Solar flux, the Ap Index and Kp Index

Solar Cycle 25

Solar cycle 25 is now showing signs of life. What happens on our sun affects radio propagation around the world.

Solar indices are a measure of activity on the sun.   These indices can be used by amateur radio operators to get and sense of how radio signals will propagate globally and when to expect radio blackout. Sites like Solar Ham give a good insight into the sun’s current activity, presenting easy to consume solar weather. But what does it mean? What’s good for radio and what’s not?

The solar cycle is a roughly 11-year periodic change in the Sun’s sunspot activity, measured by the variation in the number of sunspots observed.  The Sun just ended its 24th solar cycle in 2020 and is now entering Solar Cycle 25.

Our Sun is a huge ball of electrically-charged hot gas. This charged gas moves, generating a powerful magnetic field.

The Sun’s magnetic field goes through a cycle, called the solar cycle. Every 11 years or so, the Sun’s magnetic field completely flips. This means that the Sun’s north and south poles switch places.

The solar cycle affects activity on the surface of the Sun, such as sunspots which are caused by the Sun’s magnetic fields. As the magnetic fields change, so does the amount of activity on the Sun’s surface. Giant eruptions on the Sun, such as solar flares and coronal mass ejections, also increase during the solar cycle.

This activity can have effects on Earth. For example, eruptions can cause lights in the sky, called aurora, or impact radio communications. Extreme eruptions can even affect electricity grids on Earth.

Solar Cycle 25 is coming to life

This split image shows the difference between an active Sun during solar maximum (on the left, captured in April 2014) and a quiet Sun during solar minimum (on the right, captured in December 2019). December 2019 marks the beginning of Solar Cycle 25, and the Sun’s activity will once again ramp up until solar maximum, predicted for 2025.
Credits: NASA/SDO

Last Updated on November 11, 2022

3 Band Condition Indicators

Solar Flux

One of the key solar indices is a measure known as solar flux. It’s used as the basic indicator of solar activity, and to determine the level or amount of radiation being received from the Sun.

The higher the solar flux, the better for amateur radio.

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. 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.

Solar Flux Units (SFU)

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 HF conditions will not be very good. Conversely, high values generally indicate there is sufficient ionization to support long-distance communication at higher-than-normal frequencies.

Solar flux index

Typically values in excess of 200 will be measured during the peak of a sunspot cycle with high values of up to 300 being experienced for shorter periods.

Apart from the Solar Flux, another important influence on the ionosphere, and hence radio propagation prediction, is the level of geomagnetic activity. While the geomagnetic activity is a measure of the state of the Earth’s magnetic field, this, in turn, is influenced by the Sun. To indicate the state of Geomagnetic activity, there are two indices that are used that are related to each other.

The K Index and the A index.

Although different, both these indices give indications of the severity of magnetic fluctuations, and hence the level of disturbance to the ionosphere.

The K index is a three hourly measurement of the variation of the Earth’s magnetic field compared to what are “quiet day” conditions. The measurement is made using a magnetometer. This indicates the variation of the magnetic flux in nanoTeslas. This reading is then converted to the K index. The relationship is quasi-logarithmic, that is, an almost directly proportional on a logarithmic scale.

The A index is a linear measure of the Earth’s field. As a result of this, its values extend over a much wider range. It is derived from the K index by scaling it to give a linear value which is termed the “a” index. This is then averaged over the period of a day to give the A index. Like the K index, values are averaged around the globe to give the planetary Ap index.


Values for the A index range up to 100 during a storm and may rise as far as 400 in a severe geomagnetic storm.

Ap and Kp Indecies

There are two solar indices that are used to determine the level of geomagnetic activity… the A index and the K index.

These indices give indications of the severity of the magnetic fluctuations and hence the disturbance to the ionosphere. The first of the two indices used to measure geomagnetic activity is the K index.

K index

The K-index quantifies disturbances in the horizontal component of earth’s magnetic field with an integer in the range 0–9 with 1 being calm and 5 or more indicating a geomagnetic storm.


The K index is a “quasi logarithmic” number and as such cannot be averaged to give a longer-term view of the state of the Earth’s magnetic field, so the A index was created which is a daily average.

A index

During very severe geomagnetic storms the A index can reach values of up to 200 and very occasionally more.


The A index reading varies from one observatory to the next since magnetic disturbances can be local. To overcome this, the indices are averaged from Northern and Southern Hemisphere monitoring stations to provide the Ap index or the planetary value.

Similarly, the Kp index is the planetary average of all the K indices at observatories around the globe.

K values between 0 and 1 represent quiet magnetic conditions and this would indicate good HF band conditions, subject to a sufficient level of solar flux.

Values between 2 and 4 indicate unsettled or even active magnetic conditions and are likely to be reflected in the degradation of HF conditions. Moving up the scale, 5 represents a minor storm. A K value of 6 represents a larger storm and 7 through 9 represents a very major storm that would result in a radio blackout of HF communications.

Relationship between Kp index and A Index
Ap Index Kp Index Description
0 0 Quiet
4 1 Quiet
7 2 Unsettled
15 3 Unsettled
27 4 Active
48 5 Minor storm
80 6 Major storm
132 7 Severe storm
208 8 Very major storm
400 9 Very major storm
What to look out for, for good DX
Solar Indices for best conditions

Generally, the higher the flux, the better the conditions will be for the higher HF frequencies and even 6 meters. However, the levels need to be maintained for some days. In this way, the overall level of ionization in the F2 layer will build up. Typically values of 150 and more will ensure good HF band conditions, although levels of 200 and more will ensure they are at their peak.


The level of geomagnetic activity has an adverse effect, lowering the maximum usable frequencies. The higher the level of activity as reflected in higher Ap and Kp indices, the greater the depression of the MUFs. The actual amount of depression will depend not only on the severity of the storm but also its duration.

For best conditions, the solar flux should remain above about 150 for a few days with the K index below 2.

When these conditions have been met, check out the bands and expect some good DX to be about!

For the latest solar indices, visit Solar Ham. This is not the only sight for current information, but it’s an easy to follow site which has current the latest images of the sun as well as graphs and predictions. You can also check out spaceweather.com which is another reputable site. And don’t forget closer to home, the Australian Bureau of Meteorology has a Space Weather Service. Finally, for more information about the Ionosphere, you’ll find details on the Sporadic E page.

The Carrington Event – The biggest solar storm on record

The Carrington Event was a powerful geomagnetic storm on September 1–2, 1859, during solar cycle 10. A solar coronal mass ejection hit Earth’s magnetosphere and induced the largest geomagnetic storm on record. The event was named after British astronomer Richard Carrington who witnessed the event. He was the first to realize the link between activity on the sun and geomagnetic disturbances on Earth.


People in the northeastern United States could read a newspaper by the aurora’s light. The aurora was visible from the poles to low latitude areas such as south-central Mexico, Queensland, Cuba, Hawaii, southern Japan and China, and even at lower latitudes very close to the equator, such as in Colombia.


The geomagnetic disturbances were strong enough that the United States that telegraph operators reported sparks leaping from their equipment. Telegraph systems all over Europe and North America failed, in some cases giving telegraph operators electric shocks.


Modern society depends on a variety of technologies susceptible to the extremes of space weather. Strong electrical currents driven along the Earth’s surface during auroral events disrupt electric power grids and contribute to the corrosion of oil and gas pipelines.


Changes in the ionosphere during geomagnetic storms interfere with high-frequency radio communications and GPS navigation. During polar cap absorption events caused by solar protons, radio communications can be compromised.

Sunspots as drawn by Richard Carrington
Sunspots of September 1, 1859, as sketched by Richard Carrington. A and B mark the initial positions of an intensely bright event, which moved over the course of five minutes to C and D before disappearing.

Exposure of spacecraft to energetic particles during solar energetic particle events and radiation belt enhancements cause temporary operational anomalies, damage critical electronics, degrade solar arrays, and blind optical systems such as imagers and star trackers.


So as solar cycle 25 comes to life, keep an eye on the solar indices charts and like everything in nature, expect the unexpected.