Auroral latitude Pc 5 field line resonances: Quantized frequencies, spatial characteristics, and diurnal variation

Abstract

On the basis of observations with the Goose Bay Radar, several stations of the the Canadian Auroral Network for the Origin of Plasmas in the Earth's Neighborhood (OPEN) Unified Study (CANOPUS) magnetometer array, and a CANOPUS meridian scanning photometer, recent work has indicated the existence of Pc 5 ULF waves with discrete and remarkably stable frequencies (1.3, 1.9, 2.7 and 3.3 mHz with <10% variation) in the local midnight and early morning (predawn) sectors. These ULF waves were interpreted as the consequence of field line resonances driven by MHD waveguide/cavity modes, possibly excited by impulsive stimulation of the magnetopause or by Kelvin-Helmholtz instabilities in the low-latitude boundary layer. However, the reported stability of these frequencies over long periods of time remains a puzzle, as it seems to be inconsistent with the dynamic behavior of the magnetosphere. In order to establish if these frequencies are also observed in the dayside magnetosphere, where one of the proposed source mechanisms predicts they should be seen, we have examined the temporal and spatial characteristics of Pc 5 pulsations recorded recently with the CANOPUS magnetometer array. Pure state filtering techniques were used to determine the Pc 5 signal characteristics and their temporal and spatial variation. The results show that ULF wave signals with quantized frequencies seem to be observed throughout the dayside and dusk magnetosphere. However, frequencies both higher than and between previously reported values were also seen. The difficulty in correctly determining fundamental frequencies and their stability in the presence of polychromatic, highly amplitude-modulated signals is noted. The signal characteristics observed in the posmoon sector were found to be significantly different compared with those seen at prenoon. While local morning and noon Pc 5 revealed typical field line resonance characteristics, toroidal mode signal characteristics prevailed in the local afternoon and evening. The reason for this asynmaetry is unknown. The azimuthal wave numbers were generally small and showed a linear dependence on the Pc 5 signal frequency, implying antisunward phase velocities independent of the wave frequency.

Pc5 geomagnetic field fluctuations at discrete frequencies at a low latitude station

“…∼0.1 mHz). On the other hand, Ziesolleck and McDiarmid (1995) and, more recently, Mathie et al (1999) suggested that there is no unique set of waveguide mode frequencies; the CMS are dominant against a background spread of frequencies.…”

“…Pioneering investigations by several authors Samson et al, 1991Samson et al, , 1992Walker et al, 1992;Ziesolleck and McDiarmid, 1994) reported evidence at auroral latitudes of long period ULF waves in F-region drift velocities and in the local geomagnetic field components at discrete frequencies (of the order of 1.3, 1.9, 2.6 and 3.4 mHz). According to the theoretical suggestions provided by Radoski (1974) and extended by Southwood (1985, 1986), these modes were interpreted in terms of field line resonances driven by compressional modes of the magnetosphere which would act as a resonant cavity (with eigenfrequencies depending on its dimensions and physical characteristics) when excited by external mechanisms such as pressure pulses of the solar wind (SW).…”

“…Samson et al (1991) and Walker et al (1992) also found that the discrete frequencies (also known as CMS frequencies, after the cavity mode model proposed by Samson et al, 1992) were stable within 5-10% over time intervals ranging between a few hours and several months. On the other hand, Ziesolleck and McDiarmid (1995), who analyzed data from the CANOPUS array (corrected geomagnetic latitude between 61 • and 73 • ), suggested that these apparently stable frequencies do not necessarily represent a unique set. In a recent paper, Mathie et al (1999) examined the statistical characteristics of ULF pulsations observed by the IMAGE array (corrected geomagnetic latitudes between 56 • and 76 • ) and found that CMS frequencies (1.2-1.4, 1.8-2.0, 2.4-2.6, 3.2-3.4 mHz) are very prominent in ground observations but their results do not support a day-to-day stability of the spectral peaks.…”

Some evidence of ground power enhancements at frequencies of global magnetospheric modes at low latitude

“…Samson et al (1992) and Ziesolleck and McDiarmid (1994) showed that these peaks of power can also be identified in the simultaneous fluctuations of the local ground magnetic field. These events typically are of long duration, have stable frequencies (within 10%) and most frequently occur near local midnight and in the very early morning, although Ziesolleck and McDiarmid (1994) recently proposed similar observations in the dayside hemisphere. Following the original suggestion by Kivelson and Southwood (1985), these oscillations are currently interpreted in terms of field line resonances excited by compressional modes of the magnetospheric cavity Walker et al, 1992).…”

A comparison of field-line resonances observed at the Goose Bay and Wick radars

“…Thus the significance of these observations has been open to debate. Ziesolleck and McDiarmid (1994) employed ground-magnetometer data in search of these monochromatic field-line resonances, but their results proved inconclusive. In view of this a study of the data from the SABRE (Nielsen et al, 1983) radar at Wick has been undertaken to establish whether observations taken from a lower latitude, at a different longitude and with a different radar system would exhibit the same structured spectra.…”