Abstract for Paper 1: The 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 (SW). Charged particles in the SW are able to follow these open field lines into Earth's upper atmosphere. The OCB also defines the polar cap boundary (PCB). 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 interesting insights into the precipitation of energetic protons into the ionosphere. Using the Tsyganenko model of the geomagnetic field 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 (PCA) events which adversely affect high frequency radio wave propagation in polar regions.

Abstract for Paper 2: The first well-documented polar cap absorption event occurred on 23 February 1956. Since that time much has been learned and explored regarding these events and their effect on high-frequency radio wave propagation in earth's polar regions. An important boundary when considering geomagnetic field topology and polar cap absorption events is commonly referred to as the cutoff latitude or energy cutoff latitude. The cutoff latitude is dependent on several factors, including solar wind parameters and proton energy. There exists an extensive body of research involving the cutoff latitude. One common approach in finding the cutoff latitude is to use a model of the geomagnetic field and trace proton trajectories through the field. Using this approach and the 1996 version of the Tsyganenko model, we demonstrate a unique method to organize three cutoff latitude boundary dependencies. The first, which we call a Universal Time or UT effect, is independent of proton energy and is based on geomagnetic alignment. The second, a local time or LT effect, exhibits an energy-dependent modulation as well as solar wind dependence. The third, a longitudinal effect, is also energy-dependent. Together, these effects can significantly alter the solar proton energy cutoff latitude, especially in the energy regime most responsible for polar cap absorption events.

Author ORCID Identifier

David Alan Smith



Document Type




File Format


Publication Date



Utah State University


The data were collected using the Tsyganenko Model of the Geomagnetic field, freely available at the CCMC. We used our own FORTRAN code in conjunctions with the T96 model to determine the OCB as well as the cutoff latitude for various proton energies.

Scientfic Names

Tsyganenko, FORTRAN code, T96 model

Referenced by

Smith, D. A., & Sojka, J. J. (2019). Model-Based Properties of the Dayside Open/Closed Boundary: Is There a UT-Dependent Variation? Space Weather, 17(12), 1639–1649.



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