Global and frequent appearance of small spatial scale field-aligned currents possibly driven by the lower atmospheric phenomena as observed by the CHAMP satellite in middle and low latitudes

Kunihito Nakanishi,Kento Taira,Hermann Lühr,Toshihiko Iyemori

doi:10.1186/1880-5981-66-40

Abstract

Using magnetic field data obtained by the Challenging Minisatellite Payload (CHAMP), we show global and frequent appearance of small-amplitude (1 to 5 nT on the dayside) magnetic fluctuations with period around a few tens of seconds along the satellite orbit in middle and low latitudes. They are different from known phenomena, such as the Pc3 pulsations. The following characteristics are presented and discussed in this paper: (1) The magnetic fluctuations are perpendicular to the geomagnetic main field, and the amplitude of the zonal (east–west) component is larger than that of the meridional component in general. (2) As latitude becomes lower around the dip equator, the period tends to become longer. (3) The amplitudes have clear local time dependence, which is highly correlated to the ionospheric conductivities in local time (LT) 06–18. (4) The amplitude of the fluctuations shows magnetic conjugacy to a certain extent. (5) The amplitude shows no dependence on solar wind parameters nor geomagnetic activity. (6) A seasonal dependence is seen clearly. The amplitudes in the northern summer and winter are larger than those in the equinoxes. In the northern summer, the amplitudes above the Eurasian and South American continents and their conjugate areas are larger. In the northern winter, those above the eastern Pacific Ocean are larger. We suggest that the above characteristics, (1) to (6), can be attributed to the small spatial scale field-aligned currents having a lower atmospheric origin through the ionospheric dynamo process.

Highlights

High-precision magnetic field data have been obtained by low-altitude satellites such as Ørsted, Challenging Minisatellite Payload (CHAMP), and SAC-C since Magsat was launched in 1979
The characteristics of the magnetic fluctuations treated in this paper cannot be explained by the instrumental effect nor the phenomena reported before
Our analysis of the CHAMP magnetic data shows existence of small-amplitude (1 to 5 nT) magnetic fluctuations with period around a few tens of seconds along the orbits of the satellite at middle and low latitudes, i.e., at the geomagnetic dipole latitudes within 50°, most the time and at any local time

Summary

Introduction

High-precision magnetic field data have been obtained by low-altitude satellites such as Ørsted, Challenging Minisatellite Payload (CHAMP), and SAC-C since Magsat was launched in 1979. The CHAMP satellite kept the most stable attitude, and the resolution of the magnetic data is so far the highest. Three kinds of phenomena have been reported on the basis of the magnetic data obtained by the CHAMP satellite. As a temporal variation, the Pc3 micro-pulsation has been reported (e.g., Vellante et al 2004; Heilig et al 2007; Ndiitwani and Sutcliffe 2009). Pressure inside the plasma is smaller than that outside. Balancing the total pressures on both sides, magnetic pressure inside increases compared to that outside, which means that the parallel component to the geomagnetic main field is not zero

Methods

Results

Conclusion