Dr. SVALGAARD on SOLAR SYSTEM Oct 1, 2009 7:49:04 GMT
Post by Solarius on Oct 1, 2009 7:49:04 GMT
Heliospheric Current Sheet [HCS]
The solar wind speed itself has little to do with the GCR count. What matters is the amount of turbulence and tangled magnetic fields. These things occur in regions where there are differences in solar wind speed.
To get a full understanding of this we have to get an image of the Heliospheric Current Sheet [HCS] into our mind. Below I show the picture most people have in mind of the HCS (it is from chapter 3 of the Oulu textbook at www.physics.oulu.fi/fysiikka/oj/766656S/2006/Lectures ). Note the HCS tilted away from the heliographic equator and how the sector structure is explained by the rotation of this tilted plane:
This is the WRONG [or too simplistic] image to have as the current sheet is not a plane. Here is a better one [by Pete Riley]:
It shows what the HCS looks like at different phases of the solar cycle, from rather flat at minimum, swelling up towards maximum, before flattening again at the next minimum.
But this is complicated to comprehend for most people, so one often resorts to showing various cuts through the surface. The following is due to Miyake and Yanagita showing North-South cuts through the surface:
What you see is an undulating boundary oscillating in an angular section about the equator [goes out to 80 AU - almost to the termination shock]. These are meridional (North-South) plots. Below is a blow-up of one of them. The colored dots show how a sector change occurs. The red is in one polarity and the green is in the other polarity. Instead of moving the solar wind, I moved the dot, but I think you get the point. Note that there is no overall tilt of the wavy area:
So far so good. Now for the real crux of the matter: the solar wind speed is low at the boundary and high in the middle of a sector. This means that the 'waves' will be compressed by the faster wind plowing into the slower wind and piling up the plasma, like a snow plow piling up snow., as shown in this movie:
The compressed 'onion peels' have a compressed and disturbed magnetic field. It is this field that scatters the GCRs out of the solar system. Now you can see that the wider the angle is, that is the broader the region with tangled fields is, the more GCRs will be scattered away. In addition, sporadic CMEs also plays a role as the give you extra compression. Because CMEs and more importantly the opening angle of the wavy area [volume in 3D] varies with the phase of the cycle, the GCRs will also vary as we explained in our 1976 Nature paper on this.
So, the modulation of GCRs does not depend on the speed as such but on the much more complicated shape changes of the wavy volume that contains the HCS.
We know from cosmic rays that the solar cycle was still operating, during Maunder Minimum so the HCS would still go through its normal variation from flat to convoluted and back to flat. Thus there would be little difference with now.