E se le macchie solari sparissero?

Il sito Spaceweather ha appena pubblicato un approfondimento della notizia relativa a uno studio sulla variabilità magnetica delle macchie solari, le quali, secondo due astrofisici del National Solar Observatory di Tucson, stanno perdendo visibilmente intensità. Il sito è andato a chiedere direttamente il parere dei due studiosi, che ammettono che a fronte di una simile perdita di intensità non è del tutto da escludere l'ipotesi che le macchie solari, che sono appunto legate all'intensità dei campi magnetici, spariscano quasi completamente. Ci sono insomma condizioni solari compatibili con il celebre minimo di Maunder, che iniziò nel 1645 e si prolungò per oltre 70 anni. Altri esperti ritengono che il sole si è semplicemente addormentato un po', che entro un anno tutto ripartirà.
Un paio di giorni prima il sito aveva pubblicato un altro articolo che fa riflettere sulle possibile conseguenze di questo profondo minimo solare sul sistema geomagnetico. Il 2 settembre 1859 durante un massimo a bassa intensità come quello che si prevede nei prossimi 3 o 4 anni, fu registrata a terra una tempesta geomagnetica di proporzioni colossali, con l'aurora boreale visibile addirittura a Cuba. Il giorno prima l'astronomo dilettante Richard Carrington aveva avvistato e descritto un potentissimo brillamento sulla superficie del sole, in corrispondenza di un esteso gruppo di macchie. A terra, i fili dei telegrafi si caricarono al punto da provocare scintille, danneggiando gli apparecchi riceventi e addirittura infiammando le strisce di carta degli inchiostratori. La morale? Che anche un minimo solare o un ciclo a bassa intensità possono fare da sfondo a tempeste geomagnetiche di grandi proporzioni. Anche dal punto di vista propagative, la mia esperienza di cicli solari e correlazioni con la ricezione di segnali lontani mi induce a credere che non si debba temere un blackout delle onde corte o delle aperture in E sporadico in FM. In questo fine settimana sentivo cose molto interessanti, incluso il servizio per le forze armate americane ripetuto dalla base nel Pacifico alle isole Hawaii, e molti segnali interessanti nelle bande alte radioamatoriali. La stagione dell'E sporadico non è stata eccezionale, ma sembra ormai chiaro che il fenomeno non è fortemente correlato all'andamento del ciclo.

Are Sunspots Disappearing?
09.03.2009

September 3, 2009: The sun is in the pits of the deepest solar minimum in nearly a century. Weeks and sometimes whole months go by without even a single tiny sunspot. The quiet has dragged out for more than two years, prompting some observers to wonder, are sunspots disappearing?
"Personally, I'm betting that sunspots are coming back," says researcher Matt Penn of the National Solar Observatory (NSO) in Tucson, Arizona. But, he allows, "there is some evidence that they won't."
Penn's colleague Bill Livingston of the NSO has been measuring the magnetic fields of sunspots for the past 17 years, and he has found a remarkable trend. Sunspot magnetism is on the decline: "Sunspot magnetic fields are dropping by about 50 gauss per year," says Penn. "If we extrapolate this trend into the future, sunspots could completely vanish around the year 2015."
This disappearing act is possible because sunspots are made of magnetism. The "firmament" of a sunspot is not matter but rather a strong magnetic field that appears dark because it blocks the upflow of heat from the sun's interior. If Earth lost its magnetic field, the solid planet would remain intact, but if a sunspot loses its magnetism, it ceases to exist.
"According to our measurements, sunspots seem to form only if the magnetic field is stronger than about 1500 gauss," says Livingston. "If the current trend continues, we'll hit that threshold in the near future, and solar magnetic fields would become too weak to form sunspots."
"This work has caused a sensation in the field of solar physics," comments NASA sunspot expert David Hathaway, who is not directly involved in the research. "It's controversial stuff."
The controversy is not about the data. "We know Livingston and Penn are excellent observers," says Hathaway. "The trend that they have discovered appears to be real." The part colleagues have trouble believing is the extrapolation. Hathaway notes that most of their data were taken after the maximum of Solar Cycle 23 (2000-2002) when sunspot activity naturally began to decline. "The drop in magnetic fields could be a normal aspect of the solar cycle and not a sign that sunspots are permanently vanishing."
Penn himself wonders about these points. "Our technique is relatively new and the data stretches back in time only 17 years. We could be observing a temporary downturn that will reverse itself."
The technique they're using was pioneered by Livingston at the McMath-Pierce solar telescope near Tucson. He looks at a spectral line emitted by iron atoms in the sun's atmosphere. Sunspot magnetic fields cause the line to split in two—an effect called "Zeeman splitting" after Dutch physicist Pieter Zeeman who discovered the phenomenon in the 19th century. The size of the split reveals the intensity of the magnetism.
Astronomers have been measuring sunspot magnetic fields in this general way for nearly a century, but Livingston added a twist. While most researchers measure the splitting of spectral lines in the visible part of the sun's spectrum, Livingston decided to try an infra-red spectral line. Infrared lines are much more sensitive to the Zeeman effect and provide more accurate answers. Also, he dedicated himself to measuring a large number of sunspots—more than 900 between 1998 and 2005 alone. The combination of accuracy and numbers revealed the downturn.
If sunspots do go away, it wouldn't be the first time. In the 17th century, the sun plunged into a 70-year period of spotlessness known as the Maunder Minimum that still baffles scientists. The sunspot drought began in 1645 and lasted until 1715; during that time, some of the best astronomers in history (e.g., Cassini) monitored the sun and failed to count more than a few dozen sunspots per year, compared to the usual thousands.
"Whether [the current downturn] is an omen of long-term sunspot decline, analogous to the Maunder Minimum, remains to be seen," Livingston and Penn caution in a recent issue of EOS. "Other indications of solar activity suggest that sunspots must return in earnest within the next year."
Whatever happens, notes Hathaway, "the sun is behaving in an interesting way and I believe we're about to learn something new."

GEOMAGNETIC MEGA-STORM: On Sept. 2nd, a billion-ton coronal mass ejection (CME) slammed into Earth's magnetic field. Campers in the Rocky Mountains woke up in the middle of the night, thinking that the glow they saw was sunrise. No, it was the Northern Lights. People in Cuba read their morning paper by the red illumination of aurora borealis. Earth was peppered by particles so energetic, they altered the chemistry of polar ice.
Hard to believe? It really happened--exactly 150 years ago. This map shows where auroras were sighted in the early hours of Sept. 2, 1859:
As the day unfolded, the gathering storm electrified telegraph lines, shocking technicians and setting their telegraph papers on fire. The "Victorian Internet" was knocked offline. Magnetometers around the world recorded strong disturbances in the planetary magnetic field for more than a week.
The cause of all this was an extraordinary solar flare witnessed the day before by British astronomer Richard Carrington. His sighting marked the discovery of solar flares and foreshadowed a new field of study: space weather. According to the National Academy of Sciences, if a similar flare occurred today, it would cause $1 to 2 trillion in damage to society's high-tech infrastructure and require four to ten years for complete recovery.
A repeat of the Carrington Event seems unlikely from our low vantage in a deep solar minimum--but don't let the quiet fool you. Strong flares can occur even during weak solar cycles. Indeed, the Carrington flare itself occured during a relatively weak cycle similar to the one expected to peak in 2012-2013. Could it happen again? Let's hope not.