Astronomy Home Page
Heartbeat of the Sun - By Nick Kollerstrom
Above: 31st August 2012 - a giant prominence erupts, sending out particles and a shock wave that traveled near Earth.
Diameter: 1,390,000 km
Mean Surface Temperature: 6,000ºC
Rotational Period: 25 to 36 days
(From Equator to Poles)
Our Sun is now (2013) at the peak of a very half-hearted solar maximum, as if it hasn’t quite got the energy to do it properly! Its huge magnetic field North-South is dying away, and then shortly after it will re-appear in reverse polarity, growing again. Its whole field reverses every 11 years, at solar maxima. After this maximum, its solar flares and sunspots will decrease and its chromosphere will shrink, as the solar North-South magnetic field increases, and builds up to its maximum. Our Sun's majestic heartbeat is twenty-two years. The previous solar maximum had two peaks, one in 2000 and the other in 2002.
Sol is the fiery, pulsating heart-centre of the solar system, which is something more than the burnt-out 'yellow dwarf' as astronomers regard it. Just as cardiac fibre has a periodic contraction built into it, so likewise the Sun's surface has a self-reversing magnetic field within it, which flips over every eleven years. That is something very unearthly! The word 'plasma' refers both to the fluid of the bloodstream, and to the solar matter which streams out along with the solar wind, bathing the Earth.
At a sunspot maximum, the Sun's magnetic field becomes stretched out around its equator. It is in a highly stressed condition, so that sunspots are gradually pulled nearer to the solar equator by this stretching process. The magnetic-field tubes emerge through a sunspot, then re-enter the Sun by another. This field at the equator flips over at each sunspot minimum. In contrast, the solar magnetic field at its poles reverses at each sunspot maximum. Slowly, we begin to acquire a feeling for the heart-like functioning of that tremendous solar magnetosphere.
Sunspots have symmetry above
and below the solar equator, and magnetic flux streams out of the Sun from one
sunspot and back in again through another, which we may compare with the
arterial and venous system of the blood. Sunspots are magnetic orifices whose
polarity expresses whether a magnetic filament is emerging from or returning
into the Sun. They move round differentially on the solar surface, at different
speeds at different latitudes, and this gradient makes the equatorial magnetic
field grow stretched out and tangled-up. Yes, it is rather complicated. This
stretching field 'pulls' the sunspots nearer to the equator, so that they move
to lower solar latitudes as the cycle progresses. Then, at sunspot minimum
this whole equatorial solar magnetic field flips over and the cycle of sunspots
Early in 2001, NASA websites announced that the North-South magnetic field of the Sun was reversing its polarity, was flipping over, as it does every eleven years. This rhythm makes up the well-known 22-year cycle. Solar satellites recorded the event as it happened, making this the first time ever that the event was followed in real time. There are four stages in this cycle, just like there are in a heartbeat, and let's go over them. The North-South magnetic field emerges from its poles, reaching out above and below the ecliptic. As the solar equator is tilted merely six degrees to the ecliptic, this field is more or less perpendicular to it. It grows strongest as the sunspots are at their minimum (the sunspots are then small, and there are only a few around the solar equator) and then grows weak as the sunspot maximum approaches - when, dramatically, it flips right over, between North and South, then starts increasing again with polarity reversed. Then as we saw, around the solar equator, the magnetic field becomes stretched-out and stressed at sunspot maximum, with strands threading in and out of the sunspots. Astronomers talk of a solar 'dynamo' as having a 'self-reversing magnetic field' and this is something unearthly - no magnetic field self-reverses here on Earth.
We are used to thinking of
the 11-year sunspot cycle, but let�s try instead to visualise two 22-year
cycles, proceeding out of phase at the poles and equator. Thus the full solar
heartbeat is a four-stage process, as might remind us of the human heartbeat.
Between arteries and veins this goes through its four-stage systole-diastole
process. Thus the emerging modern picture endorses Galileo�s view of the Sun,
rather like the heart of an animal, in which
The solar wind spirals out
from the Sun, and it normally divides up into four sectors, of alternating
magnetic polarity. But, these do fragment into a larger number of streams, eight
or so, during a sunspot maximum. Thus in the ecliptic, there is normally a
fourfold 'rotating sector structure' of the solar plasma The spiral is caused by
the Sun's rotation. Solar wind flows along one sector out to the planets, and
then back again into the Sun via another. Does this indicate that the Sun has an
inner structure differentiated by sectors, and, if so, would this be comparable
to the four ventricles of the heart?
We experience the mean rotation period of the solar plasma (3) as 27.3 days, more or less. That is the mean rotation period of sunspots as we see them - and it also happens to be the lunar orbit period. If there are four plasma sectors, then this gives a change in sectors bathing the Earth, once a week. Thus Earth gets a fourfold experience of the Sun per sidereal lunar month! The polarity reverses, weekly. There are quite a lot of biological and climatological effects that result from this solar wind change.
The solar corona is far hotter than the Sun's surface, which has rather puzzled astronomers, rising to several millions of degrees in temperature. X-ray pictures of this solar corona are obtained from satellites. The corona streaming out far from the Sun is visible during a solar eclipse. Sunspots in contrast are cooler than their surroundings, making them darker. Modern studies have come a long way towards a more biological image of our Sun. Even Eudoxos, who wrote:
"...Willingly would I burn to death like Phaeton, were this the price for reaching the Sun and learning its shape, its size and its substance."
might be pleased.
A solar eclipse
happens, when the Moon comes exactly in front of the Sun. We may experience this
as a primal miracle of Mother Nature, as Luna eclipses the Sun's light. In 2005
an 'annular' eclipse happened, and that means that the Moon was then near its
apogee (further away) and so didn't quite get to cover all of the Sun, leaving a
ring. The 'path of totality' of this eclipse moved across Spain, where these
pictures were taken. One can see solar flares all around its edge.
Click on this link to see the full series of these annular solar eclipse images (courtesy of Ben Cavenna) taken on October 3rd 2005, in Spain.'
A Lemniscate in the sky
At noon, the sun casts a shadow. From some high building, say a church steeple, one plots the shadow of its tip on the ground, and that shadow at noon slowly weaves out a figure of eight, during one year. Astronomers used to call this the ‘Equation of Time’ because it indicates a difference between mean and apparent time. Set an astrology chart to noon, and see how far away the Sun is from the MC: up to 18 minutes away, this will vary through the course of a year and is in essence the 'Equation of Time.' For the first time ever, astronomer-photographer Tunc Tezel has recorded this not as a shadow, but as the sun’s position in the sky at noon. He is here viewing it from the shores of the Caspian Sea, during a year 2011-12, using a wide-angle lens so we can see the horizon. Many thanks to Tunc for permission to use this image. For more info click on the photo above to go to Tunc's page.
- See it on Video:
letter to Pietro Dini of March 1615:
Frazier, 1982, p151: