Magnetic Local Time and Interplanetary Magnetic Field By Variation of Cusp Location Dependence on Dipole Tilt

Abstract

AbstractWe produced a database of over 41,000 ionospheric cusp locations using 40 years of energetic particle measurements from 14 Defense Meteorological Satellite Program (DMSP) satellites. We limited the database to periods when the Auroral Electrojet (AE) was <100 nT and the Interplanetary Magnetic Field (IMF) measurements were available, then calculated the magnetic latitude (MLAT), magnetic local time (MLT) and the dipole tilt (Λ) for each boundary, multiplying Λ by −1 in the southern hemisphere. We then binned the data in running one‐hour bins from 8.5 to 15.5 MLT. To obtain the dependence of the MLAT of the cusp boundary on Λ and IMF By, we performed linear fits on the MLAT versus Λ distributions, separated by ±IMF By. The dependence (degrees MLAT/degrees Λ) is asymmetric in both MLT and hemisphere as well as IMF By. In the northern hemisphere, the dependence for By > 0 (By < 0) is greatest in the afternoon (morning) sector for both poleward and equatorward boundaries. The opposite is true in the southern hemisphere. However, in the noon sector, the dependence is nearly the same for all boundaries and By sign. In addition, separation by Λ sign shows dramatically greater dependence variance for Λ > 0 than for Λ < 0 in the afternoon and morning sectors (but not in the noon sector). The observed asymmetries revealed in this work are thought to be due to the effect of By on reconnection and the location of the reconnection X line.