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Papers by Vasyl Yurchyshyn

Research paper thumbnail of RELATIONSHIP BETWEEN EARTH-DIRECTED SOLAR ERUPTIONS AND MAGNETIC CLOUDS AT 1AU: A BRIEF REVIEW

Advances in Geosciences - A 6-Volume Set, 2010

We review relationships between coronal mass ejections (CMEs), EIT post eruption arcades, and the... more We review relationships between coronal mass ejections (CMEs), EIT post eruption arcades, and the coronal neutral line associated with global magnetic field and magnetic clouds near the Earth. Our previous findings indicate that the orientation of a halo CME elongation may correspond to the orientation of the underlying flux rope. Here we revisit these preliminary reports by comparing orientation angles of elongated LASCO CMEs, both full and partial halos, to the post eruption arcades. Based on 100 analysed events, it was found that the overwhelming majority of halo CMEs are elongated in the direction of the axial field of the post eruption arcades. Moreover, this conclusion also holds for partial halo CMEs as well as for events that originate further from the disk center. This suggests that the projection effect does not drastically change the appearance of full and partial halos and their images still bear reliable information about the underlying magnetic fields. We also compared orientations of the erupted fields near the Sun and in the interplanetary space and found that the local tilt of the coronal neutral line at 2.5 solar radii is well correlated with the magnetic cloud axis measured near the Earth. We suggest that the heliospheric magnetic fields significantly affect the propagating ejecta. Sometimes, the ejecta may even rotate so that its axis locally aligns itself with the heliospheric current sheet.

Research paper thumbnail of Evidence of a Flux‐Rope Model for Corona Mass Ejections Based on Observations of the Limb Prominence Eruption on 2002 January 4

The Astrophysical Journal, 2002

We report on a prominence eruption as seen in Kanzelhö he Solar Observatory H images, Solar and H... more We report on a prominence eruption as seen in Kanzelhö he Solar Observatory H images, Solar and Heliospheric Observatory (SOHO) EUV Imaging Telescope (EIT) 195 Å images, and coronal SOHO Large Angle Spectrometric Coronograph C2 images. Our data favor the flux-rope model for coronal mass ejections (CMEs), which suggests that a flux rope is formed long before the eruption. Our conclusion is based on a three-part structure of the pre-erupted configuration of the magnetic field and on the fact that the first H, SOHO EIT 195 Å brightenings occurred some 15 minutes after the filament began to ascend. The data also clearly demonstrate two rarely observed components of the standard flare model: (1) magnetic loops that overlay the pre-erupted filament and (2) magnetic field lines stretched vertically by the ascending filament. These field lines are compressed horizontally and move toward each other where they reconnect to form an apparently growing post-flare loop system.

Research paper thumbnail of Relationship between EIT Posteruption Arcades, Coronal Mass Ejections, the Coronal Neutral Line, and Magnetic Clouds

The Astrophysical Journal, 2008

There is observational evidence that the elongation of an Earth directed coronal mass ejection (C... more There is observational evidence that the elongation of an Earth directed coronal mass ejection (CME) may indicate the orientation of the underlying erupting flux rope. In this study, we compare orientations of CMEs, MCs, eruption arcades and coronal neutral line (CNL). We report on good correlations between i) the directions of the axial field in EIT arcades and the elongations of halo CMEs and ii) the tilt of the CNL and MC axis orientations. We found that majority of the eruptions that had EIT arcades, CMEs and MCs similarly oriented also had the CNL co-aligned with them. To the contrary, those events that showed no agreement between orientations of the EIT arcades, CMEs and MCs, had their MCs aligned with the coronal neutral line. We speculate that the axis of the ejecta may be rotated in such a way that it is locally aligns itself with the heliospheric current sheet.

Research paper thumbnail of Structure of magnetic fields in NOAA active regions 0486 and 0501 and in the associated interplanetary ejecta

Space Weather, 2005

Spectacular burst of solar activity in October -November 2003, when large solar spots and intense... more Spectacular burst of solar activity in October -November 2003, when large solar spots and intense solar flares dominated the solar surface for many consecutive days, caused intense geomagnetic storms. In this paper we analyze solar and interplanetary magnetic fields associated with the storms in October -November 2003. We used space and ground based data in order to compare the orientations of the magnetic fields on the solar surface and at 1AU as well as to estimate parameters of geomagnetic storms during this violent period of geomagnetic activity. Our study further supports earlier reports on the correlation between the CME speed and the strength of the magnetic field in an interplanetary ejecta. A good correspondence was also found between directions of the helical magnetic fields in interplanetary ejecta and in the source active regions. These findings are quite significant in terms of their potential to predict the severity of geomagnetic activity 1 -2 days in advance, immediately after an earth-directed solar eruption.

Research paper thumbnail of FORMATION AND ERUPTION OF A SMALL FLUX ROPE IN THE CHROMOSPHERE OBSERVED BY NST, IRIS , AND SDO

The Astrophysical Journal, 2015

Using high-resolution images from 1.6 m New Solar Telescope (NST) at Big Bear Solar Observatory (... more Using high-resolution images from 1.6 m New Solar Telescope (NST) at Big Bear Solar Observatory (BBSO), we report the direct evidence of chromospheric reconnection at the polarity inversion line (PIL) between two small opposite polarity sunspots. Small jet-like structures (with velocities of ∼20-55 km s −1 ) were observed at the reconnection site before the onset of the first M1.0 flare. The slow rise of untwisting jets was followed by the onset of cool plasma inflow (∼10 km s −1 ) at the reconnection site, causing the onset of a two-ribbon flare. The reconnection between two sheared J-shaped cool Hα loops causes the formation of a small twisted flux rope (S shaped) in the chromosphere. In addition, Helioseismic and Magnetic Imager (HMI) magnetograms show the flux cancellation (both positive and negative) during the first M1.0 flare. The emergence of negative flux and cancellation of positive flux (with shear flows) continue until the successful eruption of the flux rope. The newly formed chromospheric flux rope becomes unstable and rises slowly with the speed of ∼108 km s −1 during a second C8.5 flare that occurred after ∼3 hours of the first M1.0 flare. The flux rope was destroyed by repeated magnetic reconnection induced by its interaction with the ambient field (fan-spine toplology) and looks like an untwisting surge (∼170 km s −1 ) in the coronal images recorded by Solar Dynamic Observatory/Atmospheric Imaging Assembly (SDO/AIA). These observations suggest the formation of a chromospheric flux rope (by magnetic reconnection associated with flux cancellation) during the first M1.0 flare and its subsequent eruption/disruption during the second C8.5 flare.

Research paper thumbnail of Correlation between speeds of coronal mass ejections and the intensity of geomagnetic storms

Space Weather, 2004

We studied the relationship between the projected speed of coronal mass ejections (CMEs), determi... more We studied the relationship between the projected speed of coronal mass ejections (CMEs), determined from a sequence of Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph Experiment (SOHO/LASCO) images, and the hourly averaged magnitude of the Bz component of the magnetic field in an interplanetary ejecta, as measured by the Advanced Composition Explorer (ACE) magnetometer in the Geocentric Solar Magnetospheric Coordinate System (GSM). For CMEs that originate at the central part of the solar disk we found that the intensity of Bz is correlated with the projected speed of the CME, Vp. The relationship is more pronounced for very fast ejecta (Vp > 1200 km/s), while slower events display larger scatter. We also present data which support earlier conclusions about the correlation of Bz and the Dst index of geomagnetic activity. A possible application of the results to space weather forecasting is discussed.

Research paper thumbnail of Photospheric Plasma Flows Around a Solar Spot

We study photospheric plasma flows in an active region NOAA 8375, by using uninterrupted high-res... more We study photospheric plasma flows in an active region NOAA 8375, by using uninterrupted high-resolution SOHO/MDI observations (137 intensity images, 44 hours of observations). The active region consists of a stable large spot and many small spots and pores. Analyzing horizontal flow maps, obtained with local correlation tracking technique, we found a system of stable persistent plasma flows existing in the active region. The flows start on either side of the sunspot and extend over 100 to the east. Our measurements show that the speed of small sunspots and pores, averaged over 44 hours, was about 100 m s −1 , which corresponds to root-mean-square longitudinal drifts of sunspots of 0.67 • -0.76 • day −1 . We conclude that these large-scale flows are due to faster proper motion of the large sunspot relative to the ambient photospheric plasma. We suggest that the flows may be a good carrier to transport magnetic flux from eroding sunspots into the outer part of an active region.

Research paper thumbnail of Magnetic Topology in 1998 November 5 Two-Ribbon Flare as Inferred from Ground-based Observations and Linear Force-free Field Modeling

Research paper thumbnail of Intermittency and multifractality spectra of the magnetic field in solar active regions

We present results of a study of intermittency and multifractality of magnetic structures in sola... more We present results of a study of intermittency and multifractality of magnetic structures in solar active regions (ARs). Line-of-sight magnetograms for 214 ARs of different flare productivity observed at the center of the solar disk from January 1997 until December 2006 are utilized. Data from the Michelson Doppler Imager (MDI) instrument on-board the Solar and Heliospheric Observatory (SOHO) operating in the high resolution mode, the Big Bear Solar Observatory digital magnetograph and Hinode SOT/SP instrument were used. Intermittency spectra were derived via high-order structure functions and flatness functions. The flatness function exponent is a measure of the degree of intermittency. We found that the flatness function exponent at scales below approximately 10 Mm is correlated to the flare productivity (the correlation coefficient is -0.63). Hinode data show that the intermittency regime is extended toward the small scales (below 2 Mm) as compared to the MDI data. The spectra of multifractality, derived from the structure functions and flatness functions, are found to be more broad for ARs of highest flare productivity as compared to that of low flare productivity. The magnetic structure of high-flaring ARs consists of a voluminous set of monofractals, and this set is much richer than that for low-flaring ARs. The results indicate relevance of the multifractal organization of the photospheric magnetic fields to the flaring activity. Strong intermittency observed in complex and high-flaring ARs is a hint that we observe a photospheric imprint of enhanced sub-photospheric dynamics.

Research paper thumbnail of Magnetic Flux Cancellation Observed in the Sunspot Moat

In this paper we study the evolution of magnetic fields of a 1F/2.4C solar flare and following ma... more In this paper we study the evolution of magnetic fields of a 1F/2.4C solar flare and following magnetic flux cancellation. The data are Big Bear Solar Observatory and SOHO/MDI observations of active region NOAA 8375. The active region produced a multitude of subflares, many of them being clustered along the moat boundary in the area with mixed polarity magnetic fields. The study indicates a possible connection between the flare and the flux cancellation. The cancellation rate, defined from the data, was found to be 3 × 10 19 Mx h −1 . We observed strong upward directed plasma flows at the cancellation site. Suggesting that the cancellation is a result of reconnection process, we also found a reconnection rate of 0.5 km s −1 , which is a significant fraction of Alfvén speed. The reconnection rate indicates a regime of fast photospheric reconnection happening during the cancellation. Solar Physics 202: 309-318, 2001.

Research paper thumbnail of On the Temporal Relationship between H? Filament Eruptions and Soft X-Ray Emissions

On the Temporal Relationship between H? Filament Eruptions and Soft X-Ray Emissions

Research paper thumbnail of Transverse Motions of Chromospheric Type II Spicules Observed by the New Solar Telescope

Using high resolution off-band \ha\ data from the New Solar Telescope and Morlet wavelet analysis... more Using high resolution off-band \ha\ data from the New Solar Telescope and Morlet wavelet analysis technique, we analyzed transverse motions of type II spicules observed near the North Pole of the Sun. Our new findings are that i) some of the observed type II spicules display kink or an inverse "Y" features, suggesting that their origin may be due to magnetic reconnection, and ii) type II spicules tend to display coherent transverse motions/oscillations. Also, the wavelet analysis detected significant presence of high frequency oscillations in type II spicules, ranging from 30 to 180 s with the the average period of 90 s. We conclude that at least some of type II spicules and their coherent transverse motions may be caused by reconnection between large scale fields rooted in the intergranular lanes and and small-scale emerging dipoles, a process that is know to generate high frequency kink mode MHD waves propagating along the magnetic field lines.

Research paper thumbnail of Statistical Assessment of Photospheric Magnetic Features in Imminent Solar Flare Predictions

Solar Physics, 2008

In this study we used the ordinal logistic regression method to establish a prediction model, whi... more In this study we used the ordinal logistic regression method to establish a prediction model, which estimates the probability for each solar active region to produce X-, M-or C-class flares during the next 1-day time period. Three predictive parameters are: (1) total unsigned magnetic flux T flux , which is a measure of an active region's size, (2) the length of stronggradient neutral line L gnl , which describes the global non-potentiality of an active region, and (3) total magnetic dissipation E diss , which is another proxy measure of an active region's non-potentiality. They are all derived from SOHO MDI magnetograms.

Research paper thumbnail of STATISTICAL DISTRIBUTION OF SIZE AND LIFETIME OF BRIGHT POINTS OBSERVED WITH THE NEW SOLAR TELESCOPE

The Astrophysical Journal, 2010

We present results of two-hour non-interrupted observations of solar granulation obtained under e... more We present results of two-hour non-interrupted observations of solar granulation obtained under excellent seeing conditions with the largest aperture ground-based solar telescope -the New Solar Telescope (NST) -of Big Bear Solar Observatory. Observations were performed with adaptive optics correction using a broad-band TiO filter in the 705.7 nm spectral line with a time cadence of 10 s and a pixel size of 0.0375". Photospheric bright points (BPs) were detected and tracked. We find that the BPs detected in NST images are co-spatial with those visible in Hinode/SOT G-band images. In cases where Hinode/SOT detects one large BP, NST detects several separated BPs. Extended filigree features are clearly fragmented into separate BPs in NST images. The distribution function of BP sizes extends to the diffraction limit of NST (77 km) without saturation and corresponds to a log-normal distribution. The lifetime distribution function follows a log-normal approximation for all BPs with lifetime exceeding 100 s. A majority of BPs are transient events reflecting the strong dynamics of the quiet sun: 98.6% of BPs live less than 120 s. The longest registered life time was 44 minutes. The size and maximum intensity of BPs were found to be proportional to their life times.

Research paper thumbnail of Successive flaring during the 2005 September 13 eruption

Successive flaring during the 2005 September 13 eruption

Research paper thumbnail of ROTATION OF WHITE-LIGHT CORONAL MASS EJECTION STRUCTURES AS INFERRED FROM LASCO CORONAGRAPH

The Astrophysical Journal, 2009

Understanding the connection between the magnetic configurations of a coronal mass ejection (CME)... more Understanding the connection between the magnetic configurations of a coronal mass ejection (CME) and their counterpart in the interplanetary medium is very important in terms of space weather predictions. Our previous findings indicate that the orientation of a halo CME elongation may correspond to the orientation of the underlying flux rope. Here we further explore these preliminary results by comparing orientation angles of elongated LASCO CMEs, both full and partial halos, to the EUV Imaging Telescope post-eruption arcades (PEAs). By analyzing a sample of 100 events, we found that the overwhelming majority of CMEs are elongated in the direction of the axial field of PEAs. During their evolution, CMEs appear to rotate by about 10 • for most of the events (70%) with about 30 • -50 • for some events, and the corresponding time profiles display regular and gradual changes. It seems that there is a slight preference for the CMEs to rotate toward the solar equator and heliospheric current sheet (59% of the cases). We suggest that the rotation of the ejecta may be due to the presence of a heliospheric magnetic field, and it could shed light on the problems related to connecting solar surface phenomena to their interplanetary counterparts.

Research paper thumbnail of Limb Prominence Eruption on 11 August 2000 as Seen From Ground and Space-Based Observations

Solar Physics - SOL PHYS, 2002

We report on a prominence eruption as seen in Halpha with the Crimean Lyot coronagraph, the globa... more We report on a prominence eruption as seen in Halpha with the Crimean Lyot coronagraph, the global Halpha network, and coronal images from the LASCO C2 instrument on board SOHO. We observed an Halpha eruption at the northwest solar limb between 07:38:50 UT and 07:58:29 UT on 11 August 2000. The eruption originated in a quiet-Sun region and was not associated with an Halpha filament. No flare was associated with the eruption, which may indicate that, in this case, a flux rope was formed prior to the eruption of the magnetic field. The Halpha images and an Halpha Dopplergram show a helical structure present in the erupted magnetic field. We suggest that the driving mechanism of the eruption may be magnetic flux emergence or magnetic flux injection. The limb Halpha observations provide missing data on CME speed and acceleration in the lower corona. Our data show that the prominence accelerated impulsively at 5.5 km s-2 and reached a speed slightly greater than 800 km s-1 in a narrow re...

Research paper thumbnail of PHOTOSPHERIC SIGNATURES OF GRANULAR-SCALE FLUX EMERGENCE AND CANCELLATION AT THE PENUMBRAL BOUNDARY

The Astrophysical Journal, 2011

We studied flux emergence events of sub-granular scale in a solar active region. New Solar Telesc... more We studied flux emergence events of sub-granular scale in a solar active region. New Solar Telescope (NST) of Big Bear Solar Observatory made it possible to clearly observe the photospheric signature of flux emergence with very high spatial (0 ′′ .11 at 7057Å) and temporal (15 s) resolution. From TiO observations with the pixel scale of 0 ′′ .0375, we found several elongated granule-like features (GLFs) stretching from the penumbral filaments of a sunspot at a relatively high speed of over 4 km s −1 . After a slender arched darkening appeared at a tip of a penumbral filament, a bright point (BP) developed and quickly moved away from the filament forming and stretching a GLF. The size of a GLF was approximately 0 ′′ .5 wide and 3 ′′ long. The moving BP encountered nearby structures after several minutes of stretching, and a well-defined elongated shape of a GLF faded away. Magnetograms from SDO/HMI and NST/IRIM revealed that those GLFs are photospheric indicators of small-scale flux emergence, and their disappearance is related to magnetic cancellation. From two well-observed events, we describe detailed development of the sub-structures of GLFs, and different cancellation processes that each of the two GLFs underwent.

Research paper thumbnail of PROPERTIES OF UMBRAL DOTS AS MEASURED FROM THE NEW SOLAR TELESCOPE DATA AND MHD SIMULATIONS

The Astrophysical Journal, 2012

We studied bright umbral dots (UDs) detected in a moderate size sunspot and compared their statis... more We studied bright umbral dots (UDs) detected in a moderate size sunspot and compared their statistical properties to recent MHD models. The study is based on high-resolution data recorded by the New Solar Telescope at the Big Bear Solar Observatory and three-dimensional (3D) MHD simulations of sunspots. Observed UDs, living longer than 150 s, were detected and tracked in a 46 minute long data set, using an automatic detection code. A total of 1553 (620) UDs were detected in the photospheric (low chromospheric) data. Our main findings are (1) none of the analyzed UDs is precisely circular, (2) the diameter-intensity relationship only holds in bright umbral areas, and (3) UD velocities are inversely related to their lifetime. While nearly all photospheric UDs can be identified in the low chromospheric images, some small closely spaced UDs appear in the low chromosphere as a single cluster. Slow-moving and long-living UDs seem to exist in both the low chromosphere and photosphere, while fast-moving and short-living UDs are mainly detected in the photospheric images. Comparison to the 3D MHD simulations showed that both types of UDs display, on average, very similar statistical characteristics. However, (1) the average number of observed UDs per unit area is smaller than that of the model UDs, and (2) on average, the diameter of model UDs is slightly larger than that of observed ones.

Research paper thumbnail of MAGNETIC ENERGY SPECTRA IN SOLAR ACTIVE REGIONS

The Astrophysical Journal, 2010

Line-of-sight magnetograms for 217 active regions (ARs) of different flare rate observed at the s... more Line-of-sight magnetograms for 217 active regions (ARs) of different flare rate observed at the solar disk center from January 1997 until December 2006 are utilized to study the turbulence regime and its relationship to the flare productivity. Data from SOHO/MDI instrument recorded in the high resolution mode and data from the BBSO magnetograph were used. The turbulence regime was probed via magnetic energy spectra and magnetic dissipation spectra. We found steeper energy spectra for ARs of higher flare productivity. We also report that both the power index, α, of the energy spectrum, E(k) ∼ k −α , and the total spectral energy W = E(k)dk are comparably correlated with the flare index, A, of an active region. The correlations are found to be stronger than that found between the flare index and total unsigned flux. The flare index for an AR can be estimated based on measurements of α and W as A = 10 b (αW ) c , with b = −7.92 ± 0.58 and c = 1.85 ± 0.13. We found that the regime of the fully-developed turbulence occurs in decaying ARs and in emerging ARs (at the very early stage of emergence). Well-developed ARs display under-developed turbulence with strong magnetic dissipation at all scales.

Research paper thumbnail of RELATIONSHIP BETWEEN EARTH-DIRECTED SOLAR ERUPTIONS AND MAGNETIC CLOUDS AT 1AU: A BRIEF REVIEW

Advances in Geosciences - A 6-Volume Set, 2010

We review relationships between coronal mass ejections (CMEs), EIT post eruption arcades, and the... more We review relationships between coronal mass ejections (CMEs), EIT post eruption arcades, and the coronal neutral line associated with global magnetic field and magnetic clouds near the Earth. Our previous findings indicate that the orientation of a halo CME elongation may correspond to the orientation of the underlying flux rope. Here we revisit these preliminary reports by comparing orientation angles of elongated LASCO CMEs, both full and partial halos, to the post eruption arcades. Based on 100 analysed events, it was found that the overwhelming majority of halo CMEs are elongated in the direction of the axial field of the post eruption arcades. Moreover, this conclusion also holds for partial halo CMEs as well as for events that originate further from the disk center. This suggests that the projection effect does not drastically change the appearance of full and partial halos and their images still bear reliable information about the underlying magnetic fields. We also compared orientations of the erupted fields near the Sun and in the interplanetary space and found that the local tilt of the coronal neutral line at 2.5 solar radii is well correlated with the magnetic cloud axis measured near the Earth. We suggest that the heliospheric magnetic fields significantly affect the propagating ejecta. Sometimes, the ejecta may even rotate so that its axis locally aligns itself with the heliospheric current sheet.

Research paper thumbnail of Evidence of a Flux‐Rope Model for Corona Mass Ejections Based on Observations of the Limb Prominence Eruption on 2002 January 4

The Astrophysical Journal, 2002

We report on a prominence eruption as seen in Kanzelhö he Solar Observatory H images, Solar and H... more We report on a prominence eruption as seen in Kanzelhö he Solar Observatory H images, Solar and Heliospheric Observatory (SOHO) EUV Imaging Telescope (EIT) 195 Å images, and coronal SOHO Large Angle Spectrometric Coronograph C2 images. Our data favor the flux-rope model for coronal mass ejections (CMEs), which suggests that a flux rope is formed long before the eruption. Our conclusion is based on a three-part structure of the pre-erupted configuration of the magnetic field and on the fact that the first H, SOHO EIT 195 Å brightenings occurred some 15 minutes after the filament began to ascend. The data also clearly demonstrate two rarely observed components of the standard flare model: (1) magnetic loops that overlay the pre-erupted filament and (2) magnetic field lines stretched vertically by the ascending filament. These field lines are compressed horizontally and move toward each other where they reconnect to form an apparently growing post-flare loop system.

Research paper thumbnail of Relationship between EIT Posteruption Arcades, Coronal Mass Ejections, the Coronal Neutral Line, and Magnetic Clouds

The Astrophysical Journal, 2008

There is observational evidence that the elongation of an Earth directed coronal mass ejection (C... more There is observational evidence that the elongation of an Earth directed coronal mass ejection (CME) may indicate the orientation of the underlying erupting flux rope. In this study, we compare orientations of CMEs, MCs, eruption arcades and coronal neutral line (CNL). We report on good correlations between i) the directions of the axial field in EIT arcades and the elongations of halo CMEs and ii) the tilt of the CNL and MC axis orientations. We found that majority of the eruptions that had EIT arcades, CMEs and MCs similarly oriented also had the CNL co-aligned with them. To the contrary, those events that showed no agreement between orientations of the EIT arcades, CMEs and MCs, had their MCs aligned with the coronal neutral line. We speculate that the axis of the ejecta may be rotated in such a way that it is locally aligns itself with the heliospheric current sheet.

Research paper thumbnail of Structure of magnetic fields in NOAA active regions 0486 and 0501 and in the associated interplanetary ejecta

Space Weather, 2005

Spectacular burst of solar activity in October -November 2003, when large solar spots and intense... more Spectacular burst of solar activity in October -November 2003, when large solar spots and intense solar flares dominated the solar surface for many consecutive days, caused intense geomagnetic storms. In this paper we analyze solar and interplanetary magnetic fields associated with the storms in October -November 2003. We used space and ground based data in order to compare the orientations of the magnetic fields on the solar surface and at 1AU as well as to estimate parameters of geomagnetic storms during this violent period of geomagnetic activity. Our study further supports earlier reports on the correlation between the CME speed and the strength of the magnetic field in an interplanetary ejecta. A good correspondence was also found between directions of the helical magnetic fields in interplanetary ejecta and in the source active regions. These findings are quite significant in terms of their potential to predict the severity of geomagnetic activity 1 -2 days in advance, immediately after an earth-directed solar eruption.

Research paper thumbnail of FORMATION AND ERUPTION OF A SMALL FLUX ROPE IN THE CHROMOSPHERE OBSERVED BY NST, IRIS , AND SDO

The Astrophysical Journal, 2015

Using high-resolution images from 1.6 m New Solar Telescope (NST) at Big Bear Solar Observatory (... more Using high-resolution images from 1.6 m New Solar Telescope (NST) at Big Bear Solar Observatory (BBSO), we report the direct evidence of chromospheric reconnection at the polarity inversion line (PIL) between two small opposite polarity sunspots. Small jet-like structures (with velocities of ∼20-55 km s −1 ) were observed at the reconnection site before the onset of the first M1.0 flare. The slow rise of untwisting jets was followed by the onset of cool plasma inflow (∼10 km s −1 ) at the reconnection site, causing the onset of a two-ribbon flare. The reconnection between two sheared J-shaped cool Hα loops causes the formation of a small twisted flux rope (S shaped) in the chromosphere. In addition, Helioseismic and Magnetic Imager (HMI) magnetograms show the flux cancellation (both positive and negative) during the first M1.0 flare. The emergence of negative flux and cancellation of positive flux (with shear flows) continue until the successful eruption of the flux rope. The newly formed chromospheric flux rope becomes unstable and rises slowly with the speed of ∼108 km s −1 during a second C8.5 flare that occurred after ∼3 hours of the first M1.0 flare. The flux rope was destroyed by repeated magnetic reconnection induced by its interaction with the ambient field (fan-spine toplology) and looks like an untwisting surge (∼170 km s −1 ) in the coronal images recorded by Solar Dynamic Observatory/Atmospheric Imaging Assembly (SDO/AIA). These observations suggest the formation of a chromospheric flux rope (by magnetic reconnection associated with flux cancellation) during the first M1.0 flare and its subsequent eruption/disruption during the second C8.5 flare.

Research paper thumbnail of Correlation between speeds of coronal mass ejections and the intensity of geomagnetic storms

Space Weather, 2004

We studied the relationship between the projected speed of coronal mass ejections (CMEs), determi... more We studied the relationship between the projected speed of coronal mass ejections (CMEs), determined from a sequence of Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph Experiment (SOHO/LASCO) images, and the hourly averaged magnitude of the Bz component of the magnetic field in an interplanetary ejecta, as measured by the Advanced Composition Explorer (ACE) magnetometer in the Geocentric Solar Magnetospheric Coordinate System (GSM). For CMEs that originate at the central part of the solar disk we found that the intensity of Bz is correlated with the projected speed of the CME, Vp. The relationship is more pronounced for very fast ejecta (Vp > 1200 km/s), while slower events display larger scatter. We also present data which support earlier conclusions about the correlation of Bz and the Dst index of geomagnetic activity. A possible application of the results to space weather forecasting is discussed.

Research paper thumbnail of Photospheric Plasma Flows Around a Solar Spot

We study photospheric plasma flows in an active region NOAA 8375, by using uninterrupted high-res... more We study photospheric plasma flows in an active region NOAA 8375, by using uninterrupted high-resolution SOHO/MDI observations (137 intensity images, 44 hours of observations). The active region consists of a stable large spot and many small spots and pores. Analyzing horizontal flow maps, obtained with local correlation tracking technique, we found a system of stable persistent plasma flows existing in the active region. The flows start on either side of the sunspot and extend over 100 to the east. Our measurements show that the speed of small sunspots and pores, averaged over 44 hours, was about 100 m s −1 , which corresponds to root-mean-square longitudinal drifts of sunspots of 0.67 • -0.76 • day −1 . We conclude that these large-scale flows are due to faster proper motion of the large sunspot relative to the ambient photospheric plasma. We suggest that the flows may be a good carrier to transport magnetic flux from eroding sunspots into the outer part of an active region.

Research paper thumbnail of Magnetic Topology in 1998 November 5 Two-Ribbon Flare as Inferred from Ground-based Observations and Linear Force-free Field Modeling

Research paper thumbnail of Intermittency and multifractality spectra of the magnetic field in solar active regions

We present results of a study of intermittency and multifractality of magnetic structures in sola... more We present results of a study of intermittency and multifractality of magnetic structures in solar active regions (ARs). Line-of-sight magnetograms for 214 ARs of different flare productivity observed at the center of the solar disk from January 1997 until December 2006 are utilized. Data from the Michelson Doppler Imager (MDI) instrument on-board the Solar and Heliospheric Observatory (SOHO) operating in the high resolution mode, the Big Bear Solar Observatory digital magnetograph and Hinode SOT/SP instrument were used. Intermittency spectra were derived via high-order structure functions and flatness functions. The flatness function exponent is a measure of the degree of intermittency. We found that the flatness function exponent at scales below approximately 10 Mm is correlated to the flare productivity (the correlation coefficient is -0.63). Hinode data show that the intermittency regime is extended toward the small scales (below 2 Mm) as compared to the MDI data. The spectra of multifractality, derived from the structure functions and flatness functions, are found to be more broad for ARs of highest flare productivity as compared to that of low flare productivity. The magnetic structure of high-flaring ARs consists of a voluminous set of monofractals, and this set is much richer than that for low-flaring ARs. The results indicate relevance of the multifractal organization of the photospheric magnetic fields to the flaring activity. Strong intermittency observed in complex and high-flaring ARs is a hint that we observe a photospheric imprint of enhanced sub-photospheric dynamics.

Research paper thumbnail of Magnetic Flux Cancellation Observed in the Sunspot Moat

In this paper we study the evolution of magnetic fields of a 1F/2.4C solar flare and following ma... more In this paper we study the evolution of magnetic fields of a 1F/2.4C solar flare and following magnetic flux cancellation. The data are Big Bear Solar Observatory and SOHO/MDI observations of active region NOAA 8375. The active region produced a multitude of subflares, many of them being clustered along the moat boundary in the area with mixed polarity magnetic fields. The study indicates a possible connection between the flare and the flux cancellation. The cancellation rate, defined from the data, was found to be 3 × 10 19 Mx h −1 . We observed strong upward directed plasma flows at the cancellation site. Suggesting that the cancellation is a result of reconnection process, we also found a reconnection rate of 0.5 km s −1 , which is a significant fraction of Alfvén speed. The reconnection rate indicates a regime of fast photospheric reconnection happening during the cancellation. Solar Physics 202: 309-318, 2001.

Research paper thumbnail of On the Temporal Relationship between H? Filament Eruptions and Soft X-Ray Emissions

On the Temporal Relationship between H? Filament Eruptions and Soft X-Ray Emissions

Research paper thumbnail of Transverse Motions of Chromospheric Type II Spicules Observed by the New Solar Telescope

Using high resolution off-band \ha\ data from the New Solar Telescope and Morlet wavelet analysis... more Using high resolution off-band \ha\ data from the New Solar Telescope and Morlet wavelet analysis technique, we analyzed transverse motions of type II spicules observed near the North Pole of the Sun. Our new findings are that i) some of the observed type II spicules display kink or an inverse "Y" features, suggesting that their origin may be due to magnetic reconnection, and ii) type II spicules tend to display coherent transverse motions/oscillations. Also, the wavelet analysis detected significant presence of high frequency oscillations in type II spicules, ranging from 30 to 180 s with the the average period of 90 s. We conclude that at least some of type II spicules and their coherent transverse motions may be caused by reconnection between large scale fields rooted in the intergranular lanes and and small-scale emerging dipoles, a process that is know to generate high frequency kink mode MHD waves propagating along the magnetic field lines.

Research paper thumbnail of Statistical Assessment of Photospheric Magnetic Features in Imminent Solar Flare Predictions

Solar Physics, 2008

In this study we used the ordinal logistic regression method to establish a prediction model, whi... more In this study we used the ordinal logistic regression method to establish a prediction model, which estimates the probability for each solar active region to produce X-, M-or C-class flares during the next 1-day time period. Three predictive parameters are: (1) total unsigned magnetic flux T flux , which is a measure of an active region's size, (2) the length of stronggradient neutral line L gnl , which describes the global non-potentiality of an active region, and (3) total magnetic dissipation E diss , which is another proxy measure of an active region's non-potentiality. They are all derived from SOHO MDI magnetograms.

Research paper thumbnail of STATISTICAL DISTRIBUTION OF SIZE AND LIFETIME OF BRIGHT POINTS OBSERVED WITH THE NEW SOLAR TELESCOPE

The Astrophysical Journal, 2010

We present results of two-hour non-interrupted observations of solar granulation obtained under e... more We present results of two-hour non-interrupted observations of solar granulation obtained under excellent seeing conditions with the largest aperture ground-based solar telescope -the New Solar Telescope (NST) -of Big Bear Solar Observatory. Observations were performed with adaptive optics correction using a broad-band TiO filter in the 705.7 nm spectral line with a time cadence of 10 s and a pixel size of 0.0375". Photospheric bright points (BPs) were detected and tracked. We find that the BPs detected in NST images are co-spatial with those visible in Hinode/SOT G-band images. In cases where Hinode/SOT detects one large BP, NST detects several separated BPs. Extended filigree features are clearly fragmented into separate BPs in NST images. The distribution function of BP sizes extends to the diffraction limit of NST (77 km) without saturation and corresponds to a log-normal distribution. The lifetime distribution function follows a log-normal approximation for all BPs with lifetime exceeding 100 s. A majority of BPs are transient events reflecting the strong dynamics of the quiet sun: 98.6% of BPs live less than 120 s. The longest registered life time was 44 minutes. The size and maximum intensity of BPs were found to be proportional to their life times.

Research paper thumbnail of Successive flaring during the 2005 September 13 eruption

Successive flaring during the 2005 September 13 eruption

Research paper thumbnail of ROTATION OF WHITE-LIGHT CORONAL MASS EJECTION STRUCTURES AS INFERRED FROM LASCO CORONAGRAPH

The Astrophysical Journal, 2009

Understanding the connection between the magnetic configurations of a coronal mass ejection (CME)... more Understanding the connection between the magnetic configurations of a coronal mass ejection (CME) and their counterpart in the interplanetary medium is very important in terms of space weather predictions. Our previous findings indicate that the orientation of a halo CME elongation may correspond to the orientation of the underlying flux rope. Here we further explore these preliminary results by comparing orientation angles of elongated LASCO CMEs, both full and partial halos, to the EUV Imaging Telescope post-eruption arcades (PEAs). By analyzing a sample of 100 events, we found that the overwhelming majority of CMEs are elongated in the direction of the axial field of PEAs. During their evolution, CMEs appear to rotate by about 10 • for most of the events (70%) with about 30 • -50 • for some events, and the corresponding time profiles display regular and gradual changes. It seems that there is a slight preference for the CMEs to rotate toward the solar equator and heliospheric current sheet (59% of the cases). We suggest that the rotation of the ejecta may be due to the presence of a heliospheric magnetic field, and it could shed light on the problems related to connecting solar surface phenomena to their interplanetary counterparts.

Research paper thumbnail of Limb Prominence Eruption on 11 August 2000 as Seen From Ground and Space-Based Observations

Solar Physics - SOL PHYS, 2002

We report on a prominence eruption as seen in Halpha with the Crimean Lyot coronagraph, the globa... more We report on a prominence eruption as seen in Halpha with the Crimean Lyot coronagraph, the global Halpha network, and coronal images from the LASCO C2 instrument on board SOHO. We observed an Halpha eruption at the northwest solar limb between 07:38:50 UT and 07:58:29 UT on 11 August 2000. The eruption originated in a quiet-Sun region and was not associated with an Halpha filament. No flare was associated with the eruption, which may indicate that, in this case, a flux rope was formed prior to the eruption of the magnetic field. The Halpha images and an Halpha Dopplergram show a helical structure present in the erupted magnetic field. We suggest that the driving mechanism of the eruption may be magnetic flux emergence or magnetic flux injection. The limb Halpha observations provide missing data on CME speed and acceleration in the lower corona. Our data show that the prominence accelerated impulsively at 5.5 km s-2 and reached a speed slightly greater than 800 km s-1 in a narrow re...

Research paper thumbnail of PHOTOSPHERIC SIGNATURES OF GRANULAR-SCALE FLUX EMERGENCE AND CANCELLATION AT THE PENUMBRAL BOUNDARY

The Astrophysical Journal, 2011

We studied flux emergence events of sub-granular scale in a solar active region. New Solar Telesc... more We studied flux emergence events of sub-granular scale in a solar active region. New Solar Telescope (NST) of Big Bear Solar Observatory made it possible to clearly observe the photospheric signature of flux emergence with very high spatial (0 ′′ .11 at 7057Å) and temporal (15 s) resolution. From TiO observations with the pixel scale of 0 ′′ .0375, we found several elongated granule-like features (GLFs) stretching from the penumbral filaments of a sunspot at a relatively high speed of over 4 km s −1 . After a slender arched darkening appeared at a tip of a penumbral filament, a bright point (BP) developed and quickly moved away from the filament forming and stretching a GLF. The size of a GLF was approximately 0 ′′ .5 wide and 3 ′′ long. The moving BP encountered nearby structures after several minutes of stretching, and a well-defined elongated shape of a GLF faded away. Magnetograms from SDO/HMI and NST/IRIM revealed that those GLFs are photospheric indicators of small-scale flux emergence, and their disappearance is related to magnetic cancellation. From two well-observed events, we describe detailed development of the sub-structures of GLFs, and different cancellation processes that each of the two GLFs underwent.

Research paper thumbnail of PROPERTIES OF UMBRAL DOTS AS MEASURED FROM THE NEW SOLAR TELESCOPE DATA AND MHD SIMULATIONS

The Astrophysical Journal, 2012

We studied bright umbral dots (UDs) detected in a moderate size sunspot and compared their statis... more We studied bright umbral dots (UDs) detected in a moderate size sunspot and compared their statistical properties to recent MHD models. The study is based on high-resolution data recorded by the New Solar Telescope at the Big Bear Solar Observatory and three-dimensional (3D) MHD simulations of sunspots. Observed UDs, living longer than 150 s, were detected and tracked in a 46 minute long data set, using an automatic detection code. A total of 1553 (620) UDs were detected in the photospheric (low chromospheric) data. Our main findings are (1) none of the analyzed UDs is precisely circular, (2) the diameter-intensity relationship only holds in bright umbral areas, and (3) UD velocities are inversely related to their lifetime. While nearly all photospheric UDs can be identified in the low chromospheric images, some small closely spaced UDs appear in the low chromosphere as a single cluster. Slow-moving and long-living UDs seem to exist in both the low chromosphere and photosphere, while fast-moving and short-living UDs are mainly detected in the photospheric images. Comparison to the 3D MHD simulations showed that both types of UDs display, on average, very similar statistical characteristics. However, (1) the average number of observed UDs per unit area is smaller than that of the model UDs, and (2) on average, the diameter of model UDs is slightly larger than that of observed ones.

Research paper thumbnail of MAGNETIC ENERGY SPECTRA IN SOLAR ACTIVE REGIONS

The Astrophysical Journal, 2010

Line-of-sight magnetograms for 217 active regions (ARs) of different flare rate observed at the s... more Line-of-sight magnetograms for 217 active regions (ARs) of different flare rate observed at the solar disk center from January 1997 until December 2006 are utilized to study the turbulence regime and its relationship to the flare productivity. Data from SOHO/MDI instrument recorded in the high resolution mode and data from the BBSO magnetograph were used. The turbulence regime was probed via magnetic energy spectra and magnetic dissipation spectra. We found steeper energy spectra for ARs of higher flare productivity. We also report that both the power index, α, of the energy spectrum, E(k) ∼ k −α , and the total spectral energy W = E(k)dk are comparably correlated with the flare index, A, of an active region. The correlations are found to be stronger than that found between the flare index and total unsigned flux. The flare index for an AR can be estimated based on measurements of α and W as A = 10 b (αW ) c , with b = −7.92 ± 0.58 and c = 1.85 ± 0.13. We found that the regime of the fully-developed turbulence occurs in decaying ARs and in emerging ARs (at the very early stage of emergence). Well-developed ARs display under-developed turbulence with strong magnetic dissipation at all scales.