Featured Pub: What Causes the Seasons of Solar Activity?

Recently it was demonstrated from many observations of solar activity that the variability within a sunspot cycle consists of quasi-periodic bursts of activity followed by quieter intervals; these are called the "seasons" of solar activity. Complete intervals of burstiness followed by quiet are 6-18 months in duration; bursts often persist at certain longitudes for several rotations. These bursts include enhanced coronal mass ejections (CMEs) and flares from which magnetic fields and energetic particles that travel through interplanetary space to interact with the Earth, producing  ‘space weather’. Currently space weather events, hazardous to our technological society, are predicted only after the CMEs and flares have been observed at the Sun, leaving only a few days to prevent disruptions of power grids, radio communications and global positioning systems (GPS). Numerous industries rely on GPS for accurate orientation and navigation. On flights over polar regions during space weather events, airplanes can experience radio blackouts and equipment disruptions, and satellites can go out of control. About three decades ago, a CME about the size of 36 Earths erupted from the Sun’s surface and ripped through the space at a million miles per hour speed. Two days later this huge plasma ball crashed against the Earth’s magnetosphere, and most significantly Canada’s Hydro-Quebec power utility grid crashed, knocking out electricity to six million people for nine hours. 

Tachocline fluid shell image
Two perspective snapshots of top-surface (color-shade) of a tachocline fluid shell, viewed respectively along longitude (left panel) and latitude (right panel), during its MHD evolution; red/orange represents swelling of the fluid and blue/sky-blue the depression. Yellowish-green represents neutral thickness. The shallow-water tachocline model has a rigid bottom and deformable top; vertical extent denotes the tachocline thickness (20 times enlarged). Portions of the toroidal magnetic bands (two white tubes one each in the North and South hemispheres) that coincide with swelled fluid are shown encircled by black ellipses -- these portions start entering the convection zone, and hence are more likely to buoyantly erupt at the surface.


Composite image

Left panel (a) shows oscillation between differential rotation kinetic energy (solid red curve) and perturbation Rossby waves kinetic energy (red dashed curve). Typically up to 42 modes in longitude were included in these nonlinear simulations. The units in x and y axis are dimensionless. 100 dimensionless time units correspond to approximately one year; thus the TNO has a period of about 6 months in this case. Frames (b-d) show perturbation flow patterns (in arrow vectors), and thickness of tachocline fluid shell (in color shades, red shade representing swelling, blue depression). Tilts of perturbations are eastward in (b), extracting energy from differential rotation until Rossby waves’ energy is at a maximum; perturbations then go through neutral tilts (c), and then overshoot to acquire westward tilts (d). Enhanced bursts of activity (shown by a semi-transparent gray arrow pointed towards a local peak (yellow-filled ellipse) in sunspot number curve in panel (e)) occurs when perturbation Rossby wave kinetic energy is at its maximum, followed by a relatively quiet season (second semi-transparent gray arrow pointing towards a local dip (the second yellow-filled ellipse) in panel (e)). 

In a paper just published in the Nature Journal [https://www.nature.com/articles/s41598-017-14957-x] Dikpati, Cally, McIntosh and Heifetz discovered for the first time the physical origin of the "seasons" in space weather. They demonstrated, using a global shallow-water model of the Sun's shear-layer (called the tachocline), that magnetic fields can more easily penetrate the convection zone and rise buoyantly to the solar surface to erupt in the form of bursts when the deformable top surface of the tachocline most strongly bulges into the convection zone above, carrying the partially frozen magnetic fields with it (see figure). 

A Bursty season is more likely to occur when the Rossby waves are at or near peak amplitude in the oscillation on the Sun, and is followed by a quieter season due to oscillatory nonlinear exchange of energies among the Sun's Rossby waves, differential rotation and sunspot-producing magnetic fields. Under a wide variety of conditions, the period of these Tachocline Nonlinear Oscillations (TNOs) is within 2-20 months, closely matching with the range of quasi-periodic solar seasons observed on the surface.

Dikpati and colleagues are working on using the model — fed with observations of magnetic fields on the front and back sides of the Sun — to make predictions of seasonal changes up to a year in advance.