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

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. Background 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. In particular, the CHAMP satellite kept the most stable attitude, and the resolution of the magnetic data is so far the highest. The magnetic fluctuations with period around a few tens of seconds along the orbits are the objective of this research. So far, three kinds of phenomena have been reported on the basis of the magnetic data obtained by the CHAMP satellite. Firstly, 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). It is thought to be a * Correspondence: [email protected] Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan Full list of author information is available at the end of the article © 2014 Nakanishi et al.; licensee Springer. This Attribution License (http://creativecommons.or in any medium, provided the original work is p shear Alfvén wave to which the field line resonance (FLR) converts fast magnetosonic wave generated by a solar wind upstream wave (e.g., Yumoto 1985 and references therein). Secondly, as a spatial variation, the equatorial plasma bubble occurring after post-sunset has been identified (e.g., Stolle et al. 2006; Park et al. 2009a). 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. Lastly, as a spatial variation, the mid-latitude magnetic fluctuation (MMF) on the nightside has also been investigated recently (Park et al. 2009b). The authors claim that the MMFs are a result of the Perkins instability forming the medium-scale traveling ionospheric disturbances (MSTIDs), and they report the following characteristics: (1) the MMFs are perpendicular to the geomagnetic main field and thought to be the effect of field-aligned current is an Open Access article distributed under the terms of the Creative Commons g/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction roperly credited. Nakanishi et al. Earth, Planets and Space 2014, 66:40 Page 2 of 16 http://www.earth-planets-space.com/content/66/1/40 (FAC), (2) their occurrence is rare above the southern Atlantic ocean, and (3) the occurrence rate bears little connection to the geomagnetic activity. Contrary to the above three phenomena, our analysis of the magnetic field observed by the CHAMP satellite shows global and frequent appearance of small-amplitude magnetic fluctuations with period around a few tens of seconds along the satellite orbit nearly all the time and at