近两年开始回答客人问题，开始关注北极光的信息。 常听客人说起他们做攻略，Kp值，什么极光大年小年之类， 我的反应是，“是吗？ 没听说过诶。” 这几天看阿拉斯加大学的地球物理研究所的网站，发现别人说的那些都在上面。
How often can I see aurora? 看到极光的机会是多少
VIEWING THE AURORA
- How often can I see aurora?
- There is always some aurora at some place on earth. When the solar wind is calm, the aurora might only be occurring at very high latitudes and might be faint, but there is still aurora. In order for us to see the aurora, however, the sky must be dark and clear. Sunlight and clouds are the biggest obstacles to auroral observations.
- Where is the best place to see aurora?
- What is the best time of day to see aurora?
- The best time to watch for aurora is the three or four hours around midnight, but aurora occurs throughout the night. Active auroral displays tend to be more diffuse and fragmented later in the night, which means that 9 p.m. to 3 a.m. is typically the time period with the highest probability of seeing spectacular auroral displays over interior Alaska in winter. Since clear sky and darkness are both essential to see aurora, the best time is dictated by the weather and by the sunrise and sunset times. The moon is also very bright and can make it more difficult to view the aurora, so lunar cycles should be taken into account.
- What is the best time of year to see aurora?
Clear skies are a requirement, so you should try to choose a location and a season that is blessed with the clearest skies. The continental locations in Russia, Alaska and western Canada under the auroral zone statistically have the clearest skies. Note that during the spring, the skies of Iceland and Scandinavia are usually clear. So the dark of the moon in March is the best time of year to travel to the auroral zone since the yearly cycle of auroral activity also peaks around the equinox.
Sunlight prevents viewing the aurora during the summer at high latitudes. The skies at night are simply too bright as the sun climbs in the sky until the June 21 solstice and then descends. Here in Fairbanks, at about latitude 64.8, our aurora viewing “season” is August 21 to April 21. The odds are in your favor between those dates: if the weather is clear and you stay for at least three nights, it’s highly likely (though never certain!) that you will see the aurora. From April 22 to August 20 although the sun does set the sky never gets dark enough in Fairbanks to see the aurora even if it was a spectacular display.
- Why are some years better than others to view the aurora?
- The number of sunspots on the Sun's surface changes on a fairly regular cycle, which scientists refer to as the sun's 11-year cycle variation. Sunspot activity, and hence auroral activity, tends to peak every 11 years. This peak is called the solar maximum. The last solar maximum was in 2014; the next is expected around 2025. The chances of seeing the aurora at lower latitudes increase when the sunspot cycle is at a maximum, but chances at higher latitudes are not as dependent on the solar maximum because the auroral oval is normally present.
- Where should I view the aurora when I'm in Fairbanks, Alaska?
SCIENCE OF THE AURORA
- What is the aurora?
- What is the auroral oval?
The two figures below show the locations with the most frequent occurrences of aurora borealis (left) and aurora australis (right) during the period of best viewing around the middle of the night.
This level of auroral activity, index Kp=2, will occur often enough that you will probably see the aurora if you travel to these regions when the nights are dark and stay for three days to a week, assuming that the skies are clear. If the auroral activity level is higher than 2, you will still observe the stronger motions and color changes, etc., that are seen farther equatorward.
- What makes the colors of the aurora?
- What is the altitude of the aurora?
- What are the types of aurora?
- Can you hear the aurora?
The best answer is “Maybe.”
It is easy to say that the aurora makes no audible sound. The upper atmosphere is too thin to carry sound waves, and the aurora is so far away that it would take a sound wave five minutes to travel from an overhead aurora to the ground. But many people claim that they hear something at the same time when there is aurora in the sky. We are aware of only one case where a microphone has been able to detect audible sound associated with aurora (visit Auroral Acoustics: the web site does not have sound samples, but you'll find a link to an in-depth paper and recent news on this topic.). The sound is often described as whistling, hissing, bristling or swooshing. What it is that gives people the sensation of hearing sound during auroral displays is an unanswered question. By searching for an answer to that question, we will probably learn more about the brain and how sensory perception works than about the aurora.
- Can you predict when and where there will be aurora?
- What is the solar wind?
- Streams of charged particles that produce the aurora come from the corona, the outermost layer of the sun's atmosphere. The corona is exceedingly hot, measuring more than one million degrees. The high temperature causes hydrogen atoms to split into protons and electrons. The resulting gas of charged particles is called plasma, which is electrically conductive. The solar plasma is so hot that it breaks free of the sun's gravitational force and blows away from the surface in all directions. The movement of this plasma is called solar wind. The intensity of the solar wind and the magnetic field carried by it change constantly. When the solar wind blows stronger, we see more active and brighter aurora on Earth.
- What is the 11-year solar cycle?
- What are Kp numbers?