Space Weather

Preamble

1. What is a solar activity "grand minimum"? 
2. Absence of renewed "torsional oscillation"
3. Weak "rush to the poles" phenomenon
4. Long-term weakening trend in the magnetic field strength of sunspots 

This article is a sequel of the one titled "The latest prediction of solar activity" in the last e-bulletin. Although the activity of the current Solar Cycle 24 is in its rising trend and the solar maximum is expected to occur in 2013, scientists have already hinted what the scenario could be after the current Cycle. This article will give an overview of what might happen then.

 

1. What is a solar activity "grand minimum"? 
   
   A grand minimum is an exceptional extended period of unusually low solar activity.  In June 2011, some solar physicists (Hill et al., Altrock, Livingston et al.) from the American Astronomical Society predicted that the sun might enter a grand minimum at the end of the current solar cycle 24.  Their respective findings are based on the study of "torsional oscillation", "rush to the poles" phenomenon and the long-term trend of sunspots magnetic field strength, as explained below.
   
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2. Absence of renewed "torsional oscillation"
   
   There are zonal (west-east) "jet streams" that rotate at a high speed around the sun.  They initially form at the mid-latitudes during the solar minimum of a solar cycle.  Then the "jet streams" migrate towards the equator and poles as the cycle progresses to the solar maximum (Fig.1).  Such patterns of migrating zonal "jet streams" are called "torsional oscillations".  Sunspots appear to emerge near the branches of jet streams that move from higher latitudes towards the equator, forming the sunspot migration patterns in the so-called Butterfly Diagram (Fig.2).
   
   The formation of zonal "jet streams" at the mid-latitudes is expected to recur in the current solar cycle 24.  However, so far no new poleward flow of "jet streams" have been observed at the mid-latitudes about two years after the solar minimum of solar cycle 23 in 2008.  While a solar minimum has been expected to occur between the end of solar cycle 24 and the start of solar cycle 25, Hill et al. opined that in view of such delayed or missing jet stream formation, the start of solar cycle 25 might be delayed to 2021 or 2022, or even later, hence giving rise to a prolonged period of low solar activity, i.e. the solar activity "grand minimum". 
   
    
   Fig. 1: A schematic diagram showing the migration of "jet streams" towards the solar equator and poles.
   
    
   Fig. 2: The Butterfly Diagram in March 2012 (upper panel) and the corresponding average daily sunspot area coverage (lower panel).  (Image source: Courtesy of NASA)
   
   
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3. Weak "rush to the poles" phenomenon
   

   The two solar magnetic poles reverse in a cycle of 11 years (see an article of "HKO e-Bulletin on Educational Resources", Issue No.26, June 2010).  During a typical cycle, new solar activity usually occurs at around 70-degree latitude at the beginning of a solar cycle.  The formation of sunspots will gradually move towards the equator later in the cycle, consistent with the Butterfly Diagram (Fig.2).  At the same time, magnetic fields of the new cycle push the magnetic remnant activities of the previous cycle towards the poles as far as 85-degree latitude.  This is called the "rush to the poles" phenomenon.
   
   Iron ions are easily concentrated by magnetism rising from the solar interior.  The coronal features displaying regional brightness, called "emission features", form in the light emitted by highly ionized iron (Fe XIV) in the solar corona.  They are related to the strong magnetic structure inside the sun, reflecting changes in the magnetic fields within the solar interior.  The "emission features" start to appear near 55-degree latitude and take three or four years to reach the poles.  The maximum of solar activity tends to occur about 14 months before the "emission features" reach and disappear at the poles, at an average latitude of 76 degrees as deduced from "rush to the poles" in previous solar cycles (Fig.3).

   
   
    
   Fig. 3: A simplified schematic diagram showing the regions of coronal "emission features of highly ionized iron (Fe XIV)".
   
   

   By analyzing and tracing the regions of these coronal "emission features", the measured parameter was found to be well below the annual average value, indicating that the solar maximum of solar cycle 24 would be weak and would occur in 2013. Altrock also found that the "rush to the poles" phenomenon was delayed and weak in the current solar cycle 24. Hence, the remnant magnetic field of solar cycle 23 might still persist near the polar regions, creating uncertainty for subsequent solar activities.

   
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4. Long-term weakening trend in the magnetic field strength of sunspots 
   
   Livngston et al. analyzed the magnetic field strength at the darkest position in the umbrae of sunspots, and a long-term weakening trend in the average magnetic field strength is observed. Extrapolating the trend suggests that sunspots are not likely to form by solar cycle 25 at the time between 2021 and 2022.
   
   
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   References:

1. This link will open in a new windowAmerican Astronomical Society (AAS) press release of 14 June 2011.
2. This link will open in a new windowFull abstract book for the 2011 Meeting on 12-16 June 2011 of the American Astronomical Society, Solar Physics Division.
3. "Large-Scale Zonal Flows During the Solar Minimum -- Where Is Cycle 25?" by Frank Hill, R. Howe, R. Komm, J. Christensen-Dalsgaard, T.P. Larson, J. Schou & M. J. Thompson, presented at the Annual Meeting of the Solar Physics Division of the American Astronomical Society, New Mexico State University, Las Cruces, NM, June 13-16, 2011.
4. "A Decade of Diminishing Sunspot Vigor", by W. C. Livingston, M. Penn & L. Svalgard, presented at the Annual Meeting of the Solar Physics Division of the American Astronomical Society, New Mexico State University, Las Cruces, NM, June 13-16, 2011.
5. "Whither Goes Cycle 24? A View from the Fe XIV Corona", by R. C. Altrock, presented at the Annual Meeting of the Solar Physics Division of the American Astronomical Society, New Mexico State University, Las Cruces, NM, June 13-16, 2011.
6. "The relationship between sunspot activity and the reversal of the sun's magnetic polarity", by CHIU Hung-yu, HKO e-Bulletin on Educational Resources, Issue No.26 (June 2010).
7. "This link will open in a new windowNew Method for Determining the Time and Amplitude of Solar Maximum", by Dick Altrock, Todd Brown, Joe Elrod, John Cornett, Tim Henry, NOAO (National Optical Astronomy Observatory) Newsletter - National Solar Observatory - March 1999 - Number 57.
8. "This link will open in a new windowThe Sunspot Cycle (Updated 2012/3/2)", Marshall Space Flight Center, NASA.
9. "The latest prediction of solar activity", by CHIU Hung-yu, HKO e-Bulletin on Educational Resources, Issue No.32 (December 2011). 

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