S. T. Ables

Multipoint observations of Pc1-2 waves in the afternoon sector

[1]xa0Coordinated observations from GOES-9, DMSP F-13, and Chokurdakh (CHD) have shown concurrent Pc1-2 band wave activity in the late afternoon sector, close to 16 MLT. The left-hand polarization of the waves in space indicates that these are electromagnetic ion cyclotron (EMIC) waves. In the region of field line conjunction, DMSP also observed 6–30 keV energy ion precipitation. We have examined the propagation of the EMIC waves from the magnetosphere to the ionosphere using both time series analysis and a 2-D magnetohydrodynamic model. Our analysis suggests that the EMIC are generated by interactions with cold plasma within a drainage plume, consistent with theory, and that the waves primarily propagate earthward along geomagnetic field lines at the eastward (outer) edge of the plume.

Monitoring cusp/cleft topology using Pc5 ULF waves

Induction magnetometer data recorded at three closely spaced sites (∼120 Km) in Antarctica (mlat ∼ −75°) have been examined for ionospheric signatures of the cusp/cleft region of the magnetosphere. Crossphase analysis of the 1–10 mHz band, using pure-state filtering techniques reveal diurnally varying field line resonances embedded in the spectra, while interstation phase lag measurements indicate azimuthal propagation of waves away from local magnetic noon. Using the T89 external field model crossphase measurements are put in the context of diurnally changing field line topology due to compression at the subsolar region and stretching along the dawn and dusk flanks. On six of the eight days of this study we have identify a consistent two dimensional phase pattern projected in the dayside ionosphere, indicating closed field lines thread these sites during periods of low to moderate geomagnetic activity (Kp<3).

Phase structure of Pc3 waves observed by Cluster and ground stations near the cusp

[1]xa0Ground observations of Pc3 ULF waves at high latitude show enhanced pulsation activity near the cusp. To investigate this, typical Pc3 pulsation events from 0920 to 1010 UT on 1 April 2004 have been selected that are simultaneously observed by the Cluster satellites and Zhongshan, Davis, and Mawson magnetometers, Antarctica. When Cluster was located on closed field lines equatorward of the exterior northern cusp, Zhongshan and Davis were near the ionospheric footprint of the southern cusp. Assuming linear phase variation between the four Cluster spacecraft, the Pc3 waves with a frequency of ∼25 mHz have a wavelength of ∼5200 ± 2400 km and wave vector directed almost perpendicular to the geomagnetic field (94.6° ± 10.4°) pointing toward the Earth. The Pc3 waves are transverse waves over the interval and different from the broadband perturbations and compressional waves generally observed in the cusp and the magnetosheath, respectively. The wave Poynting flux indicates energy is transported predominantly along the magnetic field line toward the ionosphere. These results indicate that Pc3 waves near the cusp are shear Alfvenic waves guided by closed field lines in the low-latitude boundary of the cusp and reflected from the ionosphere. We suggest that the waves may result from incoming compressional fast waves from the upstream solar wind that couple to shear Alfven waves guided by the magnetic field at high latitudes and excite field line resonance in various harmonics at middle and low latitudes.

The annual and longitudinal variations in plasmaspheric ion density

[1]xa0This paper shows that at solar maximum, equatorial ion densities at L = 2.5 are substantially higher at American longitudes in the December months than in the June months. This arises because the configuration of the geomagnetic field causes a longitude-dependent asymmetry in ionospheric solar illumination at conjugate points that is greatest at American longitudes. For example, at −60°E geographic longitude the L = 2.5 field line has its foot point near 65° geographic latitude in the Southern Hemisphere but near 42° latitude in the Northern Hemisphere. We investigated the consequent effects on equatorial electron and ion densities by comparing ground-based observations of ULF field line eigenoscillations with in situ measurements of electron densities (from the CRRES and IMAGE spacecraft) and He+ densities (IMAGE) for L = 2.5 at solar maximum. Near −60°E longitude the electron and ion mass densities are about 1.5 and 2.2 times larger, respectively, in the December months than in the June months. Over the Asia-Pacific region there is little difference between summer and winter densities. Plasmaspheric empirical density models should be modified accordingly. By comparing the electron, helium, and mass densities, we estimate the annual variation in H+, He+, and O+ concentrations near −3°E longitude and −74°E longitude. In each case the He+ concentration is about 5% by number, but O+ concentrations are substantially higher at −3°E longitude compared with −74° E. We speculate that this may be related to enhanced ionospheric temperatures associated with the South Atlantic anomaly.

Interstation Pc3 coherence at cusp latitudes

Magnetic fluctuations in the 22–100 millihertz (Pc3) band are a consistent indicator of the presence of the cusp in the overhead ionosphere at high latitudes. Correlation of the signals from a variety of instruments have shown that the sources of these pulsations are local (ionospheric) rather than distant (magnetospheric) [Engebretson et al., 1990]. Modulated electron precipitation is presumed to be the source of the fluctuations through the modulations in ionospheric conductivity that they produce. Olson and Szuberla [1997] used data from a pair of cusp stations to deduce the scale size of the precipitating beams using a simple model in which the beams were assumed to have circular cross section. They obtained an upper bound for the coherence length of the order of 200 km. In this paper we extend the analysis of Olson and Szuberla by incorporating data from the Magnetometer Array for Cusp and Cleft Studies (MACCS) magnetometer array and the Australian ANARE antarctic sites to give a broader range of station separations. Using a statistical approach we computed the cumulative distribution function of the interstation coherence and from that distribution we established a measure of coherence, CL. The result of this analysis is a coherence that diminishes with inter-station distance as CL ≈ 1.4 exp(−S/250) where S is the station separation in km. When this result is interpreted in the context of the simple model mentioned above we find a coherence length of 140–180km.

Conjugate ULF field line resonances at cusp latitudes

Abstract Significant similarities have been identified in the dynamic power spectra of induction magnetometer data recorded at the conjugate cusp stations of Davis, Antarctica (mlat −74.3, mlong 101.5) and Longyearbyen, Svalbard (mlat 75.0, mlong 114.5). A 60 day period in 1993 (January–February) has been studied to characterise two common features, 1) Broadband Pc5 bursts which predominantly occur 2–4 hours before and after local magnetic noon, and which exhibit polarisations suggestive of a Kelvin-Helmholtz-like source mechanism, and 2) A resonance structure which has an arch-shaped local time dependence. Interhemisphere phase measurements indicate odd-mode toroidal standing-waves after field line resonance and azimuthal propagation effects are taken into account. Phase studies such as this may be used to study conjugacy at high latitudes and identify the location of the open and closed field line boundary, an important diagnostic parameter in space weather studies.

Transverse-scale size of Pc3 ULF waves near the exterior cusp

[1]xa0The concept of a geomagnetic field line has been widely used in the study of magnetospheric physical phenomena. For example, the mode in which ULF waves propagate relates to the direction of the field, with the Alfven wave mode propagating along the field direction. Very little is known about the perpendicular extent of the propagating wave. In this paper, wave coherency methods are utilized to analyze ULF waves in the Pc3 band that were simultaneously observed by the Cluster satellites near the exterior cusp and by ground stations at local magnetic noon near the footprint of the cusp. The results show that the coherency of waves observed at the ground on the H component is much larger than that on the D component, which is opposite to that seen by Cluster in space. The coherency between the H component on the ground and the y component in space was higher than the other combination of pairs, with the coherency between the satellite and the Daneborg (DNB) station having the maximum value. These results suggest that the polarization of the waves are rotated by 90° after propagating through the ionosphere, and the magnetic footprint of Cluster is closest to the DNB station at this time. The coherency of the Pc3 waves between the satellites is highly related to the alignment of satellite pairs with respect to the geomagnetic field direction. This alignment may provide a transverse-scale size of the geomagnetic Pc3 ULF waves near the exterior cusp at ∼900 km with a coherency of 0.65.

FIELDS AND FLOWS AT GEOTAIL DURING A MODERATE SUBSTORM

The behavior of the tail field and plasma during a moderate substorm with onset at -1120 UT on Dec. 13, 1994 is examined using data from the ISTP spacecraft GEOTAIL and WIND, synchronous orbit and the ground. Five substorms were observed on this day while GEOTAIL was located near the center of the tail at Xgsm = -46 &. In each substorm the field and plasma variations were similar to those observed during substorms by spacecraft closer to the earth. A southward turning of the IMF caused accumulation of lobe flux, development of a more tail-like field, and eventually an expansion phase and its consequences. The -1120 UT onset immediately followed a northward turning which ended a lengthy interval in which the IMF was alternately northward and southward. Shortly after the onset a flux rope passed GEOTAIL with a delay consistent with its formation at 2030 R, several minutes earlier than a Pi 2 burst began at midnight. Immediately after the onset the lobe field decreased and the plasma sheet disappeared. During the substorm recovery phase the plasma sheet reappeared with plasma moving earthward. The plasma data show that the tailward flow is a combination of convecting and streaming plasma. All of the substorms exhibited multiple onsets. The main onset of each can be determined by a combination of negative bay onsets, Pi 2 bursts, synchronous field-aligned currents, dispersionless particle injection, and midlatitude positive bays. In one event the flux rope at 46 R, arrived before dispersionless injection at synchronous orbit suggesting that reconnection in the tail begins at or before major onsets. In fact most of the major onsets were preceded by pseudo breakups early in the growth phase, and weak tailward flows carrying a weak vertical field fluctuating about zero. These observations suggest that reconnection begins in the middle tail early in the growth phase and that it is substantially intensified or begins again at another location at the expansion onset. 01997 COSPAR. Published by Elsevier Science Ltd.