Det interplanetära magnetfältet - IMF

Under solminimum ser solens magnetfält, precis som jordens, ut som en helt vanlig magnet med stängda linjer nära ekvatorn och med öppna linjer nära polerna. Forskare kallar dessa områden för dipoler. Solens dipolfält är ungerfär lika starkt som en kylskåpsmagnet (cirka 50 gauss). Jordens magnetfält är cirka 100 gånger svagare.

Runt solmaximum, när solen når sin maximala aktivitet, är flera solfläckar synliga på soldisken. Solfläckarna är starkt påverkade av magnetfältet och magnetfältslinjerna som transporterar partiklar. Dessa fältlinjer är oftast flera hundra gånger starkare än omkringliggande dipol. Detta gör att magnetfältet runt solen är mycket komplext med många påverkande fältlinjer.

Solens magnetfält stannar inte vid bara solen. Solvinden flyttar magnetfältet ut i solsystemet tills det når heliopausen. Heliopausen är där solvinden upphör, på grund av att den kolliderar med det insterstellära mediet. Det är därför som solens magnetfält kallas för det interplanetära magnetfältet eller IMF. Eftersom solen roterar runt sin egna axel (var 25e dag) har IMF en spiralform som kallas Parkerspiral.

Bt value

The Bt value of the interplanetary magnetic field indicates the total strength of the interplanetary magnetic field. The higher this value, the better it is for enhanced geomagnetic conditions. Moderate Interplanetary Magnetic Field strength values start at 15nT but for middle latitude locations, values of 25nT or more are desirable.

Bx, By and Bz

The interplanetary magnetic field is a vector quantity with a three axis component, two of which (Bx and By) are orientated parallel to the ecliptic. The Bx and By components are not important for auroral activity. The third component, the Bz value is perpendicular to the ecliptic and is created by waves and other disturbances in the solar wind.

The three axes of the IMF.

Interaction with Earth's magnetosphere

The north-south direction of the interplanetary magnetic field (Bz) is the most important ingredient for auroral activity. When the north-south direction (Bz) of the the interplanetary magnetic field is orientated southward, it will connect with Earth's magnetosphere which points northward. Think of the ordinary bar magnets that you have at home. Two opposite poles attract each other! With a southward Bz, solar wind particles have a much easier time entering our magnetosphere. From there they are guided into our atmosphere by Earth's magnetic field lines where they collide with the oxygen and nitrogen atoms that make up our atmosphere, which in turn causes them to glow and emit light.

For a geomagnetic storm to develop it is vital that the direction of the interplanetary magnetic field (Bz) turns southward. Continues values of -10nT and lower are good indicators that a geomagnetic storm could develop but the lower this value goes the better it is for auroral activity. Only during extreme events with high solar wind speeds it is possible for a geomagnetic storm (Kp5 or higher) to develop with a northward Bz.

A schematic diagram showing the interaction between the IMF with a southward Bz and Earth's magnetosphere.

Image: A schematic diagram showing the interaction between the IMF with a southward Bz and Earth's magnetosphere.

Measuring the interplanetary magnetic field

satellite which is stationed at the Sun-Earth Lagrange Point 1. This is a point in space always between the Sun and Earth where the gravity of the Sun and Earth have an equal pull on satellites meaning they can remain in a stable orbit around this point. This point is ideal for solar missions like ACE, as this gives ACE the opportunity to measure the parameters of the solar wind and the interplanetary magnetic field before it arrives at Earth. This gives us a 15 to 60 minute warning time as to what kind of solar wind structures are on their way to Earth. It is actually possible to calculate how long it will take for the solar wind to travel from ACE to Earth. On the graphs that we show on our website you can find the letters "ETA" and next to that you will find a number which show in minutes how long it takes for the solar wind to travel from ACE to the Earth.

The location of ACE at the Sun-Earth L1 point

Animation: The location of ACE at the Sun-Earth L1 point.

Deep Space Climate Observatory (DSCOVR)

In February of 2015, NASA launched the successor of ACE: the Deep Space Climate Observatory (DSCOVR) mission. DSCOVR will send real-time information about the solar wind and the interplanetary magnetic field back to Earth as ACE is doing right now. DSCOVR has arrived at L1 in the summer of 2015 and will go fully operational in October. ACE will then continue as a spare satellite.

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Senaste X-utbrottet:10/09/2017X8.2
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I dag i historien*

Solutbrott
12015M2.1
22002M1.8
31998M1.8
42015M1.5
52002M1.5
ApG
1201544G3
2199634G2
3200327G1
4201618
5200412G1
*sedan 1994