A frequently observed geophysical phenomenon in the daytime equatorial ionosphere are 150-kilometer radar echoes. These very high frequency (VHF) radar echoes were first detected more than 50 years ago, however, it remains unclear how exactly they are generated. A recent study by Oppenheim and Dimant  suggested the Sun’s extreme ultraviolet (EUV) radiation as the main cause of the 150-kilometer radar echoes. Patra et al.  present new observational evidence of an anti-correlation between solar EUV radiation and these echoes, and therefore contradicts with the recent study that would otherwise expect a positive relationship. This calls for further investigation into the generation mechanisms of the 150-kilometer radar echoes as they are manifestations of plasma density irregularity that may affect communications in the near-Earth space environment.
Citation: Patra, A. K., Pavan Chaitanya, P., St.-Maurice, J.-P., Otsuka, Y., Yokoyama, T., & Yamamoto, M. . The solar flux dependence of ionospheric 150 km radar echoes and implications. Geophysical Research Letters, 44, 11,257–11,264. https://doi.org/10.1002/2017GL074678
—Gang Lu, Editor, Geophysical Research Letters