What is a Kp index? Why do we talk about BWith and a Bt where the? What does it take to cause aurora borealis? You will find out in this article! We have to keep a close eye on the tables and graphs that show the many data and we have to wait very patiently until the data becomes favorable.
We keep an eye on the data so that we do not unnecessarily stand outside and look to the North. However, it is not because one graph shows favorable measurement data that aurora can be seen. Only if all parameters and data are favorable for our regions is there a good chance that we will see aurora in Belgium and the Netherlands (if the weather is favourable). Via our specialized aurora borealis website www.poollicht.be you have all the necessary data and graphs on one website and you will be warned in case of favorable activity on the Sun and on Earth.
Low, Medium and High Latitude
When there are aurora chances, there is often talk about great aurora chances in the high latitudes. But what are the high, medium and low latitudes? There is no precise definition of where that boundary is now, but generally speaking, the high latitude is around the 60th parallel and above. The average latitude is between the 50th and 60th parallels, and the low latitude is anything below the 50th parallel.
The chart below will make everything much clearer:
Poollicht.be/SpaceWeatherLive provides real-time alerts on its page, via Twitetr and the mobile website. As soon as a significant activity takes place on the Sun or around aurora, this is automatically reported on the aforementioned channels.
We briefly explain what this means.
There are 5 different types of alerts:
- Favorable KP value
- solar flares
- aurora alerts and/or activity summaries
The aurora activity
How active is the Aurora right now? We can see this on the special aurora website www.poollicht.be. On the world map on the front page you can immediately see how big the aurora oval is at that moment and whether there are opportunities for Belgium and the Netherlands.
The Kp Index
The geomagnetic storm index is the estimated probabilities of at least a 3-hour long K-index, on the demonstrated value, for each of the past few days. The standard chart on our website shows the Kp value of the past 24h with a forecast of this index for the next hour. Based on that graph you can immediately deduce whether there are aurora chances. We therefore distinguish the following types of storms in the Kp index
|big storm storm||K>6|
The K-index is a three-hour, quasi-logarithmic, local index of magnetic activity relative to an assumed quiescent diurnal curve for the recorded location. The Kp index ranges from 0 to 9. Ideal for our regions is a value of 7 or more. The Kp-index measures the deviation of the most disturbed horizontal component of the magnetic field at fixed locations on Earth, each with their own specific K-index. The global Kp index is thus determined by an algorithm based on the averages of the measuring stations.
The solar wind parameters
The solar wind is monitored by the ACE satellite, which is 1/100th of the distance from the Sun to Earth. The measured values at the height of this satellite reach the Earth after about an hour.
The solar wind speed
The speed of the solar wind is very important, if the speed is too low there will be no aurora. The normal solar wind speed is around 300km/sec but increases when a coronal hole or CME arrives at Earth. Depending on the speed of the impact, it can jump to 500 to even 1000 km/sec. A good speed to reach aurora in our regions is higher than 700 km/second; but it is not excluded that it can also happen at lower speeds. On the graphs you can usually see well when a shock front has arrived, the solar wind then makes a sudden increase with a difference of a few hundredths of kilometers per second during large storms. When the shock front has arrived, keep a close eye on the other gauges! The image below shows the arrival of a shock front in 2006, the difference is clearly noticeable:
The solar wind density
This refers to the density of the particles (protons) carried by the solar wind. The more particles it contains, the more chances of aurora can be created. The scale used here is protons per cubic centimeter, abbreviated as p/cm³ (protons/cm³). A value higher than 50 is positive but does not mean that there will be effective aurora. It regularly happens that during storms the density remains low after impact and the chance of aurora continues to exist, so don’t be fooled.
The Interplanetary Magnetic Field – IMF
During the solar minimum, the Sun’s magnetic field, like the Earth’s magnetic field, resembles that of an iron bar-shaped magnet, with large closed lines near the equator and open field lines near the poles. The scientists call such an area a ‘dipole’. The Sun’s bipolar region is about as strong as a magnet on a refrigerator (50 gauss). The Earth’s magnetic field is 100 times weaker.
Around solar maximum, when the Sun is at its most active, sunspots adorn the face of the Sun. These sunspots are permeated with magnetism, and from these sunspots run gigantic magnetic field lines along which material from the Sun also travels. These field lines are often hundreds of times stronger than the surrounding dipole region. This causes the magnetic field around the Sun to become confused with field lines.
The Sun’s magnetic field is not limited to our star itself. The solar wind carries it through the solar system and extends to the heliopause where the solar wind stops and collides with interstellar space. That is why we call this magnetic field of the sun the interplanetary magnetic field or IMF for short. Because the Sun rotates on its axis (once every 27 days), the IMF has a spiral shape (called the “Parker Spiral”, named after the scientist who first described it).
The IMF is a vector quantity with a three-way component, two of which (Bx and By) are oriented parallel to the ecliptic. The third component, the Bz value, is perpendicular to the ecliptic and is created by waves and other disturbances in the solar wind. When the IMF and geomagnetic field lines are inversely oriented or not perpendicular to each other, they cannot reconnect with each other resulting in a transfer of energy, mass and momentum from the solar gust to the magnetosphere. The strongest coupling, with the most dramatic magnetospheric effects, occurs when the Bz component faces south. The IMF is a weak field near Earth, ranging in strength near Earth from 1 to 37nT with an average value of about 6nT.
Bt where the
The Bt value refers to the strength of the interplanetary magnetic field. The stronger this pressure (values above +40 nT are already favourable) the higher the chance of aurora here, provided, of course, that the IMF also turns south.
BWith where the
A strong southbound BWith value often heralds widespread auroras, coupled with good IMF strength. The further south the Bz value, the better chance we have of seeing aurora in our region.
This magnetogram shows the values measured in the Kiruna measuring station (Sweden, Europe). This gives a better representation than the American Canopus oval because the Kiruna measuring station is located in Europe and is therefore much more reliable. To have favorable conditions and a good chance of aurora in our regions, this value must be lower than -1300nT. From then on, the chance of aurora is slightly greater again. To demonstrate that it is possible, see below the magnetogram of October 30, 2003 where the shock front of the X17 and X11 arrived on Earth.
Photo: Kiruna IRF
Daily activity report
On www.poollicht.be you will find daily reports (partly in English and partly in Dutch) about the activity on the Sun and the geophysical activity on Earth. This contains a lot of useful information about the sunspot groups, eruptions, aurorae probabilities and much more. Belgium and the Netherlands reside in the Middle-latitude zone, better known as the mean latitude. When a Middle-latitude watch or warning is active, we have a chance of aurora borealis in our regions. The reports contain the details and possibly the hours of a possible impact on Earth as well as the probabilities at the various latitudes from which aurora may be seen. From then on it’s just a matter of keeping an eye on the data and waiting for the shock front and hopefully enjoying a beautiful spectacle!
More info and more data
For more information and real-time data related to the Sun, space weather and aurora, please refer to our popular daughter page poollicht.be!