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K-V-L: Hvað eru norðurljós?

Hvað eru norðurljós?

Úr tímaritinu Lifandi vísindi 1.tölublað 2011.

 

Norðurljós eru óneitanlega mjög falleg en af hverju stafa þau og er þau einnig að finna á öðrum plánetum?

Norðurljósin, eins og við þekkjum þau, er einnig að finna á suðurhveli jarðar en þar kallast þau raunar suðurljós. Latneska heitið er aurora, en það var einmitt nafn rómversku dagrenningargyðjunnar, og myndir af henni minna mikið á litadýrðina sem við þekkjum af norðurljósunum. Norðurljósin myndast í kringum segulpólana þegar hlaðnar agnir frá sólu rekast á lofthjúp Jarðar.

Norðurljós sjást aðallega á kraga kringum segulpólana, á milli 60. og 70. breiddargráðu, og suðurljósin sjást svo á sambærilegum breiddargráðum suðurs. Breytingar í sólvindum valda því hins vegar að kragarnir geta stækkað eða minnkað og sjást þá ljósin á mismunandi breiddargráðum. Sem dæmi um þess konar breytingar má nefna að sólin sendir stöku sinnum frá sér gífurlegt magn af efni út í geiminn, svokallaða sólstróka. Þegar þeir ná til jarðarinnar geta norður- og suðurljósakragarnir náð mjög langt í átt að miðbaug og dæmi eru um að orðið hafi vart við ljósaganginn á sjálfum miðbaugnum.

Yfirborð sólarinnar sendir í sífellu frá sér svokallaðan sólvind, en um er að ræða straum hlaðinna agna, aðallega róteinda og rafeinda. Segulsvið jarðar hrindir flestum þessum ögnum frá sér svo að þær streyma umhverfis hana eins og vatn um kjöl. Undantekning frá þessu er kringum segulpólana en það eru pólarnir sem segulnál vísar á, annar á norðurhveli og hinn á suðurhveli jarðar, gagnstætt við hinn. Á svæðum kringum þessa póla sleppur lítill hluti þessara agna inn í segulsvið jarðar. Svæðið þar sem flestar agnirnar sleppa inn myndar kraga utan um segulpólana.

Hlöðnu eindirnar sem fara inn í segulsvið jarðar hreyfast á miklum hraða eftir gormlaga brautum kringum segulsviðslínurnar milli segulskautanna. Rafeindir og róteindir streyma þannig í átt að segulpólunum og þegar nær dregur pólunum rekast eindirnar á lofthjúpinn, oftast í 100 til 250 km hæð. Orkan í rafeindunum og róteindunum örvar sameindir og frumeindir í lofthjúpnum en þær senda aftur á móti frá sér orkuna sem sýnilegt ljós sem við köllum norðurljós eða suðurljós, eftir því við hvorn pólinn þau sjást. Litirnir sem við sjáum oftast eru grænn og rauð-fjólublár, en þeir stafa frá örvuðu súrefni annars vegar og örvuðu köfnunarefni eða nitri hins vegar.

Norður- og suðurljós er ekki eingöngu að finna á Jörðinni. Allar plánetur með lofthjúp bregðast við eindum sólvinda með litabreytingum en hins vegar þarf segulsvið til að ná fram álíka litabrigði og við sjáum á Jörðu. Satúrnus og Júpíter hafa segulsvið sem liggja samsíða snúningsöxli þeirra og fyrir vikið myndast þar ljóshringir kringum norður- og suðurskautin. Þegar segulsviðin eru ekki samsíða öxli plánetnanna, eins og þekkist t.d. á Úranus og Neptúnus, verða ljósin óreglulegri að sjá.

Síðast breytt: mivikudagur, 20 jl 2011, 11:01 fh