Space weather and aurora

Space weather

"Space weather" is a modern term to denote physical conditions in space which are ultimately determined by solar activity. Space weather manifests itself through various physical phenomena such as enhanced intensity of hard radiation, increased strength of electric and magnetic fields and elevated magnitude of electric currents, to name only a few. A "magnetic storm", which may also be called "space storm", is a rather violent phase of space weather and is often caused by solar outbursts such as flares and coronal mass ejections. Solar outbursts create disturbances of the solar wind which may impact the Earth environment with a delay of a few days after their eruption from the Sun.

Solar outbursts can be compared to volcano eruptions on Earth. They are fairly frequent during the maximum phase of the 11-year solar cycle when the sunspot number is high. During solar maximum geoeffective outbursts with subsequent magnetic storms can occur several times per month while they are observed only a few times per year during solar minimum. The most recent solar maximum occurred in the years 2001-2002 and the most recent minimum in the years 1994-1997.

The state of space weather has substantial impact on the performance of satellites and space probes. High energy particle radiation can damage solar panels, cause upsets in the onboard computer systems and can lead to destructive electric discharges in space instruments. The atmosphere can heat up and expand which causes satellite deceleration and premature reentering.

Space weather conditions may also affect technical systems at ground. During geomagnetic storms with brilliant aurora, electric currents of millions of Ampére may flow in the upper atmosphere at an altitude of some 100 km. Spatial and temporal variations of these currents induce secondary electric fields and currents - called "geomagnetically induced currents" (GIC) - which flow, for instance, in the conducting ground and in high voltage power lines. In the latter they can trigger malfunctioning of line protection relays and lead to damage to transformers resulting in power outages over large regions. Long telephone and railway signal lines may also be disturbed and generate false signals, or they may even be damaged.

DMI conducts ground-based and satellite observations in the auroral and polar regions of magnetic disturbances and of high energy particle precipitation including associated atmospheric effects. In combination with data from other sources these observations are used for monitoring actual space weather conditions and for research in understanding and modelling space weather effects.

Further information contact Peter Stauning, (pst@dmi.dk)

The Aurora

Magnetic disturbances of the Earth's environment are often associated with energetic particle precipitation. Electrons with energies corresponding to electric acceleration potentials of some thousand to some hundred thousand volt constitute the dominant part of this population. The most striking effect of their interaction with the atmopshere is the aurora. Aurora occurs when clouds of such energetic electrons hit and excite the atmospheric atoms and molecules. The latter loose their excitation energy by radiating photons, and this radiation can then be detected with suitable optical sensors, e.g., photo cameras. During high solar activity aurora typically occurs several times a day in the auroral zones.

Aurora occurs on most days in the polar areas, e.g. in Northern Scandinavia, Iceland and Greenland, and can be seen mainly in the hours around midnight. Aurora is seen in Denmark only at times of strong magnetic activity, that means, during major magnetic storms.

Visual appearance of the aurora

In the auroral zones (annular bands at some 20-30 degrees distance from the magnetic poles), the aurora is most often green-white and shaped like long wavy curtains hanging from one end of the horizon to the other - often for hours. Sometimes the quiet aurora becomes active and very dynamic and can suddenly explode in wild motions and intense colors.

Aurora appears over Denmark normally only during the short intervals when the magnetic activity reaches a maximum. The aurora has mostly an intense reddish colour and is shaped like broad beams. The following samples of auroral observations in Denmark are reproduced from photos taken by members of the Astronomical Club who are mainly teachers and students from the High School and the Technical School in Sønderborg, South Jutland. The Astronomical Club has developed an "aurora alert" which is based on the magnetic disturbances which are typically associated with auroral bursts. The equipment and the photographic technique were described in articles published in the periodical "Amatørastronomen" (the Amateur Astronomer), September and November 1991. Guided by the aurora alert several club members have succeeded in taking photos of particularly spectacular aurorae. Examples of magnetic activity together with images of coincident aurorae are shown below. DMI's homepage has a link to a display of the magnetic declination over the last 48 hours, recorded at the Brorfelde (Zealand) observatory. Information about the geomagnetic field and magnetic measurements is given in another section.

Images of aurora over Denmark

The photo to the left shows intense reddish aurora recorded in South Jutland on March 24, 1991 at 22:38 Central European Time, CET (21:38 Universal Time, UT, which is equivalent to Greenwich Mean Time, GMT). Please, observe the Copyright Conditions.

The aurora was caused by a severe magnetic storm which led to a very strong disturbance of the geomagnetic field. Large deviations of the magnetic field vector components from their quiet level values were recorded at the Danish Brorfelde (Zealand) observatory, see the magnetogram below the aurora image. The figure shows the deviation of the northward magnetic field component (ΔH) from its quiet level (ΔH=0) as a function of time (UT). The time of the observation of the aurora (21:40 UT) is indicated by a vertical line segment.

The magnetic storm started in the morning shortly before 04 UT. This appears in the magnetogram as a short and strong upward impuls followed by a 2-hour long depression of the magnetic field. After that interval the magnetic field variation became smaller and of shorter period and after several hours calmed down. In the evening, between 20 and 21 UT, the magnetic field disturbance increased again, and eventually the storm produced aurora over Denmark.

Other observations of magnetic activity and aurora in Denmark

Aurora seen in Jutland during a magnetic storm, March12, 1990, approx. 23:45 UT (00:45 CET).

The Hale Bobb comet together with a faint auroral display over Sønderborg during a weak magnetic disturbance on April 11, 1997 at 22:16 UT (23:16 CET). This fine shot has been taken by Jens Ravsbech, 14 years old. Please note that the small white spots on the photo are stars and not dust on the negative. These pictures and other beautiful photographs of aurora, stars and comets can be found at http:/www.amtssgym-sdbg.dk/as/galleri.htm

December 2003