Noctilucent Clouds

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 best able to see them are shown.

They are much rarer than auroras, typically seen fewer than 15 nights per year. They are clearly visible by eye, even in towns and cities. There is no way to predict them, you just have to look at the optimum time and hope.

Acknowledgements

Global, real-time tracking of auroral substorms is only possible due to some incredible organisations allowing me to access the live data from their magnetometer arrays.

I am indebted to the kindness and generosity of the Tromsø Geophysical Observatory, Swedish Institute of Space Physics, the US Geological Survey and the Finnish Meteorological Institute for letting me access their live data feeds.

I am also indebted to I.R. Mann, D.K. Milling and the rest of the CARISMA team for use of data from the Canadian magnetometer array. CARISMA is operated by the University of Alberta, funded by the Canadian Space Agency.

Mann, I. R., et al. (2008), The upgraded CARISMA magnetometer array in the THEMIS era, Space Sci. Rev., 141, 413–451, doi:10.1007/s11214-008-9457-6.

Substorm Tracker

The headline figure -100 nT is the strength of the aurora in the sky right now, called the substorm strength.

Watch this number! You want it negative. The more negative it is, the stronger the aurora is at that minute. Falling is good, rising is bad. It tracks the strength of the actual aurora in the sky in real-time. It is the single most important number on the app and overrides all others.

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 trendline shows the changes in strength over the last hour. The line dropping sharply downwards is good. Rising is bad. The green shaded area indicates the ambient level.

IMF

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.

If the 'now' and 'next' rows are both lit up, the substorm will continue if one is active or will start in about an hour if one isn't active.

If the 'next' row is not lit up, the activity will start to wane if a substorm is active but there can be an extra burst of activity, so wait an hour.

"Turns bad" is "good" because it can trigger expansion in an active substorm. It is "bad" for future substorms.

When a substorm is in its expansion phase all of these values are irrelevant to that substorm. Substorm strength overrides all other metrics.

The trendline shows the Bz and Bt that is arriving at Earth now and as far into the future as it is possible to see. You want the bottom line to be below the green shaded (ambient) area.

Solar Wind

This is the mean velocity, density, pressure and power of the solar wind that is currently arriving at the Earth. They don't really matter that much for aurora. IMF is much more important. A good IMF will produce aurora even when speed and density are at ambient levels.

The trendline shows the solar wind speed and density that is arriving at Earth now and as far into the future as it is possible to see. When both of these are rising it normally indicates arrival of a co-rotating interaction region. When speed rises and density falls this normally indicates a coronal hole stream. The green shaded area indicates the ambient level.

When a substorm is active, all of these values are irrelevant. Substorm strength overrides all other metrics.

When the IMF is strong and well-aligned, these values are irrelevant. IMF overrides solar wind speed & density.

Webcams

Webcams supplement live user reports by allowing us to see whether aurora is being detected on static cameras.

Coronal Holes

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 earth-facing position 4 days ago.

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

CMEs

Coronal Mass Ejections are bursts of plasma that sometimes erupt from sunspots during a solar flare. They can trigger the strongest auroras if they hit Earth.

You are looking for plasma surrounding more than 50% of the solar disk in this graphic (a partial halo). The ideal is a 'full halo', where plasma appears to surround the entire solar disk. It then takes 2 to 3 days for the CME to reach Earth.

Even when there appears to be an Earth-directed component or it looks like a direct hit, the CME can still miss completely or bounce off the Earth's magnetosphere. Don't get too excited until the app alerts that a shock has been detected!

Don't take any ETAs too seriously, either here or on other sites, as they are only a very rough guide.

CME data derived by Glendale App from solar imagery kindly provided by SOHO.

Flares

Solar flares from sunspots sometimes produce a CME which, if directed earthward, can cause strong substorms between two and four days after the eruption.

Most flares don't produce a CME and most CMEs miss the Earth, so don't get too excited about flares.

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

After a flare, it can take several hours before imagery becomes available that enables us to determine whether there was a CME and whether that CME was Earth-directed. After a flare, keep checking the Coronal Mass Ejections panel for the latest updates as imagery arrives.

Long-Range Forecast

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, Scandinavia, North America and Australasia that you will find anywhere in the world.

It is essential to check the coronal holes panel, as we get closer to the dates that interest you, to confirm the current status of the hole that caused the activity, as it might have closed.

This forecast is based on my own data collected in the UK and is valid worldwide. The more negative the figure, the stronger the aurora could be.

Dates that are coloured black are uncertain. Dates shown in colour are more likely.

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