Space Weather. This example of a large Coronal Hole that formed, then split into two and then dissipated at the end of April 2010 resulted in conditions that were considered to be 'storm' like.

Image from Spaceweather.com (c) NASA

Note the 2 to 3 day lag that occurs between the event occuring and the effect that is observed. Typical undulating shifts of up to 200nT are typical of a large Coronal Hole.

Coronal Mass Ejections

These are clearly much more violent events and are a great cause of concern as the associated radiation can wipe out Satellites and can cause harm to astronauts in Space.

In early November 2004, the Earth was the unwitting target for a series of CME's that occurred over a 4 day period. The effect on the magnetosphere was dramatic and occurred within a few hours of the first CME.

These magnetograms from the first and last day of this storm clearly show rapidly changing geomagnetic shifts of approximately 400nT that can occur within a few minutes. This is the exception rather the norm but a dramatic indication of the value of a logging magnetometer.

Incidently, this event provided early warning of an Aurora that was visible as far south as Southern Europe.

Other Disturbances

A full description of the types of magnetic events can be found here in the BAA Aurora Section pages.

A full description of the magnetosphere and how it interacts with the Solar wind and the Interplanetary Magnetic Field can be found on Wikipedia

Magnetogram and magnetosphere observations should be sent to Mr. David Pettitt in the BAA who correlates the data on behalf of the Aurora Section

Martyn Kinder



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