C. L. Waters

Estimation of global field aligned currents using the iridium® System magnetometer data

The Iridium® System satellite constellation consists of 66 satellites in circular, polar, 780 km altitude orbits in six equally spaced planes. Each satellite carries an engineering magnetometer which has sufficient resolution to sense the Birkeland currents. This paper presents a spherical harmonic fitting (SHF) technique for estimating field aligned currents (FACs) using the cross track component of the magnetic field measurements from the Iridium satellites. The SHF magnetic field perturbations along with Amperes law are used to derive the global FACs from 40° MLAT to the pole in either hemisphere. Data for 10–11 UT, 23 August, 1999 were obtained from Iridium, Defense Meteorology Satellite Program (DMSP) Fl3 and the Ultraviolet Imager (UVI) onboard the POLAR satellite. The SHF data were evaluated along the DMSP F13 track. The range of east-west component, SHF magnetic perturbations from Iridium was (−530,465) nT compared with ( −630,634) nT from DMSP F13. The derived upward FACs ranged from −0.8 to 0.9 µAm−2. Upward FACs were co-located with bright dayside UVI emissions.

The resonance structure of low latitude Pc3 geomagnetic pulsations

The spectral difference in ULF wave amplitude between closely spaced meridional ground stations may be used to measure the eigenfrequency of magnetospheric field lines (Baransky et al., 1985). A more reliable technique based on the crossphase spectrum has been used to identify eigenfrequencies and study the temporal evolution of local field line resonances. Pc3 (22-100 mHz) pulsations recorded with two pairs of low latitude ground stations have been specifically examined

Propagation of electromagnetic ion cyclotron wave energy in the magnetosphere

[1] 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/m 2 , averaging range 1.3 μW/m 2 , 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.

Monitoring spatial and temporal variations in the dayside plasmasphere using geomagnetic field line resonances

It is well known that the resonant frequency of geomagnetic field lines is determined by the magnetic field and plasma density. We used cross-phase and related methods to determine the field line resonance frequency across 2.4≤<L≤4.5 in the Northern Hemisphere at 78°–106° magnetic longitude and centered on L=2.8 in the Southern Hemisphere at 226° magnetic longitude, for several days in October and November 1990. The temporal and spatial variation in plasma mass density was thus determined and compared with VLF whistler measurements of electron densities at similar times and locations. The plasma mass loading was estimated and found to be low, corresponding to 5–10% He+ on the days examined. The plasma mass density is described by a law of the form (R/Req)−p, where p is in the range 3–6 but shows considerable temporal variation, for example, in response to changes in magnetic activity. Other features that were observed include diurnal trends such as the sunrise enhancement in plasma density at low latitudes, latitude-dependent substorm refilling effects, shelves in the plasma density versus L profile, and a longitudinal asymmetry in plasma density. We can also monitor motion of the plasmapause across the station array. Properties of the resonance were examined, including the resonance size, Q, and damping. Finally, we note the appearance of fine structure in power spectra at these latitudes, suggesting that magnetospheric waveguide or cavity modes may be important in selecting wave frequencies.

Development of large‐scale Birkeland currents determined from the Active Magnetosphere and Planetary Electrodynamics Response Experiment

The Active Magnetosphere and Planetary Electrodynamics Response Experiment uses magnetic field data from the Iridium constellation to derive the global Birkeland current distribution every 10 min. We examine cases in which the interplanetary magnetic field (IMF) rotated from northward to southward resulting in onsets of the Birkeland currents. Dayside Region 1/2 currents, totaling ~25% of the final current, appear within 20 min of the IMF southward turning and remain steady. Onset of nightside currents occurs 40 to 70 min after the dayside currents appear. Thereafter, the currents intensify at dawn, dusk, and on the dayside, yielding a fully formed Region 1/2 system ~30 min after the nightside onset. The results imply that the dayside Birkeland currents are driven by magnetopause reconnection, and the remainder of the system forms as magnetospheric return flows start and progress sunward, ultimately closing the Dungey convection cycle.

The temporal variation of the frequency of high latitude field line resonances

The diurnal variation in the frequencies of the continuum of ULF field line resonances has been calculated by using the cross-spectral phase of the north-south components of data from latitudinally spaced ground magnetometers in the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) array. On most days the continuum is seen only during the local daytime, and only a single harmonic with an inverted U-shaped temporal variation in frequency is seen. At 67° geomagnetic latitude (L = 6.6) the general trend is a resonant frequency around 2 mHz near local dawn, increasing up to ∼5 mHz by 0600-0700 local time, followed by a decrease in frequency to 2 mHz by 1500–1600 local time. Near local noon, the fundamental resonant frequency is ∼3 mHz at 71° (L = 11.3), increasing monotonically to 7 mHz at 65° (L = 6.1). The waves appear to be a part of the resonant Alfven mode continuum as opposed to the single-frequency, driven magnetic field line resonances often seen at high latitudes. The cross-phase spectra show evidence of impulsively driven resonances that energize the continuum over the latitudinal range of the CANOPUS magnetometers. The temporal variation in the resonant frequency is modeled by using the Tsyganenko (1987) magnetic field model and cold plasma MHD theory. With the use of the observed resonant frequencies, the plasma density for June 1, 1990, was 4.2 × 106 H+/m³ at L = 6.6 while the data for June 7, 1990, showed densities up to 100×106 H+/m³. These results suggest that observations of the magnetohydrodynamic continuum in the magnetometer data may give a very effective method for ground-based time-dependent mapping of the equatorial plasma density.

Low latitude geomagnetic field line resonance: Experiment and modeling

Geomagnetic field line resonances may be identified in ground magnetometer data by comparing the difference in amplitude and phase of signals recorded at two closely spaced sites or by examining the latitudinal variation in polarization properties across a more extended array. These two methods give comparable results for values of the resonant frequency and width at low latitudes (L < 3). We have also found an upper limit for the damping factor, γ∼0.07 at L=1.8, by applying a damped simple harmonic oscillator model. The field line resonance structure observed in 5 weeks of data showed only one resonant frequency at L=1.8 but up to four harmonies concurrently at L=2.8. An early local morning decrease in eigenfrequency was usually present at L=1.8. This is attributed to dynamic heavy ion mass loading effects in the ionosphere where the plasma density increases around dawn. The observed eigenfrequencies were used to evaluate two plasma density models. Calculations using a combined IRI-90 and diffusive equilibrium (DE) model gave eigenfrequencies which are considerably smaller than the experimentally observed values at both L=1.8 and L=2.8. Furthermore, the calculated harmonic spacings at L=2.8 do not agree with the experimental values, although the diurnal trends were successfully modeled using the IRI-DE plasma description. The low-latitude plasma density model described by Bailey (1983) yields eigenfrequencies which show good agreement with the experimentally observed values at both latitudes.

Field line resonances and waveguide modes at low latitudes: 1. Observations

Field line resonances (FLRs) are an important mechanism for the generation of Pc3–4 (∼7–100 mHz) geomagnetic pulsations. There is considerable observational evidence for the existence of FLRs at middle latitudes, both in satellite and ground data. However, the low-latitude regions are less accessible for such studies, and consequently many aspects of low-latitude FLRs are not well understood. A temporary 12-station magnetometer array spanning eastern Australia from L= 1.3–2.0 was used to investigate the variation in Pc3–4 power with latitude, the nature and low-latitude limit of FLRs, and properties of spectral components below the local resonant frequency. Examples are presented for representative days. Power spectra are remarkably similar over this range of latitudes and often exhibit a multitude of peaks separated by ∼3–5 mHz. Using cross-phase techniques, we find that the resonant frequency increases with decreasing latitude to L∼1.6, then decreases at lower latitudes. This is due to the effect of ionospheric heavy ions at low altitudes. The characteristic size of the resonances is L∼0.15, the resonance Q is ∼2 at L=2.0 and 1.3–1.4 at L=1.3, and the normalized damping factor γ/ωR∼0.2–0.4. The low-latitude detection limit of FLRs depends on a number of factors, but on a day examined in detail it was L∼1.4. For signals below the local resonant frequency, amplitude decreased with latitude at ∼3 dB/0.1 L. Interstation phase delays are not consistent with the time of flight of radially propagating fast-mode waves in the equatorial plane, although a peak occurs in the region where the Alfven velocity peaks. We conclude that these results are consistent either with modulation of the incoming fast-mode waves or the existence of cavity or waveguide modes which drive discrete forced oscillations of low-latitude field lines across a range of frequencies, and which couple to the local FLR where the frequencies match.

Birkeland current system key parameters derived from Iridium observations: method and initial validation results

[1] The Iridium satellites in 780 km altitude, circular polar orbits provide continuous global monitoring of the Birkeland current system via engineering magnetometer data. These data have been used to characterize basic features of the global field-aligned currents (FACs) with a time window of 45 min and a time step of 15 min. The three sigma magnetometer data noise threshold is 93 nT on average. The fraction of measurements above the noise is used to provide one measure of the location of the auroral FACs. Measures are also presented for the mean latitude and equatorward/poleward extent of the region 1/region 2 FAC system. The equatorward latitude of region 1/region 2 FACs is anticorrelated with Kp, r = 0.68. Indices are presented for the net FAC intensity in terms of the eastward (westward) magnetic perturbation in the northern (southern) hemisphere by analogy with the AE, AU, and AL indices. The Iridium system indices show high correlation with the quick look auroral electrojet indices both in individual cases and statistically, r = +0.73 between their logarithms. Results are presented for two storms, 22–23 September 1999, Dst minimum approximately 160 nT, and 21–22 October 1999, Dst minimum approximately 230 nT, reflecting that intensification and equatorward expansion of the global FACs occur in response to southward IMF. Enhanced dynamic pressure promotes more rapid equatorial expansion, 10� in 1.5 hours for the September storm, for which the dynamic pressure was enhanced, 15–20 nPa, at southward IMF turning, as opposed to the October case, 13� over 8 hours, for which the southward turning occurred during nominal dynamic pressure, 5 nPa. In both storms the current intensity decreases to prestorm levels within an hour when the IMF turns northward or nearly horizontal, at the beginning of storm recovery. The key parameters are a useful means of accessing the Iridium system data for preliminary analyses, and the initial results provide motivation for future analyses to quantify the accuracy and reliability of products derived from the Iridium system data. INDEX TERMS: 2708 Magnetospheric Physics: Current systems (2409); 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions; 2407 Ionosphere: Auroral ionosphere (2704); 2794 Magnetospheric Physics: Instruments and techniques; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: Iridium constellation, Birkeland field-aligned currents

Variation of plasmatrough density derived from magnetospheric field line resonances

The diurnal variation of ULF field line resonant frequencies has been calculated using the cross phase of data from the north-south components recorded at seven latitudinally spaced ground magnetometers in the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) array. CANOPUS magnetometers range in latitude from Rankin Inlet (L = 12.4) south to Pinawa (L =4.3). Using cold plasma MHD theory, an R−4 plasma density function, and the T87 magnetic field model, the variation of plasma density in the equatorial region has been calculated from the experimentally determined resonant frequencies. Consecutive, adjacent magnetometer pairs provide six local daytime spatial estimates of the variation in plasma mass density between 4 and 11 RE. Typical values are 1–20 H+cm−3 for the plasmatrough and 50–200 H+cm−3 for the plasmasphere. The data recorded on June 7, 1990, shows an afternoon increase in density near geosynchronous orbit in agreement with convection models of the magnetosphere. The ground-based measurements of plasma mass density have been compared with data from the Los Alamos Magnetospheric Plasma Analyser on board the 1989-046 geosynchronous spacecraft. These comparisons show that the ground-based technique should allow a robust procedure for calculating dayside, time-dependent mappings of the equatorial plasma mass density from the plasmapause to the magnetopause in near real time.