Abstract
AbstractThe open‐closed boundary (OCB) defines a region of significant transformation in Earth's protective magnetic shield. Principle among these changes is the transition of magnetic field lines from having two foot points, one in each hemisphere, to one foot point at Earth, the other mapping to the solar wind. Charged particles in the solar wind are able to follow these open field lines into Earth's upper atmosphere. The OCB also defines the polar cap boundary. Being able to identify and track the OCB allows study of several components of the geomagnetic system. Among them are the electrodynamics of the geomagnetic field and the reconnection balance between the dayside and nightside of the geomagnetic field. Furthermore, the OCB can provide insights into the precipitation of energetic protons into the ionosphere. Using the Tsyganenko model of the geomagnetic field (T96), we demonstrate a diurnal fluctuation which we call the Universal Time (UT) effect of the OCB. This UT effect is independent of all other inputs. We anticipate this UT effect to have important consequences in modeling the OCB and other polar cap‐associated structures, especially polar cap absorption events that adversely affect high‐frequency radio wave propagation in polar regions.
Highlights
It is well known that the open‐closed boundary (OCB) defines the region, moving poleward, where geomagnetic field lines transition from being closed to open
Using the T96 model of the geomagnetic field and tracing magnetic lines originating from the Earth, we studied the location of the modeled OCB
Our findings are the following: The geomagnetic latitude of the OCB depends on Universal Time (UT), even given all Space Weather drivers are held constant
Summary
It is well known that the open‐closed boundary (OCB) defines the region, moving poleward, where geomagnetic field lines transition from being closed to open. The location of the OCB as a reference latitude is utilized by a wide range of scientists and space weather forecasters and indirectly by amateur (Ham) radio operators This OCB usage is to regard its latitude as an approximation of the equatorward edge of a polar region that is being impacted by energetic particles, causing PCA events. Defining open versus closed field lines on the nightside of Earth can be arbitrary (e.g., Kabin et al, 2004) This is due, in part, to the great distances to which geomagnetic field lines may be stretched in the magnetotail. Values we used to define quiet, moderate, and severe geomagnetic conditions
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