Fernando C. P. Bertoni

Solar flare detection sensitivity using the South America VLF Network (SAVNET)

[1]xa0We 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/m2 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/m2. 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-α radiation outside solar flare periods.

Effects of the fringe field of Rayleigh‐Taylor instability in the equatorial E and valley regions

[1]xa0We present a unified algorithm for the collisional interchange instabilities (Rayleigh-Taylor and gradient drift instabilities) occurring in the F and E regions of the equatorial ionosphere. The similar underlying mechanism of both instabilities enables us to derive the general two-dimensional continuity and potential equations. The equations are integrated numerically to study the nonlinear evolution of the polarization field (fringe field) associated with the generalized Rayleigh-Taylor instability. In particular, the effects of the fringe field into the equatorial E and transition (or valley) regions are investigated. The characteristics of the fringe field beneath the F region are investigated quantitatively for the first time. It is shown that the fringe field is capable of transporting plasma from the region where ions are highly magnetized (i.e., from the valley region). It is further shown that only under strongly driven but realistic conditions, the fringe field recognizes the part of the E region plasma where ions are marginally magnetized. The E region irregularities which are often located in such a region (near 120 km) during evening and nighttime can be effectively convected by the fringe field across the valley region and to the higher altitudes. On the other hand, because of the small ratio of ion-gyro-frequency to collision frequency below 120–115 km altitude the fringe field is unable to convect the E region irregularities lying below this region. These characteristics are important in the context of observed valley region echoes which are yet to be explained quantitatively.

Response of the low ionosphere to X‐ray and Lyman‐α solar flare emissions

[1] Using soft X-ray measurements from detectors onboard the Geostationary Operational Environmental Satellite (GOES) and simultaneous high-cadence Lyman-a observations from the Large Yield Radiometer (LYRA) onboard the Project for On-Board Autonomy 2 (PROBA2) ESA spacecraft, we study the response of the lower part of the ionosphere, the D region, to seven moderate to medium-size solar flares that occurred in February and March of 2010. The ionospheric disturbances are analyzed by monitoring the resulting sub-ionospheric wave propagation anomalies detected by the South America Very Low Frequency (VLF) Network (SAVNET). We find that the ionospheric disturbances, which are characterized by changes of the VLF wave phase, do not depend on the presence of Lyman-a radiation excesses during the flares. Indeed, Lyman-a excesses associated with flares do not produce measurable phase changes. Our results are in agreement with what is expected in terms of forcing of the lower ionosphere by quiescent Lyman-a emission along the solar activity cycle. Therefore, while phase changes using the VLF technique may be a good indicator of quiescent Lyman-a variations along the solar cycle, they cannot be used to scale explosive Lyman-a emission during flares.

Long-Term Power Transmission Failures in Southeastern Brazil and the Geophysical Environment

High-voltage transmission networks represent a large electrical circuit just above the ground subjected to a number of transient overcharges of various kinds, some of which may lead to failures. Some failures might be related to anomalies of the geophysical environment. We have analyzed one unprecedented long series of transmission grid failures (9xa0years) on high-voltage networks located in São Paulo state, southeastern Brazil, from 1998 to 2006, which includes an important fraction of the past solar activity cycle 23. Ninety-five distinct failure causes were given by the power line operator to explain the transmission grid shut downs. Most failures were attributed to atmospheric discharges, corresponding to 1,957 failures out of a total of 4,572 for the whole period at 138xa0kV, and 170 out of 763 at 440xa0kV, respectively. They correspond to less than one ten thousandth of the actual number of atmospheric discharges recorded in the same area, demonstrating the grid’s high resilience to breakdowns due to lightning. A clear concentration of failures in the region’s thunderstorm season has been found. A significant 67 and 77xa0% reduction in the number of failure rates per year has been found for the 138 and 440xa0kV grids, respectively, for the period studied, in good correspondence with the decay in the sunspot numbers. No obvious correlation was found between power failures and the planetary index of geomagnetic activity or major geomagnetic storms in the period, either on short or on long time scales. Assuming that the dependence of the electrosphere/ionosphere-ground coupling on the external geophysical environment plays a major role in explaining the reduction in power failures as the solar cycle wanes, it is suggested that the increase in atmosphere conductivity caused by the larger cosmic ray flux then reduces the threshold voltage required to produce lightning strokes, so reducing their effectiveness in disrupting high-voltage power lines.

Periodic and quiescent solar activity effects in the low ionosphere, using SAVNET data

Important results have been acquired using the measurements of VLF amplitude and phase signals from the South America VLF Network (SAVNET) stations. This network is an international project coordinated by CRAAM, Brazil in cooperation with Peru and Argentina. It started operating in April 2006, and now counts on eight stations (Atibaia, Palmas, Santa Maria and Estacao Antartica Comandante Ferraz in Brazil; Piura, Punta‐Lobos and Ica, in Peru; CASLEO, in Argentina). Researches, through the last decades, have demonstrated the versatility of the VLF technique for many scientific and technological purposes. In this work, we summarize some recent results using SAVNET data base. We have obtained daily maximum diurnal amplitude time series that exhibited behavior patterns in different time scales: 1) 1ong term variations indicating the solar activity level control of the low ionosphere; 2) characteristic periods of alternated slow and fast variations, the former being related to solar illumination conditions, and...

Long‐term and transient forcing of the low ionosphere monitored by SAVNET

In this paper we present the main findings obtained by the South America VLF NETwork since its installation in the South America territory. In particular we show the capability of the VLF technique to monitor the long‐term solar activity and transient solar and geomagnetic disturbances. On long timescales they are indices on the possibility of monitoring the Lyman‐α solar radiation. On shorter timescales we show that the VLF technique is a very sensitive mean of detecting solar X‐ray flares. Those small events with a peak power ⩾5×10−7u2009W/m2 are detected with a 100% probability. A lower limit for the X‐ray power of ∼2.7u200910−7u2009W/m2 has been found in order to produce a ionospheric disturbance, and we confirm the important role of the Lyman‐α radiation to form and maintain the low ionospheric D‐region. SAVNET has also observed for the first time the ionospheric disturbances produced by outbursts from the magnetar SGR 1550–5408. This genuine detection suggests the possibility of monitoring on a routine basis th...