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POLAR CAP NEUTRAL DENSITY ANOMALY
POLAR CAP NEUTRAL DENSITY ANOMALY
Polar cap neutral density with large mass density enhancements over the background has been frequently observed in the polar cap during magnetic storms. Density anomalies were frequently accompanied by plasma clouds. Modeling of plasma cloud drift paths suggests that plasma clouds originating in the dayside mid-latitudes could convect through the polar cap following the zero-potential line and reach the nightside polar cap. Modeling suggests that plasma clouds undergo frequent ion-neutral collisions with the thermosphere neutral background to produce density anomalies. [3]
CHAMP satellite
KEY POINTS:
KEY POINTS:
CHAMP satellite observed polar cap neutral density anomaly (PCNDA) occurring frequently between the open-closed field line boundary (OCB) and the nightside auroral zone
The polar cap neutral density anomaly sizes suggest an energy deposition duration for producing traveling atmospheric disturbances (TADs) in the range from 0.5 to 2.5 hr
Density anomalies are frequently accompanied by plasma clouds originating from the dayside ionosphere or the vicinity of the dayside cusp
Lin, C. S., Sutton, E. K., Huang, C. Y., & Cooke, D. L. (2018). Occurrence locations, dipole tilt angle effects, and plasma cloud drift paths of polar cap neutral density anomalies. Journal of Geophysical Research:Space Physics, 123. https://doi.org/10.1002/2017JA024657. [pdf file]
Abstract
Abstract
Polar cap neutral density anomaly (PCNDA) with large mass density enhancements over the background has been frequently observed in the polar cap during magnetic storms. By tracing field lines to the magnetosphere from the polar ionosphere, we divide the open field line polar cap into two regions, an open field line (OFL) region with field lines connecting to the magnetopause boundary and a distant tail field line (TFL) region threaded with magnetotail lobe field lines. A statistical study of neutral density observed by the CHAMP satellite during major magnetic storms with Dst < -100 from July 2001 to 2006 indicates that over 85 percent of density anomalies were detected in the TFL region, at about 18o to 25o equatorward the center of the OFL region. PCNDAs were frequently accompanied by plasma clouds with peak density greater than 105 #/cm3. Modeling of plasma cloud drift paths suggests that plasma clouds originating in the dayside mid-latitudes could convect through the OFL region following the zero-potential line and reach the PCNDA locations. Plasma clouds could become stagnate in the TFL region, allowing a long duration of collisions with the neutral gas and possibly contributing to heating of PCNDAs. The PCNDA observations are interpreted as evidence that traveling atmospheric disturbance (TADs) could be generated in the nightside polar cap. From the PCNDA size and speed of sound at 400 km, we derive an initial energy deposition duration for producing TADs in the range from 0.75 to 1.75 hr.
Occurrence summary map of density peak locations in the OFL-centered coordinates. The common locations of polar cap neutral density anomalies (PCNDAs) are between the auroral zone and the nighttime convection throat along the solar wind direction (X-axis direction). The origin is at the OFL center for each PCNDA event. XOFL is the distance in km along the OFL X-axis, and YOFL is the distance in km along the OFL Y-axis. Each bin is 2.5o in the polar angle from the OFL center and 15o in the azimuthal angle from the OFL X-axis. The number of PCNDA events in each bin is shown in color according to the color scale from 0 to 20 given on the right-hand side of the map. The mean open-closed field line boundary (OCB) is plotted as a red dashed-line ellipse. The mean position of the dayside cusp is plotted at the red dot on the positive OFL X-axis, whereas the mean position of the nighttime convection throat is at the red dot on the negative OFL X-axis. The mean position of the geomagnetic axis is displayed at a black dot with vertical and horizontal bars, which are the standard deviations in the X-axis and Y-axis directions, respectively.
The origin is at the OFL center for each PCNDA event. XOFL is the distance in km along the OFL X-axis, and YOFL is the distance in km along the OFL Y-axis. Each bin is 2.5o in the polar angle from the OFL center and 15o in the azimuthal angle from the OFL X-axis. The number of PCNDA events in each bin is shown in color according to t
REFERENCES
REFERENCES
[1] Lin, C. S., S. B. Cable, and E. K. Sutton, F. A. Marcos, J. M. Retterer and S. H. Delay (2013), Satellite Drag Validation of the Thermosphere-Ionosphere Electrodynamics General Circulation Model, DTIC Technical memorandum AFRL-RV-PS-TM-2013-0002. [pdf file]
[2] Lin, C. S., S. B. Cable, and E. K. Sutton (2016), A trade study of thermospheric empirical neutral density models, DTIC Technical Memoradium, AFRL-RV-PS-TR-2016-0008. [pdf file]
[3] Lin, C. S., Sutton, E. K., Huang, C. Y., & Cooke, D. L. (2018). Occurrence locations, dipole tilt angle effects, and plasma cloud drift paths of polar cap neutral density anomalies. Journal of Geophysical Research:Space Physics, 123. https://doi.org/10.1002/2017JA024657. [pdf file]
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