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Date: Wed, 16 Feb 2000 14:47:50
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: Re SIDs and Transatlantic
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>So can we assume that the exceptional conditions reported by Larry Kaiser
were 
>related to this Flare on 5 feb.if so there is a delay of one week between
the 
>event and the change in propagation! is this unique or has it been observed 
>before?
The MDE (or shortwave blackout) is caused by particles (from the sun) that
travel at much slower speed than light. Delays of 24h and more between the
(visual) occurence of a flare and MDE are common.

>Usually SIDs appear to be associated with increased D layer absorbtion and 
>consequent radio blackouts.Perhaps there is another mechanism in the
recovery 
>of the ionosphere from such an event which results in the dramatic
improvement 
>in propagation noted by Larry.
Increased D-layer absorption is causing the 'blackout' on HF. Some theories
say that under extreme conditions the ionisation of the D-layer can be that
strong that it will reflect LF (and eventually MF) instead of absorb it.
I remind an article in CQ-DL (somewhere in the 80's) that handles about
this and it even mentioned QSO's between Germany and Malta (abt. 2000km) at
noon (during summer) on 1.8MHz during a strong MDE.

In the past I have noted increased signalstrengths (on 136kHz) from
northern Europe (OH and SM) whan the A-index was high. At the same time
signalstrengths from southern Europe (I) was hardly affected.

73, Rik  ON7YD