About

David Alexander is a Professor of Physics and Astronomy at Rice University, where he also serves as the Director of the Rice Space Institute and the Department Ombudsperson. His main area of interest is the study of the dynamic solar corona through the analysis and theoretical interpretation of thermal and non-thermal radiation. His primary contributions have been in the field of solar flare and coronal mass ejection (CME) physics, where he has developed theoretical models for the production of gamma-rays, hard X-rays, and soft X-ray line broadening. He is involved in projects aimed at understanding the initiation and evolution of solar flares and CMEs by exploring particle production in relation to magnetic topology, helicity injection, and filament eruptions. Currently, he leads the NSF-funded INSPIRE project to study magnetic interactions between stars and planets and is working on devising a novel snapshot hyperspectral imager for Earth remote sensing in collaboration with bioengineering colleagues. Dr. Alexander is also the author of the book "The Sun" (2009) published by Greenwood Press.

Research topics

• Astronomy
• Physics
• Classical mechanics
• Nuclear physics
• Computational physics
• Remote sensing
• Geography
• Astrobiology
• Astrophysics

Selected publications

• Gravity and Human Respiration: Biophysical Limitations in Mass Transport and Exchange in Spaceflight Environments

  Research Square · 2025-11-29

  preprintOpen access
  PublisherOA PDFDOI

• Exploring the Effects of Stellar Magnetism on the Potential Habitability of Exoplanets

  The Astrophysical Journal · 2024-07-01 · 6 citations

  articleOpen access

  Abstract Considerable interest has centered on Earth-like planets orbiting in the circumstellar habitable zone (CHZ) of its star. However, the potential habitability of an exoplanet depends upon a number of additional factors, including the presence and strength of any planetary magnetic field and the interaction of this field with that of the host star. Not only must the exoplanet have a strong enough magnetic field to shield against stellar activity, but it must also orbit far enough from the star to avoid direct magnetic connectivity. We characterize stellar activity by the star’s Rossby number, Ro, the ratio of stellar rotation rate to convective turnover time. We employ a scaled model of the solar magnetic field to determine the star’s Alfvén radius, the distance at which the stellar wind becomes super-Alfvénic. Planets residing within the Alfvén surface may have a direct magnetic connection to the star and therefore not be the most viable candidates for habitability. Here, we determine the Rossby number of a sample of 1053 exoplanet-hosting stars for which the rotation rates have been observed and for which a convective turnover time can be calculated. We find that 84 exoplanets in our sample have orbits which lie inside the CHZ and that also lie outside the star’s Alfvén surface: 34 of these have been classified as terran (11) or superterran (23) planets. Applying the Alfvén surface habitability criterion yields a subset of the confirmed exoplanets that may be optimal targets for future observations in the search for signatures of life.

  PublisherOA PDFDOI

• Expanding Heliophysics to Engage in Interdisciplinary Star-Planet Interactions Studies

  2023-07-31

  articleOpen access

  Traditionally, heliophysics is characterized as the study of the near-Earth space environment, where plasmas and neutral gases originating from the Earth and the Sun, and to a lesser extent other solar system bodies, interact in ways that are detectable only through in-situ or close-range (usually within ~10 AU) remote sensing.As a result, heliophysics has data from the space environment around a handful of solar system objects, in particular the Sun and the Earth.Comparatively, Astrophysics has data from an extensive array of objects, but is more limited in temporal, spatial, and wavelength information from any individual object.Thus, our understanding of planetary space environments as a complex, multi-dimensional network of specific interacting systems may in the past have seemed to have little to do with the highly diverse space environments detected through astrophysical methods.Recent technological advances have begun to bridge this divide.Exoplanetary studies are opening up avenues to study planetary environments beyond our solar system, with missions like Kepler, TESS and JWST.Radio observatories such as LOFAR are also being utilized in the search for star-planet interaction signatures.At the same time, heliophysics studies are pushing beyond the boundaries of our heliosphere with Voyager, IBEX, and the future IMAP mission.The interdisciplinary field of star-exoplanet interactions is a critical emerging area of study that enriches heliophysics.A multidisciplinary approach to heliophysics enables us to better understand environments beyond our solar system and universal processes that operate in diverse environments, as well as the evolution of our solar system and extreme space weather.The expertise, data, theory, and modeling tools developed by heliophysicists are crucial in understanding the space environments of exoplanets, their host stars, and potential habitability.The high value of interdisciplinary studies promises to go far beyond any individual topic by enriching all subfields involved in the collaborations.The mutual benefit that heliophysics and exoplanetary studies offer each other depends on strong, continuing solar system-focused and Earth-focused heliophysics studies.It is crucial for the heliophysics discipline to receive new targeted funding to support inter-divisional opportunities, including small multi-disciplinary research projects, large collaborative research teams, and observations targeting the heliophysics of planetary and exoplanet systems.Here we discuss areas of heliophysics-relevant exoplanetary research, observational opportunities and challenges, and ways to promote the inclusion of heliophysics within the wider exoplanetary community.

  PublisherOA PDFDOI

• Star-exoplanet interactions: A growing interdisciplinary field in heliophysics

  Frontiers in Astronomy and Space Sciences · 2023 · 10 citations

  • Physics
  • Astronomy
  • Astrobiology

  Traditionally, heliophysics is characterized as the study of the near-Earth space environment, where plasmas and neutral gases originating from the Earth, the Sun, and other solar system bodies interact in ways that are detectable only through in-situ or close-range (usually within ∼10 AU) remote sensing. As a result, heliophysics has data from the space environment around a handful of solar system objects, in particular the Sun and Earth. Comparatively, astrophysics has data from an extensive array of objects, but is more limited in temporal, spatial, and wavelength information from any individual object. Thus, our understanding of planetary space environments as a complex, multi-dimensional network of specific interacting systems may in the past have seemed to have little to do with the highly diverse space environments detected through astrophysical methods. Recent technological advances have begun to bridge this divide. Exoplanetary studies are opening up avenues to study planetary environments beyond our solar system, with missions like Kepler, TESS, and JWST, along with increasing capabilities of ground-based observations. At the same time, heliophysics studies are pushing beyond the boundaries of our heliosphere with Voyager, IBEX, and the future IMAP mission. The interdisciplinary field of star-exoplanet interactions is a critical, growing area of study that enriches heliophysics. A multidisciplinary approach to heliophysics enables us to better understand universal processes that operate in diverse environments, as well as the evolution of our solar system and extreme space weather. The expertise, data, theory, and modeling tools developed by heliophysicists are crucial in understanding the space environments of exoplanets, their host stars, and their potential habitability. The mutual benefit that heliophysics and exoplanetary studies offer each other depends on strong, continuing solar system-focused and Earth-focused heliophysics studies. The heliophysics discipline requires new targeted funding to support inter-divisional opportunities, including small multi-disciplinary research projects, large collaborative research teams, and observations targeting the heliophysics of planetary and exoplanet systems. Here we discuss areas of heliophysics-relevant exoplanetary research, observational opportunities and challenges, and ways to promote the inclusion of heliophysics within the wider exoplanetary community.

  PublisherOA PDFDOI

• Incorporating Inner Magnetosphere Current-driven Electron Acceleration in Numerical Simulations of Exoplanet Radio Emission

  The Astrophysical Journal · 2021 · 10 citations

  • Physics
  • Astronomy
  • Astrophysics

  Abstract We present calculations of auroral radio emission for an Earth-like planet produced by field-aligned current (FAC) driven electron acceleration using a coupled global magnetohydrodynamic (MHD) and inner magnetosphere model, extending the capabilities of previous works which focus solely on the direct transmission of magnetic energy between the stellar wind and ionosphere. Magnetized exoplanets are expected to produce radio emission via interaction between the host star’s stellar wind and planetary magnetosphere-ionosphere system. The empirically derived Radiometric Bode’s Law (RBL) is a linear relation between the magnetic solar wind power and total emitted radio power from magnetized Solar System planets, and is often extrapolated to extreme exoplanet systems. It has been shown that the magnitudes of the FACs coupling the stellar wind to planetary ionospheres are likely to be significantly limited (often referred to as ionospheric saturation), resulting in an estimated radio power up to several orders of magnitude less than that predicted by RBL. In this paper, we demonstrate the significance of intense, sporadic FACs, driven by nightside magnetic reconnection and inner magnetosphere plasma flow, to the total radio power produced by wind–ionosphere interaction in terrestrial planets. During periods of strong stellar wind variability, the contribution from these secondary currents can be over an order of magnitude greater than the primary current systems that previous models describe. The results highlight the role of the variability of the stellar wind on the magnitude and location of the resulting emission, subsequently affecting the conditions for detectability.

  PublisherOA PDFDOI

• Compact, high performance, snapshot imaging spectrometers for environmental imaging

  2021-11-15

  articleSenior author

  Here we present implementations snapshot–image reorganization/mapping techniques including IMS (Imaging Mapping Spectrometry), and high density fiber image processors. These instruments are compact, non-scanning / high speed and provide high light throughput. The general principle of these methods is based on mapping areas (pixel, lines) of the object/scene on a large format CCD/CMOS to create void regions for spectral spread and simultaneous 3D+ data acquisition. This allows for a rapid detection and/or for increased SNR. High light throughput allows imaging at ultra-short acquisitions (entire cubes) down to microsecond levels. Our newest implementations include gas detection, environmental imaging and compact UAV applications. This presentation describes advantages of IMS, and fiber based systems and compares them to other snapshot techniques (computational methods). Examples of remote sensing results are presented (vegetation/crops assessment).

  PublisherDOI

• Acceleration of Non-Maxwellian Electron Distributions and Estimates of Radio Emission Observables

  AGU Fall Meeting Abstracts · 2020-12-01

  articleSenior author
  Publisher

• Glossary

  Rutgers University Press eBooks · 2020-12-31

  book-chapter1st authorCorresponding
  PublisherDOI

• Light-guide snapshot imaging spectrometer for remote sensing applications

  Optics Express · 2019-05-20 · 28 citations

  articleOpen access

  A fiber-based snapshot imaging spectrometer was developed with a maximum of 31853 (~188 x 170) spatial sampling and 61 spectral channels in the 450nm-750nm range. A compact, custom-fabricated fiber bundle was used to sample the object image at the input and create void spaces between rows at the output for dispersion. The bundle was built using multicore 6x6 fiber block ribbons. To avoid overlap between the cores in the direction of dispersion, we selected a subset of cores using two alternative approaches; a lenslet array and a photomask. To calibrate the >30000 spatial samples of the system, a rapid spatial calibration method was developed based on phase-shifting interferometry (PSI). System crosstalk and spectral resolution were also characterized. Preliminary hyperspectral imaging results of the Rice University campus landscape, obtained with the spectrometer, are presented to demonstrate the system's spectral imaging capability for distant scenes. The spectrum of different plant species with different health conditions, obtained with the spectrometer, was in accordance with reference instrument measurements. We also imaged Houston traffic to demonstrate the system's snapshot hyperspectral imaging capability. Potential applications of the system include terrestrial monitoring, land use, air pollution, water resources, and lightning spectroscopy. The fiber-based system design potentially allows tuning between spatial and spectral sampling to meet specific imaging requirements.

  PublisherDOI

• Simulating the Inner Asterospheric Magnetic Fields of Exoplanet Host Stars

  American Astronomical Society Meeting Abstracts #234 · 2019-06-01

  article
  Publisher

Recent grants

• INSPIRE: Modeling the Magnetic Interactions between Stars and Planets

  NSF · $1000k · 2015–2022

• Solar, Heliospheric, and Interplanetary Environment (SHINE): Understanding the Role of Reconnection in Coronal Mass Ejection (CME) Initiation

  NSF · $310k · 2004–2008

Frequent coauthors

• A Kuznetsov

  Kuzbass State Technical University

  32 shared

• Jean‐Marie Lauenstein

  Goddard Space Flight Center

  16 shared

• Heather Quinn

  Los Alamos National Laboratory

  16 shared

• Leon ft

  Ball (France)

  16 shared

• A. Keating

  16 shared

• Jeffrey W. Tripp

  Optech (Canada)

  16 shared

• Richard Horton

  Lancet Laboratories

  16 shared

• David Llulluy Nuñez

  16 shared