Magnetic Observatory Viby
Sollentuna, Sweden · geographic coordinates: 59°27' N 17°54 E · geomagnetic coordinates (2017): 57.69°N, 106.22°E

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Online determination of auroral substorms

The primary mission of this magnetometer is to issue timely alerts at times of likely occurrence of visible or photographic aurora borealis at the location of the magnetometer - Stockholm, Sweden. Of course, the magnetometer data also can be helpful for forecasting opportunities to observe aurora borealis elsewhere. A primary determinator of magnetic disturbance usually is the magnitude of change of the horizontal magnetic components. Besides that, for the Magnetometer Viby we implemented an online determination of auroral substorms based on the speed of magnetic field changes.

A transparent way of displaying this activity analysis, based on flux change speed is given by the plots: They analyze the size and speed of disturbances of the horizontal components. In the upper plots, the evolution of the horizontal magnetic field components is plotted for the last four hours, with rise and fall timescales of the disturbances color-encoded.

Rise/fall times in excess of 40 nT/10 min are considered significant. Rise/fall times in excess of 80 nT/10 min are considered to be caused by magnetic field disturbances connected to a possible occurrence of aurora borealis. Blue colors represent a fast rise of magnetic flux, red colors a fast drop of magnetic flux. In the lower plots, the absolute magnitude of the disturbance speed is given in units of nT per 10 minutes.

The Finnish Metereological Institute employs a similar estimator for assessing the probability of visible auroras. Disturbed geomagnetic conditions are assumed for changes larger than 0.3 nT s-1 at the station in Nurmijärvi (57.72N GCM). This corresponds to 180 nT/10 min in our notation. The corresponding threshold for the Sodankylä station (64.01N) is 0.5 nT s-1. Their bar plots show the hourly maximal changes in the horizontal magnetic field components. Currently, for SAM Viby we use 180 nT/10 min and for SAM Haimhausen (43.51N GCM), 90 nT/10 min to characterize conditions as disturbed, however these thresholds are preliminary and subject to change.

Peak analysis

no significant peaks found.

Horizontal component speed vs. declination

Alerts based on the evolution of the horizontal magnetic field components

For concurrent forecasts, the evolution of the horizontal magnetic field components is analyzed. Depending on the activity within the last hour, green/yellow/red alerts are displayed in plot.

Activity alert criteria assess the last 60 minutes:
Quiet geomagnetic conditions Disturbed geomagnetic conditions Considerable geomagnetic activity
Within the last hour, less than ΔB<80 nT, corresponding to <K3 Within the last hour, less than ΔB<140 nT, corresponding to K4 Within the last hour, ΔB>140 nT, corresponding to K5

The derivative of the magnetic field evolution, dB/dt characterizes the speed of the magnetic flux change. Peaks hint at fast change, which is a neccessary condition for the occurrence of aurora. Depending on the activity within the last four hours, green/yellow/red alerts are displayed in plot.

Flux change speed alert criteria assess the last four hours:
Quiet geomagnetic conditions Disturbed geomagnetic conditionsons Considerable geomagnetic activity
Outburst less steep than ΔB<40 nT/10min Outburst less steep than ΔB<80 nT/10 min Outburst less steep than ΔB<180 nT/10 min Outburst steeper than ΔB>180 nT/10 min

In the plot, also high values of the second derivative, d2B/dt2 are marked (by arrows). These characterize high curvatures of the flux curves and thus a fast change from rising to falling wings, prevalent in magnetic substorms, and may allow to identify a substorm earlier than just the first derivates. The analysis of these features is still experimental and is not being used to issue aurora activity alerts.

Current trend data for transfer to the SAM magnetometer network

From about the year 2002 on, a pan-European magnetometer network emerged, based on the SAM magnetometer design. To identify magnetic storm episodes, a common analysis and alarm feature was implemented. By a common analysis false alarms of individual stations can be effectively avoided. These data resemble dB/dt data with a step width of Δt=5 min. The data format follows the format put forward by the German magnetometer network at This webpage provides a central data analysis mainly for German SAM stations. These data resemble ΔBt data with a step width of Δt=5 min, which can be useful for aurora borealis prediction as put forward in a blog by Steve Marple of Aurorawatch UK.

To facilitate an online analysis for central/northern Europe, recent magnetometer data is provided here. These data describe te evolution of the horizontal components of the local magnetic field within the last hour.

59.46N 17.89E
K: 0 - - - - - - - 
23:19 - 00:19 UT
Min. X Y Z Trend
now +0 +17 -13 o
- 5 +2 +18 -11 o
-10 +4 +18 -10 o
-15 +3 +19 -9 o
-20 +4 +16 -8 o
-25 +5 +17 -8 o
-30 +4 +17 -9 o
-35 +4 +16 -9 o
-40 +4 +15 -6 o
-45 +4 +15 -7 o
-50 +4 +15 -7 o
-55 +2 +14 -8 o

A difference of 80 nT/5 min is needed for a "++" and a "--" trend; 20 nT/5 min for "+" and "-" trends. SAM Haimhausen utilizes this information for formulating aurora alerts, which are issued for "++" trends in presence of K5 and "--" trends. This scheme has been incorporated by the SAM magnetometer network; network alerts are only issued upon coincidence of two or more magnetometers.


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