Comparison of equatorial plasma mass densities deduced from field line resonances observed at ground for dipole and IGRF models

Abstract

The technique to remotely sense the plasma mass density in magnetosphere using field line resonance frequencies detected by ground-based magnetometers has become more and more popular in the last few years. In this paper we examine the error that would be committed at low and middle latitudes (L < 4) in estimating the equatorial plasma mass density if dipole field lines are assumed instead of the more realistic representation given by International Geomagnetic Reference Field (IGRF) lines. It is found that the use of the centered dipole model may result in an error in the inferred density appreciably larger than what is usually assumed. In particular, it has a significant longitudinal dependence being, for example, greater than +30% in the Atlantic sector and about À30% at the opposite longitude sector for field lines extending to a geocentric distance of 2 Earth radii. This may result in an erroneous interpretation of the longitudinal variation in plasmaspheric density when comparing results from ground-based arrays located at different longitudes. We also propose simple modifications of the standard technique, such as the use of an effective dipole moment or the eccentric dipole model, which allow to keep using the dipole field geometry but with a significant error reduction.

Control of ULF Wave Accessibility to the Inner Magnetosphere by the Convection of Plasma Density

“…Linear mode conversion from MHD fast modes to Shear Alfvén eigenmodes occurs at locations within the magnetosphere where the MHD fast mode driver frequency matches the Shear Alfvén wave eigenfrequency, forming highly localized features known as field line resonances (FLRs) (Allan & Poulter, 1992;Rae et al, 2005;Walker, 2000). These have been used to determine the equatorial cold plasma density from ground-based observations (Dent et al, 2006;Vellante, Piersanti, Heilig, et al, 2014;Vellante, Piersanti, & Pietropaolo, 2014).…”

Observing the cold plasma in the Earth's magnetosphere with the EMMA network

“…As regards the geomagnetic field model, a dipole ap− proximation has been usually adopted for deriving the equatorial density from FLRs detected at low and mid latitudes (L < 4) [e.g., Menk et al, 1999;Berube et al, 2005;Villante et al, 2006;Chi et al, 2013;Wang et al, 2013]. Vellante et al, [2014a], however, found that the use of the dipole model at these latitudes may result in an error in the inferred density appreciably larger than what is usually assumed, up to 40% at L = 2 when com− pared with the results obtained using the International Geomagnetic Reference Field (IGRF) model. At higher latitudes, the currents associated with the DEL CORPO ET AL.…”

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“…Their leading edges usually drive shock waves, called interplanetary shocks (ISs; Araki & Shinbori, ; Lugaz et al, ; Oliveira et al, ; Piersanti et al, ). The impact of IS onto the magnetopause compresses the magnetosphere (Vellante et al, ; Villante & Piersanti, , ), causing a sudden increase in the geomagnetic field intensity, called a Sudden Impulse (SI), often followed by intense fluctuations of the magnetospheric field (Di Matteo & Villante, ; Piersanti et al, ; Villante et al, ; Zhang et al, ). While traveling in the interplanetary space, ICMEs carry magnetic structures, often recognizable as magnetic clouds (MC), which are characterized by an intense magnetic field whose north‐south component smoothly rotates (Gosling, ; Shen et al, ).…”