Next: The Polar Cusp Location Up: The Polar Cusp Location and Dipole Tilt Previous: Introduction

Data Analysis

At high-altitudes, because of the diamagnetic effects of the plasma in the polar cusp, and the weak background magnetic field, a depression in the magnetic strength and some fluctuation in the magnetic field can be expected. We can also expect to see the magnetosheath-like (high density and low energy) plasma in this region. Based on these expectations our criteria to identify the cusp crossings are a sudden increase in the low-energy ion and electron density (greater than 5 cm^-3), an electron thermal energy less than 100 eV, the presence of significant He⁺⁺ (usually greater than 0.5 cm^-3) which signifies a solar wind origin, and a decrease of the magnetic field strength from the background field greater than 1 nT associated with the presence of enhanced plasma. Protracted encounters with cusp/magnetosheath plasma such as seen in the mantle or when POLAR approached the magnetopause as on May 29, 1996 [Russell et al., 1998] are not included in this survey.

Figure 1 shows an example of a polar cusp crossing. The upper panel shows the residual of the magnitude of POLAR MFE data with Tsyganenko 96 model [Tsyganenko and Stern, 1996]. The second and third panel show the Hydra electron density and energy. The fourth and fifth panels show the Timas low energy range H⁺ density and temperature. The bottom panel shows He⁺⁺ density. We determine the edges of the cusp by the combined signature of the sharp depression in the magnetic field, a high plasma density, and a significant fraction of alpha particles. The vertical dashed lines indicate the region we have designated to be the polar cusp. It extends from 0755-0821 UT on May 31, 1996, corresponding to 1221-1302 MLT at the footpoint on the surface of the Earth. The location in GSM is from (4.01, -0.66, 4.63) Re to (4.02, -0.24, 5.24) Re. The residual between the MFE data and the Tsyganenko 96 model is about 70 nT, the highest full range energy electron density is 35 cm^-3, the electron energy is about 60 eV. The H⁺ observations from the Timas low energy range shows a density of 12 cm^-3, and an energy of 90 eV. These are all consistent with the criteria we discussed above. In this case, the solar wind dynamic pressure is 3.4 nPa, IMF By= -1 nT, Bz= ~0 nT. The solar wind conditions are from the WIND key parameters and are time-shifted according to the distance from the spacecraft to 10 Re and the solar wind velocity Vx component in GSM. The invariant latitude of cusp lies in the range 79.5^°-81.6^° . The dipole tilt angle is 15.14^° .

Next: The Polar Cusp Location Up: The Polar Cusp Location and Dipole Tilt Previous: Introduction