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(meteorobs) Clarification of terms regarding aurora..

Hi folks... I have recently had a couple of requests to explain some of the terms and numbers that appear in these e-mails..
Ok. Here we go...
There are four key factors that affect auroral activity:
 
1. Solar wind velocity. This is measured in kilometres per second, and the higher the better.. I start to pay attention when this gets above 450 - 500 km/ps. (800 or 900 is even better)
 
2. Bz. Now don't ask me what this stands for, because I don't know... However, this is related to the orientation of the Sun's magnetic field relative to Earth. Positive numbers ( 0, 1, 2... 12..) are said to be 'North.' Generally, this is not good for aurora, even if the other factors are good. What we want to see is negative Bz. (-1, -2, ... -12..) This represents 'South'. Once again, the higher the negative values, the better the show... -12 is better than -1.
 
3. Kp index. As I understand it, this is related to the amount of energy available in our own magnetic field. Kp 1 is low. Kp 4 can result in a faint, but definite glow on the horizon at mid latitudes. (most of New Zealand fits in this area) Kp 9 is a full blown geomagnetic storm. If you ever see numbers like this, get off the computer and get to a clear sky!!!
 
4. Proton density. This is the number of solar protons to be found in one cubic centimetre of space above the Earth's magnetic field. Commonly, this hangs around 0.5 - 2.0. Strong shows can occur while this low, but higher values are better.. Say 15 - or higher.
 
A combination of all the above factors can lead to truly breath-taking display.
 
Now for a couple of terms related to the sun...
 
Solar flares. Eruptions on the surface of the sun are caused when magnetic fields become 'tangled' and suddenly unwind. C flares are relatively low in power, M are moderate, and X are exciting!
Any size flare can result in a CME.
 
CME. Coronal mass ejection. This is material that has been thrown out from the sun in a violent explosion (hopefully towards the Earth) These can result in the most powerful auroras.
 
Coronal hole. The corona is the atmosphere of the sun. Due to magnetic activity, this sometimes becomes thin enough to allow passage of material directly from the surface of the sun into space. When Earth encounters a wind stream from a coronal hole, there may be a prolonged period (2-5 days) of low to moderate activity in the sky.
 
Well that just about sums it up... I am not an expert in this field, so there may be errors in my thinking. If you find any, let me know. Or, If you want to add something or ask questions, please feel free to reply...
 
Good luck and clear skies, Graham.

Graham Palmer
1/622 Nelson St. North,
Mahora.
Hastings, New Zealand.
Home phone: 06-8735951
Graham's cell No: 021-2650487