Pc5 modulation of high energy electron precipitation: particle interaction regions and scattering efficiency

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 subauroral, 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 K p selection towards mid (i.e. 3-4) activity levels which mimics the product of the K p 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 Correspondence to: E. Spanswick (emma@phys.ucalgary.ca) 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 (diffusio...

Pc5 geomagnetic pulsations, pulsating particle precipitation, and VLF chorus: Case study on 24 November 2006

“…Nosé et al [1998] used DE-satellite and ground magnetometer and riometer data to argue that the riometer signature, accompanying Pc5 pulsations was due to parallel accelerated electrons in FLRs. However, contrary to that, Spanswick et al [2005] using the NORSTAR riometer and CANOPUS magnetometer arrays, and CRRES satellite concluded that high energy electron precipitation associated with Pc5pulsations is caused by pitch angle scattering. The relationship between quasi-periodic VLF emissions and geomagnetic pulsations has been discussed by some authors [e.g., Kimura, 1974;Manninen et al, 1996].…”

“…Moreover, the statistical results, obtained by Spanswick et al [2005], suggest there is a localized region in the pre-noon magnetosphere where Pc5 band ULF activity can modulate high-energy electron precipitation, and a necessary condition for the pulsations observed by riometer is magnetic pulsations.…”

“…This wave component could be enhanced by the 3-4 mHz fluctuations in SW dynamic pressure and simultaneous antiphase IMF Bz ( Figure 5) variations supporting a possible magnetosphere compression. Moreover, Spanswick et al [2005] showed that FLRs are very efficient for modulating particle precipitation. One possible mechanism of interaction between VLF and ULF waves seems to be the selfmodulation of the wave-particle system [e.g., Bespalov and Kleimenova, 1989].…”

“…There is clearly a relationship between the fluxes of tens to hundred of keV electrons and ULF waves. A number of ground-based, satellite, and theoretical studies have been documented the particle flux modulation causing by ULF waves in the Pc5 frequency band [e.g., Kokubun et al, 1977;Kremser et al, 1981;Kleimenova et al, 1997Kleimenova et al, , 2005Spanswick et al, 2005;Sarris et al, 2007]. Nosé et al [1998] used DE-satellite and ground magnetometer and riometer data to argue that the riometer signature, accompanying Pc5 pulsations was due to parallel accelerated electrons in FLRs.…”

Pc1–2 auroral pulsations

“…Roldugin and Roldugin [2010] studied 12 events of Pc4-5 pulsations recorded during photometric observations within the 2007-2008 polar night season in Spitsbergen, and in all events, similar auroral fluctuations were present. The occurrence of Pc5 fluctuations in the riometer absorption within a substantial data set was earlier mentioned by Spanswick et al [2005]: in the morning sector, 70 per cent Pc5 magnetic pulsations were accompanied by riometer pulsations. That the riometer pulsations do not occur in the other 30 per cent events may be explained by the fact that riometer's field of view is narrower than that of the magnetometer, or by a soft spectrum of precipitating particles when they do not reach the D region.…”