[1] We present recent observations of Sudden Phase Anomalies due to subionospheric propagation anomalies produced by solar X-ray flares. We use the new South America VLF Network (SAVNET) to study 471 ionospheric events produced by solar flares during the period May 2006 to July 2009 which corresponds to the present minimum of solar activity. For this activity level, we find that 100% of the solar flares with a X-ray peak flux above 5 × 10 −7 W/m 2 in the 0.1-0.8 nm wavelength range produce a significant ionospheric disturbance, while the minimum X-ray flux needed to do so is about 2.7 × 10 −7 W/m 2 . We find that this latter minimum threshold is dependent on the solar cycle, increasing when the Sun is more active, thus confirming that the low ionosphere is more sensitive during periods of low solar activity. Also, our findings are in agreement with the idea that the ionospheric D-region is formed and maintained by the solar Lyman-a radiation outside solar flare periods.
Se presenta la red South America VLF Networks (SAVNET) que es una nueva instalación de observación en frecuencias muy bajas. SAVNET se instaló recientemente en diversas localidades en América LAtina (Brasil, Perú y Argentina) Consiste en una red de siete receptores cuyo principal objetivo científico es monitorear la actividad solar en escalas temporales cortas (minutos/horas) y extensas (años). Otros objetivos incluyen una mejor comprensión de la estructura espacial de la anomalía Magnética del Atlántico sur, el estudio de fenómenos atmosféricos y la búsqueda sistemática de efectos sismico-electromágneticos genuinos.
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