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Simultaneous observations from a 630 nm all-sky airglow imager, GNSS-TEC receivers, and an ionosonde are used to investigate the role of E- and F-region coupling on the generation of medium-scale traveling ionospheric disturbances (MSTIDs). The primary observations are OI 630 nm airglow images taken by an all-sky imager in Kühlungsborn (54.07°N; 11.46°E, 53.79°N Mlat.), a site in northern Germany. Out of 226 nights of observations, MSTIDs were found only in 18 nights, demonstrating the low occurrence rate over Kühlungsborn. We focused on four MSTIDs events: two during the vernal equinox and two during summer. Coincident measurements of detrended GNSS-TEC supported the presence of MSTIDs during the selected events, and simultaneous observations from the ionosonde in Juliusruh (54.60°N, 13.4°E, 54.02°N Mlat.) showed sporadic-E (Es) layer and spread-F activity in the E- and F-region, respectively. We observed the onset of the observed MSTIDs to be around the 15°–20°E longitude and 60–45°N latitude belts. Additionally, we found that in each case, the onset of MSTIDs coincides with the presence of an Es layer with sporadic-E trace is observed (foEs) exceeding 4 MHz. This suggests that an Es layer with foEs ≥ 4MHz was a source of the generation of these MSTIDs. Altitude of the Es layer could be another important factor in generating MSTIDs. The Es layer should exist at an altitude where Hall conductivity is large, as happened in the present study.

This work proposes a range spread-F (RSF) prediction model using the neural network (NN) over the equatorial Chumphon (CPN) region in Thailand. The RSF model is constructed by using five input spaces including the diurnal variations, seasonal variations, geographic latitude, solar flux index ( F 10.7 ), and magnetic index ( A p ). The RSF NN model is trained with three years of RSF data during 2013 to 2015 from Chumphon (CPN) station (Latitude = 10.7 ° N , Longitude = 99.4 ° E ) and the performance of the proposed RSF NN model is validated using the dataset of 2016. As a result, the RSF NN model achieves 98.3% accuracy of all correct predictions even with the limited available data. The results show that the proposed NN model yields a lower RSF probability than the actual observation by about 7.3%, but the overestimation of the proposed NN model is 2.5% in both the equinoxes and solstices. In addition, we discover that the IRI-2016 model mostly overestimates the RSF probability when compared with the actual observation for all seasons in 2016, particularly, in equinoctial months over Chumphon station.

The characteristics of global electron density variations in the ionosphere during a geomagnetic storm on 7 and 8 November 2004 were investigated using total electron content (TEC) obtained from the global navigation satellite system (GNSS). The regions of enhanced TEC over North America, Europe, and Japan first appeared in the middle‐latitude regions. The TEC enhancements over North America showed a rapid longitudinal expansion and reached a wide longitudinal extent during the initial and main phases of the geomagnetic storm. TEC enhancements were simultaneously observed in both North America and Japan at 05:00 UT on 8 November. Observation data from the Defense Meteorological Satellite Program showed a slight enhancement of electron density at 850 km below the equatorward boundary of the middle‐latitude trough (45–48°N in geomagnetic latitude) over the Pacific Ocean. This electron density variation may correspond to the TEC enhancements observed in both Japan and North America. These results imply that an enhanced TEC region existed between North America and Japan. The TEC enhancement in Japan appeared with a magnetic conjugacy in the Southern Hemisphere, indicating one of the characteristics of storm‐enhanced density (SED). Moreover, TEC enhancements simultaneously appeared from Japan to central Asia at 11:00 UT on 8 November, corresponding to the early recovery phase of the geomagnetic storm. From the above results, it is suggested that SED phenomena can be simultaneously generated over a wide longitudinal width (~100°). The longitudinal extent of this SED event is 2.5–5.0 times longer than those reported by previous studies., ファイル公開：2020-06-01

Relationship between the locations of the midlatitude trough minimum in the ionosphere and plasmapause in the inner magnetosphere has been statistically investigated using global navigation satellite system (GNSS)-total electron content (TEC) and electron density data obtained from the Arase satellite from March 23, 2017 to May 31, 2020. In this analysis, we identify the midlatitude trough minimum as a minimum value of GNSS-TEC at subauroral and midlatitude regions, and determine the plasmapause as an electron density decrease by a factor of 5 or more within ΔL < 0.5 in the inner magnetosphere. As a result, the plasmapause does not always coincide with the midlatitude trough minimum in all magnetic local time (MLT) sectors under all geomagnetic conditions. During the geomagnetically quiet periods, the midlatitude trough minimum is located at higher and lower geomagnetic latitudes (GMLATs) of the plasmapause in the MLT ranges of 5–21 and 21–5 h, respectively. This implies that both the features could not be on the same magnetic field line. On the other hand, during the storm main phase, the midlatitude trough minimum and plasmapause move toward a low-latitude region with day-night and dawn-dusk asymmetries and the correlation becomes highest, compared with that under other geomagnetic conditions. Especially, both the features mapped on the ionosphere at a height of 300 km exist near GMLAT in the afternoon-midnight sectors. This suggests that the midlatitude trough and plasmapause are formed at almost the same location due to an enhanced subauroral polarization stream during the storm main phase.

This paper proposes a new expression for computing the bottomside thickness parameter at equatorial latitude station, Chumphon (10.72°N, 99.37°E), Thailand. Its diurnal variations from 2004 to 2006 at this location are then studied. The proposed expression is derived based on two experimental data sources: FMCW ionosonde and dual-frequency GPS system, and some expressions of the NeQuick 2 model. Hence, after both the bottomside thickness parameter computed by the proposed equation, B2bot_Pro, and the bottomside shape parameter (namely, B1_Pro in this work) are computed, the bottomside electron density and the height where the bottomside electron density drops down to be 24% of the NmF2 (namely, h0.24) can be computed and shown in this work using the analytical functions of the IRI model. Moreover, the diurnal variations of the B2bot_Pro are compared with those computed from the NeQuick model, B2bot_NeQ, and the predicted B0 of the IRI-2012 model with ABT-2009 and Bil-2000 options (namely, “B0_ABT” and “B0_Bil”, respectively). The averaged, minimum, and maximum values of percentage deviations among these bottomside thickness parameters are also computed and shown in this work. Our results show that the diurnal variations of B2bot_Pro at Chumphon station have the following patterns: they start to increase during nighttime to the first peaks during pre-sunrise hours, and then decrease abruptly to their minimum values during sunrise hours. Afterward, they increase again to reach the second peaks around local noontime and fall gradually to their starting times during 20–04 LT. The diurnal variations of B2bot_Pro follow generally the same trends as those of the B2bot_NeQ and the B0_ABT, except pre-sunrise hours. The pre-sunrise peaks and sunrise collapses in both the B2bot_NeQ and the B0_ABT can be found occasionally. On the other hand, the diurnal variations in B2bot_Pro differ from those in B0_Bil due to the flattened variation in B0_Bil and the pre-sunrise peaks as well as sunrise collapses in B0_Bil disappear. The pre-sunrise peaks of the B2bot_Pro at the Chumphon station are higher than those of the B2bot_NeQ, the B0_ABT, and the observed B0 at other regions. Furthermore, the percentage deviations between the B2bot_Pro and the B0_ABT (PD_B2B0ABT) are mostly lower than 30% for all seasons of the studied years, opposite to the other percentage deviations studied in this work. The proposed B2bot_Pro parameters in this work follow a similar trend to the B2bot_NeQ and the B0_ABT, but it is not conclusive that the proposed values are equivalent to them.

It is well-known that the equatorial anomaly at equatorial and low latitudes is caused by the fountain effect, therefore, the modeling of topside electron density profiles (EDPs) and the knowledge of electron density distribution in the region are particular challenging. Chumphon station, Thailand, is located within this region. However, at this station, since only an FMCW ionosonde system is installed, the topside EDP cannot be automatically obtained. Therefore, in this work, the topside EDPs are derived using the NeQuick topside formulation of the NeQuick 2 model (namely, NeQuick-derived topside EDPs) and then compared with those of three options of the IRI-2012 model including NeQuick, IRI01-corr, and IRI2001 options. The results show that, at Chumphon station, the NeQuick-derived topside EDPs are generally closer to the topside EDPs of IRI01-corr option in winter season and the topside EDPs of NeQuick option in equinox and summer seasons. When analyzing the topside TEC values obtained from each profile and the IGS TEC, it is found that the topside TEC values integrated from NeQuick-derived topside EDPs and predicted by both IRI01-corr and IRI-2001 options of the IRI-2012 model are sometimes higher than the IGS TEC values observed at Chumphon. In addition, we study the diurnal variations of the scale height computed from NeQuick topside formulation (Hsc), hmF2 and foF2 parameters. The correlation of Hsc with the parameters hmF2 and the bottomside thickness parameter of NeQuick model (B2bot). The computed Hsc values during daytime are lower than those during nighttime and they show non-linear correlations with hmF2 and B2bot. The main cause of these discrepancies are possibly due to the B2bot expressions used to compute the Hsc.

The ionospheric peak height in the F layer (hmF2) varies with not only thermospheric conditions but also dynamic processes in the upper atmosphere. At mid-latitudes, the field-aligned diffusion and recombination loss determine the hmF2 in the absence of applied vertical drift. Vertical drifts displace the hmF2 to a new equilibrium position in conjunction with the field-aligned redistribution of the plasma. In the vicinity of the magnetic equator, however, the equilibrium state would be different from low and mid-latitudes because the direct vertical coupling of plasma through the diffusion process is not allowed. Thus the behavior of hmF2 cannot be simply an extrapolation of that at low latitudes. In this paper, ionosonde measurements of the hmF2 near the magnetic equator and off-equatorial latitudes are compared with the IRI output.

This paper presents the development of Neural Network (NN) model for the prediction of the F2 layer critical frequency (foF2) at three ionosonde stations near the magnetic equator of Southeast Asia. Two of these stations including Chiang Mai (18.76°N, 98.93°E, dip angle 12.7°N) and Kototabang (0.2°S, 100.3°E, dip angle 10.1°S) are at the conjugate points while Chumphon (10.72°N, 99.37°E, dip angle 3.0°N) station is near the equator. To produce the model, the feed forward network with backpropagation algorithm is applied. The NN is trained with the daily hourly values of foF2 during 2004–2012, except 2009, and the selected input parameters, which affect the foF2 variability, include day number (DN), hour number (HR), solar zenith angle (C), geographic latitude ( θ ), magnetic inclination (I), magnetic declination (D) and angle of meridian (M) relative to the sub-solar point, the 7-day mean of F10.7 (F10.7_7), the 81-day mean of SSN (SSN_81) and the 2-day mean of Ap (Ap_2). The foF2 data of 2009 and 2013 are then used for testing the NN model during the foF2 interpolation and extrapolation, respectively. To examine the performance of the proposed NN, the root mean square error (RMSE) of the observed foF2, the proposed NN model and the IRI-2012 (CCIR and URSI options) model are compared. In general, the results show the same trends in foF2 variation between the models (NN and IRI-2012) and the observations in that they are higher during the day and lower at night. Besides, the results demonstrate that the proposed NN model can predict the foF2 values more closely during daytime than during nighttime as supported by the lower RMSE values during daytime (0.5 ≤ RMSE ≤ 1.0 for Chumphon and Kototabang, 0.7 ≤ RMSE ≤ 1.2 at Chiang Mai) and with the highest levels during nighttime (0.8 ≤ RMSE ≤ 1.5 for Chumphon and Kototabang, 1.2 ≤ RMSE ≤ 2.0 at Chiang Mai). Furthermore, the NN model predicts the foF2 values more accurately than the IRI model at the three sites on average, as clearly seen on the yearly RMSE averages. The RMSE values of NN model are lower than those of both CCIR and URSI options, and in terms of the yearly percentage improvements, the NN model gives improvement of around 10–15% in 2009 and 10% in 2013 for Chiang Mai, 20–25% in 2009 and 5–10% in 2013 for Chumphon, and around 18–25% in 2009 and 20–30% in 2013 at Kototabang. Although the NN model predicts the foF2 values closely to the observed data and produces more accurate prediction than the IRI models, in some cases, the IRI model performs better than the NN model. Hence, there is still room for further improvement of the proposed NN model.

The occurrence rate of the equatorial plasma bubble (EPB) with season, solar activity, and geomagnetic conditions are investigated using long-term data sets of Malaysia Real-Time Kinematics Network (MyRTKnet) from 2008 to 2013. The rate of TEC (total electron content) change index (ROTI) in 5 min was derived from MyRTKnet data to detect the EPB with scale sizes around tens of kilometers. Then, the daily east-west cross sections of 2-D ROTI maps were used to examine the EPB features over 100°E–119°E longitudes. The EPBs tend to occur successively in one night along the observational coverage of MyRTKnet during equinoxes in high solar activity years. The perturbations in a form of wavelike structures along the observed longitudes might be responsible for the development of successive EPBs due to high growth rate of the Rayleigh-Taylor instability (RTI) process. On the contrary, the occurrence of successive EPBs is infrequent and the occurrence day of EPB remains active during equinoctial months in low solar activity years. The small growth rate of the RTI process during low solar activity years might require a strong seed perturbation to generate the EPB structure. The occurrence probability of the EPB was found to be similar during quiet and disturbed geomagnetic conditions. The results imply that the strong perturbations play an important role in the development of the EPB in low solar activity years. Nonetheless, the high growth rate of the RTI could cause the successive occurrence of the EPB in high solar activity years.

Temporal and spatial evolutions of total electron content (TEC) and electron density in the ionosphere during a geomagnetic storm that occurred on 27 and 28 September 2017 have been investigated using global TEC data obtained from many Global Navigation Satellite System stations together with the ionosonde, geomagnetic field, Jicamarca incoherent scatter and Super Dual Auroral Radar Network (SuperDARN) radar data. Our analysis results show that a clear enhancement of the ratio of the TEC difference (rTEC) first occurs from noon to afternoon at high latitudes within 1 hr after a sudden increase and expansion of the high‐latitude convection and prompt penetration of the electric field to the equator associated with the southward excursion of the interplanetary magnetic field. Approximately 1–2 hr after the onset of the hmF2 increase in the midlatitude and low‐latitude regions associated with the high‐latitude convection enhancement, the rTEC and foF2 values begin to increase and the enhanced rTEC region expands to low latitudes within 1–2 hr. This signature suggests that the ionospheric plasmas in the F2 region move at a higher altitude due to local electric field drift, where the recombination rate is smaller, and that the electron density increases due to additional production at the lower altitude in the sunlit region. Later, another rTEC enhancement related to the equatorial ionization anomaly appears in the equatorial region approximately 1 hr after the prompt penetration of the electric field to the equator and expands to higher latitudes within 3–4 hr., ファイル公開：2021-01-01

In this research, as a part of working towards improving the IRI over magnetic equatorial region, the critical frequency of E layer in the ionosphere ( fo E) derived from the ionogram at Chumphon station (10.72°N, 99.37°E), Thailand, during 2005–2008 is analyzed. The Chumphon station is located in the magnetic equatorial region at the magnetic latitude of 3.22°N. The seasonal variation of the fo E measurements is compared to the IRI fo E predictions with the optional input in the sunspot number (Rz12) and the solar radio noise flux (F10.7). For a declining phase of the solar cycle 23 during the year 2005–2008, the study shows that the IRI fo E prediction is similar to the fo E observation during the period of 2005–2008. The maximum differences between the IRI fo E prediction and fo E observation are about 500 kHz during daytime period of 2007.

In this study, the probability of equatorial spread-F (ESF) occurrence at the conjugate stations in Southeast Asia: Chiangmai station (CMU), Thailand, and Kototabang station (KTB), Indonesia, and near the magnetic equator, Chumphon station (CPN), Thailand, is presented. We analyze the ionogram data recorded by the Frequency Modulated Continuous Wave (FM/CW) ionosondes for the periods of minimum solar activity from September 2008 to April 2009 and in the equinoctial months (March and April) from 2006 to 2013. The spread-F signatures are manually categorized into three types: the frequency spread-F (FSF), the range spread-F (RSF) and the mixed spread-F (MSF) and the monthly average percentage of the occurrence of each ESF type is presented. The results show that the percentage of RSF occurrence at CPN, which is located around the magnetic equator, is higher than at other stations and the RSF mostly occurs during the equinoctial months. On the other hand, the FSF occurrence at CMU and KTB, that are located in the northern and southern hemispheres, respectively, are higher than at CPN. The RSF occurrence typically has the peaks before midnight, while the maximum occurrence rate of FSF is after midnight. Furthermore, the RSF onsets normally precede the FSF onsets by about 1–2 h. As the solar activity levels go up, the percentages of RSF occurrence increase, but the percentages of FSF tends to decrease. In addition, we compare the statistics of observed RSF occurrence with the prediction of the IRI-2012 model. The results show that the IRI model overestimates the observed RSF occurrence at all stations during most of the solar activity levels and seasons, except in June 2008 (21:00–03:00 LT), March 2011 (23:30–01:00 LT) and March 2013 (01:00–02:30 LT) when the IRI model underestimates our observations. However, the IRI model gives closer probability of RSF occurrence to our observed values at CPN, especially in equinoctial months and during the periods of medium solar activity. This work is important for an improvement of the IRI model in the prediction of the spread F occurrence probability in the low-latitude region.

This is the first paper that reports simultaneous observations of zonal drift of plasma bubbles and the thermospheric neutral winds at geomagnetically conjugate points in both hemispheres. The plasma bubbles were observed in the 630 nm nighttime airglow images taken by using highly sensitive all-sky airglow imagers at Kototabang, Indonesia (geomagnetic latitude (MLAT): 10.0°S), and Chiang Mai, Thailand (MLAT: 8.9°N), which are nearly geomagnetically conjugate stations, for 7 h from 13 to 20 UT (from 20 to 03 LT) on 5 April 2011. The bubbles continuously propagated eastward with velocities of 100–125 m/s. The 630 nm images at Chiang Mai and those mapped to the conjugate point of Kototabang fit very well, which indicates that the observed plasma bubbles were geomagnetically connected. The eastward thermospheric neutral winds measured by two Fabry-Perot interferometers were 70–130 m/s at Kototabang and 50–90 m/s at Chiang Mai. We compared the observed plasma bubble drift velocity with the velocity calculated from the observed neutral winds and the model conductivity, to investigate the F region dynamo contribution to the bubble drift velocity. The estimated drift velocities were 60–90% of the observed velocities of the plasma bubbles, suggesting that most of the plasma bubble velocity can be explained by the F region dynamo effect.

Using the fan sector backscatter maps of 47 MHz Equatorial Atmosphere Radar (EAR) at Kototabang (0.2°S geographic latitude, 100.3°E geographic longitude, and 10.4°S geomagnetic latitude), Indonesia, the spatial and temporal evolution of equatorial plasma bubbles (EPBs) were examined to classify the evolutionary-type EPBs from those which formed elsewhere and drifted into the field of view of radar. A total of 535 EPBs were observed during the low to moderate solar activity years 2010–2012, out of which about 210 (~39%) are of evolving type and the remaining 325 (~61%) are drifting-in EPBs. In general, both the evolving-type and drifting-in EPBs exhibit predominance during the postsunset hours of equinoxes and December solstices. Interestingly, a large number of EPBs were found to develop even a few minutes prior to the apex sunset during equinoxes. Further, the occurrence of evolving-type EPBs exhibits a clear secondary peak around midnight (2300–0100 LT), primarily, due to higher rate of occurrence during the postmidnight hours of June solstices. A significant number (~33%) of postmidnight EPBs generated during June solstices did not exhibited any clear zonal drift, while about 14% of EPBs drifted westward. Also, the westward drifting EPBs are confined only to June solstices. The responsible mechanisms for the genesis of fresh EPBs during postmidnight hours were discussed in light of equatorward meridional winds in the presence of weak westward electric fields.

Accepted: 2014-09-01, 資料番号: SA1005061000

The spatial large-scale wave structure (LSWS) at the base of F layer is the earliest manifestation of seed perturbation for Rayleigh-Taylor instability, hence, found to play a deterministic role in the development of Equatorial Plasma Bubbles (EPBs). Except for a few case studies, a comprehensive investigation has not been conducted on the characteristics of LSWS because of the complexity involved in detecting the LSWS, particularly, in spatial domain. In this scenario, a comprehensive study is carried out, for the first time, on the spatial and temporal characteristics of LSWS observed in spatial domain over African and Southeast Asian longitudinal sectors during the year 2011. The observations indicate that these wave structures can be detected a few degrees west of E region sunset terminator and found to grow significantly at longitudes past the sunset terminator. The phase fronts of these spatial structures are found to align with the geomagnetic field (B→) lines over a latitudinal belt for at least 5−6° (~500–600 km) centered on dip equator. The zonal wavelengths of these structures are found to vary from 100 to 700 km, which is consistent with the earlier reports, and the EPBs were consistently observed when the amplitudes of LSWS were grown to sufficient strengths. These results would provide better insights on the underlying physical processes involved in excitation of LSWS in terms of important roles being played by E region electrical loading and polarization electric fields induced via spatially varying dynamo current due to neutral wind perturbations associated with atmospheric gravity waves.

We present the results of a comprehensive study of traveling ionospheric disturbances (TIDs) occurring over Europe during the total solar eclipse of 20 March 2015. For detection of wave structures and estimation of TID parameters, two remote sensing techniques were combined: incoherent scatter (IS) radars and European and Finnish dense GPS receiver networks. Similar procedures were applied for processing both IS and GPS data. We developed a new method enabling to analyze TEC data separately in the temporal and spatial domain. For the first time, we produced maps of band-pass filtered TEC variations and reported both large- and medium-scale prevailing TIDs observed during this solar eclipse, both having similar periods of about 50 – 60 min. The downward phase progression indicates that TIDs were induced by atmospheric gravity waves generated at lower altitudes. The variations in IS power attained peak relative amplitudes of 0.22 (22%) at 220 km over Tromso and of 0.17 (17%) at 200 km over Kharkiv. The vertical phase velocity was about 57 m/s over Tromso. It increased from 25 to 170 m/s over Kharkiv with altitude increasing from 120 to 310 km. Over Western Europe, large-scale TIDs (LSTIDs) had prevailing north-east direction over the region from 45°to 50°N and 2°W to 8°E. Here, their average horizontal phase velocity V m was 803 ± 281 m∕s and the absolute amplitudes of TEC variations usually do not exceed 0.17 TECU. For this region, we found strong differences in LSTID propagation azimuth between the solar eclipse day and the two adjacent days of 19 and 21 March 2015, used as reference. This most likely indicates that these LSTIDs were directly caused by the solar eclipse through local heating/cooling processes occurring during the passing of the Moon penumbra. Over another region, limited by 44°–50°N and 13°–19°E, the LSTIDs had south-east propagation. Over Finland, the LSTIDs also propagated southeastward having V m = 774 ± 202 m∕s and TEC amplitudes up to 0.6 TECU. A possible evidence of LSTID generation at high latitudes indirectly by this solar eclipse through an excitation of slow magnetosonic waves was experimentally detected. Medium-scale TIDs (MSTIDs) propagated southeastward over both regions having V m values of 144 ± 54 m∕s over Western Europe and of 104 ± 43 m∕s over Finland. Over Northern Europe, the maximum MSTID amplitudes were greater by a factor of 5 compared to those over Western Europe and reached 0.4 TECU. We did not detect a clear difference in MSTID propagation between solar eclipse and reference days. The IS and GPS results are in consistency with each other. The detected TID parameters of predominant periods, relative amplitudes, altitude range and MSTID horizontal propagation direction generally correspond to the results of other studies.

[1] We detected clear concentric waves and short-period oscillations in the ionosphere after an Enhanced Fujita scale (EF)5 tornado hit Moore, Oklahoma, U.S., on 20 May 2013 using dense wide-coverage ionospheric total electron content (TEC) observations in North America. These concentric waves were nondispersive, with a horizontal wavelength of ~120 km and a period of ~13 min. They were observed for more than 7 h throughout North America. TEC oscillations with a period of ~4 min were also observed to the south of Moore for more than 8 h. A comparison between the TEC observations and infrared cloud image from the GOES satellite indicates that the concentric waves and short-period oscillations are caused by supercell-induced atmospheric gravity waves and acoustic resonances, respectively. This observational result provides the first clear evidence of a severe meteorological event causing atmospheric waves propagating upward in the upper atmosphere and reaching the ionosphere.

The equatorial spread-F (ESF) is a phenomenon of ionopheric irregularities which are mainly generated by the generalized Rayleigh–Taylor (R–T) instability mechanism in conjunction with the other physical mechanisms, originated at the bottom side of the F-layer in the equatorial region after sunset. It degrades the quality of signals that propagate through these irregularities, especially in the navigation satellite system, which requires the high integrity signals. In this work, we analyze the ESF statistics obtained from the FM/CW ionosonde stations over Thailand longitude sector. One is at Chumphon (10.72°N, 99.37°E, dip latitude 3.0°), located near the geomagnetic equator, and the other station is located at Chiangmai (18.76°N, 98.93°E, dip latitude 12.7°). Both stations are as part of the South-East Asia Low Latitude Ionospheric Network (SEALION) project. The ionograms are obtained at every 15 min from September 2004 to August 2005, which has the monthly mean of solar 10.7 cm flux (F10.7) from ∼80 to ∼110. In addition, we compare the diurnal patterns between the ESF occurrences and the variation of virtual height of the F-layer bottom side (h’F) of these two stations. The results show that the ESF occurrences at Chumphon stations are higher than Chiangmai station in all seasons. The high ESF occurrences of both stations mostly occur in equinoctial months corresponded with the rapid rising of the monthly mean h’F in the post-sunset. However, some inconsistent results are still observed, implying the role of other factors such as gravity waves and planetary waves to ESF occurrences.

In this work, the foF2 and hmF2 parameters at the conjugate points near the magnetic equator of Southeast Asia are studied and compared with the International Reference Ionosphere (IRI) model. Three ionosondes are installed nearly along the magnetic meridian of 100°E; one at the magnetic equator, namely Chumphon (10.72°N, 99.37°E, dip angle 3.0°N), and the other two at the magnetic conjugate points, namely Chiang Mai (18.76°N, 98.93°E, dip angle 12.7°N) and Kototabang (0.2°S, 100.30°E, dip angle 10.1°S). The monthly hourly medians of the foF2 and hmF2 parameters are calculated and compared with the predictions obtained from the IRI-2007 model from January 2004 to February 2007. Our results show that: the variations of foF2 and hmF2 predicted by the IRI-2007 model generally show the similar feature to the observed data. Both parameters generally show better agreement with the IRI predictions during daytime than during nighttime. For foF2, most of the results show that the IRI model overestimates the observed foF2 at the magnetic equator (Chumphon), underestimates at the northern crest (Chiang Mai) and is close to the measured ones at the southern crest of the EIA (Kototabang). For hmF2, the predicted hmF2 values are close to the hmF2(M3000F2OBS) during daytime. During nighttime, the IRI model gives the underestimation at the magnetic equator and the overestimation at both EIA crests. The results are important for the future improvements of the IRI model for foF2 and hmF2 over Southeast Asia region.

[1] Unusual ionospheric variations were observed in the M9.0 Tohoku-oki earthquake on 11 March 2011. Among various kinds of features in the ionosphere, significant depletion of total electron content (TEC) near the epicenter was observed after the earthquake. Although previous studies have suggested that the coseismic ionospheric variations are associated with atmospheric perturbation caused by vertical displacement of the sea surface, the mechanism of the TEC depletion has not been fully understood. In this paper, a two-dimensional nonlinear nonhydrostatic compressible atmosphere-ionosphere model is employed to investigate the ionospheric variations in the vicinity of the epicenter. The simulation results reveal that an impulsive pressure pulse produced by a sudden uplift of the sea surface leads to local atmospheric expansion in the thermosphere and that the expansion of the thermosphere combined with the effect of inclined magnetic field lines in the ionosphere causes the sudden TEC depletion above the epicenter region.

[1] Geomagnetic disturbances from 7 to 12 November 2004 were quite intense, and the maximum excursion of Dst reached −374 nT. Unusual ionospheric phenomena have been observed around the world that have been associated with successive magnetic storms during this period. The ionospheric total electron content (TEC) was increased at the longitudes of Japan within a short time after sunset on 8 November, from 20 TEC units at 1830 JST to 97 TEC units at 2015 JST (JST = UT + 9 h), where 1 TEC unit = 1×1016 el/m2. The enhanced TEC was significant over Hokkaido (∼43°N), and the center of the enhanced region was well within the plasmasphere as an L value of approximately 1.5. The drift velocity of the plasma in the density-enhanced region measured by the Defense Meteorological Satellite Program (DMSP) satellite was westward with a peak value of 250 m/s in the Earth's frame, demonstrating a positive correlation between density and drift velocity. A similar TEC event was observed after sunset on 10 November: TEC was enhanced, from 15 TEC units at 1830 JST to 45 TEC units at 2030 JST. During the second event, the ionosphere was highly structured and the rate of the TEC index (ROTI) increased. The two-dimensional map of a ROTI-enhanced region exhibited the west to northwest transportation of plasma in which density irregularities were entrained. A physical mechanism is proposed to explain these disturbances, i.e., storm-induced plasma stream, which is different from a phenomenon called storm-enhanced density at midlatitudes.

There is mounting evidence that plasma structure in nighttime equatorial F layer evolves from large-scale wave structure (LSWS) in the bottomside F layer. This process cannot be ignored because equatorial plasma bubbles (EPBs), arise from large-amplitude LSWS; and, because intense radiowave scintillations are associated with EPBs, understanding the LSWS-to-EPB process is a crucial step toward reliable space-weather forecasting. In this regard, the transequatorial propagation (TEP) experiment appears to be most useful among available research instruments. After a lapse of 30 years, the TEP experiment has been resurrected [e.g., Maruyama and Kawamura, 2006]; a goal of this research is to understand TEP measurements well enough so that they can be used to diagnose the LSWS-to-EPB process. Toward this end, new results are presented in two companion papers. Herein (P1), off-great-circle (OGC) propagation paths are shown to consist of two types, discrete and diffuse. The new findings include: (1) a generalized multi-reflection model that can explain most of the observed properties; (2) the discrete type is supported by multi-reflections from an unstructured upwelling, (3) the diffuse type is supported by reflections from plasma structure in EPBs; and (4) the observed east-west (EW) asymmetry can be explained in terms of a distorted upwelling or plasma structure along the west wall of an upwelling. In Paper 2 (P2), a second form of observed EW asymmetry is explained in terms of plasma structure, which is not aligned with the geomagnetic field. The findings strongly confirm a close relationship between upwellings, ESF patches, and OGC paths.

Numerical simulations are performed to simulate atmospheric perturbations observed at ionospheric heights just after the 2011 off the Pacific coast of Tohoku Earthquake. A time-dependent, two-dimensional, nonlinear, non-hydrostatic, compressible and neutral, numerical model is developed to reproduce the atmospheric perturbations. An impulsive upward surface motion is assumed as the source of the perturbations. Simulated atmospheric perturbations at 300-km altitude show remarkable agreement with oscillations observed in the ionospheric total electron content (TEC) when the source width is about 250 km. In the vicinity of the source, the acoustic resonance modes between the ground surface and the lower thermosphere are dominant. They have three dominant frequencies for the interval between 20 and 60 min after the impulsive input. The perturbation with the maximum amplitude has a frequency of 4.4 mHz. The other dominant modes have frequencies of 3.6 and 5.1 mHz. The beats between the dominant modes are also seen. In the distance, the gravity modes are dominant. The horizontal phase velocities are about 220 to 300 m/s, and the horizontal wavelengths are about 200 to 400 km. The good agreement between the simulation and the observations indicates that ionospheric oscillations generated by the earthquake are mainly due to the motion of the neutral atmosphere.

High-density GPS receivers located in Southeast Asia (SEA) were utilized to study the two-dimensional structure of ionospheric plasma irregularities in the equatorial region. The longitudinal and latitudinal variations of tens of kilometer-scale irregularities associated with equatorial plasma bubbles (EPBs) were investigated using two-dimensional maps of the rate of total electron content change index (ROTI) from 127 GPS receivers with an average spacing of about 50–100 km. The longitudinal variations of the two-dimensional maps of GPS ROTI measurement on 5 April 2011 revealed that 16 striations of EPBs were generated continuously around the passage of the solar terminator. The separation distance between the subsequent onset locations varied from 100 to 550 km with 10 min intervals. The lifetimes of the EPBs observed by GPS ROTI measurement were between 50 min and over 7 h. The EPBs propagated 440–3000 km toward the east with velocities of 83–162 m s−1. The longitudinal variations of EPBs by GPS ROTI keogram coincided with the depletions of 630 nm emission observed using the airglow imager. Six EPBs were observed by GPS ROTI along the meridian of Equatorial Atmosphere Radar (EAR), while only three EPBs were detected by the EAR. The high-density GPS receivers in SEA have an advantage of providing time continuous descriptions of latitudinal/longitudinal variations of EPBs with both high spatial resolution and broad geographical coverage. The spatial periodicity of the EPBs could be associated with a wavelength of the quasiperiodic structures on the bottomside of the F region which initiate the Rayleigh-Taylor instability.

A multipoint ionosonde observation campaign was conducted along the magnetic meridional plane in Southeast Asia to study ionosphere-thermosphere coupling. One station was near the magnetic equator and two of the other stations were at off-equatorial latitudes (∼10° magnetic latitude). The daytime ionospheric peak height (hmF2) was analyzed for each season during the solar minimum years, 2006–2007 and 2009. The peak height increased for ∼3 h after sunrise at the magnetic equator and off-equatorial latitudes, as expected from the daytime upward E × B drift. The apparent upward drift at the magnetic equator ceased before noon, while the drift at the off-equatorial latitudes continued upward and the layer height exceeded the equatorial height around noon. The noontime limited layer peak height at the magnetic equator, which was termed the ionospheric ceiling, did not depend on the season, while the maximum peak height at the off-equatorial latitudes largely varied with each season. Numerical modeling using the SAMI2 code was conducted and the features of the ionospheric ceiling were reproduced quite well. The dynamical parameters provided by the SAMI2 modeling runs showed that the ionospheric ceiling is formed by the field-aligned plasma diffusion, which is a part of the fountain effect.

Quiet-time daily variations of the geomagnetic field near the magnetic equator due to the equatorial electrojet are simulated using the National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere Electro- dynamics General Circulation Model (TIE-GCM), and compared to those observed by ground-based magnetometers. Simulations are run both with and without tidal forcing at the height of the model lower boundary (∼97 km). When the lower-boundary forcing is off, the wind that generates an electro- motive force in the model is primarily the vertically non-propagating diurnal tide, which is excited in the thermosphere due to daytime solar ultra-violet heating. The lower-boundary tidal forcing adds the effect of upward-propagating tides, which are excited in the lower atmosphere and propagate vertically to the thermosphere. The main objective of this study is to evaluate the relative importance of these thermospherically-generated tides and upward-propagating tides in the generation of the equatorial electrojet. Fairly good agreement is obtained between model and observations when the model is forced by realistic lower-boundary tides based on temperature and wind measurements from the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite, as determined by Wu et al. [2012]. The simulation results show that the effect of upward-propagating tides increases the range of the geomagnetic daily variation in the magnetic-northward component at the magnetic equator approximately by 100%. It is also shown that the well-known semiannual change in the daily variation is mostly due to upward-propagating tides, especially the migrating semidiurnal tide. These results indicate that upward-propagating tides play a substantial role in producing the equatorial electrojet and its seasonal variability.

[1] The strong westward electrojet and simultaneous upward drift of the equatorial ionospheric peak observed over South-East Asia and Indian equatorial regions during the prolonged Dst minimum phase of an intense geomagnetic storm during 14–15 December 2006 are investigated for the altitudinal variation of zonal electric field polarity using ground based and space-borne observations. The results show first observational evidence for simultaneous existence of daytime westward and eastward zonal electric fields at equatorial E and F region altitudes, respectively, in a wide longitude sector. While the westward electric fields at E region altitudes cause westward electrojet, at the same time, the eastward zonal electric fields atFregion altitudes cause the upward drift of the equatorial ionospheric peak and reinforcement of the equatorial ionization anomaly (EIA) even in the topside ionosphere (∼660 km). The reversal of the electric fields is found to occur at ∼280 km height. A clear bifurcation of F region plasma at ∼280 km is evident in the iso-electron density contours due to these oppositely polarized zonal electric fields, which manifests as an unusually deep cusp between F1 and F2 layers on equatorial ionograms.

[1] After the magnitude 9.0 earthquake off the Pacific coast of Tohoku (near the east coast of Honshu, Japan), which occurred on 11 March 2011, an unusual multiple-cusp signature (MCS) was observed in ionograms at three ionosonde stations across Japan. To investigate the general characteristics of MCSs, we examined ionograms obtained during the period 1957 to 2011 from five ionosonde stations in Japan after earthquakes with a seismic magnitude of 8.0 or greater. A total of 43 such earthquakes occurred after 1957, as recorded in the database of the United States Geological Survey. MCSs were observed in 8 of 43 earthquakes, allowing us to investigate the specific conditions of MCS appearance. The appearance of MCSs at different epicentral distances exhibits traveling characteristics at a velocity of ∼4.0 km/s, which is in the range of Rayleigh waves. There is a ∼7 min offset in delay time after the earthquake at each epicentral distance in the travel-time diagram. This offset is consistent with the propagation time of acoustic waves from the ground to the ionosphere launched by Rayleigh waves. MCSs in ionograms provide a snapshot of vertical disturbances induced by a wave because the sweep time for the whole ionospheric echo traces is much shorter than the propagation time of acoustic waves. The vertical scale of the disturbances is 10∼30 km, which corresponds to an acoustic wave period of 20∼50 s at ionospheric heights.

[1] We investigated the relationship between post-midnightF-region field aligned irregularities (FAIs) andF-layer altitude by analyzing data of a 30.8-MHz radar installed 5at Kototabang, Indonesia (0.2°S, 100.3°E; geomagnetic latitude 10.4°S) and an ionosonde installed at Chumphon, Thailand (10.7°N, 99.4°E; geomagnetic latitude 3.3°N). Chumphon is located near the geomagnetic equator on approximately the same meridian as Kototabang. Case studies show that the altitude of theF-layer rose at Chumphon a half hour before the post-midnight FAIs appeared at Kototabang. The Doppler velocity of theE-region FAIs observed simultaneously by the 30.8-MHz radar was downward, indicating that theF-layer uplift was not caused by the electric field. We also investigated seasonal variations of the post-midnight FAI occurrence and theF-layer altitude. Both the post-midnight FAIs and the uplift of theF-layer were frequently seen around midnight between May and August. The seasonal variation of the midnightF-layer uplift around the geomagnetic equator coincided with that of the post-midnight FAI occurrence at Kototabang. These results suggest that the uplift of theF-layer would play an important role in the generation of post-midnight FAIs. We evaluated the linear growth rate of the Rayleigh-Taylor instability based on the altitude of theF-layer observed at Chumphon. The result shows that the uplift of theF-layer can enhance the growth rate because gravity-driven eastward electric current increases. Therefore, we interpret that the observed FAIs were accompanied by plasma bubble, the growth rate of which was reinforced by the upliftedF-layer.

[1] Using the 47 MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (10.36°S dip latitude), it is shown that postmidnight irregularities during solar minimum are morphologically different from those detected during solar maximum and are quite similar to those observed with the middle and upper atmosphere (MU) radar in midlatitudes (29.3°N dip latitude). Utilizing the rapid beam-steering capability of the EAR, the spatial structure of the postmidnight irregularities is clearly presented for the first time. It is found that they usually propagate westward and can be categorized into two types. One shows sharp upwelling plumes near local midnight, which should not be a mere passage of fossil plasma bubbles. The other has successive tilted structures which have the same orientation as medium-scale traveling ionospheric disturbances typically observed at midlatitudes. We suggest that the convergence of the equatorward thermospheric wind which is believed to be responsible for the midnight temperature maximum may be an important factor to produce a preferable condition for the upwelling plumes in the postmidnight sector. The displacement between geographic and magnetic equators may also be important for seasonal/longitudinal variation of the postmidnight irregularities.

[1] Understanding the day-to-day variability in occurrence of equatorial spread F (ESF) remains as a high-priority objective in space weather research. A major difficulty has been an inability to resolve the roles being played by large-scale wave structure (LSWS) and the post-sunset rise (PSSR) of the equatorial F layer, in the production of ESF. In this paper, we show conclusively that total electron content (TEC), measured as a function of latitude and longitude, provides clear, routine descriptions of LSWS. Then, together with ionosonde data, we show, for the first time, that while a seed for LSWS can occur in the late afternoon, its amplification takes place mostly during the PSSR. Implications of these findings are discussed in light of existing theories.

[1] Using multi-instrumental and multistation data, we present low-latitude ionospheric-thermospheric behavior during the geomagnetic storm of 15 May 2005. The diurnal pattern of total electron content (TEC) at a chain of equatorial to low-latitude stations shows strong positive ionospheric storm on 15 May. Latitudinal variation of TEC shows development of strong equatorial ionization anomaly (EIA) on the same day. Evidence, in terms of equatorial electrojet (EEJ) and magnetogram signatures, is presented for the prompt penetration of interplanetary electric field (IEF) as the cause of the positive ionospheric storm. Consequent to the storm time circulation resulting from the extra energy deposition via Joule heating over high latitudes, compositional changes occur in the global thermosphere. TEC enhancements on 16 May are attributed to enhancement of atomic oxygen at equatorial and low latitudes and the negative ionospheric storm on 17 May observed beyond certain low latitudes is explained in terms of enhancement of molecular species because of the storm time neutral composition changes. Strong ESF plume structures on range time intensity (RTI) map and L-band scintillation and TEC depletions in GPS measurements are observed in the longitude sectors where the local time of sudden storm commencement (SSC) falls after the post sunset hours. The ionospheric zonal electric fields are altered by the combined effects of eastward disturbance dynamo electric fields and direct prompt penetration of eastward electric fields associated with the northward turning of interplanetary magnetic field (IMF) Bz leading to subsequent development of ESF after midnight.

[1] During a magnetic storm on 14–16 December 2006, a polar cap tongue of ionization (TOI) was detected by an all-sky imager (ASI) at Resolute Bay, Canada (74.73°N, 265.07°E). We investigate the temporal evolution and spatial structure of the TOI in detail by combining the optical data with other observations (e.g., solar wind, GPS total electron content, SuperDARN, and DMSP and NOAA POES satellites). The TOI was observed as a bright and elongated 630 nm airglow plume for 4 h during the main phase of the storm. This interval corresponded to a period of prolonged stable large-amplitude southward IMF during a coronal mass ejection (CME). One to one and a half hours before the appearance of TOI, the polar cap boundary expanded rapidly far equatorward, and a positive ionospheric storm occurred. This implies that both the “expansion of the high-latitude plasma convection” and “build up of the source plasma in the midlatitudes” are necessary conditions for the formation of a TOI. Because both of them were triggered by a major southward turning of the IMF, the prolonged large-amplitude southward IMF orientation in the trailing part of the CME was primarily responsible for the generation of TOI. After its appearance, the TOI exhibited dynamic motion in the dawn to dusk direction. Simultaneous SuperDARN data suggest that a longitudinal progression of subauroral polarization stream controlled this dynamic motion. The optical TOI was found to be a continuous stream elongated in the noon-midnight direction although it contained some mesoscale patterns. Absence of large-scale temporal changes in the cusp plasma flow during the stable IMF period allowed the TOI to remain continuous without being broken into polar cap patches. The mesoscale structures within the TOI were probably produced by small-scale velocity fluctuations in the cusp plasma flow. The TOI as visualized with the all-sky airglow imager was found to be much more dynamic and much more complicated than we ever thought. The current study indicates that such a behavior of the TOI was presumably caused by a combination of temporal variations in the global-scale plasma circulation system, expansion and contraction of the polar cap area, and plasma density changes in the dayside low to midlatitudes.

[1] On 15 December 2006, during the main phase of a relatively large storm, Doppler velocity data from the Super Dual Aural Radar Network (SuperDARN) Hokkaido radar, together with total electron content (TEC) data from the GPS Earth Observation Network (GEONET), recorded daytime large-scale traveling ionospheric disturbances (LSTIDs). We studied two disturbances propagating southward and one disturbance propagating northward between 0000 and 0600 UT on 15 December 2006. The former disturbances were LSTIDs typical of those reported in many previous studies, whereas the latter was confirmed as an LSTID propagating from the Southern into the Northern Hemisphere, reported in a few past studies. From comparisons of SuperDARN Hokkaido radar Doppler velocity and GEONET TEC, we found a positive correlation between downward ionospheric motion and increasing TEC. This relationship is consistent with results of model calculation. This is the first observation of LSTIDs ranging from high to low latitude combining simultaneous SuperDARN HF radar and GPS network observations.

[1] Medium-scale traveling ionospheric disturbances (MSTIDs) whose peak-to-peak amplitude was larger than 20 TECU (=1016el/m2) were observed at midlatitude during the geomagnetic storm on 10 November 2004. This amplitude was more than 10 times larger than that of the average MSTID. High-resolution data of the GPS Earth Observation Network (GEONET) clarified the characteristic of the total electron content (TEC) disturbances over Japan on 10 November 2004. The disturbances started around 1000 UT in the central part of Japan. The maximum of TEC temporal change was 7.2 TECU in 30 s. The disturbances had several wave fronts which extended from northwest to southeast and propagated from northeast to southwest. TEC data around Japan revealed that the disturbances were mainly observed from 18°N/S to 34°N/S of the geomagnetic latitude in the both hemispheres. Since those characteristics were similar to those of MSTIDs in spite of the unusual large amplitude, the MSTIDs are referred as “super-MSTIDs” in this paper. TEC variations of the super-MSTIDs were also observed at 460 km altitude by the GRACE satellite. The ion density fluctuations of the super-MSTIDs were observed in situ by the CHAMP and DMSP-F15 satellites, which flew at 360 km and 850 km, respectively. It is found that the plasma density variations of the super-MSTIDs occurred mainly above 360 km altitude. The characteristics that distinguish the event from plasma bubbles are its successive wave fronts, constant northwest-southeast direction along which the wave fronts stretched, and late local time of the occurrence. It is found that the uplift of the ionosphere around sunset excited the super-MSTIDs at midlatitudes. The uplift was attributed to the strong eastward electric field during the geomagnetic storm.

[1] We report an event of unusually elongated, bright airglow plume, which is considered as an optical manifestation of tongue of ionization (TOI) in the central polar cap. This optical structure was detected with an all-sky airglow imager at Resolute Bay (74.73°N, 265.07°E) during a large magnetic storm on December 15, 2006. The absolute optical intensity of the plume was ≈1 kR, which is much brighter than that of non-stormtime polar cap patches. Two-dimensional imaging capability of the all-sky imager demonstrates that some meso-scale structures (≈250–600 km) were embedded within the plume. Simultaneous ion density and drift measurements with the DMSP spacecraft strongly suggest that the plume was extending from the dayside as a narrow stream of dense plasma and thus is an optical manifestation of polar cap TOI. The DMSP data also implies that the possible source of the plume is a narrow stream of storm enhanced density (SED) transported from lower latitudes. The DMSP auroral particle observation demonstrates that the polar cap extremely expanded equatorward during this interval. This extreme expansion allowed the anti-sunward convection to capture plasmas within the SED and deliver them deep into the polar cap as a luminous airglow plume. This observation claims that the plasma transport from the dayside lower latitudes plays an important role in controlling the plasma environment in the polar cap ionosphere during magnetic storms.

[1] We present midlatitude medium-scale traveling ionospheric disturbances (MSTIDs) observed with a Super Dual Auroral Radar Network (SuperDARN) HF radar at around 10 MHz in Hokkaido, Japan, in combination with a 630-nm all-sky imager and a GPS network (GEONET) that provides total electron content (TEC) data. MSTIDs propagating southward from high latitudes are detected at first with the HF radar and then with the imager and GEONET. We analyze two MSTID events, one in winter (event 1) and the other in summer (event 2), to find that MSTIDs appear simultaneously, at least, at 55°–25°N. It is shown that nighttime MSTIDs propagate toward the southwest over a horizontal distance of about 4000 km, and daytime MSTIDs do so toward the southeast. Daytime radar echoes are due to ground/sea surface (GS) scatter, while nighttime echoes in event 1 return from 15-m-scale F region field-aligned irregularities (FAIs) and those in event 2 are due to GS scatter. Doppler velocities of the nighttime F region FAI echoes in event 1 are negative (motion away from the radar) within strong echo regions and are positive (motion toward the radar) within weak echo regions. This fact suggests that the strong (weak) echoes return from suppressed (enhanced) airglow/TEC areas, in line with previous observations over central Japan. The nighttime MSTIDs in events 1 and 2 are often accompanied by concurrent coherent echoes from FAIs in sporadic E (Es) layers. The Es echo areas in event 2 rather coincide with suppressed airglow/TEC areas in the F region that are connected with the echo areas along the geomagnetic field, indicating the existence of E and F region coupling at night.

International audience; We present simultaneous observations of daytime medium-scale traveling ionospheric disturbances (MSTIDs) over North America, using measurements of the Total Electron Content (TEC) by the US GPS network and ionospheric plasma data from the DEMETER micro-satellite. Several events show latitudinal variations of the plasma parameters at satellite altitude corresponding to the MSTID structures revealed on 2D TEC maps. In a case study with a very well defined MSTID, quasi periodic variations of the plasma density and of the ion velocity parallel to the Earth's magnetic field are observed along the satellite orbit that match the signature of the MSTID on the TEC maps. We believe it is the first simultaneous observation of parallel plasma motion in the topside ionosphere and propagating MSTID structures in the F-region. An initial analysis of this event is performed in the light of results from simple model of ionospheric disturbances associated with Atmospheric Gravity Waves (AGWs).

[1] Nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed in 630-nm airglow images at middle latitudes are known to have a predominantly northwest-southeast phase surface and to move southwestward in the Northern Hemisphere of Earth. However, the mechanisms of MSTID generation and their systematic southwestward motion have not been clarified. In this paper, we report the “northeastward” motion of the MSTIDs observed at Paratunka, Far East Russia (52.97°N, 158.25°E), using an all-sky 630-nm airglow imager at 2000–2300 LT on 19 August 2007. The MSTIDs moved first southwestward but then back northeastward in the northern part of the images. The northeastward motion of the MSTIDs took place coincident with a F layer height decrease observed by an ionosonde at Paratunka. The F layer height decrease was also confirmed by an enhancement of the 630-nm airglow intensity, which seemed to propagate from northeast to southwest. This fact suggests that the F layer height decrease was caused by poleward wind enhancement rather than westward electric field. These observations imply that the F layer height decrease or the poleward thermospheric wind has some role in the northeastward turning of the MSTID propagation direction.

[1] A numerical simulation was performed to investigate quasiperiodic ionospheric oscillations that were observed with periods of 4–5 h by the ionosonde network (Okinawa, Yamagawa, Kokubunji, and Wakkanai) in Japan during the 15 December 2006 magnetic storm. This simulation used the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) 2.0 model. The CMIT model reproduced the main characteristics of the observed ionospheric oscillations, although it remains a challenging task to simulate the observations in a quantitative sense. Term analysis of the ion continuity equation demonstrated that the ionospheric oscillations in this event were mainly induced by the disturbed neutral winds, which were associated with the large scale thermospheric circulation and traveling atmospheric disturbances (TADs) during the storm. The TADs simulated from the model were then compared with those observed by the GPS Earth Observation Network (GEONET) in Japan to validate the simulation results. A prominent northward propagating large-scale traveling ionospheric disturbance (LSTID) during daytime, seen by the GEONET total electron content (TEC) data, was captured by the CMIT model. Two southward LSTIDs observed by GEONET GPS network were also reproduced by the CMIT model. However, the model gave faster phase speeds for the southward propagating LSTID occurred during 0620–0800 UT and the northward propagating LSTID; furthermore, the model missed the LSTID seen in the TEC perturbation data during 0140–0220 UT. Finally, both observations and simulations showed a strong hemispheric asymmetry for the TAD propagation that occurred during 0000–0400 UT, which may be associated with the hemispheric asymmetry of the change of Joule heating at high latitude.

This paper reports simultaneous observations of ionospheric scintillation during an auroral substorm that were made using an all-sky full-color digital single-lens reflex (DSLR) camera (ASC) and a Global Positioning System (GPS) ionospheric scintillation and total electron content monitor (GISTM) in Tromso (69.60 N, 19.20 E), Norway. On the night of November 19, 2009, a small substorm occurred in northern Scandinavia. The ASC captured its temporal evolution from the beginning of the growth phase to the end of the recovery phase. The amplitude scintillation, as monitored by the S4 index from the GISTM, did not increase in any substorm phase. By contrast, phase scintillation, as measured by the σ φ index, occurred when discrete auroral arcs appeared on the GPS signal path. In particular, the phase scintillation was significantly enhanced for a few minutes immediately after the onset of the expansion phase. During this period, bright and discrete auroral forms covered the entire sky, which implies that structured precipitation on the scale of a few kilometers to a few tens of kilometers dominated the electron density distribution in the E region. Such inhomogeneous ionization structures probably produced significant changes in the refractive index and eventually resulted in the enhancement of the phase scintillation.

We report the first observation of the disappearance of a plasma bubble over geomagnetically conjugate points. It was observed by airglow imagers at Darwin, Australia (magnetic latitude: −22°N) and Sata, Japan (21°N) on 8 August 2002. The plasma bubble was observed in 630-nm airglow images from 1530 (0030 LT) to 1800 UT (0300 LT) and disappeared equatorward at 1800 to 1900 UT (0300 to 0400 LT) in the field of view. The ionograms at Darwin and Yamagawa (20 km north of Sata) show strong spread-F signatures at approximately 16 to 21 UT. At Darwin, the F-layer virtual height suddenly increased from approximately 200 to approximately 260 km at the time of bubble disappearance. However, a similar F-layer height increase was not observed over the conjugate point at Yamagawa, indicating that this F-layer rise was caused not by an eastward electric field but by enhancement of the equatorward thermospheric wind over Darwin. We think that this enhancement of the equatorward neutral wind was caused by an equatorward-propagating large-scale traveling ionospheric disturbance, which was identified in the north-south keogram of 630-nm airglow images. We speculate that polarization electric field associated with this equatorward neutral wind drive plasma drift across the magnetic field line to cause the observed bubble disappearance.