Hong Kong Space Museum

The Sun rises every morning in the east. This celestial clockwork masks the Sun's variability and the potential havoc it could cause. If the surface temperature of the Sun drops by a few percent, that would be enough to throw the Earth into deep freeze. While dramatic changes of such extreme magnitude are unlikely, consequential solar variations are known to occur historically, and the Sun's energy output is known to be varying from year to year. The variations of the Sun and the effects of solar activities on Earth have received increasing interest and concern in recent years. "Solar-Terrestrial Relations" is gaining importance as a branch of space science.

Progress of a Coronal Mass Ejection (CME) observed over an eight hour period on 5-6 August 1999.

In the later 1600s, for a period of almost half of a century, a prolonged cold spell occurred in Europe. During this long period, the number of sunspots on the sun got very low. The 11-year sunspot cycle essentially disappeared. This sunspot deficient period is called the "Maunder Minimum". A number of other similar periods have been recorded, including the "Sporer Minimum" in the mid-1400 to early 1500s and the "Wolf Minimum" in the early 1300s. Since sunspot number is closely related to solar activities like flares and coronal mass ejection, the Sun might have entered a low-activity state for a rather long period of time during these Minimums.

The intriguing historical events raise one natural question: Does the total solar radiation output (solar irradiance) really correlate with solar activity? Continuous monitoring of the solar irradiance have been carried out since the early 80s by instrument in space. This is essential because several portions of the solar radiation spectrum such as ultraviolet and far infrared are blocked by Earth's atmosphere. By now, two solar cycles of data have been acquired. The answer to the above question is a definite `yes'. When the Sun is active, it radiates more. The solar irradiance correlates positively and strongly with the number of sunspots. The difference between the maximum and minimum is about 0.1%. The amount is small, yet possible climatic consequences are suspected and being studied.

Sunspots

The time scale for substantial solar irradiance change is in the order of a century or centuries. In time periods shorter than a century, the most striking influences caused by the Sun are associated with its magnetic activities such as flares and prominence. Our Earth is constantly bombarded by high-speed high energy particles( 400 km/sec) and radiation from the Sun. We are shielded from these hazards by Earth's atmosphere and magnetic field. The field forms a protective shell called magnetosphere that has a radius around 10 times that of the solid Earth. But strong eruptions on the solar surface can seriously distort this shell, creating geomagnetic storms. One famous example occurred in 1989 when a storm-induced current caused a failure in Hydro-Quebec's electric power system. Six million people in Canada and the USA ran out of electricity for over 9 hours. The same storm caused the atmosphere to inflate and dragged a satellite to a lower orbit prematurely. Magnetic storms can also disrupt long-distance radio communication essential for ocean-faring ships and intercontinental flights. The enhanced levels of high-energy particles and radiation can even pose serious biological threats to human beings working or travelling in space.

The number and strength of energetic eruptions on the solar surface peak every 11 years, in phase with the sunspot number. Sunspots are islands of intense magnetic bundles punching through the solar surface. The magnetic bundles constantly interact with each other and sometimes create unstable configurations that then lead to eruptions in the form of flares and plasma ejection. The prediction of energetic events and "space weather" therefore depends on observation of sunspots and active regions. A very recent achievement in solar research is the ability to detect sunspots on the invisible far side of the Sun. This is done through tracing sound waves that bounce between different depths of the Sun. The technique, called helioseismology, is similar to the use of seismic waves for detecting underground features on Earth. Being able to reveal features beneath the surface of the Sun, helioseismolgy has contributed a lot to modern understanding of the Sun. It is becoming also an important tool for studying the solar-terrestrial relations.