Polar Field Reversal as observed with Hinode
19 April 2012
Naional Astronomical Observatory of Japan (NAOJ/NINS)
Japan Aerospace Exploration Agency(JAXA)
National Aeronautics and Space Administration (NASA)
Science and Technology Facilities Council(STFC)
European Space Agency(ESA)
The polarity of the extended uni-polar magnetic field in the solar polar region is known to reverse every 11 years, and the reversal occurs at around the maximum of solar activity, which is expected to take place at around 2013 May (NOAA ).
High latitude magnetic fields have been observed with solar telescopes on the ground. However, actual process of the polar field reversal is poorly understood because of the difficulty of the observations on the extreme limb combined with atmospheric seeing effect. The solar optical telescope aboard the Hinode satellite allows us for the first time to perform extremely high-quality observations of the deep polar region of the Sun (Figure A). The initial discoveries include that there are many magnetic patches with intense magnetic field in the polar regions. Their field strength is close to that of sunspots, and their size is as large as small sunspots called pore.
The international research team led by Saku Tsuneta, a professor at NAOJ, has been performing the monthly polar observations with Hinode from September 2008. We here report the discovery that the average magnetic flux of the north polar region is rapidly and steadily decreasing during the period of 2008 and 2012 (Figure B). The reversal (from minus to plus polarity) is taking place in sequence from lower latitude to higher latitude. The average magnetic flux of the polar region soon becomes zero. The estimated completion of the reversal of the north polar region will take place in 1 months or so, about one year earlier than the nominal expected reversal time.
In striking contrast to the north polar situation, the magnetic flux of the south polar region has been very stable, and maintains the plus polarity (Figure C). These latest Hinode observations suggest that the global magnetic field of the Sun will become different from the normal bipolar configuration.
Observations of the polar magnetic fields are the key for understanding the cyclic solar dynamo. Their results will shed light on the origin of the solar magnetism, and will contribute to our understating on the Sun's effect to the solar-terrestrial environment.
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