YO3FVR

Credits: M0OBU

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SFI = Solar Flux Index ~ A measure of solar activity .
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A measure known as the solar flux is used as the basic indicator of solar activity, and to determine the level or radiation being received from the Sun. 
The solar flux is measured in solar flux units (SFU) and is the amount of radio noise or flux that is emitted at a frequency of 2800 MHz (10.7 cm). The Penticton Radio Observatory in British Columbia, Canada reports this measure daily. 
The solar flux is closely related to the amount of ionization and hence the electron concentration in the F2 region.    As a result it gives a very good indication of conditions for long-distance communication.

The figure for the solar flux can vary from as low as 50 or so to as high as 300.    Low values indicate that the maximum useable frequency will be low and overall  conditions will not be very good, particularly on the higher HF bands.   Conversely, high values generally indicate there is sufficient ionization to support long-distance communication at higher than normal frequencies.  However, remember that it takes a few days of high values for conditions to improve.   Typically values in excess of 200 will be measured during the peak of a sunspot cycle with high values of up to 300 being observed.

SN = Sunspot Number ~ A number over 100 is good.

'A' Index ~ 0 to 3 is Good - 0 is really good, higher, progressively worse

'K' Index ~ 0 is good ~ higher progressively worse.

Signal to Noise level - 'S' 0 is good - higher, progressively worse.

 
HF Propagation predictions are just that, - predictions ... and are calculated using the available data.   Actual conditions at your location depend on a lot of additional items.    

​Solar Flares, and Geomagnetic Storms.

Since the late 1800s, it was noted solar activity affected telegraphic lines, and later, radio communications. However, there was no scientific proof for this link. From the 1920s onward, radio amateurs clearly correlated HF propagation and the MUF to the solar cycle. But again, there was no scientific proof. Astronomers and physicists knew there was a sun–earth connection, but without direct observational data, it remained an un proven scientific theory. The scientific proof did not come until quite recently – basically, the space age – when we got our first look at the sun from outside our protective atmosphere.

In the 1970s, the Voyager spacecrafts were the first to confirm the existence of the solar wind. It was not until Skylab that increases in radiation and the solar wind were linked to solar flares, and coronal mass ejections (CME) were first detected. The sun-earth interconnect finally became a scientific fact. Since then, numerous satellites and ground based instruments monitor the sun and our geomagnetic field in realtime. Today, the radio amateur and QRPer has a wealth of solar information available via the internet that professional astronomers did not have a decade ago. This article, in part, describes how to interpret this internet data, and some of the terminology encountered in the daily reports and solar data from NOAA. Much of the solar physics in this article has been developed by astrophysicists in the past 15 years, and not yet available in other than scientific journals
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If the sun radiated as a thermal source only, the received brightness would vary directly with frequency – from ultraviolet and visable light down into the radio spectrum. This is called Plank’s black body radiation law. Optical observations at different wavelengths does follow the black body radiation, proving the visible and optical wavelengths from our sun are thermally generated. However, radio energy does not follow black body radiation, proving the radio energy from our sun is being generated by processes other than heat

. Deep in the core of the sun is a massive thermo-nuclear reactor generating very short wavelength energy (gamma and x-rays). As this energy works its way to the surface of the sun, the wavelength gets elongated, or stretched, into the radio wavelengths, becoming the background radiation from the sun – called the solar flux (SF). It is measured at several observatories and reported daily by the National Oceanographic and Atmospheric Administration (NOAA) at their website: http://www.sec.noaa.gov/today.html. The solar flux is low during the quiet sun (SF <100) and elevated during the active sun (SF >100). In short, the solar flux is a measure of the ionizing radiation from the sun, and an indicator of the electron density of our ionosphere. The higher the electron density, the more reflective our ionosphere is to HF signals, and the higher the maximum usable frequency (MUF).

The solar flux is measured at 2880 MHz (10 cm), a frequency not generally affected by solar flares, and one where our atmosphere is very transparent. Occasionally, a large solar flare will increase the 2880 MHz solar flux. NOAA will report this event as a ten-flare, indicating the 10cm solar flux value has been contaminated by a solar flare. Most people ignore the elevated solar flux from a ten-flare. However, it does indicate the earth was exposed to increased ionizing radiation from the solar flare – ionizing our E and F layers above the normal solar flux. QRPers should check the higher bands for openings for several hours following a solar flare, or a ten-flare event, due to the enhanced E/F layer ionization, possibly temporarily raising the MUF. The daytime ionizing radiation from the sun strips electrons away from their host molecules in our upper atmosphere. These free electrons increase the electron density of the ionosphere, stratify into layers, and called the D, E and F layers. The E/F layers are reflective to HF signals below the MUF, reflecting them back to earth for long distance communications. This is generally called skip propagation. The HF signals must also pass through the D-layer, the closest to the earth’s surface. This is called the absorption layer, since some of your HF signal will be absorbed by the D-layer – in fact, twice – going to, and coming back from the E/F layers for 2-6dB total path loss. At night, solar radiation ceases and the free electrons recombine with their host molecules. The D-layer completely disappears and offers no signal loss. The E/F layers merge into a single layer, but remain reflective to HF signals. However, this combined layer has a lower electron density than daytime levels, lowering the MUF. Astronomers call these ionization layers plasma layers and the lowest frequency that escapes into space the plasma frequency, fp. QRPers look at it just opposite – what is the highest frequency that does not escape into space? We call this the maximum usable frequency or MUF. In reality, the MUF and plasma frequency are exactly the same. During the active sun, Earth’s plasma frequency is about 18 MHz (nighttime) to 30+ MHz (daytime), and during the quiet sun, varies from about 10MHz (nighttime) to around 20 MHz (daytime). Interestingly, the sun’s plasma frequency varies between 300–1000 MHz.

The only time strong HF radiation escapes the sun is during a solar flare, and when it does, it is called a solar storm. QRP Propagation Hint: Ionization Sunspots and the Solar Cycle The solar cycle was first realized by noting that sunspots “come and go” over a 7–11 year cycle. Sunspots are cooler areas on the the solar surface. More recently, sunspots have been identified as regions with strong magnetic fields called Alpha, Beta and Delta groups. These active regions are carefully watched for possible flare activity. An Alpha group are sunspots with no bipolar magnetic fields, and seldom produce a flare. When bipolar magnetic fields (with N-S, or +/– polarization) develop between sunspots, it is called a Beta group. When a Beta group becomes particularly intense, with strong, bipolar magnetic fields between sunspots, it is called a Delta group. Amajor flare alert is issued by NOAA when a Delta configuration develops. A major solar flare will always occur from a Beta or Delta group, but, not all Beta or Delta groups will produce a flare. Terminology seen in the NOAA reports will be the umbra, the central core area of the ALPHA Unorganized, unipolar Little threat, but watched magnetic fields for further growth BETA Bipolar magnetic fields C class flares and between sunspots possible M class flares DELTA Strong, compact bipolar High potential for large fields between sunspots M or X class flares Sunspot Class Description of the Active Region Potential for Solar Flare Activity

It is believed the filamentary structure of the penumbra is “painting” a picture of the magnetic field lines emanating from the sunspot. Often NOAA will report that a Beta group shows rapid growth in the penumbra. This means the magnetic field lines of the sunspot disturbance are rapidly growing, likely into a Delta group, becoming a high candidate for a major flare. Until recently, the physics behind a solar flare was not well known. They are extremely energetic events on the sun that can produce emissions across the entire spectrum – in the optical wavelengths, gamma and x-rays, down to the HF frequencies. The Physics of a Solar Flare In many ways, a solar flare is very similar to a nuclear detonation. Imagine for a moment, you are about to witness the detonation of an atomic bomb, say the Trinity test in New Mexico on July 16, 1945. Sitting in the main bunker, you are surrounded by radio equipment to see what affect, if any, will occur to the HF and VHF frequencies. At 5:29 am, the world’s first atomic bomb is detonated. You see a tremendous flash of light; the gamma and x-ray detectors are immediately triggered, and you hear deafening blasts of noise coming from your receivers. Light, ionizing radiation and radio waves are the first forms of energy to arrive at the bunker – instantly. They are traveling at relativistic speeds – the scientific terminology for the “speed of light.” For the first few seconds, you see a huge, brilliant bubble of plasma and burning gasses. Everything inside this bubble is vaporized from the extreme temperatures. After several seconds, the production of gamma and x-rays begins to subside. After about 10 seconds, the rising gas cloud begins to make the familiar mushroom shape. These are the hot burning gasses, electrons, protons and debris rising at about the speed of sound, or at sonic speeds. Along the ground, you can see a wall of debris being blown away from the explosion by the shock wave. Several seconds after the detonation, you hear the thunderous boom of the air shock arriving. Several minutes later, the shockwave hits the bunker. This is a blast of hot wind traveling over 100 mph, carrying with it dirt, rocks and other debris carried along the way. A solar flare is not much different.

While the exact mechanism triggering a flare is not precisely known, it is believed that the strong magnetic field lines emanating from the sunspots becomes so strong, hot burning gasses , White Sands Missile Range from the sun are suddenly sucked out of the interior and carried along the magnetic field lines of the disturbance in a violent explosion. While the interior of the sun is exposed at the flare site, gamma and x-rays are allowed to escape, traveling outward at relativistic speeds. This explosion creates a shockwave, at supersonic  speeds, usually around 1,200–2,000 km/sec. Being well above the 350 km/sec. escape velocity of the sun, this shockwave carries some of the burning solar mass out into space. This shockwave and rising gas cloud of solar mass, being ejected from the sun, is called a Coronal Mass Ejection, or CME. It is traveling outwards at supersonic speeds and could strike the earth if the trajectory and geometry is correct. Some of the burning mass gets caught in the magnetic field lines of the disturbance, forming an illuminated loop or halo, and called a full-halo CME.

The key point is a solar flare releases several major forms of energy that can effect VHF and HF propagation on earth:
1. Ionizing radiation, electrons and protons at relativistic speeds (arrives at earth immediately and for the duration of the flare event)
2. A supersonic shockwave riding along with the solar wind
3. Dense particles behind the shockwave (2 and 3 arrives at earth 2-3 days after the flare event) The emissions from a solar flare in the optical (visible) wavelengths are, for a typical short-duration flare and the less common long-duration flare. An actual brightness plot of a flare, measured by photon counts from satellite instrumentation, is also shown. This is the optical evidence of the flare, which is actually very difficult to detect due to the normally bright surface of the sun. For this reason, flares are now determined by the x-ray radiation, detected onboard the GEOS, LASCO and SOHO satellites, not optically.

The optical properties of a flare are not particularly important to the ham or QRPer, other than indicating that other things are about to come!  The hard x-rays, those >30 kev, is the ionizing radiation striking the earths atmosphere. The hard x-rays last only a minute or two, while the soft x-rays can persist from tens of minutes to over an hour – all the while showering the earth with ionizing radiation. X-rays from very large flares can also penetrate our atmosphere, all the way to the ground (a GLE, or ground level event). This will highly ionize the D-layer as well, causing an HF radio blackout for several tens of minutes following a major flare. This is fairly rare, occurring only a few times each solar cycle. If you’re in a QSO when a major flare causes an HF blackout, it seldom lasts more than an hour. If you’re working a contest, this hint could be useful. Take a break, but don’t QRT! These x-rays do provide extra ionization to the E/F layers for improved reflectivity and a higher MUF.

Exploit the benefits of a solar flare. Good DX contacts are possible immediately following a solar flare until sundown due to the improved reflectivity (better signal-to-noise ratio for QRP signals) and the higher MUF opening the higher bands – especially during the solar minimum years. OPTICAL EMISSIONS X-ray and Gamma radiation from a Solar Flare QRP Propagation Hint: QRP Propagation Hint:... Actual brightness plot of a short duration M-class solar flare Optical brightness 0 10 20 30 Time after Flare (minutes) .

For the radio amateur and QRPer, the real interest lies in what happens to the HF bands. Radio emissions from a flare can cause noise bursts, buzzing sounds, sudden QSB, continuum noise, and occasionally, a temporary HF blackout. After about 30 minutes following the flare, HF noise levels and propagation return to normal. The most important thing to remember about a solar flare is this: the HF effects are generally only for the duration of the flare event (20-60 minutes) and seldom effect frequencies <10 MHz. The most damaging effects of a solar flare is actually the arrival of the shockwave 2-3 days later, triggering a geomagnetic storm. This is discussed beginning on the next page (Geomagnetic Storms). The following details of a solar storm is offered for completeness only. This is relatively new solar physics theories, and presented for those so interested, as the information is currently available only in professional astrophysical journals, and certainly not in amateur publications. The first radio emissions to arrive on earth following a flare is the bursty Type III storm occuring for the first 5-6 minutes following a flare. These are relativistic electrons released by the flare traveling through QRP Propagation

. The radio emissions begin around 300 MHz and drift downward in frequency at about 20 MHz/sec.. They sound like ignition noise from a fast running engine, or sometimes a “buzz” as they sweep past your frequency. Seldom will these bursts be heard below 10 MHz. Some of these electrons migrate and travel along the open field lines in a spiraling motion, still about the speed of light, producing continuum noise (wideband) from 10–300 MHz. As the shockwave travels through the sun’s magnetic field lines, electric currents and bursty radio emissions are generated by the dynamo effect, called a Type II storm. The sun’s plasma frequency becomes lower at greater distances. Therefore, as the shockwave travels away from the sun, the bursts are heard at lower and lower frequencies on earth, .This is important to astronomers. By measuring the time is takes for the bursts to drift from one frequency to a lower one, the velocity of the shockwave can be determined. . It can produce powerful radio energies for several minutes following a flare, sometimes disrupting satellite and VHF communications. Radio telescopes use 2–10 GHz (S,C and X band) to make maps of the fine structures of the solar flare. 1.4 GHz, the spectral line of hydrogen (L band), is also mapped to show the intensities of local hydrogen and HII during a flare. This reveals the amount of ionization, and recombination near the sun’s surface. This is interesting from a science viewpoint, but not necessarily for ham radio. Approximate plasma freq. vs. distance from the sun solar wind about 350 km/sec., an increase to 600 km/sec. generally triggers a minor geomagnetic storm, around 1000 km/sec. a major storm, and much above that, a severe storm. These, of course, are all rough estimates. The shockwave also travels through the strong magnetic field lines of the disturbance, where the electrons and particles get trapped in the closed field lines. This also produces a bursty radio emission called a Type I storm. These drift downward in frequency at about 2 MHz/sec. and sound like ignition noise from an idling car.

A Type I storm can extend to around 10 MHz and persist for 20-30 minutes following a major flare. Behind the shockwave is a gas cloud of particles from the flare, generating wideband noise called a Type IV Continuum Storm. The noise begins around 1 GHz. The higher the gas cloud rises, the lower in frequency will the noise escape the sun. (That solar plasma frequency thing again). These particles rise until the pressure of the gas cloud equals the pressure of the solar atmosphere. At this point (about 15-30 minutes following the flare), the particles become stationary and generate noise down to 10-20MHz, depending upon the height of equilibrium. The Type IV storm can persist for hours following the flare and is an overall elevation of noise on HF. The exact mechanism of this noise emission from the gas cloud is not well understood. Disturbances to the solar wind, from a solar flare or coronal hole, can cause serious disruptions to HF by triggering a geomagnetic storm. The solar wind is the constant outflow of gasses, electrons, and particles from the sun and travel along the ecliptic plane, as shown in Fig. 12. It was long believed that the solar wind was fairly constant, at around 350 km/sec., the escape velocity of the sun. We now know that the solar Radio Emissions due to the Gas Cloud 

A Few Final Thoughts
The solar flare occurs at time 0, noted on earth by 10-30 minutes of noise bursts (Type I, II, III bursts) and elevated noise. Almost immediately, the ionizing radiation increases the MUF (whether or not it increases the solar flux). 30 minutes or more after the flare, HF noise levels return to normal, quiet conditions. This is an excellent window for QRPers, right after the flare. As soon as the solar storm ceases, HF noise levels become quiet with an elevated MUF, lasting until sundown. Night time conditions on 80-40M can be excellent. The daytime MUF the next day may be elevated as well. Shortly after day 2, the shockwave arrives, compressing our magnetic field and triggering a major geomagnetic storm. HF noise levels immediately rise, and in severe cases, may cause an HF blackout. Electrons from the shockwave enter the earth at the poles, causing a Polar Cap Absorption (PCA) event. This causes blackout conditions on HF in the higher latitudes. The next 3-hourly K-Index will be high (6–9), sufficient to also trigger auroral activity. A major geomagnetic storm (K>6) can last 12–24 hours. When it finally subsides, our magnetic field often becomes very quiet, producing low noise levels on HF. This is the other window for QRPers, when the geomagnetic storm subsides. Night time noise levels on 40-80M can be very low.
1. The solar flux, indicating the level of ionization, affects HF propagation above about 10 MHz. The solar flux does not affect 40M and below, since the MUF seldom drops below 10 MHz. This is why the lower bands are always open.
2. The K-index, indicating the geomagnetic condition, indicates HF noise primarily below about 10 MHz, except in severe cases. During a storm, high noise levels on 40M doesn’t mean high noise on 20M.
3. 30M is the ham band caught between the 2 worlds. It can be affected by both solar flux and the K-index. On the other hand, it is more often not bothered by either. It is a good band throughout the solar cycle.
4. Every solar flare and the resultant storm is different. No two are alike, nor accurately predictable.
5. Never let reports of flares or geomagnetic storms scare you from getting on the air and checking it out.

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Credits: M0OBU