Sluggish flow inside the sun may cause late sunspot cycle

The sun undergoes an activity cycle during which the number of sunspots on its surface increases and decreases over a period of approximately 11 years.¹ Sunspots² are huge areas of intense magnetic fields. They are the locations of flares,³ explosive events that hurl large amounts of charged particles into space. When these particles hit the earth (a phenomenon known as space weather),^4,5 they disrupt and can damage many of our technological systems, such as global-positioning systems, telecommunications, polar airline routes, satellites, and power grids. The radiation also poses a danger to astronauts. Currently, we are in an unusual and extremely quiet period of solar activity, with several record-setting aspects. But what is causing this?

The sunspot cycle is thought to arise from large-scale motions inside the sun: north-south and east-west flows (meridional circulation⁶ and torsional oscillation,⁷ respectively) and differential rotation⁸ in which the solar equator rotates faster than the poles. The combination of these flows and their interaction with the magnetic field set up by the moving, electrically charged particles in the solar plasma is believed to create the sunspot cycle through a dynamo mechanism.⁹ The advent of helioseismology has made it possible to probe the solar interior and watch these flows evolve as the cycle progresses.

Helioseismology is the study of the sound waves that fill the solar interior.¹⁰ The acoustic waves are trapped in the thermal gradient inside the sun, and measurements of their properties, in particular their temporal frequencies, can be analyzed to estimate the direction and magnitude of the flows as a function of depth, horizontal position, and time. Using data from the Global Oscillation Network Group¹¹ (GONG) facility of the National Solar Observatory and from the Stanford University Michelson Doppler Imager¹² (MDI) on NASA's Solar and Heliospheric Observatory (SOHO) spacecraft, we have constructed a map of the torsional-oscillation flow inside the sun over the past 14 years.

GONG has been continuously observing solar oscillations since 1995 with a set of six instruments located in California, Hawaii, Australia, India, Spain, and Chile. This network of telescopes allows us to see the sun 24 hours per day. The data is returned to Tucson, Arizona, where it is calibrated and the frequencies of the oscillations of roughly 200,000 vibration modes are extracted. These frequencies are supplemented with those from SOHO/MDI, which is located in space at a point where it obtains uninterrupted solar observations. The frequencies are affected by motions inside the sun, which essentially create a Doppler shift of the waves that depends on the internal flow and the sensitivity of each mode as a function of depth.

The frequencies are analyzed with a technique known as inversion,¹³ and the flows as a function of depth and solar latitude can be inferred. The analytical process is very similar to that used for terrestrial earthquakes, and creates a sonogram of the flows. The result,¹⁴ presented in Figure 1, shows the torsional oscillation at a depth of 1000km below the solar surface as a pair of red-and-yellow chevron patterns, one for the past cycle, and one for the cycle that is now beginning. Comparison of the two patterns, shown by the blue lines, reveals that the flow for the current cycle is sluggish in its travel from the poles to the equator. To date, compared with the previous cycle, the flow has taken 1.5 years longer to reach the latitude where sunspots typically begin to emerge in large numbers. This delay is equal to the unexpected, extra duration of the current minimum, which suggests that the patterns are related.

Figure 1. Torsional oscillation (an east-west flow) inside the sun at a depth of 1000km as a function of date and solar latitude. The color scale on the right shows the amplitude of the flow (δ2π, where δΩ is the change in rotation rate) in units of nanohertz, 1nHz ≈ 5m/s. The torsional oscillation appears as the chevron-shaped bands of yellow and red. The black contours represent the surface solar magnetic field, which appears in close association with the position of the torsional oscillation. The blue lines compare the progress of the torsional oscillation as it moves from the solar poles to the equator during the previous and current solar cycles (left and right sides of image, respectively). We can see that the current flow is taking approximately 1.5 years longer to reach the latitude where solar activity generally rises rapidly at the start of a cycle. This delay in the movement of the torsional oscillation may account for the current extended minimum.

We do not know why the torsional oscillation is slowly migrating. We also monitor the north-south meridional flow and the differential rotation, but neither is significantly different between the two cycles. The meridional flow is thought to play an important role in determining the timing and amplitude of the solar-activity cycle,¹⁵ but at a depth far below what we can presently sample reliably.

Thus, several mysteries remain in our quest to unravel the cause of the solar cycle. Because of our analysis technique, the flows in Figure 1 are precisely symmetric across the equator, but the sun shows significant differences between the northern and southern hemispheres that may play a role in the cycle behavior. These differences can be studied using other helioseismological methods, such as ring diagrams¹⁶ and time distance.¹⁷ In addition, we are developing methods to search for the deep meridional flow that should exist about 200,000km below the surface. These techniques will enable us to learn more about the roots of the sunspot cycle.