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Locations of Chorus Emissions Observed by the Polar Plasma Wave Instrument K JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, A00F12, doi:10.1029/2009JA014579, 2010 Click Here for Full Article Locations of chorus emissions observed by the Polar Plasma Wave Instrument K. Sigsbee,1 J. D. Menietti,1 O. Santolík,2,3 and J. S. Pickett1 Received 18 June 2009; revised 20 November 2009; accepted 17 December 2009; published 8 June 2010. [1] We performed a statistical study of the locations of chorus emissions observed by the Polar spacecraft’s Plasma Wave Instrument (PWI) from March 1996 to September 1997, near the minimum of solar cycles 22/23. We examined how the occurrence of chorus emissions in the Polar PWI data set depends upon magnetic local time, magnetic latitude, L shell, and L*. The Polar PWI observed chorus most often over a range of magnetic local times extending from about 2100 MLT around to the dawn flank and into the dayside magnetosphere near 1500 MLT. Chorus was least likely to be observed near the dusk flank. On the dayside, near noon, the region in which Polar observed chorus extended to larger radial distances and higher latitudes than at other local times. Away from noon, the regions in which chorus occurred were more restricted in both radial and latitudinal extent. We found that for high‐latitude chorus near local noon, L* provides a more reasonable mapping to the equatorial plane than the standard L shell. Chorus was observed slightly more often when the magnitude of the solar wind magnetic field BSW was greater than 5 nT than it was for smaller interplanetary magnetic field strengths. We also found that near solar minimum, chorus is twice as likely to be observed when the solar wind speed is greater than 450 km/s than it is when the solar wind speed is less than 450 km/s. Citation: Sigsbee, K., J. D. Menietti, O. Santolík, and J. S. Pickett (2010), Locations of chorus emissions observed by the Polar Plasma Wave Instrument, J. Geophys. Res., 115, A00F12, doi:10.1029/2009JA014579. 1. Introduction Meredith et al., 2001, 2002; Summers et al., 2004]. Chorus emissions are thought to be generated through nonlinear [2] Whistler mode chorus emissions are electromagnetic processes involving an electron‐cyclotron resonance between waves at frequencies between about one tenth of the equa- whistler mode waves and energetic electrons in the outer torial cyclotron frequency, f /10, up to near f , which typi- ce ce radiation belt [Helliwell, 1967; Nunn et al., 1997]. These cally covers a frequency range from a few hundred Hz to waves have also been associated with solar wind pressure several kHz [see Burtis and Helliwell, 1969; Tsurutani and pulses [Lauben et al., 1998] and they may play an important Smith, 1974; Burtis and Helliwell, 1976]. When viewed on role in geomagnetic storms [Santolík et al., 2004; Li et al., timescales of minutes or hours, chorus typically appears in 2007]. Chorus emissions may be able to accelerate elec- time‐frequency spectrograms as narrow band emissions that trons in the outer radiation belts to energies greater than sometimes have a gap near f /2 that divides these waves into a ce 1 MeV through a stochastic process occurring in the low lower and an upper band. When viewed on timescales of a few density region outside the plasmapause [Horne and Thorne, seconds, a time‐frequency spectrogram of chorus emissions 1998; Summers et al., 2002; Horne et al., 2003, 2005a, reveals discrete chorus elements, which correspond to individual 2005b]. Recent simulation results have suggested that rela- whistler mode wave packets. Chorus elements typically tivistic turning acceleration due to nonlinear particle consist of rising tones, but falling tones and elements with dynamics could also accelerate electrons from a few hundred more complicated structures have also been observed [Helliwell, keV up to a few MeV through a resonant wave trapping 1965; Burtis and Helliwell, 1976; Lauben et al., 2002]. process [Omura et al., 2007]. High‐energy electron micro- [3] Substorm electron injections can excite intense whis- bursts are thought to be associated with chorus [e.g., Oliven tler mode chorus emissions near the geomagnetic equator, and Gurnett, 1968; O’Brien et al., 2003] and may be outside the plasmapause [Tsurutani and Smith, 1974, 1977; related to auroral X rays observed on balloon experiments [Anderson and Milton, 1964]. Other research has suggested 1Department of Physics and Astronomy, University of Iowa, Iowa City, that the scattering of electrons by chorus emissions may Iowa, USA. contribute to particle precipitation in the diffuse aurora [Inan 2Institute of Atmospheric Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic. et al., 1992; Ni et al., 2008; Meredith et al., 2009]. 3Faculty of Mathematics and Physics, Charles University, Prague, [4] Although chorus emissions are thought to be gener- Czech Republic. ated in the equatorial plane, chorus‐like electromagnetic emissions can be found at higher latitudes. Some studies Copyright 2010 by the American Geophysical Union. have suggested that the whistler mode waves observed at 0148‐0227/10/2009JA014579 A00F12 1of17 A00F12 SIGSBEE ET AL.: LOCATIONS OF POLAR PWI CHORUS EMISSIONS A00F12 high latitudes are chorus generated near the magnetic (PWI) [Gurnett et al., 1995], which used three orthogonal equator that has propagated to high latitudes [e.g., Bortnik et electric dipole antennas provided by the Electric Field al., 2007]. Under certain conditions, it appears that these Instrument (EFI) [Harvey et al., 1995], two in the spin plane waves can be reflected back toward the equator after prop- and one aligned along the spacecraft spin axis. Magnetic agating to high latitudes [Chum and Santolík, 2005; Santolík fields were measured using the PWI’s magnetic loop et al., 2006]. There has also been evidence that some of the antenna and triaxial magnetic search coil antenna. The Polar off‐equatorial whistler mode emissions may be generated in PWI used five receiver systems to process signals from minimum B pockets at high latitudes on the dayside, near these antennas: a digital wideband receiver (WBR), a high‐ the cusp [Tsurutani and Smith, 1977; Pickett et al., 2001; frequency waveform receiver (HFWR), a low‐frequency Vaivads et al., 2007; Tsurutani et al., 2009]. waveform receiver (LFWR), two multichannel analyzers [5] In this paper, we will explore how the occurrence (MCA), and a pair of sweep frequency receivers (SFR). A probability of chorus emissions in the Polar Plasma Wave key feature of the Polar plasma wave instrument was the Instrument (PWI) data set depends upon magnetic local time, ability to make simultaneous measurements from six magnetic latitude, L shell, and L*. We will also discuss how the orthogonal electric and magnetic field sensors. This feature chorus occurrence probability depends upon the Kp and Dst permitted the analysis of the chorus wave vectors [LeDocq geomagnetic indices, as well as upstream solar wind conditions. et al., 1998; Sigsbee et al., 2008]. The WBR was capable [6] The Polar spacecraft was launched on 24 February of using up to 8‐bit resolution and sample rates as high as 1996 into a highly elliptical orbit, initially with apogee at 249 k samples/s, providing sufficient time and frequency ∼9 Earth radii (RE) in the Northern Hemsiphere, perigee resolution for viewing individual chorus elements. These at 1.8 RE in the Southern Hemisphere, and an approximately features of the Polar PWI are essential for identifying chorus 90° inclination [Acuña et al., 1995]. The Polar PWI made events and analyzing the properties of the chorus emissions. observations of plasma waves in the Earth’s magnetosphere The Polar PWI provided plasma wave data from 26 March between frequencies of 0.1 Hz and 800 kHz from March 1996 to 16 September 1997 (or 540 days). 1996 to September 1997. During 1996 and 1997, as the [9] In our data analysis, we were careful to avoid known Polar spacecraft traveled from high latitudes in the Northern difficulties with the PWI data set. During intervals of Hemisphere down to the equatorial plane, it often skimmed roughly 1 h or less centered on the lowest L shell values of along the boundary of the plasmapause, cutting through the the Polar orbit (twice per orbit), almost all PWI data taken Van Allen radiation belts and entering the plasmasphere at with the electric and loop antennas are contaminated with low altitudes near the equator. After reaching perigee in the interference and crosstalk. This contamination resulted from Southern Hemisphere, Polar encountered similar regions as electric field antenna preamplifier oscillations in regions of it traveled back to high latitudes in the Northern Hemi- high plasma density and interference from the EFI. Polar sphere. The 18 h period of Polar’s orbit provided roughly was generally inside the plasmasphere, where chorus is not two passes through the radiation belts every day. likely to be observed, when these oscillations occurred so [7] The extended coverage at high latitudes provided by eliminating these intervals should not adversely affect our Polar is especially important near local noon, where struc- statistics. These oscillations can be clearly seen in the SFR tured, chorus‐like emissions can often extend into the electric field spectrograms at, or slightly below, the upper vicinity of the cusp. These high‐latitude dayside emissions hybrid resonance frequency and its harmonics. The PWI can feature the classic rising elements of equatorial chorus, electric field measurements taken during the approximate 5– but we have also found falling tones, elements with hook‐ 10 min periods near the end of lengthy eclipses are also like structures, combinations of different spectral forms, and considered uninterpretable due to abnormal behavior of the possible triggered emissions [see, e.g., Helliwell, 1967, EFI electric antennas, so we did not include data from p.
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