Statistical survey of auroral latitude Pc 5 spectral and polarization characteristics

Abstract

Recent magnetometer and HF radar observations have indicated the existence of multiple, discrete field line resonances with remarkably stable frequencies of 1.3, 1.9, 2.6, and 3.4 mHz, apparently driven by MHD waveguide/cavity modes in the magnetosphere. Given the dynamic nature of the magnetosphere, the apparent stability of these frequencies is difficult to understand, and it is therefore important to examine this question on the grounds of a larger database. In this paper we present a statistical survey of the occurrence of Pc 5 field line resonance frequencies based on 1 year of data from several stations of the CANOPUS magnetometer array. Pure state filtering techniques were used to investigate various statistical aspects of auroral latitude Pc 5 characteristics, with emphasis on the occurrence of preferential, discrete spectral peaks. A number of case studies exploiting the full spatial extent of the CANOPUS array were carried out to complement the statistics. Although the results provide further support for the existence of discrete (spatially monochromatic) field line resonances, we find that the apparently stable frequencies do not seem to be particularly distinguished from other Pc 5 frequencies, except perhaps the 1.9‐mHz peak. Discrete field line resonance characteristics were also observed for other sets of frequencies, and we therefore conclude that the apparently stable waveguide/cavity mode frequencies do not necessarily represent a unique set. The implications of these results with respect to various possible Pc 5 wave generation mechanisms are discussed.

Pc5 field line resonance frequencies and structure observed by SuperDARN and CANOPUS

“…Using 1 year of data from five CANOPUS magnetometer stations, Ziesolleck and McDiarmid [1995] examined the spectral and polarization characteristics of'auroral latitude Pc5 pulsations at all local times. Although they found further evidence of discrete FLRs, they also showed that the apparently stable frequencies of 1.3, 1.9, 2.6, and 3.4 mHz did not seem particularly distinguished from other Pc5 signal frequencies.…”

Geomagnetic field fluctuations during the passage at the Earth’s orbit of the tail of the 15–16 July 2000 ejecta

“…In addition, studies of individual events reported evidence of oscillations at the cavity/waveguide mode frequencies, which were associated with SW pressure pulses and extended to a major portion of the Earth's magnetosphere, confirming the global character of these fluctuations (Shimazu et al, 1995;Villante et al, 1998;Lepidi et al, 1999). It was observed that, at auroral latitudes, low-frequency pulsations show a clear morning/afternoon asymmetry, with an higher occurrence rate in the morning (Rostoker and Sullivan, 1987;Ziesolleck and McDiarmid, 1995;Chisham and Orr, 1997;Mathie et al, 1999) and that afternoon pulsations are driven dominantly by sporadic impulses in the SW (Rostoker and Sullivan, 1987). Consistent with the theory by Mann et al (1999), who investigated the energization of waveguide modes by magnetosheath flows on the magnetopause flanks, and with the observations by Mathie et al (1999) and Matthie and Mann (2000), the observed morning/afternoon asymmetry can be explained in terms of a greater stability of the postnoon magnetopause to shear-flow instabilities than the dawn flank; in this sense, pulsations driven by magnetopause instabilities during intervals of enhanced SW speed occur predominantly in the morning, while impulsively driven pulsations may extend over a wide range of local times.…”

ULF waves: 1997 IAGA division 3 reporter review

“…The question is how often these solar-windinduced magnetic pulsations contribute to the pulsations observed on the ground by radar or magnetometers. Most statistical studies of mHz-range pulsations on the ground (e.g., Ziesolleck and McDiarmid, 1995) have not fully incorporated solar wind data. It is of great importance to examine the state of the solar wind at times when magnetic pulsations are observed on the ground.…”

“…Ziesolleck and McDiarmid (1995) present a critical examination of the CMS frequencies based on the CANOPUS magnetometer data covering L 4.2±12.3. Although they con®rm the signature of multiple discrete ®eld-line resonances, they do not ®nd the pulsation frequencies to be as stable as claimed from radar observations (Samson et al, 1991;Walker et al, 1992), except perhaps for the 1.9 mHz component.…”