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

Abstract

[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.

Electron acceleration at Jupiter: input from cyclotron-resonant interaction with whistler-mode chorus waves

“…The orbit of Galileo was such that it covered regions close to the magnetic equator, but the quantity of wave data falls off dramatically at latitudes λ 10 • . Evidence from both the Earth and Saturn, where chorus is observed up to 45 • (Meredith et al, 2012;Bunch et al, 2012;Menietti et al, 2012), suggest that chorus may be present at Jupiter far from the equator.…”

“…At Jupiter wave observations have only been made for latitudes 10 • in the region outside Io; previous work to calculate the electron acceleration due to chorus waves assumed the waves only extended to 10 • . However, observations at Earth and Saturn show that waves are observed up to latitudes of 45 • (Meredith et al, 2001(Meredith et al, , 2012Bunch et al, 2012;Menietti et al, 2012). Since the plasma density drops significantly with increasing latitude (Bagenal, 1994), the conditions for acceleration may be more favorable at higher latitudes.…”

Field-aligned chorus wave spectral power in Earth's outer radiation belt

“…Because of the large latitudinal extent of chorus wave power in the day side (e.g. Bunch et al, 2012;Tsurutani et al, 2012;Agapitov et al, 2013), parameters of chorus wave distributions have been determined recently along magnetic field lines up to high latitudes. These parameters have made it possible to estimate the effects of the latitudinal dependence of chorus wave-normal angle and amplitude on the electron diffusion rates.…”

“…However, observed field-aligned dependence of chorus spectral parameters have not been provided nor explained yet. These parameters would enable the accurate calculation of chorus spectral extent impact on the outer radiation belt dynamics, notably at high latitudes (Bunch et al, 2013). Bunch et al (2013) have used observations carried out on POLAR spacecraft to determine the off-equatorial chorus spectral extent for different L-shells and latitude ranges, and to deduce first estimations of their impact on diffusion rates.…”

“…These parameters would enable the accurate calculation of chorus spectral extent impact on the outer radiation belt dynamics, notably at high latitudes (Bunch et al, 2013). Bunch et al (2013) have used observations carried out on POLAR spacecraft to determine the off-equatorial chorus spectral extent for different L-shells and latitude ranges, and to deduce first estimations of their impact on diffusion rates. However, due to the POLAR orbit and the time range of the statistics used, the full range of chorus latitudinal extent could not be covered for each specific L-shell.…”