Locations of chorus emissions observed by the Polar Plasma Wave Instrument

Sigsbee, K.; Menietti, J. D.; Santolı́k, O.; Pickett, J. S.

doi:10.1029/2009ja014579

Cited by 21 publications

(27 citation statements)

References 71 publications

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“…Waves on the dayside occur over a wider range of activity, and even during quiet conditions wave mean and time‐averaged amplitudes at noon significantly exceed disturbance amplitudes at midnight (at least at latitudes of >15°). High occurrence in the noon sector is not fully understood [ Tsurutani et al , 2009; Santolík et al , 2010; Sigsbee et al , 2010; Spasojevic and Inan , 2010] but is likely a combination of several factors including higher magnetic field homogeneity and lower damping leading to superior generation or propagation conditions as well as additional contributions to chorus source energy aside from injected electrons. It has been suggested that this could be related to the local magnetospheric configuration because of solar wind speed or dynamic pressure [e.g., Tsurutani and Smith , 1977; Koons and Roeder , 1990; Santolík et al , 2005b; Li et al , 2009; Spasojevic and Inan , 2010].…”

Section: Discussionmentioning

confidence: 99%

“…For example, Shprits et al [2006] showed that energy and pitch angle diffusion coefficients for pitch angles of 20°–40° for 1 MeV electrons at L = 3.5 can shift from being negligible when considering only equatorially confined waves ( λ < 15°), to ∼10 −5 s −1 (similar to that for pitch angles >40°) by extending waves to just 25° latitude. Studies by Sigsbee et al [2010] and Bunch et al [2011] have begun to address this need and have shown significant chorus occurrence rates and intensities, respectively, in the off‐equatorial region by employing observations from the Plasma Wave Instrument (PWI) onboard the Polar spacecraft, the polar orbit of which (∼18 h, ∼2 × 9 R E , ∼90° inclination, North Pole apogee) has allowed for extension of the known distribution of chorus activity to much higher latitudes.…”

Section: Introductionmentioning

confidence: 99%

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Off‐equatorial chorus occurrence and wave amplitude distributions as observed by the Polar Plasma Wave Instrument

2012

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[1] Determining the global distribution of chorus wave power in the off-equatorial region (i.e., magnetic latitude l > 15 ) is a crucial component of understanding the contribution of chorus to radiation belt acceleration and loss. In this paper we use a database of chorus power spectral density observations from the Plasma Wave Instrument (PWI) Sweep Frequency Receiver (SFR) on the Polar spacecraft to generate separate distributions of wave occurrence rate and magnetic field amplitude as a function of space and geomagnetic activity. Previous studies focused on a band-integrated and time-averaged data product to characterize the global distribution of wave power. Using a slightly different technique, we first estimate the wave amplitude from the peak wave power spectral density for times when chorus is observed. The mean wave amplitude at a given location is then multiplied by the wave occurrence rate to yield the time-averaged amplitude. We present the spatial distributions of wave occurrence rate, mean amplitude, and time-averaged amplitude in the region of maximum statistics, l > 15 and R = 4À8 R E . We find that waves of significant amplitude (>10 pT) can be observed in all local time sectors, but significant wave occurrence (>20%) is confined to the dawn and noon local time sectors. Wave mean and time-averaged amplitudes are also highest in the dawn and noon sectors. The spatial extent of regions with high time-averaged amplitude is primarily defined by regions of high occurrence rate. Time-averaged amplitudes exceeding $6 pT are observed up to a magnetic latitude of 40 at dawn and 50 at noon, while at midnight and dusk the time-averaged amplitude tends to be below that value. We also examine the geomagnetic and solar wind dependence of the spatial distribution of wave occurrence, mean amplitude, and time-averaged amplitude. In the off-equatorial region (l > 15 ), wave amplitude and occurrence on the nightside increase dramatically during disturbed geomagnetic and solar wind conditions. In contrast, waves on the dayside occur over a wider range of activity, and even during quiet conditions, mean and time-averaged amplitudes at noon significantly exceed amplitudes at midnight for disturbed times. In the dusk sector, observation of waves is mostly limited to quiet conditions, and during those times, mean amplitudes at dusk exceed those at midnight and approach amplitudes observed in the dawn sector. Parallel investigation of the independent variability of occurrence and amplitude provides a more complete picture of the chorus wave environment, particularly for application to modeling radiation belt dynamics on both short and long time scales.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Off‐equatorial chorus occurrence and wave amplitude distributions as observed by the Polar Plasma Wave Instrument

2012

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“…It is well known that chorus waves are an important mechanism for energization and loss processes outside the plasmasphere in the outer radiation belt [e.g., Summers et al , 2007]. Chorus waves have been reported to be associated with substorm activity [e.g., Tsurutani and Smith , 1974, 1977]; hence, previous observational chorus wave statistical studies [e.g., Li et al , 2009; Sigsbee et al , 2010; Bunch et al , 2011] were often binned according to the maximum AE value in the previous 3 h ( AE *), magnetic local time ( MLT ), and/or solar wind speed ( V sw ). Due to limitations with SCATHA data, we are unable to bin SC3 data according to MLT or substorm events for a meaningful statistical study.…”

Section: Discussion and Summarymentioning

confidence: 99%

SCATHA measurements of electron decay times at 5 < L ≤ 8

Su¹,

Johnston²,

Albert³

et al. 2012

J. Geophys. Res.

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[1] The electron decay timescale (t) is well known to be associated with radiation belt loss processes. Knowledge of t is important for understanding pitch angle and radial diffusion mechanisms. Previous studies reported decay timescales from the inner belt to geosynchronous orbits; however, relatively few statistical studies have been focused on the region beyond the traditional outer belt. In this paper, a systematic calculation of electron decay times at 5 < L ≤ 8 is performed using 11 years (1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989) of electron data from the Spacecraft Charging AT High Altitudes (SCATHA) satellite. The decay timescale is determined using daily median fluxes, providing resolution of t > 1 day. t is examined as a function of energy, pitch angle, L shell, Kp, and AE during magnetically disturbed periods when Dst ≤ À50 nT. Results show that t increases with increasing electron energy at L < 6.6 for electron energies from 50 to 300 keV, but is independent of energy at L > 6.6. This suggests radial transport as the dominant effect at L > 6.6. Additionally, t decreases with increasing L-shell. This dependence has the strongest correlation and is seen in all energies and pitch angles. However, t has no systematic dependence with pitch angle suggesting that pitch angle diffusion also plays a key role in the electron loss process. Based on our results, t can be expressed as a function of energy and L, and coefficients are provided for a two-variable fit. Surprisingly, t is slightly longer for higher activity cases at L < 6.6, which is inconsistent with the current radial or pitch angle diffusion models. Global effective decay times on the timescale of days place an upper bound on the true loss timescale.

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“…Though the above estimations are based upon a number of simplified models and thus approximate only, they guide us to tentatively suggest that the ECH be a more promising candidate for higher L shell PsAs as in our event. However, one should be noted that the whistle-mode chorus may well have an off-equatorial source [Tsurutani and Smith, 1977;Sigsbee et al, 2010] which could not be observed by THEMIS satellites. We thus may not entirely exclude, from both theoretical and observational viewpoints, the possible existence of the chorus waves as well as their potential role in generating the observed PsAs in our event.…”

Section: Discussionmentioning

confidence: 99%

THEMIS observations of electron cyclotron harmonic emissions, ULF waves, and pulsating auroras

et al. 2010

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[1] We present multiprobe, multi-instrument observations of the electron cyclotron harmonic (ECH) emissions and ultralow-frequency (ULF) waves from Time History of Events and Macroscale Interactions during Substorms (THEMIS) and explore their potential linkage to the concurrent ground-observed pulsating auroras (PsA) on 4 January 2009. The ECH emissions were observed as discrete packets modulated by the ULF flapping motion of the neutral sheet around the probes. Combining different data sets of the ECH observations, we infer that the ECH emission intensities were strongly fluctuating and contained multi-time scale fine structures. The distribution of PsA patches featured longitudinal "wavelength" in concert with the in situ ULF wave characteristics inferred from a cross-phase analysis. The overall activeness of the PsA correlated with the in situ-measured energetic electron fluxes and ECH wave intensities. We suggest that ECH waves played the key role in the pitch angle diffusion of the plasma sheet electrons that led to the PsA, while the ULF waves structured the plasma sheet and imposed a macroscopic effect over the spatial distribution of the PsA.

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Locations of chorus emissions observed by the Polar Plasma Wave Instrument

Cited by 21 publications

References 71 publications

Off‐equatorial chorus occurrence and wave amplitude distributions as observed by the Polar Plasma Wave Instrument

Off‐equatorial chorus occurrence and wave amplitude distributions as observed by the Polar Plasma Wave Instrument

SCATHA measurements of electron decay times at 5 < L ≤ 8

THEMIS observations of electron cyclotron harmonic emissions, ULF waves, and pulsating auroras

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