Multiple-Harmonic ULF waves in the Plasma Sheet Boundary Layer Observed by Cluster
The passage of the Cluster satellites in a polar orbit through Earth's magnetotail has provided numerous observations of harmonically related Pc 1-2 ULF wave events, with the fundamental near the local proton cyclotron frequency. Broughton et al. [JGR 2008] reported observations by Cluster of three such events in the plasma sheet boundary layer, and used the wave telescope technique to determine wave propagation nearly perpendicular to B0. We report here on a survey of the entire set of Cluster observations in Earth's magnetotail during 2003: 34 harmonically related wave events were observed during 13 of the 42 tail passes of Cluster from July 22 to October 28. Event durations were commonly < 5 min, but ranged up to 40 min. Wave events were distributed rather evenly from -7 RE out to the Cluster apogee distance of -18 RE, with one event also observed at X = -4 RE. The distribution in the YGSE coordinate was also rather even but asymmetric, between -9 and + 17 RE. Events occurred for ZGSE values from -10 to -3 RE and +3 to +7 RE; i.e., none was observed for |Z| < 3 RE. Each wave event was associated with signatures of the PSBL in the CIS instrument: elevated fluxes of ions with energies from 1 keV to over 30 keV, and highly variable, often counterstreaming ion velocities.
Magnetic-Field Strength and Electron Density Measured in the Earth's Plasmasphere by the WHISPER Relaxation Sounder Onboard CLUSTER
The WHISPER relaxation sounder that is onboard the four CLUSTER spacecraft has for main scientific objectives to monitor the natural waves in the 2 kHz 80 kHz frequency range and mostly to determine the total plasma density from the solar wind down to the Earth's plasmasphere. To fulfil these objectives, WHISPER uses the two long double sphere antennae of the Electric Field and Wave experiment as transmitting and receiving sensors. In its active working mode WHISPER works according to principles that have been worked out for topside sounding. A radio wave transmitter sends a wave train during a very short time period, 1 or 0.5 ms, covering a frequency band of 1 or 2 kHz. Then, a receiver listens to the surrounding plasma response a few milliseconds after. Strong and long lasting echoes are usually received whenever the transmitting frequencies coincide with characteristic plasma frequencies. Provided that these echoes, also called resonances, may be identified, the WHISPER relaxation sounder becomes a reliable and powerful tool for plasma diagnosis. When the transmitter is off, WHISPER behaves like a passive receiver, allowing natural waves to be monitored. The paper aims mainly at the resonance recognition process and interpretation inside the Earth's plasmasphere. Several types of resonances are usually seen in this region, they allow the magnetic field strength, the total electron density and sometimes the hot-to-cold electron density ratio to be determined.
Probing the Relationship Between EMIC Waves and Plasmaspheric Drainage Plumes near Geosynchronous Orbit
Plasmaspheric plumes created during disturbed geomagnetic conditions have been suggested as one cause of increased occurrences of electromagnetic ion cyclotron (EMIC) waves at these times. Murphy et al. [Fall AGU, 2008] cataloged all occurrences of Pc1 EMIC waves from 1996 through 2003 at three automated geophysical observatories (AGOs) at auroral zone latitudes in Antarctica (L = 6.28, 7.68, and 8.07; GLON = 20.4 W, 3.0 E, and 23.9 W). They found increased wave activity during the initial stages of convection during high speed streams, using data from 1996 and 2003, consistent with the observations [Borovsky and Denton, JGR, 2008; Denton and Borovsky, JGR, 2008] of increased density of plume material convecting to the dayside magnetopause at these times. In this study we further explore the correlation between the occurrence of Pc1 wave activity at these ground stations and the occurrence of plasmaspheric drainage plumes in space, using 1996-2002 data from the Magnetospheric Plasma Analyzer (MPA) on the LANL 1990-095 spacecraft, in geosynchronous orbit near 38 degrees west longitude.
Storm-time occurrence of relativistic electron microbursts in relation to the plasmapause
Microbursts, or short duration bursts of precipitating relativistic electrons observed at low altitudes, are associated with whistler-mode chorus and have been proposed as one of the primary mechanisms for storm- time depletion of the outer radiation belt. Whistler chorus in turn occurs outside the plasmapause. We use low-altitude identifications of the plasmapause from DMSP and microburst observations from SAMPEX to investigate the correlation between plasmapause and microburst locations. Examination of an 82-day period in 2001 shows microbursts move radially inward during storm-time in concert with plasmapause movement, with microbursts consistently located outside the plasmapause. In a superposed epoch analysis of seven moderate storms, microbursts occur near storm peak at a given L-value within hours of erosion of the plasmapause at that location. These observations support association of microbursts with chorus and the suggestion that microbursts contribute significantly to storm-time radiation belt depletion.
Pressure Gradient Effect on a Particle Velocity Distribution
In space missions, measurements of the three-dimensional particle velocity distribution are used to examine the dynamical evolution of plasmas. The first velocity moment of the measured particle velocity distribution is often regarded as the bulk flow of the population. Accurate determination of bulk flows is crucial if the frozen-in- condition is judged by E + V x B = 0, where E is the electric field, V is the bulk flow, and B is the magnetic field. In this paper, we show that this first velocity moment computed from the measured particle velocity distribution can deviate substantially from the bulk motion of the particle population when a significant pressure gradient exists at the measurement location. The discrepancy, which arises from the diamagnetic drift with a finite Larmor radius effect, increases with increasing energy range used in the computation. The result calls for caution in the proper interpretation of this velocity moment and suggests a means to avoid error in the determination of bulk flow due to pressure gradient effects.
One Minute USGS Dst
The storm-time disturbance index Dst is used widely in the space weather community as a measure of geomagnetic activity and magnetospheric ring-current intensity. The standard Dst is a one-hour index, routinely produced by the World Data Center in Kyoto using data from four magnetic observatories (HON, KAK, SJG, HER). Traditionally, calculation of the standard index is done in the time domain, with estimates of secular and solar-quiet (Sq) variation being removed from the data of each station in order to obtain a local, disturbance time series. The four resultant time series are then weighted by latitude and averaged to produce the Dst index. Here we report on an updated and somewhat simpler method for calculating the Dst index, and the application of this method to one-minute observatory data. Secular variation is subtracted in the time domain using Chebyshev polynomials. Then, through Fourier transformation, the data are taken into the frequency domain. Here, Sq is revealed to be dominated by periodic forcing related to the Earth's rotation, the Moon's orbit around the Earth, the Earth's orbit around the Sun, and, to some extent, the solar cycle. In the time domain, cross- coupling of the associated harmonics gives an Sq time series that is seemingly complicated. In the frequency domain Sq is simply represented as a discrete set of identifiable Fourier terms. These can be removed through band-stop filtering. With inverse Fourier transformation, the disturbance time series for each observatory is obtained. Examples of the 1-minute USGS Dst are presented and comparisons to the Kyoto hourly Dst are made.