V. L. Khalipov

Formation of a Polarization Jet during the Expansion Phase of a Substorm: Results of Ground-Based Measurements

The ground-based polarization jet measurements at the Yakutsk ionosphere station (L= 3.0) for the years 1989–1991 (110 events) are compared with variations of the AE-index and with parameters of the local magnetic activity. It is shown that polarization jet development in the near midnight sector can occur during a period of no longer than 10 min on the expansion phase of a substorm. The formation of the polarization jet is accompanied by a specific magnetic field variation corresponding in shape to a fast passage of the Harang Discontinuity above the station. Statistical data are given on ground level observations of the polarization jet, which are close to those measured from satellites. The mean delay (averaged over the full data bank) between the onset of a substorm with AE≥ 500 nT and the moment of the polarization jet appearance at L= 3.0 is equal to 0.5 h near midnight and to 1.0 – 1.5 h in the evening sector. Estimations show that the duration of the polarization jet formation when energetic ions are injected into the Harang Discontinuity region above the ground station can last for about 10 min, and during this time the Harang Discontinuity can be shifted to the west. This is in qualitative agreement with the described observations.

FORMATION OF POLARIZATION JET DURING INJECTION OF IONS INTO THE INNER MAGNETOSPHERE

Long-lasting ground based measurements of a polarization jet (PJ) by the latitudinal chain of ionospheric stations in Yakutia (3 500 nT.

Position variations of the polarization jet and injection boundary of energetic ions during substorms

The comparison of selected cases of polarization jet observation at ground stations and measurements of energetic ions at the AMPTE/CCE satellite shows that these phenomena occur simultaneously and on the same L shells. Polarization jet observations at DMSP satellites make it possible to statistically determine the dependence of its equatorial boundary position on the AE-index value. It is also shown that, in the case of isolated magnetic disturbances, the position of the inner boundary of injection of energetic ions measured at the AMPTE/CCE satellite depends on the AE index. It was found that the dependences of both boundaries on the AE index match over a wide range of AE variations. This is evidence that the equatorial boundary polarization jet band and the inner boundary of the injection of energetic ions are physically interconnected and are formed on the same L shells during substorms.

Structural features of the subauroral ionosphere during the origination of the polarization jet

Narrow jets of rapid westward ion drifts were registered near the plasmapause projection at the F-region altitudes on the Cosmoc-184 satellite and were called “a polarization jet.” In this work, the effect of this polarization jet on the ionospheric structure has been studied, using a three-dimensional model of the high-latitude ionosphere, when strong local magnetospheric electric fields were originated. The calculations indicated that a narrow trough in the latitudinal variations in the electron density at the F-region maximum was formed in the zone where the electric field was switched on. This trough was more pronounced in the early evening hours, when the electron background density was still high, and was less distinct at low back-ground levels during premidnight hours. A comparison of the calculations and experimental data indicated that they were in good agreement with one another, which made it possible to state that the polarization jet was the main mechanism by which narrow electron density troughs were formed in the subauroral ionosphere.

Observations of large-scale plasma convection in the magnetosphere with respect to the geomagnetic activity level

The data of the ionospheric observations (the daily f plots) at the Yakutsk meridional chain of ionosondes (Yakutsk–Zhigansk–Batagai–Tixie Bay) with sharp decreases (breaks) in the critical frequency of the regular ionospheric F2 layer (foF2) are considered. The data for 1968–1983 were analyzed, and the statistics of the foF2 break observations, which indicate that these breaks are mainly registered in equinoctial months and in afternoon and evening hours under moderately disturbed geomagnetic conditions, are presented. Calculations performed using the prognostic model of the high-latitude ionosphere indicate that the critical frequency break position coincides with the equatorial boundary of large-scale plasma convection in the dusk MLT sector.

A comparison of polarization jet characteristics at the greatly separated stations Yakutsk and Podkamennaya Tunguska

Characteristics of the polarization jet (PJ) are considered on the basis of measurements made in 1989–1992 at the ionospheric stations Yakutsk (L = 3.0, λ = 129.6°) and Podkamennaya Tunguska (L = 3.0, λ = 90.0°) separated in longitude. Using the data of these stations, the result obtained earlier (that the formation of PJ during disturbances in the near-midnight sector occurs at the expansion phase of a substorm) is confirmed. At isolated magnetic disturbances with AE > 500 nT in the 11:00–16:00 UT interval, the PJ band covers an MLT sector of 3 h between the Yakutsk and Podkamennaya Tunguska stations. The time of the PJ beginning at the Podkamennaya Tunguska station for the majority of events is 45–60 min behind the PJ beginning at the Yakutsk station. This corresponds to the westward motion of a PJ source with a velocity of ∼3 MLT h per hour.

Vertical plasma drift velocities in the polarization jet observation by ground Doppler measurements and driftmeters on DMSP satellites

Vertical and horizontal plasma drifts are investigated during the polarization jet (PJ) detection in the F2 ionospheric layer based on the Doppler measurements at the Yakutsk meridian chain of subauroral ionospheric stations. It is shown that the velocities of vertical and horizontal drifts are significantly higher than the background motion during PJ observation periods. The ionospheric plasma motion direction changes from upward to downward on the polar edge of the main ionospheric trough. Doppler measurements on the DPS-4 ionosondes are compared with the simultaneous measurements of the plasma drift on the DMSP satellites during their passage near the Yakutsk meridian. The two kinds of measurements are in good agreement with each other. During the magnetic storm of June 23, 2005, by measurements of the DMSP satellites, the velocities of upward plasma flows were 1.0–1.4 km/s at a satellite altitude of 850 km. In the ionospheric F region, this speed corresponds to 150 m/s. According to satellite measurements, the westward drift velocity reached 2.5 km/s. The development of the polarization jet in the ionosphere was accompanied by a tenfold decrease in the electron density in 15–30 min.