# Travelling Ionospheric Disturbance

**Title:** Travelling Ionospheric Disturbance (Large Scale)

**Contributed by:** Bob Mattaliano N6RFM and Gwyn Griffiths G3ZIL

**Reviewed by:**

**Receiving Station Location:** EM12jw, near Fort Worth, TX 19January2026

**Receiving Station Details:** Grape-1 DRF

**Feature Description**

![WWV 10MHz 19January2026](/_static/images/image003.png)

Travelling Ionospheric Disturbance (TID) - a wave-like disturbance in the ionosphere that propagates over long distances. This example is a Large Scale TID (LSTID) given its period of about one hour. LSTIDs are characterized by periods of between 30 minutes and three hours, with a wavelength of over 1000 km and propagation speeds of 300 to over 1000 km per hour. These are not hard limits, and these indicative limits do vary in the literature. LSTIDs often propagate toward the equator from northern or southern auroral regions. Dr Nathaniel Frissell W2NAF and colleagues have described LSTIOD observations using amateur data in [this paper](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022GL097879).  

In this example, just after the seond positive peak a weaker trace, with larger spectral width appears for a few minutes. This is from two-hop propagation. The TID, during its peak positive to zero Doppler phase has caused the eletron density to increase, and to increase sufficiently for two-hop propagation to take place. Figure 1 in [this paper](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017RS006263) by Reinisch and colleagues shows, in an ionosonde plot, regions of higher critical frequency (electron density) appear at the crest of TIDs. That's how a TID can open up two-hop or open a higher frequency band.