The Latest Solar Differential Rotation Information.  Feb 17/09

Some may be aware I have been trying to track down if we are experiencing any change to the differential rotation rates of the Sun. If angular momentum changes can be tied to solar differential rotation rates, we might have a solid causation. Solar differential rotation can be measured in different ways but basically, different parts of the Sun rotate at different speeds, keeping in mind the Sun is not a solid object. The equator rotation rate is around 27 days, with the poles at around 34 days, as seen from earth. But there is a lot more to consider. Recently I heard back from Dr. Rachel Howe from NOAE who specializes in this area, and the results were quite amazing.

Her hot off the press paper here:

The graphic above is a Doppler image from GONG which shows the regular patterns of the changing rotation speed of the Sun with the yellow sections being the fastest. The image is taken at 0.99R which is around 7000Km beneath the top of the Convection zone. It is thought these patterns (darker zones mostly) are generated at the Tachocline (a thin sheer layer between the radiative core and the Convection zone at around 0.71R) and they spiral out to the top of the Convection zone and are subject to spin momentum like a garden sprinkler. As we go deeper, like the graphic below (0.99R to 0.84R) the patterns break up, which is not what I would expect. The overall scale of the pattern seems to remain but loses clarity. Perhaps this is a condition of going deep with Doppler equipment or maybe the frequency is a lot higher as we go down?

If so, then maybe what is driving the pattern at the surface is something of much higher frequency down at the Tachocline, like the highly elliptical orbit of Mercury every 88 days. There is a reasonable tidal effect from Mercury which may warrant further investigation. It is thought the rotating central radiative core spins at an even speed.

Dr. Howe's paper suggests that the travel time interval from Tachocline to the solar surface is around 2 years and, as we will see, these Doppler patterns align very closely to the sunspot butterfly image when overlayed. If so, then sunspot activity is a historical account of what happened at the Tachocline 2 years ago.

In the next graphic we go back further and it displays exactly what I have been looking for. During past grand minima, observations suggest a faster rotating Sun, as can be seen at the equator. After 2000 there is a greater acceleration occurring.

Also noted is the much longer length of cycle 23 (96- ?) compared to cycle 22 (86-96). So here we see two direct links to the highly possible effect from Angular Momentum created by the Gas Giants, increased rotation rate and the stretching of the cycle length as the Sun takes its abnormal path every 172 years.

The last time this occurred was in SC4 just before the Dalton Minimum and the retrograde motion we are now part of is very similar to SC4. On average the retrograde orbit (purple) is around 9 years, SC23 & SC4 have orbits of over 11 years.

Below is a graphic I produced by rescaling the SIDC sunspot butterfly plots then overlaying over Dr. Howe's graphic, as can be seen there is a direct correlation between the two sets of data. It is thought the sunspots are created at the poleward edge of the fast and slow zones (yellow /green), but I am sceptical. The slower zones are what travel all the way to the Tachocline (its like a "brakes on" feedback) and we might have to possibly shift the overlay to the right to allow for angular momentum as the Doppler image is taken from 7000Km below. I have asked Dr. Howe her opinion.

Animation: Dr.Howe's diagram with the SIDC butterfly sunspot scatter with re scaled X & Y coordinates to match..

You can see in the pattern why early cycle sunspots appear at high latitudes then gravitate to the equator. Looking at butterfly graphs of solar activity, they suggest the majority of sunspots occurring between -40 & +40 latitude, ruling out the dark blue/blacks areas at high latitudes. I speculate they are reserved for Coronial Mass Ejections. The purple arrows show where a "flow" starts.

Some more speculation:  Some suggest the modulation of a sunspot cycle is governed by the strength of the solar poles, which are a late product of the previous cycle (this product can also be severely downgraded as seen in SC19) . Maybe the poles are a player, but not the main influence as the main source of power is coming from the Tachocline. This is where the poles eventually get their flow from. It may explain why we have a weak cycle after a grand minima cycle, as the following cycle after could have very low pole strength.

Worth saving and blowing up to see the detail.


This would be an invaluable tool if we could access it easily like the WSO polar strength graph and watch each move through the cycle. I informed Dr. Howe of this, lets hope it gets something going.

Geoff Sharp.



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