Different Spectral Shapes of Whistler-mode Chorus Emissions

Transcription

1 WDS'0 Proceedings of Contributed Papers, Part II,, 00. ISBN MATFYZPRESS Different Spectral Shapes of Whistler-mode Chorus Emissions E. Macúšová and O. Santolík Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic. Institute of Atmospheric Physics, ASCR, Prague, Czech Republic. Abstract. The whistler-mode emissions are occurring in the frequency range from a few hundreds of Hz to several khz. This frequency interval is well covered by the high-time resolution measurements provided by the Wideband (WBD) instrument onboard the four Cluster spacecraft. The time resolution of the WBD instrument allows us to identify whistler mode emissions as shapeless hiss or as chorus composed of discrete narrow-band tones rising or falling in frequency. The STAFF-SA instrument allows us to determine their source region. Our study consists of categorization of unusual spectral shapes of chorus emissions: different shapes of elements and the number of chorus emissions bands. We estimate occurrence rates of the discrete falling tones, rising tones, and shapeless hiss in their source region and we analyze the main factors determining the spectral shapes of chorus wave packets. Our study also includes discrete spectral structures in the whistler-mode range observed close to the plasmapause. Introduction Chorus emissions are very intense electromagnetic whistler-mode waves, which are generated in the low-density region outside the plasmapause due to the injection of plasmasheet electrons during geomagentically disturbed times [Burtis and Helliwell, 99; Tsurutani and Smith, 9; Meredith et al., 00]. This emission is a potential source of the acceleration of energetic electrons in the outer Van Allen radiation belt to relativistic energies [Horne and Thorne, 99; Meredith et al., 00; Santolik et al., 00] and [Demekhov et al., 00]. The Poynting flux measurements made by the Spatio-Temporal Analysis of Field Fluctuations (STAFF) instrument confirm that the chorus source is localized in the magnetic equatorial plane [Santolik et al., 00]. Chorus emissions usually occur near the source region in two distinct frequency bands lying at frequencies from a few hundred Hz to several khz and separated by a gap at one half of the electron cyclotron frequency (/f ce ). This configuration is called the banded chorus [Burtis and Helliwell, 99; Nunn, 9] and it is positively correlated with magnetic activity [Anderson and Maeda, 9]. The Landau damping is one of the possible mechanisms that explain the existence of the gap [Tsurutani and Smith, 9; Borntnik et al., 00]. On the other hand [Bell et al., 009] discuss the role of ducts in the formation of the gap. The frequency range of the lower band extends from 0. to 0. f ce and the upper band extends from 0. to 0. f ce [Burtis and Helliwell, 99; Meredith et al., 00]. Frequency bands most often consist of combinations of fine structure of individual chorus wave packets occurring in one frequency band and of shapeless hiss forming the second band. Trakhtengerts [99, 999] introduced a theory of backward wave oscillator to explain the discrete structures of chorus and the formation of the chorus frequency spectrum [Trakhtengerts et al., 00]. The others nonlinear theories published by [Omura et al., 00, 009; Nunn et al., 009] also try to explain the source mechanism of the discrete wave packets. Nunn et al. [009] explains discrete structures by using an analogy with trigger emissions, Omura et al. [00, 009] on the other hand have theoretically analyzed the mechanism of the rising chorus emissions (explained below) in terms of nonlinear wave growth due to the formation of an electromagnetic electron hole in velocity phase space.

2 Earlier studies have found that the frequency of individual chorus wave packets is usually rising with time [Sazhin and Hayakawa, 99]. We called these elements risers or rising tones, however, however, falling tones (fallers) were also observed [Pickett et al., 00]. Frequency of all of them is changing from a few khz to a few tens of khz per second. Santolik et al. [00]; Chum and Santolík [00] mentioned that chorus emissions propagate in the Earth s magnetosphere from the geomagnetic equator toward larger magnetic latitudes usually approximately along the magnetic field lines, but oblique propagation was also detected [Santolik et al., 009b]. Parrot et al. [00] have presented an event, where magnetospherically reflected chorus waves were observed. A ray tracing study based on measurements from four Cluster spacecraft has shown that chorus waves have suffered a Lower Hybrid Resonance (LHR) reflection at low altitudes and return to the equator. Reflected waves have still a high degree of polarization, even if they started to lose the coherent structure of the chorus elements. These emissions could be a source of hiss [Parrot et al., 00]. Recent studies have shown that chorus is probably the origin of the plasmaspheric hiss [Bortnik et al., 00; Bornik et al., 009; Santolik and Chum, 009a], which is another type of whistler-mode waves. It is an incoherent type of wave in frequency band between a few hundred Hz and several khz, and it is generally confined within the plasmasphere. The purpose of this paper is to analyse the spectral structure of chorus events observed simultaneously by the wave experiments STAFF and WBD, and to establish their main characteristics. Section will briefly describe these two instruments and WHISPER active sounder situated on board the four Cluster spacecraft and the processed data set obtained by them. Examples of typical and untypical chorus spectral shapes are presented. Discussion and conclusions are given in Section. Structure of chorus emissions The unique Cluster multi-spacecraft mission provides us with a large data set with adequate time and frequency resolution for our study. The primary observations discussed below, are from the Cluster Wideband (WBD) plasma wave receiver [Gurnett et al., 99, 00] which makes one-axis measurement of the electric or magnetic field. We use data from the lowest frequency band of the WBD instrument, with a lower frequency of approximately Hz and a total bandwidth of 9. khz. The WBD data are sampled at high time resolution, 0. µs (sample rate is. khz). Simultaneous measurements of two components of the electric field and three components of the magnetic field at frequencies between Hz and khz are made by the STAFF-SA instrument [Cornilleau-Wehrlin et al., 99, 00]. They allow us to determine the value of the parallel component of the Poynting flux normalized by its standard deviation, elipticity, planarity, the angle between ambient magnetic field and the wave vector, and power spectral density of magnetic and electric field fluctuations. The data from WBD Survey Plots were sensitive enough for a rough estimation of the chorus structures. These data are available on the web page They are accessible as ten minutes or thirty seconds time-frequency spectrograms. The WHISPER active sounder situated onboard Cluster spacecraft monitor the natural waves in the khz - 0 khz frequency range. Data form this instrument allow us to determine the position of the plasmapause. When the Cluster spacecraft is crossing the plasmapause, large density irregularities are there observed. They can thus be better qualified as a structured plasmasphere boundary layer. Plasmapause crossing area should be also determined by plasma frequency that increases there extremely quickly. When value of the plasma frequency is out of the range of the WHISPER active sounder we know that we are inside the plasmapause. Outside the plasmapause is value of the plasma frequency between a few khz and a few tens of khz. Chorus emission is mostly observed outside the plasmapause.

3 We examined chorus events measured in the time period between the January 00 and the January 00. All of them occurred in the interval of the dipole magnetic latitude (λ m ) from -0 degrees to 0 degrees. From previous studies we knew how the typical spectral shapes of chorus look like. We found during our study several typical (T) and untypical (UT) spectral shapes of chorus emissions outside and close to plasmapause. From this reason, we created categories of chorus spectral shapes and we visually classified all chorus events into these categories. Some of them fell into the more than one category. The typical chorus (T) outside the plasmapause usually consists of two frequency bands separated by a gap localized close to the source region at /f ce. These frequency bands either contain shapeless hiss, a combination of hiss and discrete structures or are just from individual wave packets. Typical whistler-mode waves observed inside and close to plasmapause are lightning produced whistlers (their frequency is falling with increasing time). The other spectral shapes belong in this paper to UT. The categories of spectral structures with numbers of events belong to them are named and classified below. The classification (T or UT) is shown in front of the name of each category. The appropriate number of all chorus events fulfilling criteria of this category, with the numbers of different types of chorus spectral structure are mentioned after the description of each category. The types of the chorus spectral structures are: risers their frequency is rising with the increasing time (r), fallers their frequency is falling with the increasing times (f), hooks they have curved ends (h), shapeless hiss (sh), and the others vertical lines etc. (o). In some cases chorus elements belong to more than one type of spectral structures, it means risers can be simultaneously hooks. Outside the plasmapause: (T) Individual elements combined with shapeless hiss (see Fig.)... 0 (r, f, h) (T) Individual wave packets with spectral shapes looking like straight lines (rising or falling tones) (r, f) (T) Individual wave packets with shape as hooks (rising or falling tones)... 9 (9r) (T) Shapeless hiss (no discrete structure)... (UT) Risers and fallers observed simultaneously in both frequency bands... (h) (UT) Three or more frequency bands (see Fig.).... (r, sh, f, o) (UT) Chorus elements as broadband vertical lines...0 (UT) Risers changing to fallers (or oppositely) during their propagation away from the source region... (UT) Different inclination of elements in the area of positive and negative magnetic latitudes, for an example in the northern hemisphere were viewed risers and in the south hemisphere were fallers.... (UT) Starting or ending frequency of the element is hidden in the shapeless hiss (Fig. ) (h) (UT) Hissy elements (The difference between the intensity of the element and of its closest background is less then one order of the magnitude)... Close to plasmapause: (T) Pure whistlers... (T) Shapeless hiss... (UT) Whistlers combined with individual elements at - 0 > λ m > 0 (see Fig.) (r,f,o) (UT) Shapeless hiss combined with individual wave packets at - 0 > λ m > 0... (r,f) (UT) Hissy elements (The same as the hissy elements observed outside the plasmapause)... (r, o)

4 CLUSTER WBD :: ::.00 SC SC SC SC UT: 0:00 0:0 0:0 0:0 0:0 0:0 0: 0: SC-λ m ( o ): SC-R(R E ): SC-MLT(h): mv m - Hz - mv m - Hz - mv m - Hz - mv m - Hz Figure. Frequency-time spectrogram of the power-spectral density of electric field fluctuations of typical spectral structure of chorus measured by Cluster,, and on April, 00. The upper band consists of shapeless hiss and the lower band consists of individual rising tones. Some of them have hook shape. The position of the spacecraft is on the bottom: UT represents universal time; λ m is magnetic dipole latitude; R E is the Earth radius and MLT is the magnetic local time. CLUSTER WBD :: ::0.000 SC UT: :0 :0 : :0 SC-λ m ( o ): SC-R(R E ):.... SC-MLT(h):.... mv m - Hz Figure. Example of chorus emissions with more then two frequency bands and more than one gap measured by Cluster on January, 00. One gap localized at the /f ce is marked with the black thin horizontal line. Second gap is below khz and third is around khz. Spacecraft position has the same format as in Fig.. Figures through show a few examples of untypical chorus spectral shapes. These spectrograms of power spectral density of electric field fluctuations were obtained by the WBD instrument. Spacecraft positions are given on the bottom of the spectrograms: UT represents universal time; λ m is magnetic dipole latitude; R E is the Earth radius and MLT is the magnetic local time. These spectrograms display just a small example of the diversity and variability of chorus emissions. Subtle differences are also observed between selected events of spectral structures in one category. This high variability of the chorus emissions are likely due to a nonlinear generation mechanism, wave-particle interactions and their propagation in the Earth magnetosphere. Conclusions and discussion Chorus emissions with more then two frequency bands (Fig.) and with more then one frequency gap were observed in percent of the total number of events situated outside the 9

5 CLUSTER WBD :: ::0.00 SC mv m - Hz SC mv m - Hz SC mv m - Hz SC mv m - Hz UT: 0:0 0:0 0: 0:0 0: 0:0 SC-λ m ( o ): SC-R(R E ): SC-MLT(h): Figure. Example of rising tones with their starting points hidden in hiss. They were observed by all Cluster satellites on December, 00 further from the source region at λ m around - o. We have never observed this type of the spectral structure in the source region. It is impossible to identified the starting time and starting frequency of risers/hooks, which are visible in this time-frequency spectrogram. Spacecraft position is given on the bottom of the Figure with the same format as in Fig.. CLUSTER WBD :: ::00.00 SC UT: 0: 0:0 0: 0:0 0: 0:00 SC-λ m ( o ): SC-R(R E ): SC-MLT(h): mv m - Hz Figure. An example of the abnormal spectral structure of whistler-mode chorus emission, observed close to plasmapause by Cluster on August, 00. All previous events were measured outside the plasmapause. At least one whistler (intense vertical line situated between and. khz at λ m equal -. o ) is visible together with a few risers (short inclined lines at the beginning of the interval at λ m equal -. o and frequencies from. to. khz) in the frequency-time spectrogram. The black horizontal line is the /f ce. Spacecraft position is given on the bottom in the same format as in Fig.. plasmapause (OPP). To our knowledge, this phenomena has not been published in any previous studies of banded chorus. At least percent of all events OPP with three or more frequency bands were observed at MLT interval (-). We didn t observe this type of banded chorus at dipole magnetic latitudes greater than o. However, the individual wave packets with their starting frequency hidden in the shapeless hiss was only observed at magnetic latitudes greater than degrees (see Fig.). Outside the plasmapause we saw the banded chorus with two frequency bands in percent of the cases and in percent of the cases, only one frequency band was detected. The second gap is within the source region usually above /f ce and at magnetic latitudes between and degrees it is usually below the one half of electron cyclotron frequency and above the lower hybrid 0

6 frequency. Perhaps the reflected chorus plays a role in the formation of the third frequency band at magnetic latitudes ( o - o ). An examination of the parallel component of the Poynting flux normalized by its standard deviation may give an insight on the possible role of reflected chorus in the formation of the third or other frequency band and it will be examined in a future study. Chorus emissions containing three or more frequency band usually have an oblique propagation (the value of the angle between the wave vector and the ambient magnetic field was larger than 0 degrees). Our conclusions confirmed some findings of previous studies when the measurements were obtained outside the plasmapause. Individual wave packets occurred mostly as rising tones ( percent) in the dawn and day MLT sector, with the K p index varying between values of slightly less than to as large as. The plasma density was usually a few tens of particles per cc and the McIlwain parameter was around R E. Hooks covered the dayside with the probability 0 percent and comprise percent of the total number of events situated OPP. The frequency of all hooks rise with time. Elements with characteristics as broadband vertical lines and undefined inclination were found in 0 percent of the total number. The banded chorus with shapeless hiss in one frequency band and with visible individual wave packets in second band were observed in percent of the cases. An interesting conclusion was that lightning produced whistlers were detected together with chorus elements in almost percent of all events located close to plasmapause. Our investigation did not show any evident dependence on such significant parameters, as for example the Kp index, MLT or magnetic latitude are. These preliminary results help to explain some of behaviors and properties of chorus emissions, and they can help theoretical physicists with the estimation of some input parameters necessary in their studies. Acknowledgments. The authors thank the WBD and STAFF teams for the magnetic and electric field data. We acknowledge the support of the Ministry of Education, Youth, and Sports of the Czech Republic under contracts ME and ME000. References Anderson R. R., and K. Maeda, VLF Emissions Associated With Enhanced Magnetospheric Electrons, J. Geophys.Res.,,, 9. Bell T. F., U. S. Inan, N. Haque, and J. S. Pickett, Source regions of banded chorus, Geophys. Res. Lett.,, L0, doi:0.09/009gl09, 009. Bortnik J., U. S. Inan and T. F. Bell,Landau damping and resultant unidirectional propagation of chorus waves, Geophys. Res. Lett.,, L00, doi:0.09/00gl0, 00. Bortnik, J., R. M. Thorne, and U. S. Inan, Nonlinear interaction of energetic electrons with large amplitude chorus, Geophys. Res. Lett.,, L0, doi:0.09/00gl000, 00. Bortnik J., W. Li, R. M. Thorne, V. Angelopoulos, C. Cully, J. Bonnell, O. Le Contel, and A. Roux, An Observation Linking the Origin of Plasmaspheric Hiss to Discrete Chorus Emissions, Science,,, 009. Burtis W. J. and R. A. Helliwell, Banded chorusa new type of vlf radiation observed in the magnetosphere by OGO and OGO, J. Geophys.Res.,, p. 00, 99. Cornilleau-Wehrlin, N., et al., The Cluster spatio-temporal analysis of field fluctuations (STAFF) experiment,space Sci. Rev., 9, 0-, 99. Cornilleau-Wehrlin, N., Chanteur, G., Perraut, S., Rezeau, L., Robert, P., Roux, A., de Villedary, C., Canu, P., Maksimovic, M., de Conchy, Y., Lacombe, D. Hubert C., Lefeuvre, F., Parrot, M., Pinon, J. L., Dcrau, P. M. E., Harvey, C. C., Louarn, Ph., Santolik, O.l Alleyne, H. St. C.l Roth, M.l Chust, T.l Le Contel, O., Staff Team, First results obtained by the Cluster STAFF experiment, Annales Geophysicae,,, 00. Demekhov, A. G., V. Y. Trakhtengerts, M. J. Rycroft, and D. Nunn, Electron acceleration in the magnetosphere by whistler-mode waves of varying frequency, Geomang. Aeron.,, No.,, 00. Gurnett, D. A., Huff, R. L., and Kirchner, D. L, The Wide-band plasma wave investigation, Space Sci. Rev., 9:90, 99.

7 Gurnett, D. A., Huff, R. L., Pickett, J. S., Persoon, A. M., Mutel, R. L., Christopher, I. W., Kletzing, C. A., Inan, U. S., Martin, W. L., Bougeret, J.-L., Alleyne, H. St. C., Yearby, K. H., First results from the Cluster wideband plasma wave investigation, Annales Geophysicae, 9, 9, 00. Horne, R. B. and R. M. Thorne, Potential waves for relativistic electron scattering and stochastic acceleration during magnetic storms, Geophys. Res. Lett.,, 0 0, 99. Chum J. and O. Santolík, Propagation of whistler-mode chorus to low altitudes: divergent ray trajectories and ground accessibility, Annales Geophysicae,,, 00. Meredith, N. P., Horne, R. B., and Anderson, R. R.: Substorm dependence of chorus amplitudes: Implications for the acceleration of electrons to relativistic energies, J. Geophys.Res., 0, -, 00. Meredith, N. P., R. B. Horne, D. Summers, R. M. Thorne, R. H. A. Iles, D. Heynderickx and R. R. Anderson, Evidence for acceleration of outer zone electrons to relativistic energies by whistler mode chorus,annales Geophysicae, 0, 9 99, 00. D. Nunn, A theoretical investigation of banded chorus, J. Plasma Phys.,, 9, 9. Nunn, D., O. Santolik, M. Rycroft, and V. Trakhtengerts, On the numerical modelling of VLF chorus dynamical spectra, Annales Geophysicae,, -9, 009. Omura, Y., Y. Katoh, and D. Summers, Theory and simulation of the generation of whistler-mode chorus, J. Geophys.Res.,, A0, doi:0.09/00ja0. Omura, Y., M. Hikishima, Y. Katoh, D. Summers, and S. Yagitani, Nonlinear mechanisms of lowerband and upper-band VLF chorus emissions in the magnetosphere, J. Geophys.Res.,, A0, doi:0.09/ 009JA00, 009. Parrot, M., Santolík, O., Cornilleau-Wehrlin, N., Maksimovic, M., and Harvey, C. C.: Magnetospherically reflected chorus waves revealed by ray tracing with CLUSTER data, Annales Geophysicae,,, - 0, 00. Parrot, M., O. Santolík, D.A. Gurnett, J.S. Pickett, and N. Cornilleau-Wehrlin, Characteristics of magnetospherically reflected chorus waves observed by CLUSTER,Annales Geophysicae,, 9 0, 00. Pickett, J. S., O. Santolik, S. W. Kahler, A. Masson, M. L. Adrian, D. A. Gurnett, T. F. Bell, H. Laasko, M. Parrot, P. Decreau, A. Fazakerley, N. Cornilleau-Wehrlin, A. Balogh, and M. Andre, Multi-point Cluster Observations of VLF Risers, Fallers, and Hooks at and near the Plasmapause, in Multiscale processes in the Earth s magnetosphere: From Interball to Cluster, Eds: J.-A. Sauvaud and Z. Nemecek, NATO Science Series II: Mathematics, Physics and Chemistry, Vol., ISBN: -00--, 00. Santolík O., D. A. Gurnett, J. S. Pickett, M. Parrot and N. Cornilleau-Wehrlin, A microscopic and nanoscopic view of storm-time chorus on March 00, Geophys. Res. Lett.,,, CiteID L00, 00a. Santolík, O., J. Chum, M. Parrot, D. A. Gurnett, J. S. Pickett, and N. Cornilleau-Wehrlin (00), Propagation of whistler mode chorus to low altitudes: Spacecraft observations of structured ELF hiss, J. Geophys.Res.,, A00, doi:0.09/00ja0, 00. Santolík O. and J. Chum, The origin of plasmaspheric hiss, Science, (9), 9 0, doi: 0./Science., 009a. Santolík, O., D. A. Gurnett, J. S. Pickett, J. Chum, N. Cornilleau-Wehrlin (009), Oblique propagation of whistler mode waves in the chorus source region, J. Geophys.Res.,, A00F0, doi:0.09/009ja0, 009b. Sazhin, S. S., and M. Hayakawa, Magnetospheric chorus emissions: A review, Planet. Space Sci., 0(), -9, 99. Trakhtengerts, V. Y., Magnetosphere cyklotron maser: Backward wave oscillator generation regime, J. Geophys.Res., 00, 0 0, 99. Trakhtengerts, V.Y., A generation mechanism for chorus emission, Annales Geophysicae,, 9 00, 999. Trakhtengerts, V. Y., A. G. Demekhov, E. E. Titova, B. V. Kozelov, O. Santolik, D. Gurnett, and M. Parrot, Interpretation of Cluster data on chorus emissions using the backward wave oscillator model, Phys. Plasmas,,, 00. Tsurutani, B. T. and E. J. Smith, Postmidnight Chorus: A Substorm Phenomenon, J. Geophys.Res., 9,, 9.