Equatorial plasma depletion precursor signatures and onset observed at 11° south of the magnetic equator

Abstract

Coordinated radio and optical measurements of the structure and dynamics of the postsunset equatorial ionosphere were conducted on October 1, 1994, from Agua Verde, Chile (11.3°S magnetic latitude (MLat)). The measurements clearly show a north‐south aligned undulation or ripple on the bottomside of the F layer at 2000 LT, appearing as an eastward propagating decrease in the 630.0‐nm airglow, resembling a traveling ionospheric disturbance in the digital portable ionosonde measurements and causing a total electron content decrease in the Global Positioning System (GPS) satellite measurements. The initial development of this feature, toward the east and away from the magnetic equator, took place in an otherwise smooth, unstructured ionosphere. Spread F began to develop in the ionograms at 2020 LT, and, at this same time, local onset of satellite signal scintillation was detected using the multiple ray paths throughout the sky available from the GPS satellite constellation transmitting at L band frequencies. UHF scintillation measurements from Ancon, Peru, along the same magnetic field line, show that intense scintillation and ionospheric irregularities had developed over the magnetic equator almost 60 min prior to their development at 11°S MLat. The observations suggest that the east‐west electric field expected to be present within the earlier developed depletion and scintillation region at the magnetic equator mapped along magnetic field lines to lower altitudes and higher latitudes, resulting in an undulation or dome‐shaped structure, before evolving into a fully developed depletion (with associated ionospheric irregularities) all along the magnetic flux tube.

Short‐term delays (hours) of ionospheric spread F occurrence at a range of latitudes, following geomagnetic activity

“…More recent reports have essentially confirmed the existence in equatorial regions of wavelike isoionic contours [Flaherty et al, 1996;Sales et al, 1996, Weber et al, 1996. Recent analyses also give support to the hypothesis that most of the ESF traces on ionograms are produced by off-vertical specular reflections [Wright et al, 1996;Sales et al, 1996].…”

Observations in equatorial anomaly region of total electron content enhancements and depletions

“…All such cases have been excluded from the identification of TEC depletion. Using the 630.0-nm airglow emissions, Weber et al (1996) have shown that the TEC depletions are a manifestation of the equatorial plasma depletion. The general morphology of the TEC depletions and their association with strong VHF scintillations, using the data from the South American zone, were described by Das Gupta et al (1983).…”

Multi-technique investigations of storm-time ionospheric irregularities over the São Luís equatorial station in Brazil

“…Woodman and La Hoz, 1976;Ossakow, 1981;Abdu et al, 1985Abdu et al, , 2002Kelley et al, 1996;Weber et al, 1996;Fejer, 1997b;Hysell and Woodman, 1997;Musman et al, 1997;Kil and Heelis, 1998;Beach and Kintner, 1999;Fejer et al, 1999;Basu et al, 2001). Long-term climatological and persistence studies of the irregularities have been recently performed with the JULIA radar at Jicamarca Burcham, 1998, 2002).…”