Ionospheric Sluggishness: A Characteristic Time‐Lag of the Ionospheric Response to Solar Flares

The term “sluggishness” was coined by E. V. Appleton in the 1950s to describe the time delay between peak irradiance at solar noon and the resulting peak in ionospheric electron density. Sluggishness can be understood as an inertial property of the ionosphere that manifests as a lag of the ionospheric response to a solar driver. As shown by Appleton, estimates of sluggishness can be used to study the chemistry of the lower ionosphere, of the D‐region in particular. In this study, for the first time, we have examined ionospheric sluggishness in terms of the time delay between the peak irradiance during a solar flare and the resulting peak in ionospheric electron density using HF instruments. Estimates of the delay are obtained using HF observations from riometers and SuperDARN radars that are primarily sensitive to absorption in the D‐region. Two new methods for measuring delay are introduced. Sluggishness is shown to be anti‐correlated with peak solar X‐ray flux and positively correlated with zenith angle and latitude. The choices of instrument, method, and reference solar waveband affect the sluggishness estimation. A simulation study was performed to estimate the effective recombination coefficient in the D‐region. The coefficient was found to vary by orders of magnitude with peak flare intensity. We argue that the variation in effective recombination coefficient with peak flare intensity is highly sensitive to changes in the negative and positive ion chemistry of the D‐region, which is sensitive to the incoming solar X‐ray and EUV radiation. Plain Language Summary A systematic time delay between peak incoming solar radiation during a solar flare and peak electron density in the ionosphere is known as ionospheric sluggishness. Ionospheric sluggishness is known to be maximized around D‐region heights (∼60–90 km altitude). This article is our first attempt to estimate ionospheric sluggishness using high frequency (3–30 MHz) instruments. In addition, we statistically characterize the observed sluggishness and provide an insight into D‐region photochemical processes. In this article, we also demonstrate how to extract the D‐region's recombination coefficient using a theoretical model and measured sluggishness. Key Points The choice of ionospheric sounding techniques and reference solar irradiance wavebands affects the estimation of ionospheric sluggishness A simulation study shows the D‐region effective recombination coefficient varies by 4–6 orders of magni