Ionospheric conductivity modulation in ULF pulsations

Abstract

Abstract. Using incoherent scatter radar and magnetometer measurements, we report that during terrestrial magnetic Pc5 pulsations in the afternoon sector, a modulation of particle precipitation and ionospheric conductivities by a factor of 2 occurs in addition to high-amplitude variations of electric and magnetic fields. The event thus seems to be considerably more complicated than previously studied ones where information about conductivities was mostly not available. Our groundbased data set gives us several clues about magnetospheric processes. The origin of the conductivity variations seems to be periodically modulated diffusion of hot electrons into the loss cone that is in turn caused by a ring current instability. The direction of the phase propagation of the observed disturbances is also consistent with the hypothesis of a ring current source. From the ionospheric electron densities we can roughly estimate the equatorial phase space diffusion rate which seems relatively high. In addition, strong electric field and Poynting flux variations suggest that intense coupling to shear Alfv•n modes happens in the magnetosphere. The latitudinal variation of power and wave polarization shows features of a field line resonance. Furthermore, power spectral analysis of conductivities, electric and magnetic fields, reveals that there is a turbulent-like background in all three parameters, which is of magnetospheric origin but modified by the ionosphere. The power law slope of the conductivity spectra is comparable to that of the electric field, while the ground magnetic field shows a steeper decrease with frequency because of the shielding of small-scale current structures. A clear anticorrelation between conductivities and the eastward electric field is interpreted as an ionospheric polarization effect, which transmits Alfv•n waves from the ionosphere upward. Finally, we show that due to the time-varying conductivities only the handedness (ratio of left-and right-handed components) of the Hall current is very close to that of the magnetic field, while the electric field has a significantly different polarization.

A statistical analysis of ionospheric velocity and magnetic field power spectra at the time of pulsed ionospheric flows

“…Previously, Buchert et al [1999] found similar differences between the power spectra of magnetometer data and electric field and conductivity measurements made with the EISCAT incoherent scatter radar. They suggested that these differences were due to the magnetometer being influenced by ionospheric currents flowing over a large area (of the order of a few hundred kilometers square), while the EISCAT radar measurements were taken from a much smaller ionospheric area (of the order of a few kilometmers square).…”

“…[10] Besides the Buchert et al [1999] case study, we know of only two other studies that have measured the ionospheric electric field spectrum (see Table 1). Furthermore, we have found no other estimates in the literature of the power spectral exponent of the ionospheric electric field or E Â B velocity measured directly with radar in this frequency range (though similar work has been carried out using SuperDARN radar to look at the power spectrum of gravity waves, e.g., Bristow and Greenwald [1997]).…”

“…[9] A complication in the comparison of the temporal structure of solar wind and ground geomagnetic data has been raised by Buchert et al [1999] who suggested that the exponent of a power law magnetic field power spectrum measured at the ground in the mHz frequency range is generally greater than that of the associated ionospheric current spectrum owing to the shielding of small-scale current structures from the ground. They observed a power law-like spectra with a = À4.5 for 3 F 20 mHz, when examining the X and Y components of the magnetic field measured by the IMAGE magnetometers at Tromsø and Kilpisjärvi from one 2.5 h sample on 18 October 1993 in the afternoon auroral zone.…”

“…The grey straight line indicates a power law with a = 1.4. Buchert et al [1999] 4.5 3 -20 2.5 hours 66°N…”

“…Electric Field Measurements Buchert et al [1999] 2 4 -9 2.5 hours 66°N Bering et al [1995] 1.5 0.4 -30 8 hours 74°S Weimer et al [1985] 1.8 0.001 -0.1 km À1 2 passes 55 -70°inv. lat threshold, with the same power law exponent, independent of threshold, for durations between $1 and >100 min.…”