Antarctic Space Science - Publications
2002
[back]ULF Pc5-6 magnetic activity in the polar cap as observed along a geomagnetic meridian in Antarctica
Authors: Yagova, N. V.; Lanzerotti, L. J.; Villante, U.; Pilipenko, V. A.; Lepidi, S.; Francia, P.; Papitashvili, V. O.; Rodger, A. S.
Journal: Journal of Geophysical Research (Space Physics), Volume 107, Issue A8, pp. SMP 22-1, CiteID 1195, DOI 10.1029/2001JA900143
Date: Aug 2002
Abstract: Latitudinal and diurnal distributions of spectral power and spatial coherency parameters of the geomagnetic variations in the Pc5-6 (1-6 mHz) frequency range are analyzed using data of magnetometer stations in Antarctica. The available stations give the possibility to form a latitude chain along the geomagnetic meridian 40°E stretching from magnetic latitude 69°S to 86°S. Long-period ULF activity at polar cap latitudes is characterized by lower amplitudes and wider spectra with lower central frequencies as compared with typical auroral Pc5 pulsations. The meridional distribution of average Pc5-6 spectral power is nonmonotonic and has a minimum near 80°. In general, the low-frequency broadband ULF activities in the polar cap and at auroral latitudes seem to be decoupled. This long-period ULF activity in the polar cap could be an image of wave activity in the tail lobes or the manifestation of turbulent component of the ionospheric convection at very high latitudes, but this requires further investigation.
Statistical study of auroral roar emissions observed at South Pole Station
Authors: LaBelle, J.; Weatherwax, A. T.
Journal: Journal of Geophysical Research (Space Physics), Volume 107, Issue A7, pp. SIA 17-1, CiteID 1140, DOI 10.1029/2001JA000319
Date: Jul 2002
Abstract: Auroral roar is a band-limited natural auroral radio emission observed at ground level in the auroral zone at frequencies near 3 MHz and 4.5 MHz, close to 2 and 3 times the electron gyrofrequency. Using semiautomated data analysis techniques, we generate a database of frequencies, amplitudes, and universal times of 2fce-auroral roar emissions observed at South Pole Station (74° invariant latitude) during 1999. These data confirm that auroral roar emissions are observed at ground level only during times when the ionosphere is not illuminated, and magnetic local time plays a significant role in controlling the occurrence rates of auroral roar emissions observed at ground level. Auroral roar occurrence rates and the ranges of auroral roar amplitudes and frequencies increase during times of high geomagnetic activity. The distribution of auroral roar frequencies is bimodal for large-amplitude events, which we interpret as evidence that a substantial fraction of the auroral roar emissions observed at ground level originate in the topside ionosphere. Finally, superposed epoch analyses of occurrences of auroral roar relative to substorm onsets inferred from magnetometer and VLF data show that auroral roar emissions occur favorably preceding such events rather than after them, consistent with previous case study and anecdotal evidence.
Observations of two types of Pc 1-2 pulsations in the outer dayside magnetosphere
Authors: Engebretson, M. J.; Peterson, W. K.; Posch, J. L.; Klatt, M. R.; Anderson, B. J.; Russell, C. T.; Singer, H. J.; Arnoldy, R. L.; Fukunishi, H.
Journal: Journal of Geophysical Research (Space Physics), Volume 107, Issue A12, pp. SMP 20-1, CiteID 1451, DOI 10.1029/2001JA000198
Date: Dec 2002
Abstract: Analysis of high time resolution magnetometer data from the Polar satellite and from an array of high-latitude ground stations in Antarctica has identified 20 simultaneously observed Pc 1-2 wave events in the outer dayside magnetosphere during the first 17 months of Polar operations. In contrast to most earlier satellite studies of Pc 1-2 waves, based on data from equatorial satellites near apogee which moved only slowly across L shells if at all, the initial orbit of Polar allowed it to rapidly cross outer magnetospheric L shells, but significantly away from the magnetic equator. Consistent with several previous studies of outer magnetospheric Pc 1-2 waves, the majority of these events (75%) were associated with significant compressions of the magnetosphere. Seven of the events occurred simultaneously in satellite and ground data, with wave bursts temporally associated with compressions. These events, most at L values > 9, confirm the suggestion of Anderson and Hamilton [1993] that the outer dayside magnetosphere is often near marginal stability for electromagnetic ion cyclotron (EMIC) wave generation, so waves can be stimulated by even modest magnetospheric compressions. However, 10 of the wave events (five associated with compressions, and five not) were highly localized in L shell. In these ``spatially localized'' cases, continuous wave emissions were seen on the ground for extended periods of time, while in space waves were observed for only a few minutes and occurred only in radially narrow regions. The noncompressional events, all spatially localized, appear to be the first examples identified in space of the class of wave events known as Pc 1-2, sustained narrowband emissions which have durations at high-latitude ground stations of the order of 10 hours in the day and dusk local time sectors. All 10 of the spatially localized events, whether compressional or not, followed intervals of at least 12 hours of sustained very quiet magnetospheric conditions. Energetic ion observations from Polar confirm earlier suggestions that drifting plasma sheet ions are the source of dayside Pc 1-2 waves in the outer dayside magnetosphere, but they also show different particle configurations for the spatially localized and temporally ordered event categories. Events in both categories occurred within radially extended regions with ring-like, moderately anisotropic distributions of >=5 keV protons and with deep minima in the flux distributions at energies <5 keV. However, spatially localized wave events occurred only in association with radially localized regions that also contained highly anisotropic fluxes of ~0.5-3 keV protons, at a considerable distance from the magnetospheric boundary. In contrast, no such radial structure was evident in any of the temporally ordered events, or in three ``uncertain'' events. The association of the spatially localized events with highly structured populations of plasma sheet protons of keV and higher energies indicates an important but unanticipated role for these protons in destabilizing plasma in the outer dayside magnetosphere, possibly through increasing the local plasma beta near the magnetic equator.
Traveling convection vortices induced by solar wind tangential discontinuities
Authors: Kataoka, R.; Fukunishi, H.; Lanzerotti, L. J.; Rosenberg, T. J.; Weatherwax, A. T.; Engebretson, M. J.; Watermann, J.
Journal: Journal of Geophysical Research (Space Physics), Volume 107, Issue A12, pp. SMP 22-1, CiteID 1455, DOI 10.1029/2002JA009459
Date: Dec 2002
Abstract: Two typical magnetic impulse events (MIEs) accompanied by traveling convection vortices (TCVs) are investigated. The analysis of their conjugate equivalent convection patterns is performed using magnetic field data obtained from high-latitude ground magnetometer networks in the Northern and Southern Hemispheres. A three-dimensional analysis of solar wind structures is also performed using solar wind data obtained from multiple International Solar-Terrestrial Physics satellites. In the first event observed at ~1310 UT on 22 May 1996, a westward moving TCV appeared simultaneously in the noon-to-dawn sector in the Northern and Southern Hemispheres. The solar wind source of this TCV is found to be a tangential discontinuity (TD), which causes a rapid northward turning of the interplanetary magnetic field (IMF) and abrupt dynamic pressure changes. In the second event observed at ~1610 UT on 27 May 1998, an eastward moving TCV appeared in the noon sector in the Northern and Southern Hemispheres, with a timing delay of 2 to 3 min in the Southern Hemisphere. The solar wind source of this TCV is found again to be a TD, which causes a rapid IMF By negative turning and an abrupt enhancement of dynamic pressure. Analyses show that the TDs driving these events have their motional electric fields pointing toward the TDs and their normal vectors with large cone angles from the sunward direction. These TDs satisfy the conditions for the formation of a hot flow anomaly (HFA) at the bow shock. The sweeping motion across the magnetosphere of the intersection of the TD and the bow shock is found to be consistent with the observed TCV motion in each event. Magnetopause deformations due to HFAs can explain all the observed morphological features and the triggering process of these two MIEs. It is suggested, however, that bursty merging and/or pressure pulses would reinforce the processes produced by the HFAs, since the TDs are usually accompanied by both abrupt IMF changes and pressure enhancements. Consequently, it seems reasonable to conclude that the integrated processes of HFA, bursty magnetic field merging, and pressure pulses produce the evolution of these MIEs and TCVs.
Interpreting observations of MF/HF radio emissions: Unstable wave modes and possibilities to passively diagnose ionospheric densities
Authors: Weatherwax, A. T.; Yoon, Peter H.; LaBelle, J.
Journal: Journal of Geophysical Research (Space Physics), Volume 107, Issue A8, pp. SIA 26-1, CiteID 1213, DOI 10.1029/2001JA000315
Date: Aug 2002
Abstract: Recent satellite observations by the Wind spacecraft, as well as earlier observations by the ISIS 1 and 2 satellites, of MF/HF radio emissions have been attributed to upper hybrid/Z mode waves generated in the topside ionosphere at two and three times the local electron gyrofrequency fce. We calculate the growth rates of such emissions for various wave modes as a function of altitude in the topside ionosphere and for a variety of electron density profiles, employing a modified loss-cone/horseshoe electron distribution function as the free energy source. These calculations represent the first application of this instability model to the topside ionosphere, in parallel to earlier efforts involving wave excitation in the bottomside of the F region. In addition to the Z mode, which is known to be unstable at points where the upper hybrid frequency matches cyclotron harmonics, the X mode can be directly generated near the first harmonic of the electron cyclotron frequency for certain density profiles. We further put forth how these emissions might be used as a remote diagnostic of ionospheric plasma properties.
Conjugate observations of traveling convection vortices: The field-aligned current system
Authors: Murr, D. L.; Hughes, W. J.; Rodger, A. S.; Zesta, E.; Frey, H. U.; Weatherwax, A. T.
Journal: Journal of Geophysical Research (Space Physics), Volume 107, Issue A10, pp. SIA 14-1, CiteID 1306, DOI 10.1029/2002JA009456
Date: Oct 2002
Abstract: Analysis of a single traveling convection vortex (TCV) event that combines magnetometer, auroral imager, and riometer observations from conjugate hemispheres provides a complete view of the field-aligned current (FAC) system associated with the TCV. Conjugate observations clearly show that the TCV is a pair of FACs that flow alternatively out of and then into both ionospheres. Imaging observations at 427.8 nm indicate that the upward FAC region is associated with a narrow arc 50-100 km wide and at least 600 km long. Mapping the imaging observations made in the Southern Hemisphere to the Northern Hemisphere shows excellent collocation of the precipitation with the flow vortex associated with the upward FAC as inferred from a large two-dimensional network of magnetometers. Also consistent with the flows inferred from the magnetometers, the arc is found to be skewed with respect to the direction of motion of the transient. Low-altitude spacecraft passes near the time of the event suggest that the FAC is coincident with, and possibly confined to, a relatively narrow region (less than 3° in latitude) in which 1-10 keV electrons characteristic of the central plasma sheet are present.
A traveling convection vortex event study: Instantaneous ionospheric equivalent currents, estimation of field-aligned currents, and the role of induced currents
Authors: Amm, O.; Engebretson, M. J.; Hughes, T.; Newitt, L.; Viljanen, A.; Watermann, J.
Journal: Journal of Geophysical Research (Space Physics), Volume 107, Issue A11, pp. SIA 1-1, CiteID 1334, DOI 10.1029/2002JA009472
Date: Nov 2002
Abstract: We analyze a traveling convection vortex (TCV) event on 31 January 1997 using ground magnetometer data of the CANOPUS, MACCS, Geological Survey of Canada, Greenland, and IMAGE networks. For the first time, spatial and instantaneous distributions of the mesoscale ionospheric equivalent currents associated with a TCV are obtained. We apply the method of spherical elementary currents (SECS) to calculate these currents, as well as to infer the part of the ground magnetic signatures that is caused by internal currents induced in the Earth. The resulting ionospheric equivalent currents consist of a leading clockwise vortex centered at 71° CGM latitude which moves westward with ~7.3 km s-1 and a trailing anticlockwise vortex at 77 ° latitude which moves southwestward away from noon at ~3.0 km s-1. In an area of ~200 km around the center of the twin vortices the derived equivalent current densities are less than 70 mA m-1 but reach 100-160 mA m-1 in a broad channel of equatorward currents between the vortices. Using the assumption of a uniform Hall to Pedersen conductance ratio and the assumption that conductance gradients perpendicular to the ionospheric electric field are vanishing, we can estimate the field-aligned current (FAC) associated with the TCV. The maxima of ~1 μA m-2 of downward FAC in the leading western vortex, and of ~0.4 μA m-2 of upward FAC in the trailing eastern vortex occur along the perimeter of the vortices, not in their centers, in contrast to the prediction of ionosphere-magnetosphere coupling theories for TCVs. The integrated FAC are not fully balanced between the two vortices but show an excess of downward FAC. While the ratio between the internally generated and the total horizontal ground magnetic field for most magnetometer sites in the TCV area amounts to around 20-40%, at some stations it can reach values of 50% and larger.
Alfven resonator in the topside ionosphere beneath the auroral acceleration region
Authors: Pilipenko, V. A.; Fedorov, E. N.; Engebretson, M. J.
Journal: Journal of Geophysical Research (Space Physics), Volume 107, Issue A9, pp. SMP 21-1, CiteID 1257, DOI 10.1029/2002JA009282
Date: Sep 2002
Abstract: The possible occurrence of a new resonator in the topside ionosphere at auroral latitudes is proposed. This resonator is formed between the E-layer of the ionosphere and the bottom boundary of the auroral acceleration region (AAR), which has a localized field-aligned potential drop. The AAR is shown to effectively reflect Alfven waves with transverse scales less than the Alfven transit scale λA. The proposed resonator can trap and accumulate Alfvenic small-scale disturbances with periods from a few seconds to a few tens of seconds, and with transverse scales from a kilometers to a few tens of kilometers. The eigenfrequencies of the AAR-associated resonator are estimated to be lower than that of the ionospheric Alfven resonator. The small-scale Alfvenic structures commonly observed by satellites can be generated by a nonsteady field-aligned current transported by precipitating electrons and trapped in the AAR-associated resonator. Observations by ground-based magnetometers at auroral latitudes often show the occurrence of intensifications in the dynamic spectra of Pi1 pulsations in accordance with the predictions of the model.
A statistical study of traveling convection vortices using the Magnetometer Array for Cusp and Cleft Studies
Authors: Zesta, E.; Hughes, W. J.; Engebretson, M. J.
Journal: Journal of Geophysical Research (Space Physics), Volume 107, Issue A10, pp. SIA 18-1, CiteID 1317, DOI 10.1029/1999JA000386
Date: Oct 2002
Abstract: The Magnetometer Array for Cusp and Cleft Studies (MACCS) is primarily a longitudinal magnetometer array located in the eastern Canadian arctic at cusp latitudes. We present a statistical study of traveling convection vortices (TCVs) as observed by MACCS from the first year of operations, August 1992 to July 1993. For this period, data are routinely available from three stations, extending more than 2 hours in magnetic local time (MLT) and located roughly on a line of constant geomagnetic latitude at 75°. With the selection criteria used, we find an average of 38 TCV events per month. We characterize the events as ``isolated'' and ``nonisolated''. The isolated events have the typical, accepted signature of a TCV while the nonisolated ones are events that satisfy the selection criteria but occur among relatively strong magnetic activity. The diurnal distribution of these events shows a prenoon-afternoon asymmetry, but the distribution has a single peak at 10 MLT and then falls off smoothly in the afternoon hours. It does not show the previously reported minimum of event occurrence around magnetic local noon except for isolated events occurring during higher Kp values. The most likely explanation for this discrepancy is the wide longitudinal extent of MACCS. The majority of the events have propagation velocities between 3 and 11 km/s, which can be a factor of 2 larger than the events reported in previous studies. The majority of the events move away from noon, as is expected, with a small fraction moving toward noon. Most (80%) of the studied events exhibit all the characteristics of a typical TCV, while the remaining 20% show some form of irregularity. We find four different types of irregularities. The isolated and nonisolated events have many similar morphological and propagation characteristics. However, quantitative comparison of the statistical distributions of the two types of TCVs indicates that they belong to different populations. We also find that nonisolated TCVs may occur for somewhat higher Kp values. We therefore suggest that the two classes of TCVs have different source mechanisms.
Quiet time magnetotail dynamics and their implications for the substorm trigger
Authors: Ohtani, S.; Yamaguchi, R.; Nosé, M.; Kawano, H.; Engebretson, M.; Yumoto, K.
Journal: Journal of Geophysical Research (Space Physics), Volume 107, Issue A2, pp. SMP 6-1, CiteID 1030, DOI 10.1029/2001JA000116
Date: Feb 2002
Abstract: The present study seeks to systematically examine the fast plasma flow in the plasma sheet at geomagnetically quiet time. The study uses plasma measurements made by the Geotail satellite in the midnight sector at x > -50 RE during quiet intervals with a total duration of 446 hours. Comparison with the results of previous studies suggests that the occurrence frequency of the perpendicular flow velocity VX_\bot depends more clearly on the radial distance than on geomagnetic activity. Two extreme events were selected for detailed studies; they occurred on 23 November and 10-11 December 1994. In the 23 November event, Geotail was located ~37 RE from Earth and observed a fast tailward flow (VX_\bot ~ -1250 km s-1) with a strongly southward magnetic field (BZ ~ -8.9 nT). The signature indicates that a near-Earth neutral line was formed earthward of the satellite and the reconnection reached the lobe magnetic field. On the ground, however, only weak (<100 nT) magnetic disturbances were observed at high (~75°) latitudes but not at auroral zone stations. The result strongly suggests that lobe reconnection is not sufficient for the global development of a substorm. The 10-11 December 1994 event is very similar to the 23 November 1994 event except that Geotail observed a fast (VX_\bot ~ +1500 km s-1) earthward flow rather than a tailward flow, along with the dipolarization of the local magnetic field. It is asserted that the near-Earth substorm process, that is, tail current disruption, controls the development of a substorm. The fast plasma flow may set a favorable condition for this process to proceed, and therefore the substorm may tend to develop following generation of the fast flow, but the result of the present study indicates that near-Earth reconnection does not necessarily trigger the global substorm.
Studies of some statistics of the interplanetary magnetic field and implications for discrete modes
Authors: Thomson, D. J.; Lanzerotti, L. J.; Maclennan, C. G.
Journal: Advances in Space Research, Volume 29, Issue 12, p. 1911-1916.
Date: n/a 2002
Abstract: We present some results from a statistical study of the levels of the power spectrum of the interplanetary magnetic field as measured by the Ulysses spacecraft near the ecliptic plane between 1 and 5 AU. We conclude from the studies of the probability distributions of the power spectral levels that the interplanetary medium is not totally chaotic, but can sustain high percentages (21% to 30% for the examples shown here) of discrete, low frequency modes, probably of solar origin, over a long interval of time.