HAO 2011 Profiles In Science: Dr. Delores Knipp
Contact:
303-497-1547
knipp@ucar.edu
Publications
(1) Knipp, D.; S. Eriksson; L. Kilcommons; G. Crowley; J. Lei; M. Hairston; and K. Drake. 2011: Extreme Poynting flux in the dayside thermosphere: Examples and statistics.
Abstract: Using Defense Meteorological Satellite Program F-15 we calculated Earth-directed, magnetospheric Poynting flux across large swaths of the polar caps and auroral zones for the interval 2000–2005. While examining the database for this key element of the upper atmosphere energy budget, we found a pattern of extreme Poynting flux deposition associated with a large east-west (By) interplanetary magnetic field (IMF) component. At such times the dayside high-latitude Poynting flux may exceed 170 mW/m2 —an order of magnitude above typical values. The likely source of these events is merging at the magnetopause flank and lobe. A significant fraction of these events occur with high-speed solar wind. Energy deposition at these high rates is a likely source of previously reported, but poorly understood, near-cusp neutral density enhancements. The figure below illustrates the localization of the Poynting flux energy deposition. For northward and southward IMF the Poynting flux deposition focuses on the dayside where the conductivity is enhanced by solar extreme ultraviolet radiation and the strong plasma flow channels develop under the influence of the IMF By component.
Figure 1 caption: Survey of Maximum Poynting Flux During IMF |By| >10 nT Events from 2000–2005, Poleward of 60°. The top panel shows the maximum Poynting flux for intervals of Bz >0 nT and |By| >10 nT. Each marker locates the maximum Poynting flux for a northern or southern pass. The color bars extend from 0–100 mW/m2. a) Northern hemisphere By− and Southern hemisphere By+ events. b) Northern hemisphere By+ and Southern hemisphere By− events. The bottom panel shows the maximum Poynting flux for intervals of Bz <0 nT and |By| >10 nT; c) Northern hemisphere By− and Southern hemisphere By+ events; d) Northern hemisphere By+ and Southern hemisphere By− events. Poynting flux exceeding 75mW/m2 is highlighted by colored "+" (Northern hemisphere) and "x" (Southern hemisphere). Poynting flux exceeding 100mW/m2 is highlighted by colored upward triangles (Northern hemisphere) and colored downward triangles (Southern hemisphere).
(2) Li, Wenhui; Delores Knipp; Jiuhou Lei; and Joachim Raeder. 2011: The relation between dayside local Poynting flux enhancement and cusp reconnection.
Abtract: We use the Open Geospace General Circulation Model (OpenGGCM) coupled magnetosphere-ionosphere-thermosphere code to study several events with large east-west interplanetary magnetic field (IMF) component and neutral to positive vertical component. During such conditions, unusually large earthward Poynting flux has been observed. We find that the Joule heating rate in the model agrees well with the observed Poynting flux from the Defense Meteorological Satellite Program spacecraft. Analysis of the model results shows that the strong Poynting flux hot spots are physically linked with magnetic reconnection at the high-latitude cusps. The solar wind mechanical force and the J×B force act on the newly opened field lines created by cusp reconnection to produce a Pedersen current, which consequently generates an intense Joule heating region, and a pair of adjacent and opposite field-aligned currents (FACs) connecting to the magnetopause currents forming a closed circuit. The intense Joule heating region is also the region with strong downward Poynting flux. The distribution, scale, and magnitude of this Joule heating region and corresponding FACs in the polar regions are mainly controlled by IMF clock angle, IMF magnitude, and solar wind dynamic pressure. A northward IMF condition with a large By component results in an extended region with intense Joule heating and FAC, thus making a spacecraft transiting the dayside region more likely to observe a strong downward Poynting flux.
Figure 2 caption: Distributions of (left) Joule heating rate and (right) Field Aligned Current in the northern polar region at 07:50 UT August 24, 2005. The red arc indicates the strong Joule heating rate region. The thick red line and the arrowhead indicate that the DMSP F15 satellite was passing through this region around 07:50 UT. The DMSP satellite recorded a peak Poynting flux value of ~100 mW/m2. The enhanced Joule heating region (~120 mW/m2) is located between two adjacent and opposite high-latitude strong field aligned currents shown in b. The Joule heating occurs in the narrow plasma flow channel where horizontal currents close the field aligned currents on either side. The IMF clock angle near the magnetopause is about 75° with a 20.5 nT IMF By and a 5.1 nT IMF Bz.