A new look at the Sun’s magnetic field

Posted: March 28, 2015 by oldbrew in predictions, Solar physics
Tags: ,

The Sun from NASA's SDO spacecraft

The Sun from NASA’s SDO spacecraft

According to new research entitled: “The crucial role of surface magnetic fields for the solar dynamo”, a prediction method for solar cycles, first proposed decades ago, has been validated:
‘As the dipole field [of the Sun] is the source of the toroidal field of the next cycle, its strength should be a measure of the activity of the next cycle.’

Phys.org reports:
Sunspots, bursts of radiation and violent eruptions are signs that our sun is permanently active. Researchers have long known that this activity varies in a cycle of around eleven years’ duration. Even if many questions are still unresolved, one thing is certain: magnetic fields which emerge on the surface of our sun from within its depths are the cause of the manifold activities.

Robert Cameron and Manfred Schüssler from the Max Planck Institute for Solar System Research in Göttingen have now proved that it is possible to deduce what the internal mechanism is simply by observing the magnetic processes on the surface. This even allows predictions to be made about the strength of a forthcoming activity cycle.

The sun is a huge ball of gas in whose interior hot gases flow, rise and sink. Inside this inferno, a magnetic field is generated whose fundamental structure is similar to that of Earth’s magnetic field. It takes the form of a dipole whose magnetic field lines emerge from the surface at the solar poles.

The magnetic fields are, however, bound to the hot, electrically conducting gas, and this gas stretches and warps the fields in a complicated way – like rubber bands in honey which is stirred. A magnetic field line originally running parallel to the axis of rotation is thus entrained by the rotating gas.

The gas in the equatorial region moves much faster than in middle and high latitudes, however. This means that the field lines in the equatorial region are stretched lengthwise and actually twist tight in the course of several rotations: a ring-shaped magnetic field forms in the east-west direction, also called a toroidal field.

These magnetic field lines can merge to form thick bundles that rise up until they eventually emerge from the surface and form a loop. The familiar, dark solar spots form at the two points of emergence. They therefore usually occur in pairs in the east-west direction and form a magnetic north and south pole respectively. Over an eleven-year cycle, the magnetic orientation of all spots is identical. The toroidal field is therefore always in the same direction.

“Until now, many experts believed that the magnetic phenomena which manifest themselves to the outside are only the symptoms of the processes taking place in the interior,” says Manfred Schüssler from the Max Planck Institute for Solar System Research in Göttingen. “We have now applied a mathematical theorem that the Irish mathematician and physicist George Gabriel Stokes proved in the 19th century.”

This theorem relates the fields at the surface to those in the interior of a body. The scientists used this purely mathematical argument to prove that the magnetic field which can be measured at the surface of the sun is the sole source for the ordered toroidal field in its interior, which, in turn, causes the activity phenomena of the subsequent eleven-year cycle.

“What we see on the surface is the relevant field,” says Schüssler. “Figuratively speaking, the surface phenomena are not the tail of the dog, but the dog itself.”

By comparing them with observational data, Robert Cameron and Manfred Schüssler were able to show that the dipole field is far and away the dominant source of the toroidal field. They have thus confirmed a model which the American astronomers Horace Babcock and Robert Leighton had already proposed in the 1960s.

This also now allows predictions to be made about the strength of an upcoming activity cycle. Over the course of an eleven-year cycle, the dipole field changes its direction: the magnetic north pole becomes the south pole and vice versa. The new dipole field reaches its maximum strength roughly in the phase of minimal solar activity.

As the dipole field is the source of the toroidal field of the next cycle, its strength should be a measure of the activity of the next cycle. Such a correlation has already been established: “During the phase of the last minimum around 2009, the strength of the dipole field was relatively low, and the present cycle is correspondingly weak,” says Schüssler.

In future it will be possible to test the predictive power further. Until now, it has been very difficult to measure the strength of the dipole field, as the solar poles are hardly visible from Earth. This should change when, in 2017, the Solar Orbiter is launched. This solar telescope will approach the sun to within one-third of the distance Earth-Sun and also rise above the plane of the terrestrial orbit. It will thus afford us a view over the polar regions.

The Max Planck Institute for Solar System Research is involved in the construction of four instruments on board the Solar Orbiter, and also has overall responsibility for a camera known as the Polarimetric and Helioseismic Imager.

Phys.org source here.

  1. Bob Weber says:

    Gas? What, no mention of plasma?

    EEs know that all the solar magnetic fields come from electric currents caused by the motion of charged particles, the plasma constituents, mainly protons and electrons, that behave electrodynamically (also magnetohydrodynamically – MHD).

    Confirmation of the Babcock-Leighton model. Let’s see what, if any, clues are there for SC25 now:

  2. oldbrew says:

    If I’m reading it right (using black PF avg.) the green line for SC 24 is crossed at about 50. SC23 was about 95, SC22 was 125. So: 125,95,50 – the direction of travel is clear.

    According to the theory the indicator for the current cycle was 95, and for the next one it’s 50.

  3. Bob Weber says:

    OB – yea, it looks like the next cycle is going to be pretty small. If we were to overlay the SIDC cycles 21-24 onto the above plot, we’d see the maximum amplitudes of each respective cycle corresponding well to the polar field maxes at the previous cycle minimums, verifying the B-L model.

    We can therefore expect a scaled version of SC24 for SC25 based on the numbers you posted, indicating an est. SC25 magnitude of just over half of SC24 – a ssn of about 40 max. Sound right?

    From the drawn out polar field timeline between SC23 & 24 maxes, it would appear that less plasma at the poles during the prior minimum means a slower transport to the solar equator, hence a longer next cycle in general, and applying that to cycle 25, it will also be long, probably longer than SC24.

    The question is, how is it that polar plasma concentrations at the minimums vary so much? I for one don’t believe that an internal dynamo causes that, but that’s just a hunch at this point. Wonder what the PRP guys would have to say about that….

    A sunspot number of 40 corresponds to less than 120 sfu of F10.7cm solar radio flux, meaning SC25 might not add much warmth to the Earth. Imagine that, a 13+ year SC24 followed by maybe a 14 year SC25, with little to no solar warming until possibly no sooner than the SC26 max in about 2040 or so!

  4. oldbrew says:

    I’m not an expert on it but the SC escalator is certainly heading down.

    The theory says the next indicator comes in SC 25 sometime so we’ll look out for that – a few years after 2020 maybe 😉

    The average solar cycle was well below 11 years in the last century, so longer cycles were bound to arrive eventually, and here they are.

  5. Bob Weber says:

    David Archibald had this to say here http://wattsupwiththat.com/2013/07/28/solar-cycle-24-update-2/

    “Just over two years ago, Richard Altrock of the National Solar Observatory at Sacramento Peak published the latest version of his green corona emissions diagram.

    He stated at the time that the progression of the Solar Cycle 24 was 40% slower than the average of the previous two cycles. That would make it 15.5 years long. Given that the cycle started in December 2008 and solar maximum is in 2013, that makes the Solar Cycle 24 fall time 11.5 years.

    Figure 9 shows the strong relationship between fall time and the time from maximum to maximum. Based on that relationship, the Solar Cycle 24 fall time derives a period of 17 years from the Solar Cycle 24 maximum to the Solar Cycle 25 maximum – putting it in 2030.”

    Figure 9:

    SC24 maximum had a second peak in 2014.

  6. oldbrew says:

    Related: ‘What drives the solar cycle?’

    ‘Magnetic fields on solar-type stars’
    Quote: ‘Surveys using these proxies have found clear dependencies between rotation and the stellar dynamo and the star’s magnetic cycles, among other things.’


    Magnetism everywhere.