06 maggio 2008

1859, l'aurora boreale arriva a Kingston, Jamaica

Il primo settembre del 1859 l'astronomo inglese Richard Carrington stava studiando con l'aiuto del suo telescopio un imponente gruppo di macchie solare quando sulla superficie dell'astro cominciarono a formarsi due strane macchie bianche. Ancora più bianche della luce solare. Era l'inizio del "brillamento di Carrington", un'esplosione magnetica di inusitata potenza. Lo studio dei ghiacci perenni dell'Antartide permette di classificare il Carrington flare come il più intenso degli ultimi 500 anni. Quella stessa sera, dopo il tramonto, il cielo del mondo si illuminò come un lampione. L'aurora boreale venne osservata in Jamaica, a Cuba, alle Hawaii e sembra che alle latitudini più settentrionali la luminosità fosse così intensa da permettere di leggere il giornale. La tempesta elettrica associata al fenomeno mandò in tilt la giovane rete telegrafica mondiale. I rotoli di carta degli apparecchi presero fuoco, gli operatori riuscivano a trasmettere i messaggi anche dopo aver staccato le batterie che alimentavano le linee.
Raramente una storia letta sul sito che la NASA dedica allo space weather, al tempo geomagnetico, mi era parsa altrettanto avvincente. Giustamente gli autori si chiedono quale possa essere la probabilità che un Carrington flare si verifichi ancora (statisticamente sappiamo che potrebbe verificarsi tra 350 anni, ma anche in coincidenza col prossimo massimo solare). E quali potrebbero essere le conseguenze oggi, visto che non c'è solo il telegrafo in gioco. Secondo alcune stime, solo per i 900 satelliti e rotti in orbita potremmo avere danni compresi tra i 30 e i 70 miliardi di dollari, ma dopo il flop del baco del 2000 forse è meglio non sbilanciarsi con previsioni catastrofiche. Quel che è certo è che nel cielo di Milano ormai sarebbe molto difficile osservare un'aurora.

A Super Solar Flare


At 11:18 AM on the cloudless morning of Thursday, September 1, 1859, 33-year-old Richard Carrington—widely acknowledged to be one of England's foremost solar astronomers—was in his well-appointed private observatory. Just as usual on every sunny day, his telescope was projecting an 11-inch-wide image of the sun on a screen, and Carrington skillfully drew the sunspots he saw.
On that morning, he was capturing the likeness of an enormous group of sunspots. Suddenly, before his eyes, two brilliant beads of blinding white light appeared over the sunspots, intensified rapidly, and became kidney-shaped. Realizing that he was witnessing something unprecedented and "being somewhat flurried by the surprise," Carrington later wrote, "I hastily ran to call someone to witness the exhibition with me. On returning within 60 seconds, I was mortified to find that it was already much changed and enfeebled." He and his witness watched the white spots contract to mere pinpoints and disappear.
It was 11:23 AM. Only five minutes had passed.
Just before dawn the next day, skies all over planet Earth erupted in red, green, and purple auroras so brilliant that newspapers could be read as easily as in daylight. Indeed, stunning auroras pulsated even at near tropical latitudes over Cuba, the Bahamas, Jamaica, El Salvador, and Hawaii.
Even more disconcerting, telegraph systems worldwide went haywire. Spark discharges shocked telegraph operators and set the telegraph paper on fire. Even when telegraphers disconnected the batteries powering the lines, aurora-induced electric currents in the wires still allowed messages to be transmitted.
"What Carrington saw was a white-light solar flare—a magnetic explosion on the sun," explains David Hathaway, solar physics team lead at NASA's Marshall Space Flight Center in Huntsville, Alabama.
Now we know that solar flares happen frequently, especially during solar sunspot maximum. Most betray their existence by releasing X-rays (recorded by X-ray telescopes in space) and radio noise (recorded by radio telescopes in space and on Earth). In Carrington's day, however, there were no X-ray satellites or radio telescopes. No one knew flares existed until that September morning when one super-flare produced enough light to rival the brightness of the sun itself.
"It's rare that one can actually see the brightening of the solar surface," says Hathaway. "It takes a lot of energy to heat up the surface of the sun!"
The explosion produced not only a surge of visible light but also a mammoth cloud of charged particles and detached magnetic loops—a "CME"—and hurled that cloud directly toward Earth. The next morning when the CME arrived, it crashed into Earth's magnetic field, causing the global bubble of magnetism that surrounds our planet to shake and quiver. Researchers call this a "geomagnetic storm." Rapidly moving fields induced enormous electric currents that surged through telegraph lines and disrupted communications.
"More than 35 years ago, I began drawing the attention of the space physics community to the 1859 flare and its impact on telecommunications," says Louis J. Lanzerotti, retired Distinguished Member of Technical Staff at Bell Laboratories and current editor of the journal Space Weather. He became aware of the effects of solar geomagnetic storms on terrestrial communications when a huge solar flare on August 4, 1972, knocked out long-distance telephone communication across Illinois. That event, in fact, caused AT&T to redesign its power system for transatlantic cables. A similar flare on March 13, 1989, provoked geomagnetic storms that disrupted electric power transmission from the Hydro Québec generating station in Canada, blacking out most of the province and plunging 6 million people into darkness for 9 hours; aurora-induced power surges even melted power transformers in New Jersey. In December 2005, X-rays from another solar storm disrupted satellite-to-ground communications and Global Positioning System (GPS) navigation signals for about 10 minutes. That may not sound like much, but as Lanzerotti noted, "I would not have wanted to be on a commercial airplane being guided in for a landing by GPS or on a ship being docked by GPS during that 10 minutes."
Another Carrington-class flare would dwarf these events. Fortunately, says Hathaway, they appear to be rare: "In the 160-year record of geomagnetic storms, the Carrington event is the biggest." It's possible to delve back even farther in time by examining arctic ice. "Energetic particles leave a record in nitrates in ice cores," he explains. "Here again the Carrington event sticks out as the biggest in 500 years and nearly twice as big as the runner-up."
These statistics suggest that Carrington flares are once in a half-millennium events. The statistics are far from solid, however, and Hathaway cautions that we don't understand flares well enough to rule out a repeat in our lifetime.

And what then?

Lanzerotti points out that as electronic technologies have become more sophisticated and more embedded into everyday life, they have also become more vulnerable to solar activity. On Earth, power lines and long-distance telephone cables might be affected by auroral currents, as happened in 1989. Radar, cell phone communications, and GPS receivers could be disrupted by solar radio noise. Experts who have studied the question say there is little to be done to protect satellites from a Carrington-class flare. In fact, a recent paper estimates potential damage to the 900-plus satellites currently in orbit could cost between $30 billion and $70 billion. The best solution, they say: have a pipeline of comsats ready for launch.
Humans in space would be in peril, too. Spacewalking astronauts might have only minutes after the first flash of light to find shelter from energetic solar particles following close on the heels of those initial photons. Their spacecraft would probably have adequate shielding; the key would be getting inside in time.
No wonder NASA and other space agencies around the world have made the study and prediction of flares a priority. Right now a fleet of spacecraft is monitoring the sun, gathering data on flares big and small that may eventually reveal what triggers the explosions. SOHO, Hinode, STEREO, ACE and others are already in orbit while new spacecraft such as the Solar Dynamics Observatory are readying for launch.
Research won't prevent another Carrington flare, but it may make the "flurry of surprise" a thing of the past.

Authors: Trudy Bell & Dr. Tony Phillips | Editor: Dr. Tony Phillips | Credit: Science@NASA

more information

Description of a Singular Appearance seen in the Sun on September 1, 1859, Monthly Notices of the Royal Astronomical Society, Vol. 20, p.13-15 -- the original report by R.C. Carrington

An engaging book on the history of the 1859 Carrington flare and the detective work to sleuth its cause and significance is Stuart Clark's The Sun Kings: The Unexpected Tragedy of Richard Carrington and the Take of How Modern Astronomy Began (Princeton University Press, 2007).

One recent analysis on the effects of a potential future solar flare of similar magnitude is "The Carrington event: Possible doses to crews in Space from a comparable event," by L. W. Townsend et al., Advances in Space Research 38 (2006): 226–231--one of 16 articles in an entire special issue devoted to the 1859 Carrington flare.

See also "The 1859 Solar–Terrestrial Disturbance and the Current Limits of Extreme Space Weather Activity," by E. W. Cliver and L. Svalgaard, Solar Physics (2004) 224: 407–422 (available at ) and "Forecasting the impact of an 1859-caliber superstorm on geosynchronous Earth-orbiting satellites: Transponder resources," by Sten F. Odenwald and James L. Green, Space Weather (2007) 5: 1-16.

NASA is well aware of radiation hazards in space and taking mitigation measures. A book-length report on a 2005 workshop exploring the subject is Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop published by the National Research Council in 2006.

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