Unusual ionospheric absorption characterizing energetic electron precipitation into the South Atlantic Magnetic Anomaly

Abstract

An imaging riometer (IRIS) was installed newly in the southern area of Brazil in order to investigate precipitation of energetic electrons into the South Atlantic Magnetic Anomaly (SAMA). An unusual ionospheric absorption event was observed in the nighttime (∼20 h LT) near the maximum depression (D st ∼ −164 nT) and the following positive excursion during the strong geomagnetic storm on September 22-23, 1999. The unusual absorption that has short time-duration of 30-40 min shows two characteristic features: One feature is a sheet structure of the absorption appearing at the high-latitude part of the IRIS field-of-view, showing an eastward drift from the western to the eastern parts and subsequent retreat to the western part. Another feature is a meridionally elongated structure with a narrow longitudinal width (100-150 km) appearing from the zenith to the low-latitude part of the IRIS fieldof-view, enhanced simultaneously with the sheet absorption, and is subsequently changed to a localized structure. These features likely characterize precipitation of energetic electrons into the SAMA ionosphere, associated with substorm occurrences during the strong geomagnetic storm. From the eastward drift (∼250 m/s) of the sheet absorption, precipitating electrons are estimated to be ∼20 keV energies, assuming plasmaspheric electric fields of 1.8 mV/m. However, no ionospheric effect due to the precipitating electrons was definitely detected by the ionosonde measurements at Cachoeira Paulista, separated eastward by about 1000 km from the IRIS station.

A discussion about some aspects of the solar-terrestrial systems coupling with regard to space weather effects

“…The instrument designed to evaluate the cosmic noise absorption under the SAMA region is the Relative Ionospheric Opacity Meters (RIOMETER), which is essentially a very sensitive radio receivers that have been used to study the CNA since the early 60's (Little and Leinbach, 1959;Lusignan, 1960;Abdu et al, 1967;Abdu et al, 1973;Nishino et al, 1993;Nishino et al, 2002;Nishino et al, 2006). The present one, used in this study, is an Imaging Riometer for Ionospheric Studies (IRIS) installed at the Southern Space Observatory (SSO, 29°4'S, 53°8'W, 480 m a.s.l.…”

A comparison of two different techniques for deriving the quiet day curve from SARINET riometer data

“…One of the twodimensional array is aligned in the geographic north-south (N-S), while the others is in quadrature and, therefore, is set in the east-west (E-W) direction. The field-of-view of the IRIS at 100 km of altitude is approximately 330 km in both N-S and E-W directions (Nishino et al, 2002). By connection of the dipole array with a Butler-matrix phasing circuit and four channel radio receivers, 4 × 4 two-dimensional beams with a half-power beam-width of 22 • are directed upward to the ionospheric area centred at the zenith.…”

“…The riometer has been used to study the cosmic noise absorption (CNA) since the early 1960s (Little and Leinbach, 1959;Lusignan, 1960;Abdu et al, 1967Abdu et al, , 1973Nishino et al, 1993Nishino et al, , 2002Nishino et al, , 2006. The CNA generally occurs in the Dregion of the ionosphere, which is the region with the highest concentration of molecular gases.…”

Main patterns of the geomagnetic field: A case study using principal component analysis

“…Ref. [9] verified variations on some ionospheric parameters measured by ionosonde at Cachoeira Paulista (Brazil). The processes which cause energetic electrons to precipitate in the atmosphere are: magnetospheric wave-particle interaction, lightning or artificially induced wave-particle interaction, driftresonance interactions and wave-particle interactions generated by plasma instabilities [10].…”