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(plots last generated: 24.06.2018 11:05)

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 evolution of the local magnetic field. It allows drawing conclusions particularly on the probability of observing northern lights (aurora borealis) at the location of the magnetometer. Earth rotates below the auroral oval. Directly opposite the dayside, the auroral oval is elongated towards south. As the Swedish east coast is substantially more eastern than central and western Europe, the area around Stockholm will rotate below the auroral oval about 30 minutes before central European longitudes. Therefore, this Stockholm-based magnetometer can provide a first idea of geomagnetic activity at northern-central latitudes before the peak of the oval reaches locations further west.

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 magnetic 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 a well-suited tool for forecasting northern lights.

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:	By: change of the East-West component of the magnetic flux density:	Bz: change of the vertical component of the magnetic flux density:
the smaller Bx, the higher the probability for polar lights	evening hours: the higher By, the higher the probability for polar lights morning hours: the smaller By, the higher the probability for polar lights	the higher Bz, the higher the probability for polar lights
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.		WMM 2015 model, Chulliat et al., doi: 10.7289/V5TH8JNW. Picture credit: NOAA/NGDC/CIRES

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, 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 with the size of disturbance is chosen depending 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 a 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 comparable 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 or by looking at a Magnetometer Stack covering your region.

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 60 nT disturbed 4 below 105 nT active 5 below 180 nT minor storm G1 6 below 300 nT moderate storm G2 7 below 495 nT strong storm G3 8 below 750 nT severe storm G4 9 exceeding 750 nT extreme storm G5