Space Weather

Space... the final frontier... these are the voyages of the solar wind...

Well yes, the solar wind embarks on her lasting voyage quite some times each and every day. When it hits a planets upper atmosphere and, if present, the planets magnetic field reacts to the impact, the elusive aurorae will make an appearance.

The solar wind is a stream of electrons and protons launched in a Coronal Mass Ejection emanating from the solar flares and coronal holes. Simplified: The flares come from strong magnetic currents and when available from the sun spots. Coronal holes send out a stream of the electrons and protons. The sun spots are not always there. The sun has its 11-year cycle in which she flips her polarity. The north and south pole change place. The forces involved seek a way out and flames of millions of kilometerscan launch solar material into space.

Contents of the Solar Wind

The material that is send out, the Coronal Mass Ejection or CME for short, has quite a few parameters to it:

speed, there is no preferred speed but higher speeds have sometimes been giving more colours.
density, no preferred density. Though when the speed increases and density drops some very good displays occur! My thoughts to this (in no way scientifically proven) are like the ball bath at Ikea (no space to move and after you drop in the stable state returns very quickly) versus the situation on a pool table where a single fast shot creates a chain reaction and havoc amongst the other balls.
magnetic force, this is preffered to be 10 or higher.
magnetic orientation has to any minus value meaning it is oriented 'south'.

Then a series of Earths parameters have to be in a certain state when it hits the Earths most upper atmosphere. There forms an energy in those upper altitudes between 80 and 400km which is called the Hemispheric Power. The amount of this power is given in gigawatts and a measure of the severity of the impact. Its value should be above 20 GW.

Next thing to act is the Earths magnetic core and field. The Disturbance Storm Time Index is a measure of the amount of engery that is stored in the magnetic field of the planet. The lower its value gets the more energy is available in the magnetic field. For an aurora to appear this value should be negative, preferably -10 or lower.

When this all is in place the Earths magnetic field has to react. This is probably the most critical parameter. The more it reacts the more aurora will show. It is this disturbed magnetic field that plays a crucial role as to where the aurora appears. Rarely the fields disturbance influenced by all mentioned parameters goes all the way down to Denmark or even lower to Austria and Portugal. The latter was the case at halloween 2003 when the aurora stood in the zenith of Lisbon.

The interpretation of all the data

Well this is a bit tricky. All the values are hard solid data. Only Lady Aurora seems to have a mind of her own quite sometimes. And there are a few reasons for that.

First of all: we do not fully understand the process yet.
Second: the CME is kind of like a soup. Imagine it like a clear vegetable soup in which you see these fatty bubbles. When you envision it like that you can imagine that it is far from consistent. That means that what has been measured is not necessarily fully correct as parts may pass the satellites sensor unnoticed. The more if you realize that that sensor is just a tiny spot in an ocean of CME material literally like a single lonely buoy in the middle of the Pacific Ocean.
So when all parameters say nay... small quantities of CME, denser, faster, stronger, whatever different, may cause a short auroral burst after all.
Third: A misstake in calculations. Keep in mind that the DSCVR satellite isn in the Lagrange point 1 million km from Earth. It measures amongst many other things the speed of the solar wind and sends that with the other measured data to Earth. Calculating effects on impact needs to be done with regards to the timeframe that the satellite gave. That means a correction on average between 30 and 90 minutes. The Hemispheric Power is also measured/calculated with a delay which is some 50 minutes. I checked this with the speed of the solar wind at that time and it differed 8 minutes less. This may be the time needed to calculate this.

So there is a time delay. In the time between measuring values and their arrival there may be a slight shift in those values. The material won't change that much in the last 1 million km after having soared for 148 of those but you can't rule that out. When it is overtaken by a following CME's material with a different composition called a cannibal CME, the effect will be different, usually spectacular, than expected in the first place.

Then... finally, and that has a reason, the parameter everybody is looking for first but has the least value of all... the KP value.
The KP value is an overall number resulting from a myriad of processes, calculations and overviews. It is most often presented as THE sign to look at if an aurora will appear at your place.
Well... It's wake up time! Not for an aurora but for what it most likely seems to represent: the pressure of the CME pushing on the upper atmosphere and, IF there is an aurora, how far southward over the face of the planet it will reach.
Example: the aurora would need a KP8 to reach a lattitude on which it would be visible in the Netherlands.
So... yes it does. BUT... I have photographed auroras from my home in Anhem during a KP5. How that is possible you have been reading moments ago. It is just like fishing: you aim for the big adult meaty big game which is not around and a hungry little basterd ravages your bait and disappears with it...

Having said that about the KP-scale... it isn't worthless of course. No one would make any effort producing it if it was. It just isn't that important. But it does give a good impression of the overall severeness of geomagnetic storms.
List of A and K scales

The Best Time To 'Hunt' For The Aurora

A FAQ category question. Is there a 'best' period?
Yes in theory there are a few good/better moments for the CME to connect with the Earths magnetic field but bear in mind that by all the variables this is no physics law!
The periods to look for are:

the top of the 11 year solar cycle, including 1 year before and one year after
the four years after that top
the time of the equinox, the magnetic axle of the planet then is perpendicular to the direction of the solar wind and the effect is different. There is no direct 'better' or other way to describe that correctly but it is the time that the dynamics tend to be more active.

Obviously a combination of the latter with the first or second period is the optimum moment to be there. I have witnessed two cycle tops and the long period in between and I have seen weird things on that top in 2014 (top of cycle 24) and the four years after that I haven't seen later.