Silent Sun

A photograph of the Sun's full disk. Image: NASAWhy has the Sun been so quiet?

The Sun ought be awash with activity right now.  But space scientists are baffled…  The Sun has reached its solar maximum: the point in its 11-year cycle where activity is at a peak.  Yet it has hit a lull.  And to see when the Sun was this inactive last… you’ve got to go back about 100 years… 

This solar lull in activity is simply puzzling astrophysicists at the moment, because the Sun has reached the solar maximum of its 11-year cycle.  And for this very reason, the Sun should be at the peak of its activity – bursting with flares and coronal mass ejections (See NASA picture).


At the Sun’s Surface

The Sun’s visible surface is about 1.4 x 106 km in diameter, but it is not really a surface at all.  The solar luminosity L_{\bigodot} is about 3.84 x 1026J s-1, that is the rate at which energy is radiated by the Sun.  The light does not come from a single well-defined surface, but rather from a thin, semi-transparent shell of gaseous material, more like an atmospheric layer.

A close-up photograph showing sunspots on the surface of the Sun.Almost all of the Sun’s light comes from a gaseous layer about 500 kilometres thick, called the photosphere.  The average temperature at the photosphere is around 6000 K, and it is marked by important dark features of the Sun’s magnetic field, called sunspots.

Sunspots are periodic temporary surface phenomena, large and relatively cool regions of the photosphere of the Sun that appear as visibly dark spots compared to the surrounding regions.  Sunspots are caused by intense magnetic activity, which inhibits plasma convection, forming areas of reduced temperature.

The temperature of sunspots is lower than their surrounding, at around 4200 K.  They tend to form in pairs of opposite polarity, roughly aligned along lines of constant latitude, with the ‘leading’ sunspot – that is, by definition, further ahead in the sense of the rotation of the Sun – is closer to the equator than the ‘trailing’ sunspot.


The Solar Cycle

A graph showing the number of sunspots over the Solar Cycle 23 to 24 from1995 to 2018.All phenomena associated with solar activity, including the number of sunspots, undergo cyclical variability with a period of about 11 years.  This recurrence of the Sun’s activity is what is called the solar cycle.

The Sun’s magnetic field depends very strongly on the solar cycle.  Many regions of increased magnetic field appear at solar maximum, while they are scarce around the solar minimum.  Those are called the active regions of the Sun.

At the start of the solar cycle, when active regions begin to re-appear after a solar minimum, the active regions tend to be located at high latitudes, both north and south of the Sun’s equator.  As the solar cycle progresses, the band in which active regions occur migrates towards the equator.

Solar Cycle 24 is the 24th solar cycle since 1755, when recording of solar sunspot activity began.  It is the current solar cycle, and began on 4th January 2008, but there was minimal activity until early 2010.  Recent footage was captured by spacecraft with their sights on our star.  The Sun is revealed in exquisite detail, but its face is strangely featureless.  Our familiar giant ball of plasma ought to be pockmarked with sunspots, exploding with solar flares and spewing out huge clouds of charged particles into space in the form of coronal mass ejections (CME).

Apart from the odd event, however, like some recent solar flares, the Sun has been very quiet lately.  And this rather damp squib of a maximum follows a solar minimum – the period when the Sun’s activity troughs – that was longer and lower than scientists expected.  This drop-off in activity is happening surprisingly quickly, and scientists are now watching closely to see if it will continue to plummet.

Dr Lucie Green, from University College London‘s Mullard Space Science Laboratory, says: “It’s completely taken me and many other solar scientists by surprise.  It could mean a very, very inactive star, it would feel like the Sun is asleep… a very dormant ball of gas at the centre of our Solar System.”

However, such a level of solar inactivity would certainly not be happening for the first time.


Maunder Minimum and mini-Ice Age: Frost Fairs on the River Thames

An old illustration showing the Frost Fair that took place on the Thames during the so-called 'Little Ice Age' in London, during one of the Sun's Maunder minimum.Between 1309 and 1814, the river Thames froze at least 23 times.  On five of these occasions (1683-84, 1716, 1739-40, 1789 and 1814), the ice was thick enough to hold a fair.  1814 was the third coldest January on records since 1659 – when the Central England Temperature records began.  The reason was low sun spot activity and the North Atlantic Oscillation sucking in weather from Siberia.

During the latter half of the 17th Century, the Sun went through an extremely quiet phase of activity, a period called the “Maunder Minimum“.  The era of solar inactivity in the 17th Century coincided with a period of bitterly cold winters in Europe, when Londoners enjoyed frost fairs on the Thames after it froze over.  Snow cover across the continent increased.  The Baltic Sea iced over.  The conditions were so harsh that some describe it as a mini-Ice Age…

Of course, the Thames has frozen since.  But could such a dramatic climate event be repeated?


1963’s ‘Big Freeze’

A 1963 black and white photograph of the 'Big Freeze'. A man can be seen walking and another one cycling on the frozen River Thames.The winter of 1962-63 is known as the ‘Big Freeze’.  Cold weather reached the United Kingdom on 22 December 1962.  Snow started falling across Scotland on Christmas Eve before sweeping South of the Borders.  Central England experienced its coldest winter since 1740.  Rivers, lakes, and even the sea, froze over.  The conditions made for surreal scenes: a man was spotted cycling on the Thames near Windsor Bridge, people skating in front of Buckingham Palace, a milkman doing his deliveries on skis…  The Arctic weather didn’t relent until March 1963.  For 62 consecutive days, snow lay on the ground in the south of England.  The next highest number of snow lying days since then is a mere 10 in 1987.

But it was only parts of the Thames that froze further upriver and for short periods, and not long enough to turn the capital’s river into a mixture of circus and rave.

The frost fairs took place during the ‘Little Ice Age’, roughly between 1350-1850.  It was colder in those days.  Historical records have revealed that sunspots almost disappeared during this time.  And it could well be that the Sun is currently behaving in the way that it did in the run-up to the Maunder Minimum.

Currently, sunspot numbers are running at less than half  those seen during cycle peaks in the 20th Century.  Analysis of ice-cores, which hold a long-term record of solar activity, have suggested that the decline in solar activity is the fastest seen in 10,000 years.  An unusually rapid decline!

According to a paper by the Met  Office, there is a  92% chance that both Cycle 25 and those taking  place in the following decades will be as weak as, if not weaker than, the ‘Dalton  minimum’ of 1790 to 1830.  During this period, named after the meteorologist John  Dalton, average temperatures in parts of Europe fell by 2°C.

If the Sun continues to wane, this climate phenomenon could theoretically happen again.


Implications for Local Climate Variations

In a recent report by the United Nations’s climate panel, scientists concluded that they were 95% certain that humans were the “dominant cause” of global warming since the 1950s, and if greenhouse gases continue to rise at their current rate, then the global mean temperature could rise by as much as 4.8°C.  While some scientists have argued that the ebbs and flows of the Sun’s activity are driving the climate, overriding the effect of greenhouse gas emissions, the IPCC (Intergovernmental Panel on Climate Change) concluded that solar variation only makes a small contribution to the Earth’s climate.

However, Mike Lockwood, a professor of Space Environment Physics from the University of Reading, thinks there is a significant chance that the Sun could become increasingly quiet: “We estimate that within about 40 years or so there is a probability of 10% to 20% – nearer 20% – that we’ll be back in Maunder Minimum conditions”.  He believes a local effect happens because the amount of ultraviolet (UV) light radiating from the Sun dips: “It’s a very active research topic at the present time, but we do think there is a mechanism in Europe where we should expect more cold winters when solar activity is low.”

When the Sun’s activity is low, less UV radiation hits the stratosphere – the spherical layer of air that sits high above the Earth.  In turn, this feeds into the jet stream, the fast-flowing air current in the upper atmosphere that drives the weather.  Large meanders in the jet stream are blocking off the normal moist, and mild winds from the Atlantic.  Instead, cold air is dragged down from the Arctic and from Russia.  The results are pre-dominantly felt above Europe.

Prof Lockwood says that while UV light varies with solar activity, other forms of radiation from the Sun that penetrate the troposphere (the lower layer of air that sits around the Earth) do not change that much: “If we take all the science that we know relating to how the Sun emits heat and light and how that heat and light powers our climate system, and we look at the climate system globally, the difference that it makes even going back into Maunder Minimum conditions is very small.  I’ve done a number of studies that show at the very most it might buy you about five years before you reach a certain global average temperature level.  But that’s not to say, on a more regional basis there aren’t changes to the patterns of our weather that we’ll have to get used to.”

Cold, snowy winters could become the norm again for Europe.


Quiet Space Weather – Looking Ahead at Cycle 25…

Harsh weather would not be the only consequence of a drawn-out period of solar inactivity.  Polar lights – one manifestation of solar activity in the Earth’s magnetosphere – may dim.  “If the Sun were to get very quiet, one of the few things that would happen is that we’d have very few displays of the northern lights.  They are driven by solar activity, and we’d miss out on this beautiful natural phenomenon,” Dr Green explains.

The good news is that there could be positive effects to this solar lull.  “Solar activity drives a whole range of space weather, and these have ultimately effects on the electricity networks, on satellites, on radio communications and GPS on your sat-nav,” she explains.  While scientists cannot discount that the random bursts of activity still occur, calmer periods of space weather would help to maintain the technological infrastructure that we rely so heavily on.  Up to this point, solar cycle 24 has not produced much in the way of flare-triggered fireworks, so we have been dodging the powerful solar storms that threaten orbiting satellites and our power grid.

The bad news is…

Sunspot numbers are down to less than half  those seen during previous cycle peaks over the 20th Century.  Expert analyses by NASA and the  University of Arizona, derived from magnetic-field measurements 120,000 miles  beneath the Sun’s surface, suggest that Cycle 25, whose peak is due in 2022,  will be a great deal weaker still.

The full implications of a quietening Sun are not fully understood.