arXiv subjects

Jason E. Kooi

Publications and source records attributed to Jason E. Kooi.

8 recordsLinked to original sources

arXiv2026

Observations of Disrupted CME Material Falling Back Into the Low Corona

We present an empirical study of a disrupted CME, parts of which fall back to the Sun, using observations from SOHO, STEREO-A, and SDO. At UT 18:00 on 2024 August 16, a slow CME is overtaken by a faster CME. A leg of the second CME carries part of the slower CME out with it, resulting in an unusually well-defined flux rope leg for this CME. This second CME is observed in radio by the VLA, with Faraday rotation measurements showing a clear magnetic flux rope signature. A strong response is also later seen when the radio-observed line of sight enters the CME leg enriched by material from the disrupted CME. Outside this leg, the rest of the disrupted CME simply disappears and is replaced by a large number of small jet-like downflows. We see clear evidence of this plasma falling back to the low corona in EUV images from SDO, roughly 6-17 hours after the CME is disrupted, with an inferred downward velocity of V=-30 km/s. There is a clear temperature dependence, with the downflows seen first in the 211 bandpass, followed successively by responses at 193, 171, and 304. The downflows are much slower than would be expected for a ballistic descent, so we model the downflows using a kinematic drag model. In the 304 bandpass, coronal rain activity is triggered by the downflowing CME material, suggesting that downflows from the upper corona could be contributing to coronal rain more generally.

astro-ph.SR↗

arXiv2025

Modeling Polarized Radio Sounding Observations of a Coronal Mass Ejection

Coronal Mass Ejections (CMEs) evolve significantly as they propagate from the Sun to the Earth, so remote observations of their changes in speed, strength of the magnetic field, density, and overall structure are critical for predicting their arrival time and geoeffectiveness. Radio line-of-sight observations of Faraday rotation and Total Electron Content combined with white-light observations enables the measurement of these properties with careful analyses. This paper describes the analysis techniques and evaluates their accuracy with regard to measuring a CME's complex evolving structure and speed. The approach utilizes the layout of the Faraday effect tracker of coronal and heliospheric structures (FETCH), a purely space-based instrument concept, with Alfven Wave Solar atmosphere Model (AWSoM) simulations as input for evaluating these radio-based measures. Focusing on density and velocity/speed, we find that in-situ measurements of CME properties observe similar but different aspects of the distinct CME structure. The AWSoM model suggests that Faraday rotation may be a more sensitive measure of structure than Total Electron Content (TEC). Finally, we discuss the difficulty the simulation reveals in determining the trailing edge location of a magnetic flux rope.

astro-ph.IM↗

arXiv2023

Defining the Middle Corona

The middle corona, the region roughly spanning heliocentric altitudes from $1.5$ to $6\,R_\odot$, encompasses almost all of the influential physical transitions and processes that govern the behavior of coronal outflow into the heliosphere. Eruptions that could disrupt the near-Earth environment propagate through it. Importantly, it modulates inflow from above that can drive dynamic changes at lower heights in the inner corona. Consequently, this region is essential for comprehensively connecting the corona to the heliosphere and for developing corresponding global models. Nonetheless, because it is challenging to observe, the middle corona has been poorly studied by major solar remote sensing missions and instruments, extending back to the Solar and Heliospheric Observatory (SoHO) era. Thanks to recent advances in instrumentation, observational processing techniques, and a realization of the importance of the region, interest in the middle corona has increased. Although the region cannot be intrinsically separated from other regions of the solar atmosphere, there has emerged a need to define the region in terms of its location and extension in the solar atmosphere, its composition, the physical transitions it covers, and the underlying physics believed to be encapsulated by the region. This paper aims to define the middle corona and give an overview of the processes that occur there.

astro-ph.SR↗

arXiv2023

Radio Studies of the Middle Corona: Current State and New Prospects in the Next Decade

The "middle corona," defined by West et al. (2022) as the region between ~1.5-6 solar radii, is a critical transition region that connects the highly structured lower corona to the outer corona where the magnetic field becomes predominantly radial. At radio wavelengths, remote-sensing of the middle corona falls in the meter-decameter wavelength range where a critical transition of radio emission mechanisms occurs. In addition, plasma properties of the middle corona can be probed by trans-coronal radio propagation methods including radio scintillation and Faraday rotation techniques. Together they offer a wealth of diagnostic tools for the middle corona, complementing current and planned missions at other wavelengths. These diagnostics include unique means for detecting and measuring the magnetic field and energetic electrons associated with coronal mass ejections, mapping coronal shocks and electron beam trajectories, as well as constraining the plasma density, magnetic field, and turbulence of the "young" solar wind. Following a brief overview of pertinent radio diagnostic methods, this white paper will discuss the current state of radio studies on the middle corona, challenges to obtaining a more comprehensive picture, and recommend an outlook in the next decade. Our specific recommendations for advancing the middle coronal sciences from the radio perspective are: (1) Prioritizing solar-dedicated radio facilities in the ~0.1-1 GHz range with broadband, high-dynamic-range imaging spectropolarimetry capabilities. (2) Developing facilities and techniques to perform multi-perspective, multiple lines-of-sight trans-coronal radio Faraday Rotation measurements.

astro-ph.IM↗

arXiv2021

Recommendations to clarify NASA open source requirements

The software community has specific definitions for terms such as "open source software," "free software," and "permissive license," but scientists proposing software development efforts to NASA are not always knowledgeable about these definitions. Misunderstandings about the meaning of these terms can result in problems of fairness with solicitations, because scientists who interpret the terms differently than NASA intends may either needlessly limit the scope of their proposed work, or unwittingly propose work that does not comply with software licensing requirements. It is therefore recommended that NASA adopt definitions of the above terms that are in line with software community usage, that these definitions be communicated as part of solicitations to ensure a common understanding, and that proposals be required to identify what software licenses the proposers expect to use.

cs.SE↗

arXiv2020

Inferences About the Magnetic Field Structure of a CME with Both In Situ and Faraday Rotation Constraints

On 2012 August 2, two CMEs (CME-1 and CME-2) erupted from the west limb of the Sun as viewed from Earth, and were observed in images from the white light coronagraphs on the SOHO and STEREO spacecraft. These events were also observed by the Very Large Array (VLA), which was monitoring the Sun at radio wavelengths, allowing time-dependent Faraday rotation observations to be made of both events. We use the white-light imaging and radio data to model the 3-D field geometry of both CMEs, assuming a magnetic flux rope geometry. For CME-2, we also consider 1 au in situ field measurements in the analysis, as this CME hits STEREO-A on August~6, making this the first CME with observational constraints from stereoscopic coronal imaging, radio Faraday rotation, and in situ plasma measurements combined. The imaging and in situ observations of CME-2 provide two clear predictions for the radio data; namely that VLA should observe positive rotation measures (RMs) when the radio line of sight first encounters the CME, and that the sign should reverse to negative within a couple hours. The initial positive RMs are in fact observed. The expected sign reversal is not, but the VLA data unfortunately end too soon to be sure of the significance of this discrepancy. We interpret an RM increase prior to the expected occultation time of the CME as a signature of a sheath region of deflected field ahead of the CME itself.

astro-ph.SR↗

arXiv2016

VLA Measurements of Faraday Rotation through Coronal Mass Ejections

Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the Sun that play an important role in space weather. Faraday rotation (FR) is the rotation of the plane of polarization that results when a linearly polarized signal passes through a magnetized plasma such as a CME. FR observations of a source near the Sun can provide information on the plasma structure of a CME shortly after launch. We report on simultaneous white-light and radio observations made of three CMEs in August 2012. We made sensitive Very Large Array (VLA) full-polarization observations using 1 - 2 GHz frequencies of a "constellation" of radio sources through the solar corona at heliocentric distances that ranged from 6 - 15 solar radii. Of the nine sources observed, three were occulted by CMEs: two sources (0842+1835 and 0900+1832) were occulted by a single CME and one source (0843+1547) was occulted by two CMEs. In addition to our radioastronomical observations, which represent one of the first active hunts for CME Faraday rotation since Bird et al. (1985) and the first active hunt using the VLA, we obtained white-light coronagraph images from the LASCO/C3 instrument to determine the Thomson scattering brightness, BT, providing a means to independently estimate the plasma density and determine its contribution to the observed Faraday rotation. A constant density force-free flux rope embedded in the background corona was used to model the effects of the CMEs on BT and FR. We demonstrate this model's ability to successfully reproduce both BT and FR profiles. The plasma densities (6 - 22 x 10$^3$ cm$^{-3}$) and axial magnetic field strengths (2 - 12 mG) inferred from our models are consistent with the modeling work of Liu et al. (2007) and Jensen & Russell (2008), as well as previous CME FR observations by Bird et al. (1985).

astro-ph.SR↗

arXiv2014

Measurements of Coronal Faraday Rotation at 4.6 Solar Radii

Many competing models for the coronal heating and acceleration mechanisms of the high-speed solar wind depend on the solar magnetic field and plasma structure in the corona within heliocentric distances of $5R_\odot$. We report on sensitive VLA full-polarization observations made in August, 2011, at 5.0 and 6.1 GHz (each with a bandwidth of 128 MHz) of the radio galaxy 3C228 through the solar corona at heliocentric distances of $4.6-5.0R_\odot$. Observations at 5.0 GHz permit measurements deeper in the corona than previous VLA observations at 1.4 and 1.7 GHz. These Faraday rotation observations provide unique information on the magnetic field in this region of the corona. The measured Faraday rotation on this day was lower than our a priori expectations, but we have successfully modeled the measurement in terms of observed properties of the corona on the day of observation. Our data on 3C228 provide two lines of sight (separated by 46'', 33,000 km in the corona). We detected three periods during which there appeared to be a difference in the Faraday rotation measure between these two closely spaced lines of sight. These measurements (termed differential Faraday rotation) yield an estimate of $2.6$ to $4.1$ GA for coronal currents. Our data also allow us to impose upper limits on rotation measure fluctuations caused by coronal waves; the observed upper limits were $3.3$ and $6.4$ rad/m$^2$ along the two lines of sight. The implications of these results for Joule heating and wave heating are briefly discussed.

astro-ph.SR↗