Short-wave fadeouts without reported flares

Abstract

Short‐wave fadeouts were sought which had no associated Hα flare. To carry out this study, Sacramento Peak flare patrol films were re‐examined for those days on which short‐wave fadeouts (SWF's) had occurred during patrol times without reported flares. Fifteen such fadeouts were reported during the IGY period. Upon re‐examination of the films, 12 of the SWF's were found to be in close time association with outstanding Hα events, usually in the nature of pronounced plage brightenings. In 2 other cases, seeing was so poor that no definite statement could be made about possible Hα activity. In only 1 case was there definitely no unusual Hα event in time association with the fadeout. For each of the 15 cases, the flare film for the entire day was re‐examined without previous knowledge of the time of the SWF. Any periods of outstanding Hα activity were selected and their times noted; only after their independent selection were their times matched to SWF times. Usually there was only 1 such outstanding period of optical activity per day. The time association with SWF was, in general, very close. The results indicate that outstanding Hα activity is essential in time relation with SWF occurrence.

Characterization of Short‐Wave Fadeout Seen in Daytime SuperDARN Ground Scatter Observations

“…Solar extreme ultraviolet (EUV) and X-rays propagate at the speed of light and take only ≈8 min to reach Earth's atmosphere, so SWF is one of the earliest space weather effects following a flare. This geophysical phenomenon was first described by J. H. Dellinger in 1935(Dellinger, 1937) and a later statistical study (DeMastus & Wood, 1960) showed that there is a one-to-one relation between solar flares and SWF. A recent study by Zawdie et al (2017) showed that plasma density enhancement is the main driver of HF radio wave absorption in D and E layers.…”

Extreme Solar Flare-driven Short-wave Fadeout Observed by SuperDARN ZHO Radar

“…SWF was discovered for the first time in 1935 (Dellinger 1937). The early statistical studies show a one-to-one correlation between SWF and solar flares, with the occurrence of SWF increasing as solar activity increases (DeMastus & Wood 1960;Hendl & Skrivanek 1973). Additionally, SWF generally occurs about 1-3 minutes after the flare erupts due to the ionospheric sluggishness (Appleton 1953;Chakraborty et al 2021).…”

A Modeling Framework for Estimating Ionospheric HF Absorption Produced by Solar Flares

“…This sudden enhancement of electron density leads to an increase in ionospheric high-frequency (HF: 3-30 MHz) radio wave absorption that disrupts over-the-horizon (OTH) communication, commonly known as the Dellinger effect or shortwave fadeout (SWF) (Dellinger, 1937). A statistical study by DeMastus and Wood (1960) showed there is almost a one-to-one relationship between an earthward directed solar flare and the occurrence of SWF. More recent studies have shown how the duration and intensity of SWF depend on duration and peak intensity of the solar flare, location of the transmitter and receiver, frequency of the radio wave, and the background ionosphere (Chakraborty et al, 2018;Fiori et al, 2018).…”