A substorm in midnight auroral precipitation

International audienceDMSP F7 spacecraft observations for the whole of 1986 were used to construct the empirical model of the midnight auroral precipitation during a substorm. The model includes the dynamics of different auroral precipitation boundaries and simultaneous changes in average electron precipitation energy and energy flux in different precipitation regions during all substorm phases, as well as the IMF and solar wind plasma signatures during a substorm. The analysis of the model shows a few important features of precipitation. (1) During the magnetic quietness and just before the beginning of the substorm expansive phase the latitudinal width of the auroral precipitation in the nightside sector is about 5 ? 6° CGL, while that of the auroral oval is about 2 ? 3° CGL during such periods. (2) For about 5 min before the substorm onset a decrease in the average precipitating electron energy in the equatorward part of auroral zone was observed simultaneously, with an increase in both the average electron energy and energy flux of electron precipitation in the poleward part of the auroral zone. (3) The isotropy boundary position in the beginning of the substorm expansive phase coincides well with the inner edge of the central plasma sheet. The analysis of interplanetary medium parameters shows that, on average, during the substorm development, the solar wind dynamic pressure was about 1.5 times that of the magnetic quietness period. Substorms occurred predominantly during the southward IMF orientation, suggesting that substorm onset often was not associated with the northern turn or decrease in the southward interplanetary Bz . The Northern Hemisphere's substorms occurred generally during the positive interplanetary By in winter, and they were observed when the interplanetary By was negative in summer

Physics of Auroral Phenomena

Abstract. The comparison between equatorial boundaries of auroral discrete forms (auroral oval) and diffuse auroral luminosity was carried out for periods of magnetic storms. Position of equatorial boundary of the night-side auroral oval depending on the value of the Dst index was taken from the paper by Starkov [1993]. The limit corrected geomagnetic latitude (CGL) of visual aurora locations was received on the basis of visoplots for IGY period [Auroral visoplots, 1964]. Twenty six magnetic storm events were used to determinate the equatorward boundary of visual auroral luminosity during Dst<-100 nT. When Dst changed from -100 nT to -400 nT the boundary of the visual auroral luminosity moves from about 51º to 44º CGL. According to visoplots the equatorward boundary of the visual auroral luminosity was found to be remove on about 7º to lower latitude from the equatorward boundary of the auroral oval in intervals of magnetic storms. The auroral luminosity was registered at all latitudes between its equatorward boundaries to the auroral oval. It means that during magnetic storm the equatorward boundary of visual luminosity according visoplots does not reflect the location of discrete auroral forms boundary. It is well known that a diffuse aurora equatorward of discrete auroral forms can intensify considerably in intervals of magnetic storms. This special type of the auroral luminosity at mid-latitudes in the forms of stretched diffuse areas is associated generally with the atomic oxygen 630.0 nm emission. In that way the lowest latitude of auroral luminosity on visoplots characterizes the equatorial boundary of diffuse auroras. The region of red diffuse luminosity spatially coincides with region of diffuse electron precipitation covering the region equatorward of the auroral oval up to latitudes of the main ionospheric trough. An electron energy flux and their average energy sharp decrease from oval to lower latitudes. It leads to decrease of the diffuse aurora intensity and respectively to the increase in the luminosity height at lower latitudes. The intensification of the 630.0 nm luminosity at mid-latitudes near of the main ionospheric trough was called as a stable auroral red arc (SAR-arcs). Between usual auroras and SAR-arcs does not exist the latitudinal gap. The latitudinal interval between the auroral oval and SAR-arcs is filled by auroral particle precipitations and diffuse luminosity

Extended period of polar cap auroral display: auroral dynamics and relation to the IMF and the ionospheric convection

An unusually extended period (5 h) of polar cap auroral display on 3 August 1986 is examined. Auroras have been investigated using ground-based data as well as measurements from the IMP-8 spacecraft in interplanetary space and simultaneous observations from the polar-orbiting satellites Viking and DE-1 in the northern and southern hemispheres, respectively. It is found that visible Sun-aligned arcs are located inside the transpolar band of the θ-aurora observed from the satellite in ultraviolet wavelengths. The transpolar band can contain several Sun-aligned arcs that move inside the band toward the morning or evening side of the auroral oval independent of the direction of the band movement. Intensifications of polar cap auroras with durations of up to about 30 min are observed. No change has been found in either IMF parameters or substorm activity that can be related to these intensifications. The θ-aurora occurred during a 2-h period when the B z-component of the IMF was negative. A tendency is noted for dawnward (duskward) displacement of the transpolar band when By>0 (By<0) in the southern hemisphere. Simultaneous observations of auroral ovals during interplanetary Bz<0, By<0 and Bx>0 in both hemispheres and convection patterns for Bz<0 and By<0 have been displayed using satellite and ground-based measurements. It was found that the transpolar band of the <theta>-aurora in the sunlit hemisphere was situated in the region of large-scale downward Birkeland currents

A substorm in midnight auroral precipitation

DMSP F7 spacecraft observations for the whole of 1986 were used to construct the empirical model of the midnight auroral precipitation during a substorm. The model includes the dynamics of different auroral precipitation boundaries and simultaneous changes in average electron precipitation energy and energy flux in different precipitation regions during all substorm phases, as well as the IMF and solar wind plasma signatures during a substorm. The analysis of the model shows a few important features of precipitation. (1) During the magnetic quietness and just before the beginning of the substorm expansive phase the latitudinal width of the auroral precipitation in the nightside sector is about 5 – 6° CGL, while that of the auroral oval is about 2 – 3° CGL during such periods. (2) For about 5 min before the substorm onset a decrease in the average precipitating electron energy in the equatorward part of auroral zone was observed simultaneously, with an increase in both the average electron energy and energy flux of electron precipitation in the poleward part of the auroral zone. (3) The isotropy boundary position in the beginning of the substorm expansive phase coincides well with the inner edge of the central plasma sheet. The analysis of interplanetary medium parameters shows that, on average, during the substorm development, the solar wind dynamic pressure was about 1.5 times that of the magnetic quietness period. Substorms occurred predominantly during the southward IMF orientation, suggesting that substorm onset often was not associated with the northern turn or decrease in the southward interplanetary Bz . The Northern Hemisphere’s substorms occurred generally during the positive interplanetary By in winter, and they were observed when the interplanetary By was negative in summer.Key words. Ionosphere (auroral ionosphere; particle precipitation) – Magnetospheric physics (storm and substorm; magnetosphere-ionosphere interaction

Extended period of polar cap auroral display: auroral dynamics and relation to the IMF and the ionospheric convection

An unusually extended period (5 h) of polar cap auroral display on 3 August 1986 is examined. Auroras have been investigated using ground-based data as well as measurements from the IMP-8 spacecraft in interplanetary space and simultaneous observations from the polar-orbiting satellites Viking and DE-1 in the northern and southern hemispheres, respectively. It is found that visible Sun-aligned arcs are located inside the transpolar band of the θ-aurora observed from the satellite in ultraviolet wavelengths. The transpolar band can contain several Sun-aligned arcs that move inside the band toward the morning or evening side of the auroral oval independent of the direction of the band movement. Intensifications of polar cap auroras with durations of up to about 30 min are observed. No change has been found in either IMF parameters or substorm activity that can be related to these intensifications. The θ-aurora occurred during a 2-h period when the <i>B</i> <i><sub>z</sub></i>-component of the IMF was negative. A tendency is noted for dawnward (duskward) displacement of the transpolar band when <i>B<sub>y</sub></i>>0 (<i>B<sub>y</sub></i><0) in the southern hemisphere. Simultaneous observations of auroral ovals during interplanetary <i>B<sub>z</sub></i><0, <i>B<sub>y</sub></i><0 and <i>B<sub>x</sub></i>>0 in both hemispheres and convection patterns for <i>B<sub>z</sub></i><0 and <i>B<sub>y</sub></i><0 have been displayed using satellite and ground-based measurements. It was found that the transpolar band of the <theta>-aurora in the sunlit hemisphere was situated in the region of large-scale downward Birkeland currents