The influence of solar wind variability on magnetospheric ULF wave power

Abstract

Abstract. Magnetospheric ultra-low frequency (ULF) oscillations in the Pc 4-5 frequency range play an important role in the dynamics of Earth's radiation belts, both by enhancing the radial diffusion through incoherent interactions and through the coherent drift-resonant interactions with trapped radiation belt electrons. The statistical distributions of magnetospheric ULF wave power are known to be strongly dependent on solar wind parameters such as solar wind speed and interplanetary magnetic field (IMF) orientation. Statistical characterisation of ULF wave power in the magnetosphere traditionally relies on average solar wind-IMF conditions over a specific time period. In this brief report, we perform an alternative characterisation of the solar wind influence on magnetospheric ULF wave activity through the characterisation of the solar wind driver by its variability using the standard deviation of solar wind parameters rather than a simple time average. We present a statistical study of nearly one solar cycle (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)) of geosynchronous observations of magnetic ULF wave power and find that there is significant variation in ULF wave powers as a function of the dynamic properties of the solar wind. In particular, we find that the variability in IMF vector, rather than variabilities in other parameters (solar wind density, bulk velocity and ion temperature), plays the strongest role in controlling geosynchronous ULF power. We conclude that, although time-averaged bulk properties of the solar wind are a key factor in driving ULF powers in the magnetosphere, the solar wind variability can be an important contributor as well. This highlights the potential importance of including solar wind variability especially in studies of ULF wave dynamics in order to assess the efficiency of solar wind-magnetosphere coupling.

Acceleration of radiation belt electrons and the role of the average interplanetary magnetic field Bz component in high‐speed streams

“…When maintaining the negative baseline, but increasing the input solar wind speed from 500 to 600 km/s, as it was the case from midday on 15 November to the beginning of the 16th, even higher enhancements in the ULF band IPSD values were found, which is in accordance with previous works that used other magnetohydrodynamic (LFM) code [e.g., Claudepierre et al , ; Huang et al , ]. In this respect, our simulation results for higher nightside, equatorial IPSD values for higher solar wind speeds are also in agreement with previous observational works that used both ground‐based and space‐based magnetometer data [see Pokhotelov et al , , and references therein]. Van Allen Probes observations indeed showed a gradual increase in the magnetic field power spectral densities in the 0.5 to 16 mHz range from 13 to 16 November, with a particularly enhanced activity on both the 15th and 16th (see Figure ).…”

Nature's Grand Experiment: Linkage between magnetospheric convection and the radiation belts

“…The 2009 period was anomalously quiet, allowing a better chance to disentangle the complex and interlinked processes. While our results are consistent with radiation belt acceleration due to enhanced convection (proxied through substorms), it is possible that they may also be consistent with other acceleration mechanisms, for example, solar wind-driven magnetospheric ULF wave [e.g., Kepko et al, 2002;Pokhotelov et al, 2015]. This deserves further examination.…”

ULF Wave Activity in the Magnetosphere: Resolving Solar Wind Interdependencies to Identify Driving Mechanisms

“…Previous studies have indicated a ULF wave power dependence on speed perturbations or variability (Pokhotelov et al, ), but the interdependence of δ v sw with v sw has not been fully explored. It is possible that the summed power δ v sw (or indeed the variance) will increase in magnitude with the speed v sw , so there is an interdependence to account for.…”

“…In particular, both the horizontal and vertical cut-throughs at constant v sw (Figure 4b) and constant v sw (Figure 4c) indicate a power dependence only on v sw , because the cut-throughs in Figure 4b are roughly horizontal. Hence, it is likely that the relationship shown in Pokhotelov et al (2015) is due to the interdependence between v sw and v.…”