If you have ever photographed the auroras you know how important it is to know when they will explode in the sky. We actually have data available making that prediction easier than just following the Kp number. There has been much written on the auroras, but when I put together my new video on photographing the auroras, I wanted to make something which wasn’t the generic aurora video telling you the basics of how they work and they’re “unpredictable.” I have learned a ton and my life-long fascination with space came in handy researching this video. I kind of felt I was back in university making a paper.
So a little introductory has to be made.
The Kp Number Is Not a Prediction
Most of us already know about the Kp value. The Kp index is scaled from 1-9, where one is weak and nine is very strong. Normally the Kp number swings between below one and two and during periods with higher than normal solar activity the number increases. During a Kp9 it is possible to see the auroras as far south as Spain or Florida. During a period of strong geomagnetic activity, as it is rightly called, geomagnetic storms can occur. Geomagnetic storms are measured on the G-scale from 1-5, which translates linearly to the Kp scale. G1 equals Kp5, G2 Kp6 and so forth. A G5 storm only happens during extreme events and it translates into a Kp-value of 9.
What most people do not know is that the Kp-value is the average global Kp-value from the past three hour period and is thus not a forecast. The three hour periods start at midnight at Universal Coordinated Time formerly known as Greenwich Mean Time. Which means the first period is from midnight to 3:00 am, the second from 3:00 am to 6:00 am and so forth. Using the Kp-value as a way of predicting the auroras assumes the activity stays the same. Which is not the best way of predicting anything. Let us imagine you are out at 2:30 am and you are checking the Kp-value, the value you have available is the global average from 9:00 pm to midnight, which does not really help you much at 2:30 am. That is why you can have all your sources giving you a Kp6, yet there are no auroras in the sky because the activity has decreased in the meantime.
Geomagnetic Storms and Substorms
Geomagnetic storms often last several days and create a “constant” flow of particles. Whereas substorms only last a couple of hours or shorter. They occur when the magnetic field lines of the magnetotail of the Earth’s magnetic field reconnect and creates a “snap-back” effect hurling the particles back towards Earth with increased velocity creating a burst in aurora activity.
We want geomagnetic storms to occur, which increases the chances of magnetic reconnections, which creates the substorms.
How to Predict the Auroras
Spaceweatherlive.com is, in my opinion, the best resource for predictions I have come across. That is if you understand all the graphs and data.
Besides having the most recent Kp-value, the graphs show the solar wind, solar wind density and the Bt and Bz values of the interplanetary magnetic field (also known as the heliospheric magnetic field), which is a component of the suns magnetic field.
Solar Wind and Density
The first two graphs on the left describe the solar wind. One is for the solar wind speeds and one is for the solar wind density. The faster the wind the harder the charged particles from the sun hit the molecules in the atmosphere, resulting in stronger auroras and the denser the wind is, the more particles hits more molecules resulting in bigger interaction, resulting in more and stronger auroras.
Dependent on the speed of the solar wind we have more or less time to prepare for the auroras. The turquoise bar tells us that time. The information of the graphs comes from the right and the black vertical line on top of the graphs represents Earth now.
Bt and Bz values
There are two more factors playing in though, which you can see on the two graphs on the right. Interplanetary magnetic field Bt and Bz values.
The Bt value is the strength of the Interplanetary magnetic field, the stronger the field the more magnetism, the more auroras. The average value of the Interplanetary magnetic field is 6nT, but for geomagnetic storms to occur you’d like values of at least 10 nT
The Bz value, which is extremely important for auroras to occur, is basically the direction of the Interplanetary magnetic field. Since space is 3-dimensional there’s also a Bx and By value, but these two does not influence aurora activity. When the direction of the IMF turns southward, the Bz value turns negative. This is good as Earth’s magnetic field lines are turned north. As you know from magnetism north and south poles attracts each other and north and north or south and south detracts each other. When the south-bound IMF meets the north-bound magnetosphere of the Earth they merge resulting in the transfer of energy, mass, and momentum from the solar wind flow to the magnetosphere, transferring all those delicious electrically charged particles towards the poles of the Earth and into our atmosphere. With a northbound IMF or positive Bz value auroras are still possible, you just need the other factors, solar wind speed and density and the Bt value to be much higher.
If you are still in doubt just aim for the colors. Low activity represented by the green color, moderate by yellow and high activity by red. If you want even more information be sure to hover over the small blue information icon.
If you need to predict the auroras within the near future, these are the factors you need to be aware of. It is still impossible to say anything about the behavior of the actual auroras and precisely where on the night sky you’ll see them. I hope you understood the graphs and how to interpret them. This should give you a better chance of predicting when you have the statistically best chance of observing the lights when you're in the field.
If you want to know even more about photographing the auroras and have a visual description of what I just wrote be sure to check out the video above.