For the second month in a row, sunspot counts in the sun's northern hemisphere are more than double the south. The asymetry is obvious in this summary of September's sunspots compiled by astronomer Senol Sanli using data from NASA's Solar Dynamics Observatory:
According to the Royal Observatory of Belgium's Solar Influences Data Analysis Center, in September the monthly sunspot number for the sun's northern hemisphere was 90, the southern hemisphere was only 44; that's a ratio of 2-to-1 in favor of the north. August was about the same. Looking back over an entire year, the north is leading the south by an average of 50%.
What's going on? In fact, it's not unusual for the sun's northern and southern hemispheres to be out of synch. As long ago as the 19th century, solar cycle pioneers Spoerer (1889) and Maunder (1890) noted that there were often long periods of time when most sunspots were found preferentially in one hemisphere and not the other. This plot from the Royal Observatory of Belgium shows assymetries throughout the last 6 solar cycles:
What's going on? In fact, it's not unusual for the sun's northern and southern hemispheres to be out of synch. As long ago as the 19th century, solar cycle pioneers Spoerer (1889) and Maunder (1890) noted that there were often long periods of time when most sunspots were found preferentially in one hemisphere and not the other. This plot from the Royal Observatory of Belgium shows assymetries throughout the last 6 solar cycles:
Until recently, Solar Cycle 25 was pretty evenly matched, north vs. south. Sunspot counts from August and September, however, suggest that the northern hemisphere may be seizing control--at least temporarily. This has happened during the upslope of all four previous solar cycles (21-24).
One possible explanation for this phenomenon may be that the two hemispheres of the sun have their own solar cycles, one out of phase with the other by about a year. Indeed, Solar Max is often double peaked. You can see it in the hemispheric sunspot plot. In the three most recent cycles (22-24), north peaked before south, creating two surges of solar activity separated by a "Gnevyshev gap." Solar Cycle 25 might continue this trend.
A complete discussion of sunspot asymmetries is included in David Hathaway's excellent review article "The Solar Cycle."
https://spaceweather.com/
One possible explanation for this phenomenon may be that the two hemispheres of the sun have their own solar cycles, one out of phase with the other by about a year. Indeed, Solar Max is often double peaked. You can see it in the hemispheric sunspot plot. In the three most recent cycles (22-24), north peaked before south, creating two surges of solar activity separated by a "Gnevyshev gap." Solar Cycle 25 might continue this trend.
A complete discussion of sunspot asymmetries is included in David Hathaway's excellent review article "The Solar Cycle."
https://spaceweather.com/
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