Northern lights now?

To determine whether a current is flowing in the ionosphere, how strong it is and in which direction, we can look at the change in the horizontal component (Bx, By) of ground-based magnetometers. If there is an electric field, i.e. a current flows, there is always a magnetic field. If the strength of a magnetic field changes, there is always a current flowing. We can therefore use magnetic field measurements to determine the electrical activity above us and deduce how strong and where the aurora is shining.

The Norwegian Geophysical Observatory in Tromsø provides a tool with which you can put together your own favourite magnetometers in a stacked diagram. Above you can see a selection of stations on the “Norwegian line” from 69°N to 59°N from Tromsø to Karmøy with the horizontal component H, which shows the current flow to the east or west. The Z component (vertical component, pointing towards or away from the centre of the earth) can be used to determine the northern and southern boundaries of the oval, but it is very difficult or impossible to determine the intensity.

With a little practice (e.g. by observing the relevant webcams at the same time), the graph can be used to determine very reliably when and where the aurora will light up. If the station “RVK” (Rørvik, 64°N) shows a deviation of -200nT in half an hour, you can almost certainly see at least photographic aurora in the far north of Central Europe (54°N). Important: The scale changes depending on the activity – at the top right you can see how much 200nT is.

Click on the image to open the page on which the graphic is embedded. Move the mouse over the abbreviations to display the names of the stations (does not work on touchscreens). In “Realtime” mode, the graphic reloads every 2 minutes. With D, H, Z, I and F you can switch between the different components (I always recommend selecting H), with the green buttons you can easily click through the past days and check whether the photo you took the day before yesterday really shows Aurora or just a purple glowing greenhouse. “To Menu” takes you to the overview page where you can also see, for example, Finnish magnetometers, those in Canada, the USA, Greenland or Germany.

On aurora.mtwetter.de by Michael Theusner there are heatmap diagrams of the Norwegian line, which I have recoloured a little so that you can better distinguish between the 5 states neutral (orange), slightly positive (magenta), strongly positive (blue, cyan), slightly negative (yellow-green, green), strongly negative (green/cyan/white/grey):

The abbreviations of the magnetometer stations are shown on the right-hand side of the image, and the latitude from 56° to 76° N on the left. Also very interesting is the table at the bottom of the page, which shows how far south the oval must be for the lower part to be visible as a green arc. If 60°N is the southern limit, the arc in Berlin (52.5°N), for example, is already 4.3° above the horizon – at least theoretically. 60°N is roughly the latitude of Oslo, Stockholm, Helsinki and St. Petersburg.

There are also diagrams on aurora.mtwetter.de that show the probability of aurora visibility calculated from the magnetometer data – with an auto-update function. The example shown here is for 52.5°N and 10°E. Even with a probability of just under 20% photographic (subvisual), I was able to make out photographic (subvisual) aurora on the northern horizon, but sometimes nothing is visible even at 70%.

At the top of the example image you can see the moon phase and the availability of the individual measuring stations marked with green lights. Also shown as triangles at the top of the diagram are the twilight phases and the moonrise and moonset at the bottom. The same graphic is also available together with a forecast for visibility within the next hour derived from measured values of the solar wind.

The R or RX index for individual stations indicates the extent to which individual components of the Earth’s magnetic field change within a certain period of time (“R” for “rate” I assume). The Rx only takes into account the x-component, i.e. towards or away from the geomagnetic north pole, while the R-index takes into account all three planes of the Cartesian coordinate system Bx, By and Bz. With a little practice, the indices can be used to estimate how the oval will develop in terms of strength and position.

The following applies to the Finnish R-index: The aurora is visible at the location from around 100nT. If there are stations in southern Finland, the aurora should also be visible on the northern horizon in central Europe – the more southerly and stronger the more likely. The Finnish Meteorological Institute website also provides diagrams with a time axis for the individual stations. The graphic with the whole of Scandinavia and the “Real-time ground disturbance” (RX) comes from the formidable TGO Tromsø – site.uit.no/spaceweather, which now also offers nowcast, +1h and +4h for the Brocken and for Hamburg with a star map and all the bells and whistles – I haven’t yet tested whether the nowcasting works. The lines of longitude and latitude shown above are geomagnetic coordinates with the geomagnetic poles as reference points. The geomagnetic North Pole is located near Thule, North Greenland

Another very reliable method of determining the exact position and strength of the auroral oval is the use of Global Navigation Satellite Systems (GNSS) such as GPS (USA), GLONASS (Russia), Galileo (Europe) and Beidou (China). To do this, the data from receiving stations on the ground is analysed, merged and a two-dimensional map is created. Based on the deviation of certain parameters, the Total Electron Content TEC of the ionosphere is first derived. The Rate of TEC Index ROTI is derived from the change in values per grid point per minute. ROTI is actually primarily intended for users of radio services in order to be able to make predictions for interference in short-wave, VHF, UHF and satellite reception (especially GNSS). A practical side benefit: We can track the location of the aurora live on the map. On this website in the sidebar (desktop: on the right, mobile devices: below the article) you can see the One minute mean ROTI, Europe from IMPC Neustrelitz (German Aerospace Centre, DLR), other ROTI data products are One minute mean ROTI, Global, One minute maximum ROTI, Europe and One minute maximum ROTI, Global. Red grid points = This is probably the aurora… Not valid south of 45°N (ROTI also displays plasma bubbles and other disturbances that are not aurora).

…, the most helpful from my point of view is the prediction for the AE index derived from solar wind data. The representation of the north-south component of the interplanetary magnetic field Bz chosen here (not to be confused with the Bz value of the Earth’s magnetic field described above – something completely different!) shows trends to the north or south very well, better than in conventional diagrams with narrow lines of the same colour.

Measure of magnetic convection determined from the solar wind (Bz, By, Bt, v). Synonyms: Asymptotic Polar Cap Potential, Polar Cap Potential (PCP), Cross Polar Cap Potential (CPCP), Cross Polar Cap Potential North (CPCPN), Cross Polar Cap Potential South (CPCPS) or in the pointer graphic simply “Convection”. The formula was determined empirically; the PCP can be used to predict the AE index and the Kp index, among other things. From a value of 100kV, the probability of seeing the aurora increases in Central Europe.

☞ agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/96JA01742 Boyle, 1997, Empirical polar cap potentials

An alternative to the polar cap potential is Newell’s non-linear coupling function, which was also determined empirically and can predict the auroral potential similarly well. It indicates the degree of coupling of the interplanetary magnetic field with the Earth’s magnetic field and also uses the parameters v, Bz, By and Bt. It is used by NOAA/SWPC to drive the Ovation Prime model and to predict the current Kp index. Optimised for Kp prediction.

dΦMP/dt = ^{v4/3BT2/3sin8/3}(_θc/2)

☞ agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2006JA012015 Newell, 2007, A nearly universal solar wind-magnetosphere coupling function inferred from 10 magnetospheric state variables

The empirical Ovation Prime model is calculated with data from the solar wind (Newell Coupling) and shows the probability of the aurora being visible at a specific location. It gives a very rough indication of what can generally be expected. It does not show the actual position of the oval! Spontaneous substorms are also not included. The brightness of the aurora can be roughly estimated from the calculated “hemispheric power”. From around 50GW, the chances of auroras increase slightly in Central Europe, and significantly from around 80GW.