Magnetospheric Boundaries and Fields

Abstract

The quantitative determination of the magnetospheric magnetic field, B, is intimately related to the study of the magnetopause, plasma sheet and other boundaries in the magnetosphere. B is formed by the main field of the Earth and the fields from currents flowing in the magnetosphere. The boundary (or magnetoapuse) currents are formed by the shocked solar wind particles. The ions and electrons are deflected in opposite directions as they encounter the magnetospheric magnetic field. This process simultaneously forms the magnetopause and the boundary currents. Recent studies of the magnetic field in the tail of the magnetosphere also illustrate the importance of boundaries. It was possible to quantitatively model the tail field only when the cross tail currents were placed on the upper and lower boundaries of the plasma sheet. In the older models currents flow only across the center of the tail and the observed features of the tail field cannot be correctly produced. It is on the edges of the plasma sheet that the diamagnetic currents are the strongest. Drift currents also flow throughout the plasma sheet due to gradients in B. Success in modeling the tail field by considering diamagnetic and drift currents has led to the concept of currents distributed throughout the magnetosphere. They exist because of gradients in plasma density and B. The distributed currents then account for both the tail currents and the quiet time ring current. The total model magnetospheric magnetic field representation of B then consists of the Earth’s main field and contributions from the boundary (magnetopause) and distributed currents (Olson, 1973).

The model representation of B has been tested in several ways using both particle and field observations. It correctly places the last closed field line at 78° magnetic latitude while older models predict 80–83°. The model yields the observed B values along the Earth-Sun line and the proper values of ΔB (observed total field minus observed main field) throughout the magnetosphere. Previous models do not produce the observed variations along the Earth-Sun line and ΔB contours calculated from them bear little or no resemblance to the observations. The observed decay of the tail field and other features back to lunar orbit are also correctly modeled. B has been approximated by expressing it analytically as a power series with exponential terms and then tested by predicting magnetospheric particle behavior. The high latitude trapping boundary for low energy particles calculated with the analytic model agrees with observations. (Previous models are in error by 4–5°.) The model was also tested by using it to compute the latitudinal cutoffs for solar cosmic rays. While older models predict cutoffs that are 6–8° in error with observations, the cutoffs computed with the present model are within 2° of the observed values.

This total magnetospheric magnetic field can also be used to redetermine the shape of the magnetopause and to study particle entry and motions in the magnetosphere. The magnetopause then exhibits the observed cusped geometry at high latitudes instead of the ‘neutral points’ produced by the older models. Low energy particles easily enter the magnetosphere through the dayside cusp regions and the flanks of the tail (through the dawn and dusk edges of the plasma sheet). These particles form currents across the tail and locally in the cusp region.

These studies point out the necessity of considering the low energy (< 50 keV protons) plasma simultaneously with currents and fields. In order to understand and predict the formation and maintenance of magnetospheric structures (magnetopause, dayside cusp, plasma shed), it is necessary to model several current systems, the shapes and locations of boundaries, and the entry of particles. Each of these depends on all of the others. The work discussed here has been guided along these lines and the resulting models have been quite successful in predicting observed magnetospheric behavior.