Abstract
Abstract. A long lasting narrow-band (4–7 mHz) Pc5 fluctuation event at geosynchronous orbit is presented through measurements from GOES-8 and GOES-10 and the response of energetic electrons with drift frequencies close to the narrow-band pulsation frequency is monitored through a spectral analysis of flux data from the LANL-SOPA energetic electron instrument. This analysis shows electron flux modulations at the magnetospheric pulsation's frequency as well as at various other frequencies in the Pc5 range, related to the particles' drift-frequencies and their harmonics. A drift resonance effect can be seen, with electron flux modulation becoming more evident in the energy channels of electrons with drift frequencies closer to the wave frequency; however no net increase or decrease in energetic electron flux is observed, indicating that the net energy transfer and transport of electrons is not significant. This Pc5 event has a long duration, being observed for more than a couple of days at geosynchronous orbit over several traversals of the two GOES satellites, and is localized in azimuthal extent. Spectral analysis shows that most of the power is in the transverse components. The frequency of the narrow-band event, as observed at geosynchronous orbit shifts during the time of the event from 7±0.5 mHz to about 4±0.5 mHz. On the ground, CARISMA magnetometers record no distinct narrow-band fluctuation in the magnetic field, and neither does Geotail, which is traversing the outer magnetosphere a few RE further out from geosynchronous orbit, at the same UT and LT that GOES-8 and -10 observe the pulsations, suggesting that that there is no connection to external fluctuations originating in the solar wind. An internal generation mechanism is suggested, such as could be provided by energetic ring current particles, even though conclusive evidence could not be provided for this particular event. Through a statistical study, it is found that this event belongs to a class of similar events, occurring predominantly in the post-noon region in the inner magnetosphere.
Highlights
Geomagnetic pulsations in the Pc5 range of frequencies are Ultra Low Frequency (ULF), large-scale oscillations in Earth’s magnetic field
There are several observations that have attributed the pulsations to external oscillatory sources in the solar wind: for example, Sarafopoulos (1995) reported Pc5 polarization signatures in the lobes that are very similar to those in the solar wind; Ohtani et al (1999) investigated a narrow-band event which showed the same oscillation frequency on the ground, at geosynchronous orbit and in the morning-side flank of the magnetosphere and attributed the excitation to a Kelvin-Helmholtz instability at the magnetopause; and Kepko et al (2002) reported correlations between multiple discrete frequencies in the solar wind number density, dynamic pressure, and in the geosynchronous magnetic field: they concluded that variations in the solar wind preceded and drove the geosynchronous signatures
In order to investigate if there are such periods of consistent phase difference we performed a comparison between the phases of GOES-8 and GOES-10 magnetic field on 18 and 19 February, when both spacecraft were in the active region
Summary
Geomagnetic pulsations in the Pc5 range of frequencies are Ultra Low Frequency (ULF), large-scale oscillations in Earth’s magnetic field Such pulsations are often observed in the magnetosphere: some events show a single spectral peak There are several observations that have attributed the pulsations to external oscillatory sources in the solar wind: for example, Sarafopoulos (1995) reported Pc5 polarization signatures in the lobes that are very similar to those in the solar wind; Ohtani et al (1999) investigated a narrow-band event which showed the same oscillation frequency on the ground, at geosynchronous orbit and in the morning-side flank of the magnetosphere and attributed the excitation to a Kelvin-Helmholtz instability at the magnetopause; and Kepko et al (2002) reported correlations between multiple discrete frequencies in the solar wind number density, dynamic pressure, and in the geosynchronous magnetic field: they concluded that variations in the solar wind preceded and drove the geosynchronous signatures. A long term study of the occurrence distribution of similar narrow-band ULF pulsations is presented, as observed during an 8-year interval by GOES-8 at geosynchronous orbit
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