Low-latitude scintillation occurrences around the equatorial anomaly crest over Indonesia

We investigated low-latitude ionospheric scintillation in Indonesia using two GPS receivers installed at Bandung (107.6 degrees E, 6.9 degrees S; magnetic latitude 17.5 degrees S) and Pontianak (109.3 degrees E, 0.02 degrees S; magnetic latitude 8.9 degrees S). This study aimed to characterise climatological and directional ionospheric scintillation occurrences, which are useful not only for the physics of ionospheric irregularities but also for practical use in GNSS (global navigation satellite system)based navigation. We used the deployed instrument's amplitude scintillation (S4 index) data from 2009, 2010, and 2011; the yearly SSN (sunspot-smoothed numbers) were 3.1, 16.5, and 55.9, respectively. In summary, (1) scintillation occurrences in the post-sunset period (18:00-01:00 LT) during equinox months (plasma bubble season) at the two sites can be ascribed to the plasma bubble; (2) using directional analyses of the two sites, we found that the distribution of scintillation occurrences is generally concentrated between the two sites, indicating the average location of the EIA (equatorial ionisation anomaly) crest; (3) scintillation occurrence enhancements for the two sites in field-aligned directions are herein reported for the first time by ground-based observation in a low-latitude region; (4) distribution of scintillation occurrences at Pontianak are concentrated in the southern sky, especially in the southwest direction, which is very likely associated with the plasma bubble tilted westward with increasing latitude; and (5) scintillation occurrence in the post-midnight period in the non-plasma-bubble season is the most intriguing variable occurring between the two sites (i.e. post-midnight scintillations are observed more at Bandung than Pontianak). Most of the post-midnight scintillations observed at Bandung are concentrated in the northern sky, with low elevation angles. This might be due to the amplitude of irregularities in certain directions, which may be effectively enhanced by background density enhancement by the EIA and because satellite-receiver paths are longer in the EIA crest region and in a field-aligned direction.