“The unusual minimum of sunspot cycle 23 caused by meridional
plasma flow variations”
Dibyendu Nandy1, Andrés
Muñoz-Jaramillo2,3 & Petrus C. H. Martens2,3
1Department of Physical Sciences,
Indian Institute of Science Education and Research, Kolkata, Mohanpur
741252, West Bengal, India
2Department of Physics, Montana State
University, Bozeman, MT 59717, USA
3Harvard-Smithsonian Center for
Astrophysics, Cambridge, MA 02138, USA
Source: Nature, 3rd March, 2011 issue (Note: Please
respect Nature’s embargo policies!)
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Graphics 1 Caption: This collage shows magnetic fields in the interior of
the Sun simulated using a solar dynamo model (center) and the observed
solar outer atmosphere (corona) at two different phases of solar activity:
A quiescent phase during the recent, unusually long minimum in solar
activity (right) and a comparatively active phase following the minimum
(left). Sunspots originate from the internal magnetic field and are the
seats of solar storms that generate beautiful auroras but are also
hazardous to our space-based technologies. This computer modeling shows
that a deep minimum in solar activity occurs when the magnetic field belts
of two successive sunspot cycles (blue and red coloured regions in the
right) become separated in space and time due to changes in solar internal
meridional plasma flow. This separation of the internal magnetic field
belts results in a lack of sunspots eruptions (and solar storms), over a
long period of time, between two successive sunspot cycles. Image credit:
NASA/Goddard/SDO-AIA/JAXA/Hinode-XRT; Artistic rendering:
Cygnus-Kolkata/William T. Bridgman; Conceptualization and simulation data:
Dibyendu Nandy (IISER Kolkata), Andrés Muñoz-Jaramillo (Harvard Smithsonian
Center for Astrophysics) and Petrus C.H. Martens (Montana State University).
Graphics 2 Caption: This computer modeling shows that a deep minimum in
solar activity occurs when the magnetic field belts of two successive
sunspot cycles (blue and red coloured regions in the right) become
separated in space and time due to changes in solar internal meridional
plasma flow. This separation of the internal magnetic field belts results
in a lack of sunspots eruptions (and solar storms), over a long period of
time, between two successive sunspot cycles. Image credit: William T.
Bridgman (NASA/GSFC), Dibyendu Nandy (IISER Kolkata), Andrés Muñoz-Jaramillo
(Harvard Smithsonian Center for Astrophysics) and Petrus C.H. Martens
(Montana State University).
Graphics 3 Caption: A different phase of sunspot cycle, now also showing
the conveyor belt-like North-South meridional flow of plasma (thick black
line with arrows indicating direction of flow). Image credit: William T.
Bridgman (NASA/GSFC), Dibyendu Nandy (IISER Kolkata), Andrés Muñoz-Jaramillo
(Harvard Smithsonian Center for Astrophysics) and Petrus C.H. Martens
(Montana State University).
Graphics 4 Caption: An image of the Sun taken with the Solar and
Heliospheric Observatory during the minimum of solar cycle 23, showing a
spotless Sun. Credit: SOHO/ESA/NASA.
Movies: Movies (animation of the movement of magnetic fields in the
Sun’s interior and how they produce sunspots) are available at: http://svs.gsfc.nasa.gov/vis/a000000/a003500/a003521/index.html.
Note that these movies are based on an earlier version of the computer
model but can be used for visually representing the movement of the Sun’s
internal magnetic field for purposes of explaining the dynamics. Movies of
plasma flows in the Sun’s interior are available at: http://svs.gsfc.nasa.gov/vis/a000000/a003400/a003496/index.html.
Please give proper credits when using these.
Background on the Work
Sunspots are visibly dark, strongly magnetized regions on the
Sun, which have been observed continuously starting with the pioneering
studies of Galileo Galilei in the early 17th Century. The
number of sunspots on the solar surface changes in time, going through
periods of highs and lows in a cyclic fashion with an average periodicity
of 11 years. This phenomenon is known as the solar magnetic cycle, often
simply referred to as the sunspot cycle.
Sunspots play an important role in space and on Earth. Solar
magnetic storms, the largest explosions in the solar system, originate from
sunspots and carry vast amounts of plasma (charged particles) into space
generating what is known as space weather. When directed towards Earth and
its orbiting satellites, these solar storms disrupt operations of sensitive
equipment in spacecrafts, affect telecommunication systems, and pose a
hazard to air-traffic on polar routes – which is the preferred route for
long distance carriers. Sunspots also control the amount of total energy
that is being radiated from the Sun; more sunspots mean more incident solar
energy on Earth – which is the primary natural driver of the climate
system. The magnetic field from Sun is also carried by the solar wind and
permeates the solar system and this magnetic field is the primary modulator
of the cosmic ray flux on Earth. Cosmic ray flux is believed to be
important in climate dynamics because it seeds cloud formation in the
Earth’s atmosphere. Therefore, variations in the number of sunspots,
including their long absence, can affect the Earth’s climate.
During the periodic low activity periods, a phase known as the
solar minimum, often no sunspots are observed on the solar surface for many
days. The recent solar minimum following sunspot cycle 23 was characterized
by an unexpectedly large number of days without sunspots (unprecedented in
almost a century), very low solar energy output and a record high cosmic
ray flux at Earth. This period also had very few solar magnetic storms,
therefore contributing to a fair weather in space.
The research by Nandy et al reported in Nature:
In the paper “The unusual minimum of sunspot cycle 23 caused
by meridional plasma flow variation” Nandy et al. present the first
consistent explanation of both the long absence of sunspots and the very
weak large-scale magnetic field of the Sun that permeated the solar system
during this deep and prolonged lull in solar activity. Through computer
simulations, they find that variations in a large-scale flow of plasma in
the Sun’s interior known as the meridional circulation caused this
unprecedented solar minimum. The meridional circulation is a loop-like flow
that stretches from the Sun’s equator to the poles near the surface, and
closes in on itself, flowing from the poles to the equator deeper within
the Sun. Their simulations show that a fast plasma flow during the early
half of a sunspot cycle, followed by a slower flow creates the weak
large-scale field of the Sun, and generates a solar minimum with a large
number of spotless days.
They predict that very deep solar minima with a large number
of spotless days are bound to be associated with weak large-scale dipolar
field strength in the Sun, thereby resulting in very weak magnetic field in
the heliosphere.
At a fundamental level, this work demonstrates how the inner
working of the Sun, and variations in the plasma flow deep within our
parent star can control its magnetic and energetic output, which in turn,
determines the environment in space and affects climate on Earth. At the
applied level, this work opens up the possibility of predicting fair
weather in space, based on computer simulations of solar activity driven by
observations of its plasma flows. Such predictions are important for
planning space missions, estimating the life-time of satellites and
scheduling air-traffic on polar routes and therefore have relevance for
space weather forecasting – which is multi-billion dollar industry
worldwide. Prior knowledge of a deep minimum and therefore low solar
radiative energy output can also be used, in principle, for short-term
predictive assessment of solar forcing of the global climate.
The research that resulted in this discovery was led from the
Indian Institute of Science Education and Research, Kolkata and supported
through the Ramanujan Fellowship of the Department of Science and
Technology of the Government of India (to Dr. Dibyendu Nandi). The work at collaborating
institutions (Montana State University and the Harvard-Smithsonian Center
for Astrophysics) was supported by a NASA Living With a Star grant.
Note
Dibyendu Nandi publishes with his last name spelt as “Nandy”.
Website: http://www.iiserkol.ac.in/~dnandi
Email: dnandi@iiserkol.ac.in; dnandi@cfa.harvard.edu
Phone: +91-974-860-6215 (GMT + 05.30 HRS)
