Auroral behavior during magnetic superstorms
1999; Pan American Institute of Geography and History; Issue: 50 Linguagem: Inglês
ISSN
2663-4015
Autores Tópico(s)Earthquake Detection and Analysis
ResumoIn this work we study the auroral oval dynamics, in terms of the displacement of equatorward auroral boundaries, besides the ionospheric dissipation during superstorms (Dst < -240nT) due to precipitating charged particles, using ionospheric and interplanetary data. Interplanetary magnetic field, density, temperature, and solar wind velocity were collected from the ISEE 3 satellite; the total energy flux of particles that precipitates on high-latitude were obtained from polar orbit NOAA satellites. The energy dissipation estimate in the auroral ionosphere via Joule heating were computed through our hybrid method described recently in a previous work. From that approach we can obtain mean values of the auroral radii to study the movement of the boundaries related with geomagnetic activity indices. Our results fit well with a linear relationship shown in previous case and statistical studies for moderate magnetic storms; however, when the mean-latitude geomagnetic index goes over a threshold value the linear relationship goes down, probably due to ionospheric saturation. Resumen Se estudia la disipacion ionosferica y la dinamica del ovalo auroral en terminos del movimiento de sus contornos hacia el ecuador, debido a la precipitacion de particulas cargadas, usando datos ionosfericos e interplanetarios obtenidos durante tempestades superintensas (Dst < -240nT). La densidad, temperatura, y campo magnetico interplanetarios, asi como la velocidad del viento solar fueron colectados por el satelite ISEE 3, y el flujo de energia total de particulas que se precipitan en altas latitudes fueron colectadas por los satelites NOAA de orbita polar. Las estimaciones de la disipacion de energia en la ionosfera auroral, via calor joule, fueron calculadas a traves de nuestro metodo hibrido especial descrito en trabajo previo reciente. Del mismo metodo se obtienen valores promedios de los radios aurorales para estudiar el movimiento de los contornos en relacion a los indices de actividad geomagnetica. Nuestros resultados muestran un buen ajuste con la relacion lineal mostrada en estudios estadisticos y de caso para tempestades magneticas moderadas. Sin embargo, cuando el indice geomagnetico de latitudes medias sobrepasa un cierto valor umbral, aparentemente, la capacidad ionosferica se satura produciendose un quiebre en la relacion lineal. INTRODUCTION This paper shows a preliminary study on the evolution of the equatorward boundaries of the auroral zone as defined by the total energy flux of charged precipitating particles, and an estimation of the dissipation in the auroral ionosphere during superstorms' events using auroral boundaries and interplanetary data. For the study we have chosen the four most intense magnetic storms occurring in the solar maximum neighborhood of the solar cycle 21 in which we could find good solar wind coverage. The onset of the storms occurred on December 19, 1980 (Dst* = 249nT); April 13, 1981 (Dst* = -311nT); July 13, 1982 (Dst* = -325 nT); September 5, 1982 (Dst*= -289 nT), where Dst* denotes the peak Dst value including solar wind ram-pressure correction [Gonzalez et al., 1994]. During the main phase of intense geomagnetic storms, auroras tend to come down, to latitudes lower than the normal auroral latitudes, reaching magnetic latitude values below 50 degrees; therefore auroral displays can be seen occasionally in populated areas.. Akasofu and Chapman [1963], and Akasofu [1964], based in observations made on storms occurred during the International Geophysical Year, were the first to show that there is a direct relationship between the latitude of auroras and the magnitude of the ring current as given by the Dst index. Now, it is widely accepted that the equatorward shift of the auroral oval boundary is related with the earthward movement of the outer boundary of the radiation belt. Diversous studies have shown that the equatorward boundaries of the diffuse auroral zone are mapped along magnetic field lines toward the magnetospheric plasma sheet [e.g., Lassen, 1974; Lui et al., 1975, 1976; Slater et al, 1980]. Considering that, the polar and auroral dynamic dues to particle precipitation have been studied by several authors [e.g., Gussenhoven et al., 1981, 1983; Meng, 1981,1984; Hardy et al., 1981; Nakai et al., 1986]. High-latitude dynamic studies related with the equatorward boundaries have reported the systematic movement in relation with geomagnetic activity and the interplanetary parameters. In fact, the southward component of the interplanetary magnetic field has been found to be the main contributor to the size of the auroral belt [e.g., Nakai et al., 1986], so it is natural to expect that the equatorward boundary of the auroral belt expands significantly during the main phase of magnetic storms [Yokoyama et al., 1998]. Additionally, several studies related with the polar cap potential have been done in the last two decades. Hill et al [1976] theorized and found in a reasonable basis that the polar cap potential should saturate during times of exceptionally strong interplanetary magnetic fields. Reiff et al [1981] and Reiff and Luhmann [1986] found a maximum value for the potential of about 160 kV. Sojka et al [1994] studied the ionospheric response to great geomagnetic storm during the sustained level of high geomagnetic activity. They show, using data obtained from the DMSP F8 and F9 satellites, the behavior of the auroral equatorward latitude in relation with the cross-tail potential variations, for a 10-day period of the superstorm occurred on March 13, 1989. The maximum potential for this severe storm, which reach a Dst peak value of Dst* = 600 nT, sometimes overpasses the maximum value suggested previously [e.g., Reiff and Luhmann, 1986] and the minimum location for an equivalent midnight boundary was about 40 degrees in magnetic latitude. THE BOUNDARIES AND THE DISSIPATION In this work, the necessary equatorward auroral boundaries were determined from the total energy flux of precipitating particles in the energy range between 0.3 and 20 keV. The data were collected mainly by the NOAA 6 satellite when it was encircling the earth in a polar orbit with an approximated speed (Vsat) of 7.5 km/s, considering an altitude about 815.5 km and a semi-period of approximately 50 minutes. The data involved approximately 1500 passes over the auroral zones allowing to determine four values during each pass, two equatorward boundaries and hypothetically two poleward boundaries. Many times, only the equatorward boundary was determined without ambiguity. The satellite passing over the auroral zone last about 15 to 25 minutes between the external boundaries. During this interval of time, (∆t), it is assumed that the relative variation between the boundaries is small. Then, we can infer the external and internal auroral zone diameter considering the projection from the satellite to ionospheric heights (120 km) from the expression Vsat (RE + 120/RE+ 815.5) ∆t , where RE is the radius of the earth in kilometers. In Figures 1, 2 , 3, and 4 are shown the equatorward auroral boundaries in corrected magnetic latitudes (top panel) besides the low-latitude geomagnetic index Dst (bottom panel) for the events in study. The dissipation of energy in the polar and auroral ionosphere that satisfies the classical expression [e.g., Cole, 1962] involves conductance and electrical fields. The determination of the auroral boundaries is a necessary task to compute both of
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