Why the Sun is flagging

How the Sun’s activity will develop cannot be predicted in the long term.

August 06, 2015

The Sun is currently unexpectedly tame. Our star’s last peak of activity in mid-2014 fell well short of those of the past decades. Comparatively few sunspots covered its surface; the number and severity of solar flares were lower than expected. Scientists from the Max Planck Institute for Solar System Research (MPS) in Germany and from the Chinese Academy of Science have now found an explanation for the Sun’s "loss of power". Decisive were unusual magnetic structures that appeared on the surface of the Sun near its equator approximately eleven years ago and weakened the Sun’s overall magnetic field in the years that followed. Since structures of this type form randomly, the researchers’ calculations especially show that the strength of an imminent activity peak can be determined only a few years in advance. Longer-term predictions are impossible.

The 24th solar cycle is noticeably weaker than its predecessors. Shown here is the number of sunspots covering the visible surface of the Sun at a given time. This number is a means of quantifying the Sun’s activity.

The Sun’s activity varies in a more or less regular cycle. Approximately every eleven years our star shows its impetuous side: In violent eruptions it hurls charged particles and radiation into space; strong, varying magnetic fields appear at its visible surface and produce dark sunspots. About six or seven years later the Sun has calmed down again.

But despite this regularity each solar cycle is different. The current cycle for example, the 24th one since the beginning of systematic solar observations in the 18th century, is significantly weaker than its predecessors. In its maximum about a year ago, only about half as many sunspots covered the surface of the Sun as in the early 90s. Particularly powerful solar flares were also absent in the 24th cycle.

Looking for an explanation for this solar weakening, researchers at the MPS and the Chinese Academy of Sciences have taken a close look at the magnetic fields on the surface of our star. In addition to a large-scale magnetic field that – like the Earth’s magnetic field - resembles that of a bar magnet, the Sun’s magnetic field is mainly characterized by strong local fields. These fields show up on the surface of the Sun as a so-called bipolar regions: two closely spaced regions of high magnetic field strength, corresponding to the two opposite magnetic poles. Often, these fields become noticeable because of the dark sunspots associated with them.

The arrangement of the magnetic poles in a bipolar region tends to follow a kind of rule of thumb: If the bipolar region appears near the solar equator, the magnetic poles are usually oriented side by side in the direction of solar rotation. With increasing distance from the equator, they are shifted more and more apart from each other. In the following years, near-surface plasma currents transport preferably those parts of the magnetic fields that are further from the equator toward the Sun’s North and South Pole. In this way, the bipolar regions add to the bar magnet component of the solar magnetic field, which is crucial for the strength of the following solar maximum. "The puzzle was why the magnetic field of the Sun was so unusually weak before the current solar cycle 24”, says Dr. Jie Jiang of the Chinese Academy of Sciences, first author of the new study. "Following the rule of thumb for the shift of the bipolar regions it should have been four times stronger," the astrophysicist adds.

To solve the puzzle, the team calculated the evolution of the Sun’s magnetic field between 1996 and 2012 on the basis of measured data for the bipolar regions during this period. The crucial factor is the precise shift of the magnetic poles within the bipolar regions. "While previous studies had to resort to a general rule and could therefore not explain the weak magnetic field, we for the first time worked with the precise data for each bipolar region, " says Prof. Dr. Manfred Schüssler from MPS, who led the new study. The magnetic maps of the Sun used in these calculations were obtained by the SOHO spacecraft, a joint project of ESA and NASA.

The data show that in the solar cycle 23, the bipolar regions on average followed the rule of thumb. However, crucial for the result were a few untypical regions that showed reversed polarities  and therefore weakened the bar magnet component of the solar magnetic field. "If such outliers - as happened in solar cycle 24 – appear near the equator, this has a particularly big effect," says Dr. Robert Cameron from MPS, a co-author of the study. As a result, the Sun's magnetic field around the minimum around 2009 was relatively weak - and the next peak of activity correspondingly tame.

The unusual bipolar regions that were seen in the 23rd cycle are a random phenomenon. Since the bipolar regions are caused by turbulent plasma flows inside the Sun, neither the place or exact arrangement of their appearance can be predicted. This makes it impossible to determine the strength of a solar cycle more than a few years in advance. The Sun remains a mysterious star.





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