Space Weather ­ An Introduction 1. What is "space weather"?
2. What is a "solar cycle"?
3. What are "sunspots"?
4. What is a "solar flare"?
5. What is a "solar wind"?
6. What is a "geomagnetic storm"?
7. What are "auroras"?
8. How is space weather monitored?
Written by : HUNG Fan­yiu September 2004 1. What is "space weather"? Space weather originates from the Sun. It generally refers to all solar activities such as sunspots
and solar flares, and the effects they may have on the Earth. The intensity varies from time to
time, sometimes strong and sometimes weak. "Good weather" means a calm period of solar
activities, while "bad weather" is a period of frequent and disturbed activities which may affect
telecommunications, navigation and power systems on Earth and the operations of satellites or
spacecraft. Back to content
2. What is a "solar cycle"? Solar activity is periodic. A solar cycle (also called a sunspot cycle) is an approximately 11­year
period with increasing and decreasing sunspot numbers. Each cycle starts from the time of
minimum activity. The cycle numbering system dates back to the eighteenth century and the
current solar cycle is cycle 23. Annual mean sunspot numbers (1900 ­ 2002) (Source/Credits: National Geophysical Data Center, US Department of Commerce) Back to content
3. What are "sunspots"? Sunspots are dark areas on the Sun's surface with relatively low temperatures (compared with
other parts of the Sun) and strong magnetic fields. They normally appear in groups. The number
of sunspots is usually taken as an indicator of solar activity. It increases significantly and can
reach hundreds at the peak of a solar cycle. A huge sunspot group may cover an area
13 times or more the Earth's entire surface
(Source/Credits: SOHO (ESA & NASA)) Back to content
4. What is a "solar flare"? Solar flare is a violent solar activity. Its occurrence is related to a sudden burst of electromagnetic
waves and a vast amount of charged particles (mostly electrons) from the Sun. These
electromagnetic waves can affect telecommunications, radio broadcast and navigation system
on Earth. Sometimes, charged particles can also endanger the operations of spacecraft and
satellites in space, and expose astronauts to higher amounts of radiation. A solar flare
(Source/Credits: Space Environment Center, Boulder, CO, National Oceanic and Atmospheric Administration, US Dept. of Commerce) Back to content
5. What is a "solar wind"? The Sun releases tremendous energy. Solar wind refers to this released energy in the form of
charged particles at high speed, reaching several hundred kilometres per second. While the
magnetic field on the Earth (which traps charged particles encircling the Earth into radiation belts
called the Van Allen belts) normally protects our planet from the solar wind, it may be deformed in
the event of violent solar wind, resulting in a geomagnetic storm on Earth. Illustration of the influence on the Earth's magnetic field when solar wind approaches the Earth. White lines represent the solar wind and the blue lines surrounding the Earth represent its magnetic field.
(Source/Credits: SOHO (ESA & NASA)) Back to content
6. What is a "geomagnetic storm"? A geomagnetic storm occurs when a violent solar wind hits the Earth and severely distorts the
Earth's magnetic field. Geomagnetic storms can seriously affect radio broadcasts and navigation
systems, and paralyze electric grids on Earth. During the great geomagnetic storm in 1989,
electricity was suspended for 9 hours in Quebec and a number of American satellites were taken
out of service. The last significant geomagnetic storm occurred in October 2003 when intense
solar flares erupted from the Sun. Among other incidents, the geomagnetic storm caused a
Japanese communications satellite to shut down temporarily. Back to content
7. What are "auroras"? Auroras are most commonly visible at high latitudes. They are associated with geomagnetic
activity brought about by solar wind. Auroras occur as a result of charged particles (mostly
electrons) from the Sun colliding with gas particles in the Earth's atmosphere, producing a glow
in different colours. The patterns and shapes of the aurora are related to the flow of charged
particles and the magnetic fields. In respect of the auroral colour, oxygen molecules produce
green and red colours while nitrogen molecules produce purplish­red and blue colours. Picture of aurora taken at Antarctica
(Source/Credits: John K.W. Chan, HKO) Back to content
8. How is space weather monitored? As space weather originates from the Sun, it is monitored by international space weather centres
using special satellites or telescopes. We will discuss more about this in the next issue. Back to content
Other related websites
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National Space Weather Monitoring and Warning Centre, China (http://www.spaceweather.gov.cn) Space Environmental Centre, United States (http://www.sec.noaa.gov)