All Has Been Quiet
on the Sun. So Where Is the Roar?
【By KENNETH CHANG／陳世欽譯】
This is the height
of the 11-year solar cycle, the so-called solar maximum. The face of the Sun should
be pockmarked with sunspots, and cataclysmic explosions of X-rays and particles
should be whizzing off every which way. Instead, the Sun has been tranquil,
almost spotless.
As W. Dean
Pesnell, a scientist with NASA, the American space agency, dryly noted, “We’re
not having much of a solar maximum.”
On September 16, a
solitary sunspot blemished an otherwise blank yellow disk. In the ensuing days,
a few more specks appeared, but even a small explosion, or coronal mass
ejection, on September 19, seemed like the halfhearted effort of a slacker
star.
One of the worries
in our highly technological 21st-century civilization is that a direct hit on
Earth by a gargantuan solar storm could disable satellites and overwhelm wide
swaths of power grids. A quiet Sun makes that much less likely.
Scientists do have
a basic understanding of the dynamics of the Sun. Inside the Sun, flows of
electrons and protons generate magnetic fields that undulate on roughly an
11-year schedule. The roiling of the fields creates regions that are cooler and
darker — sunspots. The twisting magnetic fields within sunspots periodically
snap, releasing enormous amounts of energy in solar flares and coronal mass
ejections.
But some solar
cycles are ferocious while others remain calm. Why the cycle is 11 years is
another mystery.
This cycle, Number
24 since scientists started counting, has been befuddling from the start. Some
expected an active cycle, similar to the ones of the recent past. Others
predicted that this one would be quieter than usual; those predictions looked
prescient as the lull of solar minimum stretched longer. In 2008, the Sun was
spotless on 266 days — the blankest in half a century. The following year, when
the percolating of sunspots should have picked up, the Sun was blank for 260
days.
How far back do
scientists have to look to find a solar maximum quite as weak? Cycle 14 in the
early 1900s was similarly quiet. This time, solar scientists have Sun-watching
satellites providing data for them to analyze.
Despite the
minimal sunspots, the Sun is still going through the rest of its cycle as
usual. Its magnetic field is on the cusp of flipping, as expected. At solar
maximum, the magnetic fields at the poles essentially disappear for a brief
time, and when they reemerge, they are pointing in the opposite direction.
If you had a
compass on the Sun’s north pole and it were pointing north before solar
maximum, it would be pointing south after solar maximum. The north pole has
already flipped; the south pole is behind but in August scientists at Stanford
University in California said they expected the transition to be complete soon.
“We do see
indications that solar maximum is about now,” Dr. Pesnell said.
中譯
太陽已進入11年循環的高峰，亦即太陽活躍期。表面應該出現一些太陽黑子，駭人的X射線及粒子也該往各方疾速噴發。然而太陽卻一直很平靜，幾乎沒有黑子。
美國太空總署科學家裴斯尼爾淡淡的說：「看不到什麼太陽活躍狀態。」
9月16日，這塊空白黃色圓盤表面出現一個黑子。接下來幾天，幾個黑子陸續出現，然而9月19日的小爆發，或稱日冕物質拋射，卻無甚可觀，有辱其威名。
21世紀文明高度科技化，隱憂之一是，強大的太陽風暴直撲地球，可能癱瘓人造衛星與範圍廣大的電力輸送網。太陽平靜使這種可能性大為降低。
科學家對太陽的動力原理有基本瞭解。在太陽內部，電子與質子流動產生磁場，循環周期大約是11年。磁場攪動會產生溫度較低、較暗的區域，即太陽黑子。黑子內彎曲的磁場會周期性斷裂，以太陽閃焰與日冕物質拋射的形式釋出巨大能量。
然而太陽周期時而極猛，時而平靜。周期為何是11年也是個謎。
目前的周期是科學家開始計算以來的第24個，一開始就令科學家困惑。部分科學家預期它會跟前幾次一樣相當活躍，有些科學家則認為這次會比較平靜。後者似乎有先見之明，因為太陽的安靜期持續得比較久。2008年，太陽無黑子的時間共266天，是半世紀以來最久的一次。翌年，太陽黑子應該加速出現，卻也空白了260天。
科學家須回溯多久才能發現同樣無力的太陽活躍期？1900年代初的第14周期呈現類似的安靜狀態。這一次，科學家以太陽觀察衛星取得資料加以分析。
雖然黑子極少，太陽周期的剩餘階段還是照常進行。一如預期，它的磁場處於翻轉的高峰狀態。在太陽活躍期，兩極的磁場會短暫消失，再次出現時兩磁極互換。
把指南針放在太陽的北極，指南針在太陽進入活躍期之前指向北，則太陽活躍期過後它會改而指向南。北極已經翻轉，南極則落後，然而加州史丹福大學的科學家8月表示，整個轉換應該很快就會完成。
裴斯尼爾說：「我們看到的跡象確實顯示太陽處於活躍期。」
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