Abstract:
The CHAMP magnetic field variations during international
quiet days of low solar activity period 2008–
2009 are investigated. The present paper reports the existence
of frequency peaks ≤20 mHz in the compressional
component of the magnetic field in almost all CHAMP
passes. The magnetic field variations associated with these
frequencies have amplitude of a few tens of nT during the
daytime. The geomagnetic activity and interplanetary magnetic
field parameters were observed to be low during the
period of study. The spectral powers of the observed frequencies
show no dependence on solar wind velocity and
cone angle; hence, the reported frequencies are not related to
the geomagnetic pulsations. For frequency-peaks ≤15 mHz,
strong local time dependence is observed with maximum
power near noon and minimum at night. The longitudinal
and seasonal variation of the powers of these frequency
peaks match well with those of the equator-to-middle latitude
ionospheric currents derived by the earlier studies. As
a polar Low Earth Orbiting (LEO) satellite spans the entire
range of latitudes within few minutes, it monitors the
geomagnetic field variations caused by the quiet-time ionospheric
currents flowing at different latitudes. This can result
in certain frequencies in the magnetic field recorded by LEO
satellites.We demonstrate that the frequencies <10 mHz are
mainly due to the latitudinal structure of the equatorial electrojet.
The observed frequencies in CHAMP data are therefore
attributed to the latitudinal structures of the ionospheric
currents that are monitored only by the polar LEO satellites and are found to alter the observations of geomagnetic pulsations
(Pc4–5 and Pi2) significantly.
