BOSTON UNIVERSITY
Department of Astronomy
Center for Space Physics

Analytic Theory

Rigorous analytic theory is the foundation beneath every simulation our group runs — explaining why instabilities grow, predicting where turbulence should appear, and interpreting what the codes produce. This page highlights the two people most responsible for that theoretical foundation: Senior Research Scientist Yakov Dimant and former PhD student (now Research Professor) William Longley.

Why Theory Matters

Kinetic and fluid simulations tell us what a plasma does; analytic theory tells us why — and lets us predict behavior in regimes too costly or too extreme to simulate directly. Across the group’s three research programs (ionosphere, solar chromosphere, and meteor plasma physics), closed-form theory has repeatedly done the same three jobs: derived the instability thresholds and growth rates that simulations are designed to test, explained anomalous features (conductivities, radar echoes, temperature biases) that pure simulation left unexplained, and extended results into parameter regimes — different magnetic fields, ion species, or electron temperatures — that would be prohibitively expensive to simulate one by one.

Much of this theoretical work has been led by Yakov Dimant, the group’s senior theorist for over two decades, and carried forward into new domains — incoherent scatter radar diagnostics and the 150 km echo problem — by William Longley during his PhD and beyond.

Yakov Dimant
Senior Research Scientist

Yakov Dimant

BU Center for Space Physics • leading theorist in the group for over 20 years

Yakov Dimant develops the analytic and semi-analytic theory that underlies most of the group’s simulation work. His contributions span three domains: the kinetic and fluid theory of meteor plasma trails, the theory of E-region turbulence, anomalous conductivities, and electron-precipitation effects in the ionosphere, and — more recently — the unified fluid theory of E×B and thermal Farley–Buneman instabilities, extended to the partially ionized solar chromosphere. He is a co-author on more than 40 group publications.

Meteor Plasma Trail Theory

Developed the 2-D analytic theory of meteor trail ambipolar fields and anisotropic diffusion, later extended to include external DC electric fields and validated against simulation. This theory also underlies the kinetic description of plasma formation around small meteoroids used to interpret radar head echoes.

E-Region Turbulence, Conductivity & Precipitation

Derived the energy-budget and anomalous-conductivity theory that explains how Farley–Buneman turbulence modifies magnetosphere–ionosphere coupling during storms, the theory of electric-field and current generation by neutral-wind “collisional dynamos,” and — most recently — how electron precipitation raises the instability threshold and suppresses turbulence inside the auroral oval.

Chromospheric Thermal Farley–Buneman Theory

Built the unified fluid theory of E×B and thermal-Farley–Buneman instabilities for arbitrarily magnetized, multi-species ions — the theoretical backbone connecting the group’s ionospheric turbulence theory to the newly discovered instability heating the solar chromosphere.

Selected Theory Publications — Yakov Dimant

William Longley
Research Professor, NJIT Center for Solar-Terrestrial Research

William Longley

Astronomy Ph.D., BU, 2019 • AGU BASU (Basu Early Career Award), 2023

As a PhD student and later as a collaborating researcher, William Longley built the theory and simulations connecting incoherent scatter radar (ISR) spectra to the underlying collisional plasma physics. He then used that same theoretical grounding to solve a long-standing puzzle: what drives the anomalous 150 km daytime radar echoes in the equatorial ionosphere. He has since continued collaborating with the group while a faculty member at NJIT.

ISR Spectral Theory & Simulation

Developed particle-in-cell simulations of incoherent scatter radar spectra incorporating electron–ion Coulomb collisions and nonlinear electron–electron collision effects, then validated the resulting forward models directly against Millstone Hill small-aspect-angle radar measurements — work that identified and corrected systematic biases in standard ISR temperature retrievals.

Discovery of the 150 km Echo Mechanism

Extended the group’s photoelectron-driven upper-hybrid instability theory to explain the decades-old mystery of daytime 150 km radar echoes, then followed it with a sole-authored paper identifying nonlinear wave-mode coupling as a further generation mechanism — together the group’s most cited recent theoretical contribution to equatorial aeronomy.

Open-Source Simulation Tools

Continues to contribute to the research field’s software infrastructure, including the open-source electromagnetic FDTD code for modeling radio-wave propagation through structured ionospheric plasma.

Selected Theory Publications — William Longley

Theory and simulation are not separate tracks in this group — they are the same research program viewed from two directions. See how these theoretical results connect to simulation work on the Ionosphere, Chromosphere, and Meteor Physics pages, or how they are computed on the Simulations page.

Analytic Theory

Rigorous closed-form and semi-analytic theory, developed alongside massively parallel simulation, is what lets this group explain — not just reproduce — the plasma physics of the ionosphere, chromosphere, and meteor trails.

Research in the group runs through the BU Astronomy PhD program: bu.edu/astronomy/graduate.

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