Noctilucent clouds are lit by sunlight in an area of the night sky called the Twilight Arc.

My twilight-arc tracker is constantly monitoring the position of the sun at your location and calculating the position and height of the twilight arc.

The centre of the twilight arc is at the sun's azimuth. The sun's elevation is its angle above (or below, if -ve) the horizon.

The 'Max Heights' are the maximum angles above the horizon that Noctilucent Clouds and Cirrus can be lit directly by sunlight.

NLCs are only visible when the sun is between 6° and 16° below the horizon and the times when you are able to see them are shown.

The headline figure (with nT beside it) equates to the strength of the aurora in the sky right now. You want it negative. The more negative it is, the stronger the aurora is. Falling is good, rising is bad.

When it says 'growth', the substorm is charging up like a battery. When it says 'expansion' that energy is being released as aurora. When it says 'recovery', the aurora will slowly begin to fade.

The trend-line shows the changes in strength over the last hour. The line dropping sharply downwards is good. Rising is bad.

I am indebted to the kindness and generosity of the Tromsø Geophysical Observatory and Swedish Institute of Space Physics for letting me access their live data feeds and use them to alert you when the aurora can be photographed in the UK, Iceland & Europe.

The table shows an analysis of the interplanetary magnetic field in the 30 minute window that is currently arriving at Earth, together with the 30 minute windows either side of it.

The 'percentage' figure indicates how negative the Bz was, the ideal is 100% negative.

This the the mean velocity, density, pressure and power of the solar wind that is currently arriving at the Earth. The higher the pressure and power of the solar wind the better.

The trendline shows the changes in the solar wind speed over the last 24 hours, with the density shown behind. When both of these are rising it normally indicates arrival of a co-rotating interaction region. When speed rises and density falls this indicates a coronal hole stream.

Coronal holes cause most of our auroras and they are regular, repeating on a 27 day cycle. Plasma from an Earth-facing coronal hole typically takes 3 to 4 days to reach us and spark auroras.

Flipping between the 'now' and '-27 days' tabs allows you to see how coronal holes have changed since their last rotation 27 days ago.

The '-4 days' tab allows you to see how the coronal hole looked when it was in the geoeffective position 4 days ago.

Solar imagery courtesy of NASA/SDO and the AIA, EVE, and HMI science teams.

Long-duration solar flares often produce a CME which, if directed earthward, can cause geomagnetic substorms between two and four days after the eruption.

Flares are classed B, C, M or X with B being weakest and X being strongest. The number indicates how strong the flare was within its class.

These are the dates when solar wind streams that gave us good auroras will rotate around again. There is no guarantee that the coronal holes that caused the substorms on the previous rotation won't have closed but they also may have got larger. You can only use these as a guide but this is the most accurate long-range forecast for the UK, Ireland, Iceland and Scandinavia that you will find anywhere in the world.

This forecast is based on my own data collected in the UK and is specific to Western Europe & Iceland. You may see more than one entry per day if there were multiple, strong substorms in the same day on the previous rotation.

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