* Astronomy

Hinode and SOHO Paint An Asymmetrical Picture Of the Sun

Approximately every 11 years the magnetic field on the sun reverses completely - the north magnetic pole switches to south, and vice versa. It's as if a bar magnet slowly lost its magnetic field and regained it in the opposite direction, so the positive side becomes the negative side. But, of course, the sun is not a simple bar magnet and the causes of the switch, not to mention the complex tracery of moving magnetic fields throughout the eleven-year cycle, are not easy to map out.
Mapping such fields, however, is a crucial part of understanding how - and, in turn, when - the sun will exercise its next flip. This flip coincides with the greatest solar activity seen on the sun in any given cycle, known as "solar maximum."
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Title: Variation in sunspot properties between 1999 and 2011 as observed with the Tenerife Infrared Polarimeter
Authors: R. Rezaei, C. Beck, W. Schmidt

We study the variation in the magnetic field strength and the umbral intensity of sunspots during the declining phase of the solar cycle no.23 and in the beginning of cycle no.24. We analyse a sample of 183 sunspots observed from 1999 until 2011 with the Tenerife Infrared Polarimeter at the German Vacuum Tower Telescope. The magnetic field strength is derived from the Zeeman splitting of the Stokes-V signal in one near-infrared spectral line, either Fe I 1564.8 nm, Fe I 1089.6 nm, or Si I 1082.7 nm. This avoids the effects of the unpolarised stray light from the field-free quiet Sun surroundings. The minimum umbral continuum intensity and umbral area are also measured. We find that there is a systematic trend for sunspots in the late stage of the solar cycle no.23 to be weaker, i.e., to have a smaller maximum magnetic field strength than those at the start of the cycle. The decrease in the field strength with time of about 94 G/yr is well beyond the statistical fluctuations that would be expected because of the larger number of sunspots close to cycle maximum (14 G/yr). In the same time interval, the continuum intensity of the umbra increases with a rate of 1.3 (± 0.4)% of Ic/yr, while the umbral area does not show any trend above the statistical variance. Sunspots in the new cycle no.24 show higher field strengths and lower continuum intensities than those at the end of cycle no.23, interrupting the trend. Sunspots have an intrinsically weaker field strength and brighter umbrae at the late stages of solar cycles compared to their initial stages, without any significant change in their area. The abrupt increase in field strength in sunspots of the new cycle suggests that the cyclic variations are dominating over any long-term trend that continues across cycles. We find a slight decrease in field strength and an increase in intensity as a long-term trend across the cycles.

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Title: Precursor of Sunspot Penumbral Formation discovered with Hinode SOT Observations
Authors: Toshifumi Shimizu, Kiyoshi Ichimoto, Yoshinori Suematsu

We present observations of a precursory signature that would be helpful for understanding the formation process of sunspot penumbrae. The Hinode Solar Optical Telescope successfully captured the entire evolution of a sunspot from the pore to a large well-developed sunspot with penumbra in an emerging flux region appeared in NOAA Active Region 11039. We found an annular zone (width 3"-5") surrounding the umbra (pore) in Ca II H images before the penumbra is formed around the umbra. The penumbra was developed as if to fill the annular zone. The annular zone shows weak magnetogram signals, meaning less magnetic flux or highly inclined fields there. Pre-existing ambient magnetic field islands were moved to be distributed at the outer edge of the annular zone and did not come into the zone. There is no strong systematic flow patterns in the zone, but we occasionally observed small magnetic flux patches streaming out. The observations indicate that the annular zone is different from sunspot moat flow region and that it represents the structure in the chromosphere. We conclude that the annular zone reflects the formation of a magnetic canopy overlying the region surrounding the umbra at the chromospheric level, much before the formation of the penumbra at the photospheric level. The magnetic field structure in the chromosphere needs to be considered in the formation process of the penumbrae.

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Title: Distribution of the daily Sunspot Number variation for the last 14 solar cycles
Authors: Mihail-Ioan Pop

The difference between consecutive daily Sunspot Numbers was analysed. Its distribution was approximated on a large time scale with an exponential law. In order to verify this approximation a Maximum Entropy distribution was generated by a modified version of the Simulated Annealing algorithm. The exponential approximation holds for the generated distribution too. The exponential law is characteristic for time scales covering whole cycles and it is mostly a characteristic of the Sunspot Number fluctuations and not of its average variation.

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