http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:2808 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). 2012-03-12T07:22:24.080Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:7818 The 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. 2011-06-02T06:00:30.473Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:1995 This work attempts to fill a gap in comparative studies of upstream-generated Pc3–4 waves and broad band ULF noise observed at cusp latitudes. We performed a statistical analysis of the spectral properties of three years of cusp-latitude ground magnetometer data, finding that the average daytime Pc3–4 spectra are characterized by two principal components: an upstream-related band-limited enhancement (‘signal’) and a power-law background (‘noise’) with S(f ) α f⁻⁴. Based on this information we developed an algorithm allowing for the deconvolution of these two components in the spectral domain. The frequency of the signal enhancement increases linearly with IMF magnitude as f [mHz] ≃ 4.4 | BIMF | [nT], and its power maximizes around IMF cone angles θxB ≃ 20 and 160° and at 10:30–11:00 MLT. Both spectral components exhibit similar semiannual variations with equinoctial maxima. The back-ground noise power grows with increasing southward Bz and remains nearly constant for northward Bz. Its diurnal variation resembles that of Pc5 field-line resonance power, with a maximum near 09:00 MLT. Both the band-limited signal and broad band noise components show power-law growth with solar wind velocity α V⁵‧⁷¹sw and α V⁴‧¹²sw, respectively. Thus, the effective signal-to-noise ratio increases with in-creasing Vsw. The observations suggest that the noise generation is associated with reconnection processes. 2010-04-27T06:45:46.921Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:2908 Magnetometer data from Davis, Antarctica (74.49° S, 100.03° E CGM) often exhibit spectral characteristics which can be interpreted as field line resonance (FLRs) signatures of the last closed field-lines in the dayside magnetosphere. In particular, cross-phase measurements in the Pc5 band (1-10 mHz) show spatial gradients which, after allowing for mapped propagation from the equatorial magnetopause, can provide information on field-line topology via small variations in the resonance frequency. We present here examples of diurnal cross-phase data from two pairs of closely spaced (~110 km), azimuthally separated stations in a square array including Davis, and define a new index - Φ5 as averaged cross-phase between 2 and 4 mHz. We have constructed a model based on the T01 geomagnetic field, and a simple power law plasma density model which allows us to determine the time of flight of Alfvén waves along closed field lines, and thence resonance frequency gradients. For quiet to moderate conditions we find this model shows good functional agreement with the data. During disturbed conditions we find propagation signatures suggestive of reconnection replace/mask the FLR phase pattern. 2010-04-27T06:21:30.954Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:2807 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. 2010-04-27T06:18:44.517Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:1089 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 oddmode 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. 2010-04-27T06:06:33.073Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:4853 Ground 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 Alfvénic 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 Alfvén waves guided by the magnetic field at high latitudes and excite field line resonance in various harmonics at middle and low latitudes. 2010-04-27T05:00:21.433Z ]]>