SM41A-01  

Stationary Inertial Alfvén Waves and Auroral Morphology

* Knudsen, D J (knudsen@phys.ucalgary.ca), University of Calgary, Department of Physics and Astronomy, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
Koepke, M E (mkoepke@wvu.edu), West Virginia University, Physics Department, PO Box 6515, Morgantown, WV 26506-6315, United States

Auroral arcs are sometimes observed to oscillate at frequencies related to the field-line resonance frequency of shear-mode dispersive Alfvén waves, indicating that these waves are responsible for the spatial and temporal behavior of such arcs. However, resonant frequencies are not apparent in most arcs. This may be due in part to experimental limitations, yet some arcs are observed to last much longer than the FLR period, indicating that some other mechanism is at play. This talk shows how Alfvén waves can be created in a way which is analogous to time-stationary water waves downstream from a stationary perturbation in a river. Stationary Alfvén waves are predicted by a two-fluid theory of low-beta current sheets with a normal component of plasma flow. The waves manifest as spatially periodic oscillations in field-aligned electron velocity, which can become much larger than the Alfvén speed, and in plasma density, which can vary by tens of percent. Both of these are observed properties of auroral arcs, as is the natural scale of the oscillations direction perpendicular to B: tens of electron inertial lengths, or several to tens of kilometers above the auroral ionosphere. Stationary Alfvén waves offer a mechanism to produce the key features found in FLR arcs, but within a quasi-static structure that can last indefinitely with no time variation.

SM41A-02  

Modeling Feedback Between Ion Outflows and Electromagnetic Alfven Ion Cyclotron Waves

* Johnson, J R (jrj@pppl.gov), Princeton University, Princeton University, Plasma Physics Laboratory, PO Box 451, Princeton, NJ 08525, United States

Satellites in the auroral region often detect energetic heavy ion outflows together with electromagnetic ion cylotron wave activity (1-100Hz). Because the Poynting flux of the waves is directed into the ionosphere the waves can energize ionospheric ions at lower altitude leading to ion outflow from the topside ionosphere. One difficulty with relating the ion outflows to the wave activity is the nonlocality of the heating process---much of the heating occurs between the ionosphere (where the ions originate) and the spacecraft. A common practice is to assume a heating rate based upon the spectrum observed locally by the satellite. However, nonlocal wave solutions suggest that propagation and dissipation of the wave spectrum depends sensitively on the heavy ion plasma profiles in the topside ionosphere as well as the collisional ionospheric model. Consequently, the heating rate is strongly dependent on the plasma density profile. Because the heating rate determines the plasma profiles and the background profiles determine the heating rate, it is necessary to account for the feedback in a self-consistent manner. We successively iterate (1) a wave propagation code based on background plasma profiles (which solves the full electromagnetic equations including a realistic ionospheric model) and (2) a Monte Carlo simulation code to obtain the ion profiles based on heating rates obtained from the results of the wave propagation code. These wave solutions include the possibility of mode conversion among the propagating wave modes, dissipation at the cyclotron resonance, and collisional dissipation and reflection of the wave in the ionosphere. The method converges rapidly to a stable state, and the results suggest that the temporal evolution of the plasma profiles may involve a two-step process where helium is first heated then oxygen. We also discuss how primary cyclotron resonant heating differs from nonlinear stochastic ion heating that can occur at lower frequency in large amplitude Alfven waves. Finally, we discuss the challenges of incorporating such physical processes in global models.
http:w3.pppl.gov/~jrj/icw.html

SM41A-03  

Generation and propagation of Pc1-2 waves in the magnetosphere: Effects of heavy ions

Lee, D (dhlee@khu.ac.kr), Kyung Hee University, Department of Astronomy and Space Science, Yongin, 449-701, Korea, Republic of
Kim, K , Kyung Hee University, Department of Astronomy and Space Science, Yongin, 449-701, Korea, Republic of
Kim, K , Ajou University, Division of Energy Systems Research, Sowon, 442-749, Korea, Republic of
* Johnson, J R (jrj@pppl.gov), Princeton University, Plasma Physics Laboratory, Princeton, NJ 08543, United States
Kim, E , University of Sydney, School of Physics, New South Wales, 2006, Australia

Previous statistical studies show that Pc1-2 waves are found frequently with linear polarization near the equatorial region. Since the dispersion relation indicates that the polarization of most Pc1-2 wave events should be either left-handed or right-handed, it has remained as a long-standing, unanswered question. Our study suggests that these linearly-polarized events are generated by mode conversion of compressional waves into heavy ion resonances, called Buchsbaum-Bers and ion-ion hybrid resonances, which correspond to Alfven resonances in the low frequency limit. We examine how these resonances occur for various heavy ion population and source sizes in detail by performing an accurate calculation on the mode conversion efficiency. The dependence of the efficiency on the relative abundance among different ions is investigated and found to be consistent with observational features. From tha fact that the Buchsbaum-Bers resonance frequency is determined only by the magnetic field and relative population density of heavy ions, we suggest that monitoring the local heavy ion population is possible simply by examining peak frequencies of the linearly polarized wave events.

SM41A-04  

Seasonal Variation of Substorm Recovery Time Scales

* Fillingim, M O (matt@ssl.berkeley.edu), Space Sciences Laboratory, University of California, Berkeley, 7 Gauss Way, Berkeley, CA 94720-7450, United States
Chua, D , E. O. Hulbert Center for Space Research, Naval Research Laboratory, Washington, DC 20375, United States
Germany, G A, Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, Huntsville, AL 35899, United States
Spann, J F, NASA, Marshall Space Flight Center, Huntsville, AL 35812, United States

The hypothesis that ionospheric conductivity plays a major role in the global-scale dynamics of the aurora is further evaluated in this study. The substorm recovery time scale during auroral intensifications are computed for over three hundred substorms observed by the Polar Ultraviolet Imager (UVI) and the IMAGE Far Ultraviolet Imager (FUV) in both hemispheres and then sorted according to season. The substorm recovery times are well ordered by whether or not the nightside auroral region is sunlit: substorms occurring in the winter and equinox periods have similar recovery time scales which are both roughly a factor of two longer than that for summer when the auroral oval is sunlit. These results strongly suggest that simultaneous auroral intensifications in the northern and southern hemispheres develop differently during solstice conditions. We expect the auroral breakup in the dark (winter) hemisphere to be more intense and longer lived than that observed in the sunlit (summer) hemisphere. This also implies that more energy is deposited by electron precipitation in the winter hemisphere than in the summer one during a substorm. Simultaneous, conjugate auroral substorm observations by Polar UVI and the IMAGE FUV instrument are used to confirm this behavior. The observed hemispheric asymmetry and non-conjugacy of auroral substorms is consistent with the suppression of discrete aurora in sunlight and highlights the importance of this effect in magnetosphere-ionosphere coupling.
http:sprg.ssl.berkeley.edu/matt/AGUS2007/aurora/

SM41A-05  

A Statistical Examination of the Relation between Substorm Onsets and Magnetic and Plasma Changes in the Tail

Hsu, T (thsu@igpp.ucla.edu), IGPP/UCLA, 405 Hilgard Ave., Los Angeles, CA 90095, United States
* McPherron, R (rmcpherr@igpp.ucla.edu), IGPP/UCLA, 405 Hilgard Ave., Los Angeles, CA 90095, United States
Borovsky, J (jborovsky@lanl.gov), Los Alamos National Laboratory, Los Alamos, Los Alamos, NM 87545, United States

Substorms are so complex that many issues related to them remain very controversial. Many models have been proposed to explain substorm activity. These models can be broadly classified into two categories depending on the cause of the expansion onset. The first invokes processes in the near-Earth region (|X| < 15 Re) such as current sheet disruption, instability triggered by reduction of interplanetary electric field, or some feed back instabilities near the ionosphere. The other invokes mid-tail magnetic reconnection beyond |X| ~ 15 Re as a source of plasma flowing earthward. Deceleration of this earthward flowing plasma and pileup of magnetic flux close to the Earth is then the cause of near-Earth disturbances. A possible way to distinguish the cause of substorm expansion onset is to examine the relative timing of magnetotail disturbances. If the near-Earth region is the source region of substorm onset, a disturbance should be first observed in the near-Earth region and later in the midtail. Similarly, the opposite time delay should be observed if the mid-tail reconnection is the source of substorm expansion onset. In this study, we have identified several candidate events with chance conjunctions of GOES, GEOTAIL, POLAR, and Cluster. Data from auroral imagers, ground magnetometers, Pi 2 pulsation detectors, and synchronous particle detectors will be used to establish accurate onset times. IMF observation will be accurately time propagated by Weimer et al [2002] algorithm. The starting time of near and mid-tail disturbance from these multi- satellite observations will be compared with substorm onset time to establish a relative timing pattern during substorms.

SM41A-06  

Field-Aligned Current at Plasma Sheet Boundary Layers During Storm Time: Cluster Observation

* Shi, J (jkshi@center.cssar.ac.cn), Key Laboratory for Space Weather, CSSAR, Chinese Academy of Sciences, Beijing, 100080, China
Cheng, Z (zwcheng@spaceweather.ac.cn), Key Laboratory for Space Weather, CSSAR, Chinese Academy of Sciences, Beijing, 100080, China
Zhang, T (Teilongzhang@aoew.a.at), Space Research Institute, Austrian Academy of Sciences, Graz, A-8042, Austria
Dunlop, M (mwdunlop@rl.ac.uk), SSTD, Rutherford Appleton Laboratory, Chilton, Didcot, United Kingdom
Liu, Z (Liu@cssar.ac.cn), Key Laboratory for Space Weather, CSSAR, Chinese Academy of Sciences, Beijing, 100080, China

The magnetic field data from the FGM instruments on board the four Cluster spacecrafts were used to study Field Aligned Current (FAC) at the Plasma Sheet Boundary Layers (PSBLs) with the so called "curlometer technique". We analyzed the date obtained in 2001 in the magnetotail and only two cases were found in the storm time. One (August 17, 2001) occurred from sudden commencement to main phase, and the other (October 1, 2001) lay in the main phase and recovery phase. The relationship between the FAC density and the AE index was studied and the results are shown as follows. (1) In the sudden commencement and the main phase the density of the FAC increases obviously, in the recovery phase the density of the FAC increases slightly. (2) From the sudden commencement to the initial stage of the main phase the FAC increases with decreasing AE index and decreases with increasing AE index. From the late stage of the main phase to initial stage of the recovery phase, the FAC increases with increasing AE index and decreases with decreasing AE index. In the late stage of the recovery phase the disturbance of the FAC is not so violent, so that the FAC varying with the AE index is not very obvious.

SM41A-07  

The time-scale for the formation of the cold-dense plasma sheet: A case study

Meng, C (ching.meng@jhuapl.edu), the Jons Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, United States
* Wing, S (simon.wing@jhuapl.edu), the Jons Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, United States
Johnson, J R (jrj@pppl.gov), Princeton University, POB 451 MS 28, Princeton, NJ 08543-0000, United States
Fujimoto, M (fujimoto@stp.isas.jaxa.jp), ISAS/JAXA, Space Plasma Dvision 3-1-1 Yoshinodai, Kanagawa, 229-8510, Japan

The formation of cold-dense plasma sheet ions during northward interplanetary magnetic field (IMF) was investigated with DMSP and in situ satellite observations. The connection between solar wind ions and plasma sheet cold-component ions is demonstrated. The hot-component ions, which are typically found during southward IMF, are also present during northward IMF. The densification of the plasma sheet can be attributed to the influx of the cold-component (magnetosheath/solar wind origin) ions. The cooling of the plasma sheet can be attributed not only to the influx of the solar wind ions, but also the cooling of the hot-components. In the immediate aftermath of the IMF northward turning, the cooling of the hot components can play a more significant role before the arrival of the bulk of the cold components. Order of magnitude calculations of plasma sheet filling rate from reconnection and diffusion suggest that both entry mechanisms could result in roughly comparable filling rates. Hence, the dawn-dusk asymmetries would be key in distinguishing the roles of the various proposed entry mechanisms. The specific entropy of the northward IMF plasma sheet is discussed.

Authors (2007), Title, Eos Trans. AGU, 88(23), Jt. Assem. Suppl., Abstract xxxxx-xx