1 Introduction

Low frequency (1–100 Hz) oscillations of the geomagnetic field, also called geomagnetic pulsations, are the ground signatures of magnetospheric ultra-low-frequency (ULF) hydrodynamic waves which, particularly at low and mid frequencies (1–100 mHz), originate from the interaction of the solar wind (SW) with the magnetosphere. In the magnetosphere, both transverse (or Alfven) waves and compressional waves can be excited (Samson 1991). When a perturbation sets field lines into azimuthal motion, transverse waves occur. They propagate along the field lines and may resonate by forming standing waves between conjugate ionospheres where the ends of field lines are fixed. Like waves in a string, Alfven waves can satisfy the reflection condition only for selected wavelengths. Then, the periods of the resulting standing waves are quantized and are characteristic of the local field line depending on both the length of the line and the distribution of plasma along it. So, longer field lines, which map at higher latitudes, have longer resonant periods. On the other hand, compressional waves occur when a perturbation compresses the field lines producing changes in the meridional plane. They propagate across field lines and can be trapped in resonant cavities. At frequencies of the order of few mHz, modes can be trapped between the magnetopause and an inner boundary (or turning point) just outside the plasmapause where the increasing Alfven speed leads to reflection (Fig. 3 in Waters et al. 2000). Beyond the inner turning point, compressional modes damp but the evanescent waves can excite the resonance on the field lines with matching eigenfrequencies. On the ground, at both high and low latitudes, pulsations with frequencies of few mHz (Pc5 pulsations, 1–7 mHz) have been observed at stable, discrete frequencies, namely at ∼1.3, 1.9, 2.4, 3.2 mHz, often associated with the arrival of interplanetary shocks (Samson et al. 1991; Francia et al. 1997; Mathie et al. 1999; Villante et al. 2001). They were interpreted in terms of magnetospheric cavity modes. More recently, however, Kepko et al. (2002) presented several cases in which fluctuations in the SW pressure and magnetospheric field occurred at the same discrete frequencies which often matched some of the frequencies attributed to cavity modes. They suggested that fluctuations at discrete frequencies may be an inherent property of the SW and a source of magnetospheric pulsations. Moreover, Kepko and Spence (2003) speculated that solar p-mode oscillations might be the ultimate source for such fluctuations in the SW and magnetosphere. The availability of long series of data at our station of L’Aquila (Italy, CGM latitude ∼36° N, MLT = UT + 1:37, MLT being the magnetic local time) allowed us to conduct a statistical analysis through 1998–2002. We found several events of pulsations driven directly by SW compressive fluctuations and, in some cases, also evidence for amplification processes possibly due to cavity mode resonances (Villante et al. 2007). Now, we will further examine one of such events, which is clearly observed along a meridional array in Europe consisting of eleven stations, approximately at the same MLT as L’Aquila, spanning the latitude range between ∼42° N and ∼67° N.

2 Experimental Results and Conclusions

In Fig. 1 (left) we show the geomagnetic field variations along the array and the SW density data during the event A analyzed by Villante et al. (2007). The event occurred on August 1, 1998, in the time interval 1730–1930 UT corresponding to evening hours at the array stations.

Fig. 1
figure 1

Left: From the bottom: the solar wind number density (WIND spacecraft) and the 1-min values of the geomagnetic field horizontal component H along the array during the event A (Villante et al. 2007). The station corrected geomagnetic latitude is listed in the right side of each panel. Right: Power spectra (mT2Hz) of the H component at the different stations during the interval 1730–1930 UT

Full size image

The field variations are very similar up to 65° and closely correspond to the variations in the SW density (lower panel), while at higher latitudes the pattern is modified by the auroral ionospheric currents. For the time interval of interest, the power spectra (Fig. 1, right) confirm up to 61° the presence of power peaks at ∼1.3, 2.2, and 3.2 mHz, which were already observed at L’Aquila and in the SW density and pressure by Villante et al. (2007). At 63°–65° the 3.2 mHz peak is strongly amplified and reaches a maximum. It disappears at ∼66°, while considerable power emerges at slightly lower frequencies. Such behavior might indicate a possible field line resonance. On the basis of our results we conclude that SW compressional waves can drive magnetospheric pulsations, which in turn can be amplified through cavity and field line resonance processes. It will be interesting to search the source of the periodic SW variations, investigating the possibility that they are related to fluctuations in the outer layers of the Sun.