At the Earth's surface, these tempests can be detected by changes in the strength of the Earth's magneticfield.
在地球表面，可以通过改变地球磁场强度来检测这些风暴。
2
Recently, they were packaged with fiber optics for detecting the light signals that register magneticfieldstrength.
目前，它们被封装进检测光信号的光学器件，以记录磁场的强度。
3
It detects magnetic signals that are produced by the electrical activity of brain cells—and which have about a billionth of the strength of the Earth's magneticfield.