Schmucker-Weidelt Lecture Notes, Aarhus, 1975 - MTNet

polar substorlns is j.n mid-latitudes (e.g. Denmarlc, Germany) much
smaller than the ampliiudes of H and D
-
because the vertical field
of induction currents nearly cancels the vertical field of the
polar jet. Under "normal condirtions" the Z:I-l ratio is about 1: '10.
Assu~ning for the mid--latitude substorm field an effective wave
number of 10- 20 000 km, yielding kx 3-6*10-~ as wavenumber, depth
of penetration i.s
'bay
A
I = 150 to 300 km.
bay kx
There are indications tllat the penetration deptll of bays into -the
oceanic substructure is substantialiy smaller. The ocean itself
produces an attenuation of the H-ar~pl.itude of about 75% at the
ocean bottcm, deep basins filled with unconsolidated sediments can
yield a comparable attenua-tion &-6. North German basin).
Pulsations and VLF-emissions: Rapid oscilla-tions of the Earth's fie1
- -
with periods between 5 minu-tes and 1 second are called pu1.sati.011~.
Their ampli-tude increases 1.iIce the ainpli-tude of substorms strongly
when approaching the auroral zone. Their typical midlati.tude
ampli-tude is 1 gamma. The source field structure of bays and pul.-
sstions is similar, the depth of pene-tra-t:ion of pulsations being
largely dependent on the near-surfack conductivity. Lt may rmge
from many kilometers in exposed shield areas to a few hundred
meters and less in sedimentary basins.
The "normal" Z:H ra-tio o:E pulsations is too small to be determS.~-rccl
with any reliabili-ty outside of the auroral zone. However, "an~!naI.oi
Z-pulsations. are frequent and usually of very local. charak-ter. IF
pulsations occur in the form of las-tine harmonic oscilla-:ions , oftel
with a beat-frequency, they are called "pulsation tr>ains1' pt, sin~J.(
pul-satj.on events are ' called "pi" , pul.satj.ons which rt~arlc the be-
ginning of a bay.dis-turbance are called "pc". All three types !la\'c
a clear local time dependence, occuring almost daily:
'F\ 1-1 m- -7,. -