What is the T Index?

Thank you to VK3FS 

If you spend a bit of time hunting or activating POTA or WFF stations, you’ll no doubt know that HF propagation can be fantastic one day, and terrible the next. One day, 10 metres is wide open, and you’re working stations thousands of kilometres away. The next day, the same band is completely dead.

We often look at sunspot numbers, solar flux, and geomagnetic activity to get an idea of what the bands might be doing. But there’s another number that’s particularly useful.

It’s called the T-index.

This Ionosonde image is used in calculating the T index.

So, what exactly is the T-index, what does it tell us, and how can it help us understand HF propagation?

First, a quick refresher, HF radio relies on the ionosphere to refract radio signals back towards Earth, allowing communication over hundreds or thousands of kilometres, but there’s a limit to how high in frequency we can go.

We call this limit the Maximum Usable Frequency, or MUF.

The MUF changes throughout the day, from season to season, and over the course of the solar cycle.

When solar activity is high, the ionosphere generally supports higher frequencies. That’s why bands like 15, 12 and 10 metres can perform so well around solar maximum.

At solar minimum, those higher bands are often much less useful, and we tend to rely more heavily on lower frequencies.

Maximum Useable Frequency

The familiar way of tracking the 11-year solar cycle is through the sunspot number. The solar cycle lasts roughly 11 years, from one minimum to the next. Generally speaking, more sunspots mean greater solar activity and increased ionisation in the upper atmosphere, and that often means better propagation on the higher HF bands. But there’s a catch…

Sunspot numbers don’t tell us exactly what the ionosphere is doing right now

The ionosphere responds to solar radiation, particularly extreme ultraviolet radiation, and that doesn’t always follow the sunspot number perfectly. Geomagnetic disturbances can also change propagation conditions dramatically. So, while the sunspot number is a useful guide, it doesn’t always reflect the actual state of the ionosphere. That’s where the T-index comes in.

To understand the T-index, we need an instrument called an ionosonde. Think of an ionosonde as a radar system for the ionosphere. It transmits radio signals vertically upwards, across a range of frequencies, and measures the echoes returning from different ionospheric layers. From those echoes, we can determine important characteristics of the ionosphere. One of these is called foF2.

Ionosonde vertical radar

foF2 is the highest frequency reflected vertically by the F2 layer

foF2 isthe highest frequency reflected vertically by the F2 layer, which is usually the most important layer for long-distance HF communication. foF2 isn’t quite the same as the MUF for a long-distance radio path, but it provides vital information for calculating that MUF. Ionosondes measure the ionosphere directly, they tell us what it’s actually doing, rather than what we might expect it to be doing.

Over many decades, scientists have collected ionosonde measurements and compared them with sunspot numbers. They’ve established relationships between solar activity and the frequencies supported by the ionosphere. Using those relationships, they can work backwards.

They look at the frequencies the ionosphere is supporting and calculate the sunspot number that would normally produce those conditions. That calculated value is the T-index.

In other words, the T-index is an equivalent sunspot number.

It’s not a count of sunspots. It’s a number that describes how the ionosphere is behaving, expressed on a scale similar to the sunspot number. And that’s an important distinction.

The T-index is an equivalent sunspot number

For example, imagine the actual sunspot number is 100, but ionosonde measurements indicate the ionosphere is behaving more like it would with a sunspot number of 65. The T-index might therefore be around 65. That tells us the ionosphere isn’t supporting frequencies quite as well as the sunspot number alone might suggest.

The reverse can happen too.

The T-index can be higher than the actual sunspot number when measured ionospheric conditions are better than expected.

T-index values commonly fall between minus 50 and plus 200. A low T-index generally indicates that lower HF frequencies are likely to be more useful. This might occur around solar minimum, or during a significant ionospheric disturbance. A high T-index suggests the ionosphere is capable of supporting higher frequencies, making propagation in the upper HF bands more promising.

But remember, the T-index doesn’t guarantee that a particular band will be open. It’s an input to propagation prediction models.

T index compared to sunspot number (SSN)

In Australia, the Bureau of Meteorology’s Space Weather Services operates ionospheric monitoring facilities. Measurements from these stations are used to calculate T-index values. The information feeds into propagation prediction systems.

These propagation prediction systems uses the T-index, along with information about the radio path, time of day and season, to estimate usable HF frequencies. And that’s where the T-index becomes particularly useful.

In conclusion, the sunspot number tells us about activity on the Sun. The T-index tells us how the ionosphere is responding to that activity. For HF communication, it’s the ionosphere that ultimately determines which frequencies will work. A higher T-index generally favours higher HF frequencies, while a lower T-index suggests looking further down the bands. It won’t tell you everything about propagation, but it provides a valuable indication of the ionosphere’s ability to support HF communication.

So, the next time you see a T-index reading in a propagation report, you’ll know what it means.

It’s effectively a sunspot number derived from what the ionosphere is actually doing.

That’s what makes it such a useful tool for HF radio operators.