L.J. LANZEROTTI, Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974
A. SHONO and H. FUKUNISHI, Department of Astrophysics and Geophysics, Tohoku University, Sendai, Japan
C.G. MACLENNAN, and L.V. MEDFORD, Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974
Magnetospheric phenomena occurring at the highest geomagnetic latitudes (above approximately 70°) are produced by plasma physics phenomena on the dayside magnetopause and along the boundary of the geomagnetic tail (e.g., Troitskaya and Bol'shakova 1977; McHarg and Olson 1992). Unfortunately, measurements at very high geomagnetic latitudes, whether made in space or on the ground, present difficult challenges—for many different reasons. The plasma phenomena are most often highly localized in space and quite highly time variable on scales ranging from electron and ion gyrofrequencies to bulk plasma flow velocities. The Polar Experiment Network for Geophysical Upper-atmosphere Investigations (PENGUIN) program, using automatic geophysical observatories (AGOs) that are designed for the harsh antarctic environment, was devised specifically to place geophysical investigations at very high latitudes (the 1994 review issue of Antarctic Journal contains initial reports from the AGO program). The present AGO program is designed to investigate the latitudinal dependence of magnetospheric and ionospheric phenomena to 90°. The "final" installation of the AGOs includes two locations at approximately 74°S (around Amundsen-Scott South Pole Station), two locations at approximately 80°S, and the fifth and sixth units spanning 90°S latitude, separated by 12 hours in local time.
Active analysis is now in progress to use the acquired data to obtain new understanding of high latitude, upper atmosphere phenomena. Presented in this report is an example of some ongoing research in studies of long-period (approximately 500-second) Alfven waves at these latitudes. Data are presented from the AGO stations P3, P1, and P4, and the South Pole Station (figure 1).
Shown in figure 2 are the results of dynamic power spectral analyses of data (south-north, H, component) from the four stations for 3 March 1995. This day was quiet geomagnetically, with an average Kp=1+. Local noon and midnight are indicated by open and closed triangles, respectively. This day is only one of many being analyzed and is chosen because it illustrates some of the new information that can be derived from such spaced high-latitude locations. Four-second data values for 60 minutes were used to calculate each spectra. Each successive spectra is computed from a data set that has been incremented in time by 15 minutes from the end of the previous set. Each spectra is calculated using five prolate spheroidal data windows (e.g., Thomson 1982) in the time domain prior to calculating the spectra using a fast Fourier transform algorithm. After calculating a spectrum, a second-order polynomial is fit to the spectra. The fit is then subtracted from the computed spectrum, and the residual values are plotted as gray-coded values in figure 2.
Low decibel (dB) values of the spectra at approximately 1 millihertz (mHz) in all of the panels are artificial "enhancements" in the power, resulting from the subtraction of the quadratic fit to the individual spectra, and should not be interpreted. Such "enhancements" occur in all panels during the nighttime hours, about 20 universal time (UT) to about 03 UT. At P1 and P4, these false power enhancements occur at other times as well, because the geomagnetic fluctuations are quite low during this quiet day at these 80° latitude stations.
Enhancements in power appear at the 80° latitude stations at about 12 mHz around 10 UT and at about 8 mHz near 23 UT. There are also small increases in the power levels between approximately 1 and 2 mHz in the local morning and prenoon hours.
The most significant features in the four spectra in figure 2 are the strong enhancements of the power in the range approximately 2 to 5 mHz during the local morning hours at P3 and South Pole. No similar accompanying increases are evident at the higher latitudes at P1 and P4.
Thus, there is a dramatic difference in the dominant frequency in the Earth's space environment over a spatial scale of only about 5° to 6° in latitude. A similar change in frequency across a narrow latitude range was found in studies of Alfven waves in the region of the Earth's plasmapause (e.g., Fukunishi and Lanzerotti 1974), where ground-based data in Quebec and the northeastern United States and in the conjugate region at Siple Station, Antarctica, were used for the investigations. The "disruptions" in the dynamic spectra at P3 and South Pole that occur during hour 17 UT in figure 2 are associated with a switch in direction of the interplanetary magnetic field from a northward to a southward direction (not shown here). Such a shift in field direction produces enhanced geomagnetic activity in the magnetosphere.
Although analysis and interpretations of the new measurements, acquired over a wide range of geomagnetic states of the magnetosphere, are ongoing, it is possible to interpret qualitatively the spatial effects shown in figure 2 as evidence of a spatial gradient in the plasma parameters of the dayside magnetosphere. This gradient is probably the magnetopause. This is similar to the studies of Fukunishi and Lanzerotti (1974), where the plasma gradients they mapped were produced by the plasmapause interior to the magnetosphere. In this study, the higher frequencies seen at the lower latitude stations are characteristic of the frequencies of the "last" closed field lines of the dayside magnetopause under quiet geomagnetic conditions (Samson, Jacobs, and Rostoker 1971). Much of the time difference between the two stations South Pole and P3 when the shift to higher frequency occurs arises from the local time and slight latitudinal differences of these two locations (P3 is at a slightly lower geomagnetic latitude than is South Pole and earlier in local time; see figure 1). Thus, from a local night polar location, station P3 will rotate under the magnetopause and into the closed field, dayside magnetosphere region before South Pole.
The interpretation of the results in figure 2 in terms of the magnetopause boundary is different than that of McHarg, Olson, and Newell (1995) wherein they interpret a narrow-banded feature at approximately 5 mHz obtained by a search coil magnetometer at Svalbard (about the same magnetic latitude as South Pole) in terms of particle precipitation in the magnetospheric cusp region. At this time in our understanding, such a particle precipitation interpretation would not readily account for the facts that the frequency enhancements that we report here are seen both at South Pole and at P3 (which is at a lower geomagnetic latitude than South Pole), and for the frequency shift with local time during the time of observations, especially the decrease in frequency as local noon is approached. These aspects of the new observations are being examined further, as are other similar events that can be used to define the dayside structure of the magnetopause.
The research at South Pole and at the AGO stations was supported in part by the Office of Polar Programs of the National Science Foundation and is conducted in collaboration with colleagues at the University of Maryland.
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