那一天，太阳风消失了

The Day the Solar Wind Disappeared
那一天，太阳风消失了

—— @大杨   译自NASA/GSFC

Dec. 13, 1999: From May 10-12, 1999, the solar wind that blows constantly from the Sun virtually disappeared — the most drastic and longest-lasting decrease ever observed.

1999/12/13 ： 1999年5月10-12日，太阳风消失了 —— 有史以来最极端和最长时间的衰减。
 

Dropping to a fraction of its normal density and to half its normal speed, the solar wind died down enough to allow physicists to observe particles flowing directly from the Sun's corona to Earth. This severe change in the solar wind also changed the shape of Earth's magnetic field and produced an unusual auroral display at the North Pole.

密度降至非常小，速度减为平常的一半，太阳风衰减到物理学家们几乎无法观察日冕到地球的粒子流。 太阳风这次严重的变化改变了地球磁场的形状，导致北极点（！）不寻常的极光。

Starting late on May 10 and continuing through the early hours of May 12, NASA's ACE and Wind spacecraft each observed that the density of the solar wind dropped by more than 98%. Because of the decrease, energetic electrons from the Sun were able to flow to Earth in narrow beams, known as the strahl. Under normal conditions, electrons from the Sun are diluted, mixed, and redirected in interplanetary space and by Earth's magnetic field (the magnetosphere). But in May 1999, several satellites detected electrons arriving at Earth with properties similar to those of electrons in the Sun's corona, suggesting that they were a direct sample of particles from the Sun.

5月10日晚些时候开始持续到5月12日早上， NASA的ACE和Wind卫星各自探测到太阳风密度下降超过98%。因此，来自太阳的高能电子以细束状涌向地球，即所谓的“Strahl”（德语：束/柱）。一般情况下，来自太阳的电子是散淡的、混杂的，被行星际磁场和地球磁场（磁层）改变着方向。但是1999年5月，多个卫星探测到电子到达地球时的特性与太阳日冕中相似，可以视为太阳粒子的直接样本。
 

"This event provides a window to see the Sun's corona directly," said Dr. Keith Ogilvie, project scientist for NASA's Wind spacecraft and a space physicist at NASA's Goddard Space Flight Center, Greenbelt, Maryland. "The beams from the corona do not get broken up or scattered as they do under normal circumstances, and the temperature of the electrons is very similar to their original state on the Sun."

“这次事件提供了一个直接观察日冕的窗口，” Keith Ogilvie博士说。他是NASA Wind卫星项目科学家，NASA Goddard空间飞行中心（马里兰州，Greenbelt）空间物理学家。“来自日冕的电子束没有一般情况下的中断或发散，电子的温度也和在太阳的原始状态非常相似。”

"Normally, our view of the corona from Earth is like seeing the Sun on an overcast, cloudy day," said Dr. Jack Scudder, space physicist from the University of Iowa and principal investigator for the Hot Plasma Analyzer on NASA's Polar spacecraft. "On May 11, the clouds broke and we could see clearly."

“一般而言，从地球上看日冕，如同在阴天多云的日子看太阳，”，Jack Scudder博士说。他是Iowa大学空间物理学家、NASA Polar卫星高温等离子分析仪的研究员。“5月11日，云散了，我们看得很清楚。”

Fourteen years ago, Scudder and Dr. Don Fairfield of Goddard predicted the details of an event such as occurred on May 11, saying that it would produce an intense "polar rain" of electrons over one of the polar caps of Earth. The polar caps typically do not receive enough energetic electrons to produce visible aurora. But in an intense polar rain event, Scudder and Fairfield theorized, the "strahl" electrons would flow unimpeded along the Sun's magnetic field lines to Earth and precipitate directly into the polar caps, inside the normal auroral oval. Such a polar rain event was observed for the first time in May when Polar detected a steady glow over the North Pole in X-ray images.

14年前，Scudder博士和Goddard的Don Fairfield博士预言了类似5月11日事件的情形，认为会在南极或北极的极盖产生强电子“极雨”。正常情况下，极盖地区不能获得足够的高能电子，所以不会产生可见的极光。但是在强“极雨”的情形下，Scudder和Fairfield认为理论上这种电子束流将会不受阻碍地沿着太阳磁力线到达地球，直接下冲到极光卵内的极盖。Polar 5月X光成像探测到了北极点的稳定发光，这是“极雨”事件第一次被观测到。

In parallel with the polar rain event, Earth's magnetosphere swelled to five to six times its normal size. NASA's Wind, IMP-8, and Lunar Prospector spacecraft, the Russian INTERBALL satellite and the Japanese Geotail satellite observed the most distant bow shock ever recorded by satellites. Earth's bow shock is the shock front where the solar wind slams into the sunward edge of the magnetosphere.

与极雨事件同时，地球磁层膨胀了5-6倍。NASA的Wind、IMP-8和Lunar Prospector卫星，俄罗斯的INTERBALL卫星和日本的Geotail卫星都观测到了有卫星记录以来最远距离的弓震。

According to observations from the ACE spacecraft, the density of helium in the solar wind dropped to less than 0.1% of its normal value, and heavier ions, held back by the Sun's gravity, apparently could not escape from the Sun at all. Data from NASA's SAMPEX spacecraft reveal that in the wake of this event, Earth's outer electron radiation belts dissipated and were severely depleted for several months afterward.

根据ACE卫星的观测，太阳风中氦离子密度降到了0.1%以下，更重的粒子被太阳引力拽回，完全无法脱离太阳。NASA SAMPEX卫星揭示了这次事件的带来的后果：地球的外层电子辐射带（范艾伦带外层）在之后的几个月内耗散殆尽。
 

"The May event provides unique conditions to test ideas about solar-terrestrial interactions," Ogilvie noted. "It also strengthens our belief that we're beginning to understand how the Sun-Earth connection works."

“5月事件提供了唯一的一次验证太阳系相关活动的重要机会，”  Ogilvie说。“它也让我们对了解太阳-地球之间的关联更有信心。”

【原文出处：https://science.nasa.gov/science-news/science-at-nasa/1999/ast13dec99_1 】