One of the things that most people embarking on a break to see the Northern Lights are curious about is how are they predicted? This overview only scratches the surface of this complex science but we thought it would be of interest to people who want to know how the scientists at places like NASA manage Northern Lights Forecasting.
What are the Northern Lights?
In essence, the Northern Lights are caused when highly charged particles from the sun enter our magnetic field and react with it to create an electric current. This pushes the charged particles that are already in our magnetic field toward the Poles. These particles enter the atmosphere and collide with oxygen and nitrogen ions; transferring the particles’ energy to the ions. This ‘excites’ the electrons in the ions sending them into a high-energy state. As the electrons calm down the excess energy is re-radiated in the form of light: the Aurorae.
Fig 1. How the Northern Lights (Aurora Borealis) are generated.
The particles that create the electric current come to Earth from the outer surface of the sun. This layer is called the corona and is not normally visible to the naked eye; however, if you’ve ever looked up at the moon during a solar eclipse and seen the ‘halo’ surrounding it, you’ve seen the corona. The particles get from the corona to the Earth in three different ways.
- The first and most common way for the particles to reach us is to come from coronal holes: areas on the sun’s outer layer with open magnetic fields that release a continual stream of particles into the solar wind. The near constant release of particles from here means that whenever a coronal hole is facing our way many of the particles are carried to us.
- Secondly there are coronal mass ejections (CME); these are explosions on the sun’s surface propelling billions of tonnes of particles into the solar system. When a CME is directed towards us we get a huge quantity of high energy particles coming in our direction.
- Lastly, there are solar flares. These are also high energy explosions on the sun’s surface but these are more ‘local’ than CMEs which are huge eruptions. Another key difference between them is that a CME expels magnetised plasma on an enormous scale, whereas a solar flare super heats nearby particles (it is these that fire off into space) but also emits large amounts of radiation.
The much publicised Aurora display of July 2012 was caused by a single solar flare coming from an active sunspot region. The region’s NASA designation - AR1520 – sounds relatively uninspiring but its size – over 15 times the width of the earth – is much more impressive!
Similarly fascinating is the speed at which the particles travelled from AR1520 - some 1400 metres per second or 3132mph - that’s fast enough to do a loop of the earth in just eight hours! Following the initial speed, the particles became super-heated by the flare, giving them the energy to accelerate to near the speed of light. It is these astonishing speeds that let the particles make the 93 million mile journey to Earth in roughly four days.
Northern Lights Forecasting Methods:
1: Monitoring Long-Term Sunspot Patterns
Monitoring sunspots is one way that scientists forecast the Aurora. For an accurate prediction you have to take into account: long-term solar cycles, the results of 27/8 day solar monitoring and, of course, the immediate weather forecast.
Long range Northern Lights Forecasting is based on the solar cycle. The sun has periods of high activity: the solar maxima and periods of low activity: the solar minima. We move from the minima to the maxima during an 11 year cycle. The cycles are best thought of as a pendulum in motion: the pendulum swings up, giving us the incredible Aurora displays at the solar maxima and then takes 11 years to swing to the minima and back up.
Fig 2. A graph depicting the solar activity cycle and forecast until 2020.
Mapping these cycles has been the work of centuries. For hundreds of years Aurora spotters recorded changes in the size and intensity of the displays until eventually a pattern began to emerge. These records date as far back as John Worcester in AD 1128 and continue through to Galileo and beyond until, in the 18th and 19th centuries, astronomers identified sunspots as sources of the Aurora and noted that at fairly regular intervals sunspot numbers peaked then dropped. Drawing on these historical records, as well as on-going observations and current technology, predictions of the solar maxima are more accurate than they historically have been. But it is still far from a perfect science. Solar activity does not work to a fixed pattern and forecasting the sun’s quirks of activity is an on-going challenge. A prime example of these unexpected changes occurred only last year. All the predictions showed a solar maxima coming in the winter of 2012 to 2013 and yet solar activity rose in 2011 and then dropped in 2012. The most likely reason for this is that we are actually experiencing a double-peak solar maxima. These occur when the Northern and Southern hemispheres of the sun don’t peak at the same time and no reliable way of anticipating them has yet been found, although now it has begun it can be predicted that the double peak may continue into the winter of 2014/2015.
Fig 3. Solar activity compared between 2010 and 2012 (start of the current maximum).
2: Monitoring Solar Activity about a Month Ahead
The shorter 27/28 day forecast is based on the fact that the sun rotates fully on its axis during this time period and that any sunspots or coronal holes facing the Earth at the start of the rotation will again be facing us at the end of the rotation. These are also not guaranteed predictions as both sunspots and coronal holes can disappear as quickly as they arrive. But if a particular coronal hole showers us with particles or a sunspot causes CMEs and solar flares to give us an incredible display one night, so long as it is still active 27/28 days later, it is possible we will be treated to yet more!
3: On the Day
The final link in the Northern Lights Forecasting chain is the on-the-day forecast. To find out if there’s going to be a display, scientists use satellites to identify and measure particle-loaded solar winds. Once scientists have predicted when the lights will appear, they look into the likely impact of each appearance. If you’ve ever been on a Northern Lights holiday, for example, you may have heard the guide discussing ratings of ‘a two’ or ‘a five’. This isn’t a score out of 10, however, it is a measurement on the Kp index. This index measures the strength of disturbances in the Earth’s magnetic field. A big disturbance means a strong, particle-filled solar wind which often results in bigger displays, although that depends on your location and the weather in your area.
- A rating of 0-4 on the Kp index is categorised as mild, the Aurora will not be seen further south than usual and the display will not be particularly intense.
- Ratings of 5-7 indicate storm level intensity; this means the displays will be visible further south than usual and will be stronger and more vibrant.
- An impressive 7-9 is severe storm level and it is during this rare type of display that the Northern Lights can be visible as far south as Spain!
A high rating on the Kp Index isn’t always essential when seeing the Aurora. Around the Poles a good display is possible with a Kp of 2-4 in fact. Many of you might remember Joanna Lumley’s series where she saw the Northern Lights. This was at a period of solar minima, in which a high Kp is very unlikely. So how did she get such a good viewing? She was guided into the Auroral Oval, this is the oval shape drawn on a map that shows the common boundaries of the Aurora. The Oval is caused by the particles from the sun creating the electric current that pushes pre-existing particles into our atmosphere at the Poles; because the particles enter at the Poles it is there that they collide with the ions to create the Lights. However, a high Kp means that more particles have entered the magnetosphere and so created a stronger current: pushing more particles into the atmosphere. When more particles enter the atmosphere they have more chance of spreading further than the usual Auroral Oval, giving those of us further south a sight of the Lights.
A further weapon in the scientists’ Northern Lights Forecasting armoury is the Bz measurement. This looks at the direction of the solar wind’s magnetic field. The Earth has a northward Bz value, meaning that the solar wind needs a southward value to avoid our magnetosphere repelling most of the particles. When the solar wind has a strong southward Bz we’re in for a treat, as many more particles make it through the field to light up our skies.
Everything has built up to this point and armed with the Kp and Bz indexes Aurora spotters, like the famous guides available on our Tromsø City Break, can perform on the day Northern Lights Forecasting, assessing when and where to search. Their decision will also be based partly on cloud coverage and weather factors, i.e. dictating if you’ll be able to see the Aurora, rather than if it will appear. These sort of decisions are best left to local residents who have a detailed knowledge of where clouds usually pass, where you are best placed to get away from competing light sources and not offend other locals. This detailed knowledge of the location and local micro-climate is one of the reasons we rely on local guides on our Northern Lights hunts.
So, now you know the science, why not discover the Aurora for yourself and take a look at the magnificent Northern Lights holidays on offer at the Mighty Fine Company?