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

Abstract

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. CHAKRABORTY ET AL.

Tests of a New Solar Flare Model Against D and E Region Ionosphere Data

“…There are a couple of limitations to our model that need to be considered in this context. First, OASIS is configured to be a steady state model whereas numerous studies of flares have suggested that the time dependence is an important factor in governing the recombination rate (Basak & Chakrabarti, 2013; Chakraborty et al., 2021; Palit et al., 2015; Zigman et al., 2007). Essentially the idea is that the solar irradiance peaks while the ionospheric chemistry lags in its response.…”

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

“…It can be seen that the long onset and solar irradiance at time T end are inversely proportional, with correlation coefficients of (EUV: −0.97, SXR: −0.64), respectively. As discussed by Chakraborty et al (2021) sluggishness is shown to be anticorrelated with peak solar X-ray flux. Based on reasonable inference, the radiant fluxes and the sluggishness have the same trend at the onset end.…”

“…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). The statistical study of SWFs at Boulder, Colorado, from 1980-1987 shows that the average duration of SWFs was approximately 23 minutes, with 58.9% of events lasting less than 14 minutes, 21.4% lasting between 15 and 29 minutes, 4.3% lasting between 30 and 44 minutes, and 3% lasting more than 90 minutes (Dieminger et al 2012), respectively.…”

Driving Influences of the Doppler Flash Observed by SuperDARN HF Radars in Response to Solar Flares

“…Sudden enhancement in ionospheric electron density, referred to as a sudden ionospheric disturbance (SID) (e.g., Davies, 1990 ; Dellinger, 1937 ), following a solar flare severely disrupts trans‐ionospheric high frequency (HF: 3–30 MHz) communications by introducing HF absorption, and signal anomalies. HF absorption, commonly referred to as short‐wave fadeout (SWF) (e.g., Chakraborty et al., 2018 , 2019 ; Chakraborty, Baker, et al., 2021 ; Chakraborty, Baker, & Ruohoniemi, 2021 ; Chakraborty, Ruohoniemi, et al., 2021 ; Davies, 1990 ; Fiori et al., 2018 , 2022 ), is a well‐studied phenomenon caused by dissipation of signal energy as heat through collisions with neutral particles, leading to a partial or complete reduction in the strength of the radio signal (e.g., Browne et al., 1995 ). In contrast, frequency and phase anomalies, commonly referred to as sudden frequency and phase deviation (SFD, SPD), are less explored and understood phenomena (Khan et al., 2005 ; Kikuchi et al., 1986 ; Liu et al., 1996 ; Watanabe & Nishitani, 2013 ).…”