My students are often surprised to learn that the Sun rotates, and to convince them I usually let them view how a sunspot group takes a few days to march across the solar disk. But until recently it's been tough to make my case: throughout much of 2008-10, there were no sunspots at all. Zip. Zilch. Nada.
It turns out I wasn't the only one fretting about a spotless Sun. The same physicists who routinely worry about the consequences of a frenetic solar maximum were scratching their collective heads about this deep, prolonged solar minimum.
They'll be the first to tell you that we really don't understand what controls the Sun's cyclic behavior, but it probably has to do with meridional flow. This is a conveyor belt of sorts, not unlike permanent ocean currents on Earth, that carries the hot, near-surface plasma and its entrained magnetic fields from the equator to the poles. There the flow takes a dive and returns toward the equator deep within the Sun's convective layer. The meridional flow is slow, no more than 20 to 40 miles per hour (10 to 20 m per second), and it takes about 11 years to complete one circuit — the length of a solar cycle.
In yesterday's issue of Nature, a trio of theorists offers a new model of solar circulation that, they claim, explains why the Sun's face remained blank for so long. Dibyendu Nandy (Indian Institute of Science Education and Research), Andrés Muñoz-Jaramillo, and Petrus Martens (both jointly at Montana State University and the Harvard-Smithsonian Center for Astrophysics) say the key is how fast the meridional flow moves and how it interacts with a second wholesale flow around the Sun's midsection.
The researchers used their model to simulate 210 solar cycles, extending nearly 2,000 years, using it to match the record of sunspots over the past few centuries but also, more recently, the strength of the Sun's magnetic field across its orb.
The dearth of sunspots for the past few years — and a relatively weak magnetic field at the solar poles — would both result if the meridional flow was relatively fast during the first half of solar cycle 23 (say, from 1998 to 2003) but slowed down during its last (2004 to 2009).
There might be a "gotcha" in this new assessment: an 2010 analysis by David Hathaway (NASA-Marshall Space Flight Center) and Lisa Rightmire (University of Memphis) argues that the flow sped up during the latter half of cycle 23.
Perhaps what we can measure right at the Sun's surface isn't matching the true ebb and flow farther down, so Nandy and his colleagues might have the right answer. But this isn't the first time researchers have stepped forward with a "solution" to the recent deep solar minimum. Long-time followers of SkyandTelescope.com will recall stories we posted in 2006 and 2009 about the transition from cycle 23 to 24.
In any case, everyone now agrees that the upcoming solar maximum should be relatively weak, perhaps one of the mildest on record, which would be a comfort to those who worry about the potential harm that a too-active Sun can cause.