Spectral structure of Pc3–4 pulsations: possible signatures of cavity modes

Abstract

In this study we investigate the spectral structure of Pc3–4 pulsations observed at low and midlatitudes. For this purpose, ground-based magnetometer data recorded at the MM100 stations in Europe and at two low latitude stations in South Africa were used. In addition, fluxgate magnetometer data from the CHAMP (CHAllenging Minisatellite Payload) low Earth orbit satellite were used. The results of our analysis suggest that at least three mechanisms contribute to the spectral content of Pc3–4 pulsations typically observed at these latitudes. We confirm that a typical Pc3–4 pulsation contains a field line resonance (FLR) contribution, with latitude dependent frequency, and an upstream wave (UW) contribution, with frequency proportional to the IMF (interplanetary magnetic field) magnitude <I>B</I>_IMF. Besides the FLR and UW contributions, the Pc3–4 pulsations consistently contain signals at other frequencies that are independent of latitude and <I>B</I>_IMF. We suggest that the most likely explanation for these additional frequency contributions is that they are fast mode resonances (FMRs) related to cavity, waveguide, or virtual modes. Although the above contributions to the pulsation spectral structure have been reported previously, we believe that this is the first time where evidence is presented showing that they are all present simultaneously in both ground-based and satellite data

A comprehensive analysis of the occurrence and characteristics of midperiod ULF waves at low latitude

“…In the past, some studies indicated the presence of long‐wavelength waves propagating toward noon at λ ≈ 54°–56° [ Green , ; Mier‐Jedrzejowicz and Southwood , ]; in addition, Ostwald et al [] identified, at λ ≈ −42° in the morning sector, waves in the range f ≈ 20–25 mHz propagating sunward from source regions likely located in the nightside. It is worth noting that Sutcliffe et al [] recently interpreted the occurrence of modes at f ≈ 20 mHz on both components at low and middle latitudes in terms of fast mode resonances related to cavity or waveguide modes in the plasmasphere. In the daytime hours, the major axis of the polarization ellipses is permanently oriented in the NW/SE quadrant. A 3 day analysis, at λ ≈ 42°–44°, allowed Lanzerotti et al [] to reveal that such axis maintained a primarily NW/SE orientation; they also speculated that geological characteristics (which, in general, may influence ellipticity and azimuth) were possibly responsible for the absence of the switch of the azimuthal angle observed around noon at higher latitudes [ Fukunishi and Lanzerotti , ].…”

“…In the past, some studies indicated the presence of long-wavelength waves propagating toward noon at λ ≈ 54°-56° [Green, 1976;Mier-Jedrzejowicz and Southwood, 1979]; in addition, Ostwald et al [1993] identified, at λ ≈ À42°in the morning sector, waves in the range f ≈ 20-25 mHz propagating sunward from source regions likely located in the nightside. It is worth noting that Sutcliffe et al [2013] recently interpreted the occurrence of modes at f ≈ 20 mHz on both components at low and middle latitudes in terms of fast mode resonances related to cavity or waveguide modes in the plasmasphere. 7.…”

Nighttime Pc3 pulsations: MM100 and MAGDAS observations

“…A possible mechanism that is responsible for the equatorial peak wave amplitude can involve the enhancement of the ionospheric currents induced by Alfven waves as they spread into the equatorial ionosphere with the elevated Cowling conductance (Yumoto et al 1992). Another possible mechanism is the direct penetration of compressional wave energy toward the near-equatorial region (Yumoto et al 1985;Waters et al 2000;Heilig et al 2007;Sutcliffe et al 2013). Active generation of magnetohydrodynamic (MHD) waves along the ionosphere by fluctuations of the equatorial electrojet can also contribute to the near-equatorial ULF wave activity .…”

“…These results have suggested a global cavity mode as a mechanism responsible for the spatial structure of low-latitude Pi2 pulsations. Typical period of low-latitude Pi2 pulsations, from a fraction of a minute to few minutes, is believed to be determined either by eigenperiod of the plasmasphere cavity mode (Sutcliffe et al 2013) or by a forced response to a periodic energy release in the magnetotail (Keiling and Takahashi 2011). Takahashi et al (2005) analyzed nightside Pc4s and Pi2s and found that these two types of ULF waves were generated by different physical mechanisms, although their central frequencies were close.…”

The role of SANSA's geomagnetic observation network in space weather monitoring: A review

“…The fast mode compressional wave energy drives field line resonances (FLRs) in the plasmasphere [ Waters et al , ; Ndiitwani and Sutcliffe , ], which can be pictured as transverse standing Alfvén waves along geomagnetic field lines with ends fixed at conjugate ionospheres. However, besides the UW and FLR contributions, Pc3 and Pc4 pulsations also contain signals at other frequencies that are independent of these contributions [ Verõ et al , ; Sutcliffe et al , ].…”