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(plots last generated: 27.03.2023 19:05)

Explain these plots!

This magnetometer is located in Viby, about 15 km north of Stockholm, Sweden. It measures the horizontal components of the Earth's magnetic field. The above magnetogram shows the development of the local magnetic field and allows to draw conclusions particularly on the probability of observing norther lights (aurora borealis) at the location of the magnetometer. The data analysis is based of self-developed python and perl code.

The magnetogram shows intensity variations with time of the geomagnetic field in Viby. The rather small variations are primarily caused by the solar wind, which continuously moves onto the Earth's magnetic field. Changes in its velocity, density and magnetic orientation influence orientation and intensity of the geomagnetic field. The disturbance of the magntic field can have a considerable effect on the distribution of electrically charged molecules in the ionosphere (upper atmosphere) which gives rise to northern lights. The magnetometer is able to measure effects much smaller than causing aurora, which makes it an interesting tool for forecasting northern lights.

The magnetogram A magnetogram shows the temporal evolution of the magnetic flux density of the Earth's magnetic field at its location:
	Bx: change of the North-South component of the magnetic flux density: the smaller Bx, the higher the probability for polar lights
	By: change of the East-West component of the magnetic flux density: evening hours: the higher By, the higher the probability for polar lights morning hours: the smaller By, the higher the probability for polar lights
	Bz: change of the vertical component of the magnetic flux density: the higher Bz, the higher the probability for polar lights
Quelle: NOAA/NDGC	Often, instead of Bx und By the horizontal intensity BH (quadratic sum of Bx and By) as well as the magnetic declination D = arctan (By/Bx) is used.

Strong disturbances caused by solar activity are usually seen in the horizontal components of the magnetic field. The vertical component can be used to infer the location of the electrojets. These electric currents run along the auroral ovals and are, give and take, responsible for all magnetic disturbances resulting from magnetic substorms close to the surface. Hence changes in the vertical component represent a good measure for the strength of local substorms.

The K index is a measure of the maximum fluctuation of the horizontal magnetic field component ΔB_H in fixed three-hour intervals. It is a historically-grown historisch gewachsenes, quasi-logarithmic measure of the occurrence probability for polar lights. It is calculated as the difference of maximal and minimal magnetic field strength in eight predefined 3-hour intervals per day. The association of K values and disturbance size is chosen dependent on the latitude of the magnetic observatory in a way so that the statistical distribution of worldwide K values is reached. This means that K is an universal measure for polar light probabilities, and also that stations at higher (geomagnetic) latitudes require larger fluctuations to reach a given K level. For a given disturbance of the geomagnetic field, K values remain combarable globally. This also allows for computing a global, so-called planetary K_p Index (K_p Indices from 1868 on).

Disturbances on the K scale can roughly be translated in how far south the auroral oval will reach according to this map. Basically, for disturbances equivalent to K5 or greater (G1-level storms) there is a fair chance for northern lights in Stockholm. For a fair chance of observing northern lights in northern central Europe, a level of K6 should be reached. Only from K8 the chances for visible aurorae in central Europe are high. It is a good idea to also consider local circumstances at other magnetometers, like SAM Haimhausen.

SAM Viby measures the evolution of the x and y components of the Earth's magnetic fields. As only the relative changes with respect to the values at 00:00 UTC are regarded, an absolute calibration against long-term drifts of the instrument is not required. To determine the K value, SAM Viby uses this conversion, appropriate for latitudes on the transition between mid and high latitudes:

K value magnetic field disturbance ΔBx color code magnetic field G value 0 below 10 nT quiet G0 1 below 20 nT 2 below 40 nT unsettled 3 below 80 nT disturbed 4 below 140 nT active 5 below 240 nT minor storm G1 6 below 400 nT moderate storm G2 7 below 660 nT strong storm G3 8 below 1000 nT severe storm G4 9 exceeding 1000 nT extreme storm G5