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Katariina Nykyri

Publications and source records attributed to Katariina Nykyri.

5 recordsLinked to original sources
arXiv2026

Photospheric Kelvin--Helmholtz Vortices as Possible Drivers of Coronal Heating: Implications of the DKIST Observations

The Daniel K. Inouye Solar Telescope (DKIST) has resolved Kelvin--Helmholtz (KH) vortices at photospheric magnetic-flux boundaries with a characteristic wavelength of 65 km. I estimate whether these vortices can supply the photospheric driver for cross-scale plasma heating through reconnection across different heights from photosphere to low-corona. Using the simulated MURaM shear, density contrast, and 500 km vertical extent, together with a representative photospheric density, gives a shear-energy density of $1.35\times10^{2}$ J m$^{-3}$ and $2.2\times10^{24}$ erg per characteristic vortex. Magnetic fields $1^\circ$--$7^\circ$ from the exact perpendicular orientation ($ {\bf{B}}\perp{\bf{k}}$) remain KH unstable in an idealized calculation and provide an in-plane component that can be wound or compressed into current layers. The limiting case, in which the center-of-momentum shear reservoir becomes new magnetic free energy, gives $b_{\rm cs}=184$ G, identical to the ideal marginal-stability field and equivalent to a $7.6^\circ$ effective twist. This stores at most 135 J m$^{-3}$ in the layers. Using empirical collisionless reconnection heating fractions of 0.28--0.44, the same twist mapped to weakly collisional heights gives ion heating from $\approx$20 eV at the photosphere to $\approx$1.4 keV in the low corona. For an illustrative, snapshot-based KH-active surface fraction of 0.03, quiet-Sun and coronal-hole losses require 5--8\% and 14--21\%, respectively, of the shear reservoir to become reconnecting magnetic free energy that reaches such heights. Active regions likely require a separate guide-field twist and helicity reservoir. The required upward transport has not been measured by DKIST, but it is directly testable.

astro-ph.SR↗
arXiv2025

Observation of Large-Scale Kelvin-Helmholtz Instability Wave Driven by a Coronal Mass Ejection

The Kelvin-Helmholtz instability (KHI) can occur when there is a relative motion between two adjacent fluids. In the case of magnetized plasma, the shear velocity must exceed the local Alfvén speed for the instability to develop. The KHI produces nonlinear waves that eventually roll up into vortices and contribute to turbulence and dissipation. In the solar atmosphere KHI has been detected in coronal mass ejections (CMEs), jets, and prominences, mainly in the low corona. Only a few studies have reported the KHI in the upper corona, and its vortex development there has not been previously observed. We report the event with large-scale KHI waves observed from $\sim 6$ to 14~$R_{\odot}$ on 2024-Feb-16 using SOHO/LASCO and STEREO-A coronagraphs. KHI appeared during the passage of a fast CME and evolved into the nonlinear stage showing evidence of vortices. A closely timed subsequent CME in the same region has further developed the fully nonlinear KHI waves along its flank. We find that the radial speed of the CMEs exceeds the estimated local Alfven speed obtained from in-situ Parker Solar Probe (PSP) magnetic field data at perihelia. We propose that such events are rare because the fast CME created specific conditions favorable for instability growth in its trailing edge, including radial elongation of magnetic-field lines, reduced plasma density, and enhanced velocity and magnetic-field shear along the developing interface. The observed growth rate of KHI wave is in qualitative agreement with the theoretical predictions.

astro-ph.SR↗
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↗
arXiv2018

Magnetospheric Multi-Scale Observations of High-Energy Electrons and Protons in the Vicinity of Southern High-Altitude Cusp Boundaries

Recent Magnetosphere Multiscale (MMS) observations found 238 high energy (> 40 keV) electron "leaking" events in the the magnetosheath [Cohen et al., 2017]. While several sources have been proposed, the dominant mechanism or origin of these particles is not well understood. We have analyzed MMS locations during these these events and found that most of these electron leaking events were observed close to the southern high-altitude cusp and associated boundaries, so these events may have a high-latitude source. Here we present a new case study of observations of a MMS encounter of southern magnetospheric cusp boundaries. In the magnetosheath side of the southern cusp, MMS observes both parallel streaming energetic ions, as well as a population with 90 degree pitch angles. Subsequently, MMS detects plasma flows and magnetic field rotation consistent with magnetic reconnection (operating above and dawn-ward of the MMS spacecraft), finally entering a depressed magnetic field region with nearly stagnant flow. The depressed field region is occupied by high-fluxes of trapped high-energy electrons and protons, which resemble the Cluster observations of cusp diamagnetic cavities at the northern hemisphere [Nykyri et al., 2011a]. However, the local magnetic field geometry during the prevailing IMF orientation is not favorable for magnetic reconnection to directly create a cavity in this location. We show that the plasma flows that were observed prior to cavity observation satisfy the onset condition for the Kelvin-Helmholtz instability for 50 percent of all available k-vector directions.

physics.space-ph↗