Eine wirklich geballte Ladung PL-Wissen...

[Ulrich Rieth]

...von Cary Oler. Gepostet im STD Forum( http://solar.spacew.com/aurora/forum.html) :

Name: Cary
Date: Mon, 15 Apr 2002 12:59:27 -0600
Subject: (PETER) Re: IMF

Peter Nilsson, the ability of the solar wind to couple with the Earth's magnetosphere to produce auroral activity is dependent upon
several primary factors: the velocity of the solar wind, the density of the solar wind, the strength of the interplanetary magnetic field (IMF)
and in particular the direction of the IMF with respect to the Earth's magnetic field.

The URL you mentioned (http://www.sec.noaa.gov/ace/MAG_SWEPAM_6h.html) contains all of this information provided by the ACE
spacecraft, which is located at the L1 orbital position which is where the gravitational pull of the Sun balances with that of the Earth
and the moon, thereby always keeping the spacecraft positioned between the Earth and the Sun - an ideal location for monitoring solar
wind conditions before those conditions are observed at the Earth.

We'll start at the top panel and work down.

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Bt (the white plot line): Bt is the total strength of the IMF in the solar wind. It is simply the vector sum of all three orthogonal IMF
components (Bx, By and Bz which each represent the strength of the IMF in three-dimensional space). Each of the three components
(bx/by/bz) cannot ever exceed Bt. That is, if Bt is 15 nanoteslas (nT), then Bz can never go below -15 nT or above +15 nT. The same
rule applies to By and Bz.

For completeness, Bx represents the magnetic field magnitude in the east-west direction. By represents the field strength in a direction
that is directed either toward or away from the Sun. Bz denotes the field strength that is directed either northward or southward (in a
direction parallel with the Earth's coordinate system).

The Bz component is perhaps the most influential in terms of the IMFs ability to couple with the Earth's magnetic field. Anytime the Bz
component swings southward (ex. anytime the red Bz plot line drops to negative values) for an extended period of time, the potential for
geomagnetic and auroral substorming increases. The potential for substorming is also somewhat proportional to the velocity of the solar
wind. So if the solar wind is fast (say 700 km/sec) and the IMF Bz component drops to -15 nT for more than about an hour, you can
reasonably expect with fair confidence that some strong auroral storming will occur with some equatorward migration of the auroral oval
into more southern locations perhaps into the middle latitudes.

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PHI: The direction of the IMF in a plane parallel to the ecliptic (or the plane in which the Earth and the Sun both reside). It is much like
a compass. a direction of 0 or 360 degrees means the the IMF is "pointing" directly toward the Sun. A value of 180 degrees means the
"compass needle" would be pointing directly away from the Sun. This value does not provide any information with respect to whether
the IMF is directed northward or southward as this is only a two-dimensional property and does not have a "north" or "south".

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For completeness, THETA: This angle tells you how many degrees northward or southward the IMF is pointing. It is similar to Phi except
that it only measures the inclination angle of the IMF with respect to the Earth's north/south direction. A value of +90 degrees means
that the IMF is pointing directly northward. A value of -90 degrees means the IMF is pointing directly southward.

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DENSITY: The orange plot line shows the density of the solar wind on a logarithmic scale (solar wind density, like other proton density
measurements) increases in an exponential manner, so a logarithmic scale best depicts variations in this quantity. Values between 5
and 10 are normal background values. Densities within high velocity coronal hole streams are usually near values of 1 to 3. Densities
within coronal mass ejections can vary substantially from around 1 to more than 100. The higher the density of the solar wind, the
greater the pressure it will exert upon the Earth's magnetosphere. Greater magnetospheric RAM pressures on the sunward side of the
Earth can increase the potential for geomagnetic and auroral substorming. Large fluctuations in solar wind densities (and hence
pressures against the magnetosphere) given a constant solar wind velocity can also induce larger-scale changes in magnetospheric
configurations which may also trigger substorm activity.

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SPEED: The yellow solar wind speed plot line depicts the velocity of the solar wind. Actually, it really depicts speed, since velocity is a
vector quantity and there is no directional information available from the ACE spacecraft. But the terms velocity and speed are usually
used interchangeably. Shock fronts in the solar wind (an "animal" in the solar wind that is somewhat akin to blast waves observed
when a bomb goes off on the Earth) can be detected by watching for discontinuous jumps in solar wind velocity, density, temperature
and IMF strengths. Shock fronts can drive sudden increases in magnetic field strength at the Earth. These are known as sudden
magnetic impulses (or SI's). If these impulses are followed by geomagnetic storm activity (which are large fluctuations in the strength of
the Earth's magnetic field), then the sudden impulse is usually named a "sudden storm commencement" or SSC. In general, the higher
the speed of the solar wind, the greater the potential is for periods of geomagnetic and auroral storming - but this is usually contingent
upon a favorably (southward-directed) IMF. Large fluctuations in speed can also help contribute to storminess.

-----------------------

TEMPerature: The green plot line denotes the temperature of the solar wind in degrees Kelvin. Temperatures are used (in part) to
determine the characteristics or types of mass that is flowing in the solar wind. Magnetic clouds (often associated with the core
regions of coronal mass ejections where the IMF rotates smoothly from one direction to another) usually have lower than average
temperatures because the magnetic "bubble" inside a magnetic cloud structure impedes normal temperature flow. A similar feature of
magnetic fields causes the cores of sunspots to cool and appear darker than surrounding regions. Normal temperatures are around
1.0E+5 degrees K to 1.0E+6 degrees K (in other words, 100,000 to 1 millionn degrees Kelvin). Coronal hole based disturbances and
non-magnetic-cloud related features of some coronal mass ejections are often associated with higher than normal temperatures.

For the novice, pay closest attention to the IMF Bz component and the speed of the solar wind. If Bz drops to negative values for a few
hours and the speed of the solar wind is above 500 km/sec, then conditions may become more favorable for geomagnetic and auroral
substorm activity. The largest geomagnetic storms occur with more extreme values of these parameters.

For example, a strong coronal mass ejection may be associated with solar wind speeds in excess of 1,000 km/sec and IMF Bz values
less than -60 nT - VERY favorable conditions for producing severe geomagnetic and auroral storm activity.

I hope this helps.

[Peter Wloch]

Hi Ulrich !
Schoene Erklaerung von Cary...
Ich freue mich immer, wenn ihn die Schreiblust ueberfaellt.
Da sind immer interessante Sachen zum Lernen dabei...
Das waere z.B. etwas fuer die FAQ auf Ulrichs Polarseite in deutsch.
Oder wie es ueberhaupt gut waere, wenn neben jedem Diagramm eine
schluessige Erklaerung staende.
Oder es eine Seite gaebe, die die verschiedenen Diagramme erklaert.
Also auch ACE und aehnliches...
Man laedt sich die Polarlichtvorhersage auf den Screen und kann nun in Ruhe die Messwerte studieren, zusammen mit den Erklaerungen dafuer; die, wenn es gestalterisch ginge, immer direkt unter dem betreffenden Diagramm abzulesen waeren...
Das ist zwar teilweise schon bei der AKM-Polarlichtvorhersage so,
aber die Seite koennte ja etwas aktualisiert und verbessert werden.
(Nur ein Vorschlag..)

Gruss
Peter