arXiv subjects

V. Aparna

Publications and source records attributed to V. Aparna.

9 recordsLinked to original sources

arXiv2026

Hbeta Spicules, Small-scale Jets, and Hbeta Microflashes: Sub-arcsecond Dynamic Events and their Magnetic Origins in the Lower Solar Chromosphere Observed by DKIST

We examine an on-disk solar quiet region of enhanced magnetic network using Hbeta images from Daniel K. Inouye Solar Telescopes (DKISTs) Visible Broadband Imager (VBI) and line-of-sight magnetograms from DKISTs Visible Spectro-Polarimeter (ViSP). We also compare the ViSP magnetograms with co-aligned co-temporal line-of-sight magnetograms from the Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI). We find:(i) Two types of chromospheric jet-like features: Hbeta spicules, and small-scale jets. Both are rooted near edges of magnetic network lanes. Several sit close to ViSP-detected tiny islands of either minority-polarity flux or dips in majority-polarity flux. Hence, those several plausibly stem from mixed-polarity magnetic flux, and are caused by chromospheric magnetic reconnection. For H\b{eta} spicules, the average width, length, lifetimes, and speeds are 420$\pm$400 km, 2600$\pm$1600 km, 4.3$\pm$0.25 min, and 12$\pm$4.7 kms. For small-scale jets, those are 385$\pm$100 km, 620$\pm$40 km, 5$\pm$3min, and 4.4$\pm$1.5 kms. (ii) H$\beta$ microflashes sit in evidently unipolar flux. Their widths, lengths, lifetimes, and speeds are 180$\pm$65 km, 365$\pm$100 km, 4.3$\pm$2.6 min, and 3.1$\pm$0.2 kms. (iii) At the base of a coronal plume, due to its higher spatial resolution than HMI, ViSP shows both some very strong (>800 G) majority-polarity flux not shown by HMI and some sub-arc-second minority-polarity-flux inclusions not shown by HMI. These sub-arcsecond chromospheric transients carry sufficient energy (10^{24}erg) to transiently heat the local chromosphere and corona and potentially contribute to solar-wind acceleration. They may represent the small-scale end of a continuum of magnetic-reconnection-driven activity, highlighting the importance of DKIST's high-resolution magnetic-field measurements for understanding small-scale chromospheric dynamics.

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arXiv2026

Extreme Ultraviolet Microflashes at Plume Bases: A Candidate for Powering the Corona and Solar Wind?

Solar plumes - outflows of bright coronal plasma - are a major component of the open-magnetic-field corona and solar wind, but their driving mechanism remains uncertain. Here we report on network microflashes, fine-scale bright bursts captured by Solar Orbiters Extreme Ultraviolet Imager in 174A images encompassing magnetic network at the base of plumes. Because they sit in evidently unipolar magnetic flux, they are evidently a new, previously unidentified, kind of network event. Approximately 20 microflashes are ongoing within a plume base, with a new microflash starting every second. The energy for an average microflash is 1024 erg, in the range of nanoflares. A 3D data-driven global MHD model yields open magnetic field with fast solar wind for the investigated plumes. From our findings, we suggest that network microflashes result from fine-scale bursts of reconnection of crossed legs of unipolar magnetic field, that the bursts are often triggered by 5-minute p-mode oscillations, and that the bursts are candidates for powering the open-field corona and solar wind. That is, unipolar microflashes such as ours are plausibly from unipolar-network-field reconnection bursts that sustain the heliosphere.

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arXiv2026

The Making of Delta Sunspots

We explore what fraction of delta sunspots in which the polarity inversion line (PIL) is sharp in photospheric magnetograms are made from a writhe kink in an emerging twisted flux rope. We searched simultaneous full-disk magnetograms and continuum images from Helioseismic and Magnetic Imager (HMI) on Solar Dynamics Observatory (SDO) to find 28 random sharp-PIL delta sunspots that are born well on the disk. Only one of these is made from a single newly emerged bipolar magnetic region (BMR) and therefore is a candidate for being made from a single emerging writhe-kinked flux rope. That outcome indicates that few, if any, sharp-PIL delta sunspots are made by a single emerging writhe-kinked flux rope. That is the main new finding of this paper. Each of the other 27 is made by merging of two or more emerging or emerged BMRs. We name delta-sunspot genesis from a single BMR Type I genesis. We identify another three genesis types among the other 27 delta sunspots: Type II, Type III, and Type IV. We present an observed example genesis for each of the four genesis types, and for each example present schematic drawings depicting our scenario(s) for the cause of that example genesis. The core idea of each scenario is that the delta sunspot is made by packing opposite-polarity magnetic flux together by advection into a convection downflow.

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arXiv2025

Pivot of the Emerging Bipolar Magnetic Region in the Birth of Sigmoidal Solar Active Regions

We present an augmentation to longstanding evidence from observations and MHD modeling that (1) every solar emerging bipolar magnetic region (BMR) is made by an emerging omega-loop flux rope, and (2) twist in the flux-rope field makes the emerged field sigmoidal. Using co-temporal full-disk coronal EUV images, magnetograms, and continuum images from Solar Dynamics Observatory (SDO), we found and tracked the emergence of 42 emerging single-BMR sigmoidal active regions (ARs) that have sunspots in both polarity domains. Throughout each AR's emergence, we quantified the emerging BMR's tilt angle to the east-west direction (the x-direction in SDO images) by measuring in the continuum images the tilt angle of the line through the (visually located) two centroids of the BMR's opposite-polarity sunspot clusters. As each AR emerges, it becomes either S-shaped (shows net right-handed magnetic twist) or Z-shaped (shows net left-handed magnetic twist) in the coronal EUV images. Nineteen of the ARs become S-shaped and 23 become Z-shaped. For all 42 ARs, in agreement with published MHD simulations of the emergence of a single-BMR sigmoidal AR from a subsurface twisted flux rope, if the AR becomes S-shaped, the emerging BMR pivots counterclockwise, and if the AR becomes Z-shaped, the emerging BMR pivots clockwise. For our 42 ARs, the pivot amount roughly ranges from 10{\deg} to 90{\deg} and averages about 35{\deg}. Thus, at the onset of the emergence of our average emerging omega-loop flux rope, the magnetic field's twist pitch angle at the flux rope's top edge is plausibly about 35{\deg}.

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arXiv2025

Quantifying Suppression of Solar Surface Magnetic Flux Advection with Increasing Field Strength

One of the main theories for heating of the solar corona is based on the idea that solar convection shuffles and tangles magnetic field lines to make many small-scale current sheets that, via reconnection, heat coronal loops. Tiwari et al 2017 present evidence that, besides depending on loop length and other factors, the brightness of a coronal loop depends on the field strength in the loop feet and the freedom of convection in the feet. While it is known that strong solar magnetic fields suppress convection, the decrease in the speed of horizontal advection of magnetic flux with increasing field strength has not been quantified before. We quantify that trend by analyzing 24hours of HMI SHARP vector magnetograms of each of six sunspot active regions and their surroundings. Using Fourier Local Correlation Tracking, we estimate the horizontal advection speed of the magnetic flux at each pixel in which the vertical component of the magnetic field strength (Bz) is well above (greater than or equal to 150 G) noise level. We find that the average horizontal advection speed of magnetic flux steadily decreases as Bz increases, from 110 pm 3 meters per sec for 150 G (in network and plage) to 10 pm 4 meters per sec for 2500 G (in sunspot umbra). The trend is well fit by a fourth degree polynomial. These results quantitatively confirm the expectation that magnetic flux advection is suppressed by increasing magnetic field strength. The presented quantitative relation should be useful for future MHD simulations of coronal heating.

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arXiv2024

Stealth Non-standard-model Confined Flare Eruptions: Sudden Reconnection Events in Ostensibly Inert Magnetic Arches from Sunspots

We report seven examples of a long-ignored type of confined solar flare eruption that does not fit the standard model for confined flare eruptions. Because they are confined eruptions, do not fit the standard model, and unexpectedly erupt in ostensibly inert magnetic arches, we have named them stealth non-standard-model confined flare eruptions. Each of our flaring magnetic arches stems from a big sunspot. We tracked each eruption in full-cadence UV and EUV images from the Atmospheric Imaging Assembly (AIA) of Solar Dynamics Observatory (SDO) in combination with magnetograms from SDO's Helioseismic and Magnetic Imager (HMI). We present the onset and evolution of two eruptions in detail: one of six that each make two side-by-side main flare loops, and one that makes two crossed main flare loops. For these two cases, we present cartoons of the proposed pre-eruption field configuration and how sudden reconnection makes the flare ribbons and flare loops. Each of the seven eruptions is consistent with being made by sudden reconnection at an interface between two internal field strands of the magnetic arch, where they cross at a small (10 - 20 degrees) angle. These stealth non-standard-model confined flare eruptions therefore plausibly support the idea of E. N. Parker for coronal heating in solar coronal magnetic loops by nanoflare bursts of reconnection at interfaces of internal field strands that cross at angles of 10 - 20 degrees.

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arXiv2024

Magnetic Helicity Signs and Flaring Propensity: Comparing Force-free Parameter with the Helicity signs of H{\alpha} Filaments and X-ray Sigmoids

Sigmoids produce strong eruptive events. Earlier studies have shown that the ICME axial magnetic field Bz can be predicted with some credibility by observing the corresponding filament or the polarity inversion line in the region of eruption and deriving the magnetic field direction from that. Sigmoids are coronal structures often associated with filaments in the sigmoidal region. In this study, firstly we compare filament chirality with sigmoid handedness to observe their correlation. Secondly, we perform non-linear force-free approximations of the coronal magnetic connectivity using photospheric vector magnetograms underneath sigmoids to obtain a weighted-average value of the force-free parameter and to correlate it with filament chirality and the observed coronal sigmoid handedness. Importantly, we find that the sigmoids and their filament counterparts do not always have the same helicity signs. Production of eruptive events by regions that do not have the same signs of helicities is $\sim$3.5 times higher than when they do. A case study of magnetic energy/ helicity evolution in NOAA AR 12473 is also presented.

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arXiv2020

Analysis of the Coronal Mass Ejections through Axial Field Direction of Solar Filaments and IMF Bz

In the past, there have been many studies claiming that the effects of geomagnetic storms strongly depends on the orientation of the magnetic-cloud part of the Coronal Mass Ejections (CMEs). Aparna & Martens (2020), using Halo-CME data from 2007-2017, have shown that the magnetic field orientation of filaments at the location where CMEs originate can be effectively used for predicting the onset of geo-magnetic storms. The purpose of this study is to extend their survey by analyzing the halo-CME data for 1996-2006. The correlation of filament axial direction and their corresponding Bz signatures are used to form a more extensive reasoning for the claims presented by Aparna & Martens before. This study utilizes SOHO EIT 195 \r{A}, MDI magnetogram images, KSO and BBSO H$\alpha$ images for the time period, along with ACE data for inter-planetary magnetic field signatures. Correlating all these, we have found that the trend in Aparna & Martens' study of a high likelihood of the correlation between the axial field direction and Bz orientation, persists for the data between 1996-2006 as well.

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arXiv2016

A 7~MK hot Flux Rope Observed by SDO/AIA

A filament eruption was observed on October 31, 2010 in the images recorded by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) in its Extreme Ultra-Violet (EUV) channels. The filament showed a slow rise phase followed by a fast rise and was classified to be an asymmetric eruption. In addition, multiple localized brightening which was spatially and temporally associated with the slow rise phase were identified leading us to believe that the tether-cutting mechanism to be the cause of the initiation of the eruption. An associated flux rope was detected in high temperature channels of AIA namely 94Å and 131Å corresponding to 7 MK and 11 MK plasma respectively. In addition, these channels are also sensitive to cooler plasma corresponding to ~1-2 MK. In this study we have applied the algorithm devised by Warren et al. (2012) to remove cooler emission from the 94Å channel to deduce only the high temperature structure of the flux rope and to study its temporal evolution. We found that the flux rope was very clearly seen in clean 94Å channel image corresponding to Fe XVIII emission which corresponds to a plasma at a temperature of 7 MK. This temperature matched well with that obtained using DEM analysis. This study provides important constrains in the modelling of the thermodynamic structure of the flux ropes in CMEs.

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