Abstract
Abstract. We present a case study of unusual spread-F structures observed by ionosondes at two equatorial and low-latitude Brazilian stations – São Luís (SL: 44.2∘ W, 2.33∘ S; dip angle: −6.9∘) and Fortaleza (FZ: 38.45∘ W, 3.9∘ S; dip angle: −16∘). The irregularity structures observed from midnight to postmidnight hours of moderate solar activity (F10.7 < 97 sfu, where 1 sfu = 10−22 W m−2 s−1) have characteristics different from typical post-sunset equatorial spread F. The spread-F traces first appeared at or above the F-layer peak and gradually became well-formed mixed spread F. They also appeared as plasma depletions in the 630.0 nm airglow emissions made by a wide-angle imager located at the nearby low-latitude station Cajazeiras (CZ: 38.56∘ W, 6.87∘ S; dip angle: −21.4∘). The irregularities appeared first over FZ and later over SL, giving evidence of an unusual westward propagation or a horizontal plasma advection. The drift-mode operation available in one of the ionosondes (a digital portable sounder, DPS-4) has enabled us to analyze the horizontal drift velocities and directions of the irregularity movement. We also analyzed the neutral wind velocity measured by a Fabry–Pérot interferometer (FPI) installed at CZ and discuss its possible role in the development of the irregularities.
Highlights
Equatorial spread F, representing small-scale to large-scale plasma irregularities, has been extensively studied for several decades
The airglow images of the OI 630 nm emission used in this study were measured by a Portable Ionospheric Camera and Small-Scale Observatory (PICASSO) wide-angle imaging system deployed at Cajazeiras (CZ: 6.87◦ S, 38.56◦ W; dip angle: −21.4◦), located about ∼ 352 km to the south of FZ
We have presented and discussed an unusual spread-F pattern associated with unusual depletions on the OI 630 nm airglow emission observed during geomagnetically quiet conditions during the June solstice of 2011 over the equatorial region in Brazil
Summary
Equatorial spread F, representing small-scale to large-scale plasma irregularities, has been extensively studied for several decades. During low solar activity conditions, there is a class of spread-F and plasma irregularities regularly observed in distinct longitudinal sectors, such as Brazil (Candido et al, 2011), Africa (Yizengaw et al, 2013), and Asia (Nishioka et al, 2012). They are known as postmidnight plasma irregularities (PMIs), which occur mostly in the June solstice. Yokoyama et al (2011) studied unusual patterns of echoes from coherent scatter radar data occurring around midnight during the solar minimum period They observed two principal types of irregularities: upwelling plumes and striations similar to mesoscale traveling ionospheric disturbances (MSTIDs). Plane from field-aligned irregularities and that the spread-F pattern depends on the position relative to the ionosonde and the scale sizes of the irregularities
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