Una strana smagliatura nella magnetosfera terrestre

Lo scorso anno le sonde della missione THEMIS con le loro misure hanno fatto una scoperta eccezionale proprio mentre stavano attraversando la linea di confine dove il vento solare interagisce con la magnetosfera terrestre. La missione THEMIS è stata concepita dalla NASA proprio per studiare questo tipo di interazioni, mettendo in evidenza i meccanismi che finiscono per generare fenomeni come la aurore boreali.
La scoperta consiste in una estesa smagliatura nel campo magnetico che avvolge la terra, un evento che ha stupito gli scienziati non tanto per il fatto in sé (queste smagliature sono all'ordine del giorno), ma per le modalità di comparsa. Finora si credeva che la magnetosfera si "allentasse" quando si trovava a interagire, nell'emisfero settentrionale, con un vento solare il cui campo puntasse verso sud. Questa regola, finora, era stata ferrea, ma le sonde THEMIS hanno misurato la smagliatura in una situzione del tutto opposta: con il campo sul fronte del vento solare che puntava a nord (la teoria voleva che un campo così orientato avrebbe dovuto rinforzare, non indebolire, il campo terrestre).
Quali conseguenze potrebbe avere questa scoperta? Quando nella magnetosfera si aprono questi squarci, il vento solare la carica come una pila. Una pila pronta a scatenare gli eventi aurorali quando il vento solare subisce le sue improvvise accelerazioni o in presenza di eventi coronali eiettivi. E' come se un forte colpo di frusta rompesse il vaso riempito di vento solare. Ora i fisici cominciano ad associare questa nuova osservazione con un'altra circostanza: durante i cicli solari di numero pari, come questo ciclo 24, il fronte del vento solare è orientato comunemente verso nord. Se un vento di questo tipo può "sfondare" facilmente la magnetosfera, dicono gli scienziati, ci si può aspettare eventi molto intensi nel corso delle tempeste che caratterizzeranno questo ciclo.

A Giant Breach in Earth's Magnetic Field

12.16.2008

Dec. 16, 2008: NASA's five THEMIS spacecraft have discovered a breach in Earth's magnetic field ten times larger than anything previously thought to exist. Solar wind can flow in through the opening to "load up" the magnetosphere for powerful geomagnetic storms. But the breach itself is not the biggest surprise. Researchers are even more amazed at the strange and unexpected way it forms, overturning long-held ideas of space physics.
"At first I didn't believe it," says THEMIS project scientist David Sibeck of the Goddard Space Flight Center. "This finding fundamentally alters our understanding of the solar wind-magnetosphere interaction."
The magnetosphere is a bubble of magnetism that surrounds Earth and protects us from solar wind. Exploring the bubble is a key goal of the THEMIS mission, launched in February 2007. The big discovery came on June 3, 2007, when the five probes serendipitously flew through the breach just as it was opening. Onboard sensors recorded a torrent of solar wind particles streaming into the magnetosphere, signaling an event of unexpected size and importance.
"The opening was huge—four times wider than Earth itself," says Wenhui Li, a space physicist at the University of New Hampshire who has been analyzing the data. Li's colleague Jimmy Raeder, also of New Hampshire, says "1027 particles per second were flowing into the magnetosphere—that's a 1 followed by 27 zeros. This kind of influx is an order of magnitude greater than what we thought was possible."
The event began with little warning when a gentle gust of solar wind delivered a bundle of magnetic fields from the Sun to Earth. Like an octopus wrapping its tentacles around a big clam, solar magnetic fields draped themselves around the magnetosphere and cracked it open. The cracking was accomplished by means of a process called "magnetic reconnection." High above Earth's poles, solar and terrestrial magnetic fields linked up (reconnected) to form conduits for solar wind. Conduits over the Arctic and Antarctic quickly expanded; within minutes they overlapped over Earth's equator to create the biggest magnetic breach ever recorded by Earth-orbiting spacecraft.

(a sinistra) A computer model of solar wind flowing around Earth's magnetic field on June 3, 2007; white arrows trace the extent of the breach. The model is based on actual measurements made by the THEMIS probes. Background colors represent solar wind density; red is high density, blue is low. Note the layer of relatively dense material beneath the tips of the white arrows; that is solar wind entering Earth's magnetic field through the breach.

The size of the breach took researchers by surprise. "We've seen things like this before," says Raeder, "but never on such a large scale. The entire day-side of the magnetosphere was open to the solar wind."
The circumstances were even more surprising. Space physicists have long believed that holes in Earth's magnetosphere open only in response to solar magnetic fields that point south. The great breach of June 2007, however, opened in response to a solar magnetic field that pointed north.
"To the lay person, this may sound like a quibble, but to a space physicist, it is almost seismic," says Sibeck. "When I tell my colleagues, most react with skepticism, as if I'm trying to convince them that the sun rises in the west."
Here is why they can't believe their ears: The solar wind presses against Earth's magnetosphere almost directly above the equator where our planet's magnetic field points north. Suppose a bundle of solar magnetism comes along, and it points north, too. The two fields should reinforce one another, strengthening Earth's magnetic defenses and slamming the door shut on the solar wind. In the language of space physics, a north-pointing solar magnetic field is called a "northern IMF" and it is synonymous with shields up!
"So, you can imagine our surprise when a northern IMF came along and shields went down instead," says Sibeck. "This completely overturns our understanding of things."
Northern IMF events don't actually trigger geomagnetic storms, notes Raeder, but they do set the stage for storms by loading the magnetosphere with plasma. A loaded magnetosphere is primed for auroras, power outages, and other disturbances that can result when, say, a CME (coronal mass ejection) hits.
The years ahead could be especially lively. Raeder explains: "We're entering Solar Cycle 24. For reasons not fully understood, CMEs in even-numbered solar cycles (like 24) tend to hit Earth with a leading edge that is magnetized north. Such a CME should open a breach and load the magnetosphere with plasma just before the storm gets underway. It's the perfect sequence for a really big event."
Sibeck agrees. "This could result in stronger geomagnetic storms than we have seen in many years."