J. Verö

Monitoring Schumann resonances—I. Methodology

Abstract The complex demodulation as a spectral technique has been used for the quasi-continuous determination of the actual frequencies of Schumann resonances. Applying this method, the first three modes of the vertical electric component have been measured regularly in the Nagycenk Observatory (47.6°N, 16.7°E) since May 1993.

Connections between interplanetary magnetic field and geomagnetic pulsations

Abstract On the basis of about 5000 hours of ground-based pulsation data it is shown 1. (a) that periods of the types Pc 2–4 follow the law 160 B [B being the magnitude of the interplanetary magnetic field], 2. (b) amplitudes are a maximum for the Pc 3 type, if the IMF vector has an angle of 30° with the Sun-Earth direction, 3. (c) minimum amplitudes of Pc 3–4 are found if the IMF vector is perpendicular to the Sun-Earth direction, 4. (d) longer period pulsations (Pc 5) are also influenced by IMF (decreasing amplitudes with increasing B). Furthermore the pulsation amplitudes generally depend upon geomagnetic activity (or on solar wind velocity). Some differences are also found in the daily variation of the amplitudes in cases of different IMF orientations.

Geomagnetic pulsations and parameters of the interplanetary medium

Abstract A comparison of one year of pulsation data with parameters of the interplanetary medium shows that there is a significant change in this connection at a period of about 30 s. Using other relevant material it is found that there are two types of pulsations, the first has regular shape, latitude dependent periods and strong dependence on interplanetary parameters, the other with opposite characteristics. A model is proposed to explain these two types of Pc 3–4 pulsations.

Pulsation periods at mid-latitudes—A seven-station study

Abstract A pulsation array was set up in 1984 of seven stations in Central and Northern Europe. High resolution dynamic spectra of a dozen events were analyzed. It was found that on several occasions latitude dependent and independent periods occur in the events simultaneously so that the changing period branch starts from the constant period one around L ∼ 2. The constant period corresponds to that expected on the basis of the actual scalar magnitude of the interplanetary magnetic field, the changing period increases sometimes to the highest latitude station in the array and may indicate the width of the primary spectrum. In 1984 the change of the periods with latitude is large, and that is perhaps why less latitude dependent events are found.

Whistler ducts and geomagnetic pulsation resonant field line shells near L = 2—are they identical?

Abstract Data from the processing of about 1700 whistlers recorded in Tihany ( L -value of propagation, equatorial electron density and tube content) are compared with geomagnetic pulsation characteristics (mainly of periods) as recorded at the nearby Nagycenk Observatory ( L ∼2). In addition, the number of whistlers recorded at Panska Ves (Czech Republic) for more than ten years (1971–1979, 1987, 1990) are compared with the Nagycenk pulsation activity. Both results can be reconciled with the supposition that whistler ducts and geomagnetic field line shells are closely connected with each other as they appear simultaneously with enhanced probability, and the position of these structures is similar within the magnetosphere.

Damping of geomagnetic Pc3-4 pulsations at high F2-layer electron concentrations

Abstract Analysis of Pc3–4 pulsations recorded over two years at a mid-latitude site in the northern and southern hemispheres shows that this activity bears an anticorrelation to increases in F -region electron concentration, being significantly diminished when foF 2 max exceeds some critical value, and during winter months. A smaller yearly minimum in activity is also indicated. There is a strong similarity to the F -region annual and winter anomalies. However, the likely source region of this damping lies above the F -peak, in the plasmasphere.

Changes of pulsation activity during two solar cycles

Abstract Monthly and yearly averages of the pulsation activity in the mid-latitude station Nagycenk are compared to solar wind velocity and ionospheric-plasmaspheric electron concentration data. It is found that pulsation amplitudes are correlated with solar wind velocities with the exception of some month around December in solar maximum years, when they are significantly lower than computed from the corresponding solar wind velocities. This decrease can be caused either by a cutoff of the magnetospheric shell resonances or by local ionospheric damping. In addition to these effects, pulsations amplitudes slightly depend also on the geomagnetic activity and have a semi-annual activity change with maxima around equinoxes.

Impulsive pulsation events and pulsation beats

Abstract Twenty-three pulsation events from 11 stations in Central and Northern Europe, and from two satellites, were surveyed for Impulsive Pulsation Events (IPEs) and for pulsation beats. IPEs generally had activity peaks at the same period at all stations studied, even if the spectrum of the background had latitude-dependent periods. From pulsation beats, the width of the coherent shells of field lines could be deduced; they were in coincidence with values obtained by other methods. As a conclusion, the existence (and sometimes coexistence) of slightly transformed upstream waves and shell resonances in the magnetosphere is confirmed.

Dynamic spectra of pulsation events at L ∼ 1.9 and L ∼ 3.3

Abstract High resolution dynamic spectra of 12 pulsation events are compared at NCK (L ∼ 1.9) and KVB (L ∼ 3.3), lying on the same meridian. Common features of the pulsation activity at the two stations include periods, period shifts and switches on and off. These parameters are investigated for the sample of pulsation activity and demonstrate that the two stations record activity from both a common and from different sources. Shell resonances are discussed in relation to the common source of activity.

Changes of geomagnetic micropulsations following sudden impulses

Abstract Micropulsation periods begin to drop even after negative geomagnetic impulses. This effect may be due to processes on the plasmapause, lasting about an hour after the impulse. The same process may be active after positive impulses, too.