Abstract
Abstract. Using the NORSTAR riometer and CANOPUS magnetometer arrays we have investigated the modulation of high energy electron precipitation by ULF waves in the Pc5 frequency band. We conducted two separate studies of Pc5 activity in the riometers. The first is an independent survey of three riometer stations in the Churchill line (one at each sub-auroral, auroral, and typical polar cap boundary latitudes) in which we identified all riometer Pc5-band pulsations over 11 years. All had a corresponding magnetometer pulsation implying that a magnetic pulsation, is a necessary condition for a riometer pulsation (in the Pc5 Band). We find seasonal and latitude dependencies in the occurrence of riometer pulsations. By a factor of two, there are more riometer pulsations occurring in the fall-winter than the spring-summer. At higher latitudes there is a tendency towards noon pulsations during the spring-summer, suggesting that the criteria for riometer pulsations is affected by the dipole tilt. Our second study was based on the previous magnetometer study of Baker et al. (2003). Using the database of Pc5 activity from that study we were able to select the riometer Pc5 pulsations which adhere to the strict Pc5 definition in the magnetometer. We find that roughly 95% of the riometer pulsations occurred in the morning sector compared to 70% in the magnetometer. Given a magnetometer pulsation at Gillam in the morning sector, there is a 70% chance of there being a corresponding riometer pulsation. The morning sector probabilities at Rankin (geomagnetic (PACE) latitude 74°) and Pinawa (61°) are 3% and 5%, respectively. These statistics suggest there is a localized region in the pre-noon magnetosphere where Pc5 band ULF activity can modulate high energy electron precipitation. We also find that riometer pulsations display a Kp selection towards mid (i.e. 3–4) activity levels which mimics the product of the Kp dependence of high-energy electron fluxes on the dawn side (from CRRES) and all magnetic Pc5 activity. A superposed epoch analysis revealed that the elevated electron flux needed to produce a riometer pulsation is most likely provided by substorm injections on the nightside. We also find that the amplitude of modulated precipitation correlates well with the product of the background absorption and the magnetic pulsation amplitude, again leading to the idea that a riometer pulsation needs both favorable magnetospheric electron flux conditions and large enough magnetic Pc5 wave activity. We further separate our pulsations into field line resonances (FLRs), and non-field line resonances (non-FLRs), as identified in the Baker et al. (2003) survey. We find that FLRs are more efficient at modulating particle precipitation, and non-FLRs display an amplitude cutoff below which they do not interact with the high energy electron population. We conclude that the high energy electron precipitation associated with Pc5 pulsations is caused by pitch angle scattering (diffusion) rather than parallel acceleration. We suggest two future studies that are natural extensions of this one. Keywords. Energetic Particles/Precipitating; Wave-Particle Interactions; Auroral Phenomena
Highlights
The spatio-temporal distribution of the aurora allows us to remotely sense magnetospheric processes
In total we identified over 750 h of riometer Pc5 activity in Pinawa, Gillam, or Rankin
In addition to the study of riometer Pc5 activity we conducted a separate survey of all of the Pc5 pulsations identified by Baker et al (2003) in the collocated CANOPUS magnetometer array
Summary
The spatio-temporal distribution of the aurora allows us to remotely sense magnetospheric processes. Baker et al (1981) showed that, provided that the Kp criterion for strong pitch angle diffusion was met, the integrated electron flux at a geostationary satellite was well correlated with the CNA, as observed by a magnetically conjugate riometer Their result was tantalizing: provided that the high electrons are efficiently scattered into the loss cone, riometers are able to provide quantitative information about the spatial distribution of the inner magnetospheric high energy electrons. Baker et al (1980) used Los Alamos National Laboratory charged particle analyzers on board three spacecraft to explore the local time and magnetic latitude distribution of pulsations in high energy electron fluxes at geosynchronous orbit. A single beam riometer collocated with each magnetometer used in the Baker et al (2003) study This magnetic ULF data set, combined with the collocated riometers, provides us with an excellent chance to advance our understanding of the relationship between ULF waves and the precipitation of high energy electrons. In this study we use only one-minute data from the 40–50 keV energy channel
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