Introduction
Electromagnetic features on ion gyro-radii scales are often described as kinetic Alfvén waves (KAWs). It is considered that they play an important role in particle heating/acceleration and plasma transport across the magnetopause. Some previous observations have revealed a dawn-dusk asymmetry during periods of northward interplanetary magnetic field in the distribution of cold magnetosheath ions within the magnetosphere. The magnetic local time (MLT) dependence of magnetosheath ion observations in the dayside magnetosphere implies that there is an MLT dependence in the entry mechanisms. To determine if such a distribution can be expected from KAW driven transport we have performed a statistical study using THEMIS field observations to analyze the MLT distribution of the spectral energy density of gyro-radii scale magnetic field structures. To determine how this distribution varies with the orientation of the magnetic field through the magnetosheath these results are categorized according to the orientation of BZ_sheath (GSE) component in the magnetosheath and magnetic shear angle observed across the magnetopause.
Results
Figure 1 presents a case study of a magnetopause crossing (MPC) observed from THEMIS probe B. As indicated by the labeling at the top of this figure, the spacecraft travels from the magnetosphere through a boundary layer (BL) and across the magnetopause (MP) into the magnetosheath (SH). Figures 1a - 1d show the observed particle and magnetic field data used to identify these three regions. Figure 1e shows the wavelet spectrogram in total magnetic field (Btotal). This reveals increasing spectral energy density at spacecraft frame frequencies below 1 Hz as the spacecraft transitions from the magnetosphere into the boundary layer and across the magnetopause where the largest amplitudes are found. In this study, we have compiled a database of wave observations derived from 389 THEMIS MPCs similar to the case example shown in Figure 1 for all BZ_sheath orientations from years 2007 - 2009.
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| Figure 1. THEMIS B observations on September 6, 2007. Panels from top to bottom referring to, (a) ion density and average temperature of ions and elections referring to black, blue and red lines, respectively; (b) ion energy flux; (c) ion flow velocity in GSE coordinate; (d) magnetic field in GSE and (e) spectral energy density in Btotal, black line corresponding to time series of frequency in spacecraft frame (fs/c) at fs/c ≈ vflow / (2πρi), two blue dashed lines corresponding to a fs/c range ±25% of fs/c ≈ vflow / (2πρi). Four vertical lines from left to right referring to boundary layer inner edge (magenta), magnetopause inner edge (green), magnetopause outer edge (green) and edge (light blue) in magnetosheath.. |
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Figure 2 shows distribution of spectral energy density (ε) as a function of MLT under conditions of (a) positive (northward pointing) and (b) negative (southward pointing) BZ_sheath. Moving outward from the center of the polar plots the three concentric circular grids correspond to the BL, MP and SH regions, respectively. These plots show that the largest ε irrespective of the orientation of BZ_sheath is observed near noon and peaks in the SH just outside the MP with a distribution that favors the pre-noon hours. However at MLTs more than 2 hours either side of the noon we find ε peaks at the MP and is an order of magnitude larger on the dawn (4 - 12 MLT) than duskside (12 - 20 MLT). Integrating over the entire spatial range surveyed shows that ε is larger for BZ_sheath negative than BZ_sheath positive.
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| Figure 2. The polar map of power spectral density of total magnetic field as a function of magnetic local time (MLT) in XGSE-YGSE plane under conditions of (a) positive (northward pointing) and (b) negative (southward pointing) BZ_sheath. Three concentric circular grids from inside to outside corresponding to boundary layer (BL), magnetopause layer (MP) and magnetosheath region (SH), respectively. |
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Figure 3 reveals the detailed dependence of these distributions with the degree of magnetic shear θB across the magnetopause. From Figures 4a to 4c, relationships in dawn (3 - 10 MLT), noon (10 - 14 MLT) and dusk (14 - 21 MLT) sectors are shown. In each panel, blue, green and red square lines correspond to BL, MP and SH, respectively. In general the spectral energy density (ε) in all three regions increase with θB. The most marked changes are however observed across the MP where we find nearly order of magnitude increases in ε on the dawn and dusk sectors for θB exceeding 60°. In the dawn sector ε at the MP also increases for the smallest θB.
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| Figure 3. Variation of power spectral density of total magnetic field as a function of magnetic shear angle (θB). Panels from top to bottom referring to different magnetic local time sectors, dawn, noon and dusk, respectively. In each panel, blue, green and red lines corresponding to the relationship in BL, MP and SH, respectively. Since logarithmic y-axis, error bars are plotted just in upper direction. |
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Conclusion
From a statistical study using THEMIS fields observations over a frequency range from 0.01 - 2 Hz, we find waves on scales of the order of ion gyro-radii at all magnetic local times surveyed (4 - 20 MLT). The spectral energy densities (ε) within these waves across the magnetopause are larger on the dawn (4 - 12 MLT) than duskside (12 - 20 MLT) irrespective of the direction of the magnetosheath magnetic field BZ_sheath component. Furthermore, these observations reveal that ε is magnetic shear dependent with large enhancements for shear angle > 60°. The similarity between dawn-dusk asymmetry in spectral energy density we observe and reported distribution of magnetosheath ions in the magnetosphere suggest that gyro-radii scale magnetic field structures/waves may facilitate a significant fraction of cross magnetopause plasma transport.
Source
Yao, Y., C. C. Chaston, K.-H. Glassmeier, V. Angelopoulos (2011), "Electromagnetic waves on ion gyro-radii scales across magnetopause", Geophys. Res. Lett., 38, doi10.1029/2011GL047328. In press.
Biographical Note
Yao Yao is a postdoctoral research associate at Space Sciences Laboratory of University of California, Berkeley. His current research interest is plasma transport across the magnetopause.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu
