Observations

Harry says that the magnetic fields on the Sun are reversing their polarity

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The magnetic field of the Sun at minimum in 2008 and near maximum in 2012. Courtesy Lockheed Martin Solar and Astrophysics Laboratory

Quietly, without ceremony, the sun is reversing its polar fields! What ‘on Earth’ is going on?

Although a ‘dwarf’ star our sun is a large object, about 1400Mm in diameter – and since it’s a hundred times that distance away, we can safely marvel at its doings with suitably protected ‘scopes. And it displays much strange behaviour – some I relate in these pages. Now, incredibly, for the most amazing ‘trick’ of them all: it will reverse its north and south magnetic poles! And in the ‘internet age’ with some luck, we may be able to watch the whole incredible event.

Hale and others discovered the reversal process in 1912:

“Hale et al (1919) …announced that the polarities [in each hemisphere] were reversed in the new cycle spots…. this is called the Hale Nicholson law” (Zirin, H. “Astrophysics of the Sun” 1986 Cambridge Uni Press P307). And -“The [sun’s] polar caps change polarity approximately 1 – 2 years after the sunspot maximum, to take the sign of the following (f) polarity of the hemisphere [in which they occur] during the current cycle” (Schrijver and Zwaan “Solar and Stellar Magnetic Activity” 2008. P140. (my emphases).

What does this mean? Just like Earth, the sun has magnetic north and south poles: extensive regions of ‘unipolar’ field – and at every solar cycle these polar fields reverse. Earth, by contrast, only reverses its polar fields every 100,000 years or so – something that would cause much ‘grief’ to a high-tech society!

The sun’s polarity reversal can’t be seen in amateur ‘scopes of course, although the decay of sunspots that drives the process is seen well in amateur gear. But specialised magnetograms reveal the bipolarity of spots as well as showing how ‘tails’ or ‘streaks’ of (f) polarity slowly drift polewards from ‘decaying’ groups – the process that causes the ‘polar reversal’.

Several observatories post daily (disc) magnetograms on-line, as well as monthly (Carrington rotation) synoptic maps that show the progress of reversal across the whole disc.

Polarity field simulations: The most dramatic visualisations of solar polarity, I think, are the pfss models. This is a ‘modelling’ technique that generates a 3D model of solar fields from many information sources, some cited above. I believe Schrijver (cited) was involved in the development of this methodology. A daily pfss model is posted by © solarsoft.co – a most valuable guide to solar activity.

Reversal: the progress of field reversal is now well advanced and the Fig compares a simulation from 2008 (around minimum) with a recent one of Nov 2012 (nearing maximum). Warning: no single daily ‘model’ will capture the whole disc – they are 3D models – but they give an idea of the large scale magnetic structures that emerge from our star and the progress in changing N poles into S and vice versa. Schrijver and Zwaan’s book (cited) discusses the process and it’s assumed most comparable stars have similar activity. Let’s compare the two images.

2008: at minimum we see large ‘caps’ of unipolar field at both the sun’s poles: north is pink, and south is green. These fields are ‘open’ – i.e. they ‘earth’ somewhere out in space – at times even to Earth. As such, they are called ‘coronal holes’ and the solar wind streams out through them.

Between the two poles we mostly see ‘closed’ field – in yellow – where fields of one polarity connect to opposite polarity nearby. Pink ‘open’ field left of centre is a small spot group.

As far as I know the modelling software cannot (yet) capture rapid transients in the corona like CME’s, for example.

2012: The recent model is dramatically different from that of ~5 years ago. Most striking is the reversed colours (polarity) of the polar regions – now green dominates in the north and pink in the south, so the reversal is well advanced. Caution: daily models show rapid changes and it will be a while yet before the reversal fully stabilizes.

Open and closed fields are visible in the sun’s corona – and it may be possible to ‘fit’ the relevant ‘pfss model’ to coronal images from the recent QLD eclipse. The fields we see in the pfss model ‘sculpt’ or shape the corona.

The mid latitudes show great complexity due to the many active areas (spots) and filaments present there. Consider one example in the SW.

Filament ‘trough’: in the sun’s SW a huge magnetic ‘trough’ formed by closed (yellow) and open (pink) field is seen (arrows, Fig rhs). Regularly during November filaments have been forming in this ‘trough’ and erupting into space (filament ejections). It’s the migration of this pink field into the previously green polar region that will reverse the south pole, maybe in another year or so.

During a solar cycle spots in either hemisphere migrate towards the equator. Meanwhile, the quiet region filaments (qrf) migrate pole-wards during the cycle (Schrijver and Zwaan. Ibid P197). It’s this migration of following (f) field that finally reverses the unipolar ‘caps’ at the sun’s poles. Currently many filaments are found in the high south latitudes but far fewer are seen in the north, due perhaps to the north having reached maximum a year ago, as has been suggested.

Summary: The sun’s polar reversal is now well advanced – it’s a big event for the solar system – one that occurs every eleven years. It’s likely the reversal has some effects on Earth but little is known about this.

Our star’s dynamic behaviour is remarkable: that such a large entity can change its ‘spots’ so rapidly is, I think, astonishing. While eleven years is a long time for a human it’s nothing for a star that is already ~8 billion years old. And, after all, the sun is a ‘quiet’ star! Now try to imagine an active one!

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers. He is the recipient of the Astronomical Society of New South Wales’ McNiven Medal for 2012, which is the highest award from the society.

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