[CQ-Contest] X-class flares, propagation, and... aurora?

NW7US, Tomas nw7us at hfradio.org
Fri Jul 16 11:19:47 EDT 2004


at the time of writing this NW7US Propagation Bulletin, we are recovering  
 from an X3-class X-ray Flare.  This is the fifth X-class flare in the last  
day and a half.  This flare measured X3.6, the strongest of the five.  And  
there is more on the way.

Active Region 649 (as numbered by NOAA, 10649) is about to cross the  
central part of the Sun.  Within about 3 days it will directly facing us.  
Although this region is not particularly large (it is not visible to the  
unaided eye), it is demonstrating its volatility.  Each of these flares  
has been spaced by approximately 8 hours of relative inactivity.

Region 649 maintains its beta-gamma-delta topology indicating that more  
strong flaring activity is very likely.  Meanwhile, a new, very dynamic,  
active region is popping up from around the East limb promising continued  
strong flaring activity.

The compact magnetic structure known as a "magnetic delta," is where  
opposite polarity sunspot umbrae are located within a single penumbra.   
This particular type of magnetic configuration supports unusually high  
magnetic gradients as well as strong levels of magnetic shear.  This is  
what we're seeing in the beta-gamma-delta topology of Region 649.

When we see strong atmospheric pressure gradients in Earth's atmosphere,  
combined with changes in wind direction with height (wind shear), we  
typically see the formation of tornadoes.  This is the same sort of thing  
that is happening on the Sun.  Strong magnetic gradients along with strong  
magnetic shear with the complex twisting produce powerful magnetic  

So far, these X-class flares eruptions have not apparently been associated  
with significant coronal mass ejection (CME) activity. We don't yet know  
 from this latest flare if there was a CME associated with this one.  It is  
expected that one or more of these events will be associated with a  
coronal mass ejection. If that happens in the next few days and into next  
week, we'll see an increase in auroral activity (northern lights).  Those  
of you interested in this phenomenon are encouraged to stay informed  
during the next week.

These flares cause nearly immediate disruption of HF ionospheric radio  
communications.  The stronger the x-ray flare is, the higher the  
frequencies will be that are absorbed.  X-class flares are in the  
strongest flare category.  Flares are classified by their intensity, the  
lowest category being A, followed by categories B, C, M, and finaly X.

X-class flares can cause the absorption of radio signal energy well past  
20 MHz.  Minor solar flares (C-class) will not affect frequencies much  
beyond 5 MHz.  The x-rays from these events penetrate into the lower  
ionosphere and ionize the D layer, that layer which acts as a sponge,  
soaking up radio signals.  The more ionized the D layer, the higher the  
frequencies that are absorbed.  Thus, radio signals from distant locations  
that travel through the daylit ionosphere that is influenced by a flare's  
x-ray radiation are absorbed and become inaudible.  These fade-outs last  
only minutes for minor flares, to maybe an hour or so for the largest of  
flares.  Once the flare is exhausted, the x-ray radiation fades, and the  
ionosphere recovers to its normal level of ionization.

If a CME arrives, however, we could see the geomagnetic activity turn  
stormy, which will cause longer-term degradation of HF propagation, as  
well as trigger auroral conditions.  Geomagnetic activity has the affect  
of lowering the ionization of the various ionospheric layers, which brings  
down the maximum usable frequency over a given signal path.  This lowering  
is much like what happens at night, when the ultraviolet radiation of the  
sun is blocked, and the ionosphere settles down.  The stronger and longer  
the geomagnetic storm, the more depressed the ionospheric propagation  

So, over the course of the next few days, we can expect periods of radio  
fade-out.  If there are coronal mass ejections associated with this flare  
or new flares, we can expect next week to become a bit rough in terms of  
overall propagation on MW and HF.  However, the stronger the geomagnetic  
activity, the more likely-hood of some VHF activity via aurora.

The good news?  The increase in solar activity is opening up the middle  
and high portion of the HF radio spectrum.  We expect the 10.7cm radio  
flux to increase to 150 or better over the next week.  This is great for  
DXing on the higher frequencies.  Will there be any F layer VHF openings?   
It is possible!

Let's see what transpires.  Tune in and stay alert!

73 de Tomas, NW7US (AAR0JA/AAA0WA)

: Propagation Editor for CQ, CQ VHF, and Popular Communications :
: Quarterly Propagation Columnist for Monitoring Times Magazine :
: Creator of live propagation center - http://prop.hfradio.org/ :
: Member, US Army Miltary Affiliate Radio Service (MARS) AAR0JA :
: 122.93W 47.67N / Brinnon, Washington USA - CN87 - CW/SSB/DIGI :
: Website, software, database design - http://newwebmakers.com/ :
: Washington State Army MARS, State Army MARS Director - AAA0WA :
: 10x56526, FISTS 7055, FISTS NW 57,  AR Lighthouse Society 144 :

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