Abstract
Abstract. We present the results of a comparative study of spread-F signatures over five low-latitude sites: Chiangmai (CGM; 18.8° N, 98.9° E, mag. Lat. 8.8° N), Thailand; Tanjungsari (TNJ; 6.9° S, 107.6° E, mag. Lat. 16.9° S), Indonesia; Palmas (PAL; 10.2° S, 311.8° E, mag. Lat. 0.9° S) and São José Dos Campos (SJC; 23.2° S, 314.1° E, mag. Lat. 14.0° S), Brazil; and Tucumán (TUC; 26.9° S, 294.6° E, mag. Lat. 16.8° S), Argentina. The investigation was based on simultaneous ionograms recorded by an FMCW (frequency-modulated continuous-wave) at CGM, an IPS-71 (digital ionosonde from KEL aerospace) at TNJ, a CADI (Canadian Advanced Digital Ionosonde) at PAL and SJC, and an AIS-INGV (Advanced Ionospheric Sounder – Istituto Nazionale di Geofisica e Vulcanologia) at TUC, during the equinoctial periods March–April (R12 = 2.0 and R12 = 2.2) and September–October (R12 = 6.1 and R12 = 7.0) 2009, for very low solar activity. Spread-F signatures were categorized into two types: the range spread-F (RSF) and the frequency spread-F (FSF). The study confirms that the dynamics and the physical processes responsible for these phenomena are actually complicated. In fact, the features that arise from the investigation are different, depending on both the longitude sector and on the hemisphere. For instance, TUC, under the southern crest of the ionospheric equatorial ionization anomaly (EIA), shows a predominance of RSF signatures, while both SJC, under the southern crest of EIA but in a different longitude sector, and CGM, under the northern crest of EIA, show a predominance of FSF signatures. Moreover, the spread-F occurrence over the longitude sector that includes CGM and TNJ is significantly lower than the spread-F occurrence over the longitude sector of PAL, SJC, and TUC.
Highlights
Equator, after sunset the F-region may rapidly rise and de- Lat. 16.8◦ S), Argentina
Considering that PAL, SJC, and TUC are located in the Southern Hemisphere, while CGM is located in the Northern Hemisphere, these equinoctial asymmetries underline an hemispherical asymmetry; 8. at the range spread-F phenomenon on jungsari (TNJ), CGM and TUC, satellite traces are confirmed to be a necessary precursor to the appearance of an range spread-F (RSF) trace on the ionograms; 9. the majority of no spread days is found at TNJ and CGM, even though it must be said that for those days most of the ionograms are characterized by blanketing E sporadic (Es) layers
Cabrera et al (2010) showed that the RSF occurrence at TUC, heralded tens of minutes in advance by satellite traces (ST) appearance, seems to be related to gravity waves (GWs) propagating through the F-region that are triggered by the solar terminator
Summary
In order to perform the comparative study just mentioned at the end of the previous section, we considered simultaneous ionograms recorded by the following ionosondes: an FMCW (frequency-modulated continuous wave) at CGM, an IPS-71 (digital ionosonde from KEL aerospace) at TNJ, a CADI (Canadian Advanced Digital Ionosonde) (MacDougall et al, 1993) at PAL and SJC, and an AIS-INGV (Advanced Ionospheric Sounder – Istituto Nazionale di Geofisica e Vulcanologia) (Pezzopane et al, 2007) at TUC. RSF signatures present echo spreading occurring mainly along the height axis and they are associated with plasma irregularities in the lower part of the F-region. FSF signatures present echo spreading along the frequency axis close to the critical frequencies of the ordinary and extraordinary traces of the ionograms, they are associated with irregularities nearby the F-region peak. The lowest spread-F occurrence was found in March at SJC and CGM, just close to the southern and northern crests of the ionospheric equatorial ionization anomaly (EIA), respectively; 2. The RSF occurrence is predominant at TNJ and TUC, while the FSF occurrence is predominant at SJC and CGM. In particular, close to the southern anomaly crest, the RSF is predominant at TUC and TNJ, while the FSF is predominant at SJC; 5. Considering that PAL, SJC, and TUC are located in the Southern Hemisphere, while CGM is located in the Northern Hemisphere, these equinoctial asymmetries underline an hemispherical asymmetry; 8. at TNJ, CGM and TUC, satellite traces are confirmed to be a necessary precursor to the appearance of an RSF trace on the ionograms; 9. the majority of no spread days is found at TNJ and CGM, even though it must be said that for those days most of the ionograms are characterized by blanketing Es layers
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