For those that dont understand how or why we get northern lights or how it is to predict this was a comment I pulled from space FB page by Glen Stephan the football fans out there may understand this a bit better but sums it up pretty good For those having trouble following some of the terminology and meaning, and wanting to know if youll see anything from your hood, here’s an analogy I use to explain the major parts of the aurora prediction story to my friends. It’s not perfect or entirely accurate, and does not address all the issues, but it serves well enough to have them understand the main points: Think of an American or Canadian football running play as an analogy and the quarterback is the sun. When there is a “sneeze” – i.e. an eruption – on the sun, that’s called a solar flare. This is when the QB hands the ball off to his running back. When the running back starts running with the ball, this is called a coronal mass ejection - a CME. This is when the eruption actually blows off some of the sun’s own material into space, like the moisture droplets that blow out of your mouth when you sneeze into the air. When this CME material (the sun’s sneeze spit ;) ) is headed towards earth and not out towards some other direction in space, this is when the running back is running towards the line of scrimmage (earth’s upper atmosphere and magnetic field). Just as the larger the running back’s strength and speed make it more likely that he’ll gain more yards, the larger strength and speed of the approaching CME make it more likely that the CME will cause a better or larger auroral display. The speed of the approaching CME (the solar wind) – how fast the back is running - is simply called it’s speed. The faster the better; a calm/normalish wind speed on an average day is typically somewhere around 400 kilometers per second. is has to get faster than, say, 600km/sec to get our interest, the faster the better. The “strength” of our CME, or solar wind gust, is called it’s “dynamic pressure”, and is measured on a logarithmic scale from 0 to 100. On a normal day, this value is anywhere between 0 and 10. When the pressure goes above 10 it starts getting interesting. Again, the higher the better. When that running back hits the line of scrimmage, there is quite a shock (as anybody who’s been hit by a fully-suited football player running at full speed can tell you ;) ). There is a similar shock when the CME hits our earth’s magnetic field. A main way scientists measure that shock is by what they call the “K index”. The K index is a scale of 0 to 9; the higher the number the farther the running back is likely to get should he get past the line of scrimmage, or, the further south on earth we are likely to see auroral activity. Typically K level has to reach at least 6 to make it to the far northern US (further south in the upper elevations around the rocky Mountains), 7 to make it as far south as Illinois or Ohio, 7+ to make it further south in the US than that. Just look up the current K index (Google search “K index swpc”) and look at a map of the US to figure it out. But the line of scrimmage is not just an inert line or wall, it is filled with defenders trying to tackle the running back and stop him from gaining yards. We could call the strength of this defensive line the Bz value. Bz value is measured in strength “north” or “south”. North is typically shown as a positive value and south as a negative value. If Bz is north or positive, the defense is strong and the running back will have a very hard time breaking through without being tackled – the chances of a good or large aurora are small. The greater the positive value, the smaller the chances or size. If Bz is south or negative, this opens up holes in the line of scrimmage that the running back can run through; the CME can get through to the atmosphere better to create bigger and brighter auroral displays. Speed and dynamic pressure can be well estimated and measured, both through terrestrial observations and measurements made from solar-observing satellites. K index predictions can and are made based largely upon these observations. Bz can be measured and displayed in real time, but cannot reliably be predicted ahead of time as to if, when or how much it may fluctuate between +north and –south. That’s why they play the game! ;)
Posted on: Sat, 13 Sep 2014 21:12:17 +0000
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