Solar activity dependence for the relationship between nighttime medium-scale traveling ionospheric disturbance and sporadic E ( Es ) layer activities in summer during 1998–2019 over Japan

To investigate solar activity dependence of the coupling between medium-scale traveling ionosphere disturbance (MSTID) and sporadic E ( Es ) layer, we analyzed the total electron content (TEC) obtained from a Japanese global positioning system (GPS) receivers and ionosonde at Kokubunji (35.7° N, 139.5° E) in Japan during the summer period of May–August from 1998 to 2019. To obtain perturbation TEC caused by MSTIDs, the detrended TEC is calculated by subtracting 1-h moving averages from the measured TEC for each pair of GPS satellite and receiver. The detrended TEC data are mapped on to the geographical coordinates to make detrended 2-D maps with spatial resolution of 0.15° × 0.15° in longitude and latitude. The MSTID activity is defined as a ratio of the standard deviation to the background TEC over Kokubunji in Japan. Day-to-day variations of the MSTID activity during summer nights was compared to Es layer parameters [critical frequency ( f o Es ) and (cid:31) f o − b ≡ f o Es − f b E , where f b Es is blanketing frequency] derived from ionosonde station at Kokubunji. We have found that the correlation coefficient between the MSTID activity and f o Es ( (cid:31) f o − b ) between 1998 and 2019 is 0.5 3 (0.46) on average, suggesting that there is an electrodynami-cal coupling between the Es layer and F region could generate nighttime MSTIDs. We also have found that the correlation coefficient positively correlates with solar activity. This finding indicates that in the high solar activity conditions, when the growth rate of Perkins instability is relatively low, generation of the polarization electric fields in the Es layer could play a more important role to grow MSTIDs than in the low solar activity conditions.


Introduction
Medium-scale traveling ionospheric disturbance (MSTID) is electron density perturbations in the F-region.The MSTID occurs at mid-latitudes as a manifestation of atmospheric gravity wave (AGW) in the daytime and due to electro-dynamic coupling in the nighttime (Kotake et al. 2007).Numerous characteristics of MSTID have been discovered since the late 1990s using ionospheric inferences using two-dimensional mapping techniques using all-sky airglow imagers and multipoint GPS receiver networks.Saito et al. (1998) utilized an extensive global positioning system (GPS) receiver network comprising approximately 1000 GPS receivers to generate two-dimensional map of total electron content (TEC) over Japan.They detected twodimensional structure of the TEC perturbation induced by MSTID over Japan.The resulting TEC map emphasizes the valuable insight into the propagation behavior of MSTIDs, which has a southwestward direction during nighttime.
The nighttime MSTIDs propagate mostly southwestward in the northern hemisphere and northwestward in the southern hemisphere (Otsuka et al. 2004).The propagation direction of the nighttime MSTIDs is supporting the hypothesis that polarization electric fields might have a pivotal role in generating MSTIDs during nighttime (Shiokawa et al. 2003a;Saito et al. 2002;Kotake et al. 2007).Shiokawa et al. (2003b) investigated a nighttime MSTID measured by airglow observation at Shigaraki, Japan, and compared it with ion drift data of the DMSP F15 satellite and found that perturbations in the electric field are correlated with MSTID structures which have wave fronts aligned from northwest to southeast.Their findings revealed that the orientation of the electric field coincides with the polarization of the electric fields anticipated from the continuity of the electric current in the F region.These results suggests that the nighttime MSTIDs could be caused by the of the Perkins instability (Perkins 1973).Kelley et al. (2003) proposed that the electrodynamical coupling between the E and F layers, stronger polarization electric fields, and perturbation in plasma density are enabling the role in development of nighttime MSTIDs.Saito et al. (2007) found a strong correlation between F region MSTID structures with E region fieldaligned irregularities (FAIs).Otsuka et al. (2007) reported that the Doppler velocity of echoes from FAI in the E -region showed that the MSTID-induced airglow showed an enhancement (depletion) corresponding to southeastward (northwestward) velocity in the FAI echoes.These results show a significant relationship between the F region MSTID's electric fields and the E region's electric field perturbations.
At mid-latitudes, a Hall polarization process similar to the one fueling the equatorial electrojet can occur.The polarization electric fields produced by this mechanism may be stronger than the surrounding electric fields (e.g., Haldoupis et al. 1996).Tsunoda and Cosgrove (2001) have reported that the electric fields produced through the Hall polarization process in the Es layer could propagate into the F region, causing plasma density perturbations by amplifying the polarization electric fields.Cosgrove and Tsunoda (2002) have proposed a Es layer instability in which Es layer fluctuation grows when

Graphical Abstract
the plasma density wavefronts align from northwest to southeast.Furthermore, Cosgrove and Tsunoda (2004) have put forward a coupling instability between the Es layer and F region, and shown that the coupling instability grows more rapidly than the Perkins instability alone.Shalimov and Yamamoto (2010) proposed a simple quantitative model for mapping the polarization of the electric field within the F-region over the mid-latitude region.Their findings showed that polarization electric fields associated with Es , having sizes larger than 10 km, could effectively disturb the plasma in the F region at mid-latitudes.Several research findings has demonstrated that the interaction between the E and F regions can enhance the Perkins instability (Tsunoda and Cosgrove 2001;Cosgrove and Tsunoda 2004;Haldoupis et al. 2003;Cosgrove 2007Cosgrove , 2013;;Otsuka et al. 2008;Earle et al. 2010;Hysell et al. 2018;Liu et al. 2019Liu et al. , 2020)).Otsuka et al. (2008) examined the coupling among the MSTID and status of Es layer events done with correla- tion statistics during the summer nights (May-August) of 2001-2005 using TEC data from a Japanese GPS network and ionosonde measurements taken at Kokubunji, Japan in May-August of 2001-2005.Their findings suggest that an electrodynamical coupling between Es and the F region via the polarization of the electric field could play an important role in generating nighttime MSTIDs and the spatial structure of Es .Since they ana- lyzed the data only for 5 years from 2001 to 2005, they did not assert a solar activity dependence of the coupling between MSTID and Es layer.This study aims to dis- close solar activity dependence of the coupling between MSTID activity and the Es layer by extending the period of the analyze data to 22 years from 1998 to 2019.

Data and methodology
Geospatial information authority of Japan (GSI) has been operating about 1000 dual frequency (L1 with 1.57542 GHz and L2 with 1.22760 GHz) GPS-receivers in Japan since 1999 (Sagiya et al. 2000).The carrier phase and pseudorange data at the dual-frequencies are obtained every 30 s.The carrier phase and pseudorange data of the dual-frequency GPS data are obtained every 30 s.Total electron content (TEC) along a ray path of the radio wave propagating from the GPS satellite to the receiver is obtained from the carrier phase and pseudorange data.TEC obtained from the carrier phase is more accurate than TEC from the pseudorange, but has an ambiguity due to the unknown initialization constant.The ambiguity was corrected by the TEC obtained from the corresponding pseudorange data.The interfrequency biases included in the TEC obtained from the pseudorange were subtracted by using the method by Otsuka et al. (2002) to obtain absolute TEC.
For obtaining the perturbation in TEC due to MSTID, a 1-h running average has been subtracted from the time series of TEC obtained for each pair of satellites and receivers (Saito et al. 1998).To mitigate the multipath effect caused by obstacles surrounding the receivers, we used the data with satellite elevation angle higher than 35°.In this way, the obtained slant TEC is multiplied by a factor defined as τ 0 / τ 1 to obtain the vertical TEC, where τ 1 is the distance travelled by the radio wave in the iono- sphere spanning an altitude range from 250 to 450 km, and τ 0 is the vertical extent or depth of the ionosphere (200 km).This vertical detrended TEC values were mapped onto the ionospheric shell existing at an altitude of 300 km in the geographical coordinates with a grid size of 0.15 o × 0.15° in longitude and latitude.The TEC val- ues in each grid are averaged.This method to make twodimensional maps of the detrended TEC is reported by (Saito et al. 1998).
According to Otsuka et al. (2008), the MSTID activity is defined as the ratio δI / Ī × 100[%], where δI represents the standard deviation of TEC perturbations observed in a specific region ranging from 33.75° N to 37.80° N and 137.50°E to 141.55° E over 1 h, and I denotes the back- ground TEC obtained as an average of vertical TEC in the same area and during the corresponding time span as δI .The MSTID activity in summer (between May and August) from 1998 to 2019 was used in this study.
In this statistical study, the critical frequency (  (Ogawa et al. 2002).Although spatial scale of the inhomogeneity for the electron density in Es layer is considerably smaller than that of MSTIDs, temporal and day-to-day variations of f o−b well correlate with those of the MSTID activity (Ogawa et al. 2002;Otsuka et al. 2008).Figure 4 shows the F10.7 dependence of the correlation coefficient between MSTID activity and f o Es , and between MSTID activity and f o Es for the summer nights of 19-02 h (JST) for 1998-2019.From Fig. 4, we find that the correlation coefficient of Es layer parameters and MSTID tend to increase with increasing solar activity, indicating that solar activity affects the E and F regions coupling process.It should be noted that during the low solar activity conditions, the correlation coefficient shows large variability.

Discussion
Our results show that that day-to-day variation of nighttime MSTIDs are well correlated with that of plasma density and its inhomogeneous structures in Es layer through all solar activity conditions.This result indicates that electrodynamical coupling processes between the Es layer and the F region along the geomagnetic fields could play an important role in generation of MSTIDs and inhomogeneities in the Es layer.
Mechanism for the nighttime MSTID generation can be explained in terms of the electrodynamical forces.The process of generating MSTIDs accompanying polarization electric fields are explained as follows (e.g., Otsuka et al. 2021).During the nighttime, the flow of the F region electric current ( J ) in the northeastward direc- tion is facilitated by the thermosphere neutral wind ( U ), which issoutheastward.When J flows through the elec- tron density perturbations, polarization electric fields ( E p ) are generated to maintain a divergence-free cur- rent density ( J ). E p is perpendicular to the wave fronts of MSTID, and oriented to the northeastward (southeastward) direction at the region where the height-integrated Pedersen conductivity is low (high).Due to upward and downward motion of the F-region plasma by oscillating E p × B plasma drift, the electron density perturbations could be created, forming MSTIDs.The perturbations of the electron density and electric fields grow through the Perkins instability under the condition that the wave vector of MSTID falls between the eastward and J directions (Perkins 1973).
However, the growth rate of the Perkins instability alone is inadequate to explain the amplitude of the observed MSTID (e.g., Kelley et al. 2003).This expresses that MSTIDs cannot be generated solely via the Perkins instability.Cosgrove and Tsunoda (2002) have proposed another instability, which occurs in Es layer.It is referred to Es layer instability.It operates under the condition that the frontal structures of Es layer plasma densities have wave fronts aligned from northwest to southeast.Cosgrove and Tsunoda (2004) have proposed a coupling between the Es layer instability and Perkins instabil- ity, and reported that linear growth rate of the coupling instability is larger than that of the Perkins instability (Perkins 1973).Yokoyama et al. (2009), who have carried out three-dimensional simulations of the coupled Perkins and Es layer instabilities, conclude that the Es layer instability plays a major role in seeding NW-SE structure in the F region, and the Perkins instability is required to amplify its perturbations.Our observations suggest that coupling processes between Es layer and F region could Regarding the solar activity dependence of the instabilities, the growth rate of the Perkins instability is anticorrelated with the solar activity.This is because the growth rate of the Perkins instability is inversely proportional to the neutral density, which increases with the solar activity.Airglow and GPS-TEC observations show that occurrence rate of the MSTID and MSTID activity defined as a ratio of the amplitude of perturbed TEC to the background TEC shows anti-correlation with the solar activity (Takeo et al. 2017;Otsuka et al. 2021) Pietrella and Bianchi (2009).This is probably because the Es layer is formed mainly by wind shear mechanism (Whitehead 1960).Yokoyama et al. (2009), who have carried out three-dimensional simulations for the coupling processes between the Perkins and Es layer instabilities, report that the Es layer instability plays a major role in seeding the perturbations elongating from northwest to southeast and that the Perkins instability amplify the perturbations.The current study shows that the correlation coefficient between the MSTID activity and Es layer is positively correlated with the solar activity.Considering the previous studies mentioned above, the current result suggests that under the low solar activity conditions, when the growth rate of the Perkins instability is higher, MSTIDs could grow mainly through the Perkins instability even with weak seeding by the Es layer insta- bility.On the other hand, under the high solar activity conditions, since the growth rate of the Perkins instability is smaller, seeding by the Es layer instability is needed for the MSTID growth.Consequently, the coupling between the MSTID and Es layer is intense under the high solar activity conditions.

Conclusion
We analyzed TEC data from the Japanese GPS network and ionosonde measurements taken at Kokubunji (35.7°N, 139.5°E), Japan, between May and August 1998-2019.Our objective is to explore the connection between the nighttime MSTIDs and Es layer.The obtained results regarding the correlation coefficient between the MSTID activities and f o Es, f o−b are summarized as follows: 1. Correlation coefficient between the MSTID activity and f o Es f o−b is 0.53 (0.46) on average for a period from 1998 to 2019.These finding implies that the electrodynamical coupling between the Es layer and F region could play an important role in growing MSTIDs.
2. The correlation coefficient tends to increase with increasing solar activity.Considering that the Perkins instability growth rate decreases with increasing solar activity, we can speculate that under high solar activity conditions, when the growth rate of the Perkins instability is relatively low, seeding of polarization electric field perturbations in Es layer is vital in growing MSTIDs.
f o Es ) and blanketing frequency ( f b Es ) of Es layer obtained by an ionosonde at Kokubunji (35.7°N, 139.5°E) in Japan during a period from 1998 and 2019 are analyzed.This study uses hourly values of manually scaled parameters provided by the World Data Centre (https:// wdc.nict.go.jp) of the Institute of Information and Communications Technology (NICT).Since f o Es represents the maxi- mum and f b Es represent minimum plasma frequencies in Es layer, respectively, the difference between the two plasma frequencies, f o Es − f b Es ≡ f o−b could indicate inhomogeneity of the concentration of denser plasma in Es Es layer

Figure 1
Figure 1 represents daily variations of MSTID activity (brown curve) during nighttime, f o Es(blue curve) and f o−b (green curve) for the months of (a) May, (b) June, (c) July and (d).August of 2019.To derive the average values for each day, the f o Es , f o−b and MSTID activity are averaged over a period between 19:00 and 02:00 JST.

Fig. 1 Fig. 2
Fig. 1 Day-to-day variations of the nighttime averaged MSTID activity (brown), f o Es(blue) and f o−b (green) for a May, b June, c July and d August of 2019

Fig. 4
Fig. 4 F10.7 dependence of correlation coefficient between MSTID activity and a f o Es and b f o−b during the summer nights of 19-02 h (JST) for the years of 1998-2019 . The correlation coefficient between MSTID activity and f o Es and between MSTID activity and f o−b shows large variabil- ity during the low solar activity conditions, as shown in Fig. 4.This feature can be interpreted as follows: The Perkins instability is more effective during low solar activity than during high solar activity because the growth rate of the Perkins instability increases with decreasing solar activity.The growth rate of the Perkins instability is a function of the background neutral winds.Day-to-day variation of the Perkins instability growth rate could be controlled mainly by the background neutral winds.In order to investigate solar activity dependence of Es layer, Fig. 5 shows the yearly variation of f o Es and f o−b averaged over summer nights of 19-02 h (JST).From 1998 to 2019, f o Es ranges from 3.9 to 5.4, and f o−b from 1.1 to 2.4.In the figure, yearly variation of F10.7 is also shown.Comparing f o Es and f o−b with F10.7, it is found that both f o Es and f o−b do not show distinct solar activity dependence.This feature is consistent with previous work done at mid latitudes by

Fig. 5
Fig. 5 Yearly variation of f o Es(red) and f o−b (blue) in summer nights of 19-02 h (JST) for the years of 1998-2019.F10.7 (black) is also shown in the figure