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Wetterzentrale Forum Archiv 2000 1. Halbjahr

Polarlicht Astro Alert

Geschrieben von: Mark (Niederbayern)
Datum: 6. April 2000, 22:35 Uhr


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MIDDLE LATITUDE | A s t r o A l e r t | 06 and 07 April

----------------- Sun-Earth Alert -------------------

** Potential category G3 Geomagnetic Storm **

One of the most vigorous interplanetary disturbances to impact the Earth
so far this solar cycle collided with the Earth's magnetosphere near 16:40
UTC on 06 April. The disturbance was preceded by one of the strongest shock
fronts yet observed this solar cycle. Solar wind velocities almost
instantaneously increased by almost 200 kilometers per second within a minute
or two from a background quiet-time value of 380 km/sec to a value exceeding
570 km/sec. The strength of the magnetic fields in space (which originate
from the Sun and are dragged outward by the solar wind) also increased
dramatically in magnitude by a factor of almost 4 times the normal quiet-time
background value.

The sudden increase in solar wind pressure against the Earth's
magnetosphere, combined with the favorable southward orientation of the solar
wind magnetic fields resulted in increased erosion and compression of the
sunward side of the Earth's magnetic field. This has caused the GOES-8 and
GOES-10 geosynchronously orbiting spacecraft (and all other geosynchronously
orbiting spacecraft near the "noon sector" of the Earth) to pass through the
protective barrier that separates the quiet and stable magnetic field of the
Earth from the more turbulent and hazardous environment inside what is known
as the magnetosheath which is the transition region between the solar wind
and the Earth's magnetic field. Passage of spacecraft through these regions
can affect satellite health and can make it difficult for satellite operators
to maintain spacecraft pointing stability.

The effects of this disturbance are also being strongly felt in the
Earth's ionosphere. This is producing heavy degradation in the quality of
radio signals that rely on the ionosphere to propagate to distant locations
around the world. Communications with spacecraft may also be affected. The
disturbed ionosphere can cause spacecraft signals to begin to scintillate,
which can affect communications with spacecraft.

Another adverse effect of this storm will be to increase the atmospheric
drag on satellites. This effect is expected to begin within the next 3 hours
(anytime after about 23:00 UTC on 06 April) and is caused in-part by the
heating and expansion of the Earth's upper atmospheric envelope. This
increased drag can change the orbital elements of lower orbiting spacecraft,
particularly those spacecraft with perigees below about 500 kilometers.

Intensely varying magnetic fields can also cause problems for electrical
power generating companies, particularly those whose power-generating grids
are located in the upper-middle and high latitude regions. The problems
manifest themselves as induced electrical currents that are superimposed upon
the existing electrically generated power flowing through power lines. These
induced currents can damage (and in rare cases even destroy) expensive
equipment such as electrical power transformers. In most cases, the general
public will not notice the effects of these induced currents. The equipment
that brings power to your house are well regulated against such events. The
worst that can happen is the complete loss of electrical power if damage
should occur to the large transformers and other susceptible equipment
maintained by the power companies. An extreme example of such an event
occurred in Quebec in March 1989, where power was lost to more than 6
million people. Since then, power companies such as Hydro Quebec have
expended considerable sums of money to help protect against recurrences of
geomagnetically induced power outages.

A strong geomagnetic and auroral storm is currently brewing over the
northern Russian and European regions, based on the latest spacecraft
imagery. There is uncertainty whether this storm will persist into the night
hours for North American observers. If the disturbed conditions persist,
observations well into the central U.S. states should be possible under dark
skies. If the disturbance decays before then, activity may be limited to the
southern Canadian provinces and/or the northern tier of the U.S. states. In
either case, observers over North America may wish to take special note of
this disturbance, which is occurring during the near-new phase of the moon.
Thus, optimal observing conditions and dark skies should provide a maximal
unimpeded view of possible auroral activity.

Observers who successfully spot auroral activity are asked to report
their findings to: http://www.spacew.com/www/auroras.html so others may see
the observations. All reports are permanently archived for potential future
scientific use. Space weather forecasters at the Space Environment Center and
elsewhere also make use of the reports submitted by amateurs to this site.

We are also interested in receiving digitized (scanned) photographs of
auroral activity that amateurs make during this storm. Later, we will release
a compiled list of the images we receive on a web page with appropriate
credits to those who provided them to us. We will also provide any links to
the images that are included with the images. Imagery can be e-mailed to:
Es sind eindeutig Polarlichter. Diese Warnung hab ich heute erhalten:

STD@Solar.Spacew.Com. Please include appropriate wording for credits and
links to any of your web pages for further information.

The current estimated visible limits of auroral activity may be seen
from the map provided at: http://www.spacew.com/www/aurora.html. Spacecraft
derived imagery and products are also available within that page.

The duration of this disturbance will probably continue through the mid
UTC hours of 07 April. The intensity of the storm could change rapidly if the
magnetic fields within the solar wind do not maintain their current southward
orientation. Such shifts in the orientation of the solar wind magnetic fields
can occur very rapidly. Within as little as 2 hours, a very strong auroral
storm can diminish to insignificant levels during such rapid swings in solar
wind magnetic field direction. As a result, for near-realtime information
concerning the state of auroral activity, rely on the sites above. Serious
users, media, and other organizations requiring immediate notification of the
arrival and current status of disturbances such as this one should consider
purchasing software such as SWARM (http://www.spacew.com/swarm) for staying
on top of these (and future) conditions.

This storm has the potential of being one of the largest events observed
so far this solar cycle.

** End of AstroAlert **

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