Investigating the IMF cone angle control of Pc3‐4 pulsations observed on the ground

Abstract

Many studies have shown that Pc3-4 pulsations (~0.014-0.1 Hz) observed in Earth's magnetosphere during daytime hours originate in the ion foreshock region of the solar wind, just upstream from Earth's bow shock. They occur when the interplanetary magnetic field (IMF) is primarily radial-when the IMF cone angle θ xB ≤ 45°. However, our knowledge of ion foreshock conditions is often incomplete because of the very limited number of spacecraft providing upstream data. In this study we compared 13 months of wave observations at two widely separated ground stations (Hornsund, Svalbard and Halley, Antarctica) to IMF values in the OMNI database in order to test this relation. Values of θ xB and the empirically predicted wave frequency (f calc = 0.06 B IMF ) were compared to daily Fourier spectrograms displaying pulsation power and frequency. Although there was often good temporal agreement between low θ xB and increased Pc3-4 wave power, numerous counterexamples were also evident. A statistical study of wave activity in quarter hour increments showed that Pc3-4 pulsations were associated with low θ xB values, 81% of the time at Hornsund and 83% at Halley. IMF cone angle data from all available upstream monitors were compared to wave observations for a more limited number of days; many of these showed inconsistent IMF orientations. This study indicates some of the limitations of the existing upstream monitors and provides a quantitative estimate (~80%) of the accuracy of the OMNI data set in characterizing conditions near the nose of Earth's bow shock under predominantly radial IMF conditions.

A solar-wind-driven empirical model of Pc3 wave activity at a mid-latitude location

“…The three upstream parameters are all influential in the excitation and downstream propagation of ULF waves in the region upstream of the bow shock and their importance to the NN model suggests that UWs are the dominant drivers of mid-latitude dayside Pc3 waves on the ground. This is in agreement with previous studies by Yumoto (1985), Verő (1980), Bier et al (2014), Heilig et al (2007) and Heilig et al (2010), for example. Other extra-magnetospheric input parameters, such as the F10.7 flux, may be included in future models.…”

“…1 h). Furthermore, Bier et al (2014) showed that the cone angle values derived from OMNI may be inaccurate -especially when the IMF is oriented approximately parallel to the Sun-Earth line, when high Pc3 wave activity is expected. Table 3).…”

“…In addition to errors in the estimation of propagation time through the magnetosheath and magnetosphere, we believe that the relatively lower correlation at the shortest timescales is the consequence of the derivation of the solar wind data, and that direct measurements at the bow shock nose would yield higher correlations. Indeed, Bier et al (2014) showed that IMF orientation observed near L1 is not always the same as the orientation impacting on the bow shock nose, i.e. cone angle data derived from the OMNI data set is not 100 % accurate (the data set they analysed showed 80 % accuracy).…”

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

“…In this sense, while Wolfe et al [] and Yumoto et al [] concluded for the absence of any control of the IMF direction on the (broadband) Pc3 amplitude at λ ≈ −74°, Ponomarenko et al [] determined, at the same latitude, a power maximum at θ XB ≈ 20° (narrowband events). On the other hand, a recent statistical study of the wave activity at two widely separated stations ( λ ≈ 74° and λ ≈ −62° [ Bier et al , ]) showed that the Pc3/4 power was associated with θ XB < 45°, ≈81% of the time. At lower latitudes ( λ ≈ 59°), Greenstadt et al [] determined active Pc3 intervals for θ XB < 50°; by contrast, Wolfe and Meloni [] found, at λ ≈ 56°, a very weak correspondence between the cone angle and the Pc3 power.…”

Nighttime Pc3 pulsations: MM100 and MAGDAS observations

“…The accuracy of the OMNI data set in characterizing conditions near the nose of Earth's bow shock under predominantly radial IMF is not better than 80 % (Bier et al 2014).…”