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Countries for which HF communications are important operate observatories which observe the sun for sunspots and flares, which have various impacts on the ionosphere, and thus HF propagation.
The K-index quantifies the variation in the horizontal component of the earth's magnetic field. It is calculated 8 times per day, each figure covering 3 hours. Linear magnetometer readings are converted to a pseudo-logarithmic value, ranging from 0 to 9. 1 is calm, while 5 and greater are geomagnetic storms.
NOAA converts USGS K-index numbers to a G-scale. A Kp of 5 is a minor storm at G1. A Kp of 9 is a extreme storm at G5.
The A-Index is derived by converting the K-index to a linear a-index (lowercase is derived at one station). These 8 values are averaged to produce the daily A-Index, uppercase being global).As of mid-April 2025 these are currently being published by NOAA.
A and K values are determined for a range of stations, with the Fredericksburg (Virginia) and Australian A-indexes (Afr and Aaus) apparently being of most use to Australian stations, as they are published weekly in the ARNSW News. There are also planetary Kp and Ap indexes.
Increases in these indexes indicate increasing disruption of the geomagnetic field, causing disruption to HF radio communications, including a depression in the maximum usable frequency (MUF).
Storm level variations also cause things like disruption to communications and other systems, including potentially, power. Geomagnetic storms, rated from G1 to G5 are discussed below.
See also: Wikipedia: K-index
NOAA pages up as of mid-April 2025: Planetary K index, Station K and A indices including Boulder, Colorado. They also make rather cool hand-drawn solar maps.
Essential if NOAA is DOGEd more extensively, but useful more generally, The Government of Canada provides sub-national forecasts, selected from the Regional magnetic conditions and forecasts page, including some covering the northern US.
Meanwhile Mexico's LANCE / REGMEX provides: Local Conditions at Central-East Mexico. Both the above sites are in English, as is this paper on the Kmex index: "Kmex: the Mexican Geomagnetic K Index" by Pedro Corona-Romero, et al; available with annoying animated ads on Researchgate or on printed page 97 (PDF page 97) of Proceedings of the II Pan-American Workshop on Geomagnetism - II PANGEO (2018). ITU for Iturbide is nearest the border with the former Mexican state of Tejas, with indexes Kitu and ΔHitu.
Meteologix has extensive weather related coverage, including mapping of the K index. Examples include Cuba, Denmark, Ireland, Israel, New Zealand, Australia, Norway. The left-hand grey bars allow selection, or you can change he country code in the middle of the address.
As "Definitive" historical data is published for researchers, on some sites the current (or very recent) data is called a "Now Cast".
Australians can visit Australian Region Estimated K-Index. The site includes a dashboard of current Space Weather and an Education index.
Flares cause disruption to HF communications. They are classed A, B, C, M, and X, by the X-ray energy they emit, with a numerical suffix further defining the strength. Normally ranging from 1 to 9, the suffix for X is open ended.
Descriptions of the numbers are a little confusing, but using C as an example, it ranges from 10-6 to 10-5 watts per square metre, or 1 to 10 microwatts. Thus C1 is 1 μW, C2 is 2 μW, C3 is 3 μW, etc. From C9, or 9 μW, the next step is M1, at 10 μW. X1 is 100 μW, and X10 1000 μW or 1 mW per square metre. A decimal can be used, such as X8.8.
Class M1 causes minor reductions in HF propagation, becoming worse as the class increases, especially from X1. Class X with large suffixes can cause outages and damage to cable-based telecommunications and power systems. They also cause strong aurora, and aurora at low latitudes. The late 2003 event overloaded meters, but was likely X45, or 4.5 mW / m². Some reports use the initial estimate of X28.
As X-Rays are electromagnetic radiation they reach earth at the speed of light, meaning that there can be limited warning of their arrival. Coronal mass ejections are matter, travelling at high speeds, but there can be a warning of 1 or more days.
You can read more at: Wikipedia: Solar Flare
G is for Geomagnetic, as an interesting alphabet book would tell you. Geo indicates that something relates to the Earth. Thus G1 to G5 are increasingly severe geomagnetic storms. The letter / number combinations in the distractors are for flare intensity, the cause, not the result. For communications they reduce the maximum user frequency. Currents induced into power transmission systems can cause over-voltages, including damaging transformers. This is/was also a problem with wire based systems such as telegraphs. They can impact satellites including navigation ones. Large currents are induced into oil and gas pipelines (hundreds of amps at G5), and aurora may become visible at low latitudes (dark areas west of Sydney, rather than Tasmania; northern Mexico, rather than Canada).
Various fictional accounts discuss the breakdown of society following widespread and prolonged power outages, and example being the initial series of the British TV series "COBRA", involving after a CME destroyed transformers, with the North being the area the Conservative PM left without power, due to there being one spare to few.
This is the "Voice Of America Coverage Analysis Program", and as the name indicates, this was written for the United State's government, specifically its shortwave radio based radio stations. The US Government aspect makes it ineligible for copyright, like the NEC antenna modelling software, and the NASA images I have used on my Posters.
You can thus use it online, or via download software, here: Map based online version, or Main site with download links.
While the traitor who sitteth upon the golden "throne" may have unlawfully destroyed VOA, the software in still available, as Jari OH6BG is in western Finland.
These are the actual questions from the Extra licence exam pool, as published by the NCVEC, for use in exams up to, and including, 30 June, 2028.
E3C01
What is the cause of short-term radio blackouts?
A. Coronal mass ejections
B. Sunspots on the solar equator
C. North-oriented interplanetary magnetic field
D. Solar flares
This asks about short term interruptions, so it is solar flares, answer D.
E3C02
What is indicated by a rising A-index or K-index?
A. Increasing disturbance of the geomagnetic field
B. Decreasing disturbance of the geomagnetic field
C. Higher levels of solar UV radiation
D. An increase in the critical frequency
This result in a measurable increasing disturbance of the geomagnetic field, answer A.
E3C03
Which of the following signal paths is most likely to experience high levels of absorption when the A-index or K-index is elevated?
A. Transequatorial
B. Through the auroral oval
C. Sporadic-E
D. NVIS
Renamed from "polar paths", it is paths going through the auroral oval which are are impacted, answer B.
E3C04
What does the value of Bz (B sub z) represent?
A. Geomagnetic field stability
B. Critical frequency for vertical transmissions
C. North-south strength of the interplanetary magnetic field
D. Duration of long-delayed echoes
Bz indicates the north-south strength of the interplanetary magnetic field, answer C.
E3C05
What orientation of Bz (B sub z) increases the likelihood that incoming particles from the Sun will cause disturbed conditions?
A. Southward
B. Northward
C. Eastward
D. Westward
Southward, answer A.
E3C06
How does the VHF/UHF radio horizon compare to the geographic horizon?
A. It is approximately 15 percent farther
B. It is approximately 20 percent nearer
C. It is approximately 50 percent farther
D. They are approximately the same
It is around 15% father, answer A.
E3C07
Which of the following indicates the greatest solar flare intensity?
A. Class A
B. Class Z
C. Class M
D. Class X
This is Class X, answer D.
E3C08
Which of the following is the space-weather term for an extreme geomagnetic storm?
A. B9
B. X5
C. M9
D. G5
Geomagnetic storm commences with G, and it is a 5 point scale, so it is G5, answer D.
The previous version of this question asked the meaning of "G5". Such storms can be disruptive to HF communications.
E3C09
What type of data is reported by amateur radio propagation reporting networks?
A. Solar flux
B. Electric field intensity
C. Magnetic declination
D. Digital-mode and CW signals
These systems decode Morse and digital signals, and report that they were heard, answer D.
E3C10
What does the 304A solar parameter measure?
A. The ratio of X-Ray flux to radio flux, correlated to sunspot number
B. UV emissions at 304 angstroms, correlated to solar flux index
C. The solar wind velocity at 304 degrees from the solar equator, correlated to solar activity
D. The solar emission at 304 GHz, correlated to X-Ray flare levels
This is a measure of solar radiation at Ultraviolet, answer B.
304 Å equates to 30.4 nm, a wavelength significantly shorter, and thus more energetic, than a disco or theatrical UV lamp, or even the sterilisation tubes which burnt the eyes of the Bored Ape tech-bros.
E3C11
What does VOACAP software model?
A. AC voltage and impedance
B. VHF radio propagation
C. HF propagation
D. AC current and impedance
Until they failed to dodge DOGE, VOA was the largest of the publicly funded US shortwave, aka HF band, broadcasters. Thus, it models HF propagation, answer C.
Perhaps these unconstitutional closures will be reversed one day.
E3C12
Which of the following is indicated by a sudden rise in radio background noise across a large portion of the HF spectrum?
A. A temperature inversion has occurred
B. A coronal mass ejection impact or a solar flare has occurred
C. Transequatorial propagation on 6 meters is likely
D. Long-path propagation on the higher HF bands is likely
This may indicate that a coronal mass ejection, with material impacting earth; or a solar flare has occurred, answer B.
In Australia the ABC (the larger publicly funded national broadcaster) tends to be allocated frequencies just above 500 kHz, as these provide coverage over hundreds of kilometres, especially in the evening, while the commercial stations tend to be from a little below 1000 kHz, and up. 2 CR on 549 kHz at 50 kW in Cumnock, between Orange and Peak Hill in rural NSW can cover from the Victorian border to the Queensland one, and in reality, probably further, covering an area larger that of the UK.
You can watch a 3 minute video on the Cumnock station on the ABC Central West page, or the 40 second version on Facebook.
BBC Radio Scotland on 810 kHz also provides great coverage to a significant portion of Norway, especially in the darkness of winter.
Often when you see say 70 cm and 2 metre Yagis on the same mast or pipe on a hilltop site, or on a home station, the UHF is placed above the VHF. Depending on the height, and the rate at which the ground drops away this may be OK. In other cases it is best to have the VHF at the top, due to the near-effects of the ground on the antenna, and the clearance needing to be greater, due to its longer wavelength.
"Ray tracing" describes the process of modelling a radio wave's path through the ionosphere. It was also a craze in the 1990s where a 486 or early Pentium was used to
On to: Practices 1 - Test & Measurement Gear
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, April 2024.
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