Modeling ring current ion and electron dynamics and plasma instabilities during a high‐speed stream driven storm

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94593/1/jgra21840.pd

The modulation of electromagnetic ion cyclotron waves by Pc5 ULF waves

The modulation of electromagnetic ion cyclotron (EMIC) waves by longer-period ULF waves has been proposed as a method for producing pearl structured Pc 1–2 EMIC waves. This study examines frequency and phase relationship between Pc 1 EMIC wavepacket envelopes and simultaneously occurring Pc 5 ULF waves using magnetic data measured by the CRRES spacecraft. Intervals from three days in 1991 where CRRES observed pearls are presented along with simple statistics for 58 EMIC wavepackets. The observations were dominated by EMIC waves propagating away from the equatorial region. Comparisons between pearl wavepacket envelopes and Pc 5 waves show excellent agreement. The pearl wavepacket duration times, τdur, were statistically correlated with Pc 5 wave periods, TPc5, resulting in a correlation coefficient of R=0.7 and best fit equation τdur=0.8·TPc5+6s. In general, phase differences varied although time intervals of constant in-phase or anti-phase correlation were observed. Anti-phase modulation may be explained by a decreasing background magnetic field due to the negative cycle of the ULF wave decreasing Alfvén velocity and minimum resonant energy. In-phase modulation could be the result of adiabatic modulation of temperature anisotropy in-phase with variations in the background field. Non-adiabatic processes may contribute to intervals that showed varying phase differences with time. Results suggest that future theoretical developments should take into account the full range of possible wave particle interactions inside the magnetosphere

Electromagnetic ion cyclotron waves in the magnetosphere

Electromagnetic ion cyclotron (EMIC) waves are generated in the equatorial region of the plasmasphere-magnetosphere by internal wave-particle interaction with ring current ions. In ground observations they are observed as Pc 1-2 (0.1-5 Hz) waves, and one group of waves exhibits a spectral fine structure that has been classically explained by bouncing packet field-aligned propagation. An unstructured class of Pc 1-2 waves, including intervals of pulsations with diminishing period, lacks a fine structure pattern and is the dominant emission observed in the middle and outer magnetosphere by satellites. Poynting flux observations show that wave energy propagates unidirectionally away from the equatorial region, which does not support the bouncing wave packet paradigm. The cold/cool magnetospheric plasma has a profound effect on the generation and spectral properties of EMIC waves. The waves are often observed at geostationary orbit within the outer magnetosphere extension of plasmaspheric plasma plumes seen by the IMAGE spacecraft in the extreme ultraviolet (EUV) imager instrument and associated with subauroral proton arcs seen by the far ultraviolet (FUV) imager. This provides evidence in support of a ring current loss mechanism induced by pitch angle scattering of protons by EMIC waves. Characteristic frequencies introduced into the cold/cool plasma by heavy O[+] and He[+] ions determine the EMIC wave spectra and these may be used in plasma diagnostic studies. Outstanding issues considered include the possible role of the ionospheric Alfvén resonator in EMIC wave generation, and the modulation of EMIC waves by Pc 3-5 long-period ULF waves

Estimating relativistic electron pitch angle scattering rates using properties of the electromagnetic ion cyclotron wave spectrum

An EMIC wave event observed by the CRRES spacecraft during an active period on 11 August 1991 was studied in order to estimate electron minimum interaction kinetic energy Emin and using quasilinear theory, to calculate the resonant scattering rate Dαα. The wave packet semibandwidth δω/2π full-width half maximum ranged from 0.06 Hz to 0.27 Hz. Resonant scattering was assumed to occur over the frequency interval ωm - δω to ωm + δω. Assuming typical stormtime ion concentrations, the use of realistic wave spectral properties when compared to only using the central wave frequency ωm results in 3 to 4 times as many wave packets that are able to interact with relativistic electrons below ∼2 MeV. Values of D αα associated with two of the wave packets, where E min falls to within the 1-2 MeV energy range, were comparable to the limit of strong diffusion suggesting enhanced electron precipitation. CRRES observed an ∼1 order of magnitude decrease in the 1-2 MeV electron flux levels during the EMIC wave interval. It is suggested that this flux decrease was due to EMIC waves pitch angle scattering the relativistic electrons. The EMIC waves were observed near the start of the main phase of a geomagnetic storm. This study strengthens the suggestion that relativistic electron scattering by EMIC waves can compete with the Dst effect as a mechanism of decreasing relativistic electron fluxes from the outer zone during magnetic storms

Observations at geosynchronous orbit of a persistent Pc5 geomagnetic pulsation and energetic electron flux modulations

A long lasting narrow-band (4&ndash;7 mHz) Pc5 fluctuation event at geosynchronous orbit is presented through measurements from GOES-8 and GOES-10 and the response of energetic electrons with drift frequencies close to the narrow-band pulsation frequency is monitored through a spectral analysis of flux data from the LANL-SOPA energetic electron instrument. This analysis shows electron flux modulations at the magnetospheric pulsation's frequency as well as at various other frequencies in the Pc5 range, related to the particles' drift-frequencies and their harmonics. A drift resonance effect can be seen, with electron flux modulation becoming more evident in the energy channels of electrons with drift frequencies closer to the wave frequency; however no net increase or decrease in energetic electron flux is observed, indicating that the net energy transfer and transport of electrons is not significant. This Pc5 event has a long duration, being observed for more than a couple of days at geosynchronous orbit over several traversals of the two GOES satellites, and is localized in azimuthal extent. Spectral analysis shows that most of the power is in the transverse components. The frequency of the narrow-band event, as observed at geosynchronous orbit shifts during the time of the event from 7&plusmn;0.5 mHz to about 4&plusmn;0.5 mHz. On the ground, CARISMA magnetometers record no distinct narrow-band fluctuation in the magnetic field, and neither does Geotail, which is traversing the outer magnetosphere a few <I>R_E</I> further out from geosynchronous orbit, at the same UT and LT that GOES-8 and -10 observe the pulsations, suggesting that that there is no connection to external fluctuations originating in the solar wind. An internal generation mechanism is suggested, such as could be provided by energetic ring current particles, even though conclusive evidence could not be provided for this particular event. Through a statistical study, it is found that this event belongs to a class of similar events, occurring predominantly in the post-noon region in the inner magnetosphere

Storm time observations of electromagnetic ion cyclotron waves at geosynchronous orbit: GOES results

Electromagnetic ion cyclotron (EMIC) waves may contribute to ring current ion and radiation belt electron losses, and theoretical studies suggest these processes may be most effective during the main phase of geomagnetic storms. However, ground-based signatures of EMIC waves, Pc1–Pc2 geomagnetic pulsations, are observed more frequently during the recovery phase. We investigate the association of EMIC waves with various storm phases in case and statistical studies of 22 geomagnetic storms over 1996–2003, with an associated Dst < −30 nT. High-resolution data from the GOES 8, 9, and 10 geosynchronous satellite magnetometers provide information on EMIC wave activity in the 0–1 Hz band over ±3 days with respect to storm onset, defined as commencement of the negative excursion of Dst. Thirteen of 22 storms showed EMIC waves occurring during the main phase. In case studies of two storms, waves were seen with higher intensity in the main phase in one and the recovery phase in the other. Power spectral densities up to 500 nT² Hz⁻¹ were similar in prestorm, storm, and early recovery phases. Superposed epoch analysis of the 22 storms shows 78% of wave events during the main phase occurred in the He⁺ band. After storm onset the main phase contributed only 29% of events overall compared to 71% during recovery phase, up to 3 days. Some differences between storms were found to be dependent on the solar wind driver. Plasma plumes or an inflated plasmasphere may contribute to enhancing EMIC wave activity at geosynchronous orbit

Propagation of electromagnetic ion cyclotron wave energy in the magnetosphere

Recent satellite and conjugate observations of Pc 1 electromagnetic ion cyclotron (EMIC) waves have cast doubt on the validity of the long-standing bouncing wave packet (BWP) model that describes their propagation in the magnetosphere. A study was undertaken using the Combined Release and Radiation Effects Satellite (CRRES) E and B field data to further the understanding of the propagation characteristics of Pc 1 EMIC waves in the middle magnetosphere. CRRES covered the region L = 3.5–8.0, magnetic latitude up to ±30°, and magnetic local time 1400–0800. From 6464 hours of observation a total of 248 EMIC wave events were identified. For the first time the Poynting vector for Pc 1 EMIC waves is presented in the dynamic spectral domain permitting the study of energy propagation of simultaneous waves located in different frequency bands. The maximum wave energy flux for the events was 25 μW/m2, averaging range 1.3 μW/m2, with the direction of wave energy propagation independent of wave frequency but dependent on magnetic latitude. EMIC wave energy propagation was bidirectional both away and toward the equator, for events observed below 11° ∣MLat∣. Unidirectional wave energy propagation away from the equator was observed for all events located above 11° ∣MLat∣. This supports the concept of unidirectional EMIC wave energy propagation away from a broad source region centered on the geomagnetic equator. No measurable energy was observed propagating equatorward beyond the source region, in contradiction to the BWP paradigm

Relativistic electron loss due to ultralow frequency waves and enhanced outward radial diffusion

Using the THEMIS and GOES satellites and ground-based magnetometers, the loss of outer zone radiation belt electrons through the magnetopause in response to ultralow frequency (ULF) waves is examined. A 2 orders of magnitude decrease in >2 MeV electron flux observed at geosynchronous orbit, starting at 00 UT on 25 June 2008, is attributed to a rapid (1–4 h) nonadiabatic loss process. ULF waves were observed by the THEMIS-A, -D, and -E probes in the afternoon-to-dusk sector from the magnetopause to geosynchronous altitude. Estimates of the electron resonant energies indicate strong drift resonant interactions occurring between the energetic electrons and the observed waves. The rate of outward radial diffusion was estimated for MeV electrons using the observed ULF wave azimuthal electric field and compressional magnetic field and the diffusion time (~2.5 h) was found to be in good agreement with the observed time for nonadiabatic flux decreases at geosynchronous orbit. The magnetopause was compressed inside of its nominal position because of increased solar wind dynamic pressure. The electron loss is interpreted as a combination of magnetopause shadowing (from the compressed magnetosphere) and enhanced outward diffusion from ULF wave-particle drift resonant interactions. The enhanced day-night asymmetry of the MeV electron drift path from the compression suggests that enhanced losses may have also occurred around local noon as well as in the afternoon-to-dusk sector