V.O. PAPITASHVILI and C.R. CLAUER, Space Physics Research Laboratory (SPRL), University of Michigan, Ann Arbor, Michigan
B.A. BELOV, Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN), Troitsk, Moscow Region, Russia
O.A. TROSHICHEV, Arctic and Antarctic Research Institute (AARI), St. Petersburg, Russia
Magnetic disturbances caused by magnetospheric substorms are the most prominent near magnetic midnight at latitudes of the auroral oval. Morphologically, they are divided into the substorm growth, expansion, and recovery phases. The substorm onset separates the growth and expansion phases. At that time, the DP1 westward auroral electrojet ("substorm current wedge") develops near midnight, and it can be well determined from auroral magnetograms.
The question of how deep into the polar cap the substorm current wedge can progress is still unresolved. Some evidences indicate that substorms can occur deep in the polar caps during quiet (i.e., northward interplanetary magnetic field, IMF) conditions (e.g., Nielsen et al. 1988). Sergeev, Yakhnin, and Dmitrieva (1979) have also shown that magnetic substorms usually can take place deep in the polar cap when the solar wind speed is significantly high [velocity (V) greater than 500 kilometers per second (km s-1)]. Another evidence of a substormlike event has recently been reported by Weatherwax et al. (1997) from the riometer observations made by the U.S. antarctic automatic geophysical observatories (AGO) at the corrected geomagnetic (CGM) latitude (F) equaling approximately -80°. A conjugate substorm development occurring deep in both the southern and northern polar caps has not yet been reported, however.
In 1994-1996, a cooperative project was undertaken to deploy and operate digital magnetometers at the permanent Russian antarctic stations Mirnyy and Vostok. Two additional autonomous magnetometers were also deployed at the antarctic sites called Sude (approximately 170 km southward from Pionerskaya) and Komsomolskaya by the Russian Antarctic Expedition snow traverse on the route from Mirnyy to Vostok (Musko, Clauer, and Papatashvili 1995). All sites are revisited annually.
This chain is magnetically conjugate to the northern portion of the Greenland magnetometer arrays (Clauer et al. 1995; see also figure 1 in Papitashvili et al. 1996). This allows us to investigate geomagnetic disturbances occurring simultaneously deep in both the southern and northern polar caps. We report here an event around 02 universal time (UT) on 6 May 1995 when the magnetic substorm occurred in both polar caps at F approximately 80°, but the IMF was southward, the solar wind velocity was high (V equaling approximately 700 km s-1), and the planetary geomagnetic activity index (Kp) was 5 + .
During the reported event, the IMF Bzs equaled approximately -5 nanoteslas (nT) during the preceding hour (figure 1). Figure 2 shows magnetic disturbances (horizontal components H and E, vertical component Z) in the data obtained from stations Nord (Greenland) and Sude (Antarctica). These stations are located at F = ±81° in the early morning sector (the local magnetic midnight occurred at these stations at approximately 20 UT). One can see a simultaneous development of the magnetic disturbances in both polar caps; this simultaneity is unusual because the magnetic field lines at these stations are generally considered to be open rather than closed, but the analysis presented here supports the latter.
In total, six spikelike northward fluctuations are observed in the IMF after its southward turn at 00:45 UT; the IMF turned northward again at 01:56 UT. The Greenland west coast stations located near magnetic midnight (occurring at approximately 02 UT) show clearly the poleward progression of a magnetic substorm from auroral latitudes to the deep polar cap. The first substorm onset occurred at 01:18 UT at auroral latitudes; this onset corresponds to the IMF Bzs fluctuation at 01:04 UT. Ground Z component variations show that the DP1 electrojet was located at F equal to approximately 69°; the substorm recovery phase lasts until approximately 02:05 UT.
Combined analysis of Greenland and antarctic high-latitude magnetometer data (e.g., west coast Greenland stations and antarctic stations Vostok, Sude, and AGO P1 and P4) confirms that this substorm progressed poleward simultaneously in both polar regions. The second substorm onset occurred at F equal to approximately 73° on 01:21 UT; the third at F equal to approximately 75° on 01:30 UT; and the fourth at F equal to approximately 78° on 01:45 UT. The last significant IMF excursion occurred at 01:56 UT; the IMF turned to be near 0 and then northward. This excursion is clearly manisfested on the ground magnetograms from the higher latitude Greenland and antarctic stations (F equals approximately 78-85°) as a substormlike disturbance occurred on 02:07 UT. This observation is also supported by a sharp enhancement in ionospheric absorption observed by the AGO P1 and P4 riometers (Weatherwax personal communication) and a significant brightening over the all-sky camera "field-of-view" at AGO P4 (Frey personal communication). The Defense Meteorological Satellite Program (DMSP) satellite F12 shows strong precipitating particles over the southern polar cap exactly at 02:07 UT (Rich personal communication).
Based on the observations presented here, we conclude that field-aligned currents driving intense, nearly conjugate, substorm electrojets may develop in the midnight sector at high CGM latitudes (approximately 80°) in both the northern and southern polar caps at the peak of the expansion phase. According to the Tsyganenko T89c magnetospheric field model ( http://nssdc.gsfc.nasa.gov/space/cgm/ext.html ), the magnetic field lines from approximately 80° magnetic latitude near midnight go far into the magnetospheric tail to distances of 70-80 RE (Earth radii) during the conditions of high geomagnetic activity (Kp=5+). Thus, we may infer that these high-latitude magnetic field lines may actually be closed at the peak of the substorm expansion phase.
We plan a further study of these very-high-latitude conjugate geomagnetic disturbances. Particularly, we plan to use data from other high-latitude magnetometers in both hemispheres identifying the regions over which disturbances are observed and mapping these regions to the outer magnetosphere.
This work was supported by National Science Foundation grant OPP 93-18766.
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