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

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.

Localized field-aligned currents and 4-min TEC and ground magnetic oscillations during the 2015 eruption of Chile’s Calbuco volcano

“…3b, which corresponds to zonal component of magnetic perturbations, is largest. This is consistent with Figure 8c in Zettergren and Snively (2015) and Figure 14 in Nakanishi et al (2014). The difference of the amplitude between δB y in our Fig.…”

“…Nakanishi et al (2014) suggested that the small-scale FACs observed by CHAMP are caused by the ionospheric E-layer dynamo having the lower atmospheric origin. That is the reason why we trace the MRs observed by Swarm to the E-layer altitude along the geomagnetic field.…”

“…2 in this paper and Nakanishi et al (2014), magnetic fluctuations along low-altitude satellite orbit are commonly observed in middle and low latitudes, but it may be reasonable to consider that the MRs in Fig. 3 are associated with the volcanic eruption, because the wave-packet structure of the MRs is roughly symmetric in the latitudinal direction from the volcano and from its conjugate point.…”

“…Timescale of their temporal variations is roughly estimated to Open Access *Correspondence: aoyama@kugi.kyoto-u.ac.jp 1 Graduate School of Science, Kyoto University, Kyoto, Japan Full list of author information is available at the end of the article be 200-340 s, and amplitudes of the MRs shown in their paper are about 1 nT. The typical latitudinal scale of the MRs is estimated to be 100 km in Nakanishi et al (2014).…”

“…Nakanishi et al (2014) found in the CHAMP magnetic data that small-amplitude (1-5 nT on the dayside) magnetic fluctuations with periods of a few tens of seconds (20-40 s) along the satellite orbit were almost always observed in middle and low latitudes in the topside ionosphere (see Nakanishi et al 2014, Figure 2). We name these magnetic fluctuations "magnetic ripples" (MRs).…”