Alfven Computations for Carrington Rotation 2025

(Corresponding to January 20, 2005 CME Event)

MHD results based on low resolution, polytropic run.

Main assumptions:

Low resolution – heavy filtering. Active regions which had fields in excess of 1,000 Gauss, reduced to approx. 50 Gauss.

Polytropic approximation: gamma = 1.05. Density variations are not realistic. Field is probably OK. Computation of VA relies on field, but only square root of density.

Here’s a picture of the source region, AR 720:

There’s not much else going on! Here is the raw SOLIS magnetogram for 2025:

So AR 720 is, in Carrington coordinates at approximately 180 degrees longitude, modestly in the northern hemisphere. Note that this has been scaled to pm 1000 Gauss.

Now here is the Alfven speed computed from the solution (which ran out to 200 code units of time, or about 80 hours of real time.

You can see the AR at approx. 180 deg. Longitude in the northern hemisphere. It is saturated above the peak speed indicated of 5000 km/s. Note how the features between this plot and the original SOLIS magnetogram are smeared out due to the low resolution of the run…an important caveat to bear in mind!

Now here’s a cut in the meridional plane at lon = 180 deg.:

The speed is now on a scale from zero to 1000 km/s. The AR produces higher VA speeds outward into the solar wind. Note the minima in VA in the equatorial plane, however, which would provide great conditions for a shock to form.

And here is a plot of VA in the equatorial plane. 180 deg. Longitude is marked by the white solid radial line:

More interestingly, there are two ‘channels’ of low VA bounding either side of the AR. I’m not sure of the timings for the energetic particles, but it would be nice to be able to tie them to either one of these source locations, assuming they were accelerated by a coronal shock driven by the CME.

Finally, here are a selection of line profiles showing the variation in VA as a function of height above the photosphere.

You can see how high VA is above the AR (top right), but that other profiles show lower numbers, and more interesting properties. The plot on the bottom right, for example, which is not that uncommon, has a relative minimum at R=2 RS. In these cases, it might be possible for a wave to shock as its speed rises above the ambient VA but then ‘un-shock’ further out, eventually shocking again: the two-shock scenario. (Originally proposed by Mann?)

Anyway, there are the types of data products we can derive from the models. To reiterate the limitations of this case, however, since it relies on a polytropic solution at low resolution, interpretations should be tentative. However, we have almost reached the point where we can run high-resolution cases with the thermodynamic model (although it’s a challenge) so we can probably produce a more realistic solution in the near future.