RESEARCH INTERESTS

Jeongwoo Lee

I have built my research career in various topics of solar physics including solar MHD theory, radiative transfer, and magnetic field measurement. With my thesis work, however, I have mainly involved with solar radio physics, on which my current research interests and activities are also focused. Since this field requires a serious involvement with the instrument and processing, I found it easy to summarize my research interests according to specific instruments used with.

1. Owens Valley Solar Array (OVSA)

The instrument that I have mainly worked with is the Owens Valley Solar Array (OVSA). Figure 1 shows its configuration where dots and lines represent antennas and baselines, respectively. Also shown as inset is a view of a 2-m dish and a 27-m dish. It is often said that OVSA is a good example of a small sized array (consisting only of two large 27-m antennas and five small 2-m dishes) that can be operated by an academic institute and upgradable with a modest amount of funds and students’ participation, but can nevertheless achieve the forefront science with unique capability. What makes this modest array unique is that this is the only solar-dedicated radio interferometer in the United States and only spectroscopic imager at microwaves in the world (see, for more details, the web page http://ovsa.njit.edu). I have been working on OVSA since I was a graduate student (1991-1995), when it was a form of 3-element array and through all the hardware upgrades (e.g., putting antenna into operation, exchange of cables, adding dipole feed for polarization measurement) and associated renovation of calibration and data acquisition (e.g., refinement of on-line software completed is needed whenever hardware changes, such as the addition of new antennas). My reseach interest with this instruments are as follows:

Fig. 1. The Owens Valley Solar Array (OVSA). The array configuration with black circles show stations existing and operating at present. The inset shows the view of the OVSA looking from station 7. the fore- and background objects are 2-m and 27-m dishes, respectively.

1.1 Low-level Data Analysis for OVSA: The operation of the OVSA is somewhat different from other radio facilities mainly because of the unusually large number of frequencies, which means a burden in correlating signals from pairs of antennas as many times as the frequencies. This complicates the data processing at OVSA, unlike other radio interferometers operating at a few frequencies, but is essential for keeping up its uniqueness. I experiment various ways to get the complex visibility gain solutions at each baseline/antenna and strategies for calibration and data acquisition, using known techniques such as SELFCAL and, sometimes, a new technique such as Spatio-Spectral Maximum Entropy Method. I also carry out hybrid mapping by combining the VLA frequencies with neighboring frequencies of OVSA for cross-check. Last year I have finished a version of imaging package optimized for the OVSA, which is now distributed as a part of the Solar Software system (http://sohowww.nascom.nasa.gov/solarsoft). Although not all these efforts are directly published in papers, the results are used to help deriving scientific conclusions presented in papers. All of these experiments are also very valuable in education.

1.2 Flare study

At present, the primary science of the OVSA is solar flares, since we are at the period of maximum solar activity, and many space mission including the RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager, a NASA mission) requires groundbased support. The OVSA is a central companion to the hard X ray and gamma-ray observations in space, and my specific interests are as follows:

• Nonthermal Electron Energy Distribution Like any other astronomical spectroscopy, the first goal of the microwave spectroscopy is to invert the observed spectrum to that of emitting particles. For solar microwaves bursts, the emitting particles are electrons in the range of a few tens of keV to several MeV, and the emissivity spectrum is mostly determined by the corresponding electron distribution in phase space, i.e., f (p_x , p_y , p_z) where p is electron momentum. I have developed software for calculating theoretical microwave spectrum in several forms: (i) commonly used isotropic, power-law distribution of electrons, (ii) arbitrary energy distribution, and (iii) power-law momentum distribution with anisotropic pitch-angle distribution. These sets of software enable more advanced calculation than ever and allow a more realistic comparison with the observations.

· Electron Acceleration and Transport Information of the emitting electrons obtained from microwave or hard X ray spectra can, in principle, represent either the acceleration property itself or additional transport effects. I have, in collaboration with Dale Gary, demonstrated that time-dependent microwave spectrum, among other flare radiations, is an adequate tool for addressing both electron acceleration and trapping in one context (Lee, Gary, & Shibasaki 2000, Lee & Gary 2000, Lee et al. 2002). Figure 2 shows an example in which the microwave maps evidence a weak pitch-angle diffusion within a magnetic loop (left panel) and a model fit to the observed spectral variation using numerical Fokker-Planck solution (right panel) provides quantitative information of the electron evolution in the phase space. This kind of formulation allows a premier study of electron acceleration and transport, as constrained with the microwave and hard X ray data, in a self-consistent fashion.

Fig. 2. An example of solar flare and radio bursts. The background color image in the left panel is soft X ray image from Yohkoh/SXT telescope during a solar flare, and the contours show radio intensity at two different frequencies. We interpret these SX and microwave bright features as representing the flaring magnetic loop and locations of high-energy electrons accelerated during the flare. The right panels shows a model fit to the observed light-curve (cross symbols) and spectral slope (diamond symbols) using a stochastic diffusion model for electrons.

· Flare Energy release In addition to the study of electron evolution in phase space, it is also important, for understanding of solar flares, to relate such microscopic physics inferred from microwave or hard X-rays to the macroscopic physics (e.g., magnetic reconnection) developed by solar MHD theorists. I have recently worked, in collaboration with solar optical and EUV experts (Jiong Qiu, Peter Gallagher, Haimin Wang) to compare microwave data with either high resolution Ha data (0.36” pixel and 0.1 s cadence) from Big Bear Solar Observatory (Qiu et al. 2002) or high resolution EUV images (0.5” pixel) from TRACE (Lee et al. 2002, Gallagher et al. 2002). I will continue to use the fine-scale motions of Ha brightening and the coronal EUV loops to find the link between the particle acceleration and magnetic energy release through reconnection.

2. Frequency Agile Solar Radiotelescope (FASR)

I am also working, as a co-I, on a project of a new solar radio facility called “Frequency Agile Solar Radiotelescope” or FASR, which we expect to stand in the forefront of solar physics in future as next generation of the OVSA. To briefly summarize the current status of this project, our FASR proposal was listed among the 17 priority projects in astronomy by the Astronomy and Astrophysics Decadal Survey Committee, and was recently ranked number one in the small projects (< $150 M) category of the Solar and Space Physics Survey Committee (see the web page, http://www.ovsa.njit.edu/fasr/). NJIT is the PI institution and collaboration is among the National Radio Astronomy Observatory, University of Maryland, UC Berkeley, Lucent Technologies, and National Solar Observatory. The FASR will address a wide range of sciences including not only solar but space sciences, and here are some of my best interests associated with the FASR project.

2.1 Solar Active Region Study

One of the most important applications of the FASR will be solar active region study, because solar active region corona provides gyro-resonant opacity (cyclotron radiation at local cyclotron frequency and its harmonics) and thus allows direct determination of the coronal magnetic field strength. Magnetic fields in active region corona is of central interest to understanding and predicting solar activities such as coronal heating and magnetic energy release, which leads to the following topics:

· Coronal Magnetography The determination of magnetic field and temperature of the solar corona is a goal pursued by a number of complementary techniques. From radio perspective, the most straightforward technique is the use of the spectral discontinuity in microwave spectrum, which represents a harmonic of the local magnetic field strength, as demonstrated elsewhere (Lee et al. 1993, 1997, 1998). Figure 3 illustrates the idea, which I have originally made for a purpose of presenting before a proposal review panel, and later commonly cited elsewhere. The solid lines represents magnetic field lines which are obtained by a stat-of-art, nonlinear force-free field extrapolation from actual data (grayscale images), and colored, meshed surfaces are isogauss layers which correspond to emission layers at the specific frequencies. Since the microwave emission is due to free electrons in the Rayleigh-Jeans regime, the observed intensity simply tells the electron kinetic temperature without complications that the line emissions involve. The emission boundary at each frequency outlines the area of coronal field above some strength set by the frequency, and therefore multi-frequency microwave maps can define continuous distribution of coronal field strength.

Fig. 3. A perspective view of a complex sunspot group (7 May 1991) in optical continuum is shown with field lines extrapolated into the corona using a nonlinear force-free extrapolation by Z. Mikic. The three surfaces are the calculated gyroresonant surfaces in the corona that will dominate the radio opacity at each of three radio frequencies: 5 GHz (B = 600 G), 8 GHz (B = 950 G) and 11 GHz (B = 1300 G).

· Coronal Currents and Heating Once we determine the coronal field distribution, a further application is then to locate the regions of the coronal current, where the free energy for solar activity is stored. I have achieved this goal in various proedures: (i) find the regions on the microwave map where the potential field (i.e. current-free fields) extrapolation fails to predict the coronal field strength to explain the observed microwave emission. (Lee et al. 1997). (ii) use a set of model field lines from a nonlinear force-free extrapolation to predict the magnetic field topology with height and find the one most consistent with the differing morphology of radio maps with frequency (Lee et al. 1998ab, 1999). As continuation of the project, I am collaborating with both MHD theorists (Zoran Mikic, Y. Mok) and vector magnetogram observer (K.D. Leka), the outcome from which will demonstrate not only this radio technique, but also the need for quality vector magnetogram and a more sophisticated field extrapolation and active region modeling technique.

· Mode-coupling As an interesting physical problem, the polarized microwaves can exchange their identities (i.e., whether there are in the x-mode or o-mode) upon passing through the layer of magnetic field polarity reversal if the mode coupling is weak (Cohen 1961, ApJ, 133, 978). An in-depth study of the mode coupling phenomenon requires a high fidelity imaging instruments such as the VLA or the FASR. My major interest is to use the mode-coupling in exploring the current sheet expected in a magnetic cusp structure or Helmet streamer above solar active regions where the exotic mode coupling is expected.

· Active Region Evolution and Flare Productivity A logical extension of the above studies is to proceed on time-dependent evolution of the magnetic fields, current, and temperature in the corona. Comparison of these coronal quantities with corresponding changes in the photosphere may reveal a more direct clue to the coronal heating and the solar activity problem, because it is in the corona where the energy release actually occurs, whereas this kind investigations have so far been limited to the low atmosphere.

2.2 Solar Flare Study

Solar flare study is again a major science that any solar astronomers will want to address with a microwave imaging instrument like the FASR because of its unique sensitivity to energetic electrons and magnetic fields. While I will continue to pursue the above-discussed flare topics (Sect.1.2), I can think of additional topics with the capability of FASR. (1) With the improved sensitivity in imaging of FASR, we can identify weaker sources as well as the most powerful source in a flare, which will allow a more comprehensive model for electron injection/transport, by taking into account the multiple loop interactions, which will be of great interest to solar MHD physicists. (2) With its spectral coverage expanded down to decimeter wavelengths (0.3-3 GHz), we will detect plasma emissions and associated plasma processes. Especially, the yet unknown locations of decimeter bursts become a number one priority science to be explored in this spectral regime. (3) In collaboration with Leon Kocharov (Kocharov et al. 1994ab), we have demonstrated that microwaves should be an essential tool in separating out electron contribution from other contributions (e.g. proton) to the gamma-rays, and that knowledge of the macroscopic magnetic quantities are very useful in physical interpretation of the gamma ray emissions during solar flares. I will continue to study radio counterparts to gamma-ray bursts under a similar strategy.

2.3 Study of Coronal Mass Ejection

The study of Coronal Mass Ejection (CME) is a rapidly growing field in Solar and Space Physics in relation to Space Weather. FASR will be a unique instrument for the CME study, specifically, by providing direct imaging of thermal and nonthermal particles contained in CMEs. I am interested in (1) use of spectral diagnostics to determine temperature, density, and magnetic field strength in CMEs, (2) direct imaging of shocks and wave fronts to locate them relative to other emissions, and (3) nonthermal components of filaments/prominences and other ejecta, to explore particle acceleration in the CME under interaction with shocks and other disturbances. I am also interested in (4) imaging of eruptions and interplanetary disturbances to locate their initial disturbances for the study of the coronal origins of solar energetic particle events.

2.4 Data Analysis Software for FASR

It is expected that the FASR project should also lead to a large software industry, just like we have witnessed in other space and ground-based missions such as RHESSI mission and the VLA. I have been carrying out a preliminary work in this line. With a graduate student, Su-Chan Bong, (a graduate from Korea visiting NJIT), I develop a new imaging technique, called Spatio-Spectral Maximum Entropy Method (SSMEM) which will mainly comprise his thesis. I collaborated with Stephen White in a recent paper, (2002, “The imaging capabilities of the Frequency Agile Solar Radiotelescope,” Proc. SPIE, in press) by White, Lee, Aschwanden, & Bastian.

3. Spectrometer in Decimeter Wavelengths

Spectrometers have commonly used in solar radio studies, which produce dynamic spectrum, a traditional tool for defining physical nature of various radio bursts. Unfortunately there have never been a radio spectrometer operating within the United States, although the OVSA provides such capability, in part, at microwave range (3-30 GHz and above) and decimeter range (0.3-3 GHz)—an example is shown in the below, Figure 4. Fortunately, we are supposed to obtain one or two spectrometers (0.1 to 4.0 GHz) from Prof. Arnold Benz in Swiss Federal Institute of Technology in Zurich, with which I will be interested in the following studies:

3.1 Space weather study

Recent results of radio studies of CME are CME-related shock waves and associated type II/III bursts from the Nobeyama Radioheiographs (NoRH) in Japan and Nancay Radioheliographs in France. It is now widely recognized that the decimeter wavelengths are the most important spectral regime for the Space Weather study, which has relatively been unexplored. Until the operation of the FASR, these spectrometers will be the most powerful tools for Space Weather study within the United States. With the spectrometers, I will be interested in identifying the physical process which drives the CME and its particle propagation, and analyzing the CME-related shock waves. Ongoing collaboration with YongJae Moon in BBSO for analysis of CMEs detected by LASCO is relevant to this project, because some of the results out of this collaboration will assist finding a relationship among CMEs, flares, and filament eruptions, and, in future, with radio bursts.

3.2 Solar flare study

Some of radio bursts in decimeter range (e.g., spikes and type III) are believed to be of solar origin, and therefore these radiations carry information on flare-produced energetic particles and its kinetics. As mentioned, there was never a decimeter spectrometer in the United States, the OVSA has served as a spectrometer-like instrument owing to its high spectral resolution—unusually many frequencies as an interferometer. In OVSA dynamic spectrum, we indeed find some decimetric radio bursts occurring at frequencies <3 GHz, which has nevertheless not much studied so far. Only recently, a systematic study of these low frequency activities, which is seemingly of non-gyrosynchrotron origin, is being carried out by an NJIT graduate, Gelu Nitta, as his thesis project. I participate in this research together with Dale E. Gary, Gregory Fleishman, and Victor Melnikov. Combining this dataset with the above-mentioned spectrometers will be a nice corporation to establish several theoretical speculations made toward decimetric bursts mechanism and associated plasma kinetic processes.

Fig. 4. An example of OVSA dynamic spectrum with frequency increasing upward and time to the right. A total of seven bursts as marked along the top of the figure are considered to be involved with various physical processes such as Razin suppression due to Chromospheric evaporation, magnetic trapping, Coulomb collision. From Lee, Gary, & Shibasaki (2000).

4. Other Projects

In addition to the radio studies, I have kept interest in other areas based on my experience in Big Bear Solar Observatory and some theoretical works.

· Quiet sun magnetic fields The magnetic fields in the quiet region is also challenging problem since they are in small-scales presumably under currently best spatial resolution and also highly dynamic, requiring excellent seeing and instrument. With ever-increasing instrumental capability, I hope to continue such sciences as the magnetic power spectrum of network/intranetwork fields (Lee et al. 1997ab), Moving Magnetic Features (Lee 1993), and size distribution of magnetic flux tubes (Wang et al. 1996). These sciences may be different from the above-mentioned radio sciences, but can nevertheless addressed using similar techniques as used in the radio study. For instance, the seeing correction for true magnetic power spectrum involves Fourier Transform, which is much like the beam correction in radio synthesis imaging. It is also possible to study chromospheric/transition region network structure using radio observations, which will provide spectral diagnostics of temperature, density and magnetic field strength of network elements.

· Prominence and sunspot oscillations Since the discovery of solar supergranular pattern by Leighton in 1962, a tremendous amount of investigations have been carried out for solar oscillations, a discipline known as helio-seismology. Similar, but relatively newer research area would be prominence oscillations, or prominence seismology. Although prominence oscillations are typically measured using spectrographs, we have demonstrated the importance of high-cadence imaging observations for a context observation (Jing et al. 2002). Since I was initially trained with the topic of MHD waves, I am very interested in combining my current radio studies with the topic of oscillations, such as the prominence oscillations and sunspot 3-min oscillations.

· Theoretical Study Finally I have interests in theoretical investigation of the following topics: (1) Numerical simulation of solar radio bursts, in case existing theories are found in a form not suitable for addressing new observations. Specifically, mechanism for the above-mentioned, low-frequency microwave emissions is not well established. (2) Simulation of solar magnetic turbulent spectrum including energy density transfer between magnetic field and turbulence is needed in order to account for the observed power spectrum. (3) Stochastic numerical solutions for Fokker-Planck equations for interpretation of microwave and hard X ray spectral data.

RESEARCH ACCOMPLISHMENTS

Jeongwoo Lee

Software development

· A Fourier-Transform imaging package for use with the radio interferometric data obtained from the Owens Valley Solar Array (OVSA), based on standard techniques such as CLEAN, Maximum Entropy Method, and Forward-Fitting, which is now included as a part of the Solar Software Tree in support of the High Energy Solar Spectroscopic Imager (HESSI) /Gary, Lee, Bong 2001/

· An advanced imaging technique named Spatio-Spectral Maximum Entropy Method (SSMEM), as a variant of traditional MEM to ensure spectral smoothness /Bong, Lee, Gary 2001ab/

· Microwave emission codes for scientific simulation as a part of the FASR project /see next page/

Solar Flare Study

· First-time application of a stochastic acceleration model to microwave observations /Lee & Gary 1994/

· First-time application of a stochastic transport model to microwave observations /Lee & Gary 2000/

· Radiative transfer in 3D magnetic fields for microwave imaging spectroscopy /Lee, Gary, & Zirin 1994/

· Observational study of weak/strong pitch angle diffusions /Lee, Gary & Shibasaki 2000/

· First-time demonstration of an-isotropic pitch-angle injection of electrons /Lee & Gary 2001/

· First-time comparison of electric field inferred from Ha with microwave light-curves /Qiu et al. 2001/

· First-time evidence for time-dependent injection and the electron transport /Lee et al. 2002/

· First-time application of a topological flare model to a solar microwave burst /Lee et al. 2003/

· Analysis of time-dependent change of Hydrogen Balmer lines during a solar flare /Lee et al. 1996/

Active Region Study

· A complete determination of 3D coronal magnetic field above a symmetric sunspot using microwave imaging spectroscopy /Lee et al. 1993ac/

· First-time application of a state-of-art, fully nonlinear force-free field extrapolation from the MEES vector magnetogram for prediction of the coronal heating /Lee et al. 1998a/

· First-time observation of a mode coupling in the presence of coronal currents /Lee et al. 1998b/

· First-time testing the coronal field extrapolations against microwave maps /Lee et al. 1999/

Magnetogram Study

· High-resolution flow maps of the Moving Magnetic Features around sunspots /Lee 1992/

· First-time quantification of the seeing quality in Big Bear Solar Observatory /Lee et al. 1997a/

· First-time reconstruction of the power spectra of network and intranetwork fields /Lee et al. 1997b/

Analysis of Gamma Ray Events

· Analysis of gamma ray bursts observed by the GRANAT satellite and Haleakala neutron monitor with emphasis in the observable macroscopic magnetic fields /Kocharov et al. 1994ab/

Theoretical Study

· A comprehensive formulation of wave generation in a strongly magnetized plasma /Lee 1993/

Research Grants

Jeongwoo Lee

1. Project/Proposal Title: Study of OVSA Microwave Bursts in Conjunction with HESSI Hard X-ray Bursts

Source of Support: NASA Grant NAG 5-10891
Support: Current
Total Award Amount: $145,769

Total Award Period Covered: June 1, 2001 – May 31, 2004

P.I. Effort: 50%

Location of Project: NJIT

2. Project/Proposal Title: The Frequency Agile Solar Radiotelescope:
A technical Study and Implementation Plan

Source of Support: NSF Grant AST-0138317
Support: Current
Total Award Amount: $400,572

Total Award Period Covered: March 1, 2002 – February 28, 2004

Co-Investigator Effort: 25%

Location of Project: NJIT

3. Project/Proposal Title: Solar Microwave Imaging Spectroscopy and Electron Acceleration

Source of Support: NSF Grant AST-9987366
Support: Current
Total Award Amount: $660,000

Total Award Period Covered: April 1, 2000 – March 31, 2003

Co-Investigator Effort: 25%

Location of Project: NJIT

Pending Support

4. Project/Proposal Title: Investigations of Solar Activity with the Owens Valley Solar Array

Source of Support: NSF
Support: Pending
Total Award Amount: $770,036

Total Award Period Covered: April 1, 2003 – March 31, 2006

Co-Investigator Effort: 45%

Location of Project: NJIT