Introduction

Energetic electrons injected into the inner magnetosphere during disturbed times exhibit three distinct types of pitch angle distributions after being transported to the dawn and the dayside: pancake distributions at low energies (E < a few keV), flat distributions around E ~ a few keV, and distributions with enhanced anisotropy at E > a few keV, as shown in Figure 1. The pitch angle distributions evolve because of interactions between electrons and different plasma waves in space. Among these waves, electron cyclotron harmonic (ECH) waves and chorus waves have been proposed as two candidates for formation of the pancake distributions. However, which wave mode is dominant in shaping electron pitch angle distributions has not been fully understood. This study aims to use the best plasma wave observation available from CRRES spacecraft to model interactions between electrons and ECH and chorus waves, so that we can identify which wave mode is responsible for the formation of these three types of electron pitch angle distributions.

Figure 1. (a) A schematic plot showing drift paths (red lines) of plasma sheet electrons injected into the inner magnetosphere. Squares show the rough locations where measurements in panels b, c,and d were taken. Representative electron phase space densities at different energies (represented by different colors) calculate d using data from ESA of THEMIS D (THD) or E (THE) are shown for 00:48 MLT (b), 04:00 MLT (d), and 06:36 MLT (c).

Click each image to enlarge.

Results

We use quasilinear theory to model interactions between electrons and ECH and chorus waves around L = 6. Initial and boundary electron distributions are from THEMIS observations, and the plasma waves are modeled from observations of CRRES.  The evolution of electron pitch angle distributions is shown in Figure 2, which demonstrates good agreement with observations (Figure 1). Thus chorus waves are dominant in evolution of energetic electrons pitch angle distributions and formation of three distinct types of pitch angle distributions.

Figure 2. (a) Initial distribution of electrons following an injection and the evolution of electron distribution after interactions with the combination of ECH waves, upper band chorus, and lower band chorus for (b) 0.5 h and (c) 1 h. To demonstrate the effect of each kind of waves on the electron distribution, we show the distribution of electrons after interactions for 1.0 h with (d) ECH waves only, (e) upper band chorus only, and (f) lower band chorus only.

Click each image to enlarge.

Conclusion

The main results of this work can be summarized as follows:

1): ECH waves are not responsible for producing pancake distributions near L = 6.

2): Scattering from chorus waves forms the observed pancake distributions at low energies (E < a few keV), flat distributions at intermediate energies (E ~ a few keV) and distributions with enhanced anisotropy at higher energies.

Source

Tao, X., R. M. Thorne, W. Li, B. Ni, N. P. Meredith, and R. B. Horne (2011), Evolution of electron pitch angle distributions following injection from the plasma sheet, J. Geophys. Res., 116, A04229, doi:10.1029/2010JA016245.

Biographical Note

Xin Tao is a postdoc in the UCLA Department of Atmospheric and Oceanic Sciences. His current research interest inlcudes theories of wave particle interactions and their applications in radiation belt physics.

Please send comments/suggestions to

Emmanuel Masongsong / emasongsong@igpp.ucla.edu