The Sun, our nearest star, is the fundamental source of energy and heat for the Earth, which is vital for the existence of life on our planet. We definitely cannot survive without the Sun, but it also influences us in more subtle ways, in particular its output of ultraviolet radiation, X-ray, solar wind and high-energy particles.

Corona Photo courtesy:  NASA

Phenomena such as sunspots, solar wind, flares and coronal ejection are called solar activities. Such activities undergo variations in a Solar Cycle of approximately 11 years and the most obvious observation is the increasing number of sunspots. Sunspots are dark regions on the sun's surface, where their temperatures are lower than the average temperature of the photosphere (6,000 degrees Celsius), and are presumably associated with the strong magnetic field. The strength of the magnetic fields is thousands times stronger than Earth's. Such strong magnetic field keeps sunspots at 4,500 degree Celsius and also serves as a reservoir of energy for explosive events like solar flares. Spot sizes vary from several hundred km across to more than 100,000 km for groups. With the aids of suitable light reduction device, sunspots can be easily observed through small telescope and huge sunspots can even be observed through solar filter. (Precautions: do not attempt to observe the Sun directly by naked eye or through telescope without safety devices, otherwise it will cause permanent damages to your eyes!).

Extreme ultraviolet radiation and X-ray created by magnetic field around the sunspots also cause the Earth's atmosphere to heat up and expand. This will create additional drag to satellites or space shuttles orbiting at low altitudes and eventually pull them out of orbits earlier than expected. Solar wind flows out from the corona (the Sun's outer atmosphere) into interplanetary space at speed of between 300 and 900 km per second. It causes fluctuation to the Earth's magnetic field, which we call magnetic storm. Such event will cause deflection of compass needles. In extreme cases, it will bring disruption to electric supply and telephone connection. Solar flares and energetic particle events also complicate matters by interrupting satellite and shortwave radio communications.

It is not all bad news. When high-energy particles interact with atoms and molecules in the Earth's upper atmosphere, light will be emitted. This is the well-known phenomenon called aurora. Aurora is a shifting pattern of white, green, violet or red light that are normally seen in polar areas. With the increasing level of solar activities, it will likely trigger dazzling auroral displays at mid-latitudes (40 or above degrees north/south from the equator).

The recent increase in sunspot number offers a hint of the coming solar maximum of its 11-year cycle. More recently, David Hathaway, a solar physicist at the Marshall Space Flight Center/NASA and his colleagues predict the peak of solar activity will take place around the middle of the year 2000. Their prediction is based on the recent sunspot number and geomagnetic activity. Moreover, the expected sunspot number will be a bit lower than the last two maxima (in 1989 and 1978), but still can be classified as above average. This maximum will last nearly one year and begin to drop gradually in 2002. It is therefore a prime time to watch sunspots in the coming two years and aurora will even perhaps appear in the sky above the mid-latitude regions.

(Click here if you want to learn more on how to observer the Sun safety)