Ionospheric Sluggishness: A Characteristic Time-Lag of the Ionospheric
Response to Solar Flares
Abstract
The term “ionospheric sluggishness” is used to describe the time delay
between maximum radio absorption in the ionosphere following the time of
maximum irradiance during a solar flare. Sluggishness is one of the
characteristic properties known to be maximized around D-region heights
and can be used for studying lower ionospheric (D-region) and
mesospheric chemistry. This article is our first attempt to estimate
ionospheric sluggishness using high frequency (HF, 3 – 30 MHz)
instruments. Specifically, we report on first estimates of sluggishness
from riometer and SuperDARN observations following a solar flare and
propose two new methods to estimate sluggishness. Sluggishness is shown
to be anti-correlated with the peak solar X-ray flux and positively
correlated with solar zenith angle and geographic latitude. The choice
of instrument, method, and reference solar waveband effects the
sluggishness estimation. A simulation study was performed to estimate
the effective recombination coefficient, which was found to vary between
4-5 orders of magnitude. We suggest that the effective recombination
coefficient is highly sensitive to D-region’s negative and positive ion
chemistry.