2009 THEMIS SCIENCE NUGGETS

Global Distribution of Whistler-mode Chorus Waves Observed on the THEMIS Spacecraft

by Wen Li

Introduction

Chorus waves are intense electromagnetic emissions, which typically occur in the range of 0.1-0.8 fce, with a gap near 0.5 fce, where fce is the equatorial electron cyclotron frequency. The top panel in Figure 1 shows the spatial distribution of chorus waves indicated by the blue color and the yellow region represents the high-density plasmasphere. Chorus waves are usually observed in the low-density region, outside the plasmapause from midnight through dawn to the afternoon sector. Chorus is excited through cyclotron resonance with anisotropic electrons (1-100 keV) injected from the plasma sheet during geomagnetically active times, and the source region is generally believed to be near the equator. Previous studies have shown that nightside chorus tends to be confined near the equator, while dayside chorus can extend to higher magnetic latitudes. Whistler-mode chorus has recently received increased attention due to its important role in both acceleration and loss processes of energetic electrons in the inner magnetosphere. Therefore, understanding the generation mechanism and characteristics of chorus waves is very important in radiation belt electron dynamics.

Figure 1. (GRL Cover) Chorus waves (blue region) are generated outside the plasmapause (yellow color represents the high density plasmasphere) by the injection of electrons (red arrow) from the plasma sheet. Chorus wave intensities measured by THEMIS spacecraft with 5 probes in the near-equatorial orbits are used to provide a global survey of chorus distribution from 1 Jun 2007 to 1 Feb 2009. Statistical results show new findings on high occurrence rate of strong chorus waves (30-100 pT) in the prenoon sector between 7 and 9 RE both near the equator (|MLAT| < 10°) and in the mid-latitude region (10° < |MLAT| < 25°).
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Global distribution of chorus wave amplitudes

THEMIS wave magnetic field amplitudes from filter bank data are used to obtain the global distribution of chorus emissions over the radial range between 5 and 10 RE at all MLTs. The average Root Mean Square (RMS) chorus amplitudes and the number of samples in each bin are shown as a function of L and MLT in Figure 2 for three different levels of AE* (the mean value of AE over the previous 1 hour). In the near-equatorial region, chorus intensity on the nightside (up to L ~ 8) depends strongly on AE* with stronger intensity during higher geomagnetic activity. However, around noon, wave intensity is less dependent on AE* and moderate chorus is present even during quiet times (AE* < 100 nT). Interestingly, strong nightside chorus is confined to L ≤ 8, while strong dayside chorus can extend to higher L-shells. In the mid-latitude region, nightside chorus is very weak even during strong magnetic activity (AE* > 300 nT), while strong dayside chorus can be present up to at least L ~ 10, peaking around L ~ 8. Our new statistical survey is consistent with the previous statistical analysis (using CRRES data) of Meredith et al. [2003] in the region of overlap between the THEMIS and CRRES spacecraft (5 ≤ L ≤ 7). However, the THEMIS data provides compelling new evidence for the persistent presence of chorus on the dayside, during moderate and even weak geomagnetic activity.

Figure 2. Global distribution of chorus waves observed at the L-shells between 5 and 10 categorized by different AE* in the near equatorial (|MLAT| < 10°) (rows a and b) and mid-latitude regions (10° ≤ |MLAT| < 25°) (rows c and d). The larger plots (rows a and c) show RMS chorus wave amplitudes (pT) and the smaller plots (rows b and d) indicate the number of samples in each bin.
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Global occurrence of chorus waves

To evaluate the global occurrence rate of chorus, the wave magnetic field data are also sorted into different levels of wave magnetic field amplitudes (moderate: 10 ≤ Bw < 30 pT, strong: 30 ≤ Bw ≤ 100 pT, and extremely strong: Bw > 100 pT). Figure 3 shows the occurrence rate (%) of different levels of wave amplitudes in the near-equatorial region (|MLAT| < 10°) (row a) and mid-latitude region (10° ≤ |MLAT| < 25°) (row b). In the near-equatorial region, the occurrence rate of both moderate (column 1) and strong (column 2) chorus is larger on the dayside than that on the nightside, and this MLT asymmetry is most apparent at larger L-shells. However, for the extremely strong (Bw > 100 pT) chorus, the occurrence rate tends to be larger in the region between the nightside and the dawnside. In the mid-latitude region, the asymmetry of the occurrence rate of moderate and strong chorus on the dayside and nightside is even more pronounced, with differences of almost an order of magnitude. These interesting features, which have not been reported in previous studies, provide important clues on the source mechanism for dayside and nightside chorus.

Figure 3. Global distribution of occurrence rates (%) categorized by different levels of wave amplitudes in the near-equatorial region (|MLAT| < 10°) (row a) and in the mid-latitude region (10° ≤ |MLAT| < 25°) (row b).
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Conclusions

The main conclusions of this study can be summarized as follows:

1. The RMS amplitudes of chorus are dependent on AE*; wave amplitudes tend to maximize at L < 7 on the nightside and L > 7 on the prenoon. However, moderate chorus (>10 pT) is present on the dayside even during quiet times (AE* < 100 nT).

2. Nightside chorus is stronger near the equator (|MLAT| < 10°) and becomes much weaker at higher latitudes (|MLAT| ≥ 10°), while strong dayside chorus can extend to higher magnetic latitude (|MLAT| ≥ 10°).

3. The occurrence rate of the moderate (10 ≤ Bw < 30 pT) and strong (30 ≤ Bw ≤ 100 pT) dayside chorus is much larger than that on the nightside both in the near-equatorial region and at mid-latitudes particularly at higher L-shells.

The persistence of dayside chorus during quiet conditions and the high occurrence of dayside chorus at L > 7 could result from the natural enhancement of electron anisotropy in the noon sector. Wave excitation is favored at higher L, where the critical stably trapped flux levels are low. Dayside wave excitation can be further enhanced by solar wind driven compressions and such concepts will be tested in our continuing analysis of dayside chorus events.

Source

Li, W., R. M. Thorne, V. Angelopoulos, J. Bortnik, C. M. Cully, B. Ni, O. LeContel, A. Roux, U. Auster, and W. Magnes (2009), Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft, Geophys. Res. Lett., 36, L09104, doi:10.1029/2009GL037595.

Biographical Note Wen Li is a postdoctoral research associate in the Department of Atmospheric and Oceanic Sciences at UCLA. Her current research interest is characteristics, generation, and propagation of plasma waves in the inner magnetosphere and their effects on radiation belt electron dynamics.



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Emmanuel Masongsong / emasongsong@igpp.ucla.edu