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Propagation 101: The case of the X1.6 flare on 9/10/14

– Posted in: 630 Meter Instructional Topics, 630 Meters

Last night and this morning I have been  hearing a lot of chatter on 2-meters and on the various email reflectors about the impact of yesterday’s X-flare.  While our internet driven society has so many resources available to help understand just what is happening, there still seems to be a lot of confusion.  Plasma and solar physics is extremely complicated so I will try to explain in very general, simple terms, making a few allusions to yesterday’s event while excluding the mathematics.

We know the sun is effectively a ball of plasma generated from a massive fusion reaction in its core.  Circulating (convective) currents from within drive solar mass,which includes helium and hydrogen nuclei as well as heavier elements, to the surface.  Sometimes the magnetic structure is such that sunspots form.  Sunspots are cooler  regions of very strong magnetic flux, generally known to be in the range of 1500 gauss, which emit UV and soft X-rays on a good day, contributing to ionization which helps to promote HF propagation here on earth.  The sunspot that erupted on 9/10 was known as 2158 and had recently rotated into view after  making an impressive showing to the Stereo A and B satellites that monitor the sun from the opposite side compared to the location of earth (literally 180 degree away from earth on the “back side” of the sun).

So why do sunspots erupt?  The easy answer is that sunspots are always teetering on the brink of disaster.  Magnetic flux is in a constant state of change, reacting to other magnetic fields on the sun as well as the convective currents from the core.  Go too far one way, and the UV and soft X-ray output diminishes while the other extreme results in enormous magnetic fields that lead to filaments, which are really just plasma reaching out along the lines of force created by the sunspot.  Sometimes there is enough energy that the filament breaks away.  This is what we typically call a solar flare.

Solar flares are not limited to these events that result in an significant impact to the ionosphere here on earth, however.  In fact, from a data reporting standpoint, when sunspots are present and have output, we are technically in a flaring condition.  The Space Weather Prediction Center defines these flaring conditions as A,B,C,M, and X.  In general, conditions A-C mark normal output, ‘A’ being on the low end of the output spectrum while ‘C’ involves more energy and a more unsettled sunspot.  ‘M’ is indicative of moderate flaring conditions while ‘X’ indicates an extreme or severe conditions that could have a profound impact on earth should the output be ‘geoeffective’, that is, earth-directed.   Intensity is defined for each level with a numeric value.  The 9/10 event was considered an X1.6.  In contrast, according to Solarham, the largest X-flare in cycle 24 was characterized as an X6.9.  The flare from region 2158 would not be counted in the top-ten of cycle 24 but as an X-flare is quite significant and will no doubt have an impact here on earth.

The 9/10/14 X1.6 flare was geoeffective, in fact 9 minutes after the solar emission and just moments after we here on earth knew of the event, the HF bands began to see a significant increase in noise level, signals dropped or changes significantly and there was effectively a total radio blackout on skywave paths on the illuminated side of the planet.  This behavior often confuses people because of the ambiguity of solar output:  too many UV or X-ray photons is bad, particularly on the daylight side of the planet, because the D-layer becomes excessively ionized causing excessive absorption.  In contrast,  too few UV and X-ray photons, and there is not enough ionization to support propagation.

So why is the D-layer responsible for absorption?  The short answer has to do with the number of gas molecules present in the D-layer.  Wikipedia says the D-layer resides 60-90 km above the earth’s surface.  While we could never breath at these altitudes, much like a climber of Mt. Everest at a much lower altitude who needs oxygen to summit, there exists a very significant number of gas particles, often referred to as ‘neutrals’.  In the presence of UV and X-ray photons these neutrals can become ionized, bonds are broken, electrons are lost and ionic species are formed.  Due to the large number of gas particles present, mean free path is relatively low.  Mean free path is the distance that a gas molecule or particle of any type can move freely before encountering another particle.  Logic should suggest that as pressure drops, mean free path increases until a perfect vacuum is achieved at which time mean free path is infinite.  In reality, there is no perfect vacuum and even deepest space still has gas particles floating around.   Keep those concepts in mind going forward as it should offer some additional support to why the D-layer alone is responsible for absorption on the daylight side of the planet.  Note that an equilibrium develops where neutrals are becoming ions and ions are recombining to form neutrals again.  For our purposes, that fact is not needed but that is the reality of the situation.

While volumes have been written on how RF energy interacts with these neutrals, electrons, and ions, the general idea is that RF energy excites these species, all of which have numerous resonant frequencies that correspond to fundamental and harmonic frequencies, respectively.  That excitation results in an oscillation and charge motion, which, by fundamental definition, is how radiation is emitted.   The key words there are ‘charge motion’.  When mean free path is low, charge motion is limited before radiation can occur due to inelastic collisions.  The excitation energy, which should have been re-radiated as a radio signal is lost to other species, typically as kinetic energy and heat.  As implied earlier, this discussion is much more complex than I am giving credit but for our purposes,  it should be obvious that the amount of ionization and subsequent absorption in the D-layer is much higher during periods of extreme solar flaring compared to normal solar emissions.

Once the more extreme solar emissions subside, the D-layer recovery can begin.  Recombination leading to neutral formation can take hours and is dependent on the amount of “normal” solar energy impacting the ionosphere, which stands to slow the rate of recombination.  Shortly after the 9/10 X1.6 event had subsided to M-class levels, I decided to check 15-meter CW.  The noise floor was very high, in some cases reaching S9 +10 db, which is very uncharacteristic of a band whose normal noise floor is S2.  Signals were on the band and characterized as weak and watery, coming in under the pulsating noise from the sun.  I did call CQ on CW and encountered a multitude of stations calling me but only one Canadian QRP station was strong enough to make it above the elevated noise level.  After a 10 minute chat where conditions really seemed quite stable in spite of the increased noise level, we cleared.  I am not much of a 10-meter operator but decided to check the band.  I did not have high expectations so I accessed the reverse beacon network and put out a few CQ’s in CW on 28.016 Mhz.  To my surprise, I was being reported 3-6 db above the noise level by stations in Curacao and Alaska, the latter being the most surprising under the circumstance.

Further evidence to the recovery was seen in the early evening during the Texas CW traffic net on 40-meters.  Stations that are normally weak were S9 + 20 db.  Clearly the D-layer was getting better (through recombination but also the fact that the solar zenith was over the south Pacific, resulting in less effects as we moved into twilight here in North America)  and the higher regions of the ionosphere where able to enjoy some of the fruits of the increased ionization.  So why were the higher layers less negatively effected by the X1.6 event?  Because of higher mean free path.  Higher altitudes result in lower gas pressures and that means that neutrals and ions can travel further before they bump into one another.  This increased time results in re-radiation actually being able to occur as it was intended.  In general, it’s business as usual for propagation in the E and F layers.

But that’s not the end of the story.

The effects of yesterday’s X1.6 event were the result of the photonic emission from the sun, that is the emission that is actually travelling at the speed of light.  What remains to be seen is the impact of the geoeffective coronal mass ejection (CME) which is travelling much slower and should arrive sometime tomorrow.  CME’s are characterized by much larger species including heavier elemental ions and large helium and hydrogen nuclei.  They pack a lot of energy due to their momentum, which is a function of their mass, but they are much less agile than electromagnetic radiation.  How do we know there was a CME accompanying the X1.6?  Probably the best indicator we have is radio emissions.  These come in various types (literally Type I, II, III, etc, and  are really beyond the scope of this discussion) and their intensity characterizes the energy associated with the solar mass as it approaches the earth’s magnetic field.

Its important to note that the earth’s magnetic field extends into the solar corona. An approaching CME will breach this magnetic field, opening up a hole that allows solar wind to flow into the ionosphere, once again causing severe and persistent HF radio blackouts, not to mention drag and electrical problems on space craft and terrestrial-based electrical systems alike.  This breach is often characterized by the planetary magnetic Bz component “going south” where the values, measured in nanoTeslas, are negative.  Furthermore, the flow of solar wind results in the increase of absorption, elevated K-index and subsequently elevated A-index.  Elaboration on these metrics will be the subject of future discussions.

Here is a very well done You Tube video that explains these concepts very succinctly:

We have some ideas about what might happen once this CME impacts the earth’s magnetic field but until it happens, the specifics are simply mere speculation.

A few very good references for learning about and studying solar activity and its impact on radio propagation are as follows:

Solar Ham

NOAA/Space Weather Prediction Center

Tamitha Skov’s very excellent space weather forecast on You Tube – This is excellent and almost like watching the TV News weather forecast.

Dr. Bob Brown, NM7M, was the former director of the Space Weather Research Center at the University of California at Berkeley and friend to the amateur radio community.  Prior to his death in the early 2000’s, Dr. Brown wrote two ground breaking books, the first is the The Little Pistol’s Guide To HF Propagation followed by the The Big Gun’s Guide To Low band Propagation.  These texts are free PDF files to download and reflect Dr. Brown’s life long body of work.  They are generally considered reasonable reads for someone with a minimal background in science.

So finally, how was 630-meters last night?  As expected the D-layer was slow to deteriorate due to increased ionization, which resulted in a slow start here in the North Texas as compared to previous operating sessions.  These conditions were complicated by a significant amount of noise from an approaching frontal boundary from the north.  Conditions were still quite good all night here in North Texas.  Of particular note was that Rudy, N6LF / WD2XSH/20, was reported in VK shortly after sunrise here in North Texas.  Prior to this report, signals from the west coast were sparse, whereas recent sessions had been characterized by significant signals in the pre-greyline and greyline periods.  As with my  recent VK and ZL excursions on 630-meters, the magnetic field was active so perhaps this is a calling card for these type of extreme DX conditions. Further study is certainly warranted.

This discussion here only scratches the surface and is in no way complete.