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

• Published in: Journal of geophysical research. Space physics Vol. 129; no. 2
• Main Authors: Anderson, Phillip C., Bukowski, Aaron L.
• Format: Journal Article
• Language: English
• Published: 01.02.2024
• Subjects: aurora; cusp; dipole tilt; energetic particles; ionosphere
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2023JA031886

We 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. Plain Language Summary The cusp is an important aspect of the near‐Earth environment where energetic particles from the solar wind have direct access to low altitudes and the ionosphere. The location of the cusp is dependent on the orientation of the Sun’s magnetic field (the Interplanetary Magnetic Field – IMF) and Earth’s geomagnetic field. We examine the impact of Earth’s dipole tilt, the angle between the Earth’s north dipole axis and the Geocentric Solar Magnetospheric (GSM) z‐axis (the vector perpendicular to the plane containing the dipole axis and the vector from the Earth to the Sun) on the cusp’s location in various Magnetic Local Time (MLT) sectors in both hemispheres. We find that the location is highly dependent on the sign of the By component of the IMF, particularly for positive values of the dipole tilt. In addition, this dependence is very asymmetric in both MLT and hemisphere. It is suggested that these dependencies are due to the effect of By on the location where the reconnection between the IMF and the geomagnetic field occurs. Key Points The dependence of cusp location on dipole tilt increases with distance from local magnetic noon The dependence is asymmetric in both MLT and hemisphere as well as IMF By Greater dependence variance is seen for positive dipole tilt away from noon (with Λ multiplied by −1 in the southern hemisphere)