HF Doppler oscillations in the low‐latitude ionosphere coherent with equatorial long‐period geomagnetic field oscillations

We present observations of HF Doppler (HFD) oscillations in the low‐latitude ionosphere seen during global long‐period (5∼15 min) geomagnetic field oscillation events, which occurred on 21 April 1993 and on 28 February 1998. In both events, daytime polar‐equatorial magnetometer data on the ground indicated that the long‐period geomagnetic field oscillations at the daytime dip equator were not only considerably enhanced but also highly correlated with those at afternoon high latitudes with no apparent time shift (within 10 s). This earlier finding [ Motoba et al. , 2002, 2003] strongly suggested that the long‐period geomagnetic field oscillations at the daytime dip equator were produced by an extension of polar‐originating ionospheric current system associated with high‐latitude geomagnetic field oscillations. In the first event on 21 April 1993, the HFD measurement at the postmidnight low latitude detected frequency oscillations coherent with the geomagnetic field oscillations at the afternoon dip equator in the Pc5 range (∼6 min). However, there was no magnetic field change at low‐latitude magnetometer stations adjacent to the reception site of the HFD. Therefore it is reasonable to consider that the HFD oscillations are not effects of directly incoming hydromagnetic waves on the nighttime low‐latitude ionosphere. In the second event on 28 February 1998, both the low‐latitude HFD and equatorial magnetometer measurements were located in the postsunrise terminator. Similar to the first event, HFD oscillations were well correlated with long‐period geomagnetic field oscillations (∼14 min) at the daytime equator, although both variations were in antiphase. In the same local time sector the corresponding low‐latitude magnetic field variations were predominant in the D component rather than the H component, suggesting that the meridional ionospheric currents originating in the polar region make a major contribution for the low‐latitude magnetic field oscillations. The two case studies presented here provide strong evidence that the observed low‐latitude HFD oscillations arose from E × B effects due to direct penetration of a polar dawn‐dusk electric field, which set up the ionospheric current system responsible for the long‐period geomagnetic field oscillations occurring on a global scale.