Comprehensive study of ULF upstream waves observed in the topside ionosphere by CHAMP and on the ground

Abstract

Abstract. Based on magnetic field measurements from the satellite CHAMP, a detailed picture could be obtained of the upstream wave (UW) distribution in the topside ionosphere. The low, near-polar orbit of CHAMP, covering all local times, allows the global distribution of this type of pulsation to be revealed. The observations from space are compared to recordings of the ground-based MM100 meridional array covering the latitude range 66 • to 42 • in magnetic coordinates. UWs show up very clearly in the compressional component of the satellite magnetic field data, whereas on the ground, their signature is found in the H component, but it is mixed with oscillations from field line resonant pulsations. Here we first introduce a procedure for an automated detection of UW signatures, both in ground and space data. Then a statistical analysis is presented of UW pulsations recorded during a 132-day period, centred on the autumn 2001 equinox. Observations in the top-side ionosphere reveal a clear latitudinal distribution of the amplitudes. Largest signals are observed at the equator. Minima show up at about 40 • latitude. The coherence between ground and satellite wave signatures is high over wide latitude and longitude ranges. We make suggestions about the entry mechanism of UWs from the foreshock region into the magnetosphere. The clear UW signature in satellite recordings between −60 • and 60 • latitude allows for detailed investigations of the dependence on solar wind conditions. We test the control of solar wind speed, interplanetary magnetic field strength and cone angle on UWs. For the first time, it is possible to derive details of the Doppler-shift effect by modifying the UW frequency from direct observations. The results reconcile foreshock wave generation predictions with near-Earth observations.

Near-equatorial Pi2 and Pc3 waves observed by CHAMP and on SAMBA/MAGDAS stations

“…The considered events are consistent with the observational results of Heilig et al (2007) for CHAMP, Han et al (2004) for Orsted, and Takla et al (2011) for MAGDAS trans-equatorial array, all showing a wide equatorial maximum of compressional Pc3/Pi2 amplitudes. According to the theoretical predictions, typical night side ionosphere should not produce any considerable attenuation of Pc3/Pi2 wave amplitude, and should introduce just a small phase delay between the ground and space ULF signals.…”

“…Similar nearly out-of phase relationships can be seen in the magnetograms of dayside Pc3 waves presented in (Heilig et al, 2007). The cross-phase between compressional Pc3 waves detected by Oersted and ground stations (Jadhav et al, 2001) in fact varied in a wide range: for morning events it tends to concentrate around 60-70°, but for near-noon events it reached $150°.…”

“…Moreover, at near-equatorial latitudes the mechanism of Alfven resonance cannot be invoked, because the field lines are totally submerged into the ionospheric plasma and Alfven field line oscillations are strongly damped (Pilipenko et al, 1998). The latitudinal distribution in the top-side ionosphere showed that dayside compressional power in the Pc3 band was most pronounced in two regions: near noon, 09-14 LT at near-equatorial latitudes (±15°from the geomagnetic equator), and at high latitudes $75° (Heilig et al, 2007). However, the overall Pc3 amplitude/phase spatial structure cannot be explained by a simple propagation of fast mode (Matsuoka et al, 1997).…”

“…The coordinated low-altitude satellite (CHAMP) and ground observations of Pc3-4 waves indicated on a significant contribution of compressional component into the wave structure in the upper ionosphere (Heilig et al, 2007). Pc3-4 waves demonstrated a localized enhancement of transverse components at midlatitudes attributed to the field line resonance, where the frequency of compressional mode transporting the wave energy from extra-magnetospheric source matches the local frequency of Alfven eigenoscillations (Vellante et al, 2004;Ndiitwani and Sutcliffe, 2009;Heilig et al, 2013).…”

“…However, a mechanism of such amplification has not been unambiguously identified yet. Possible mechanisms can involve enhancement of the ionospheric currents induced by mid-latitude Alfven wave which spread into the equatorial region with elevated ionospheric Cowling conductance (Shinohara et al, 1997); or direct penetration of compressional wave towards the near-equatorial region without conversion into Alfven field line oscillations (Yumoto and Saito, 1983;Heilig et al, 2007Heilig et al, , 2013. Most probably, combination of all these mechanisms takes place, and to resolve them is a challenging goal.…”