Sun Experiences Seasonal Changes: Study

The Sun undergoes a type of seasonal variability with its activity waxing and waning over the course of nearly two years, according to a new study which could lead to better prediction of space weather events.

The seasonal changes affect the peaks and valleys in the approximately 11-year solar cycle, sometimes amplifying and sometimes weakening the solar storms that can buffet Earth's atmosphere, said researchers led by the US National Center for Atmospheric Research (NCAR).

The quasi-annual variations appear to be driven by changes in the bands of strong magnetic fields in each solar hemisphere, researchers said.

"What we're looking at here is a massive driver of solar storms," said Scott McIntosh, lead author of the new study and director of NCAR's High Altitude Observatory.

"By better understanding how these activity bands form in the Sun and cause seasonal instabilities, there's the potential to greatly improve forecasts of space weather events," McIntosh said.

The overlapping bands are fuelled by the rotation of the Sun's deep interior. As the bands move within the Sun's northern and southern hemispheres, activity rises to a peak over a period of about 11 months and then begins to wane.

The quasi-annual variations can be likened to regions on Earth that have two seasons, such as a rainy season and a dry season, McIntosh said.

The study, published in the journal Nature Communications, can help lead to better predictions of massive geomagnetic storms in Earth's outer atmosphere that sometimes disrupt satellite operations, communications, power grids, and other technologies.

The new study is one of a series of papers by the research team that examines the influence of the magnetic bands on several interrelated cycles of solar magnetism.

In a paper last year in Astrophysical Journal, the authors characterised the approximately 11-year sunspot cycle in terms of two overlapping parallel bands of opposite magnetic polarity that slowly migrate over almost 22 years from high solar latitudes toward the equator, where they meet and terminate.

McIntosh and his co-authors detected the twisted, ring-shaped bands by drawing on a host of NASA satellites and ground-based observatories that gather information on the structure of the Sun and the nature of solar flares and coronal mass ejections (CMEs).

These observations revealed the bands in the form of fluctuations in the density of magnetic fuel that rose from the solar interior through a transition region known as the tachocline and on to the surface, where they correlated with changes in flares and CMEs.

In the new paper, the authors concluded that the migrating bands produce seasonal variations in solar activity that are as strong as the more familiar 11-year counterpart.

These variations take place separately in both the northern and southern hemispheres.