About

Steven Cranmer is a professor in the Astrophysical & Planetary Sciences department at the University of Colorado Boulder. His research interests include solar and stellar astrophysics, with a focus on the heating and energization of particles in the solar corona, the acceleration of the solar wind, and waves and turbulence in various astrophysical plasmas. Cranmer studies the hot, expanding outer atmosphere of the Sun to better understand and predict the Sun's long-term effects on Earth's climate and local space environment. His work also encompasses radiative transfer in stellar atmospheres, kinetic plasma physics, the dynamics of winds from rotating hot stars such as O, B, and Wolf-Rayet types, and nonradial stellar pulsations. Cranmer has contributed to the scientific community through publications on topics like magnetohydrodynamic mode conversion in the solar corona, stellar mass loss, and the properties of the solar corona and its connection to the solar wind.

Research topics

• Political Science
• Computer Science
• Physics
• Engineering
• Astronomy
• Systems engineering
• History

Selected publications

• Polarization Diagnostics Applied to Coronal Mass Ejections and the Background Solar Wind

  Solar Physics · 2026-02-01

  articleOpen access

  Abstract The ratio of radially to tangentially polarized Thomson-scattered white light provides a powerful tool for locating the 3D position of compact structures in the solar corona and inner heliosphere, and the Polarimeter to Unify the Corona and Heliosphere (PUNCH) has been designed to take full advantage of this diagnostic capability. Interestingly, this same observable that establishes the position of transient blob-like structures becomes a local measure of the slope of the global falloff of density in the background solar wind. It is thus important to characterize the extent along the line of sight of structures being studied, in order to determine whether they are sufficiently compact for 3D positioning. In this paper, we build from analyses of individual lines of sight to three-dimensional models of coronal mass ejections (CMEs), allowing us to consider how accurately polarization properties of the transient and quiescent solar wind are diagnosed. In this way, we demonstrate the challenges and opportunities presented by PUNCH polarization data for various quantitative diagnostics.

  PublisherOA PDFDOI

• A 7 Day Multiwavelength Flare Campaign on AU Mic. IV. Quiescent Gyrosynchrotron and Gyroresonance Radiation from 12 to 25 GHz

  The Astrophysical Journal · 2026-01-21

  articleOpen accessSenior author

  Abstract We present an analysis of the radio quiescent data from a multiwavelength campaign of the active M dwarf flare star AU Mic (dM1e) that occurred in 2018 October. Using Ku -band data (12–18 GHz) from the Karl G. Jansky Very Large Array and K -band data (17–25 GHz) from the Australia Telescope Compact Array, we find that the quiescent spectrum can be decomposed into two components: one falling with frequency and one that remains flat. The flat component has a relatively steady flux density of 0.64 ± 0.14 mJy. The falling component varies in strength, but exhibits a spectral index of α = −0.88 ± 0.10. The falling component is thus consistent with nonthermal, optically thin gyrosynchrotron radiation with a corresponding power-law index similar to flares from AU Mic. While a flat component may arise from thermal, optically thin free–free emission, the observed flux density and inferred mass-loss rate are both too large compared to previous stellar wind and X-ray emission theory and models, necessitating an alternative explanation. This flat component instead matches well with an optically thick gyroresonance component integrated over multiple source regions such that the composite spectra are reasonably flat. The persistence of these components across the rotational period suggests multiple source regions, which may help explain changes in flux density and persistent high-energy electrons.

  PublisherDOI

• Proceedings of Stan Fest: Fundamentals of Stellar Outflows: Celebrating and Amplifying the Scientific Life of Stan Owocki

  ArXiv.org · 2026-03-25

  otherOpen access1st authorCorresponding

  Over the past four decades, Professor Stanley P. Owocki and his collaborators have made great strides in understanding the physical processes that control the structure of stellar outflows, most particularly the line-driven winds of massive stars. To celebrate his formal retirement from the Bartol Research Institute of the University of Delaware, his friends and colleagues organized a meeting in his honor to review these fundamental advances and consider the prospects they provide for continued progress. This "Stan Fest," more formally titled "Fundamentals of Stellar Outflows: Celebrating and Amplifying the Scientific Life of Stan Owocki," was hosted by the Institute of Astronomy of KU Leuven from July 8 to 12, 2024 in the beautiful Belgian city of Leuven. The proceedings volume archived here contains the abstracts of every talk, full submitted papers, and links to the KU Leuven YouTube channel where videos of the talks can be found.

  PublisherOA PDFDOI

• Proceedings of Stan Fest: Fundamentals of Stellar Outflows: Celebrating and Amplifying the Scientific Life of Stan Owocki

  Zenodo (CERN European Organization for Nuclear Research) · 2026-03-25

  otherOpen access1st authorCorresponding

  Over the past four decades, Professor Stanley P. Owocki and his collaborators have made great strides in understanding the physical processes that control the structure of stellar outflows, most particularly the line-driven winds of massive stars. To celebrate his formal retirement from the Bartol Research Institute of the University of Delaware, his friends and colleagues organized a meeting in his honor to review these fundamental advances and consider the prospects they provide for continued progress. This "Stan Fest," more formally titled "Fundamentals of Stellar Outflows: Celebrating and Amplifying the Scientific Life of Stan Owocki," was hosted by the Institute of Astronomy of KU Leuven from July 8 to 12, 2024 in the beautiful Belgian city of Leuven. The proceedings volume archived here contains the abstracts of every talk, full submitted papers, and links to the KU Leuven YouTube channel where videos of the talks can be found.

  PublisherDOI

• Ubiquitous High-frequency Waves and Disturbances in the Active Region Corona Observed with Cryo-NIRSP

  The Astrophysical Journal · 2026-02-23

  articleOpen access

  Abstract The plasma of the solar corona harbors a multitude of coronal wave modes, some of which could be dissipated to provide the required energy and momentum to heat the corona and accelerate the solar wind. We present observations of the corona acquired with the newly commissioned infrared slit spectropolarimeter Cryo-NIRSP at the Daniel K. Inouye Solar Telescope (DKIST), Haleakalā, Hawaii to study the high-frequency wave behavior in closed, active region structures. Cryo-NIRSP observes the corona off the limb in the Fe xiii 1074 and 1079 nm forbidden atomic lines. The large aperture of DKIST allows us to explore the active region corona with a temporal resolution faster than a second with an achieved spatial resolution of 2″–5″. Enhanced wave power is observed in the coronal power spectra up to 100 mHz. We report evidence for spectroscopic detection of magnetohydrodynamic (MHD) wave modes with periodicities between 30 and 100 s in the observed active region corona and report their changing properties with height. Furthermore, we report on a statistically significant anticorrelation between the Fe xiii 1074 nm peak line intensity and line width. These observations show how the powerful spectropolarimetric capabilities of DKIST offer great promise for furthering our knowledge of coronal MHD waves.

  PublisherDOI

• Polarization Diagnostics Applied to Coronal Mass Ejections and the Background Solar Wind

  ArXiv.org · 2025-11-01

  preprintOpen access

  The ratio of radially to tangentially polarized Thomson-scattered white light provides a powerful tool for locating the 3D position of compact structures in the solar corona and inner heliosphere, and the Polarimeter to Unify the Corona and Heliosphere (PUNCH) has been designed to take full advantage of this diagnostic capability. Interestingly, this same observable that establishes the position of transient blob-like structures becomes a local measure of the slope of the global falloff of density in the background solar wind. It is thus important to characterize the extent along the line of sight of structures being studied, in order to determine whether they are sufficiently compact for 3D positioning. In this paper, we build from analyses of individual lines of sight to three-dimensional models of coronal mass ejections (CMEs), allowing us to consider how accurately polarization properties of the transient and quiescent solar wind are diagnosed. In this way, we demonstrate the challenges and opportunities presented by PUNCH polarization data for various quantitative diagnostics.

  PublisherOA PDFDOI

• Solar Wind Origin

  ArXiv.org · 2025-07-17

  preprintOpen access1st authorCorresponding

  The Sun continuously expels a fraction of its own mass in the form of a steadily accelerating outflow of ionized gas called the "solar wind." The solar wind is the extension of the Sun's hot (million-degree Kelvin) outer atmosphere that is visible during solar eclipses as the bright and wispy corona. In 1958, Eugene Parker theorized that a hot corona could not exist for very long without beginning to accelerate some of its gas into interplanetary space. After more than half a century, Parker's idea of a gas-pressure-driven solar wind still is largely accepted, although many questions remain unanswered. Specifically, the physical processes that heat the corona have not yet been identified conclusively, and the importance of additional wind acceleration mechanisms continue to be investigated. Variability in the solar wind also gives rise to a number of practical "space weather" effects on human life and technology, and there is still a need for more accurate forecasting. Fortunately, recent improvements in both observations (with telescopes and via direct sampling by space probes) and theory (with the help of ever more sophisticated computers) are leading to new generations of predictive and self-consistent simulations. Attempts to model the origin of the solar wind are also leading to new insights into long-standing mysteries about turbulent flows, magnetic reconnection, and kinetic wave-particle resonances.

  PublisherOA PDFDOI

• High-frequency Coronal Alfvénic Waves Observed with DKIST/Cryo-NIRSP

  The Astrophysical Journal · 2025-03-21 · 13 citations

  articleOpen access

  Abstract The presence and nature of low-frequency (0.1–10 mHz) Alfvénic waves in the corona have been established over the past decade, with many of these results coming from coronagraphic observations of the infrared Fe xiii line. The Cryo-NIRSP instrument situated at DKIST has recently begun acquiring science-quality data of the same Fe xiii line, with at least a factor of 9 improvement in spatial resolution, a factor of 30 increase in temporal resolution, and an increase in signal-to-noise ratio, when compared to the majority of previously available data. Here we present an analysis of 1 s cadence sit-and-stare data from Cryo-NIRSP, examining the Doppler velocity fluctuations associated with the Fe xiii 1074 nm coronal line. We are able to confirm previous results of Alfvénic waves in the corona and explore a new frequency regime. The data reveal that the power-law behavior of the Doppler velocity power spectrum extends to higher frequencies. This result appears to challenge some models of photospheric-driven Alfvénic waves that predict a lack of high-frequency wave power in the corona owing to strong chromospheric damping. Moreover, the high-frequency waves do not transport as much energy as their low-frequency counterparts, with less time-averaged energy per frequency interval. We are also able to confirm the incompressible nature of the fluctuations with little coherence between the line amplitude and Doppler velocity time series.

  PublisherDOI

• Visualization of High Dynamic Range Solar Imagery and the Radial Histogram Equalizing Filter

  Solar Physics · 2025-12-01 · 1 citations

  articleOpen accessSenior author

  Abstract Standard visualizations of Extreme Ultraviolet (EUV) solar imagery often fail to convey the full complexity of the Sun’s corona, especially in faint off-limb regions. This can leave the misleading impression of the Sun as a bright ball in a dark void, rather than revealing it as the dynamic, structured source of the solar wind and space weather. A variety of enhancement algorithms have been developed to address this challenge, each with its own strengths and tradeoffs. We introduce the Radial Histogram Equalizing Filter (RHEF), a novel hybrid technique that optimizes contrast in high dynamic range solar images. By combining the spatial awareness of radial graded filters with the perceptual benefits of histogram equalization, RHEF reveals faint coronal structures and works out of the box—without requiring careful parameter tuning or prior dataset characterization. RHEF operates independently on each frame, and it enhances on-disk and off-limb features uniformly across the field of view. For additional control, we also present the Upsilon redistribution function—a symmetrized cousin of gamma correction—as an optional post-processing step that provides intuitive programmatic tonal compression. We benchmark RHEF against established methods and offer guidance on filter selection across various applications, with examples from multiple solar instruments provided in an appendix. Implemented and available in both Python and IDL, RHEF enables immediate improvements in solar coronal visualization.

  PublisherDOI

• High-frequency coronal Alfvénic waves observed with DKIST/Cryo-NIRSP

  arXiv (Cornell University) · 2025-01-07

  preprintOpen access

  The presence and nature of low-frequency (0.1-10~mHz) Alfvénic waves in the corona has been established over the last decade, with many of these results coming from coronagraphic observations of the infrared Fe XIII line. The Cryo-NIRSP instrument situated at DKIST has recently begun acquiring science quality data of the same Fe XIII line, with at least a factor of 9 improvement in spatial resolution, a factor 30 increase in temporal resolution and an increase in signal-to-noise, when compared to the majority of previously available data. Here we present an analysis of 1~s cadence sit-and-stare data from Cryo-NIRSP, examining the Doppler velocity fluctuations associated with the Fe XIII 1074~nm coronal line. We are able to confirm previous results of Alfvénic waves in the corona as well as explore a new frequency regime. The data reveals that the power law behaviour of the Doppler velocity power spectrum extends to higher frequencies. This result appears to challenge some models of photospheric-driven Alfvénic waves that predict a lack of high frequency wave power in the corona due to strong chromospheric damping. Moreover, the high-frequency waves do not transport as much energy as their low-frequency counterparts, with less time-averaged energy per frequency interval. We are also able to confirm the incompressible nature of the fluctuations with little coherence between the line amplitude and Doppler velocity time-series.

  PublisherOA PDFDOI

Recent grants

• Coronal Turbulence Driven from the Sun's Photosphere: Preparing for the Era of the Daniel K. Inouye Solar Telescope

  NSF · $279k · 2016–2022

• SHINE: Accelerating the Turbulent Solar Wind: One Flux Tube at a Time

  NSF · $193k · 2013–2015

• SHINE: Accelerating the Turbulent Solar Wind: One Flux Tube at a Time

  NSF · $193k · 2014–2019

Frequent coauthors

• J. C. Raymond

  Center for Astrophysics Harvard & Smithsonian

  52 shared

• M. P. Miralles

  Center for Astrophysics Harvard & Smithsonian

  48 shared

• S. E. Gibson

  NSF National Center for Atmospheric Research

  47 shared

• J. L. Kohl

  45 shared

• S. P. Owocki

  University of Delaware

  34 shared

• M. Maksimović

  Université de Versailles Saint-Quentin-en-Yvelines

  33 shared

• L. Strachan

  33 shared

• A. V. Panasyuk

  31 shared

Education

• Ph.D., Physics & Astronomy

  University of Delaware

  1996

• M.S., Astronomy

  Ohio State University

  1991

• B.S., Physics & Atmospheric Science

  Drexel University

  1990