Pi2 pulsations are known to be an important signature of substorm processes in the magnetosphere, and their analysis plays an important rule in understanding substorm dynamics and plasma wave processes.

The geomagnetic pulsations observatory at Teoloyucan, México, has observed various examples of Pi2 pulsations. In this work we analyze these kind of pulsations compared with those pulsations observed simultaneously in the CANOPUS network. We made a statistical analysis of some characteristics of the events. We also made the comparison of the observed morphology of the signals seen in Teoloyucan with those observed in CANOPUS for three selected days and we perform the polarization analysis to evaluate the propagation of the signals. We also discuss our results within the context of the present theoretical understanding of these pulsations events.
 
 
 
 

The geomagnetic station of Teoloyucan (L=1.31) is located in the state of Mexico near Mexico City and CANOPUS network of stations are located in Canada as you can see in this world map, Figure 1.
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The table shows the geographic and geomagnetic coordinates of the stations, Figure 2.
 
 

Electrical diagram of the induction magnetometer used to observe the signals at Teoloyucan, Figure 3.
 

We selected 40 Pi2 pulsations events observed in Teoloyucan and compared with CANOPUS data, Figure 4. Some characteristics are shown in the table as: the onset of occurrence, maximum amplitudes and the magnetic activity, Teoloyucan is not shown because the amplitude is very small. All events occur during a relative calm in the magnetic activity, that means the occurrence of many subtorms and few magnetic storms or small or weak magnetic storms.

We can not see a clear dependence between magnetic activity and latitude or local time. Also you can not see a clear dependence between the maximum amplitude and the magnetic index values.
 

All the signals occur around the midnight local time, Figure 5.
 
 

In the table you can see that the maximum amplitude of the signals did not occur at the same time in all stations. This histogram shows the intervals of dispersion in time of the maximum amplitudes. You can see that 50% of the events are dispersed between 2 – 4 minutes and 4 – 6 minutes, Figure 6.
 

This histogram shows the principal total amplitudes of the Pi2. Around 50% of the events have amplitudes between 50 – 100 nT and 100 – 150 nT. The total amplitude in Teoloyucan is always less than in any other station, Figura 7.
 

This histogram shows that the X component in most cases is greater than the other components, Figure 8.
 

This histogram shows the duration of the events. You can see 15 events have duration between 6 – 8 minutes, 8 events between 4 – 6 minutes y 8 events between 8 – 10 minutes. The rest of them correspond to shorter and longer duration, Figure 9.
 

Here we can see the X, Y, and Z components of the Pi2 pulsations observed in Teoloyucan and CANOPUS network on DAY013. From top RANK to TEOL we have a latitudinal arrange, these stations are close to the same meridian. It is also shown its corresponding polarization analysis in the peak frequency of 23.75 mHz. From these figures you can see that only GILL, ISLL and PINA shows pulsations as in Teoloyucan inside the same time interval. The other stations of CANOPUS at other longitudes show some pulsations inside same interval principally in RABB and MCMU. Polarization parameters looks like to be chaotic in high latitude except between PINA and TEOL where we can see a change only in ellipticity, Figure 10a, Figure10b, Figure 10c, Figure 10d.
 
 
 

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Here we can see the X, Y, and Z components of the Pi2 pulsations observed in Teoloyucan and CANOPUS network on DAY017. Again From RANK to TEOL we have a latitudinal arrange and its corresponding polarization analysis in the peak frequency of 21.25 mHz. From these figures you can see that only GILL and PINA shows clear pulsations as in Teoloyucan inside the same time interval. The other stations of CANOPUS at other longitudes show some pulsations inside same interval almost in all stations. Polarization parameters looks like to be not so chaotic in high latitude principally between ESKI to PINA, but between PINA and TEOL we can see a change in the ellipticity and orientation of the polarization ellipse, Figure 11a, Figure11b, Figure 11c, Figure 11d.
 
 
 

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Finally, these last figures show the X, Y, and Z components of the Pi2 pulsations observed in Teoloyucan and CANOPUS network on DAY044. Again From RANK to TEOL we have a latitudinal arrange and its corresponding polarization analysis in the peak frequency of 20.6 mHz. From these figures you can see that in all latitudinal arrange from RANK to TEOL shows clear pulsations inside the same time interval even all the other stations of CANOPUS at other longitudes show some pulsations inside same interval. Polarization parameters looks like to be chaotic in high latitude, but between PINA and TEOL we can see there are not change in the parameters, Figure 12a, Figure12b, Figure 12c, Figure 12d.
 
 
 

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Observations from other stations show that Pi2 pulsations observed at low latitudes have strong H component, but that the D component weakens close to the equator (Kitamura, et al., 1988. J. Geomagnetism and Geoelectr., 40, 621 – 634; Lin et al., 1991. J. Geophys. Res. Vol. 96, No. A12, 21, 105 – 21, 113; Yumoto et al., 1994. J. Geomagnetism and Geoelectr., 46, 925 – 935). The observations from Teoloyucan are in accordance with these results, as we see a dominant H component in all events and in general a weak, or absent, D component. Therefore we find that the signals are stronger polarized.

Present theories of Pi2 pulsations observed at low latitudes are based on ideas of cavity mode oscillations of the inner magnetosphere (Saito and Matsushita 1968, J. Geophys. Res. 73, 267; Kivelson and Southwood 1986. J. Geophys. Res. 91, 4345; Allan et al. 1986. Planet Space Sci. 12, 1189 and 34, 371; Yumoto et al. 1987.) Sutcliffe and Yumoto (1989, Geophys. Res. Lett. 16, 887) investigate Pi2 pulsations at low latitudes and interpreted these pulsations as being associated with a global cavity mode.

Yeoman and Orr (1989, Planet Space Sci. Vol. 37, No. 11, 1367 – 1383) has received support from comparison between theory and observation. Fukao et al. (1993, 94^th SGEPSS Fall Meeting Abstracts, A31 – P2 – 49) proposed that at low latitudes (l£ 50°), H component Pi2 pulsations are associated with global cavity mode oscillations in the inner magnetosphere, while D components must be caused by a current intensity variation of the substorm current wedge.

All 40 events seen in Teoloyucan and compared with CANOPUS observations shown some clear coincidence in time, also we saw some similarity in their morphology and frequency content. This support the idea of cavity resonator.

The power spectrum of the selected events seen in Teoloyucan shows some maximum peaks in the frequencies: 23.75 mHz DAY013, 21.25 mHz DAY017, and 20.60 mHz DAY44, these frequencies correspond to eigenperiods of the cavity resonator plasmaspheric (Lester and Orr, 1983), they have a catalog of frequency ranges for the cavity between 20 mHz to 25 mHz (40 to 50 seconds of period).

From the polarization analysis, the change of ellipticity and the orientation of the polarization ellipse from PINA to TEOL shows maybe a change of plasma environment during the propagation of the signal as it is shown in the DAY013 and DAY017, but in the DAY044 maybe PINA and TEOL are in the magnetic field lines inside the plasmasphere. Changes in the polarization parameters near the auroral oval and the plasmapause have been seen in some results by Saito, Sakurai and Koyama (1976) and Yeoman and Orr (1989).

However, to have a better analysis using data of Teoloyucan it is necessary to get data from other station in midlatitudes as in Boulder site and low latitudes like in Mexico itself to verify the morphology and frequency content of the signals and to check possible harmonics. Also it is important to have satellite data to verify the plasma involve and localize the generation region.

Finally I would like to mention that we have new equipment installed in Teoloyucan made by the Professor C.T. Russell group of the University of Los Angeles, UCLA and this system has been working from 1998, below are shown some photographs of the equipment installed: The Figure a
shows the sensors installation, Figure b shows the background of Teoloyucan observatory where we can see the position of the sensors, in the Figure c  we can see the PC computer where it is installed the acquisition system and GPS system, finally the Figure d shows other aspect of Teoloyucan station where we can see the building where it is installed the instrumentation.
 
 
 

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This work was supported by research grant project G102 by the Geophysical Institute, University of Mexico, UNAM. We would like to thank Professor G. Rostoker to facilitate the data of CANOPUS network of stations used in this work and for his helpful advice and suggestions. We also want to thank Professor J.V. Olson to permit to use his codes for polarization analysis and for his also helpful advice and suggestions.