Les dejo esto por aquí, de interés:
https://www.facebook.com/photo.php?fbid=467684236612138&set=a.118111491569416.9250.118107518236480&type=1&theaterOne of our friends on here (hello Maria) asked us a question yesterday. She wanted to know if the Sun is actually entering a deep minimum, as was the famous Maunder Minimum.
So I asked one of our solar scientists, Dr. Leif Svalgaard to give us his take. Enjoy a deeper look into solar variability.
The Maunder Minimum is an example of a Grand Solar Minimum. Solar activity modulates the number of Galactic Cosmic Rays reaching the inner solar system and the Earth. Records in ice cores and tree-rings of the level of radioactive nuclei created by Galactic Cosmic Rays bombarding the Earth’s atmosphere have been interpreted to suggest that such Grand Minima occur from time to time every several hundred years but without any established periodicity, so it is not clear that we are ‘due’ for another one. In addition, solar activity has varied with an apparent 100-yr ‘cycle’ the past 300 years with low sunspot numbers around the beginning of each century.
For example, solar activity right now in solar cycle 24 is on par what it was in cycle 14, a bit more than a century ago [the approximately 11-yr solar cycles are numbered, with number 1 starting in 1756 from when reasonably reliable estimates are available].
We do not know the cause of these longer cycles so they are difficult to predict. Current understanding, such as it is, can with some confidence predict one cycle ahead, based on observations of the strength of the Sun’s magnetic field at its poles and the current cycle 24 was indeed [with some success] predicted to be on the low side. Judging from the past, we can extrapolate that the next few cycles might also be low, but that is not really a prediction, just a guess.
The Maunder Minimum was an extended period [1645-1715] when very few sunspots were observed, although astronomers kept a watchful eye on the Sun throughout that time. On the other hand, the ice cores show that solar modulation of cosmic rays on a time scale of the usual sunspot cycle continues during Grand Minima.
The Solar dynamo that creates solar activity does not shut down and a significant solar magnetic field persisted, so the solar wind modulating cosmic rays was present as we also know from observations of comet tails during that time. This suggests that Grand Minima are times when solar activity behaves qualitatively differently than during more ‘normal’ times.
Scientists are usually a conservative lot and take a dim view of such speculation involving ‘unusual’ conditions. On the other hand, there are indications that the Sun may be entering a different ‘regime’ with fewer visible sunspots for a given amount of overall magnetic flux. Over the past decade the sunspot number has been steadily decreasing compared to what we expected from observations of the magnetic flux, the F10.7 microwave flux, the ultraviolet flux, and even the total solar irradiance. The number of sunspots per active region and the number of small spots generally are also decreasing. The magnetic field in sunspots is decreasing, the spots are getting warmer, and their visibility is decreasing. All these indications are unprecedented in the solar record of the past many well-observed solar cycles, and we have no good explanation for this. Should this trend continue, it may signal a descent into another Grand Minimum [if so, we have already a name for it: “the Eddy Minimum” – after the late Jack Eddy who drew attention to the importance of Grand Minima in a famous 1976 paper].
Sunspots form by assembly or compacting of many small magnetic elements into stronger magnetic regions, which when strong enough interfere with the transport of heat from the deeper layers and thus effectively cool the photosphere making the region appear as a ‘dark’ sunspot [it is only dark by contrast to the brighter surroundings – in fact, if we take the smallest sunspot that we can see and remove all the rest of the Sun, leaving only that tiny spot in the sky it would shine brighter than the full Moon].
So, one hypothesis might be that the Maunder Minimum was not a serious deficit of magnetic flux, but a lessening of the efficiency of the process compacting magnetic fields into visible spots. This may now be happening again. If so, there is new solar physics to be learned and solar physics will become even more exciting; let us not shy away from that!
Captions for Figure panels
A: Ratio of sunspots observed and those expected from F10.7 cm microwave flux from the Sun
B: Ratio of sunspots observed and those expected from the Total Magnetic Field in plages
C: Intensity of darkest point in sunspots compared to surrounded photosphere outside the spot [when equal to one, the spot is not visible]
D: The number of sunspots per sunspot group
E: The magnetic field of the darkest point in sunspots
F: The percentage of small sunspot groups of all groups
G: Modulation of cosmic ray intensity
H: The red flash of the chromosphere seen at eclipse
I: The tails of the great comet of 1680
Credit: Dr. Leif Svalgaard
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