A significant mass density increase during a large magnetic storm in October 2003 obtained by ground-based ULF observations at L ≈ 1.4

Satoko Takasaki,Yuki Obana,Yoshimasa Tanaka,Natsuo Sato,Masahide Iizima,Masahiro Seto,Akimasa Yoshikawa,Hideaki Kawano,Kiyohumi Yumoto

doi:10.1186/bf03351959

Abstract

Abstract During 28–31 October, 2003, a series of coronal mass ejections hit the magnetosphere and triggered two consecutive large storms. Three ground magnetometers at L = 1.32–1.41 recorded field-line resonances (FLRs) during this interval. The FLR frequencies decreased from 0600 LT on 31 October 2003 during the main phase of the second storm until 12 LT when the recovery phase of this storm began. After the decrease, the FLR frequencies returned to its pre-storm value (at 0600 LT on 31 Oct.) in a few hours. The measured decrease in the FLR frequency suggests a relative increase in mass density along the field lines during the magnetic storm. On the other hand, the total electron content (TEC) data suggest that the ionospheric plasma number density during this storm was similar to that during quiet times. A possible explanation for the increase in mass density would be an outflow of the heavy ions (e.g., O+) from the ionosphere to the plasmasphere.

Highlights

It is well known that hydromagnetic waves are observed everywhere in the Earth’s magnetosphere
If both methods identified field-line resonance (FLR) within a given interval, we examined if the two values of the FLR frequencies derived from the two methods were in agreement
Summary This paper shows a significant decrease in the FLR frequency at L ∼ 1.4 during the large magnetic storm

Summary

Introduction

It is well known that hydromagnetic waves are observed everywhere in the Earth’s magnetosphere. FLRs are considered as the oscillation to propagate with shear Alfven waves traveling along the magnetic field lines between the northern and southern ionospheric boundary (Obayashi and Jacobs, 1958). The Alfven speed (VA) is defined as VA = (4πρ)−1/2 B, where ρ is the plasma mass density and B is the magnetic field intensity. The field-line eigenfrequency (i.e. the FLR frequency) is expected to be a function of the field-line length, the magnetic field intensity, and the mass density along the field line (Kitamura and Jacobs, 1968; Troitskaya, 1997). FLR frequency observed at a fixed point shows inverse proportion to the plasma mass density along field lines.

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Conclusion