Pages 857-860

STATISTICAL STUDY ON ELECTRON CYCLOTRON HARMONIC WAVES OBSERVED IN THE DAYSIDE MAGNETOSPHERE

H. Usui, J. Koizumi, and H. Matsumoto

Radio Atmospheric Science Center, Kyoto University, Gokasyo, Uji, Kyoto 611, Japan,
E-mail: usui@kurasc.kyoto-u.ac.jp

ABSTRACT

Dynamic spectra obtained with the Plasma Wave Instrument (PWI) onboard the GEOTAIL spacecraft show a variety of electron cyclotron harmonic (ECH) waves in the dayside equatorial magnetosphere. To see the spatial distribution of these ECH waves in terms of field intensity, we statistically analyzed the wave data obtained in a period from September 18, 1992 to April 10, 1995. It is found that intense waves ( -130 dBV/m/Hz1/2) mainly occur at the dawnside region near the magnetopause, while weak diffuse ECH waves broadly appear along the dayside magnetopause.

INTRODUCTION

Electron cyclotron harmonic (ECH) waves, which are banded electrostatic emissions at frequencies between harmonics of the electron cyclotron frequency fce, were first observed by the plasma wave experiment aboard Ogo 5 [Kennel et al., 1970]. They were also found in data from other satellites [Anderson and Maeda, 1997; Shaw and Gurnett, 1975; Christiansen et al., 1978].

Associated with these observations, theoretical studies regarding the generation of ECH waves have been intensively conducted in the 1970s and 1980s. Most of the studies focused on microscopic instabilities due to anisotropic electron velocity distributions [e.g. Young et al., 1973; Ashour-Abdalla et al., 1979]. Based on the observational and theoretical knowledge of the ECH emissions, Hubbard and Birmingham [1978] classified them into four types depending on their spectral appearances. According to the linear theory, the harmonic band of ECH emissions depends on the density ratio of hot and cold electrons. To confirm this relation with observation data, Hubbard et al. [1979] statistically studied ECH wave events obtained by IMP 6. They also obtained spatial distributions of each class of emission.

In this paper, we report statistical analysis of ECH waves observed by the Plasma Wave Instrument (PWI) [Matsumoto et al., 1994] onboard the GEOTAIL spacecraft. Unlike the study by Hubbard et al. [1979], we classified the wave data in terms of field intensity. We used the Sweep Frequency Analyzer (SFA) dynamic spectra obtained from September 18, 1992 to April 10, 1995. We have sufficient data for statistical analysis so that we can obtain occurrence probability in which a bias by the orbit trajectory is subtracted. Since the GEOTAIL orbits are within the geomagnetic latitude -20°< m < 20°and mainly concentrated around m =0°, we neglected the dependence of the geomagnetic latitude by projecting the data on the equatorial plane.

WAVE SPECTRA

Before we describe the statistical results, we briefly show a typical signature of ECH waves observed at the dayside magentosphere. Figure 1 shows a SFA dynamic spectra of the electric field component obtained

(a) (b)

Fig. 1. (a) Wave spectra of electric field obtained with the PWI/SFA from 22:00 UT to 23:00 UT on Oct. 17, 1992. (b) Orbit of the GEOTAIL spacecraft in SM coordinates on Oct. 17, 1992.

from 22:00 UT to 23:00 UT on October 17, 1992 [Matsumoto and Usui, 1996]. The orbit in the SM coordinates on that day is also plotted in Figure 1. Wave spectra of two frequency bands are displayed in the figure. One is the frequency band #2 from 196 Hz to 1.57 kHz, and the other is the frequency band #3 from 1.57 kHz to 12.5 kHz. Within each band, a linear frequency scale is used to display the SFA data. The white solid line in the figure indicates fce calculated from the DC magnetic field intensity.

Intense and almost continuous chorus emissions appeared near f ~ 200 Hz, which is seen near the bottom of the spectra. In the upper frequency range approximately above 6 kHz, almost steady continuum radiation (CR) was also seen. Between the chorus frequency and the CR lower cutoff frequency, we see multiple bands of emissions with frequency ranging from fce up to approximately the CR lower cutoff frequency. These waves are the ECH waves of interest. Two types of ECH waves can be identified in the figure. One is a diffuse emission with respect to frequency, in which each multiple band emission has a rather weak amplitude (around -130 dBV/m/Hz1/2) and a broad frequency band. This type of emission was almost continuously observed. The other type of emission was observed sporadically in time, superimposed on weak diffuse ECH waves. We name this burst of ECH waves "Totem Pole" emission (TP emission), after the spectral shape in the dynamic spectrum. The maximum intensity of the electric field for the TP emission is approximately -100 dBV/m/Hz1/2 according to the SFA data with the high gain mode. The duration is less than a few minutes. The TP emissions are often observed near the magnetopause; they are not observed elsewhere.

STATISTICAL ANALYSIS

In the statistical analysis, we used the SFA dynamic spectra obtained from September 18, 1992 to April 10, 1995. We extracted ECH events out of the wave data by checking the dynamic spectra within every 5 min interval. ECH waves with field intensity higher than -130 dBV/m/Hz1/2 are classified as `Intense waves'. Other ECH waves are classified into `Weak waves'. According to this criteria, the TP emissions and diffuse ECH waves, which are shown in Figure 1, are classified into `Intense waves' and `Weak waves', respectively.

Figure 2 depicts the count numbers of ECH occurrence versus radial distance from the Earth. Dotted and solid lines represent variations for weak and intense waves, respectively. Plasma conditions are not constant with respect to the azimuthal direction because of the compression by the solar wind pressure. Therefore the count numbers fluctuate with respect to distance from the Earth. Note that the count number is very low within 6Re from the Earth because the spacecraft rarely passes within this distance, where Re denotes one Earth radius. Primarily, weak waves are broadly distributed approximately from 6Re to 11Re. The maximum count number is found around 7.5 Re from the Earth. Intense waves are distributed from 7Re to 11Re. In contrast to the distribution of weak waves, the profile is more like a Gaussian and its peak is found around 9Re. The main difference between the two distributions is found in a region from 6Re to 8Re. In this region, weak waves are dominantly observed. According to the SFA spectra, we find that most of them are diffuse ECH waves which are considerably broad in band with the highest frequency band near the local electron plasma frequency fpe. As stated in Hubbard et al. [1979], the diffuse type ECH occurs at the inner magnetosphere region where cold electrons ( < 10 eV ) are dominant. Our result basically agrees with theirs.

Fig. 2. Count number of ECH waves observed in the equatorial plane versus distance from the Earth. Dotted and solid lines represent variations for weak ( < -130 dBV/m/Hz1/2) and intense ( -130 dBV/m/Hz1/2) waves, respectively.

(a) (b)

Fig. 3. Spatial distribution of ECH waves in the equatorial plane. (a) weak waves (< -130 dBV/m/Hz1/2) , (b) intense waves (-130 dBV/m/Hz1/2). Black and gray boxes represent count densities of more than and less than 20%, respectively.

Spatial distributions of ECH events are obtained on the SM equatorial plane. Figure 3 shows distributions of `Intense waves' and `Weak waves'. In mapping the events on the equatorial plane, we need to remove a bias by the orbit trajectory. To do so, we define `Count density' as a ratio of the number of events to the one of orbits at each 1Re X 1Re cell in the dayside equatorial magnetosphere. Thin curves in the figure represent the orbit trajectories. Dotted curve shows an approximate location of the bow shock. Although not clearly shown in the figure, a solid thick curve which crosses the positions (x,y) = (-20,-20), (10, 0) and (-20, 20) represents the location of the magnetopause. Small boxes in the figure indicate each 1Re X 1Re cell where ECH waves are observed. Gray and black boxes represent count densities of less than and more than 20%, respectively. We observed no ECH emission in the areas with no boxes.

As shown in Figure 3(a), weak waves are broadly observed along the magnetopause. Although the orbit is somewhat biased to the duskside, the distribution of waves is almost symmetric with respect to the x axis. In the night side (x < 0), the wave occurrence decreases compared with the dayside region. In the magnetosheath region, almost no ECH emission is detected, which implies that ECH waves are typical phenomena in the magnetosphere. Figure 3(b) shows a spatial distribution for `Intense waves'. The region of the wave occurrence is primarily the same as the one shown in panel (a) indicating that waves are mainly observed near the magnetopause. In comparison with panel (a), however, the distribution is obviously asymmetric with respect to the x axis. This implies that intense waves, which correspond to black boxes, appear more frequently at the dawnside than at the duskside. To confirm this statistical result, we examined a couple of magnetopause crossing events. The preference of intense ECH waves at the dawnside is observed for individual orbits as well as in a statistical sense.

SUMMARY AND CONCLUSIONS

We reported statistical analysis of ECH waves observed in the dayside equatorial magnetosphere by the GEOTAIL spacecraft. We used the Sweep Frequency Analyzer (SFA) data obtained from September 18, 1992 to April 10, 1995. Spatial distributions show that intense waves ( -130 dBV/m/Hz1/2) such as the TP emissions mainly occur at the dawnside region near the magnetopause while weak diffuse ECH waves broadly appear along the dayside magnetopause. Dependence of wave occurrence on m is not considered because the GEOTAIL orbits are limited within 20 degrees from the equatorial plane. In fact, we examined the effect of m of the intense ECH events. We found no significant dependence on m.

Although not displayed here, our preliminary analysis shows that `Intense waves' are likely to be observed at the dawnside when the geomagnetic activity is high with Kp 2. As well known, the IMF orientation can affect the geomagnetic activity in the magnetosphere. In a case of southward IMF, dayside reconnection is likely to occur at the magnetopause, which could enhance the activity of ECH waves. The asymmetry of ECH intensity can be due to the difference of plasma conditions between the dawnside and duskside. It could arise from the region of the magentopause which lies downstream of either a quasi-parallel bow shock which usually occurs at the dawnside, or a quasi-perpendicular bow shock at the duskside. The plasma convection from the magnetotail could also cause the asymmetric plasma conditions between the dawnside and duskside on the equatorial plane. To examine the plasma conditions, we need to analyze plasma particle data. Particularly, electron data will give us a clue to understand the asymmetry of ECH intensity.

ACKNOWLEDGMENTS

We thank H. Kojima, I. Nagano, R. R. Anderson, and other GEOTAIL/PWI members for their advice and suggestions.

REFERENCES

Anderson, R. R., and K. Maeda, VLF emissions associated with enhanced magnetospheric electrons, J. Geophys. Res., 82, 135 (1977)

Ashour-Abdalla, M., C. F. Kennel, and W. Liversey, A parametric study of electron multiharmonic instabilities in the magnetosphere, J. Geophys. Res., 84, 6540 (1979)

Christiansen, P., M. P. Gough, G. Martelli, J. -J. Bloch, N. Cornilleau, J. Etcheto, R. Gendrin, C. Bèghin, P. Decreau, and D. Jones, GEOS-1 observations of electrostatic waves and their relationship with plasma parameters, Space Sci. Rev., 22, 838 (1978)

Hubbard, R. F., and T. J. Birmingham, Electrostatic emissions between electron gyroharmonics in the outer magnetosphere, J. Geophys. Res., 83, 4837 (1978)

Hubbard, R. F., T. J. Birmingham, and E. W. Hones, Jr., Magnetospheric electrostatic emissions and cold plasma densities, J. Geophys. Res., 84, 5828 (1979)

Kennel, C. F., F. V. Scarf, R. W. Fredricks, J. H. Mcgehee, and F. V. Coroniti, VLF electric field observations in the magnetosphere, J. Geophys. Res., 75, 6136 (1970)

Matsumoto, H., I. Nagano, R. R. Anderson, H. Kojima, K. Hashimoto, M. Tsutsui, T. Okada, I. Kimura, Y. Omura, and M. Okada, Plasma wave observations with GEOTAIL spacecraft, Geo. Geomag. Geoelectr., 46, 59 (1994)

Matsumoto, H. and H. Usui, Intense bursts of electron cyclotron harmonic waves near the dayside magnetopause observed by GEOTAIL, Geophys. Res. Lett., in press (1996)

Shaw, R. R., and D. A. Gurnett, Electrostatic noise bands associated with the electron gyrofrequency and plasma frequency in the outer magnetosphere, J. Geophys. Res., 80, 4259 (1975)

Young, T. S. T., J. D. Callen, and J. E. MaCune, High frequency electrostatic waves in the magnetosphere, J. Geophys. Res., 78, 1082 (1973)