Der Sonnenwind

The solar wind is a stream of charged particles (a plasma) released from the Sun. This stream constantly varies in speed, density and temperature. The most dramatic difference in these three parameters occur when the solar wind escapes from a coronal hole or as a coronal mass ejection. A stream originating from a coronal hole can be seen as a steady high-speed stream of solar wind as where a coronal mass ejection is more like an enormous fast-moving cloud of solar plasma. When these solar wind structures arrive at Earth they encounter Earth's magnetic field where solar wind particles are able to enter our atmosphere around our planet's magnetic north and south pole. The solar wind particles collide there with the atoms that make up our atmosphere like nitrogen and oxygen atoms which in turn gives them energy which they slowly release as light.

Bild: Artist impression of the solar wind as it travels from the Sun and encounters Earth's magnetosphere. This image is not to scale.

Die Geschwindigkeit des Sonnenwindes

The speed of the solar wind is an important factor. Particles with a higher speed hit Earth's magnetosphere harder and have a higher chance of causing disturbed geomagnetic conditions as they compress the magnetosphere. The solar wind speed at Earth normally lies around 300km/sec but increases when a coronal hole high speed stream (CH HSS) or coronal mass ejection (CME) arrives. During a coronal mass ejection impact, the solar wind speed can jump suddenly to 500, or even more than 1000km/sec. For the lower middle latitudes a decent speed is required and values higher than 700km/sec are desirable. This is however not a golden rule and strong geomagnetic storming can also occur during lower speeds if the interplanetary magnetic field values are favorable for enhanced geomagnetic conditions. On the data plots you can easily see when a coronal mass ejection shock has arrived: the solar wind speed increases with sometimes several 100km/sec. It will then take about 15 to 45 minutes (depending on the solar wind speed at impact) before the shock wave passes Earth and the magnetometers start to respond.

Bild: Das Eintreffen eines koronalen Massenauswurfs im Jahr 2013, der Geschwindigkeitsunterschied ist offensichtlich.

Die Dichte des Sonnenwinds

This parameter shows us how the dense the solar wind is. The more particles in the solar wind, the more chances we get for an auroral display as more particles collide with Earth's magnetosphere. The scale used in the plots on our website is particles per cubic centimeter or p/cm³. A value above 20p/cm³ is a good start for a strong geomagnetic storm but it is no guarantee that we get to see any aurora as the solar wind speed and the interplanetary magnetic field parameters also need to be favorable.

Messung des Sonnenwinds

Die Echtzeitdaten zum Sonnenwind und zum interplanetaren Magnetfeld, die Sie auf dieser Website finden, stammen vom Satelliten Deep Space Climate Observatory (DSCOVR), der in einer Umlaufbahn um den Sonne-Erde-Lagrange-Punkt 1 stationiert ist. Dies ist ein Punkt im Weltraum der sich immer zwischen Sonne und Erde befindet, wo die Schwerkraft von Sonne und Erde die gleiche Anziehungskraft auf Satelliten hat, was bedeutet, dass sie in einer stabilen Umlaufbahn um diesen Punkt bleiben können. Dieser Punkt ist ideal für Sonnenmissionen wie DSCOVR, da DSCOVR die Möglichkeit gibt, die Parameter des Sonnenwinds und des interplanetaren Magnetfelds zu messen, bevor er die Erde erreicht. Dies gibt uns eine Vorwarnzeit von 15 bis 60 Minuten (je nach Sonnenwindgeschwindigkeit), welche Art von Sonnenwindstrukturen auf dem Weg zur Erde sind.

Bild: Die Position eines Satelliten am Punkt Sonne-Erde L1.

There is actually one more satellite at the Sun-Earth L1 point that measures solar wind and interplanetary magnetic field data: the Advanced Composition Explorer. This satellite used to be the primary data source up until July 2016 when the Deep Space Climate Observatory (DSCOVR) mission became fully operational. The Advanced Composition Explorer (ACE) satellite is still collecting data and now operates as a backup to DSCOVR.

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