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Ionospheric HF Radio Propagation Theory in a Nutshell
By Rick Johnson, AE3C
(This is an excerpt from an article in production -- "Quick Post" 30 October 2003)

When we transmit on our radios, we use such words as 'joy' and 'magic' to describe the feeling we have when we make contact.

Knowing that pounding brass, speaking into a microphone, or typing into our radio/computers ("compradios?") causes a tone to sound, a voice to be heard, or words to appear on a screen -- over a great distance and by a completely invisible means -- amazes us.

Conversations that might otherwise seem banal or even incomprehensible take on meaning and significance to us when we accomplish them by radio.

Those of us who enjoy the HF ("High Frequency," 3-30 MHz) aspect of our hobby are generally aware that how we do this is in large part determined by the Sun. Somehow, in a not completely understood way, our efforts to qso can be thwarted or enabled by this very bright starlight.

Article Copyright 2003 Rick Johnson //  AE3C // Pittsburgh
Solar Activity
(courtesy Solar & Heliospheric Observatory

From millions of miles away, the Sun affects our ionosphere and our HF radio activity from that great distance and by a spectacular but largely invisible means. When we understand how what happens on the Sun changes our Ionosphere and our radio results, we enjoy our hobby more, and we increase our radio contacts.

The study of the Sun's effect on our radio efforts is as fascinating -- and as complex -- as the study of radio itself; both involve action over a distance by largely invisible device. This article's goals are to explain this phenomenon in complete and simple terms, and to provide a ready-reference glossary for Ionospheric propagation theory.

The basic theoretical concepts are deceptively simple; most of us studied these ideas when we first attained our licenses. What comes from the Sun and what it affects here on Earth can be summarized in three (seemingly) simple sentences.

(1) Ultraviolet radiation from Sunspots affects the F-Region of the Ionosphere.
(2) Soft X-rays from the Sun affect the E-Layer of the Ionosphere; hard X-rays affect the D-Layer.
(3) Plasma & energetic particles from Solar Flares, CMEs, and the Solar Wind affect the ionization, deionization, and thinning of the D- and E-Layers, and the F-Region.

Coronal Activity
(courtesy Solar & Heliospheric Observatory [ESA & NASA])

But here, fellow Radio Amateurs, is where the simplicity ends. Scratch the surface just a little deeper, and you will quickly find enormously intricate interactions requiring hyper-complex mathematical modeling using measurements from multi-million dollar devices.

There is hardly a middle ground between stating the simple concepts and understanding the complex theory. How can we enjoy the field of Ionospheric HF radio propagation theory without having to learn the equivalent of a post-doctoral degree course?

A good working knowledge of Ionospheric propagation theory can be had by an understanding of (1) what happens on the surface and corona of the Sun, (2) how what occurs on and near the Sun travels to the Earth, and (3) how this affects the magnetosphere and Ionosphere of the Earth.
This, coupled with (4) a good "Solar & Geomagnetic Report" tracking the effect of particles and electromagnetic radiation from the Sun ("Solar" report) upon the Ionosphere of the Earth ("Geomagnetic" report), and (5) an idea of how HF propagation is affected, gives us what we need.

This article attempts to give a thorough but comprehensible statement of the theory of HF Ionospheric propagation in a way that can be used as a refresher and reference tool, better to understand the theory and the tracking & measuring numbers. We begin with what happens on and around the Sun.

. . . What Happens on the Sun?

To answer the question of how activity on the Sun can make our radio equipment work or fail, first we must know what happens on the Sun. Of course, the Sun emits both electromagnetic radiation and energetic particles. Some comes from the Sun's surface, and some from its corona. The key point to remember is that different emissions from the Sun are at the exact wavelengths that most affect the Ionosphere, namely ultraviolet (UV) and X-rays.

First, let's look at the very surface of the Sun, the "photosphere." To understand the ultimate effect (propagation), we look first at the cause from the solar surface. We need to examine three things to understand radiation from the Sun's surface, sunspots, solar flux, and solar flares.

Approximately 50% of the Sun's radiation is in the form of infrared, 40% visible light, and about 10% ultraviolet. It is the ultraviolet radiation that interests us, because UV affects radio propagation by ionizing our Ionosphere's F-Layer. Most of the UV radiation comes from sunspots.

Coronal Mass Ejection
7 August 2002
(courtesy Solar & Heliospheric Observatory

-- Sunspots are dark regions on the solar surface (photosphere), or large concentrations of strong magnetic flux, many times the size of the Earth. They usually occur in pairs that often last for several days, and they release from their "plages" (edges) ultraviolet radiation.

UV travels to Earth via photons energized at UV wavelengths, both as particles and as waves, depending on how measured. More sunspots mean more ultraviolet radiation, increasing the F-Region ionization, and creating the F2-Layer during the day. Fewer sunspots weaken the F-Region and lower the MUF.
Sunspots generally follow an 11-year cycle, and are measured on a scale from 0-200. These sunspots numbers are a subjective visual measurement reported either under the "Boulder Sunspot Number" (NOAA Space Environment Center) or the "International Sunspot Number" (Sunspot Index Data Center, Belgium). The Smoothed Sunspot Number (SSN) is an averaged value used for tracking sunspot cycles.

SOLAR FLUX -- The measure of total radio emissions from the sun at 10.7cm (2800 MHz), on a scale of 60 (no sunspots) to 300, generally corresponding to the sunspot level, but being too low in energy to cause ionization, not related to the ionization level of the Ionosphere. Higher Solar Flux generally suggests better propagation on the 10, 12, 15, 17, & 20 Meter Bands; Solar Flux rarely affects the 30, 40, 60, 80, & 160 Meter Bands.

SOLAR FLARES -- Explosions usually near sunspots which eject particles and electromagnetic energy (X- & Gamma-Rays and increased UV) and energetic particles. Flares are measured by NOAA/GOES satellites, and are difficult to predict because visual evidence arrives simultaneously with the other electromagnetic radiation. Energetic particles from flares arrive later. Flares are classified according to their X-ray brightness (in the 1-8 angstrom band), among four categories, B-class, C-class (C1-C9) (small), M-class (M1-M9) (medium), and X-class (X1-X9) (large). X-Class Flares emitting in the 1-8 angstrom band can increase suddenly the ionization of the D-Layer and E-Layer, causing them to absorb higher frequencies, increasing the LUF. Flares can disturb the F-Region, cause it to absorb HF, and lower the MUF. When the LUF reaches the MUF, a Radio Blackout has occurred.

. . . What Happens Between the Sun and the Earth?

CORONAL HOLES -- An X-Ray & Ultraviolet darkened region of the sun's corona observed by the "Solar and Heliospheric Observatory" (SOHO). Coronal Holes are associated with an open region of the sun's magnetic field from which plasma escapes, causing the solar wind.

Ionospheric Layers and
their predominant ion population
(courtesy NOAA/SEC)

Solar Wind Dial
(courtesy NOAA/SEC)

CORONAL MASS EJECTION (CME) -- electrically conductive magnetic plasma released from the Corona into the solar system (and beyond) at speeds of 50-2000km/s. Because the Sun rotates every 27/28 days, CMEs can be periodic. CMEs interact with the Earth's magnetic field typically two to three days after they occur, altering the field, provoking Aurora, and causing geomagnetic storms. When the Earth's magnetic field is disrupted, the ionosphere is affected either by changes in F-Region ionization or deionization rates, or by a thinning of the F-Region, decreasing MUF, and preventing refraction of radio waves.

Coronal Mass Ejection and Aurora -- 14 July 2000 -- (courtesy Solar & Heliospheric Observatory [ESA & NASA])

SOLAR WIND -- Plasma (ionized hot gas), particles (electrons & protons), and magnetic 'clouds' from the Sun (the corona of the Sun expanding into interplanetary and interstellar space) travelling from 400 km/s (normal) to 900 km/s (high) in densities ranging from 1-100 protons/cm3. Solar Wind is measured by the "ACE" (Advanced Composition Explorer) satellite, orbiting at the Earth/Sun libration point ("L1" gravitational equilibrium). High solar wind can physically distort the Earth's magnetosphere, affecting the ionosphere by changing ionization or deionization rates or by thinning the F-Region (preventing refraction, lowering the MUF), and creating Aurora.

Quiet 0-7 0-1
Unsettled 8-15 2
Active 16-29 3
Minor Storm 30-49 4
Major Storm 50-99 5
Severe Storm >99 6-9

. . . What Happens to the Layers of the Ionosphere?

The electromagnetic radiation and energetic particles from the Sun affect the various layers of the Ionosphere. Of course, Ionospheric propagation occurs when HF radio waves are refracted by the various layers of the Ionosphere and returned to Earth. We need to understand the layers, what frequencies they absorb or refract, and what kind of radiation makes them absorb or refract differing frequencies.

D-LAYER -- The D-Layer, closest to the Earth's surface (50-90 km), ionizes most at noon (via 1-10 angstrom hard X-rays from the Sun), absorbing lower frequencies (10MHz and below), but allowing higher frequencies to pass to outer layers. Strong Solar Flares can cause sudden increases in D-Layer absorption. The D-Layer quickly deionizes at night. D-Layer absorption is monitored and reported by NOAA's Space Environment Center.

Coronal Mass Ejection -- 28 October 2003 -- 22:39 UTC
(courtesy MLSO)

E-LAYER -- The E-Layer (95-130km) is ionized (via 10-100 angstrom soft X-rays from the Sun) also only during the day. Strong Solar Flares can cause sudden increases in E-Layer absorption. The E-Layer ionizes and deionizes more slowly than the D-Layer, but can refract the higher HF & VHF frequencies that happen to penetrate the D-Layer.

F-REGION -- The F-Region (160-400km), ionized via 100-1000 angstrom ultraviolet radiation from the Sun. The daytime F2-Layer (250-400km) is the major factor in HF propagation, and can retain much of its ionization through the night. At night the two F-Layers (F1 & F2) combine into the F-Region, and continue to refract lower HF (10-15MHz) which at night are not absorbed by the D- & E-Layers. The highest frequency refracted by the F-Region is the "MUF."

. . . How are Ionospheric Events Observed and Measured?

The geomagnetic condition of the Ionosphere can be measured by a magnetometer. The state of the Ionosphere can be observed at a particular place, a particular time, or the measurements can be averaged over time or over the whole planet. The K-Index, Kp-Index, A-Index, and Ap-Index achieve this.

K-INDEX -- The overall geomagnetic condition of the ionosphere ("Kp" if averaged over the planet) over the past 3 hours, measured by 13 magnetometers between 46 & 63 degrees of latitude, and ranging quasi-logarithmically from 0-9. Designed to detect solar particle radiation by its magnetic effect. A higher K-index generally means worse HF conditions. A lower K-Index generally suggests better propagation on the 10, 12, 15, 17, & 20 Meter Bands; a low & steady Kp-Index generally suggest good propagation on the 30, 40, 60, 80, & 160 Meter Bands.

A-INDEX -- The overall geomagnetic condition of the ionosphere ("Ap" if averaged from the Kp-Index) (an average of the eight 3-hour K-Indices) ('A' referring to amplitude) over a given 24 hour period, ranging (linearly) typically from 1-100 but theoretically up to 400. A lower A-Index generally suggests better propagation on the 10, 12, 15, 17, & 20 Meter Bands; a low & steady Ap-Index generally suggest good propagation on the 30, 40, 60, 80, & 160 Meter Bands.

AURORA -- Visible regions of splendorously colorful lights radiated by charged particles trapped in the northern and southern polar regions of the Earth's magnetic field, exacerbated by solar flares and CMEs. Northern Hemisphere Aurora is reported by NOAA's Space Environment Center.
GEOMAGNETIC STORM -- A planetary disturbance of the Earth's magnetic field, which may affect the Ionosphere. Storms are caused by Solar Flares and by CMEs. Measured by the Ap-Index, the storms are classified as minor (Ap between 29 & 50) major (Ap between 50 & 100), and severe (Ap over 100).

IONOSPHERIC STORM -- A planetary disturbance of the Ionosphere, which may affect HF propagation. Sudden increases in HF absorption by the D- and E-Layers is called a sudden Ionospheric disturbance ("SID"). Six Ionospheric conditions are index by the A- & K-Indices.

Quiet 0-7 0-1
Unsettled 8-15 2
Active 16-29 3
Minor Storm 30-49 4
Major Storm 50-99 5
Severe Storm >99 6-9

. . . How is HF Radio Propagation Affected?

The entire net effect of changes in Ionospheric refraction and absorption is nothing more than changes in the MUF and LUF.

MAXIMUM USABLE FREQUENCY (MUF) -- The maximum radio frequency that the ionosphere can refract over a selected point-to-point path. Higher ionization of the F-Region increases the MUF because the F-Region becomes more refractive; lower ionization decreases the MUF. MUF is lowered by the effect of solar flares and CMEs.

Radio Blackout occurs when the LUF exceeds the MUF, often following a severe X-Ray Event (courtesy NOAA/SEC)

LOWEST USABLE FREQUENCY (LUF) -- The lowest radio frequency that the ionosphere can refract (with an intelligible signal-to-noise ratio) over a selected point-to-point path. Higher ionization of the D- & E-Layers increases LUF because the D-Layer absorbs higher frequencies; lower ionization decreases the LUF. Geomagnetic storms increase the LUF

RADIO BLACKOUT LEVEL -- A condition where the MUF is lowered by decreased ionization of the F-Region (less ultraviolet radiation from sunspot plage, lower Solar Flux, disturbance by X-Class Flares, deionization or thinning by CMEs or high solar wind often accompanied by Aurora), and simultaneously the LUF is increased by more D-Layer absorption (X-Class Flares ionizing the D-Layer)

. . . Conclusion

The next time you look at a "Solar & Geomagnetic Report," pay close attention to the measurements of the Sun (Sunspot Number, Solar Flux, Flares, CMEs, and Solar Wind). Next, look at the condition of the Ionosphere (K-Index, A-Index). Interpret the numbers in light of these explanations, and try to predict the MUF and LUF. I assure you, your HF radio propagation attempts will either work or not; your understand of why or why not will bring you more joy.

73 de AE3C / Rick
< ae3c @ >

. . . More Information / Bibliography [by NW7US] -- "Flash" Primer
G3YWX, QST September 2002
NJ2L QST Nov 1991
ARRL Handbook 2002
Additional Links:
Solar Flux

Draft 09 July 2003 12:24z
Quick Post 30 October 2003

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