Antarctic Space Science - Publications
2007
[back]Magnetic Pulsation and Riometer Absorption Signatures During Two Geomagnetic Storms in Late 2006
Authors: Posch, J.; Engebretson, M. J.; Lessard, M. R.; Detrick, D. L.; Weatherwax, A. T.; Manninen, J.; Rose, M. C.
Journal: American Geophysical Union, Spring Meeting 2007, abstract #SM33C-02
Date: May 2007
Abstract: Late in 2006 two geomagnetic storms occurred with minimum Dst of -96 nT and -187 nT on November 30 at 14 UT and December 15 at 8 UT, respectively. The occurrences and latitudinal patterns of broadband and narrowband magnetic pulsations in the Pc1-2 frequency band (0.1-5 Hz) at various latitudes in both hemispheres were compared to the signatures of riometer absorption at the same locations for these two storm intervals. We used data from Antarctic search coil magnetometers and riometers located at Halley (-62° MLAT), AGO P2 (-70° MLAT), South Pole (-74° MLAT), and McMurdo (-80° MLAT) and from Northern hemisphere sites Sondrestrom, Greenland (74° MLAT) and the seven-station Finnish chain (57° - 72° MLAT). During the main phase of the December 2006 storm and continuing into the local morning sector of the following day, Pi1-2 broadband noise was the dominant feature at all latitudes in both hemispheres and was accompanied by increased riometer absorption. Narrowband Pc1-2 waves began to occur 5 hours after minimum Dst at the lowest latitude Finnish station and 6 hours after at the higher latitude Finnish stations. The Antarctic stations, located ~5 hours MLT farther west, detected similar Pc1-2 activity 8 hours after minimum Dst. In contrast, during the November 2006 storm Pc1-2 activity occurred 3 hours after minimum Dst at Halley and not until the following day at the Finnish stations and other Antarctic stations. We believe this is a local time effect; during both storms the Pc1-2 activity occurred first at sites which were in the noon-afternoon sector. Both arrays showed continued Pc1-2 activity for 2-3 days after each storm, while broadband ULF noise and riometer absorption diminished.
Quasi-stationary auroral patches observed at the South Pole Station
Authors: Ebihara, Y.; Tanaka, Y.-M.; Takasaki, S.; Weatherwax, A. T.; Taguchi, M.
Journal: Journal of Geophysical Research, Volume 112, Issue A1, CiteID A01201
Date: Jan 2007
Abstract: We present quasi-stationary auroral patches (QSAPs) observed by an all-sky imager at the South Pole Station (-74.3° CGLAT) on the dayside between ~0900 and ~1400 MLT. QSAPs appeared in a closed field line region where I(557.7 nm) $\gg$ I(630.0 nm) and each patch tends to preserve its form, luminosity, and location for up to several hours. The quasi-stationary nature of the QSAPs cannot be explained by the traditional role of the corotation electric field because the South Pole is located on the axis of Earth's rotation. The most plausible scenario is that a corotation electric potential, which dominates the convection potential, results in a trapping region of the magnetospheric cold plasmas. On the analogy of the formation of the plasmasphere, the trapping region would capture cold plasmas originated from the ionosphere and result in a locally enhanced plasma density (miniplasmasphere) that may lead to electron scattering through cyclotron wave-particle interactions. This circumstance may occur when the South Pole is located on a closed field line and the local electric potential is dominated by the corotation potential. QSAPs were also accompanied with periodic fluctuations in the Pc 5 range, which coincide closely with ground magnetic fluctuations. Assuming that the pulsation is associated with field line resonances, we could estimate the equatorial mass density of thermal plasmas, which gradually increased from 0.13 to 0.44 amu cm-3. The increase in the thermal plasma density would support the hypothesis that a miniplasmasphere exists, though further investigation is necessary to confirm it.
Suppression of resonant field line oscillations by a turbulent background
Authors: Coult, N.; Pilipenko, V.; Engebretson, M.
Journal: Planetary and Space Science, Volume 55, Issue 6, p. 694-700.
Date: Apr 2007
Abstract: Stochastic fluctuations of the magnetospheric plasma and background magnetic field, especially intense during geomagnetically active periods, can provide an additional mechanism of damping of Alfvén field line oscillations. To quantify this hypothesis, we consider a driven Alfvén field line resonator with stochastic fluctuations of the Alfvén resonant frequency. This problem is first considered analytically for a low level of fluctuations, then a more general numerical approach is introduced. The results of analytical calculations and numerical modeling both indicate the deterioration of resonant properties of the resonator owing to stochastic background fluctuations.
Methods in the study of discrete upper hybrid waves
Authors: Yoon, P. H.; Ye, S.; LaBelle, J.; Weatherwax, A. T.; Menietti, J. D.
Journal: Journal of Geophysical Research, Volume 112, Issue A11, CiteID A11305
Date: Nov 2007
Abstract: Naturally occurring plasma waves characterized by fine frequency structure or discrete spectrum, detected by satellite, rocket-borne instruments, or ground-based receivers, can be interpreted as eigenmodes excited and trapped in field-aligned density structures. This paper overviews various theoretical methods to study such phenomena for a one-dimensional (1-D) density structure. Among the various methods are parabolic approximation, eikonal matching, eigenfunction matching, and full numerical solution based upon shooting method. Various approaches are compared against the full numerical solution. Among the analytic methods it is found that the eigenfunction matching technique best approximates the actual numerical solution. The analysis is further extended to 2-D geometry. A detailed comparative analysis between the eigenfunction matching and fully numerical methods is carried out for the 2-D case. Although in general the two methods compare favorably, significant differences are also found such that for application to actual observations it is prudent to employ the fully numerical method. Application of the methods developed in the present paper to actual geophysical problems will be given in a companion paper.
Statistical relations between the probability of occurrence of Pc3-4 pulsations at high latitudes in the antarctic regions and the solar wind and IMF parameters
Authors: Chugunova, O. M.; Pilipenko, V. A.; Engebretson, M.; Rodger, A.
Journal: Geomagnetism and Aeronomy, vol. 47, issue 2, pp. 205-215
Date: Apr 2007
Abstract: Not Available
Ground and satellite observations of the evolution of growth phase auroral arcs
Authors: Lessard, M. R.; Lotko, W.; LaBelle, J.; Peria, W.; Carlson, C. W.; Creutzberg, F.; Wallis, D. D.
Journal: Journal of Geophysical Research, Volume 112, Issue A9, CiteID A09304
Date: Sep 2007
Abstract: Auroral substorms are the result of a complex process involving coupling between the solar wind, magnetosphere and ionosphere. In this study, the objective is to understand the processes that precede what is perhaps the most fundamental aspect, the brightening of a preexisting arc at onset. The notion that this brightening provides an indicator of substorm onset remains one of the few aspects of substorms that is not disputed. In order to understand the processes that lead up to onset, auroral arcs that form during substorm growth phases are examined using ground-based (CANOPUS) data with in situ measurements acquired by the FAST satellite. Four cases have been studied and are discussed in detail here. Consistent with other reports, the data show that the growth phase arcs persist for 30 min or more. During this time, the arcs often show little or no fluctuations, although a gradual intensification is clear in the ground-based optical measurements. Two important results emerge from this study. First, FAST observations show that growth phase arcs develop within the region of proton precipitation but generally poleward of most intense precipitation, a topic that has been addressed (and disputed) using ground-based observations of luminosity, a secondary effect. Second, we show that FAST observations before the growth phase arc is established show scattered plasma sheet electrons with no significant structure. However, once the growth phase is well developed, FAST confirms that the arcs are excited via inverted V precipitation, implying that an inverted V potential drop gradually develops in conjunction with the evolution of the growth phase.
In search of a new ULF wave index: Comparison of Pc5 power with dynamics of geostationary relativistic electrons
Authors: Kozyreva, O.; Pilipenko, V.; Engebretson, M. J.; Yumoto, K.; Watermann, J.; Romanova, N.
Journal: Planetary and Space Science, Volume 55, Issue 6, p. 755-769.
Date: Apr 2007
Abstract: A new ULF wave index, characterizing the turbulent level of the geomagnetic field, has been calculated and applied to the analysis of relativistic electron enhancements during space weather events in March May 1994 and September 1999. This global wave index has been produced from the INTERMAGNET, MACCS, CPMN, and Greenland dense magnetometer arrays in the northern hemisphere. A similar ULF wave index has been calculated using magnetometer data from geostationary (GOES) and interplanetary (Wind, ACE) satellites. During the periods analyzed several magnetic storms occurred, and several significant increases of relativistic electron flux up to 2 3 orders of magnitude were detected by geostationary monitors. However, these electron enhancements were not directly related to the intensity of magnetic storms. Instead, they correlated well with intervals of elevated ULF wave index, caused by the occurrence of intense Pc5 pulsations in the magnetosphere. This comparison confirmed earlier results showing the importance of magnetospheric ULF turbulence in energizing relativistic electrons. In addition to relativistic electron energization, a wide range of space physics and geophysics studies will benefit from the introduction of the ULF wave index. The ULF index database is freely available via anonymous FTP for all interested researchers for further validation and statistical studies.
Experimental tests of the eigenmode theory of auroral roar fine structure and its application to remote sensing
Authors: Ye, Shengyi; LaBelle, J.; Yoon, P. H.; Weatherwax, A. T.
Journal: Journal of Geophysical Research, Volume 112, Issue A12, CiteID A12304
Date: Dec 2007
Abstract: Auroral roar is an electromagnetic emission near two and three times the electron gyrofrequency. The generation mechanism is believed to involve mode conversion of upper hybrid waves generated where the upper hybrid frequency matches a cyclotron harmonic and trapped in density enhancements. Theory predicts that if the density enhancement has dimensions comparable to the wavelength, the waves will be discretized, and indeed discrete features have been observed with the electric field detector on the HIBAR rocket experiment and with ground-level instruments. We have generated a database of eigenfrequencies predicted by theory as a function of the dimension and amplitude of the trapping density enhancement, assuming a Lorentzian-shaped cyclindrical trapping density enhancement embedded in the auroral F-region ionosphere. A fitting method is developed to test these predictions against the rocket and ground-based data. The fitting shows a significant discrepancy between theoretical predictions of density profiles and HIBAR observations. The predicted density enhancement scale size is of order ~10 m, whereas rocket observations suggest scale size of order ~100 m. A correction factor of 30 is used to reconcile the difference between the theoretical predictions and observation. A Versatile Electromagnetic Waveform (VIEW) receiver, operated at South Pole station during 2003, recorded many examples of auroral roar fine structures. Twenty-one relatively stationary examples of fine structures were selected, and the fitting procedure was applied to them, resulting in statistics of the trapping density enhancements associated with them, assuming that the eigenmode theory applies. In this case, the eigenmode theory indicates that irregularities in the range 7-40 m, or 200-1200 m if the correction factor is applied, would be responsible for generating observed auroral roar fine structures.
Cluster observations of Pc 1 2 waves and associated ion distributions during the October and November 2003 magnetic storms
Authors: Engebretson, M. J.; Keiling, A.; Fornacon, K.-H.; Cattell, C. A.; Johnson, J. R.; Posch, J. L.; Quick, S. R.; Glassmeier, K.-H.; Parks, G. K.; Rème, H.
Journal: Planetary and Space Science, Volume 55, Issue 6, p. 829-848.
Date: Apr 2007
Abstract: Unusual wave activity in the Pc 1 2 frequency band (0.1 5 Hz) was observed by the Cluster spacecraft in association with the two large geomagnetic storms of late 2003. During the onset of the Hallowe’en storm on October 29, 2003, intense broadband activity between ˜0.1 and 0.6 Hz appeared at all 4 spacecraft on both sides of the magnetic equator at perigee (near 1400 UT and 08:45 MLT). Power was especially strong and more structured in frequency in the compressional component: a minimum in wave power was observed at 0.38 Hz, corresponding to the oxygen ion cyclotron frequency. Poynting vector calculations indicated that wave power was primarily directed radially inward rather than along the magnetic field. Narrowband purely compressional waves near 0.15 Hz appeared at higher dayside latitudes in the southern hemisphere. CIS ion spectrometer data during this pass revealed that O+ was the dominant energetic ion. During the recovery phase of the November storm, on November 22, 2003, predominantly transverse 1.8 Hz waves with peak-to-peak amplitude of 10 nT were observed by all four spacecraft near perigee at L=4.4. During this more typical Pc 1 event, wave power was directed along B, toward the northern ionosphere. An unusually polarized 2.3 Hz emission (with power in the radial and compressional, but not azimuthal directions) was observed at L=5.4 5.9, 10 15° south of the magnetic equator. We infer that this wave event may have been generated on lower L shells and propagated obliquely to Cluster's location. Consistent with other recent observations, anisotropic plasma sheet/ring current proton distributions appeared to be a necessary condition for occurrence of waves during both passes, but was not always a sufficient condition. The transverse waves of November 22 occurred in regions which also contained greatly increased fluxes of cool ions (E<1 keV). On both days, Cluster observed features not previously reported, and we note that the purely compressional nature of the October 29 events was not anticipated in previous theoretical studies. The fact that these unusually polarized waves occurred in association with very intense geomagnetic storms suggests that they are likely to be extremely rare.