Evidence of transverse magnetospheric field line oscillations as observed from Cluster and ground magnetometers

Abstract

Abstract. The dynamic spectrum of ULF waves from magnetic field data obtained by the elliptically orbiting Cluster satellites (with an apogee of 119 000 km, perigee of 19 000 km and the orbital period of 57 h) have been prepared in the frequency range 0 to 120 mHz when the satellite was near its perigee. The existence of field line oscillations, with increasing frequency in the inbound sector and decreasing frequency in the outbound sector, is seen in the transverse components, indicating the presence of independently oscillating local magnetic flux tubes in the form of transverse standing Alfvén waves. The results show that toroidal and poloidal modes are excited simultaneously. The analysis of simultaneous ground magnetometer data at the footprint of the satellite suggests that these modes are also excited due to coupling to magnetospheric waveguide modes. The clear signature of a resonant fundamental mode is seen in the ground data whereas Cluster detects a harmonic of this frequency. Lower frequency modes indicative of waveguide oscillations are seen in both the ground data and the compressional field at Cluster.

Sudden impulses at geosynchronous orbit and at ground

“…C. Low frequency waves, occurring in relative localized regions soon after the SI manifestation have be usually interpreted in terms of field line resonant oscillations with latitude dependent frequency (Saito and Matsushita, 1962;Araki, 1994). Rather, we presented a case in which a transversal oscillation mode at fE3.2 mHz appears at E04:15 LT at geosynchronous orbit; a wave activity at the same frequency is practically ubiquitously observed between midnight and noon in ground measurements (Sinha et al, 2005), although with amplitude progressively decreasing with the LT distance approximately from 04:00 LT. Interestingly, the observed frequency attains to the set of ''discrete'' frequencies extensively detected in magnetospheric and ground observations and tentatively interpreted in terms of global cavity/waveguide modes of the entire magnetosphere (fE1.3, 1.9, 2.6-2.7 and 3.2-3.4, 4.0-4.2 mHz; Samson et al, 1991Samson et al, , 1992Walker et al, 1992;Mathie et al, 1999;Villante et al, 2001, Kepko et al, 2002Kepko and Spence, 2003;Viall et al, 2009). It reinforces the conclusions that the onset of such discrete frequency modes can be related to external stimuli such as the magnetopause impact of SW discontinuities as well as the Kelvin-Helmholtz instability of the magnetopause flanks.…”

Observations of PC5 pulsations near field-aligned current regions

“…The existence of discrete spectrum, the frequency dependence of Pc5 pulsations, the relationship between auroral electrojet and Pc5 ULF waves are still considered to be timely and important (Fedorov et al, 2001;Pilipenko et al, 2001;Baker et. al., 2003;Martines-Bedenko et al, 2003;Volwerk et al, 2005;Sinha et al, 2005;Nenovski et al, 2007;Sarris et al, 2007 and references there in).…”

Toroidal quarter waves in the Earth’s magnetosphere: theoretical studies

“…There have been many theoretical as well as observational studies related to the toroidal and poloidal oscillations which are excited on geomagnetic field lines (Cummings et al 1969;Anderson et al 1990;Engebretson et al 1986;Sinha et al 2005;Pilipenko et al 2005;Mager et al 2009;Bochev and Sinha 2010;Liu et al 2013;Takahashi et al 2013) The conversion of poloidal to toroidal mode is reported in few studies (Mann and Wright 1995;Leonovich 2001). Mann and Wright (1995) suggested that compressional poloidal waves have finite lifetime and gradually gets converted to Alfvén waves of toroidal polarization.…”