5:15 PM - 7:15 PM
[PEM12-P10] Statistical Relationship between Medium-Scale Traveling Ionospheric Disturbance and Sporadic E-layer: Geomagnetic Conjugate Observations by GPS Network and Ionosondes
Keywords:MSTID, Sporadic E-layer, TEC, Ionosonde, Perkins instability, Solar activity
Medium-scale traveling ionospheric disturbances (MSTIDs) are wave-like structures of electron density variations that primarily occur in the F-region at mid-latitudes. In particular, nighttime MSTIDs are known to grow through coupling with sporadic E-layer (Es-layer) instabilities, which are more likely to occur in the summer hemisphere. Additionally, the F-region in the northern and southern hemispheres, located along the same geomagnetic field line, is known to exhibit similar structures. This suggests that electric fields generated in the Es-layer may be mapped along geomagnetic field lines, leading to electromagnetic coupling of MSTIDs between the two hemispheres.
In this study, we conducted a statistical analysis of this ionospheric coupling between the northern and southern hemispheres. We examined ionosonde data from four locations in Japan and total electron content (TEC) data obtained from GNSS systems in both Japan and its geomagnetically conjugate region in Australia, covering the period from 2011 to 2020.
TEC data were derived from the phase difference and signal delay between GNSS satellites and receivers. To extract short-term variations corresponding to MSTID periods, we subtracted a one-hour moving average from the TEC data. MSTID activity was then quantified using the percentage ratio (delta-I/I-bar) × 100%, where delta-I represents the standard deviation of TEC fluctuations observed over a 4.05°× 4.05° area within one hour, and I-bar is the background TEC observed in the same region and time period. This MSTID activity index was calculated at an altitude of 100 km above the ionosonde locations in Japan and at 300 km altitude in the conjugate region in the southern hemisphere.
The MSTID activity index during local time 19:00-02:00 was compared with two Es-layer parameters: foEs (the critical frequency of the Es-layer) and delta-fo-b (≡ foEs - fbEs, where fbEs is the blanket frequency of the Es-layer). The parameter delta-fo-b is considered indicative of electron density irregularities in the Es-layer. We calculated the correlation coefficients between the diurnal variations of MSTID activity and the two Es-layer parameters from May to August.
The cross-correlation coefficients between Es parameters and MSTID activity in Japan were consistently high (0.2–0.8) across all years at the three observation sites, except for Okinawa. The correlation values followed the order: Wakkanai > Kokubunji > Yamagawa > Okinawa. This suggests that, at higher latitudes, the conductivity ratio of the Es-layer to the F-region is greater, and Perkins instability in the F-region is weaker. As a result, MSTID growth becomes highly dependent on seeding from the Es-layer, leading to stronger coupling between MSTIDs and Es-layer dynamics.
The correlation coefficients of MSTID activity between Japan and Australia from May to August were larger at higher latitudes and exhibited an inverse correlation with solar activity. Furthermore, the cross-correlation between Es parameters in Japan and MSTID activity in Australia showed a strong conjugate relationship under low solar activity but weakened during high solar activity. Under high solar activity conditions, the increased conductivity of the F-region facilitates Pedersen currents, which may cause electric fields associated with MSTID-scale horizontal wavelengths (hundreds of kilometers) to propagate along geomagnetic field lines, while electric fields with spatial scales of a few tens of kilometers or less are likely to attenuate. Since MSTID activity includes TEC fluctuations with spatial scales ranging from approximately 15 to 400 km, TEC variations with scales of a few tens of kilometers or less may experience reduced geomagnetic conjugacy under high solar activity.
In this study, we conducted a statistical analysis of this ionospheric coupling between the northern and southern hemispheres. We examined ionosonde data from four locations in Japan and total electron content (TEC) data obtained from GNSS systems in both Japan and its geomagnetically conjugate region in Australia, covering the period from 2011 to 2020.
TEC data were derived from the phase difference and signal delay between GNSS satellites and receivers. To extract short-term variations corresponding to MSTID periods, we subtracted a one-hour moving average from the TEC data. MSTID activity was then quantified using the percentage ratio (delta-I/I-bar) × 100%, where delta-I represents the standard deviation of TEC fluctuations observed over a 4.05°× 4.05° area within one hour, and I-bar is the background TEC observed in the same region and time period. This MSTID activity index was calculated at an altitude of 100 km above the ionosonde locations in Japan and at 300 km altitude in the conjugate region in the southern hemisphere.
The MSTID activity index during local time 19:00-02:00 was compared with two Es-layer parameters: foEs (the critical frequency of the Es-layer) and delta-fo-b (≡ foEs - fbEs, where fbEs is the blanket frequency of the Es-layer). The parameter delta-fo-b is considered indicative of electron density irregularities in the Es-layer. We calculated the correlation coefficients between the diurnal variations of MSTID activity and the two Es-layer parameters from May to August.
The cross-correlation coefficients between Es parameters and MSTID activity in Japan were consistently high (0.2–0.8) across all years at the three observation sites, except for Okinawa. The correlation values followed the order: Wakkanai > Kokubunji > Yamagawa > Okinawa. This suggests that, at higher latitudes, the conductivity ratio of the Es-layer to the F-region is greater, and Perkins instability in the F-region is weaker. As a result, MSTID growth becomes highly dependent on seeding from the Es-layer, leading to stronger coupling between MSTIDs and Es-layer dynamics.
The correlation coefficients of MSTID activity between Japan and Australia from May to August were larger at higher latitudes and exhibited an inverse correlation with solar activity. Furthermore, the cross-correlation between Es parameters in Japan and MSTID activity in Australia showed a strong conjugate relationship under low solar activity but weakened during high solar activity. Under high solar activity conditions, the increased conductivity of the F-region facilitates Pedersen currents, which may cause electric fields associated with MSTID-scale horizontal wavelengths (hundreds of kilometers) to propagate along geomagnetic field lines, while electric fields with spatial scales of a few tens of kilometers or less are likely to attenuate. Since MSTID activity includes TEC fluctuations with spatial scales ranging from approximately 15 to 400 km, TEC variations with scales of a few tens of kilometers or less may experience reduced geomagnetic conjugacy under high solar activity.