Hemispheric asymmetries in thermospheric structure and dynamics

• Main Authors: Rees,David, Fuller-Rowell,Timothy J.
• Format: Report
• Language: English
• Published: Department of Physics and Astronomy, University College London 1987
• Subjects: Geomagnetic Pole; Memoirs of National Institute of Polar Research; Polar Research
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=2181; http://id.nii.ac.jp/1291/00002181/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=2181&item_no=1&attribute_id=18&file_no=1

Hemispheric differences in the structure and dynamics of the polar regions of the thermosphere and ionosphere are caused by seasonal/latitudinal asymmetries of solar insolation acting in addition to the effects of the distinctive asymmetries of the main geomagnetic field. Viewing the earth from a satellite, the longitudinal and universal time variations of the thermosphere and ionosphere are far more spectacular in the southern polar thermosphere and ionosphere than in the northern polar regions. This is simply caused by the greater offset of the geomagnetic pole from the geographic pole in the southern hemisphere, and the associated geomagnetic forcing of the polar thermosphere and ionosphere. The southern polar cusp is a localised region of intense forcing with a diurnal migration in geographic latitude from 55° to the southern geographic pole. At times of major geomagnetic disturbances, the combination of the equatorward expansion of the auroral oval and the greater offset of the southern geomagnetic pole from the rotational pole causes dramatic effects in southern mid-latitude and near-equatorial regions. The northern auroral oval rarely expands within 40° of the equator (at mid-American longitudes). In the Australian sector, equivalent major geomagnetic disturbances may extend to a southern latitude of 30°. Wind, gravity wave and ionospheric perturbations originating in the auroral ovals decrease rapidly in amplitude with increasing distance from the source region. The greater frequency and amplitude of auroral-related disturbances reaching a particular mid- or low-latitude region in the southern hemisphere, particularly in the Australian sector is thus explained. A survey of recent satellite data will be presented, highlighted by simulations using a global theoretical model, to demonstrate the causes of these hemispheric asymmetries. Future complementary ground-based and space measurements can exploit the hemispheric asymmetries to clearly identify geomagnetic forcing mechanisms. The greater UT and ...