The response of the azimuthal component of the ionospheric electric field to auroral arc brightening
We have analyzed the response of azimuthal component of the ionospheric electric field to auroral arc activity. We have chosen for analysis three intervals of coordinated EISCAT and TV observations on 18 February, 1993. These intervals include three kinds of arc activity: the appearance of a new auroral arc, the gradual brightening of the existing arc and variations of the arc luminosity. The arcs were mostly east-west aligned. In all cases, the enhancement of arc luminosity is accompanied by a decrease in the westward component of the ionospheric electric field. In contrast, an increase of that component seems to be connected with arc fading. The observed response is assumed to have the same nature as the “short circuit” of an external electric field by the conductor. The possible consequence of this phenomenon is discussed.
Geomagnetic disturbances on ground associated with particle precipitation during SC
We have examined several cases of magneto- sphere compression by solar wind pressure pulses using a set of instruments located in the noon sector of auroral zone. We have found that the increase in riometric absorption (sud- den commencement absorption, SCA) occurred simultane- ously with the beginning of negative or positive magnetic variations and broadband enhancement of magnetic activity in the frequency range above 0.1 Hz. Since magnetic varia- tions were observed before the step-like increase of magnetic field at equatorial station (main impulse, MI), the negative declinations resembled the so-called preliminary impulse, PI. In this paper a mechanism for the generation of PI is intro- duced whereby PI's generation is linked to SCA - associ- ated precipitation and the local enhancement of ionospheric conductivity leading to the reconstruction of the ionospheric current system prior to MI. Calculation showed that PI po- larity depends on orientation of the background electric field and location of the observation point relative to ionospheric irregularity. For one case of direct measurements of elec- tric field in the place where the ionospheric irregularity was present, the sign of calculated disturbance corresponded to the observed one. High-resolution measurements on IRIS facility and meridional chain of the induction magnetome- ters are utilized for the accurate timing of the impact of solar wind irregularity on the magnetopause.
Noctilucent cloud observations over Greenland by a Rayleigh lidar
We report on the detection of noctilucent clouds (NLCs) by a Rayleigh lidar system located on the west coast of Greenland during operations in July and August 1994. The lidar detected NLCs in the zenith on three nights (July 28–29, July 31–August 1, and August 4). The lidar measurements are presented at temporal (1 minute) and spatial (192 meter) resolutions exceeding previously reported lidar measurements of NLCs and reveal significantly more structure in the clouds. Unique cloud features are observed for each night NLCs were detected. Because of the large backscatter ratio associated with NLCs, the volume backscatter coefficient for the cloud particles can be determined more accurately from the lidar signal than the backscatter ratio as the upper mesospheric molecular number density is negligible in the volume backscatter coefficient calculation. The volume backscatter coefficient values determined from the three nights of NLC observations would be classified by current models as moderate to extreme. The extreme backscatter from NLC measurements on August 4 tests the assumptions used in existing models.