Simultaneous Dayside Broad-Band Spectra Observed over Two Different Magnetic Latitudes in Eastern Europe
Walter Goedecke
Zoltan Voros, Geophysical Institute of Slovak Academy of Science
Alpar Kormendi, Eötvös Loránd Geophysical Institute
Chris T Russell, Institute of Geophysics and Planetary Physics at UCLA
Presented at Spring 1999 GEM Meeting
Introduction
In Fall of 1996 two ground magnetometer stations were installed in Eastern Europe at nearly the same magnetic meridian, but differing magnetic latitudes to allow for phase difference measurements. The purpose of these stations is to provide greater observational coverage of the inner magnetosphere for both space physics and space weather needs. Typical geomagnetic field line resonances were observed, including some very unusual dayside broadband events featured in this report.
The station locations are at Srobarova, near Hurbanovo, in the Slovak Republic, and its complementary station south of this at Tihany, Hungary, with magnetic arc separation of 1.080 (see Figure 1, Figure 2). The Slovakian and Hungarian stations are run by the Geophysical Institute of Slovak Academy of Science (GISAS) and by the Eötvös Loránd Geophysical Institute (ELGI) respectively. The latter area is where Loránd Eötvös conducted gravity experiments around the turn of the century. The magnetometer station locations are tabulated below.
Magnetometer Station Locations |
|||||
| Location | Geographic |
Geomagnetic (degrees) |
L Shell | ||
| Latitude | Longitude |
Latitude |
Longitude |
||
| Srobarova, Slovakia | N 47.8136 | E 18.3111 | N 42.86 | E 93.10 | 1.861 |
| Tihany, Hungary | N 46.9029 | E 17.8843 | N 41.78 | E 92.51 | 1.799 |
The data acquisition system consist of the following components: a highly accurate fluxgate magnetometer, a personal computer, a 23 bit analog-to-digital converter, a global positioning (GPS) receiver, and a separate hard drive with removable disk. Figure 3a shows the data acquisition system at Tihany, Hungary, and Figure 3b shows a block diagram. The three measured magnetometer components are sampled every second, with resolution to .01 nT.
The Amplitude and Phase gradient Technique
Using the amplitude and phase gradient methods developed by Baransky et al. [1985] and Waters [1991] respectively between two stations, magnetospheric field line resonances stand out against other coherent magnetospheric events. Anomalous low power and incoherent high frequency events are masked out on spectra plots for visual clarity.
In the first method the magnetic field amplitude at the equatorward station is divided by that of the poleward station. During a typical dayside pulsation event as seen in Figure 5a, the amplitude ratio traced from low to high frequency reveals a minimum and then crosses to a maximum. The unity ratio crossing is where the resonant frequency is midway between the individual L-shell resonances. The second phase method compares spectral phases; a maximum indicates a resonant condition (see Figure 5b).
Distinguishing Time Delayed Signals from Field Line Resonances
When similar magnetospheric signals are observed at two stations with merely a time shift, time lag analysis is used. Geomagnetic field lines may not be resonating, but alternatively, or at least in part function as energy conduits, transferring energy from the magnetosheath, with little or no standing wave structure due to ionospheric reflection. The result would be an absence of frequency dependent phase shifts.
A time shift analysis is chosen over a cross-phase analysis is this case; units of seconds will measure the time lead, unlike a phase shift analysis using degrees. When signals are time shifted, the advantage of analyzing the data is that the time shift will be constant for all frequencies; phase difference analysis will distort this information. The technique is exemplified in the next section.
Examples of Field Data
The pulsation activity observed in Eastern Europe is very diverse, as given in the following three examples.
Typical Resonance Example observed on January 9, 1998
Simultaneous power plots for both locations are shown in Figure 4. Figure 5 depicts a 50 mHz monochromatic pulsation signature of about 8 hours in duration. Figure 5a, the ratio of amplitude of magnetic field at Tihany to Srobarova, shows a minimum to maximum crossing at 50 mHz indicating a resonating system. The equatorward station (Tihany) will observe higher field line resonance than the poleward station (Srobarova) since the former has a footprint of shorter field line on it. 50 mHz represents a frequency between the two frequencies observed at the two stations. Underscoring this, the phase difference plot of Figure 5b shows a 400 phase maximum also at 50 mHz.
Example with Bursts on March 6, 1997
In Figure 6, there is prominent pulsation activity starting at 14:00 and 16:00 UT, in addition to less powerful events. Frequent daytime observations in Eastern Europe showed short-duration broadband events similar to Pi’s but of longer duration. In Figure 6b, these two prominent pulsation bursts have a phase difference that ranges from 200 to 1400 over the 10 to 60 mHz range. A conversion from phase to a time lead basis reveals constant time shifts between the two stations, as seen in Figure 6c. The inner L-shell station receives the signal first, then the outer L-shell station. Several time shifted segments standout: a 6 second shift at 14:00, a 9 second shift at 16:00, a less prominent 6 second shift at 6:00, and some 2 second shifts at 8:30, 12:00, and 17:00. Such time shifts would confirm a traveling mode over a standing mode, since magnetic field lines with standing wave characteristics would have both a frequency dependent time shift and amplitude.
Other dayside time shifts appear faintly in Figure 6c. Figure 7, a cross-correlation plot, shows bursts at 14:30 and 16:30 UT as 7 and 10 second time shifts, the latter with greater coherence. Another burst appears at 17:20 of 1 second shift, again with strong coherence; this was not quite as apparent in Figure 6c, thus showing the merit of this type of plot.
Figure 8 shows as sample of time-domain waveforms at both stations at 14:30 to 14:40 UT. Note the near constant time shift of both horizontal and declination components. The ground measured declination component, representing the poloidal mode above the ionosphere, has almost the same power as the toroidal mode, represented as the horizontal component on the ground. In Eastern Europe, as in the Western United States, pulsation events are observed on the ground as both horizontal and declination components.
Analysis of the 14:00 UT burst, submitted by Zoltan Voros, are seen in Figures 9a and b. Analysis starts at 14:23 UT and continues for 1024 seconds. Here these assumed broad band events are actually distinct frequencies that vary through time. The cause of these events is as of present unknown, but are assumed to be a natural activity, since the waveform is observed at both locations 106 km apart.
Example with Large Pulsation Activity on September 21, 1997
There were several days of uncommonly large pulsation activity: one such day, on September 21, is seen in Figures 10 and 11. As in the previous example of March 6, 1997, the pulsation activity has a large bandwidth, similar to the bursts shown in Figures 10 and 11, however in this case the overall duration is long, from 6:30 to 19:00 local time (4:30 - 17:00 UT). Additionally, during this day of high pulsation activity, there are Pi events that extend into the evening.
The constant time shift of magnetic signal at Srobarova after Tihany would indicate a traveling wave structure rather than true resonance; a frequency dependent time shift and amplitude occur during a resonance. A 6 minute daytime sample is shown in Figure 12. Note the superposition of both 15 and 200 second period waves, and the nearly constant 4 second time shift between ground arrival of the signal at both stations, again indicating a propagating mode.
Summary
References
Baransky, L. N., Borovkov, Yu. E., Gokhberg, M. B., Krylov, S. M., and Troitskaya, V.
A., 1985, High resolution method of direct measurement of the magnetic field line’s
Eigen frequencies, Planet. Space Sci, v. 33, p. 1369-1380.
Waters, C. L., Menk, F. W., and Fraser, B. J., 1991, The resonant structure of low latitude Pc 3 geomagnetic pulsations, Geophys. Res. Lett., v. 18, p. 2293-2296.
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