2011 THEMIS SCIENCE NUGGETS

Modulation of Whistler-mode Chorus waves: 2. Role of Density Variations

by Wen Li

Introduction

Modulation of whistler-mode chorus waves, which plays an important role in driving the pulsating aurora and other processes related to energetic electron dynamics, is an interesting but a long-standing unresolved problem. Although the role of density in whistler wave modulation has been reported in previous studies inside or close to the plasmapause or at high geomagnetic latitudes, studies dealing with the chorus modulation associated with density variations near the chorus source region (close to the magnetic equator) are very limited. The main objective of this study is to examine the role that plasma density variations play in the modulation of chorus waves in the near-equatorial magnetosphere. Using simultaneous wave and density measurements (inferred from the spacecraft potential) from THEMIS we focus on chorus modulation on a timescale from a few seconds to tens of seconds.

Results

The events of chorus modulation associated with density variations were sorted into two categories; chorus modulation by (i) density depletions (DD) and (ii) density enhancements (DE).

Figure 1. (a) Omnidirectional electron energy flux and (b) electron anisotropy (A*) as functions of energy and time. (c) Electron energy flux over the energy of 3-30 keV roughly perpendicular (blue) and parallel (black) to the background magnetic field. (d) Total electron density inferred from the spacecraft potential, (e) total magnetic field, and (f) minimum resonant energy of electrons interacting with waves with normalized frequencies of 0.3 and 0.55 fce through the first-order cyclotron resonance. (g) Integrated chorus wave amplitude over 0.05-0.5 fce (lower-band) and 0.5-0.8 fce (upper-band). (h) Time-frequency spectrograms of wave electric and (i) magnetic field spectral intensity.

Click each image to enlarge.

We begin by presenting an example of the DD event in Figure 1, which shows an overview of the properties of the plasma associated with chorus modulation. Chorus wave intensity (Figures 1g, 1h, and 1i) exhibited substantial episodic intensification during this time interval. For resonant electrons with energies of ~1-30 keV, The omnidirectional electron energy fluxes were large (Figure 1a), the electron anisotropy (Figure 1b) was elevated, and the perpendicular flux was larger than the parallel flux (Figure 1c), all of which provides a favorable condition for chorus excitation. The total electron density (Figure 1d) varied considerably and the total magnetic field (Figure 1e) exhibited a monotonic decrease. Interestingly, density depletions were correlated remarkably well with increases in the chorus wave amplitude, whereas other parameters did not show the corresponding change in response to the chorus modulation.

Figure 2. The same parameters as shown in Figure 1, but for a DE event. In Figure 2f the minimum resonant energies of electrons are shown for waves with frequencies of 0.3 and 0.4 fce. Chorus wave amplitudes were integrated over 0.25-0.35 fce (blue) and 0.35-0.45 fce (red) and are shown in Figure 2g.

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Figure 2 shows similar parameters to Figure 1 but for a DE event. During this time interval, Figure 2d shows three intervals of significant density enhancements. Importantly, chorus wave intensity (Figures 2g-2i) exhibited pronounced wave intensification associated with these density enhancements. Figure 3 shows another DE event, in which both chorus intensity and frequency show significant variations in response to the density changes. As the total electron density increased (Figure 3a), chorus wave intensity increased and the central frequency of the generated waves decreased (Figure 3b).

Figure 3. (a) Total electron density and (b) time-frequency spectrograms of wave magnetic field intensity. In Figure 3b, the solid white line represents the calculated resonant wave frequency for electrons with energy of ~8 keV.

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The case analyses of both DD and DE events shown above indicate that chorus wave amplitudes increase in association with both density depletions and enhancements. In order to systematically investigate conditions and locations where DD and DE play a significant role in chorus modulation, we performed a comprehensive statistical analysis and the results are shown in Figure 4. The majority of DE events (Figure 4b) occur between the dawn and the afternoon sector, whereas most DD events (Figure 4c) are distributed from the premidnight to the dawn sector. On the nightside, both DD and DE events mostly occur at L<8, while extending up to ~12 on the dayside.

We further investigate how well the chorus modulation in DD and DE events can be explained by the changes in the linear wave growth rate. Calculations of linear growth rates in each category show that both density depletion and density enhancement can lead to chorus amplification.

Figure 4. Global distributions of (a) number of samples from all available particle burst data sorted into bins of 1 L x 1 MLT (b) DE events and (c) DD events in the dominant chorus source region between 5 and 12 RE and from 22 to 14 MLT. The gray shaded regions in panels (b) and (c) represent the area, where this statistical survey is excluded.

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Figure 5 presents a schematic illustration of the dominant region where chorus modulation by various mechanisms occur, discussed in both the companion article and the present article. As electrons (indicated by black arrows) drift from the midnight through the dawn toward the noon sector, where plasma conditions are close to the marginal stability, the onset of wave instability can be triggered by a change in the background plasma parameters. Between midnight and dawn, density depletions have a remarkably good correlation with the intensification of chorus waves. In the dawnside outer magnetosphere (L>8), compressional Pc4-5 pulsations with an anti-correlation between the density and the magnetic field play an important role in modulating chorus waves on a timescale of tens of seconds to a few minutes. Between dawn and noon, modulation of chorus by density enhancements predominates at L-shells extending from outside the plasmapause up to the magnetopause.

Figure 5. A schematic illustration showing the global distribution of chorus modulation associated with three different mechanisms in the equatorial magnetosphere. The black arrows represent trajectories of electrons injected from the plasmasheet near the midnight sector and the gray region indicates high-density plasmasphere. Red, blue and green regions represent chorus modulation relevant to compressional Pc4-5 pulsations with an anti-correlation between the density and the magnetic field (shown in the figure next to the red region), density depletions (DD), and density enhancements (DE) respectively.

Click each image to enlarge.

Conclusion

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

1. In a state of marginal stability for resonant electrons responsible for chorus generation, modulation of chorus wave intensity is frequently associated with density variations on a timescale from a few seconds to tens of seconds, comparable to typical timescales of pulsating aurora.

2. Density depletions are correlated remarkably well with increases in chorus wave amplitude. Density enhancements lead to the amplification of the chorus wave intensity, thus correlating very well with modulated chorus wave amplitudes. Density enhancements also tend to reduce the central frequency of the generated chorus waves and vice versa.

3. Statistical results show that DD events predominantly occur from premidnight to dawn, whereas DE events commonly occur between dawn and afternoon. DD events are confined to L<8 on the nightside and DE events extend to higher L-shells up to 12 on the dayside.

4. Both density depletions and enhancements can lead to the increase in linear growth rates of chorus waves. However, other potential mechanisms such as chorus wave trapping in density crests and troughs cannot be excluded.

Source

Li, W., J. Bortnik, R. M. Thorne, Y. Nishimura, V. Angelopoulos, L. Chen (2011), Modulation of Whistler-mode Chorus Waves: 2. Role of Density Variations, J. Geophys. Res., in press.

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 and pulsating/diffuse aurora.


Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu