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SSN, SFI, Solar Data for HF Radio Propagation
Here are some of the important Solar activity parametric data that are responsible for influencing the behavior of the Ionosphere on earth. These, in turn, are instrumental in determining HF radio propagation conditions on various bands. The information presented here is automatically updated on a regular basis.

The data-set is presented in two formats below. They are the graph format and a tabular metrics format. The graph which displays the history of measured SSN and SFI derived SSN – SSNf(10.7) that prevailed over the last 60 days is updated once a day, while the Solar terrestrial data metrics table is updated on an hourly basis.

This page also explains each and every Solar Data Parameter that we have covered here so that amateur radio operators irrespective of their technical understanding could get a reasonable insight into how HF radio propagation on Earth is influenced by the Sun’s activity. This way, one can watch for changes in various critical parameters and make an informed decision related to HF radio band opening prospects.

Why is SSN regarded as important parameter for HF radio Propagation assessment?
SSN and SFI derived SSN - SSNf(10.7) HF propagation chart

Smoothed Sunspot Number (SSN) and SFI derived SSN (SFIf(10.7)) data graph as they prevailed over the period of past 60 days. The general trend of expected terrestrial ionospheric activity governing HF radio propagation may be estimated from the above data. The graph is updated once a day.

The above illustration displays two graphs. The blue-colored curve represents the SSN that is measured by counting of observed Sun-Spots at several solar observatories around the world. The red-colored curve is derived from the measured Solar Flux Index (SFI). SFI is measured in the microwave band at a wavelength of 10.7 cm (2800 MHz) using a receiver with a capture window of 100 MHz. This is the global standard. The equivalent SSN, designated as SSNf(10.7) is computed from the SFI value using a standardized set of equations. This SFI derived SSN provides us a more realistic correlation to the actual effective SSN that prevails at any point in time... The SFI derived SSN (red curve) is usually a better choice and yields far more accurate results.

What are the other Solar data attributes that influence HF Radio Propagation?
Solar data chart for HF propagation

Several vital Solar Data Metrics that influence and characterize the overall health and behavior of the Earth's ionosphere are provided here and are obtained from real-time satellite data. This data metrics chart is updated once every hour.

This is a Solar Activity Data Metrics Table which is updated every hour at the turn of UTC hour. The table presents several vital parameters which determine the ionospheric condition and HF Radio propagation behavior around the world. Although some parameters change less frequently, there are others that change on an hourly basis.

SSN - The daily SSN values are derived from the visual count of sunspots and spot clusters on the surface of the sun between 16-26 degrees latitude of the Sun. They are counted using optical telescopes at various observatories around the world according to "Wolf" method. The observations are collated and averaged to arrive at the global Sunspot Number. A 24 hour smoothed value is presented as SSN. These Daily SSN values are never ever directly used for forecasting or computing propagation conditions. The ionospheric behavior is far more complex than having a direct correlation to daily SSN. Our ionosphere is very lethargic to change and has large inherent momentum.

SSNe - This is the true effective Sunspot number and is as close to realism as it could be. SSNe (Effective) is based on actual hourly measurements of F2 layer Critical frequency (f0F2) taken at 29 distinctively separate geographic locations around the world. The raw f0F2 data drawn from 29 observatories are collated and go through complex computations to arrive at SSNe value. We do automated hourly computations and updates at our end and maintain a database of SSNe values.

SSNe5 - This parameter is a 5 days exponential moving average of the hourly SSNe values. SSNe5 is also computed every hour based on previous 120 SSNe hourly values by deriving an exponential moving average.

SSNf(10.7) - This is the SSN value computed from the Solar Flux Index (SFI). Hourly SFI feed from the value measured by orbiting satellites. SFI data is processed using established algorithms to determine an equivalent SSN value. Since it is derived from SFI this parameter is termed as SSNf.

SSNf90 - This parameter is a 90 days exponential moving average of the hourly SSNf values. SSNf90 is also computed every hour based on previous 2160 SSNf hourly values by deriving an exponential moving average.

SFI - This is the Solar Flux Index (SFI). It is measured using sensitive satellite-borne microwave receivers operating at 2800MHz (10.7cm λ) with a capture bandwidth of 100MHz. The received signal is integrated over the capture (noise) window and the strength of this signal is used to determine SFI.

SFI90 - This parameter is a 90 days exponential moving average of the hourly SFI values. SFI90 is also computed every hour based on previous 2160 SFI hourly values by deriving an exponential moving average.

SWS-T - This is the T-Index from the Australian Ionospheric Prediction Service (IPS). This parameter SWS-T is also another method of measurement similar to SSN but it has no direct correlation to SSN. Like SSNe it also uses a measurement paradigm based on foF2 ionosonde measurements but the methods are different. Although SWS-T is relatively new, it tends to correlate with SSNe values to a considerable extent and appears to be very promising.

X-Ray - This is the measurement of X-Ray at the wavelength of 1-8 Angstrom region (0.1-0.8 nm). Hence it is known as "1-8A X-Ray". The values are typically displayed for instance as B2.4, where "B" represents e-7 (to the power -7). Therefore B2.4 means 2.4e-7. Similarly, "A" is e-8 and "C" is e-6 and so on. The measurements are taken by NOAA's GOES satellites with an integration interval of 1 minute. Solar X-ray emissions are measured by sensitive instruments onboard GOES satellites. The X-ray radiations from the sun take approximately 500 seconds (8.33 minutes) to cover the distance from the sun to Earth. Any solar event like Flares or CME results in a sudden increase in X-ray energy emission from the Sun. This in turn reflects itself as enhanced X-ray field strength reaching the earth. Though solar X-ray is very important in maintaining good ionospheric health and maintaining healthy slab densities, excessive X-ray often penetrates deep through the earth's upper atmosphere and the upper ionospheric region to cause excessive ionization at the D-layer region. This could lead to HF communication difficulties or blackouts.

ap Index - This an estimated Geomagnetic Index estimated by integration over 3 hours. ap index is related to the amplitude” of magnetic activity based on K index data from 11 Northern and 2 Southern Hemisphere magnetic observatories between the geomagnetic latitudes of 46 and 63 degrees. ap Index is derived from Kp index and features a scale from 0-400

Ap Index - It is a planetary average of A-Index. This is an estimated Geomagnetic Index estimated by integration over 24 hours and presented as a simple moving average of 8 values every 3 hours. Ap scale is from 0-400

Kp Index - This is an estimated Geomagnetic Index estimated by integration over 3 hours and a planetary average of K Index. This is a different unit of measurement in comparison to Ap. The Kp scale is between 0-9 with a resolution of 0.33 and has a quasi-logarithmic scale. The Kp Index is represented either a centric value like 3o meaning 3.0 or 3- meaning 2.66 or 3+ meaning 3.33 and so on. Kp Index is derived from measurements obtained from 13 geomagnetic observatories between 44 degrees and 60 degrees northern or southern geomagnetic latitude

Solar Wind Proton Flux Emissions
Proton flux emitted from the sun is particle matter streams that spread out in all directions into the entire solar system. The proton flux stream that reaches the Earth engulfs the globe and a portion of it also penetrates through the upper atmosphere's Exosphere region by piercing the Magnetopause (Magnetosphere's outer boundary) and in turn is instrumental in additional ionization of gas molecules in the Ionospheric region. This kind of enhanced ionization further strengthens the ionospheric charge density to alter (usually enhance) the HF band terrestrial propagation conditions. However, on rare occasions, turbulent proton flux might have the opposite effect.

Apart from the portion of the proton flux that penetrates the Magnetosphere, a large part of the proton stream, due to its interaction with the Earth's magnetic field, tends to bend the direction of the flux stream and align it along the Earth's magnetic field lines. These proton particles ultimately, after having traveled along the Geomagnetic field lines eventually terminate at the North and South Polar regions. While doing so, as the proton flux streams glide along the magnetic field lines, they cut through the denser atmosphere near the poles, on their journey to their termination points on the Earth's surface at the poles. These higher density gas layer molecules have a high collision rate with the high-speed proton particle. As a consequence, electrons get knocked out, ionization of gasses occurs, and this leads to the production of energy across a wide spread of the electromagnetic wave spectrum. This produces enhanced polar region radio noise as well as produces the spectacular visual display in the sky, what we call the Aurora Borealis at the North pole and Aurora Australis at the South pole. In such situations, radio communication through the polar regions is often adversely influenced throughout the HF, VHF, and UHF range.

For the purpose of determining the influence of proton flux streams, we classify them according to their energy levels. The lower-speed streams take a longer time to reach the Earth and also contain less kinetic energy in the particles. As a consequence, their ionizing capacity due to molecular collision is often below the required threshold. Therefore, low energy streams are relatively benign. However, it is the higher speed proton flux with a higher kinetic energy that usually does all the work as explained above... Hence, we measure and classify proton flux according to their energy content.

Although, currently I do not provide regularly updated information on the proton flux levels, we typically measure them in Mega electron-Volts (MeV) and classify them as streams with energy levels >1MeV, >10MeV, and so forth. An important point to note is that typically, proton flux energy levels <2MeV do not usually result in ionization of gas molecules. The higher energy streams are the one's that influence radio communication conditions.

P.Flx(>1MeV) - This term is the abbreviated form of "Proton Flux Density". This is measured by NOAA's GOES satellites using an integration interval of 5 minutes. A continuous steady stream of proton emission from the sun spreads in all directions in the Heliosphere (inter-planetary space of the solar system). They normally travel at velocities ranging from 200-400Km/sec. Therefore the kinetic energy content is relatively low. The P.Flx(>1MeV) parameter is essentially a measure of this proton flux. (>1MeV) stands for flux with energy content "Greater than 1MeV" (Mega Electron-volts). Proton Flux with energy above 2MeV is required to ionize gases in the ionospheric region. Hence this measurement of proton flux in the energy region of 2-6MeV is important to estimate ionospheric health, slab densities, and thickness.

P.Flx(>10MeV) - This is also a measurement of Proton flux Density. however, this measures only the flux with an energy level greater than 10MeV. This is also measured by NOAA's GOES satellites using an integration interval of 5 minutes. Although normally continuous emission of proton flux occurs from the sun at a more or less steady rate with energy levels between 1-8MeV, High energy Proton Flux emissions (>10MeV) are produced primarily on account of Solar "Coronal Mass Ejection" (CME) that produce "Coronal Hole High-speed Stream" (CH-HSS). A huge amount of sub-atomic particles, protons, electrons, and plasma are ejected from the sun and travel at very high velocities of 400-600Km/sec. This is the manifestation of a Solar radiation storm which eventually develops into a Geo-magnetic storm. This strikes the upper region of the Magnetosphere called the Magnetopause. The concentrated heavy impact of this creates what is called "Bow Shock". The magnetosphere gets compressed on the daylight side of earth while it creates an extended tail elevated the magnetic field lines. At the Bow Shock, the charged solar particle due to the influence of the Geomagnetic field changes orientation towards the poles with the Bow Shock acting as the canopy along which particles slide. They contain several Giga-Watts of power and reach the poles to ionize the atmospheric gases encountered at heights of 50-150 Km to produce Aurora Borealis and Aurora Australis respectively. This solar metric is an indicator of Geomagnetic storms.

SSN, SFI, Solar Data for HF Propagation 1

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SSN SSNf(10.7) – Real-time Solar Data

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