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Post-Sunrise Ionospheric Irregularities in Southeast Asia During the Geomagnetic Storm on 19–20 April 2024
(2025-08-01)
Abadi, Prayitno
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Muafiry, Ihsan Naufal
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Pratama, Teguh Nugraha
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Putra, Angga Yolanda
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Faturahman, Agri
We present new insights into post-sunrise ionospheric irregularities in Southeast Asia during the intense geomagnetic storm of 19–20 April 2024. By utilizing Total Electron Content (TEC) and Rate of TEC Change Index (ROTI) maps, along with ionosondes, we identified the emergence of post-sunset Equatorial Plasma Bubbles (EPBs)—plasma depletion structures and irregularities—in western Southeast Asia on 19 April. These EPBs moved eastward, and the irregularities dissipated before midnight after the EPBs covered approximately 10° of longitude. Interestingly, plasma density depletion structures persisted and turned westward after midnight until post-sunrise the following day. Concurrently, an increase in F-region height from midnight to sunrise, possibly induced by the storm’s electric field, facilitated the regeneration of irregularities in the residual plasma depletions during the post-sunrise period. The significant increase in F-region height was particularly pronounced in western Southeast Asia. As a result, post-sunrise irregularities expanded their latitudinal structure while propagating westward. These findings suggest that areas with decayed plasma depletion structures from post-sunset EPBs that last past midnight could be sites for creating post-sunrise irregularities during geomagnetic storms. The storm-induced electric fields produce EPBs and ionospheric irregularities at longitudes where the surviving plasma depletion structures of post-sunset EPBs are present.
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Modeling Post-Sunset Equatorial Spread-F Occurrence as a Function of Evening Upward Plasma Drift Using Logistic Regression, Deduced from Ionosondes in Southeast Asia
(2022-04-01)
Abadi, Prayitno
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Ahmad, Umar Ali
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Otsuka, Yuichi
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Jamjareegulgarn, Punyawi
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Martiningrum, Dyah Rahayu
The occurrence of post-sunset equatorial spread-F (ESF) could have detrimental effects on trans-ionospheric radio wave propagation used in modern communications systems. This problem calls for a simple but robust model that accurately predicts the occurrence of post-sunset ESF. Logistic regression was implemented to model the daily occurrence of post-sunset ESF as a function of the evening upward plasma drift (v). The use of logistic regression is formalized by ŷ = 1/[1 + exp(−z)], where ŷ represents the probability of post-sunset ESF occurrence, and z is a linear function containing v. The value of v is derived from the vertical motion of the bottom side of the F-region in the evening equatorial ionosphere, which is observed by the ionosondes in Southeast Asia. Data points (938) of v and post-sunset ESF occurrence were collected in the equinox seasons from 2003 to 2016. The training set used 70% of the dataset to derive z and ŷ and the remaining 30% was used to test the performance of ŷ. The expression z = −2.25 + 0.14v was obtained from the training set, and ŷ ≥ 0.5 (v ≥ ~16.1 m/s) and ŷ < 0.5 (v < ~16.1 m/s) represented the occurrence and non-occurrence of ESF, respectively, with an accuracy of ~0.8 and a true skill score (TSS) of ~0.6. Similarly, in the testing set, ŷ shows an accuracy of ~0.8 and a TSS of ~0.6. Further analysis suggested that the performance of the z-function can be reliable in the daily F<inf>10.7</inf> levels ranging from 60 to 140 solar flux units. The z-function implemented in the logistic regression (ŷ) found in this study is a novel technique to predict the post-sunset ESF occurrence. The performance consistency between the training set and the testing set concludes that the z-function and the ŷ values of the proposed model could be a simple and robust mathematical model for daily nowcasting the occurrence or non-occurrence of post-sunset ESFs.
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Roles of thermospheric neutral wind and equatorial electrojet in pre-reversal enhancement, deduced from observations in Southeast Asia
(2021-09-01)
Abadi, P.
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Otsuka, Y.
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Liu, Hui Xin
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Hozumi, K.
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Martinigrum, D. R.
Previous studies have proposed that both the thermospheric neutral wind and the equatorial electrojet (EEJ) near sunset play important roles in the pre-reversal enhancement (PRE) mechanism. In this study, we have used observations made in the equatorial region of Southeast Asia during March–April and September–October in 2010–2013 to investigate influences of the eastward neutral wind and the EEJ on the PRE’s strength. Our analysis employs data collected by the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite to determine the zonal (east-west direction) neutral wind at an altitude of ~250 km (bottomside F region) at longitudes of 90°–130°E in the dusk sector. Three ionosondes, at Chumphon (dip lat.: 3.0°N) in Thailand, at Bac Lieu (dip lat.: 1.7°N) in Vietnam, and at Cebu (dip lat.: 3.0°N) in Philippines, provided the data we have used to derive the PRE strength. Data from two magnetometers — at Phuket (dip lat.: 0.1°S) in Thailand and at Kototabang (dip lat.: 10.3°S) in Indonesia — were used to estimate the EEJ strength. Our study is focused particularly on days with magnetically quiet conditions. We have found that the eastward neutral wind and the EEJ are both closely correlated with the PRE; their cross-correlation coefficients with it are, respectively, 0.42 and 0.47. Their relationship with each other is weaker: the cross-correlation coefficient between the eastward neutral wind and the EEJ is just 0.26. Our findings suggest that both the eastward neutral wind and the EEJ near sunset are involved in the PRE mechanism. Based on the weak relationship between these two parameters, however, they appear to be significantly independent of each other. Thus, the wind and the EEJ are likely to be influencing the PRE magnitude independently, their effects balancing each other.
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On the solar activity dependence of midnight equatorial plasma bubbles during June solstice periods
(2021-09-01)
Ajith, K. K.
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Ram, S. Tulasi
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Li, Guo Zhu
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Yamamoto, M.
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Hozumi, K.
The occurrence of midnight Equatorial Plasma Bubbles (EPBs) during the June solstice period of the ascending phase of solar cycle 24, from 2010 to 2014, was studied using data from the 47 MHz Equatorial Atmosphere Radar (EAR) at Kototabang, Indonesia. The analysis shows that the occurrence of midnight hour EPBs was at its maximum during the low solar activity year 2010 and monotonically decreased thereafter with increasing solar activity. Details of the dependence of midnight hour EPB occurrence on solar activity were investigated using SAMI2 model simulation with a realistic input of E × B drift velocity data obtained from the CINDI-IVM onboard the C/NOFS satellite. Results obtained from term-by-term analysis of the flux tube integrated linear growth rate of RT instability indicate that the formation of a high flux tube electron content height gradient (steep vertical gradient) region at higher altitudes, due to the elevated F layer, is the key factor enhancing the growth rate of RT instability during low solar activity June solstices. Other factors are discussed in light of the relatively weak westward zonal electric field in the presence of the equatorward neutral wind and north-to-south transequatorial wind around the midnight hours of low solar activity June solstices. Also discussed are the initial seeding of RT instability by MSTIDs and how the threshold height required for EPB development varies with solar activity.
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Peculiar features of the low-latitude and midlatitude ionospheric response to the St. Patrick's Day geomagnetic storm of 17 March 2015
(2016-08-01)
Nayak, Chinmaya
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Tsai, L. C.
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Su, S. Y.
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Galkin, I. A.
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Tan, Adrian Teck Keng
The current study aims at investigating and identifying the ionospheric effects of the geomagnetic storm that occurred during 17–19 March 2015. Incidentally, with SYM-H hitting a minimum of −232 nT, this was the strongest storm of the current solar cycle 24. The study investigates how the storm has affected the equatorial, low-latitude, and midlatitude ionosphere in the American and the European sectors using available ground-based ionosonde and GPS TEC (total electron content) data. The possible effects of prompt electric field penetration is observed in both sectors during the main phase of the storm. In the American sector, the coexistence of both positive and negative ionospheric storm phases are observed at low latitudes and midlatitudes to high latitudes, respectively. The positive storm phase is mainly due to the prompt penetration electric fields. The negative storm phase in the midlatitude region is a combined effect of disturbance dynamo electric fields, the equatorward shift of the midlatitude density trough, and the equatorward compression of the plasmapause in combination with chemical compositional changes. Strong negative ionospheric storm phase is observed in both ionosonde and TEC observations during the recovery phase which also shows a strong hemispherical asymmetry. Additionally, the variation of equatorial ionization anomaly as seen through the SWARM constellation plasma measurements across different longitudes has been discussed. We, also, take a look at the performance of the IRI Real-Time Assimilative Mapping during this storm as an ionospheric space weather tool.

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