density vs velocity

[AurorahunterPA]

What matters more in terms of driving auroras southward and geomagnetic storming? plasma density or solar wind velocity?

[Guest Stephane Mabille]

Regarding density, it brings pressure on satellites rather than the earth's magnetic field, but the velocity compresses the field when negative Bz

[Marcel de Bont]

Both density and speed are important. A dense slow wave can deliver equal pressure on the magnetosphere than a fast but not so dense stream. It's all about the pressure onto our magnetosphere. The Bz is the north-south direction of the IMF and is independent of the solar wind speed or density. It is however very important for the direction of the IMF (Bz) to be southward as that causes a good connection between the solar wind and our magnetosphere which fuels aurora displays.

[Maxime Fiset]

Can a CME really "build-up" by accumulating the plasma of a slower CRIR on it's way, like on the ENLIL?

[Marcel de Bont]

Op 5-11-2015 18:07:54, Maxime Fiset zei:

Can a CME really "build-up" by accumulating the plasma of a slower CRIR on it's way, like on the ENLIL?

Build up as in getting denser? A CME shockfront often has to plow trough slower ambient solar wind and can indeed sweep up particles. It might get denser but also slow down in the process.

[Guest Lou Puls]

On 11/4/2015, 6:25:15, Marcel de Bont said:

Both density and speed are important. A dense slow wave can deliver equal pressure on the magnetosphere than a fast but not so dense stream. It's all about the pressure onto our magnetosphere. The Bz is the north-south direction of the IMF and is independent of the solar wind speed or density. It is however very important for the direction of the IMF (Bz) to be southward as that causes a good connection between the solar wind and our magnetosphere which fuels aurora displays.

I agree both are important, and often one is high while the other is low.  When they are each high in the same interval, the geomagnetic effect can be impressive.  To me this is the result of what fluid dynamicists call "dynamic pressure", the mathematical product of some measure of matter density with particulate or flow velocity.  I have long wished and promoted that solar wind data displays somehow include this kind of parameter, as it gives an immediate impression of the compounded importance of density and speed.

[Maxime Fiset]

27 minutes ago, Marcel de Bont said:

Build up as in getting denser? A CME shockfront often has to plow trough slower ambient solar wind and can indeed sweep up particles. It might get denser but also slow down in the process.

Thank you.

 

That's exactly what I wanted to make sure.

[Guest Lou Puls]

...and if the shockfront still has high enough speed after sweeping up higher density, it could have a high "dynamic pressure" to effect a significant geomagnetic storm, given the required Bz orientation.

[Marcel de Bont]

Dynamic pressure is actually used in the space weather field and is a combination of the solar wind speed and density yes. There should be a formula for it. Maybe it could be something we can display on the site.

The Bz is of course another vital piece of the puzzle.

[Vancanneyt Sander]

16 uren geleden, Marcel de Bont zei:

Build up as in getting denser? A CME shockfront often has to plow trough slower ambient solar wind and can indeed sweep up particles. It might get denser but also slow down in the process.

A CME shockwave sweeps and ploughs through, so it can either clear the path vanishing the density of the cloud that was there or drag some of it with it. Same can happen when multiple CME's are launched one after the other where the second CME catches up with the first because it was slightly faster and the void after the first CME made it catch up quickly causing the second CME ploughing through the first and sweeps through it. It can then either be a cannibalizing effect or drag some of it with it and arrive somewhat denser and stronger.

But with a CME clearing a CH along the path, it will depend on how fast that solar wind stream is of the coronal hole where the CME ploughs through i guess. 
Maybe this is a good read about "Modeling a space weather event from the Sun to the Earth: CME generation and interplanetary propagation"

[Guest Lou Puls]

Webmaster, many thanks for the modeling reference link.  In three of its four MHD conservation equations, the quantity rho*u (plasma density times velocity) is used, which is the "dynamic pressure", which again I feel is a major parameter which could be more emphasized in solar data displays.  The most recent CME (which arrived nearly a day earlier than predicted) was a case expected to have a velocity sufficient to sweep up density and still be able to have a strong dynamic pressure.  This can be seen in the WSA-Enlil image of Nov.5 : http://www.swpc.noaa.gov/products/wsa-enlil-solar-wind-prediction where at about 18:00 UT the density and speed are both peaking.  Whether this really happened in the actual early arrival has to be extracted from separate graphical or digital data.

[Guest Lou Puls]

On 11/6/2015, 5:57:28, Marcel de Bont said:

Dynamic pressure is actually used in the space weather field and is a combination of the solar wind speed and density yes. There should be a formula for it. Maybe it could be something we can display on the site.

The Bz is of course another vital piece of the puzzle.

Maybe a problem with displaying dynamic pressure is the definition of the product of density and velocity?  Is the "density" used some kind of average of the various particle densities?  Is the directional velocity represented by a similarly averaged but also normed speed?  And is the product of averages really representative of the average of the various particle products? Probably some kind of product that reflects the effects of dominant species would be a useful display parameter, even if not strictly "dynamic pressure".  

[Gabriel Hofstra]

But is it density or speed that gives the aurora a bright red colour?

[Guest Lou Puls]

I think it's neither alone -- it's the elevated combination of both together, in general.  The further specific parameters causing the relatively rare laser-like purity of 630 nm red from nitrogen at high altitude is not well understood, methinks.

E.g. is the bright red mostly from oxygen or from nitrogen at even higher altitudes?

[Tristan]

On 07/11/2015, 00:57:28, Marcel de Bont said:

Dynamic pressure is actually used in the space weather field and is a combination of the solar wind speed and density yes. There should be a formula for it. Maybe it could be something we can display on the site.

The Bz is of course another vital piece of the puzzle.

Dynamic pressure is proportional to the product of the density (p) and the square of the velocity (v).

General equation (from wikipedia) for the dynamic pressure (Q) of a fluid:    Q=1/2pv²

Hope this helps.