The influence of solar wind variability on magnetospheric ULF wave power

D Pokhotelov,I J Rae,K R Murphy,I R Mann

doi:10.5194/angeo-33-697-2015

D Pokhotelov, I J Rae + Show 2 more

Open Access

https://doi.org/10.5194/angeo-33-697-2015

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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 (1996–2004) 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.

Highlights

Ultra-low frequency (ULF) waves in the Pc 4–5 range (45– 600 s period; 2–22 mHz; Jacobs et al, 1964) were theoretically described by Dungey and Southwood (1970) and originally characterised by ground-based magnetometer observations (Singer et al, 1977; Greenstadt et al, 1979)
The interplanetary sources of standing ULF waves were proposed in the original works of Dungey and Southwood (1970), and ground magnetometer studies have confirmed the relations between solar wind parameters and the intensity of magnetospheric ULF waves
It is demonstrated that the ULF power in the magnetosphere is most sensitive to the variability of interplanetary magnetic field (IMF) and substantially less sensitive to the variability of other basic solar wind parameters

Summary

Introduction

Ultra-low frequency (ULF) waves in the Pc 4–5 range (45– 600 s period; 2–22 mHz; Jacobs et al, 1964) were theoretically described by Dungey and Southwood (1970) and originally characterised by ground-based magnetometer observations (Singer et al, 1977; Greenstadt et al, 1979). Sibeck, 1994) may involve solar wind pressure pulses, impulsive penetration, Kelvin– Helmholtz instability and flux transfer events, with each of those mechanisms requiring distinct solar wind driving properties. The ULF power is characterised here by the power spectral density (PSD) in the Pc 4–5 frequency range measured at the geostationary orbit by magnetometer on-board the GOES spacecraft over a large portion of the solar cycle 23 (years 1996–2004). This approach allows conclusions to be made about the relative importance of different solar wind variabilities responsible for the intensification of ULF power in the magnetosphere. It is demonstrated that the ULF power in the magnetosphere is most sensitive to the variability of interplanetary magnetic field (IMF) (as characterised by the hourly standard deviation of IMF vector) and substantially less sensitive to the variability of other basic solar wind parameters (solar wind proton temperature, density and bulk velocity)

GOES ULF data

Solar wind data

Geosynchronous ULF wave power as a function of solar wind variability

Discussion and summary