Drivers of chorus in the outer dayside magnetosphere

Abstract

[1] Using ELF/VLF wave data recorded in 2007 from two high-latitude (L = 69.8°, 71.8°) Antarctic ground stations, the dayside variation of chorus wave occurrence and amplitude are analyzed in conjunction with geomagnetic and solar wind driving parameters. Both stations observe chorus (defined here as discrete rising emission tones together with unstructured hiss) in a broad window of local time across the dayside when the stations are on closed magnetic field lines. Wave occurrence rates rise rapidly from ∼0.06-0.12 at dawn to their maximum value of ∼0.5-0.6 near local noon. The event amplitudes also peak near noon. Occurrence probabilities at the lower-latitude station are consistently higher with the average difference in the rate between the two stations being 0.15. In addition, ∼80% of the time, event amplitudes are larger at the lower-latitude site. When the stations are in the dawn local time sector (5.5 < MLT < 10), the onset of waves is clearly linked to substorms, as seen by the AE index as well as by energetic electron injections observed at geosynchronous orbit. However, as the stations rotate to noon (MLT > 10), wave occurrence rates appear to be relatively independent of geomagnetic activity as measured by K p and AE. Chorus near noon at times appears related to substorm activity, but intense waves can also be observed during extended quiet periods. Waves across the entire dayside are more likely during higher solar wind dynamic pressure as well as during significant changes in pressure. We attribute the high occurrence rate of outer dayside chorus to several effects resulting from solar wind compression of the dayside magnetosphere; the first is electron drift shell splitting, and the second is the creation of a region of high magnetic field homogeneity which is particularly favorable for wave growth.

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

“…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].…”

“…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]. Recent studies have shown that particles experiencing so‐called Shabansky orbits, in which their drift motion shifts to high latitudes in the outer dayside [ Shabansky , 1971], can lead to significant ion anisotropies and subsequently ion cyclotron wave growth in this region [ McCollough et al , 2010].…”

Polarization properties of Gendrin mode waves observed in the Earth's magnetosphere: observations and theory

“…Thus, these modes could be responsible for non-ducted electromagnetic wave propagation in the low-band range (Lauben et al, 2002;Helliwell, 1995). There is observational evidence that electromagnetic waves originated in the outer region of the Earth's dayside magnetosphere can propagate to the ground (Spasojevic et al, 2008). It is possible that Gendrin mode waves may be responsible for these ground observations and we suggest that further modeling efforts be undertaken to determine if this is the correct interpretation or not.…”

PENGUIn/AGO and THEMIS conjugate observations of whistler mode chorus waves in the dayside uniform zone under steady solar wind and quiet geomagnetic conditions

“…Chorus waves on the dayside, on the other hand, have a relatively high overall occurrence probability compared with other magnetospheric regions [ Tsurutani and Smith , 1977; Sigsbee et al , 2010; Li et al , 2011a] and show little dependence of occurrence on geomagnetic activity [ Tsurutani and Smith , 1977; Spasojević and Inan , 2010]. Dayside chorus has been observed over a wider range of magnetic latitudes (extending at least 25° off the equator) [e.g., Burton and Holzer , 1974; Tsurutani and Smith , 1977; Meredith et al , 2001a; Vaivads et al , 2007; Tsurutani et al , 2009].…”