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Comparison of Helioseismic Far-side Active Region Detections with STEREO Far-Side EUV Observations of Solar Activity  

Jiong Qiu   Submitted: 2017-09-25 11:30

Seismic maps of the Sun's far hemisphere, computed from Doppler data from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) are now being used routinely to detect strong magnetic regions on the far side of the Sun (http://jsoc.stanford.edu/data/farside/). To test the reliability of this technique, the helioseismically inferred active region detections are compared with far-side observation of solar activity from the Solar TErrestrial RElations Observatory (STEREO), using brightness in extreme ultraviolet light (EUV) as a proxy for magnetic fields. Two approaches are used to analyze nine months of STEREO and HMI data. In the first approach, we determine whether or not new large east-limb active regions are detected seismically on the far side before they appear Earth side and study how the detectability of these regions relates to their EUV intensity. We find that, while there is a range of EUV intensities for which far-side regions may or may not be detected seismically, there appears to be an intensity level above which they are almost always detected and an intensity level below which they are never detected. In the second approach, we analyze concurrent extreme ultraviolet and helioseismic far-side observations. We find that 100% (22) of the far-side seismic regions correspond to an extreme ultraviolet plage; 95% of these either became a NOAA-designated magnetic region when reaching the east limb or were one before crossing to the far side. A low but significant correlation is found between the seismic signature strength and the EUV intensity of a farside region.

Authors: P. C. Liewer, J. Qiu, C. Lindsey
Projects: None

Publication Status: accepted in Solar Physics
Last Modified: 2017-09-26 13:44
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Coronal Holes and Open Magnetic Flux over Cycles 23 and 24  

Jiong Qiu   Submitted: 2017-07-08 13:03

As the observational signature of the footprints of solar magnetic field lines open into the heliosphere, coronal holes provide a critical measure of the structure and evolution of these lines. Using a combination of Solar and Heliospheric Observatory/Extreme ultraviolet Imaging Telescope (SOHO/EIT), Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA), and Solar Terrestrial Relations Observatory/Extreme Ultraviolet Imager (STEREO/EUVI A/B) extreme ultraviolet (EUV) observations spanning 1996 - 2015 (nearly two solar cycles), coronal holes are automatically detected and characterized. Coronal hole area distributions show distinct behavior in latitude, defining the domain of polar and low-latitude coronal holes. The northern and southern polar regions show a clear asymmetry, with a lag between hemispheres in the appearance and disappearance of polar coronal holes.

Authors: Lowder, Chris; Qiu, Jiong; Leamon, Robert
Projects: None

Publication Status: Solar Physics, Volume 292, Issue 1, article id.18, 23 pp.
Last Modified: 2017-07-11 11:23
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Gradual Solar Coronal Dimming and Evolution of Coronal Mass Ejection in the Early Phase  

Jiong Qiu   Submitted: 2017-07-08 13:01

We report observations of a two-stage coronal dimming in an eruptive event of a two-ribbon flare and a fast coronal mass ejection (CME). Weak gradual dimming persists for more than half an hour before the onset of the two-ribbon flare and the fast rise of the CME. It is followed by abrupt rapid dimming. The two-stage dimming occurs in a pair of conjugate dimming regions adjacent to the two flare ribbons, and the flare onset marks the transition between the two stages of dimming. At the onset of the two-ribbon flare, transient brightenings are also observed inside the dimming regions, before rapid dimming occurs at the same places. These observations suggest that the CME structure, most probably anchored at the twin dimming regions, undergoes a slow rise before the flare onset, and its kinematic evolution has significantly changed at the onset of flare reconnection. We explore diagnostics of the CME evolution in the early phase with analysis of the gradual dimming signatures prior to the CME eruption.

Authors: Qiu, J., Cheng, J.X.
Projects: None

Publication Status: The Astrophysical Journal Letters, Volume 838, Issue 1, article id. L6, 6 pp. (2017)
Last Modified: 2017-07-11 11:23
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Elongation of Flare Ribbons  

Jiong Qiu   Submitted: 2017-07-08 12:59

We present an analysis of the apparent elongation motion of flare ribbons along the polarity inversion line (PIL), as well as the shear of flare loops in several two-ribbon flares. Flare ribbons and loops spread along the PIL at a speed ranging from a few to a hundred km s-1. The shear measured from conjugate footpoints is consistent with the measurement from flare loops, and both show the decrease of shear toward a potential field as a flare evolves and ribbons and loops spread along the PIL. Flares exhibiting fast bidirectional elongation appear to have a strong shear, which may indicate a large magnetic guide field relative to the reconnection field in the coronal current sheet. We discuss how the analysis of ribbon motion could help infer properties in the corona where reconnection takes place.

Authors: Qiu, J., Longcope, D. W., Cassak, P. A., Priest, E. R.
Projects: None

Publication Status: The Astrophysical Journal, Volume 838, Issue 1, article id. 17, 17 pp. (2017)
Last Modified: 2017-07-11 11:23
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Long Duration Flare Emission: Impulsive Heating or Gradual Heating?  

Jiong Qiu   Submitted: 2016-04-18 13:50

Flare emissions in X-ray and EUV wavelengths have previously been modeled as the plasma response to impulsive heating from magnetic reconnection. Some flares exhibit gradually evolving X-ray and EUV light curves, which are believed to result from superposition of an extended sequence of impulsive heating events occurring in different adjacent loops or even unresolved threads within each loop. In this paper, we apply this approach to a long duration two-ribbon flare SOL2011-09-13T22 observed by the Atmosphere Imaging Assembly (AIA). We find that to reconcile with observed signatures of flare emission in multiple EUV wavelengths, each thread should be heated in two phases, an intense impulsive heating followed by a gradual, low-rate heating tail that is attenuated over 20-30 minutes. Each AIA resolved single loop may be composed of several such threads. The two-phase heating scenario is supported by modeling with both a zero-dimensional and a 1D hydrodynamic code. We discuss viable physical mechanisms for the two-phase heating in a post-reconnection thread.

Authors: Qiu, Jiong, Longcope, Dana, W.
Projects: SDO-AIA

Publication Status: ApJ, 820, 14
Last Modified: 2016-04-18 21:08
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ULTRAVIOLET AND EXTREME-ULTRAVIOLET EMISSIONS AT THE FLARE FOOTPOINTS OBSERVED BY ATMOSPHERE IMAGING ASSEMBLY  

Jiong Qiu   Submitted: 2013-08-10 09:43

A solar flare is composed of impulsive energy release events by magnetic reconnection, which forms and heats flare loops. Recent studies have revealed a two-phase evolution pattern of UV 1600 Å emission at the feet of these loops: a rapid pulse lasting for a few seconds to a few minutes, followed by a gradual decay on timescales of a few tens of minutes. Multiple band EUV observations by the Atmosphere Imaging Assembly further reveal very similar signatures. These two phases represent different but related signatures of an impulsive energy release in the corona. The rapid pulse is an immediate response of the lower atmosphere to an intense thermal conduction flux resulting from the sudden heating of the corona to high temperatures (we rule out energetic particles due to a lack of significant hard X-ray emission). The gradual phase is associated with the cooling of hot plasma that has been evaporated into the corona. The observed footpoint emission is again powered by thermal conduction (and enthalpy), but now during a period when approximate steady-state conditions are established in the loop. UV and EUV light curves of individual pixels may therefore be separated into contributions from two distinct physical mechanisms to shed light on the nature of energy transport in a flare. We demonstrate this technique using coordinated, spatially resolved observations of UV and EUV emissions from the footpoints of a C3.2 thermal flare.

Authors: Jiong Qiu, Zoe Sturrock, Dana W. Longcope, James A. Klimchuk, and Wen-Juan Liu
Projects: None

Publication Status: ApJ, 774, 14
Last Modified: 2013-08-11 12:17
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Heating of Flare Loops With Observationally Constrained Heating Functions  

Jiong Qiu   Submitted: 2011-12-30 11:57

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Qiu J., Liu W-J, Longcope D. W.
Projects:

Publication Status: ApJ, in press
Last Modified: 2012-01-04 12:16
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RECONNECTION AND ENERGETICS IN TWO-RIBBON FLARES: A REVISIT OF THE BASTILLE-DAY FLARE  

Jiong Qiu   Submitted: 2010-12-01 12:47

We conduct a semi-quantitative analysis of two-ribbon flares to investigate the observational relationship between magnetic reconnection and energetics by revisiting the Bastille-day flare, particularly the UV and hard X-ray (HXR) observations. The analysis establishes that prominent UV emission is primarily produced by precipitating electrons that also produce HXRs. In addition, reconnection and subsequent energy release along adjacent field lines along the polarity inversion line (PIL) combined with elongated decay of UV emission may account for the observed extended UV ribbons whereas HXR sources with rapid decay appear mostly as compact kernels. Observations also show that HXR sources and UV brightenings exhibit an organized parallel motion along the magnetic PIL during the rise of the flare, and then the perpendicular expansion of UV ribbons dominate during the peak. With a 2.5 dimensional approximation with the assumed translational dimension along the PIL, we derive geometric properties of UV ribbons and infer the pattern of reconnection as with a varying magnetic guide field during reconnection. It is shown that HXR and UV emissions evolve in a similar way to reconnection rates determined by the perpendicular ''motion.'' The analysis suggests that a relatively strong guide field may be present during the rise of the flare, whereas particle acceleration and non-thermal energy release are probably more efficient with an enhanced reconnection rate with a relatively weak guide field. We discuss the role of the guide field in reconnection and particle energization, as well as novel observational experiments that may be conducted to shed new light on these issues.

Authors: Jiong Qiu, WenJuan Liu, Nicholas Hill and Maria Kazachenko
Projects: None

Publication Status: ApJ, 725, 319
Last Modified: 2010-12-01 12:55
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Evaluating Mean Magnetic Field in Flaring Loops  

Jiong Qiu   Submitted: 2009-01-07 11:10

We analyze multiple-wavelength observations of a two-ribbon flare exhibiting apparent expansion motion of the flare ribbons in the lower atmosphere and rising motion of X-ray emission at the top of newly formed flare loops. We evaluate magnetic reconnection rate in terms of V m rB m r by measuring the ribbon expansion velocity (V m r) and the chromospheric magnetic field (B m r) swept by the ribbons. We also measure the velocity (V m t) of the apparent rising motion of the loop top X-ray source, and estimate the mean magnetic field (B m t) at the top of newly formed flare loops using the relation langle V m tB m t angle approx langle V m rB m r angle, namely, conservation of reconnection flux along flare loops. For this flare, B m t is found to be 120 and 60~G, respectively, during two emission peaks five minutes apart in the impulsive phase. An estimate of the magnetic field in flare loops is also achieved by analyzing the microwave and hard X-ray spectral observations, yielding B = 250 and 120~G at the two emission peaks, respectively. The measured B from the microwave spectrum is an appropriately weighted value of magnetic field from the loop top to the loop leg. Therefore, the two methods to evaluate coronal magnetic field in flaring loops produce fully consistent results in this event.

Authors: Qiu, J., Gary, D. E., Fleishman, G. D.
Projects:

Publication Status: Solar Physics, 255, 107
Last Modified: 2009-02-24 19:12
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Observational Analysis of Magnetic Reconnection Sequence  

Jiong Qiu   Submitted: 2008-10-27 11:25

We conduct comprehensive analysis of an X2.0 flare to derive quantities indicative of magnetic reconnection in solar corona by following temporally and spatially resolved flare ribbon evolution in the lower atmosphere. The analysis reveals a macroscopically distinctive two-stage reconnection (Moore et al. 2001) marked by a clear division in morphological evolution, reconnection rate, and energy release rate. During the first stage, the flare brightening starts at and primarily spreads along the polarity inverion line (PIL) with the maximum apparent speed comparable to the local Alfvén speed. The second stage is dominated by ribbon expansion perpendicular to the PIL at a fraction of the local Alfvén speed. We further develop a data analysis approach, namely ''reconnection sequence analysis'', to determine the connectivity and reconnection flux during the flare between a dozen magnetic sources defined from partitioning the photospheric magnetogram. It is found that magnetic reconnection proceeds sequentially between magnetic cells, and the observationally measured reconnection flux in major cells compares favorably with computations by a topological model of magnetic reconnection. The 3D evolution of magnetic reconnection is discussed with respect to its implication on helicity transfer and energy release through reconnection.

Authors: Jiong Qiu
Projects: None

Publication Status: ApJ, 692, 1110
Last Modified: 2009-02-24 18:55
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On Magnetic Flux Budget in Low-corona Magnetic Reconnection and Interplanetary Coronal Mass Ejections  

Jiong Qiu   Submitted: 2006-11-03 15:39

We present the first quantitative comparison between the total magnetic reconnection flux in the low-corona in the wake of coronal mass ejections (CMEs) and the magnetic flux in magnetic clouds (MCs) that reach 1 AU 2 - 3 days after CME onset. The total reconnection flux is measured from flare ribbons, and the MC flux is computed using in-situ observations at 1 AU, all ranging from 1020-22 Mx. It is found that for the studied 9 events in which the association between flares, CMEs, and MCs is identified, the MC flux is correlated with the total reconnection flux Phi_r. Further, the poloidal (azimuthal) MC flux Phi_p is comparable with reconnection flux Phir, and the toroidal (axial) MC flux Phi_t is a fraction of Phir. Events associated with filament eruption do not exhibit a different Phit, p - Phi_r relation from events not accompanied by erupting filaments. The revealed relations between these independently measured physical quantities suggest that, for the studied samples, the magnetic flux and twist of interplanetary magnetic flux ropes, reflected by MCs, are highly relevant to low-corona magnetic reconnection during the eruption. %which contributes significantly to the formation of the helical structure of the flux ropes. We discuss the implication on the formation mechanism of twisted magnetic flux ropes, namely, whether the helical structure of the magnetic flux rope is largely pre-existing or formed in-situ by low-corona magnetic reconnection. We also measure magnetic flux encompassed in coronal dimming regions (Phi_d) and discuss its relation with reconnection flux inferred from flare ribbons and MC flux.

Authors: Jiong Qiu, Qiang Hu, Timothy, A. Howard, Vasyl, B. Yurchyshyn
Projects:

Publication Status: ApJ (accepted)
Last Modified: 2006-12-19 14:55
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Hrd X-ray and Microwave Observations of Microflares  

Jiong Qiu   Submitted: 2004-06-04 16:05

In this paper, we study solar microflares using the coordinated hard X-ray and microwave observations obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during its open-shutter operation mode and the Owens Valley Solar Array (OVSA). The events in our study are selected in the energy range of 12-25 keV and are relatively large microflares with an average GOES soft X-ray incremental flux at the B2.0 level. A total of 760 microflares are identified from the RHESSI burst catalog. Out of the 200 microflares that fall into the OVSA observing window, about 40% are detected in microwaves. Using these hundreds of events as samples, we study the event distribution with respect to the flux, the solar activity, and active regions, in comparison with flares of larger scales. Non-thermal properties of microflares are investigated through spectral analysis of X-rays and microwaves. (1) We find that the event frequency distribution with respect to the RHESSI peak count rates at 12-25 keV can be accurately described with a power-law function down to 8 cts s-1, the power-law index being 1.75pm0.03, consistent with previous studies. (2) Similar to large flares, the occurrence rate of microflares is correlated with solar activity. The studied samples of microflares are mostly produced by active regions as suggested by the large percentage of events detected by OVSA that observes target active regions. However, all active regions do not have equal productivity, and certain active regions are a lot more productive than other regions. (3) While some large and complex active regions are predominantly productive in both very weak and strong events, we also find an active region that produces many microflares and C-class events but does not produce powerful events. %It may be that frequent occurrence of small-scale %events in some active regions helps relax the configuration and %prevent build-up to large bursts. (4) Analysis of energy-dependent time profiles suggests that there is a pronounced temporal correlation between the time derivative of soft X-rays and 14-20 keV hard X-rays, i.e., the Neupert effect, in about half of the studied events. (5) Albeit small, many microflares exhibit hard X-ray emission at over 10 keV and microwave emission at around 10 GHz. Spectral analysis in these two wavelengths corroborates the non-thermal nature of these emissions. (6) In a limited number of samples, the RHESSI spectral fitting yields a photon spectral index of 4.5-7, and microwave spectral analysis on the same events shows that the power-law index of the electron spectrum is in the range of 2-5. The discrepancy in the electron spectrum index derived from hard X-rays and microwaves is substantially greater than previously reported in big flares, hinting at the existence of high-energy, microwave emitting electrons that have a much hardened spectrum compared with electrons emitting hard X-rays.

Authors: Jiong Qiu, Chang Liu, Dale E. Gary, Gelu, M. Nita, Haimin Wang
Projects: RHESSI

Publication Status: ApJ, 2004, 612
Last Modified: 2004-06-04 16:05
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Impulsive and Gradual Nonthermal Emissions in an X-Class Flare  

Jiong Qiu   Submitted: 2004-06-04 15:54

In this paper we present a comprehensive case study of an X-class flare observed on 2001 April 6. The flare consists of two episodes, the first characterized by impulsive spiky bursts and the second by gradual smooth emission at hard X-ray and microwave wavelengths. Emissions in the two episodes are regarded as the impulsive and gradual components, respectively. We compare the temporal, spatial, and spectral evolution of the two components in hard X-rays and microwaves. For this event, the most important finding is that both the impulsive and gradual hard X-rays at >=50 keV are thick-target emissions at conjugate footpoints. Evolution of hard X-rays and microwaves during the gradual phase exhibits a separation motion between two footpoint sources, which reflects progressive magnetic reconnection. Observations further reveal distinct spectral properties of the gradual component. It is most prominently observed in the high-energy (>20 keV) range, and the gradual hard X-rays have a harder and hardening spectrum compared with the impulsive component. The gradual component is also a microwave-rich event, with the microwaves lagging the hard X-rays by tens of seconds. A correlation analysis of the hard X-ray light curves shows energy-dependent time delays, with the 200 keV hard X-rays lagging the 40 keV emission by 20 s. The observations and analyses suggest that magnetic reconnection occurs during both the impulsive and gradual phases that generate nonthermal electrons, primarily precipitating at the footpoints. However, the temporal and spectral properties of the gradual component must be produced by an acceleration mechanism different from that of the impulsive phase. We propose that the ``collapsing-trap'' effect, as a product of progressive magnetic reconnection in bipolar magnetic fields, is a viable mechanism that continuously accelerates the gradual-phase electrons in a low-density trap before they precipitate into the footpoints.

Authors: Qiu, Jiong; Lee, Jeongwoo; Gary, Dale E.
Projects: None

Publication Status: 2004, ApJ, 603, 335
Last Modified: 2004-06-04 15:54
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Flare-related Magnetic Anomaly with a Sign Reversal  

Jiong Qiu   Submitted: 2004-06-04 15:50

In this paper we report a significant magnetic anomaly, specifically an apparent sign reversal of magnetic polarities in small areas of Michelson Doppler Imager (MDI) magnetograms during the impulsive phase of an X5.6 flare on 2001 April 6. Three flare kernels were observed to emit >=50 keV hard X-rays, which are located in strong magnetic fields of order ±1000-1500 G. We find that the apparent sign reversal began and persisted for a few minutes in all three kernels, in precise temporal and spatial correspondence with the hard X-ray sources. We search for a combination of instrumental and flare-induced line profile effects that can account for this behavior. Our studies provide a viable scenario that the observed transient sign reversal is likely to be produced by distorted measurements when the Ni I 6768 Å line comes into emission or strong central reversal as a result of nonthermal beam impact on the atmosphere in regions of strong magnetic fields.

Authors: Qiu, Jiong; Gary, Dale E.
Projects: None

Publication Status: 2003, ApJ, 599, 615
Last Modified: 2004-06-04 15:50
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Magnetic Reconnection and Mass Acceleration in Flare-Coronal Mass Ejection Events  

Jiong Qiu   Submitted: 2004-06-04 15:46

An observational relationship has been well established among magnetic reconnection, high-energy flare emissions and the rising motion of erupting flux ropes. In this paper, we verify that the rate of magnetic reconnection in the low corona is temporally correlated with the evolution of flare nonthermal emissions in hard X-rays and microwaves, all reaching their peak values during the rising phase of the soft X-ray emission. In addition, however, our new observations reveal a temporal correlation between the magnetic reconnection rate and the directly observed acceleration of the accompanying coronal mass ejection (CME) and filament in the low corona, thus establishing a correlation with the rising flux rope. These results are obtained by examining two well-observed two-ribbon flare events, for which we have good measurements of the rise motion of filament eruption and CMEs associated with the flares. By measuring the magnetic flux swept through by flare ribbons as they separate in the lower atmosphere, we infer the magnetic reconnection rate in terms of the reconnection electric field Erec inside the reconnecting current sheet (RCS) and the rate of magnetic flux convected into the diffusion region. For the X1.6 flare event, the inferred Erec is ~5.8 V cm-1 and the peak mass acceleration is ~3 km s-2, while for the M1.0 flare event Erec is ~0.5 V cm-1 and the peak mass acceleration is 0.2-0.4 km s-2.

Authors: Qiu, Jiong; Wang, Haimin; Cheng, C. Z.; Gary, Dale E.
Projects: None

Publication Status: 2004, ApJ, 604, 900
Last Modified: 2004-06-04 15:46
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Abstracts by Author
Comparison of Helioseismic Far-side Active Region Detections with STEREO Far-Side EUV Observations of Solar Activity
Coronal Holes and Open Magnetic Flux over Cycles 23 and 24
Gradual Solar Coronal Dimming and Evolution of Coronal Mass Ejection in the Early Phase
Elongation of Flare Ribbons
Long Duration Flare Emission: Impulsive Heating or Gradual Heating?
ULTRAVIOLET AND EXTREME-ULTRAVIOLET EMISSIONS AT THE FLARE FOOTPOINTS OBSERVED BY ATMOSPHERE IMAGING ASSEMBLY
Heating of Flare Loops With Observationally Constrained Heating Functions
RECONNECTION AND ENERGETICS IN TWO-RIBBON FLARES: A REVISIT OF THE BASTILLE-DAY FLARE
Evaluating Mean Magnetic Field in Flaring Loops
Observational Analysis of Magnetic Reconnection Sequence
On Magnetic Flux Budget in Low-corona Magnetic Reconnection and Interplanetary Coronal Mass Ejections
Hrd X-ray and Microwave Observations of Microflares
Impulsive and Gradual Nonthermal Emissions in an X-Class Flare
Flare-related Magnetic Anomaly with a Sign Reversal
Magnetic Reconnection and Mass Acceleration in Flare-Coronal Mass Ejection Events

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