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The development of a digital high‐frequency (HF) radio communication system requires an ionospheric channel model from the channel impulse response (CIR) measurement. Although the Watterson ionosphere channel model has been available and used for a long time, several CIR measurements have been conducted in all regions of the Earth in an attempt to validate or replace the Watterson channel model with a suitable model for their region. However, only a few CIR measurements were conducted in low‐latitude regions, especially over Indonesia. In this study, we develop the CIR measurement system for the near vertical incidence skywave (NVIS) propagation mode over Java Island based on the software defined radio platform to meet low‐cost and simple operation requirements. The specification of the system is based on the International Telecommunication Union ionospheric channel characteristic document and increased in order to be able to capture higher values. Results from a 1‐week campaign measurement period show the ability of the system to measure the root mean square of time delay within the range of 0.2–1.3 ms and the Doppler shift within the range of 0.7–1.1 Hz in the quiet conditions of the ionosphere. Further measurements will be conducted to obtain a comprehensive ionosphere CIR that is useful for designing the NVIS‐HF digital communication in Indonesia, which is located beneath the crest region of an equatorial ionospheric anomaly. Plain Language Summary: An ionosphere channel model based on channel impulse response (CIR) measurement is required for the development of high‐frequency (HF) digital communication systems. There are a number of in situ CIR measurements around the world with the purpose of validating or replacing the Watterson ionosphere channel model with one that is more suitable for their perspective region. However, only a few ionosphere CIR measurements were conducted in the low‐latitude region, especially over Indonesia. In this study, we develop a low‐cost and simple CIR measurement system for the near vertical incidence skywave (NVIS) propagation mode over Java Island, Indonesia, using a software‐defined radio platform. The specification of the system was based on the International Telecommunication Union ionosphere channel characteristic recommendation documents and increased in order to be able to observe higher values. Results from the 1‐week campaign measurement period show the capability of the system to measure the root mean square of time delay and Doppler frequency values. Further measurements will be taken to acquire complete CIR data for designing the NVIS‐HF communication system in Indonesia, which is under the crest area of an equatorial ionospheric anomaly. Key Points: Development of an ionosphere channel impulse measurement system based on low‐cost software defined radio implemented on Java IslandUtilizing a correlator channel sounder technique to meet a simple system and operationCampaign results show the ability of the system to observe a 0.2 ms time delay and a Doppler shift in the range of 0.7–1.1 Hz [ABSTRACT FROM AUTHOR]

We report our investigation of ionospheric effects that occurred during a total solar eclipse over the Southeast Asia‐Pacific region on 9 March 2016. In particular, here we examine rapid uplift of the ionospheric F‐layer during the eclipse, and reductions of ionospheric plasma density in areas around the eclipse totality. This study used data from ionosondes at Biak and Guam, as well as data from a bistatic HF radio link (∼1300 km apart, cutting across the eclipse totality trajectory) between Biak and Manado. Gridded total electron content (TEC) data from the Madrigal Database were also used for a cross‐comparison. The ionosonde measurements indicate an upward vertical drift velocity in the range of 21−40 m/s in the ionospheric F‐region during the eclipse. Over Biak, foF2 decreased from 10 MHz to 6 MHz (a 40% reduction) during the eclipse. Meanwhile, foF2 over Guam during the eclipse was suppressed for a few hours; lower than the 7‐day average normal level by 3 MHz. The Madrigal GPS TEC data corroborate these ionosonde measurements. Finally, data from the Biak‐Manado HF radio link indicate that the D‐region ionosphere had diminished substantially during the eclipse. Key Points: An uplift of ionospheric F‐layer with a rate of 21‐40 m/s was observed by Biak and Guam ionosondes during the 9 March 2016 solar eclipseDuring the solar eclipse, foF2 over Biak was reduced by 40%; and foF2 over Guam was suppressed by 30% below normal level for a few hoursAround the eclipse totality, ionospheric D‐region diminished ‐ as indicated by low absorption level along the Biak‐Manado HF radio link [ABSTRACT FROM AUTHOR]

We investigated a postmidnight field-aligned irregularity (FAI) event observed with the Equatorial Atmosphere Radar at Kototabang (0.2°S, 100.3°E, dip latitude 10.4°S) in Indonesia on the night of 9 July 2010 using a comprehensive data set of both neutral and plasma parameters. We examined the rate of total electron content change index (ROTI) obtained from GPS receivers in Southeast Asia, airglow images detected by an all-sky imager, and thermospheric neutral winds and temperatures obtained by a Fabry-Perot interferometer at Kototabang. Altitudes of the F layer ( h′ F) observed by ionosondes at Kototabang, Chiang Mai, and Chumphon were also surveyed. We found that the postmidnight FAIs occurred within plasma bubbles and coincided with kilometer-scale plasma density irregularities. We also observed an enhancement of the magnetically equatorward thermospheric neutral wind at the same time as the increase of h′ F at low-latitude stations, but h′ F at a station near the magnetic equator remained invariant. Simultaneously, a magnetically equatorward gradient of thermospheric temperature was identified at Kototabang. The convergence of equatorward neutral winds from the Northern and Southern Hemispheres could be associated with a midnight temperature maximum occurring around the magnetic equator. Equatorward neutral winds can uplift the F layer at low latitudes and increase the growth rate of Rayleigh-Taylor instabilities, causing more rapid extension of plasma bubbles. The equatorward winds in both hemispheres also intensify the eastward Pedersen current, so a large polarization electric field generated in the plasma bubble might play an important role in the generation of postmidnight FAIs. [ABSTRACT FROM AUTHOR]