Medium-scale Traveling Ionospheric Disturbances Research Articles

A Case Study on the Interaction Between MSTIDs' Fronts, Their Dissipation, and a Curious Case of MSTID's Rotation Over Geomagnetic Low‐Mid Latitude Transition Region

AbstractThis paper reports a rare interaction between two fronts of nighttime medium scale traveling ionospheric disturbances (MSTIDs) and associated phenomena observed over geomagnetic low‐mid latitude transition region on a geomagnetically quiet night (Ap = 5) of 21 May 2020. The event was observed for nearly 8 hours in O(1D) 630.0 nm airglow images captured by an all‐sky imager installed at Hanle (32.7°N, 78.9°E; Mlat. ∼24.1°N), Leh Ladakh, India and supplemented by a digisonde and a global positioning system receiver located within the imager's field of view (FOV). The images revealed a slow‐moving MSTID front being followed by a fast‐moving MSTID front, both traveling southwestward. The third front of the MSTID caught up with the second one, initiating the interaction. As a consequence, a plasma channel was created during the interaction and the MSTIDs' fronts began decaying. Subsequently, they became thin strips and later dissipated within the FOV of the imager. The polarization electric field of the merged region of the two interacting MSTIDs' fronts played a key role in creating the plasma channel. In the later hours of the night, another MSTID with a rotating front was observed, which is a rarely reported phenomenon. The horizontal gradient in the westward wind is a plausible reason for the observed rotation. This study elucidates the complex nature of geomagnetic low‐mid latitude transition region.

Statistical Study of Seasonal and Solar Activity Dependence of Nighttime MSTIDs Occurrence Using the SuperDARN Hokkaido Pair of Radars

AbstractWe conduct a statistical analysis on nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) using the Super Dual Auroral Radar Network (SuperDARN) Hokkaido East (43.53°N, 143.61°E) high‐frequency (HF) radar data from 2009 to 2019 and the SuperDARN Hokkaido West (43.54°N, 143.61°E) HF radar data from 2016 to 2019. We analyze the line‐of‐sight Doppler velocity data, corresponding to the magnitude of the polarization electric field associated with nighttime MSTIDs. We find that the propagation direction of the nighttime MSTIDs is mainly southwestward and rotates clockwise with progressing local time. In addition, we identify a negative correlation between the nighttime MSTIDs amplitude and the solar F10.7 index. These tendencies can be explained by the Perkins instability, which is considered to be associated with the growth of nighttime MSTIDs. This study is the first to find the solar activity dependence of the polarization electric field fluctuations associated with the nighttime MSTIDs.

Temporal Evolution of Low-Latitude Plasma Blobs Identified From Multiple Measurements: ICON, GOLD, and Madrigal TEC.

Low‐latitude plasma blobs have been studied since their first being reported in 1986. However, investigations on temporal evolution of a blob or on continental scale (>2,000 km) ionospheric contexts around it are relatively rare. Overcoming these limitations can help elucidate the blob generation mechanisms. On 21 January 2021, the Ionospheric Connection Explorer satellite encountered a typical low‐latitude blob near the northeastern coast of South America. The event was collocated with a local enhancement in 135.6 nm nightglow at the poleward edge of an equatorial plasma bubble (EPB), as observed by the Global‐scale Observations of the Limb and Disk (GOLD) imager. Total electron content maps from the Global Navigation Satellite System confirm the GOLD observations. Unlike typical medium‐scale traveling ionospheric disturbances (MSTIDs), the blob had neither well‐organized wavefronts nor moved in the southwest direction. Neither was the blob a monotonically decaying equatorial ionization anomaly crest past sunset. Rather, the blob varied following latitudinal expansion/contraction of EPBs at similar magnetic longitudes. The observational results support that mechanisms other than MSTIDs, such as EPBs, can also contribute to blob generation.

Conjugate Effect of the 2011 Tohoku Reflected Tsunami‐Driven Gravity Waves in the Ionosphere

AbstractThis study presents the conjugate ionospheric disturbances triggered by the 2011 Tohoku‐Oki reflected tsunami oceanic waves using the ground‐based Global Navigation Satellite System (GNSS) total electron content (TEC) observations. We found that the equatorward and westward propagating nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) occurred over Japan and Australia simultaneously following the tsunami oceanic waves reflected by the Emperor Seamount Chain in the northern hemisphere. The atmospheric gravity waves driven by reflected tsunami oceanic waves are hypothesized to be the source to trigger the conjugate MSTIDs by transporting the polarization electric fields along the field line to the conjugate hemisphere. Moreover, only the southwestward propagating MSTIDs have this conjugate effect, which could be due to the wavefront orientation. The Perkins instability could also be involved in the interhemispheric coupling process. This study provides the first observational evidence that the reflected tsunami can induce conjugate ionospheric disturbances through electrodynamic forcing.

Propagation Direction Analyses of Medium‐Scale Traveling Ionospheric Disturbances Observed Over North America With GPS‐TEC Perturbation Maps by Three‐Dimensional Spectral Analysis Method

AbstractWe applied a novel three‐dimensional spectral analysis method to GPS‐TEC perturbation (GPS‐TECP) maps to study the propagation direction of daytime and nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) over North America. By this method, we can automatically calculate the phase velocity spectrum and directionality of MSTIDs. We focused on the periods of high MSTIDs occurrence, namely, June–July 2006 and November–December 2006, to study nighttime and daytime MSTIDs. We divided North America into the west (100°–130°W, 25°–55°N) and east (70°–100°W, 25°–55°N) parts. Our results show that both daytime and nighttime MSTID propagations exhibit strong longitudinal variations as a function of local time and day‐to‐day variations. The power peaks of daytime MSTIDs are from 10:00–16:00 LT in the west part and 10:00–14:00 LT in the east part. The predominant propagation directions of daytime MSTIDs are southward (southeastward) in the west (east). The daytime local time variations demonstrate that the MSTIDs display directional change in the west part; however, a similar directional change is not very pronounced in the east part. The local time variations of nighttime MSTIDs shows the power peaks from 22:00–02:00 LT in the west and 20:00–00:00 LT in the east. We found that the predominant propagation direction in the west part is westward with a wider azimuthal band (∼210°–300°) than the east part (∼210°–240°). By comparing nighttime propagation directions between the western and eastern parts, we reached the conclusion that the magnetic declination angle affects the propagation direction.

Daytime and Nighttime Medium-Scale Traveling Ionospheric Disturbances to the June 2015 Geomagnetic Storm Detected by GEONET

Medium-scale traveling ionospheric disturbances (MSTIDs) to the June 22 and 23, 2015 geomagnetic storm over Japan are investigated by using the Global Navigation Satellite System Earth Observation Network of Japan (GEONET) in this letter. We have detected two groups of MSTIDs on June 22 and 23 and a negative ionospheric disturbance response on June 22 observed by the high-resolution detrended total electron content (TEC) maps from GPS observations. The negative ionospheric response (IR) performs a large-scale ionosphere depletion covering a wide range of areas (~125°E to ~140°E and ~25°N to ~40°N) simultaneously, which starts at 1900 universal time (UT) [04:00 local time (LT)] with a period of ~30 min on June 22. Compared with typical MSTIDs, the nighttime and the daytime MSTIDs in this letter both have a southwestward propagated direction and a propagated velocity of ~87.8 and ~157.8 m/s, respectively. The physical sources of the IRs over Japan during this geomagnetic storm are possibly attributed to the prompt penetration electric field (PPEF) and lower atmospheric gravity waves (AGW) referring to the previous studies. We believe that these findings can contribute to the complement of ionospheric research on MSTIDs over Japan.

Westward Drift of Ionospheric Plasma Irregularities over a Low to Mid-latitude Transition Region in Indian Sector

We report the observation of plasma depletions/plumes in the F region ionosphere over a low to middle latitude transition region in the Indian sector. The observation of these plasma depletions is based on the data obtained in May 2019 through the all-sky airglow CCD imager installed in the campus of University of Kashmir, Srinagar (34.12 °N, 74.83 °E, magnetic latitude 25.91 °N). The depletions on the two consecutive nights of 05 and 06 May 2019 are aligned along the North-South (N-S) direction and drift westward. Several depletion bands along with some enhancement bands are seen in the 630-nm airglow images throughout the two nights. The observed structures show certain characteristics similar to Medium Scale Traveling Ionospheric Disturbances (MSTIDs) but these airglow features are not completely periodic. Further, in the observed depletion bands some East-West asymmetries are observed along with the structured tree-like branches of the airglow depletions. Some depletion bands even bifurcate leading to the inference that the structures are signatures of plasma irregularities rather than the usual MSTIDs observed in low-mid latitude transition region. The westward drift of the depletions especially during geomagnetic quiet times over this region makes this study significant since it offers a possible evidence that shows extension of spread F irregularities from the mid latitude region to the low-mid latitude transition region. In this paper, we point out some possible mechanisms related to the occurrence of plasma depletions at this region and their westward movement during geomagnetic quiet times.

Northward propagation of medium scale traveling ionospheric disturbances over Srinagar, J and K India

We report some interesting medium scale travelling ionospheric disturbances (MSTIDs) observed during the night of September 20, 2020 through an all sky CCD imager installed in Kashmir University (KU), Srinagar, India (34.08° N geographic latitude, 74.79° E and 25.91° N geomagnetic latitude). Three different MSTID events including a post-sunset MSTID, a mid-night MSTID and a pre-dawn MSTID are recorded in the airglow images during this night. The post-sunset and pre-dawn MSTIDs are seen to propagate northward while the MSTID observed near midnight time propagates towards west. As the MSTIDs during the night times over the northern hemisphere generally propagate southwestward, the northward propagation of nighttime MSTIDs over Srinagar makes this result an interesting one. More importantly, three different MSTIDs are observed at different times during the single night whose directions of propagation are not same either. We provide the detailed characteristics and evolution processes of these MSTIDs in this paper. The unusual northward motion of the MSTIDs over the Srinagar region is also addressed. The upward propagation of atmospheric gravity waves during this night is believed to be the responsible candidates for such unusual propagation direction of nighttime MSTIDs over the northern hemisphere.

Long-Term Study on Medium-Scale Traveling Ionospheric Disturbances Observed over the South American Equatorial Region

Using data collected by the GNSS dual-frequency receivers network, de-trended TEC maps were generated to identify and characterize the medium-scale traveling ionospheric disturbances (MSTIDs) over the South American equatorial region (latitude: 0∘ to 15∘ S and longitude: 30∘ to 55∘ W) during solar cycle 24 (from January 2014 to December 2019). A total of 712 MSTIDs were observed during quiet geomagnetic conditions. The Frequency of occurrence of MSTID is high during the solar maximum and low in the minimum phase. This might be due to the solar cycle dependence of gravity wave activity in the lower atmosphere and gravity wave propagation conditions in the thermosphere. The predominant daytime MSTIDs, representing 80% of the total observations, occurred in winter (June-August season in the southern hemisphere) with the secondary peak in the equinox; while the evening time MSTIDs, representing 18% of the entire events, occurred in summer (December to February season) and equinox (March to May and September to November), and the remaining 2% of the MSTIDs were observed during nighttime. The seasonal variation of the MSTID events was attributed to the source mechanisms generating them, the wind filtering and dissipation effects, and the local time dependency. The horizontal wavelengths of the MSTIDs were mostly concentrated between 500 and 800 km, with the mean value of 667 ± 131 km. The observed periods ranged from 30 to 45 min with the mean value of 36 ± 7 min. The observed horizontal phase speeds were distributed around 200 to 400 m/s, with the corresponding mean of 301 ± 75 m/s. The MSTIDs in the winter solstice and equinoctial months preferentially propagated northeastward and northwestward. Meanwhile, during the summer solstice, they propagated in all directions. The anisotropy of the propagation direction might be due to several reasons: the wind and dissipative filtering effects, ion drag effects, the primary source region, and the presence of the secondary or tertiary gravity waves in the thermosphere. Atmospheric gravity waves from strong convective sources might be the primary precursor for the observed equatorial MSTIDs. In all seasons, we noted that the MSTIDs propagating southeastward were probably excited by the likely gravity waves generated by the intertropical convergence zone (ITCZ).

Rotation, bifurcation and merging of medium-scale traveling ionospheric disturbances over a low-mid latitude transition region

This paper reports the observation of propagating wave structures in OI 630.0 nm airglow images captured by a CCD imager at Kashmir University (KU), Srinagar, India (34.1 °N, 74.8 °E, and 25.9 °N magnetic latitude). These structures were detected on the night of August 17th, 2017, with an average horizontal wavelength around 330 km and average phase velocities around 92 m/s traveling in southwest (SW) direction. The SW moving wave patterns had north-west (NW) to south-east (SE) aligned phase fronts that lasted for ∼150 min and had a time period of about 1 h. Based on observed parameters such as wavelength, time span, and propagating speed, these structures are categorized as nighttime medium-scale traveling ionospheric disturbances (MSTIDs). The observed southwestward movement, phase speed, horizontal wavelength and period of these nighttime MSTIDs are consistent with the existing literature. The appearance of gravity waves in the lower atmosphere during this night means that gravity waves from below, in combination with the Perkins' instability, might be responsible candidates for generation of these MSTIDs. Near midnight the MSTID bands are seen to rotate, bifurcate and some even merging with other bands giving rise to some unique Y-shaped MSTID structures-the first such results observed from this location. These structures align along North-South direction and move westward unlike the MSTID bands before midnight which move towards southwest. In addition, we also report the shrinking phenomena of the post midnight MSTID dark bands.

The Influence of Ionospheric Neutral Wind Variations on the Morphology and Propagation of Medium Scale Traveling Ionospheric Disturbances on 8th August 2016

AbstractThis study reports a special case of the propagation and morphology of medium scale traveling ionospheric disturbances (MSTIDs) over middle‐latitude China on the night of August 8, 2016. The MSTIDs were simultaneously observed by multi‐instruments, including the all‐sky imager, Swarm satellite, and global positioning system (GPS). The MSTIDs lasted for about 6 h in the field view of airglow imager, showing typical wavelength, phase velocity of 272–296 km and 67–250 m/s, respectively. In addition, the imagers show that the inclination angles of phase fronts for some MSTIDs were decreasing during their propagation, resulting in the propagation direction changed from southwestward to nearly westward. More interestingly, the MSTIDs began to dissipate in the airglow observation when they propagated to lower latitudes (below ∼40°N) whereas the MSTIDs at higher latitudes were still visible in the later local times. Simulation results from the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model are fairly consistent with the Fabry‐Perot Interferometer (FPI) wind observations, which provide convincing explanation to show that the variations of ionospheric neutral winds might play important roles in the changes of propagation direction and the dissipation of MSTIDs.

Statistical study of medium-scale traveling ionospheric disturbances in low-latitude ionosphere using an automatic algorithm

This study investigates the medium-scale traveling ionospheric disturbances (MSTIDs) statistically at the low-latitude equatorial ionization anomaly (EIA) region in the northern hemisphere. We apply the automatic detection algorithm including the three-dimensional fast Fourier transform (3-D FFT) and support vector machine (SVM) on total electron content (TEC) observations, derived from a network of ground-based global navigation satellite system (GNSS) receivers in Taiwan (14.5° N geomagnetic latitude; 32.5° inclination), to identify MSTID from other waves or irregularity features. The obtained results are analyzed statistically to examine the behavior of low-latitude MSTIDs. Statistical results indicate the following characteristics. First, the southward (equatorward) MSTIDs are observed almost every day during 0800–2100 LT in Spring and Winter. At midnight, southward MSTIDs are more discernible in Summer and majority of them are propagating from Japan to Taiwan. Second, northward (poleward) MSTIDs are more frequently detected during 1200–2100 LT in Spring and Summer with the secondary peak of occurrence between day of year (DOY) 100–140 during 0000–0300 LT. The characteristics of the MSTIDs are interpreted with additional observations from radio occultation (RO) soundings of FORMOSAT-3/COSMIC as well as modeled atmospheric waves from the high-resolution Whole Atmosphere Community Climate Model (WACCM) suggesting that the nighttime MSTIDs in Summer is likely connected to the atmospheric gravity waves (AGWs).

Global Distribution of Nighttime MSTIDs and Its Association With E Region Irregularities Seen by CHAMP Satellite

AbstractWe investigate the correlation of sporadic E (Es) with the occurrence of medium‐scale traveling ionospheric disturbances (MSTIDs) at night in middle latitudes (25°–40°N and 25°–40°S magnetic latitudes) by examining their occurrence climatology. The occurrence climatology of Es and MSTIDs is derived using the Challenging Minisatellite Payload satellite data acquired in 2001–2008 and 2001–2009, respectively. Electron density irregularities and radio scintillations are used as the detection proxies of MSTIDs and Es, respectively. The occurrence rate of MSTIDs shows a semi‐annual variation with the primary peak during June solstices and the secondary peak during December solstices in both hemispheres. However, the occurrence rate of Es shows a seasonal variation with a pronounced peak in summer in both hemispheres. The occurrence of MSTIDs during local summer and equinoxes is correlated with the occurrence of local Es, but the high occurrence rate of MSTIDs in local winter is not correlated with local winter hemisphere Es. MSTIDs in the winter hemisphere are correlated with magnetically conjugate MSTIDs in the summer hemisphere; these summer hemisphere MSTIDs are correlated with the occurrence of Es in the summer hemisphere. The occurrence rate of MSTIDs clearly shows an increase with decreasing solar activity, but the solar cycle dependence of Es is not obvious from the data. This observation suggests that the generation of MSTIDs is significantly affected by factors other than Es such as the growth rate of the Perkins instability, atmospheric gravity waves, and the F region conductance.

Interaction between nighttime MSTID and mid-latitude field-aligned plasma depletion structure over the transition region of geomagnetic low-mid latitude: First results from Hanle, India

In this paper, we study the interaction between a nighttime Medium Scale Traveling Ionospheric Disturbance (MSTID) and geomagnetic north-south oriented field-aligned plasma depletion structure over Hanle, Leh Ladakh (32.7°N, 78.9°E; Mlat. ~24.1°N), India on a geomagnetically quiet night (06 May 2019, Ap = 4). The results are based on the observations carried out using a newly established airglow imager operating at 630.0 nm at Hanle which is a transition region of geomagnetic low-mid latitudes. A sequence of processes constituted this unique interaction that includes developing phase of the MSTID within the imager's field of view, gradual bending of the field-aligned plasma depletion structure and eventual merging with the MSTID, and propagation of the merged structure in the form of a single MSTID structure post-interaction. The mechanism appears to be controlled by unequal but oppositely directed polarization electric fields within the dark and bright bands of the developing MSTID, whose projection on the horizontal plane is observable in the airglow images. The present work is the first observation illustrating the interaction between a developing phase of MSTID and plasma depletion structure over a geomagnetic low-mid latitude transition region, which assumes importance for understanding the underlying plasma irregularity processes.

Investigation of Low Latitude Spread-F Triggered by Nighttime Medium-Scale Traveling Ionospheric Disturbance

In the current study, we investigated the mechanism of medium-scale traveling ionospheric disturbance (MSTID) triggering spread-F in the low latitude ionosphere using ionosonde observation and Global Navigation Satellite System-Total Electron Content (GNSS-TEC) measurement. We use a series of morphological processing techniques applied to ionograms to retrieve the O-wave traces automatically. The maximum entropy method (MEM) was also utilized to obtain the propagation parameters of MSTID. Although it is widely acknowledged that MSTID is normally accompanied by polarization electric fields which can trigger Rayleigh–Taylor (RT) instability and consequently excite spread-F, our statistical analysis of 13 months of MSTID and spread-F occurrence showed that there is an inverse seasonal occurrence rate between MSTID and spread-F. Thus, we assert that only MSTID with certain properties can trigger spread-F occurrence. We also note that the MSTID at night has a high possibility to trigger spread-F. We assume that this tendency is consistent with the fact that the polarization electric field caused by MSTID is generally the main source of post-midnight F-layer instability. Moreover, after thorough investigation over the azimuth, phase speed, main frequency, and wave number over the South America region, we found that the spread-F has a tendency to be triggered by nighttime MSTID, which is generally characterized by larger ΔTEC amplitudes.

Latitudinal Variations of Daytime Periodic Ionospheric Disturbances From Beidou GEO TEC Observations Over China

AbstractUsing total electron content (TEC) data from a chain of Beidou geostationary satellite TEC receivers along ∼110°E during 2016–2019, the local time, seasonal, and latitudinal occurrence characteristics of periodic TEC perturbation associated with medium scale traveling ionospheric disturbances (MSTIDs) are investigated. The results show that the daytime periodic ionospheric disturbances appear prominently in winter with dominant periods ∼30–40 min. An interesting feature is that the latitudinal variations of periodic ionospheric disturbances show two occurrence peaks at higher and lower latitudes, with a minimum centering around 30°N‐33°N, where almost no periodic ionospheric disturbance was detected under the threshold of absolute TEC perturbation greater than 0.3 TECU. Further, using two short‐baseline GNSS receiver arrays at middle and low latitudes, the propagation characteristics of the periodic ionospheric disturbances were derived. The median values of propagation azimuth, phase velocity, and period of the ionospheric disturbances at middle (low) latitude are 153.1° (157.4°), 36.7 min (33 min), and 97.2 m/s (62.5 m/s), respectively, showing typical characteristics of MSTIDs. We surmise that the perturbation source and background electron density could play important roles in causing the latitudinal variation of daytime periodic ionospheric disturbances along 110°E.

Evidence for simultaneous occurrence of periodic and single dark band MSTIDs over geomagnetic low-mid latitude transition region

We report a unique simultaneous occurrence of two plasma structures using a 630.0 nm all-sky airglow imager in Hanle, India. The characteristics indicate that the observed structures are Medium Scale Travelling Ionospheric Disturbances (MSTIDs) of different types, namely periodic and single dark band. Based on previous understandings of Perkins instability, we explore the role of polarization electric field within MSTIDs using complementary observations from a nearby digisonde. We propose that the MSTIDs involve different seeding processes. This study provides crucial information in elucidating the generation of MSTIDs and helps in understanding the F-region electrodynamics over geomagnetic low-mid latitude transition region.

Comparison of seasonal and longitudinal variation of daytime MSTID activity using GPS observation and GAIA simulations

The total electron content (TEC) data derived from the GAIA (Ground-to-topside model of Atmosphere Ionosphere for Aeronomy) is used to study the seasonal and longitudinal variation of occurrence of medium-scale traveling ionospheric disturbances (MSTIDs) during daytime (09:00–15:00 LT) for the year 2011 at eight locations in northern and southern hemispheres, and the results are compared with ground-based Global Positioning System (GPS)-TEC. To derive TEC variations caused by MSTIDs from the GAIA (GPS) data, we obtained detrended TEC by subtracting 2-h (1-h) running average from the TEC, and calculated standard deviation of the detrended TEC in 2 h (1 h). MSTID activity was defined as a ratio of the standard deviation to the averaged TEC. Both GAIA simulation and GPS observations data show that daytime MSTID activities in the northern and southern hemisphere (NH and SH) are higher in winter than in other seasons. From the GAIA simulation, the amplitude of the meridional wind variations, which could be representative of gravity waves (GWs), shows two peaks in winter and summer. The winter peak in the amplitude of the meridional wind variations coincides with the winter peak of the daytime MSTIDs, indicating that the high GW activity is responsible for the high MSTID activity. On the other hand, the MSTID activity does not increase in summer. This is because the GWs in the thermosphere propagate poleward in summer, and equatorward in winter, and the equatorward-propagating GWs cause large plasma density perturbations compared to the poleward-propagating GWs. Longitudinal variation of daytime MSTID activity in winter is seen in both hemispheres. The MSTID activity during winter in the NH is higher over Japan than USA, and the MSTID activity during winter in the SH is the highest in South America. In a nutshell, GAIA can successfully reproduce the seasonal and longitudinal variation of the daytime MSTIDs. This study confirms that GWs cause the daytime MSTIDs in GAIA and amplitude and propagation direction of the GWs control the noted seasonal variation. GW activities in the middle and lower atmosphere cause the longitudinal variation.

Ion Velocity and Temperature Variation Around Topside Nighttime Irregularities: Contrast Between Low- and Mid-Latitude Regions.

Studies on low-/mid-latitude ionospheric irregularities have a long history, but ion temperature and drift velocity variations within the irregularities have been rarely investigated, especially at mid-latitudes. To fill this knowledge gap, we statistically investigate ion temperature and velocity of mid-latitude ionospheric irregularities and compare them to those of low-latitude ones. This study mainly relies on in situ plasma moment data from the newly launched Ionospheric Connection Explorer (ICON) satellite and is supplemented by Republic of China Satellite 1 (ROCSAT-1) data in 2000-2004. At low latitudes (|MLAT|<10°) plasma density irregularities encountered by ICON have the following properties: lower-density regions have cooler ions and more poleward/westward/outward velocity than (|MLAT| = 10°∼30°), the opposite trends are observed: lower-density regions have hotter ions and more the ambient plasma, which agrees with previous reports on equatorial plasma bubbles. At mid-latitudes equatorward/eastward/inward drift than the ambient, which is a new finding in this study. The mid-latitude observations can be put into the context of Medium-Scale Traveling Ionospheric Disturbance (MSTID) properties reported previously, which evidences that topside irregularities at nighttime mid-latitudes generally represent MSTIDs. ROCSAT-1 data support the contrast between low- and mid-latitude ionospheric irregularities.

Interaction of Oppositely Traveling Medium‐Scale Traveling Ionospheric Disturbances Observed in Low Latitudes During Geomagnetically Quiet Nighttime

AbstractIn this study, an unusual event of medium‐scale traveling ionospheric disturbances (MSTIDs) that propagated in opposite directions was observed by multi‐instruments (all‐sky imager, global positioning system (GPS), and digisonde) in low latitudes during geomagnetically quiet nighttime. These MSTIDs had wavefronts aligned from northwest to southeast. One MSTID propagated southwestward and another propagated northeastward. They encountered and interacted with each other. Some MSTIDs structures merged into one and disappeared after the interaction. Furthermore, an eastward moving plasma bubble also encountered the MSTIDs and then gradually dissipated during the propagation process. The MSTIDs traveling in opposite directions should be related to ionospheric plasma motion and their trigger mechanisms. The interactive process of MSTIDS involved in plasma drifts and polarization electric fields. Meanwhile, the interaction with an ionospheric irregularity structure can change some of the MSTID characteristics.