Large-scale coordinated observations of Pc5 pulsation events

Abstract

Abstract. HF (high-frequency) radars belonging to SuperDARN (Super Dual Auroral Radar Network) receive backscatter over substantial fields of view which, when combined, allow for simultaneous returns over extensive regions of the polar caps and midlatitudes. This makes them ideal instruments for the observation of pulsations in the Pc5 (1-5 mHz) frequency band. Relatively few pulsation events observed by multiple radars have been reported in the literature. Here we describe observations of three such events which extend over more than 120 • of magnetic longitude in the Northern Hemisphere and one of which is also detected in the Southern Hemisphere. All three events show characteristics of field line resonances. In one case the pulsation has also been observed by magnetometers under or near the radar fields of view. The extensive longitudinal coverage allows accurate determination of azimuthal wave numbers. These are at the upper end of the lower values associated with external sources such as those in the solar wind. Such sources imply antisunward flow. However, the azimuthal wave number is negative, implying westward propagation at magnetic local times on both sides of noon, as would be expected from drift-bounce resonance with positive particles. Quiet conditions and a very low ring current during the events argue against this. The identification of the source of pulsations from a number of different mechanisms remains a problem of interest.

Observations of ULF Pulsations During TRINNI Events

“…In addition Mtumela et al. (2016) observed similar sunward‐propagating waves over a wide range (120°) of longitudes on the flanks of the magnetosphere. Thus the magnetometers used to detect these waves can be anywhere within a considerable range of magnetic local times before and after 06:00 and 18:00 MLT simultaneous with, or slightly later than, the TRINNI event in the tail.…”

“…The analytic signal method of complex demodulation was applied to both magnetometer and CPCP data, so that the amplitude and phase of the signal at the frequency of the significant broadband ULF peak seen in both data sets could be accurately determined and compared. This complex demodulation method has been used previously with SuperDARN data to analyze ULF pulsations (Mtumela et al., 2016; Stephenson & Walker, 2010; Walker et al., 1992) and traveling ionospheric disturbances (Atilaw et al., 2021). It has also been employed widely for signal reconstruction in a variety of fields, in particular geomagnetic data (Banks, 1975), Pi2 pulsations (Beamish et al., 1979) and solar oscillations (Thomson et al., 1995).…”

“…Mtumela et al. (2016) reported such pulsations with sunward velocity during magnetically quiet times. During these observations the Dst index varied between −10 and 0 nT, implying a very small ring current, as would be expected under these conditions (Nowada et al., 2018).…”

“…However there are numerous observations of ULF pulsations, over a range of frequencies other than the above specific values, with sunward phase velocities (Eriksson et al, 2008;Mathews et al, 2004), which suggest sources internal to the magnetosphere. Mtumela et al (2016) reported such pulsations with sunward velocity during magnetically quiet times. During these observations the Dst index varied between −10 and 0 nT, implying a very small ring current, as would be expected under these conditions (Nowada et al, 2018).…”

Periodic Modulation of the Upper Ionosphere by ULF Waves as Observed Simultaneously by SuperDARN Radars and GPS/TEC Technique

“…The morphological properties of these types of ionospheric oscillations in the Pc5 band were very different from those of classical geomagnetic Pc5 waves. Mtumela et al (2016) found several Pc5-6 global events over an unusually large range of longitudes (>120°) on the afternoon/dusk side during very quiet geomagnetic conditions (Dst~0). In contrast to typical geomagnetic Pc5 waves, their sense of azimuthal phase propagation was westward (sunward) with the azimuthal wave number m~7-12.…”