Earth’s ionosphere has a smooth density distribution during daytime when there is production of ionisation. In other words, during daytime when the E-region conductivity is strong, the ionospheric drivers such as electric fields and neutral winds and their fluctuations cannot destabilise the ionosphere. But after sunset when the E-region conductivity becomes weak, the drivers can destabilise the ionosphere and generate plasma irregularities of various scale sizes that manifest as spread-F (Booker and Wells 1938) and plasma bubbles (Woodman and La Hoz 1976) causing scintillations in communication signals (Aarons 1993). The irregularities occurring at low and mid latitudes have been studied widely using various experimental and theoretical modelling techniques for over 75 years to understand their science and variability and for mitigating their adverse effects on communication and navigation; for recent reviews, see Bowman (1990), Aarons (1993), Patra 2008, Woodman (2009), Kelley et al. (2011), and Yokoyama and Stolle (2017). The studies are to be continued for a better understanding of the generation and growth of the irregularities and their day-to-day variability to reach a level of forecasting capability.
The spread-F and plasma bubbles at equatorial latitudes where the geomagnetic field lines are horizontal are thought to originate through the Rayleigh-Taylor (RT) instability mechanism (Dungey 1956; Scannapieco and Ossakow 1976; Jayachandran et al. 1993). The main driver that destabilises the ionosphere at post-sunset hours is the pre-reversal strengthening of the eastward electric field (e.g. Heelis et al. 1974; Eccles et al. 2015), which causes the pre-reversal enhancement of the vertical upward E × B plasma drift velocity Vz referred as PRE (e.g. Fejer et al. 1979; Namboothiri et al. 1989; Balan et al. 1992; Huang and Hairston 2015). The irregularities have a linear growth rate (Ossakow 1981):
$$ \gamma =\left(1/H\right)\left(g/{\nu}_{\mathrm{in}}+\mathrm{Vz}\right)-\beta $$
(1)
The first and second terms are the gravitational and cross-field instability growth rates, β is the recombination rate, H is the electron density scale height, g is the acceleration due to gravity, and νin is ion-neutral collision frequency. The irregularities originating at the equatorial bottom-side F-region rise to the topside as plasma bubbles (Scannapieco and Ossakow 1976) and extend along the field lines to higher latitudes especially at high solar activity when PRE is strong.
Modelling studies of the growth rate have predicted a low latitude minimum. For example, Maruyama (1990) who modelled the growth rate of both the gravitational and cross-field (E × B) instability modes using flux tube integrated values (without neutral wind) showed that the net instability growth rate at March equinox under medium solar activity (F10.7 = 120) is negative in a narrow latitude region near 18° mag. lat. At lower and higher latitudes, the ionosphere becomes unstable.
The neutral wind fluctuations known as waves especially gravity waves which are always present and cause fluctuations in the E × B drift velocity (Balachandran et al. 1992; Abdu et al. 2015) are thought to act as the seed for the onset of the plasma irregularities (e.g. Osaakow, 1981; Sekar et al. 1995). The background neutral wind can also directly influence the growth of the irregularities (e.g. Maruyama and Matuura 1984). At low solar activity and when PRE is absent, the plasma irregularities generally occur at around midnight (e.g. Nishioka et al. 2012). They are thought to be caused mainly by the electric field fluctuations due to atmospheric gravity waves propagating to ionospheric heights easily at low solar activity (e.g. Tsunoda et al. 2010; Patra et al. 2013; Tulasiram et al. 2014; Narayanan et al. 2014a).
Plasma irregularities have also been observed at low and mid latitudes where PRE is absent due to inclined geomagnetic field lines (Bowman 1990 and references therein). Whalen (2002) studied the spread-F occurrence at a chain of stations up to 20° mag. lat. in American sector. Fukao et al. (1991) and Kelley and Fukao (1991) studied the F-region plasma irregularities observed by the MU (middle and upper atmosphere) radar. Otsuka et al. (2002), for the first time, observed an identical optical structure at conjugate stations beyond the EIA crests.
The mid latitude plasma irregularities were first thought to be associated with atmospheric gravity waves (e.g. Bowman 1990; Kelley and Fukao 1991) and later with medium-scale travelling ionospheric (atmospheric) disturbances (MSTIDs) (e.g. Saito et al. 2001; Makela and Otsuka 2012). The mechanism that relates the mid latitude plasma irregularities to its source (e.g. MSTID) is the so-called Perkins instability (Perkins 1973; Miller 1997) which takes the form of rising and falling sheets of ionisation when a north-south electric field is present in addition to the east-west field. Hamza (1999) updated the Perkins equations by adding neutral wind. Recent understanding is that the electro-dynamical coupling between the E- and F-regions plays an important role in enhancing the growth rate of Perkins instability (e.g. Tsunoda and Cosgrove 2001; Yokoyama et al. 2008). The instability has a linear growth rate (Tsunoda 2006; Yokoyama and Stolle 2017):
$$ \gamma =\left(\left|E\right|\ \cos I\right)\left[\sin \left(\theta \hbox{--} \alpha \right)\sin \alpha \right]/ BH $$
(2)
where E = E0 + UF × B is total effective electric field with E0 being the background field; UF is F-region neutral wind, B is geomagnetic field of magnitude B, I is magnetic inclination angle, H is atmospheric scale height including ion-neutral collision frequency, θ is the angle between E and east direction, and α is the angle between the direction normal to the frontal structure and east direction. A maximum of the growth rate γ is expected to occur at the location where α = θ/2.
As introduced, theoretical models have predicted a low-latitude minimum (e.g. Maruyama 1990) and a mid-latitude maximum (e.g. Tsunoda 2006) for the occurrence of plasma irregularities manifested as spread-F in ionograms. The possibility of the minimum has also been indicated by optical observations of MSTIDs (Shiokawa et al. 2002, 2003), which shows a low-latitude limit for their propagation at ~ 18° N mag. lat. at high solar activity (1999), and the limit is found to extend to further lower latitudes at low solar activity (e.g. Narayanan et al. 2014b). However, to our knowledge, the expected minimum and maximum in the occurrence of plasma irregularities have not yet been identified.
This short paper identifies the spread-F minimum and maximum for the first time by analysing the ionosonde data during the March equinox season when the minimum was predicted. The data used are collected at four low- to mid-latitude locations (17.0° N, 21.2° N, 26.5° N and 36.5° N mag. lat.), referred as low-mid latitudes, in Japan longitude sector (~ 135° E) in March to April (or equatorial spread-F season (Aarons 1993) at high, medium and low solar activity (2002, 2003 and 2006). The locations of the spread-F minimum and maximum are found to shift equatorward with decreasing solar activity.
