英语角

何处是观看极光的最佳地点?

英文原文

I understand that the Northern Lights activity will be starting to peak this fall. What are the best places to see them? I would like to make a vacation out of it.

John Paul Pezzulo

Yonkers, New York

I thank Chris Balch of NOAA's Space Environment Center (SEC) in Boulder, Colorado, for his help in answering this question.

The solar maximum, which occurs roughly every 11 years, marks a peak in the number of sunspots visible on the surface of the sun (which means a peak in fluctuations in the sun's output). These fluctuations, called interplanetary transients, can transfer energy to Earth's magnetic fields, causing geomagnetic storms that can lead to spectacular auroral displays.

As charged particles are energized by the disturbed geomagnetic fields, they are accelerated toward the polar regions, where they collide with neutral atoms of oxygen and nitrogen and cause them to fluoresce (emit colored light) at altitudes between 60 and 250 miles. The next solar maximum is approaching and will occur sometime during 2000 or 2001.

The aurora does not occur everywhere in the polar regions but rather in an oval band centered on the magnetic pole and generally between 60° and 70° latitude in each hemisphere. Because the intensity of light in an aurora is low, it can be seen only at night.

Since 1978, NOAA's polar-orbiting satellites have monitored the energy flux of particles into the auroral zones. The energy flux is the amount of energy passing through a given area over a given time. At least every two hours, the SEC produces maps of this energy flux, which is statistically related to the presence of auroras.

Where the energy flux is relatively high (orange and red colors), auroras are most likely. The map in Figure 1 is created from such data and shows the statistically probable location of the auroral oval for high northern latitudes on September 8, 1999. The yellow arrow pointing toward the lower right from the magnetic pole denotes the meridian of local noon.

In the Northern Hemisphere, the magnetic pole is near the northwest tip of Greenland. The auroral oval rotates clockwise (relative to the earth) around the magnetic pole as the time of day progresses. The larger energy fluxes (and the auroras) favor the night side of the hemisphere. Note that at high northern latitudes near the time of the summer solstice, nights are very short or nonexistent. For frequent viewing of auroras under normal conditions, therefore, you have to be willing to endure cold, long winter nights at high latitudes.

During the solar maximum, the increased likelihood of geomagnetic storms favors auroral sightings at lower latitudes. A strong geomagnetic storm causes large fluctuations in Earth's magnetic field and expansion of the auroral oval into the mid-latitudes. A storm typically lasts from 24 to 36 hours. The SEC foresees the possibility of an aurora being sighted as far south as the Gulf Coast states at least once during the current solar maximum.

A globally-averaged measure of the strength of fluctuations in the Earth's magnetic field is called the Kp Index. It ranges in value from one to nine. In June 1968, Sky d Telescope magazine published a map giving a general indication of the southern boundary of the auroral oval for different values of the Kp Index. (See Figure 2.) When Kp=9 during a severe geomagnetic storm, the aurora can be seen overhead as far south as the mid-Mississippi Valley to the Mid-Atlantic states. I witnessed a brilliant aurora in St. Louis during the 1960s when the ground was snow-covered; the memory is still vivid.

The SEC tracks the Kp Index daily. You can see values for the most recent three days along with other parameters at their Web site <www.sec.noaa.gov/today.html>. If you want today's location of the auroral oval (a statistical estimate), go to: <sec. noaa.gov/p map/pmapN.html>.

Prediction of individual geomagnetic storms is still in its infancy, and so it is inadvisable to plan a long trip in anticipation of seeing the aurora, unless you want to spend time at 60° -70° N where auroras are most common. If you live near 40'N or farther north, odds are high that the aurora will come to you a few times during the solar maximum. This winter and the following two winters should be favorable. I wish you clear skies and the good luck to be watching the sky when the aurora appears.

For more information on auroras, see the feature article "Awesome Auroras" in the May/June 1999 Weatherwise.

参考译文：

纽约扬克斯的John Pezzulo问：“我听说今年秋天北极光强度将达到最大，观赏极光的最佳地点是哪里？我将就此安排休假。”答：首先对科罗拉多波尔德NOAA空间环境中心的Chris Balch在回答此问题时所给予的帮助表示感谢。

大约每11年出现一次的太阳黑子极大标志着太阳表面上可见的太阳黑子数达到峰值（这也意味着太阳输出扰动中的一个峰值）。这些被称为星际间瞬态的扰动能向地球磁场传输能量，引起了能产生千姿百态极光的地磁暴。

由于受扰动的地球磁场使荷电粒子加强，令它们加速向极区移去，在60-250英哩高度上，与中性的氧原子和氮原子碰撞时，使它们发萤光（发出彩色光）。下一次太阳黑子极大将出现于2000年或2001年。

并不是极区的所有地方都能出现极光，而是出现在一个以磁极为中心的卵形带中，通常是两半球的60°-70°纬度区。由于极光的光强度很低，所以只能在夜间才能看见。

1978年以来，NOAA的极轨卫星一直对进入极光区的粒子能量通量进行检测。能量通量是一定时间内通过一定面积的能量的数量。空间环境中心至少每两小时制做一次能量图，能量通量与极光的存在具有统计相关。

在能量通量比较高的地方（图1中桔色和红色区域），最可能出现极光。图1就是根据这些资料制作的，它给出了1999年9月8日北半球高纬统计上可能的极光卵形带的位置。从磁极指向右下角的黄色箭头标出了当地中午的子午线。

在北半球，磁极位于冰岛北端附近，随着一天中时间变化，极光卵形带围绕磁极顺时针（相对于地球）旋转。较大的能量通量（和极光）最容易出现在北半球的夜间。这里请注意，夏至左右在北半球高纬夜间很短或是没有夜间。所以，为了能在正常情况下更多地观看极光，你只得心干情愿地忍受寒冷，在高纬冬季的漫漫长夜中等候。

在太阳黑子极大时期，地磁暴发生的可能性增大有利于在较低纬度观赏极光。强的地磁暴能引起地球磁场大的扰动，并使极光卵形带向中纬扩展。一般情况下，一个地磁暴持续24-36小时。空间环境中心预报当太阳黑子极大时南至墨西哥湾各州至少能见到一次极光的可能性。

全球平均地球磁场扰动强度的量度尺度被称为Kp指数，它的值在1至9之间变化。1968年6月，Sky & Telescope杂志刊载了一张给出不同Kp指数情况下极光卵形带南边缘的图（图2）。一个强的地磁暴期间，当Kp＝9时，南至密西西比河谷到大西洋中部各卅都能看到极光。60年代我就在圣路易斯目睹了一次耀眼的极光，当时地面复盖着雪，至今记忆犹新。

空间环境中心记录每天的Kp指数，你能在它们的Web站点<www.sec.noaa. gov/today.html>上看到最近三天的Kp指数和其它参数。如果你想知道今天极光卵形带的位置，请去<sec. noaa.gov/p map/pmapN.html>。

对单个地磁暴的预报还不成熟，因此，不建议你仅根据观赏极光预报就安排一次长途旅行，除非你愿意不惜在最易出现极光的60° -70° N地区消磨时光。如果你就住在40° N附近或更北一些的话，则在太阳黑子极大期间见到几次极光的机会还是很大的。今年冬天和下两个冬天应该是最好的时候。我希望届时能够晴空如洗，极光出现时你正举头观望，那真是再好不过了。

若想获得更多有关极光的信息，请参见1999年5月／6月的Weatherwise杂志上的文章“启人心扉的极光”。

（高良成）