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Coronal Thick Target Hard X Ray Emissions and Radio Emissions  

Jeongwoo Lee   Submitted: 2013-04-21 18:24

Recently a distinctive class of hard X ray (HXR) sources located in the corona was found, which implies that the collisionally thick target model (CTTM) applies even to the corona. We investigated whether this idea can independently be verified by microwave radiations that have been known as the best companion to HXRs. The study is made for the GOES M2.3 class flare occurred on 2002 September 9 that were observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Owens Valley Solar Array (OVSA). Interpreting the observed energy dependent variation of HXR source size under the CTTM the coronal density should be as high as 5x10+11 cm-3 over the distance up to 12''. To explain the cut-off feature of microwave spectrum at 3 GHz, we however, need density no higher than 1x10+11 cm-3. Additional constraints need to be placed on temperature and magnetic field of the coronal source in order to reproduce the microwave spectrum as a whole. Firstly, a spectral feature called the Razin suppression requires the magnetic field in a range of 250-350 gauss along with high viewing angles around 75 degree. Secondly, to avoid excess fluxes at high frequencies due to the free-free emission that were not observed, we need a high temperature >2x10+7 K. These two microwave spectral features, Razzin suppression and free-free emissions, become more significant at regions of high thermal plasma density and are essential for validating and for determining additional parameters for the coronal HXR sources.

Authors: Jeongwoo Lee, Daye Lim, G. S. Choe, Kap-Sung Kim, and Minhwan Jang
Projects: None

Publication Status: Accepted to ApJ Letters
Last Modified: 2013-04-22 08:56
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Frequency Distributions of Solar Microwave Burst Parameters  

Jeongwoo Lee   Submitted: 2008-12-17 12:23

Microwave bursts during solar flares are known to be sensitive to high energy electrons and magnetic field, both of which are important ingredients of solar flare physics. This paper presents such information derived from the microwave bursts of the 412 flares that were measured with the Owens Valley Solar Array (OVSA). We assumed that these bursts are predominantly due to gyrosynchrotron radiation by nonthermal electrons in a single power-law energy distribution to use the simplified formulae for gyrosynchrotron radiation in the data analysis. A second major assumption was that statistical properties of flare electrons derived from this microwave database should agree with an earlier result based on the Hard X-Ray Burst Spectrometer (HXRBS) on SMM. Magnetic field information was obtained in the form of a scaling law between average magnetic field and total source area, which turns out to a narrow distribution around ~400 gauss. The derived nonthermal electron energy is related to the peak flux, peak frequency, and spectral index, through a multi-step regression fit, which can be used for a quick estimate for the nonthermal electron energy from spatially-integrated microwave spectral observations.

Authors: Jeongwoo Lee, Gelu M. Nita, and Dale E. Gary
Projects: None

Publication Status: ApJ, in press
Last Modified: 2008-12-17 12:51
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Subject will be restored when possible  

Jeongwoo Lee   Submitted: 2008-08-21 13:48

We have investigated the variation of magnetic helicity over a span of several days around the times of eleven X-class flares which occurred in seven active regions (NOAA 9672, 10030, 10314, 10486, 10564, 10696, and 10720) using the magnetograms taken by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO). As a major result we found that each of these major flares was preceded by a significant helicity accumulation, (1.8?16)x1042 Mx^2 over a long period (0.5?a few days). Another finding is that the helicity accumulates at a nearly constant rate, (4.5?48)x1040 Mx^2 hr-1, and then becomes nearly constant before the flares. This led us to distinguish the helicity variation into two phases: a phase of monotonically increasing helicity and the following phase of relatively constant helicity. As expected, the amount of helicity accumulated shows a modest correlation with time-integrated soft Xray flux during flares. However, the average helicity change rate in the first phase shows even stronger correlation with the time-integrated soft X-ray flux. We discuss the physical implications of this result and the possibility that this characteristic helicity variation pattern can be used as an early warning sign for solar eruptions.

Authors: Sung-Hong Park, Jeongwoo Lee, G.S. Choe, Jongchul Chae, Hyewon Jeong, Guo Yang, Ju Jing, Haimin Wang
Projects: None

Publication Status: To appear in ApJ, 2008 October 1 issue
Last Modified: 2008-08-21 13:48
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THE VARIATION OF RELATIVE MAGNETIC HELICITYAROUND MAJOR FLARES  

Jeongwoo Lee   Submitted: 2008-08-21 13:48

We have investigated the variation of magnetic helicity over a span of several days around the times of eleven X-class flares which occurred in seven active regions (NOAA 9672, 10030, 10314, 10486, 10564, 10696, and 10720) using the magnetograms taken by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO). As a major result we found that each of these major flares was preceded by a significant helicity accumulation, (1.8?16)x1042 Mx^2 over a long period (0.5?a few days). Another finding is that the helicity accumulates at a nearly constant rate, (4.5?48)x1040 Mx^2 hr-1, and then becomes nearly constant before the flares. This led us to distinguish the helicity variation into two phases: a phase of monotonically increasing helicity and the following phase of relatively constant helicity. As expected, the amount of helicity accumulated shows a modest correlation with time-integrated soft Xray flux during flares. However, the average helicity change rate in the first phase shows even stronger correlation with the time-integrated soft X-ray flux. We discuss the physical implications of this result and the possibility that this characteristic helicity variation pattern can be used as an early warning sign for solar eruptions.

Authors: Sung-Hong Park, Jeongwoo Lee, G.S. Choe, Jongchul Chae, Hyewon Jeong, Guo Yang, Ju Jing, Haimin Wang
Projects: None

Publication Status: To appear in ApJ, 2008 October 1 issue
Last Modified: 2008-08-22 13:37
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Parallel Motions of Coronal Hard X-ray Source and Hα Ribbons  

Jeongwoo Lee   Submitted: 2008-08-21 12:56

During solar flares Hα ribbons form and often move away from the local magnetic polarity inversion line (PIL). The motion perpendicular to the PIL has been taken as evidence for coronal magnetic reconnection in the so-called CSHKP standard model. The other velocity component parallel to the PIL is also commonly seen but much less adopted as a property of the magnetic reconnection process. In this Letter we focus on the latter velocity component, and report an event in which the motion parallel to the PIL is found in both Hα ribbons and a thermal hard X-ray source. Such commonality would indicate a link between the coronal magnetic reconnection and footpoint emissions as in the standard solar flare model. However, its direction implies a reconnection region that is increasing in length, a feature missing from the standard two-dimensional model. We present a modified framework in which the variation along the third dimension is allowed, in order to assess the effect of such a proper motion on estimation of the magnetic reconnection rate. Data used are hard X-ray maps from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Hα filtergrams of Big Bear Solar Observatory (BBSO), and the SOHO Michelson Doppler Imager (MDI) magnetogram obtained for the 2004 March 30 flare.

Authors: Jeongwoo Lee and Dale E. Gary
Projects: None

Publication Status: To appear in ApJ Letters, v685, 2008 Sep. 20 issue
Last Modified: 2008-09-23 20:50
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Subject will be restored when possible  

Jeongwoo Lee   Submitted: 2008-08-21 12:55

During solar flares Hα ribbons form and often move away from the local magnetic polarity inversion line (PIL). The motion perpendicular to the PIL has been taken as evidence for coronal magnetic reconnection in the so-called CSHKP standard model. The other velocity component parallel to the PIL is also commonly seen but much less adopted as a property of the magnetic reconnection process. In this letter we focus on the latter velocity component, and report an event in which such a motion parallel to the PIL is found in both Hα ribbons and a thermal hard X-ray source. Such commonality would indicate a link between the coronal magnetic reconnection and footpoint emissions as in the standard solar flare model. However, its direction implies a reconnection region that is increasing in length, a feature missing from the standard two-dimensional model. We present a modified framework in which the variation along the third dimension is allowed, in order to assess the effect of such a proper motion on estimation of the magnetic reconnection rate. Data used are hard X-ray maps from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Hα filtergrams of Big Bear Solar Observatory (BBSO), and the SOHO Michelson Doppler Imager (MDI) magnetogram obtained for the 2004 March 30 flare.

Authors: Jeongwoo Lee and Dale E. Gary
Projects: None

Publication Status: To appear in ApJ Letters, v685, 2008 Sep. 20 issue
Last Modified: 2008-09-23 20:50
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Subject will be restored when possible  

Jeongwoo Lee   Submitted: 2008-08-21 12:51

During solar flares Hα ribbons form and often move away from the local magnetic polarity inversion line (PIL). While the motion perpendicular to the PIL has been taken as evidence for coronal magnetic reconnection in the so-called CSHKP standard model, the other velocity component parallel to the PIL is much less adopted as a property of the magnetic reconnection process. In this Letter we report an event in which the motion parallel to the PIL is found in both Hα ribbons and a thermal hard X-ray source. Such commonality would indicate a link between the coronal magnetic reconnection and footpoint emissions as in the standard solar flare model. However, its direction implies a reconnection region that is increasing in length, a feature missing from the standard two-dimensional model. We present a modified framework in which the variation along the third dimension is allowed, in order to assess the effect of such a proper motion on estimation of the magnetic reconnection rate. Data used are hard X-ray maps from the Reuven Ramaty High Energy Solar Spectroscopic Imager, Hα filtergrams of Big Bear Solar Observatory, and the SOHO Michelson Doppler Imager magnetogram obtained for the 2004 March 30 flare.

Authors: Jeongwoo Lee and Dale E. Gary
Projects: None

Publication Status: To appear in ApJ Letters, v685, 2008 Sep. 20 issue
Last Modified: 2008-09-23 20:50
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Magnetic Field Strength in the Solar Corona from Type II Band Splitting  

Jeongwoo Lee   Submitted: 2007-06-27 12:03

The phenomenon of band splitting in type II bursts can be a unique diagnostic for the magnetic field in the corona, which is, however, inevitably sensitive to the ambient density. We apply this diagnostic to the CME-flare event on 2004 August 18, for which we are able to locate the propagation of the type II burst and determine the ambient coronal electron density by other means. We measured the width of the band splitting on a dynamic spectrum of the bursts observed with the Green Bank Solar Radio Burst Spectrometer (GBSRBS), and converted it to the Alfvén Mach number under the Rankine-Hugoniot relation. We then determine the Alfvén speed and magnetic field strength using the coronal background density and shock speed measured with the MLSO/MK4 coronameter. In this way we find that the shock compression ratio is in the range of 1.5-1.6, the Alfvénic Mach number is 1.4-1.5, the Alfvén speed is 550-400 km s-1, and finally the magnetic field strength decreases from 1.3 G to 0.4 G while the shock passes from 1.6R to 2.1R. The magnetic field strength derived from the type II spectrum is finally compared with the potential field source surface (PFSS) model for further evaluation of this diagnostic.

Authors: K.-S. Cho, J. Lee, D. E. Gary, Y.-J. Moon, and Y. D. Park
Projects: SoHO-MDI

Publication Status: 2007 ApJ (in press)
Last Modified: 2007-06-27 13:31
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A Study of CME and Type II Burst Kinematics Based on Coronal Density Measurement  

Jeongwoo Lee   Submitted: 2007-06-27 10:00

The aim of this paper is to determine location and speed of a coronal shock from a type II burst spectrum without relying on any coronal density model, and use the result to discuss the relationship between the type II burst and Coronal Mass Ejection (CME). This study is made for the 2004 August 18 solar eruption observed by Green Bank Solar Radio Burst Spectrometer (GBSRBS) and a limb CME/streamer simultaneously detected by Mauna Loa Solar Observatory (MLSO) MK4 coronameter. We determine the background density distribution over the area of interest by inverting the MLSO MK4 polarization map taken just before the CME onset. Using the two-dimensional density distribution and the type II emission frequencies, we calculate the type II shock heights along several radial directions selected to encompass the entire position angles of the CME. We then compare these emission heights with those of CME to determine at which position angle the type II burst propagated. Along the most plausible position angle, we finally determine the height and speed of the shock as functions of time. It turns out that the type II emission height calculated along a southern streamer best agrees to the observed height of the CME flank. Along this region, both the shock and CME moved at a speed ranging from 800 to 600 km s-1. We also found that the streamer boundary already had enhanced density compared to other parts before the CME, and formed a low Alfvénic region. We therefore conclude that the type II burst was generated at the interface of the CME flank and the streamer, as was favorable for the shock formation

Authors: K.-S. CHO, J. Lee, Y.-J. Moon, M. Dryer, S.-C. Bong, Y.-H. Kim, and Y. D. Park
Projects: SoHO-MDI

Publication Status: A&A (in press)
Last Modified: 2007-06-27 10:48
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Hard X-ray Intensity Distribution Along Hα Ribbons  

Jeongwoo Lee   Submitted: 2007-06-27 09:54

Unusual ribbon-like hard X-ray sources were found with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observation of the 2B/M8.0 flare on 2005 May 13. We use this unique observation to investigate the spatial distribution of the hard X-ray intensity along the ribbons and compare it with the local magnetic energy release rate predicted by the standard magnetic reconnection model for two ribbon flares. In the early phase of the flare, the hard X-ray sources appear to be concentrated in strong field sections within the Hα ribbons, which is explicable by the model. On and after the flare maximum phase, the hard X-ray sources become spatially extended to resemble Hα ribbons in morphology, during which the spatial distribution of hard X-ray intensity lacks a correlation with that of the local magnetic energy release rate predicted by the model. We discuss the latter result as a limitation of the standard 2D model in application to a realistic 3D reconnection. More specifically we argue that the magnetic reconnection during this event may involve the magnetic field rearrangement along the magnetic arcade axis which is inevitably overlooked by the 2D model.

Authors: Ju Jing, Jeongwoo Lee, Chang Liu, Dale E. Gary, and Haimin Wang
Projects: RHESSI

Publication Status: ApJL (in press)
Last Modified: 2007-06-27 10:48
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Radio Emissions from Solar Active Regions  

Jeongwoo Lee   Submitted: 2007-06-27 09:51

Solar active region coronae are known for strong magnetic fields permeating tenuous plasma which makes them an ideal astronomical laboratory for magnetohydrodynamics research. It is, however, relatively less known that this physical condition also permits a very efficient radiation mechanism, gyro-resonant emission, produced by hot electrons gyrating in the coronal magnetic field. As a resonant mechanism, gyro-emission produces high enough opacity to fully reveal the coronal temperature, and is concentrated at a few harmonics of the local gyrofrequency to serve as an excellent indicator of the magnetic field. In addition, the polarization of the ubiquitous free-free emission and a phenomenon of depolarization due to mode coupling extend the magnetic field diagnostic to a wide range of coronal heights. The ability to measure the coronal temperature and magnetic field without the complications arising in other radiative inversion problems is a particular advantage for the active region radio emissions available only at these wavelengths. This article reviews the efforts for understanding these radiative processes, and utilizing them as diagnostic tools in addressing a number of critical issues involved with active regions.

Authors: Jeongwoo Lee
Projects: None

Publication Status: Space Science Reviews (in press)
Last Modified: 2007-06-27 10:49
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Abstracts by Author
Coronal Thick Target Hard X Ray Emissions and Radio Emissions
Frequency Distributions of Solar Microwave Burst Parameters
Subject will be restored when possible
THE VARIATION OF RELATIVE MAGNETIC HELICITY AROUND MAJOR FLARES
Parallel Motions of Coronal Hard X-ray Source and Hα Ribbons
Subject will be restored when possible
Subject will be restored when possible
Magnetic Field Strength in the Solar Corona from Type II Band Splitting
A Study of CME and Type II Burst Kinematics Based on Coronal Density Measurement
Hard X-ray Intensity Distribution Along H-alpha Ribbons
Radio Emissions from Solar Active Regions

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