arXiv subjects

Olena Podladchikova

Publications and source records attributed to Olena Podladchikova.

10 recordsLinked to original sources

arXiv2026

Riemann Map Operator for Solar Front Diagnostics

Identifying a shock from a moving solar brightness front requires relating the observed emission to changes in the plasma state. The Riemann framework describes shocks, rarefactions and other waves as parts of a pattern connecting different states. We present the Riemann Map Operator (RMO), an inverse diagnostic framework implemented as a web application to help observers identify magnetohydrodynamic (MHD) shock families from incomplete measurements. RMO tests candidate local connections against conservation laws, entropy requirements and characteristic conditions, while retaining observational uncertainties and model assumptions. We demonstrate the approach with five solar examples and a complementary in-situ comparison. An event combining extreme-ultraviolet (EUV) imaging and spectroscopy admits both fast and slow shock connections within the tested conditions. For an EUV-radio event, we derive an upstream-flow constraint that would exclude ordinary slow shocks within the adopted model, provided both diagnostics sample the same front. A flare-loop benchmark reproduces the published spectral profiles and recovers slow-shock ordering in the isothermal limit. The Solar Orbiter comparison recovers an Alfvenic relation between measured velocity and magnetic-field changes. Together, the examples show how RMO connects observations to admissible MHD interpretations and identifies the additional measurements needed to distinguish them.

astro-ph.SR↗

arXiv2026

Untangling EIT Waves: What a Measured Speed Actually Traces

Nearly three decades after the first SOHO/EIT observations, reported EUV-wave speeds still range from a few tens to more than 1000 km/s. This Perspective asks what those numbers actually trace. A measured image speed depends on passband and line-of-sight weighting, cadence, difference imaging, the selected crest or leading edge, propagation sector, fitted interval, and projection geometry. Same-event comparisons quantify this observation-operator effect. In events common to the Nitta and Muhr analyses, early or fastest-sector measurements were typically higher; five of six directly shared events differed by about 30%. For 19 May 2007, the 171 Angstrom peak speed was 475 +/- 47 km/s, compared with 238 +/- 20 km/s in 304 Angstrom, while the 171 Angstrom cadence was four times faster than the 195 Angstrom cadence. A sampling audit of the 1 April 2017 SWAP event shows that a repeated speed near 834 km/s is almost exactly one 91.6-Mm radial ring per 110-s image interval. In a reconstructed 3 April sector, AIA 171 Angstrom gives about 405 km/s, compared with the published SWAP mean of 484 km/s; an independently selected AIA 193 Angstrom ridge gives about 250 km/s under strongly shortened exposures. Such differences are not automatic evidence for different MHD modes. A same-event energy pilot separates compact-source heating, dimming/ejecta, and one weak-compression front segment. The current data do not justify a universal power law, but define a falsifiable question: is the fraction of released energy carried by the front scale invariant from compact quiet-Sun events to global waves and shocks? Establishing this dependence would show which eruption scales can contribute materially to coronal heating.

astro-ph.SR↗

arXiv2026

The Physical Origin of Periodic Density Structures in the Solar Wind: Coronal Streamers as Magnetohydrodynamic Resonators

We present a comprehensive physical model explaining the origin of Periodic Density Structures (PDS) observed in white-light coronagraphs with characteristic periods of approximately 45, 80, and 120 minutes. Through systematic investigation of potential resonant cavities in the solar atmosphere, we demonstrate that traditional large-scale cavities yield fundamentally incompatible periods: photosphere-transition region (3.3 minutes), transition region-sonic point (10.3 hours), and transition region-heliopause (7.7 years). We establish that coronal streamers act as natural magnetohydrodynamic resonators, with calculated harmonic periods of 122, 61, and 41 minutes that precisely match observations. The physical mechanism involves slow magnetoacoustic standing waves that create periodic density enhancements through wave compression, with the streamer resonator having quality factor Q ~ 10-100, enabling natural amplification of broadband coronal noise. At streamer cusps, these density enhancements trigger magnetic reconnection, releasing plasma blobs into the solar wind at resonant periods. The model provides complete energy budget calculations, wave amplitude estimates, and explains all key observational features including spatial localization, period coherence, and the relationship between remote sensing and in situ measurements. This work establishes streamer resonators as fundamental structures shaping solar wind variability and provides a new framework for understanding the emergence of coherent structures in turbulent astrophysical plasmas.

astro-ph.SR↗

arXiv2026

Can MHD Oscillations Modulate Quasi-Periodic Plasma Release from Coronal Streamers?

Periodic density structures in the slow solar wind are associated with quasi-periodic plasma release from coronal streamer and open-closed boundary regions, but the origin of their organization remains uncertain. We distinguish the process that releases plasma from the process that sets or modifies its cadence. In the proposed source-modulator framework, S-Web, interchange, cusp, or current-sheet reconnection releases the plasma, while structured MHD responses may modulate density or release rate. An observation-informed parameter-space analysis gives compact slow-mode periods of about 22-231 min; reproducing 80-130 min requires an effective path length of 0.35-1.20 solar radii. In a classical top-hat cylinder with L/a = 5-50 and rho_i/rho_e = 1.6-2.5, the fundamental fast sausage mode is not trapped and the minimum trapped harmonic is approximately n = 4-50. A transverse fast-timescale benchmark based on a = 0.10-0.45 solar radii and outer-coronal fast speeds of 250-530 km/s gives 5.7-55 min. A corresponding fast-interface proxy gives 4.4-93 min, but is not a current-sheet eigenmode solution. Kink responses are constrained empirically rather than by inserting observer-frame propagation speeds into a standing-mode formula: one COR1 event showed an approximately 25 min pulse, whereas a LASCO/COR2 survey found global streamer-wave periods of 2-8 hr, observer-frame speeds of 360-740 km/s, and typically only one or two visible cycles. Among the candidates considered here, compact slow modes are the best-constrained stable compressive modulators; confirming their role requires constraints on longitudinal reflection and damping. Surface/current-sheet responses remain high-priority coupling candidates but require a sheet-specific dispersion relation; kink responses are transient geometric candidates; and tearing/plasmoid formation remains a strong intrinsic reconnection-driven alternative.

astro-ph.SR↗

arXiv2026

Where Do Periodic Density Structures Acquire Coherence? A Low-Coronal Resonator Candidate and a 12-Event STEREO/SECCHI Transfer Test

Periodic density structures (PDS) are trains of plasma enhancements carried into the heliosphere, but their repeated timing does not by itself identify where the cadence is set. We test whether PDS selected independently in the outer corona can be traced back to a low-coronal clock, and whether that organization survives through the EUVI--COR1--COR2 observing chain. We analyze 12 STEREO-A/SECCHI events on 2008 January 11--14 along a nonradial path. EUVI 171 A at 1.10--1.20 R_sun shows an unusual concentration of power in the predeclared 80--130 minute band relative to filter-matched red-noise controls, while the ensemble is most strongly organized in upper COR1 at 2.5--3.0 R_sun. These signatures identify a low-coronal modulation candidate and an intermediate-height organization domain, but they do not form a unique phase-preserving clock extending into COR2. Instead, the more persistent observable is spatial order. Events 9 and 12 retain expansion-stable outward ordering. In polar r--theta maps, intersections of oppositely inclined, expansion-aware matched-filter ridge supports form repeated X-/diamond-like patterns. The morphology is compatible with stationary or quasi-stationary shock-cell processing, although the brightness diagnostics are insufficient to establish a stationary MHD shock branch. The observations therefore favor an intermittent source--gate--transfer picture: a low-coronal cadence may modulate plasma release, while event-dependent propagation progressively destroys exact phase coherence. The outward ordering of the density structures can survive, providing a more persistent signature of the source-to-wind transfer than phase locking itself.

astro-ph.SR↗

arXiv2026

On the Short Dissipation Scales and Current-Sheet Properties of Low-Coronal EUV Brightenings

Solar Orbiter EUV observations reveal ubiquitous small-scale brightenings in the quiet-Sun low corona. We analyze the spatial and temporal dissipation scales of these events with a focus on the formation, evolution, and dissipation of associated current sheets. The brightenings are observed at heights of 1-5 Mm and span energies of 10^20 - 10^24 erg, well below the classical nanoflare regime, with the lowest-energy brightenings preferentially originating in the lowest coronal layers. Two distinct dissipation regimes are identified: impulsive brightenings with timescales of 1-10 s, consistent with fast, Alfvenic magnetic reconnection in low-beta plasma, and longer-lived heating episodes lasting 10-100 s, indicative of slower, resistive current-sheet dissipation under higher-beta conditions. The observed dissipation scales suggest a transition from kinetic-scale reconnection to macroscopic current-sheet heating in the low corona. These results support a multi-scale energy-release framework and highlight the role of low-altitude, small-scale current-sheet dissipation in quiet-Sun coronal heating.

astro-ph.SR↗

arXiv2022

Identifying the energy release site in a Solar microflare with a jet

One of the main science questions of the Solar Orbiter and Parker Solar Probe missions deals with understanding how electrons in the lower solar corona are accelerated and how they subsequently access interplanetary space. We aim to investigate the electron acceleration and energy release sites as well as the manner in which accelerated electrons access the interplanetary space in the case of the SOL2021-02-18T18:05 event, a GOES A8 class microflare associated with a coronal jet. This study takes advantage of three different vantage points, Solar Orbiter, STEREO-A, and Earth, with observations ranging from radio to X-ray. Multi-wavelength timing analysis combined with UV/EUV imagery and X-ray spectroscopy by Solar Orbiter/STIX (Spectrometer/Telescope for Imaging X-rays) is used to investigate the origin of the observed emission during different flare phases. The event under investigation satisfies the classical picture of the onset time of the acceleration of electrons coinciding with the jet and the radio type III bursts. This microflare features prominent hard X-ray nonthermal emission down to at least 10 keV and a spectrum that is much harder than usual for a microflare with a spectral index of 2.9. From Earth's vantage point, the microflare is seen near the limb, revealing the coronal energy release site above the flare loop in EUV, which, from STIX spectroscopic analysis, turns out to be hot (at roughly the same temperature of the flare). Moreover, this region is moving toward higher altitudes over time (about 30 km/s). During the flare, the same region spatially coincides with the origin of the coronal jet. We conclude that the energy release site observed above-the-loop corresponds to the electron acceleration site, corroborating that interchange reconnection is a viable candidate for particle acceleration in the low corona on field lines open to interplanetary space.

astro-ph.SR↗

arXiv2022

Synchronization of Small-scale Magnetic Features, Blinkers, and Coronal Bright Points

We investigate the relationship between different transients such as blinkers detected in images taken at 304~Å, extreme ultraviolet coronal bright points (ECBPs) at 193~Å, X-ray coronal bright points (XCBPs) at 94~Å on AIA, and magnetic features observed by HMI during ten years of solar cycle 24. An automatic identification method is applied to detect transients, and the YAFTA algorithm is used to extract the magnetic features. Using ten years of data, we detect in total 7,483,827 blinkers, 2,082,162 ECBPs, and 1,188,839 XCBPs, respectively, with their birthrate of about $1.1\times10^{-18}$ ${\rm m}^{-2}{\rm s}^{-1}$, $3.8\times10^{-19}$ ${\rm m}^{-2}{\rm s}^{-1}$, and $1.5\times10^{-19}$ ${\rm m}^{-2}{\rm s}^{-1}$. We find that about 80\% of blinkers are observed at the boundaries of supergranules, and 57\% (34\%) are associated with ECBPs (XCBPs). We further find that about 61{--}80\% of transients are associated with the isolated magnetic poles in the quiet Sun and that \textbf{the normalized maximum intensities of the transients are correlated with photospheric magnetic flux of poles} via a power law. These results conspicuously show that these transients have a magnetic origin and their synchronized behavior provides further clues towards the understanding of the coupling among the different layers of the solar atmosphere. Our study further reveals that the appearance of these transients is strongly anti-correlated with the sunspots cycle. This finding can be relevant for a better understanding of solar dynamo and magnetic structures at different scales during the solar cycle.

astro-ph.SR↗

arXiv2019

Highly relativistic electron flux enhancement during the weak geomagnetic storm of April-May 2017

We report observations of energetic electron flux and Phase Space Density (PSD) to show that a relatively weak magnetic storm with $Sym-H_{min} \approx -50 $nT, resulted in a relativistic and ultra--relativistic electron enhancement of two orders of magnitude similar to the St. Patrick's event of 2015, an extreme storm with $Sym-H_{min} \approx -235 nT$. This enhancement appeared at energies up to $\approx 10$ MeV, lasted for at least 24 days and was not recorded in geosynchronous orbit where most space weather alert data are collected. By combined analysis of PSD radial profiles and Fokker--Planck simulation, we show that the enhancement of relativistic and ultra--relativistic electrons is caused by different mechanisms: first, chorus waves during the intense substorm injections of April 21--25 accelerate the seed electron population to relativistic energies and redistribute them while inward diffusion driven by Pc5 ULF waves further accelerates them to ultra--relativistic energies.

physics.space-ph↗

arXiv2011

Recent Developments of NEMO: Detection of Solar Eruptions Characteristics

The recent developments in space instrumentation for solar observations and telemetry have caused the necessity of advanced pattern recognition tools for the different classes of solar events. The Extreme ultraviolet Imaging Telescope (EIT) of solar corona on-board SOHO spacecraft has uncovered a new class of eruptive events which are often identified as signatures of Coronal Mass Ejection (CME) initiations on solar disk. It is evident that a crucial task is the development of an automatic detection tool of CMEs precursors. The Novel EIT wave Machine Observing (NEMO) (http://sidc.be/nemo) code is an operational tool that detects automatically solar eruptions using EIT image sequences. NEMO applies techniques based on the general statistical properties of the underlying physical mechanisms of eruptive events on the solar disc. In this work, the most recent updates of NEMO code - that have resulted to the increase of the recognition efficiency of solar eruptions linked to CMEs - are presented. These updates provide calculations of the surface of the dimming region, implement novel clustering technique for the dimmings and set new criteria to flag the eruptive dimmings based on their complex characteristics. The efficiency of NEMO has been increased significantly resulting to the extraction of dimmings observed near the solar limb and to the detection of small-scale events as well. As a consequence, the detection efficiency of CMEs precursors and the forecasts of CMEs have been drastically improved. Furthermore, the catalogues of solar eruptive events that can be constructed by NEMO may include larger number of physical parameters associated to the dimming regions.

astro-ph.SR↗