Antarctic Space Science - Publications
2008
[back]Interaction of propagating magnetosonic and Alfvén waves in a longitudinally inhomogeneous plasma
Authors: Pilipenko, V. A.; Mazur, N. G.; Fedorov, E. N.; Engebretson, M. J.
Journal: Journal of Geophysical Research, Volume 113, Issue A8, CiteID A08218
Date: Aug 2008
Abstract: The field line resonance theory has turned out to be very successful in the interpretation of the occurrence of narrow-band ULF waves at middle and auroral latitudes. However, observational evidence has emerged (such as the occurrence of monochromatic Pc3 waves deep in the polar cap, Pi1B pulsations associated with tail plasma flows, and ULF waves in the magnetospheres of giant planets) which indicates that some basic assumptions of this theory may be insufficient in regions with open or very extended field lines. To interpret these observations, we suggest a model describing the interaction of propagating magnetosonic and Alfvén waves in longitudinally inhomogeneous plasma. A set of equations for the amplitudes of the interacting modes with account of finite frequency effect is obtained and investigated in the WKB approximation. In the synchronization region, where the wave vectors of the two modes approach one another, a substantial part of the fast magnetosonic wave energy is converted into the Alfvén wave energy. The phases of the waves are matched in such a way that the phase difference is most favorable for wave conversion. This conversion has a resonant character which may explain the occurrence of monochromatic waves with frequencies much higher than the field line eigenfrequency.
Periodic modulation of Pc3 and Pc4 pulsations in the polar cap by interplanetary and atmospheric processes
Authors: Chugunova, O. M.; Pilipenko, V. A.; Shalimov, S. L.; Engebretson, M.
Journal: Geomagnetism and Aeronomy, Volume 48, Issue 3, pp.307-313
Date: Jun 2008
Abstract: The variations in the daily average energy of geomagnetic pulsations and noise in the Pc3 (20 60 mHz) and Pc4 (10 19 mHz) frequency bands in the polar cap have been studied based on the data from P5 Antarctic station (corrected geomagnetic latitude ‑87°) from November 1998 to November 1999. The daily average pulsation energy has been calculated using the method for detecting the wave packets, the spectral amplitude of which is higher than the threshold level, from the dynamic spectrum. A spectral analysis of the energy of pulsations and noise in the Pc3 and Pc4 bands, performed using the maximal entropy method, has revealed periodicities of 18 days in the local winter and 26, 13, and 7 9 days during the local summer. The simultaneous and coherent variations with periods of 26, 13, and 7 9 days in the solar wind velocity and IMF orientation indicate that the variations in the Pc3 4 wave energy in the polar cap at a sunlit ionosphere are mainly controlled by the parameters of the interplanetary medium. The variations in the Pc3 4 wave energy with a period of 18 days are observed only during the local winter and are supposedly related to the variations in the ionospheric conductivity modulated by planetary waves.
Pc1-Pc2 waves and energetic particle precipitation during and after magnetic storms: Superposed epoch analysis and case studies
Authors: Engebretson, M. J.; Lessard, M. R.; Bortnik, J.; Green, J. C.; Horne, R. B.; Detrick, D. L.; Weatherwax, A. T.; Manninen, J.; Petit, N. J.; Posch, J. L.; Rose, M. C.
Journal: Journal of Geophysical Research, Volume 113, Issue A1, CiteID A01211
Date: Jan 2008
Abstract: Magnetic pulsations in the Pc1-Pc2 frequency range (0.1-5 Hz) are often observed on the ground and in the Earth's magnetosphere during the aftermath of geomagnetic storms. Numerous studies have suggested that they may play a role in reducing the fluxes of energetic ions in the ring current; more recent studies suggest they may interact parasitically with radiation belt electrons as well. We report here on observations during 2005 from search coil magnetometers and riometers installed at three Antarctic stations, Halley (-61.84° magnetic latitude, MLAT), South Pole (-74.18° MLAT), and McMurdo (-79.96° MLAT), and from energetic ion detectors on the NOAA Polar-orbiting Operational Environment Satellites (POES). A superposed epoch analysis based on 13 magnetic storms between April and September 2005 as well as case studies confirm several earlier studies that show that narrowband Pc1-Pc2 waves are rarely if ever observed on the ground during the main and early recovery phases of magnetic storms. However, intense broadband Pi1-Pi2 ULF noise, accompanied by strong riometer absorption signatures, does occur during these times. As storm recovery progresses, the occurrence of Pc1-Pc2 waves increases, at first in the daytime and especially afternoon sectors but at essentially all local times later in the recovery phase (typically by days 3 or 4). During the early storm recovery phase the propagation of Pc1-Pc2 waves through the ionospheric waveguide to higher latitudes was more severely attenuated. These observations are consistent with suggestions that Pc1-Pc2 waves occurring during the early recovery phase of magnetic storms are generated in association with plasmaspheric plumes in the noon-to-dusk sector, and these observations provide additional evidence that the propagation of waves to ground stations is inhibited during the early phases of such storms. Analysis of 30- to 250-keV proton data from four POES satellites during the 24-27 August and 18-19 July 2005 storm intervals showed that the location of the inner edge of the ring current matched well with the plasmapause model of O'Brien and Moldwin (2003). However, the POES data showed no evidence of the consequences of electromagnetic ion cyclotron waves (localized proton precipitation) during main and early recovery phase. During later stages of the recovery phase, when such precipitation was observed, it was coincident with intense wave events at Halley, and it occurred at L shells near or up to 1 R E outside the modeled plasmapause but well equatorward of the isotropy boundary.
Interaction of Alfven waves with a turbulent layer
Authors: Pilipenko, V.; Fedorov, E.; Engebretson, M. J.
Journal: Earth, Planets and Space, Volume 60, p. 949-960.
Date: Sep 2008
Abstract: We consider the interaction of Alfven waves with a resistive turbulent layer with anomalous conductivity. High-frequency turbulence causes the occurrence of both field-aligned and transverse resistivity. The correct dispersion relationship for Alfven waves in a turbulent medium with anisotropic conductivities has been derived. Alfven waves may partially reflect from a resistive layer, be absorbed in it, or be transmitted through it. When field-aligned resistivity dominates, the relative effectiveness of these processes critically depends on the wave transverse scale. For a thin layer as compared with the wave field-aligned length, the characteristic parameter that controls the effectiveness of the wave interaction with a layer is the resistive Alfven scale λA, determined by the field-aligned resistance and Alfven velocity above the layer. Comparison of energy losses estimated from analytical relationships for a "thin" layer and from numerical calculations for a finite width layer shows that the thin layer approximation provides a reasonable estimate over a wide range of wave scales, not only very small. Estimation of the effective damping scale of the Pc1 waves in a turbulent cusp shows that the cusp proper cannot be a conduit of Pc1 wave energy from the magnetosheath to the ground. The "thin" layer model has been applied to the interpretation of the results of early studies of transient ULF wave (Pi2 pulsations) damping during substorm onset, which showed that the damping rate increased for accompanying magnetic bays stronger than ˜100 nT. Our estimates confirm that this additional damping can be caused by the occurrence of anomalous transverse resistance when magnetospheric current exceeds the threshold necessary for the excitation of high-frequency plasma turbulence.
Temporal and spatial characteristics of Pc1 waves observed by ST5
Authors: Engebretson, M. J.; Posch, J. L.; Westerman, A. M.; Otto, N. J.; Slavin, J. A.; Le, G.; Strangeway, R. J.; Lessard, M. R.
Journal: Journal of Geophysical Research, Volume 113, Issue A7, CiteID A07206
Date: Jul 2008
Abstract: We present the results of a study of Pc1 waves (0.2-5 Hz) recorded by the three spacecraft of NASA's Space Technology 5 (ST5) mission, which operated in a dawn-dusk, 300 × 4500 km Sun-synchronous orbit in a ``string-of-pearls'' configuration from 26 March through 23 June 2006. Regions with Pc1 wave activity are not only localized to rather narrow L shells but can appear and disappear on the time scales of ~10 s to 10 min as examined by ST5. Only half of the 48 identified events were observed by all three spacecraft, and five events were observed by only one spacecraft. Only seven events were observed below L = 4, and only one was observed below L = 3.6, consistent with the relatively quiet geomagnetic conditions during this interval. The temporal occurrence distribution of Pc1 events was similar to that recorded at Halley, Antarctica (L = 4.56), during this same interval in that the number and intensity of events increased during magnetospheric compressions and during the recovery phase of magnetic storms, but they were reduced or absent during main phase and early recovery phase. This agreement suggests that if Pc1 events occur during main phase, their nearly universal absence in ground records cannot be ascribed to ionospheric screening effects or obscuration by irregular ULF noise generated in the ionosphere. These findings also suggest that although electromagnetic ion cyclotron waves might theoretically cause rapid depletion of radiation belt electrons during the main phase of storms, such waves cannot be assumed to occur during the main phase of all storms.
Structure of ULF Pc3 waves at low altitudes
Authors: Pilipenko, V.; Fedorov, E.; Heilig, B.; Engebretson, M. J.
Journal: Journal of Geophysical Research, Volume 113, Issue A11, CiteID A11208
Date: Nov 2008
Abstract: Recent low-orbiting observations at satellites with high-accuracy magnetometers onboard (Oersted, CHAMP, and ST5) have provided a detailed picture of the Pc3 wave structure in the topside ionosphere. Pc3 waves were detected very clearly in the compressional component of the satellite magnetic field data, whereas on the ground their signature was found in the H component. The occurrence of a significant compressional component in Pc3 pulsations in the topside ionosphere was also evidenced by radio-sounding measurements of ionospheric plasma oscillations. The following possibilities of ULF compressional disturbance excitation are considered: (1) an incident Alfvén wave generates an evanescent fast mode as a result of its interaction with the anisotropically conducting ionosphere; (2) transport of ULF wave energy from a distant source toward the ionosphere predominantly occurs by a fast mode. We estimate quantitatively the expected relationships between the Pc3 wave magnetic components above the ionosphere and on the ground produced by these different mechanisms and have derived simple analytical relationships between the compressional and ground signals for both mechanisms. Numerical modeling with the use of exact formulas has shown that these approximations work well over a wide range of wave scales. This model has been applied to the interpretation of Pc 3 waves observed by CHAMP in the upper ionosphere and by ground stations at midlatitudes. In general, the observed ratio between the compressional component in space and the ground signal corresponds better to the scenario of direct fast mode transmission to the ground.
Pc3 4 ULF waves at polar latitudes
Authors: Pilipenko, V. A.; Chugunova, O. M.; Engebretson, M. J.
Journal: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 70, Issue 18, p. 2262-2274.
Date: Dec 2008
Abstract: A search for Pc3 4 wave activity was performed using data from a trans-Antarctic profile of search-coil magnetometers extending from the auroral zone through cusp latitudes and deep into the polar cap. Pc3 4 pulsations were found to be a ubiquitous element of ULF wave activity in all these regions. The diurnal variations of Pc3 and Pc4 pulsations at different latitudes have been statistically examined using discrimination between wave packets (pulsations) and noise. Daily variations of the Pc3 4 wave power differ for the stations at the polar cap, cusp, and auroral latitudes, which suggests the occurrence of several channels of propagation of upstream wave energy to the ground: via the equatorial magnetosphere, cusp, and lobe/mantle. An additional maximum of Pc3 pulsations during early-morning hours in the polar cap has been detected. This maximum, possibly, is due to the proximity of the geomagnetic field lines at these hours to the exterior cusp. The statistical relation between the occurrence of Pc3 4 pulsations and interplanetary parameters has been examined by analyzing normalized distributions of wave occurrence probability. The dependences of the occurrence probability of Pc3 4 pulsations on the IMF and solar wind parameters are nearly the same at all latitudes, but remarkably different for the Pc3 and Pc4 bands. We conclude that the mechanisms of high-latitude Pc3 and Pc4 pulsations are different: Pc3 waves are generated in the foreshock upstream of the quasi-parallel bow shock, whereas the source of the Pc4 activity is related to magnetospheric activity. Hourly Pc3 power has been found to be strongly dependent on the season: the power ratio between the polar summer and winter seasons is ˜8. The effect of substantial suppression of the Pc3 amplitudes during the polar night is reasonably well explained by the features of Alfven wave transmission through the ionosphere. Spectral analysis of the daily energy of Pc3 and Pc4 pulsations in the polar cap revealed the occurrence of several periodicities. Periodic modulations with periods 26, 13 and 8 9 days are caused by similar periodicities in the solar wind and IMF parameters, whereas the 18-day periodicity, observed during the polar winter only, is caused, probably, by modulation of the ionospheric conductance by atmospheric planetary waves. The occurrence of the narrow-band Pc3 waves in the polar cap is a challenge to modelers, because so far no band-pass filtering mechanism on open field lines has been identified.
Auroral medium frequency burst radio emission associated with the 23 March 2007 THEMIS study substorm
Authors: Bunch, N. L.; LaBelle, J.; Weatherwax, A. T.; Hughes, J. M.
Journal: Journal of Geophysical Research, Volume 113, Issue A6, CiteID A00C08
Date: Dec 2008
Abstract: Auroral medium frequency (MF) burst is an impulsive auroral radio emission associated with substorm onset detected by ground-based instruments between 1.3 and 4.5 MHz. On 23 March 2007 an MF burst emission was detected by the Dartmouth radio interferometer located near Toolik Lake, Alaska. This emission temporally coincides with the onset of the 23 March 2007 Time History of Events and Macroscale Interactions during Substorms (THEMIS) study substorm. Directions of arrival computed using the Dartmouth radio interferometer for this event also coincide spatially with the location of the expanding auroral arcs to the south observed by the all-sky imager at Fort Yukon, Alaska. This observation represents the first example of a direction of arrival measurement for MF burst. It strongly supports the association of MF burst with intense auroral arcs accompanying substorm onset. The direction of arrival of the MF burst is consistent with the direction to the eastern edge of the substorm onset location determined by multiple data sets during this substorm and suggests that location of MF burst radio emissions may be an effective method of locating substorm onsets, much as radio atmospherics are used to locate lightning.
Effect of Night Laboratories on Learning Objectives for a Nonmajor Astronomy Class
Authors: Jacobi, Ian C.; Newberg, Heidi Jo; Broder, Darren; Finn, Rose A.; Milano, Anthony J.; Newberg, Lee A.; Weatherwax, Allan T.; Whittet, Douglas C. B.
Journal: Astronomy Education Review, vol. 7, Issue 2, p.66-73
Date: n/a 2008
Abstract: We tested the effectiveness of hands-on nighttime laboratories that challenged student misconceptions, using a new assessment exam to measure learning in a nonmajor introductory astronomy class at Rensselaer Polytechnic Institute. We were able to increase learning at the 8.0 sigma level on one of the Moon phase objectives that was addressed in a cloudy night activity. There is weak evidence of some improvement on a broader range of learning objectives. We show evidence that the overall achievement levels of the four sections of the class are correlated with how much clear weather the sections had for observing even though the learning objectives were addressed primarily in activities that did not require clear skies. We describe our first attempt to cycle the students through different activity stations in an effort to handle 18 students at a time in the laboratories, and the lessons we learned from this.