Now that we’re trying to forecast Earth-directed solar storms, I’ve been attempting to explain how we know whether our prediction has been any good or not. This involves looking at data from a NASA spacecraft called ACE (the Advanced Composition Explorer) that sits a million km upstream of Earth and measures the Earth–directed solar wind as it blows by. The ACE data are presented as a series of wiggly lines that require an experienced eye to interpret so I was very pleased when one of the world’s experts in such data, Dr Christian Mostl from the University of Graz in Austria, agreed to give us all a lesson in how to understand what’s going on with the solar wind by studying these wiggles. So, without further delay, I will hand you over to Christian (a team can never have too many Chris’s – Chris);
What’s the solar wind doing at the moment?
Here is a little tutorial for you to understand better what is going on in the solar wind around Earth at this very moment in time. After going through this text, you will able to check for yourself in real time if the next prediction which we will issue with your help was excellent, good, bad or utterly horrific!
As pointed out earlier, this is the site to see the current space weather environment around Earth – this is one of our “weather stations” in space:
These strange, wiggly lines tell you the state of the solar wind for the last 7 days, observed by the Advanced Composition Explorer or ACE spacecraft, situated about 1.5 Million kilometers away from the Earth, in the sunward direction. So, first question: why are these lines so freakishly trembling? Answer: the solar wind is just a very turbulent medium. Even the “slow solar wind” (by definition around 400 kilometers per second – that’s over 1 million kilometers per hour!) is flowing away from the Sun so fast that its velocity always exceeds its own speed of sound by about a factor of 5. This makes it a constantly supersonic flow, and thus much, much more turbulent than, say, a wild river. By the way: why exactly the solar wind is so fast is one of the great mysteries of astrophysics!
Most of the time, the ACE plot will show you plain, normal, slow solar wind. I summarize here some of its parameters, which are shown in the plot, from top to bottom:
Bt (its total magnitude): around 5 nT, but always less than 10 nT.
Bz (its north-south component): between +/- 5 nT; be aware that this parameter is like the on/off button for Earth’s magnetosphere: the more southward or negative it is, the more energy will be transferred to the magnetosphere, resulting in auroras and magnetic storms. In contrast, for a positive or northward Bz, very little happens.
lets skip the next panel for simplicity, so lets go to…
N is the number density: between 1 and 10 protons/ccm. On the plot, there actually are two horizontal dashed lines at these levels, so you can easily spot outsiders. Don’t worry about data points which are below 1 p/ccm, this happens quite often. But be aware that this is a logarithmic scale, so the y-axis goes up all the way to 100, and intervals which appear slightly above 10 p/ccm can actually indicate much higher density!
V is the solar wind speed: between 300 and 450 km/s. Note that the solar wind flow points all the time almost perfectly in the direction away from the Sun.
T is the temperature: most of the time the protons have less than 100 000 K – again there is a horizontal line to guide the eye.
Now, if there is a slow solar wind, there must be a fast one too, right? Every few weeks on average, a gust of high speed solar wind, between 500 – 800 km/s, will hit Earth. They are strong enough to produce minor magnetic storms and are very good at creating beautiful auroras, but they usually lack the punch of a strong solar storm (a CME) to knock out any technological infrastructure. You can identify a high speed stream from these parameters:
Bt, Bz: a peak of 10-25 nT followed by values around 5 nT.
N: a peak of 10-20 p/ccm followed by low values of 1-5 p/ccm.
V: 500-800 km/s.
T: greater than 100 000 K, so hotter than slow solar wind.
Also, the magnetic field should be very wiggly throughout the interval where the speed is high.
Usually, N will peak first, followed by B, T, and V. This order is a consequence of a fast stream compressing the slow wind ahead of it.
Now, I have a little exercise for you: can you spot whats going on in the plot below?
The answer is: There is a long interval of slow solar wind followed by a high speed stream, starting on day 31. Great!
Finally, we come to the cherry on the cake: An “interplanetary coronal mass ejection” or ICME is the name given to a solar storm or coronal mass ejection (CME) you have been tracking in Solar Stormwatch when it is observed directly, on-spot or in situ by a spacecraft in the solar wind like ACE. ICME parameters are in principle elevated compared to the slow solar wind, but their signatures can vary greatly from one to another. Also, there are different regions inside an ICME. So to make it as simple as possible, watch out for intervals where B, N, V and T are much higher than normal, and yes – this can be difficult to distinguish from a high speed stream. Here is an example:
On day 06 there is a very abrupt and strong upward jump in all the parameters – this signals the arrival of the shock wave driven by the CME! That’s the time which should be compared to the forecast which was issued by Solar Stormwatch with your help.
While not well seen in the example above, there can be intervals following the shock wave, where the Bz is very smoothly changing and T is very low. With this we can identify an ICME with certainty, and this part of it is called a “magnetic cloud”. It signals that a CME hit Earth’s magnetic field head on! Most scientists think of “magnetic clouds” being at the core of many, if not all, ICMEs, and there the magnetic fields are the strongest, giving rise to the strongest geomagnetic storms.
So, now you should be able to spot for yourself if a solar storm has just hit Earth. But beware! Often it is easy to identify a shock wave driven by the ICME by very abrupt and strong upward jumps in all the parameters, but to see if a magnetic cloud has hit Earth you will have to wait for sometimes up to at least another day. Slow ICMEs do not drive a shock and can often be identified with certainty only in retrospect.