A New Take on the Spotless Sun

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.

Spotless Sun
The Sun appeared spotless on June 19, 2009 — as it did virtually every day that year.
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.

"Conveyor belt" inside the Sun
The thick black lines represent flow of ionized gas (plasma) constantly moving in the Sun's convective zone between the equator and poles. Blue and red regions on the right denote the magnetic fields of two successive sunspot cycles; blue and yellow regions on the left indicate the strength and polarity of magnetic fields near the solar surface.
NASA / W. T. Bridgman / D.Nandy & others
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.

Recent sunspot cycles
Following solar cycle 23, the Sun entered an especially long activity minimum that kept its disk free of sunspots for more than two years. Click here for larger version that begins in 1900.
Nature / D. Nandy & others
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.

For more background on the most recent explanation, see this write-up by NASA and this press release from the Center for Astrophysics.

5 thoughts on “A New Take on the Spotless Sun

  1. Eddie Lyons

    I keep a naked-eye watch on the Sun, using a set of eclipse viewers, on every clear day that I can. For such observations the spotless Sun has extended for nearly four years now: the last sunspot I recorded was on 3 May 2007.
    Daily naked-eye observations also bring home the reality of the Sun’s rotation. It’s quite fascinating to be able to determine this with such modest equipment, and raises the question of whether any ancient sunspot observers ever realised they were witnessing the Sun’s rotation.
    I’m eagerly (yet patiently!) awaiting the appearance of the first naked-eye sunspot of cycle 24.
    I would be interested to learn if there are any others out there who keep a record of naked-eye sunspots.
    Eddie Lyons
    Portsmouth, UK

  2. kdconodkconod

    Eddie – you missed it…first naked eye sunspot group was weeks ago – and there’s another one on the disk right now!

  3. Anthony BarreiroAnthony Barreiro

    In response to the previous two comments, it seems that sunspots, like so many other phenomena, are in the eye of the beholder! Eddie is observing from Portsmouth England, 51 degrees north latitude and with frequent cloud cover. Kconod, where are you?

  4. Eddie Lyons

    Kconod, Thanks for the input. What is your location? I did manage to view the Sun while that sunspot was on the disk but saw nothing — the Sun’s elevation was probably still too low, and the atmosphere too hazy. I did wonder if it would be naked-eye visible from elsewhere — now I know! I’ve also checked today (5 March) without luck; too much haze. Or maybe I need new glasses! 😎 Eddie Lyons, Portsmouth, UK

  5. Peter WilsonPeter Wilson

    "…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." Why are the sun’s poles colder than its equator? Do they get less sunlight?

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