Semester

Spring

Date of Graduation

2019

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Mark Koepke

Committee Co-Chair

Wade Huebsch

Committee Member

Wade Huebsch

Committee Member

Earl Scime

Committee Member

Weichao Tu

Committee Member

Dimitris Vassiliadis

Abstract

Earth’s magnetosphere is subject to disturbances, as evidenced by variations of the geomagnetic field in space and on the ground. It is generally understood that most such disturbances are controlled by variations in the solar wind, with interplanetary magnetic field orientations directed southward opposite to Earth’s dipole magnetic axis being most conducive to energy transfers into the magnetosphere, thus resulting in more disturbed intervals. However, the exact functional form for solar wind driving of the magnetosphere has been widely studied, with proposed functional forms varying from the simple half-wave electric rectifier to expressions with a much more complicated dependence upon solar wind parameters. We provide evidence that past empirical results favoring more complicated expressions can statistically emerge from simpler expressions when solar wind parameters are time averaged and that expressions found in past empirical studies can be at least partly explained by the use of time-averaged solar wind parameters having hourly timescales, leading to the pitfall of assigning profound physical meaning to a statistical accident. Suggestions are offered to avoid this pitfall in future investigations.

The strongest and most expansive disturbances in the magnetospheric system are magnetic storms. The signature of a geomagnetic storm is the reduction in the strength of Earth’s magnetic field at low latitudes. The conventional explanation for this storm-time geomagnetic depression is a ring shaped current system in the near Earth magnetosphere. In recent years, this conventional view has been called into question by researchers who argue that much of the depression is caused by currents in the more distant region called the magnetotail. Many researchers in the field continue to accept the conventional view. The relative contributions of the current systems are still debated.

We construct impulse response functions (IRFs) for storm-time depression to shed light on this controversy. We show that the reduced driving of the geomagnetic index SYM/H (used to measure storm magnitude) during intervals of low density solar wind is due to energy diversion to the ionosphere via burstier events called substorms. As substorm energy is derived from the magnetotail, this reduced driving of storms when substorms are enhanced implies that tail currents are significant to storm-time indices. We also note that the storm-time magnetic depression IRF has a second development several (2-7) hours after the solar wind transfers energy to the magnetosphere, which is more prominent when energy is diverted from the tail to the ionosphere. The IRF of that part of storm-time magnetic depression due (theoretically) to tail currents, as inferred from Auroral Boundary Index (ABI), is shown qualitatively similar to the IRF for SYM/H prior to the second development. We are able to show, by adding functions of AL as an additional IRF driving variable, that this second development is likely due to substorm activity. We interpret this as being consistent with the hypothesis of ionospheric O+ ions enhancing the ring current with a time delay of approximately 2-5 hours. Evaluation of IRFs for sector SMR indices (which resolve storm- time magnetic depression into zones by magnetic local time sectors) reveals a more complicated picture, with evidence for gradual symmetrization of ring current. We model an ideal IRF using our hypothesis and, by comparison to data generated IRFs, show that it presents a plausible model.

As Maltsev’s derivation [Maltsev (1996); Maltsev et al. (1996)] of tail current contributions to storm-time magnetic depression depends upon the extent of the equatorward auroral oval, the problem that K-family indices are widely regarded as auroral latitude proxies, rather than storm- time magnetic depression indices, presents itself. We show that the relation of Kp to ABI stems fromthe quasi-logarithmic scaling of Kp, and that storm-time indices, particularly Dcx when corrected for solar wind ram pressure effects, are also a good proxy for ABI when scaled logarithmically.

We use ionospheric field aligned current (FAC) maps, provided by APL’s AMPERE project, to generate statistically averaged FAC maps via the machine learning technique of k-means clustering. The region 2 (R2) currents are identified for each cluster and used as a proxy for the equatorward edge of the auroral oval. The magnetic flux in the auroral oval is then used to calculate a predicted tail current contribution to storm-time magnetic depression according to Maltsev’s theory. Re- markable agreement between the predicted and observed median pressure corrected Dcx is found, suggesting that tail contributions are a majority contribution to storm-time magnetic depression.

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