Sunspot growth on the Sun in June 2012. Image: NASA/Goddard Space Flight Centre.
The discovery of the solar cycle – which is the periodic rise and fall in the number of sunspots on the face of the Sun – is a fascinating story of serendipity and perseverance.
A German amateur astronomer named Samuel H. Schwabe, was determined to find a new planet inside the orbit of Mercury, i.e. between the orbit of Mercury and the Sun itself. He had decided to name the new planet, if he discovered it, after the Roman God of fire, Vulcan. His idea was to observe the Sun every day and look for a dark spot transiting across the bright solar disk, as the planet moved in its orbit across the face of the Sun.
On every cloudless day from 1826 to 1843, Schwabe would scan the Sun looking for Vulcan. What he observed were lots of dark spots, the sunspots, which he meticulously recorded, and noticed the regular variation of their numbers.
Rudolf Wolfe, the director of the Solar Observatory in Bern, Switzerland, followed up on these observations of sunspot activity, starting from the time of Galileo’s first observations in 1610. He calculated that the pattern of the spots’ appearance and disappearance had a period of 11.1 years, and thus established the presence of a periodic cycle in sunspot numbers known as the sunspot or solar cycle. Wolfe then numbered the 1755-1766 cycle to be solar cycle no. 1, as there had been hardly any observations prior to that period to determine a cycle.
Today, we are at the start of solar cycle 25. Although sunspot numbers vary with a relatively stable period of 11 years, their amplitudes, or numbers, vary significantly. (It is interesting that for 70 years, between 1645 and 1715, there were no sunspots recorded at all – a period called the Maunder minimum.) In 1908, the renowned astrophysicist George E. Hale discovered that sunspots were the seats of very strong magnetic fields. His work expanded the scale of magnetic fields out of Earth and into the cosmos. We know today that magnetic fields are ubiquitous in the universe.
The strong magnetic fields in sunspots are of the order of 2,000-3,000 gauss; to compare, the strength of Earth’s magnetic field is about 0.5 gauss. We know that such strong magnetic fields can inhibit efficient heat transport beneath sunspots, so these regions are somewhat cooler than their surroundings. This is why they stand out as dark spots against the bright background of the visible solar disk, a.k.a. the photosphere. If the field strength in sunspots were to decline below about 1,500 gauss, there would be no contrast between the photosphere and cooler sunspot regions, and we would not be able to see any spots.
My collaborators and I have found that solar magnetic fields have been steadily declining in strength over the past two decades. In fact, it appears from our studies that we may be headed for another long period of time similar to the Maunder minimum, when sunspots almost entirely disappear.
The figure below shows the observed decline in solar magnetic fields, starting around 1995 and continuing up to the end of 2017. Our studies show that the decline has been continuing to the present. The solid red line in Figure 1 is a straight line fit to the declining trend and the dotted red line is an extrapolation. It is apparent that if the fields continue to decline, we may have a Maunder-minimum-like period by 2034, when the polar photospheric fields would in principle drop to zero.
Solar field reversals
The Sun, like Earth, has a magnetic field with magnetic poles. But unlike Earth’s magnetic field, which reverses or flips polarity over tens of millennia, the Sun does so every 11 years – at a time when its sunspot number peaks or the solar cycle reaches its maximum amplitude. Predicting the strength or amplitude of the next solar cycle is a complicated task that requires a deep understanding of how the Sun and the so-called solar dynamo, an internal mechanism that generates the magnetic fields, behaves.
Recently, a research group I was part of discovered an unusual pattern of magnetic-field reversal at the Sun’s poles during solar cycle 24. We showed that the Sun’s southern hemisphere had reversed its polarity in mid-2013. On the other hand, the northern hemisphere had commenced its field-reversal in 2012, followed by a sustained period of near-zero field strength for 2.5 years, until late 2014, before it finally reversed polarity.
Such an asymmetry between the reversal times of the two hemispheres is probably unprecedented since solar observations first began about four centuries ago. And together with the fact of the declining solar fields, we suspect that a Maunder-minimum-like event may already be underway.
Sunspots and cosmic rays
The absence of sunspots means low solar magnetic activity, which has been previously linked to global cooling on Earth. For example, the Maunder minimum on the Sun overlapped with a period called the little ice age on Earth, when many parts of Europe and North America entered a ‘deep freeze’. More recent studies have shown that this cooling may have been global. However, we don’t have proof of a causal relationship between these two events.
In addition, it is well-known that a weaker solar magnetic field will cause an increase in the cosmic ray flux at Earth. Cosmic rays are energetic charged particles originating from outer space that impinge on Earth’s atmosphere. About 90% of these particles are protons; 9% are helium nuclei, or alpha particles; and about 1% are electrons.
Cosmic rays can charge aerosol particles in Earth’s atmosphere, and charged aerosols are thought to be more efficient in seeding clouds than uncharged aerosols. So a weaker solar magnetic field, in principle, could alter cloud cover over Earth. If the cloud cover increases, the amount of sunlight reflected back into space will also increase, thus cooling Earth’s surface. This is still a hypothesis, we don’t know this yet.
Continued observations of the solar magnetic fields and studies of the solar field reversal process in the future could help us understand the degree to which the solar cycle can affect near-Earth space and Earth’s climate.
Janardhan P., FNA, at the Physical Research Laboratory, Ahmedabad, is the principal investigator of the Aditya Solarwind Particle Experiment (ASPEX) payload on-board ISROs’ ADITYA-L1 Mission.