Abstract
Earlier incoherent scatter radar measurements revealed upward topside ion fluxes in the summer and downward fluxes in the winter at mid-latitudes at night; a summer to winter interhemispheric coupling was accordingly inferred. However, this interhemispheric coupling through the plasmasphere is difficult to confirm directly from observations. A possible result induced by this coupling is interhemispheric conjugacy of the mid-latitude ionosphere. In this paper, interhemispheric conjugate effect in longitude variations of mid-latitude total ion density (Ni) is presented, for the first time, using the Defense Meteorological Satellite Program (DMSP) measurements; northern and southern Ni longitude variations at 21:30 LT are similar between magnetically conjugate mid-latitudes around solar minimum June Solstice of 1996. The conjugate effect after sunset also occurs around the June Solstice in other solar minimum years but disappears when solar activity increases. We suggested that mid-latitude interhemispheric coupling is responsible for the conjugate effect. Neutral wind induced ionospheric transport causes topside longitude variations via upward diffusion at summer mid-latitudes; this further induces similar longitude variations of topside Ni at winter mid-latitudes via the summer to winter interhemispheric coupling. The conjugate effect occurs only inside the plasmapause where magnetic flux tubes are closed and the plasma in these tubes can stably corotate with the Earth. The conjugate effect not only proves mid-latitude interhemispheric coupling through the plasmasphere, but also implies that neutral wind induced transport can affect ionospheric coupling to the plasmasphere at mid-latitudes.
Highlights
Ionospheric longitude variations, which refer to ionospheric differences between longitudes at fixed local times, are important spatial structures of the ionosphere
We propose that the mechanism for this conjugate effect is the interhemispheric coupling of the mid-latitude ionosphere through the plasmasphere, in which neutral wind induced field-aligned transport in the summer hemisphere plays an important role during the period under consideration
We found that longitude variations of topside Ni at conjugate mid-latitudes are similar in solar minimum June Solstice season, exhibiting the interhemispheric conjugate effect
Summary
Ionospheric longitude variations, which refer to ionospheric differences between longitudes at fixed local times, are important spatial structures of the ionosphere. The summer to winter interhemispheric coupling has been used to explain some ionospheric phenomena such as the formation of nighttime winter anomaly (e.g., Jakowski et al, 2015 and references therein) and the distribution of winter nighttime enhancement in ionospheric electron density (e.g., Chen et al, 2015) Another possible result of the interaction between the northern and southern mid-latitude ionospheres is the conjugacy in the longitude variations of plasma density, especially in the topside where field-aligned diffusion plays a dominant role in the plasma density distribution. The conjugate effect disappeared since 1998 and Ni longitude variations in different years show similar patterns during 1999–2001, with one-peak structure at southern mid-latitudes and two-peak structure at northern mid-latitudes. This means that the conjugate effect after sunset only occurs at solar minimum
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