Le osservazioni eseguite dalla strumentazione della NASA
"CINDI" (Coupled Ion Neutral Dynamics Investigation)
a bordo del satellite dell'Aeronautica militare
americana C/NOFS hanno mostrato che il limite fra l'alta
atmosfera e lo spazio si è spostato a un livello
estremamente basso.
Il sistema CINDI-C/NOFS, progettato dall'Università
del Texas e lanciato nell'aprile di quest'anno, è stato
ideato per studiare i disturbi alla ionosfera terreste
che possono interferire con le comunicazioni e i segnali
GPS.
CINDI ha scoperto che la ionosfera non era dove
avrebbe dovuto essere: nel corso dei primi mesi della
sua missione ha infatti rilevato che la ionosfera si
trovava a 420 chilometri di altezza durante la notte e
a800 chilometri durante il giorno, laddove tipicamente
si situa, rispettivamente, a 640 e 960 chilometri.
Un certo abbassamento della ionosfera, dicono gli
scienziati, era atteso, dato che C/NOFS era stato
lanciato durante il minimo del ciclo undecennale di
attività solare, ma l'entità della contrazione ha
sorpreso i ricercatori.
Un'altra sorpresa è venuta dai dati rilevati da un
altra missione NASA, la THEMIS (Time History of
Events and Macroscale Interactions during Substorms),
che ha scoperto come il campo magnetico terrestre, che
funge da schermo contro le particelle provenienti dalle
tempeste solari, sviluppa spesso due "buchi" attraverso
i quali possono incunearsi grandi quantità di raggi
cosmici.
Contrariamente a quanto finora pensato, osservano i
ricercatori, "venti volte più particelle solari
attraversano lo scudo magnetico terrestre quando il
campo magnetico solare è allineato con quello della
Terra".
"La scoperta - ha detto Vassilis Angelopoulos
dell'Università della California a Los Angeles, che
sovrintende alla missione - capovolge una vecchia
convinzione sul modo e sui tempi in cui le particelle
possono penetrare il campo magnetico terrestre e potrà
essere sfruttata per prevedere quanto intense saranno le
tempeste solari".
Simulazioni al computer compiute da Wenhui Li
dell'Università del New Hampshire hanno infatti permesso
di comprendere il meccanismo con cui queste "porte si
aprono, in dipendenza dei cambiamenti di direzione del
campo magnetico solare durante i suoi cicli di attività,
fornendo le basi per previsioni sul livello di intensità
degli effetti al suolo delle tempeste solari".
(gg)
|
|
Earth's magnetic field, which shields our planet from particles
streaming outward from the Sun, often develops two holes that allow
the largest leaks, according to researchers sponsored by NASA and
the National Science Foundation.
"The discovery overturns a long-standing belief about how and when
most of the solar particles penetrate Earth's magnetic field, and
could be used to predict when solar storms will be severe. Based on
these results, we expect more severe storms during the upcoming
solar cycle," said Vassilis Angelopoulos of the University of
California, Los Angeles, Principal Investigator for NASA's THEMIS
mission (Time History of Events and Macroscale Interactions during
Substorms). THEMIS was used to discover the size of the leak.
Earth’s magnetic field acts as a shield against the bombardment of
particles continuously streaming from the sun. Because the solar
particles (ions and electrons) are electrically charged, they feel
magnetic forces and most are deflected by our planet's magnetic
field. However, our magnetic field is a leaky shield and the number
of particles breaching this shield depends on the orientation of the
sun’s magnetic field. It had been thought that when the sun’s
magnetic field is aligned with that of the Earth, the door is shut
and that few if any solar particles enter Earth’s magnetic shield.
The door was thought to open up when the solar magnetic field
direction points opposite to Earth’s field, leading to more solar
particles inside the shield.
Surprisingly, recent observations by the THEMIS spacecraft fleet
demonstrate that the opposite is true. "Twenty times more solar
particles cross the Earth’s leaky magnetic shield when the sun’s
magnetic field is aligned with that of the Earth compared to when
the two magnetic fields are oppositely directed," said Marit
Oieroset of the University of California, Berkeley, lead author of
one of two papers on this research, published May 2008 in
Geophysical Research Letters.
Researchers have long suspected that this "closed door" entry
mechanism might exist, but didn’t know how important it was. "It's
as if people knew there was a crack in a levy, but they did not know
how much flooding it caused," said Oieroset.
Previous spacecraft could only sample a small part of this enormous
layer of solar particles inside the Earth’s magnetic shield, but the
five spacecraft in the THEMIS fleet spanned the entire
rapidly-growing layer to give definitive measurements.
While the THEMIS researchers discovered the size of the leak, they
didn't know its location(s). This was discovered by Wenhui Li of the
University of New Hampshire, Durham, N.H., and his team. They used a
computer simulation to discover where two holes frequently develop
in Earth's magnetic field, one at high latitude over the Northern
hemisphere, and one at high latitude over the Southern hemisphere.
The holes form over the daylit side of Earth, on the side of the
magnetic shield facing the sun.
The simulation also showed how the leaks develop. As solar particles
flow out from the sun, they carry solar magnetic fields past our
planet. Li's team realized that the solar magnetic field drapes
against Earth's field as it passes by. Even though the two fields
point in the same direction at equatorial latitudes, they point in
opposite directions at high latitudes, When compression forces the
opposite fields together, they link up with each other in a process
called magnetic reconnection. This process tears the two holes in
Earth's magnetic field and appends the section of the solar field
between the two holes to Earth's field, carrying the solar particles
on this section into the magnetosphere, according to Li's team. "We've
found if the door is closed, the sun tears down a wall. The crack is
huge – about four times wider than Earth and more then seven Earth
diameters long," said Li, whose paper will be published in an
upcoming article of the Journal of Geophysical Research.
Solar particles by themselves don't cause severe space weather, but
they get energized when the solar magnetic field becomes
oppositely-directed to Earth's and reconnects in a different way.
The energized particles then cause magnetic storms that can overload
power lines with excess current, causing widespread blackouts. The
particles also can cause radiation storms that present hazards to
spacecraft in high orbits and astronauts passing through the storms
on the way to the moon or other destinations in the solar system.
"The more particles, the more severe the storm," said Joachim
"Jimmy" Raeder of the University of New Hampshire, a co-author of
Li's paper. "If the solar field has been aligned with the Earth's
for a while, we now know Earth's field is heavily loaded with solar
particles and primed for a strong storm. This discovery gives us a
basic predictive capability for the severity of solar storms,
similar to a hurricane forecaster's realization that warmer oceans
set the stage for more intense hurricanes. In fact, we expect
stronger storms in the upcoming solar cycle. The sun's magnetic
field changes direction every cycle, and due to its new orientation
in the upcoming cycle, we expect the clouds of particles ejected
from the sun will have a field which is at first aligned with Earth,
then becomes opposite as the cloud passes by." |
