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Determination of Magnetic Helicity Content of Solar Active Regions from SOHO/MDI Magnetograms  

Jongchul Chae   Submitted: 2004-05-14 23:44

Chae (2001) first proposed a method of self-consistently determining the rate of change of magnetic helicity using a time series of longitudinal magnetograms only, such as taken by SOHO/MDI. Assuming that magnetic fields in the photosphere are predominantly vertical, he determined the horizontal component of velocity by tracking the displacements of magnetic flux fragments using the technique of local correlation tracking (LCT). In the present paper, after briefly reviewing the recent advance in helicity rate measurement, we argue that the LCT method can be more generally applied even to regions of inclined magnetic fields. We also present some results obtained by applying the LCT method to the active region NOAA 10365 under emergence during the observable period, which are summarized as follows. (1) Strong shearing flows were found near the polarity inversion line that were very effective in the helicity injection. (2) Both the magnetic flux and helicity of the active region steadily increased during the observing period, and reached 1.2 imes 1022 Mx and 8 imes 1042 Mx2, respectively, 4.5 days after the birth of the active region. (3) The corresponding ratio of the helicity to the square of the magnetic flux, 0.05, is roughly compatible with the values determined by other studies using linear-force-free modelings. (4) A series of flares took place while the rate of helicity injection was high. (5) The choice of a smaller window size or a shorter time interval in the LCT method resulted in a bigger value of the LCT velocity and a bigger value of the temporal fluctuation of the helicity rate. (6) Nevertheless when averaged over a time period of about one hour or longer, the average rate of helicity became about the same within about 10 %, almost irrespective of the chosen window size and time interval, indicating that short-lived, fluctuating flows may be insignificant in transferring magnetic helicity. Our results suggest that the LCT method may be applied to 96-minute cadence full-disk MDI magnetograms or other data of similar kind, to provide a practically useful, if not perfect, way of monitoring the magnetic helicity content of active regions as a function of time.

Authors: Chae, J., Moon, Y.-J., Park, Y.-D.
Projects: Soho-MDI

Publication Status: 2004 Solar Physics, in press
Last Modified: 2004-06-15 18:02
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Observational Determination of Rate of Magnetic Helicity Transport through the Solar Surface via Horizontal Motion of Field Line Footpoints  

Jongchul Chae   Submitted: 2001-09-10 17:44

Magnetic helicity may be transported to the solar corona through the solar surface either via the passage of helical magnetic field lines from below or via the shuffling of footpoints of pre-existing coronal field lines. This Letter presents how to observationally determine the rate of magnetic helicity transport via photospheric footpoint shuffling from a time-series of line-of-sight magnetograms. Our approach is not confined to the previously known shear motion such as differential rotation, but can be exploited to search for the possible existence of physically significant shear motions other than differential rotation. We have applied the method to a 40 h run of high resolution magnetograms of a small active region (NOAA 8011) taken by Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SOHO). It is found in this region that the rate of magnetic helicity transport oscillates with periods of one to several hours. Our result suggests that the time-series analysis of helicity transport rate might be a useful observational diagnostic for the role of photospheric flows in the evolution of coronal magnetic fields in solar active regions.

Authors: Chae, J.
Projects:

Publication Status: ApJ Letters (in press)
Last Modified: 2001-09-10 17:44
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The Magnetic Helicity Sign of Filament Chirality  

Jongchul Chae   Submitted: 2000-10-13 15:24

A solar prominence has either dextral or sinistral chirality depending on its axial field direction. We determine the magnetic helicity sign of filaments using high-resolution observations performed by Transition Region And Coronal Explorer. At EUV wavelengths, filaments sometimes appear as mixtures of bright threads and dark threads. This characteristic has enabled us to discern overlying threads and underlying ones and to determine the sign of magnetic helicity based on the assumption that the helicity sign of two crossing thread segments is the same as that of the filament. Our results support the notion that dextral filaments have negative magnetic and that sinistral filaments have positive helicity.

Authors: Chae, J.
Projects:

Publication Status: The Astrophysical Journal, 540:L115-L118,2000 September 10
Last Modified: 2000-10-13 15:24
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Active Region Loops Observed with SUMER on board SOHO  

Jongchul Chae   Submitted: 2000-02-02 17:02

We study the emission and dynamical characteristics of transition region temperature plasmas in active region magnetic loops by analyzing a high resolution, limb observation of NOAA 7962. The observations were performed by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on board the Solar and Heliospheric Observatory ( SOHO). The SUMER observation produced a set of raster scan data of the region, in the four lines, H I Lyeta lambda1025, O VI lambdalambda1032, 1038, and C II lambda1037 lambda1037. The data were used to construct the intensity, velocity, and line width maps of the active region, from which more than ten well-resolved loops are identified and classified into four different groups. We determine physical parameters of the loops such as their diameter, length, temperature, line-of-si- line-of-sight velocity, and non-thermal broadening. It turns out that there is a distinction between stationary loops and dynamic loops. The dynamic loops have large bulk motions and large non-thermal broadenings. Some of the dynamic loops appear to be erupting, and display large velocity shears with the sign of line-of-sight velocities changing across the loop axes -- which might be attributed to rotational motions. The inferred rotational velocity has a value of up to 50 km s-1, which may imply the existence of magnetic twist with a field strength of a few Gauss. There are indications that non-thermal broadening is the result of magnetohydrodynamic turbulence inside the loops. Based on our observations, we postulate that when loops erupt, some of the kinetic and magnetic energy cascades down to turbulent energy which would be dissipated as heat.

Authors: Chae, J., Wang, H., Qiu, J., Goode, P. R., and Wilhelm, K.
Projects:

Publication Status: 2000, ApJ, v533 (Apr 20 issue), in press
Last Modified: 2000-02-02 17:02
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Small-scale Magnetic Reconnection in the Quiet Sun  

Jongchul Chae   Submitted: 2000-02-02 17:02

Recent observations indicate that small-scale magnetic reconnection ubiquitously occurs in the quiet Sun as well as in active regions. Particularly interesting is the latest finding of close association among (1) magnetic flux cancellation seen from photospheric magnetograms, (2) chromospheric upflow events seen in the Hα line and (3) transition region explosive events seen in UV lines. I present a brief review of this finding and propose a schematic two-step magnetic reconnection model to explain it. The essence of the model is in the formation of magnetic islands as a result of slow magnetic reconnection occurring in the low atmosphere. Magnetic islands are ejected and accelerated upward by the so-called diamagnetic melon seed mechanism, and are eventually annihilated by overlying magnetic field lines through fast magnetic reconnection. I consider photospheric flux cancellation as a direct result of the first magnetic reconnection, and identify chromospheric upflow events with upward moving magnetic islands, and explosive events with the hotter material ejected from the second magnetic reconnection.

Authors: Chae, J.
Projects:

Publication Status: 1999, 19th NSO/SP International Workshop on High Resolution Solar Physics: Theory, Observations, and Techniques, ASP Conf. Ser.
Last Modified: 2000-02-02 17:02
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Extreme-Ultraviolet Jets and Hα Surges in Solar Microflares  

Jongchul Chae   Submitted: 2000-02-02 16:05

We analyzed simultaneous EUV data from the Transition Region and Coronal Explorer and Hα data from Big Bear Solar Observatory. In the active region studied, we found several EUV jets that repeatedly occurred where pre-existing magnetic flux was "canceled" by newly emerging flux of opposite polarity. The jets look like Yohkoh soft X-ray jets, but are smaller and shorter lived than X-ray jets. They have a typical size of 4000- 10000 km, a transverse velocity of 50-100 km s-1, and a lifetime of 2-4 minutes. Each of the jets was ejected from a looplike bright EUV emission patch at the moment that the patch reached its peak emission. We also found dark Hα surges that are correlated with these jets. A careful comparison, however, revealed that the Hα surges are not cospatial with the EUV jets. Instead, the EUV jets are identified with bright jetlike features in the Hα line center. Our results support a picture in which Hα surges and EUV jets represent different kinds of plasma ejection -- cool and hot plasma ejections along different field lines -- which must be dynamically connected to each other. We emphasize the importance of observed flux cancellation and a small erupting filament in understanding the acceleration mechanisms of EUV jets and H surges.

Authors: Chae, J., Qiu, J., Wang, H., and Goode, P. R.
Projects:

Publication Status: 1999, ApJ, 513, L75
Last Modified: 2000-02-02 16:05
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Non-Coplanar Magnetic Reconnection as a Magnetic Twist Origin  

Jongchul Chae   Submitted: 2000-02-02 16:04

Recent studies show the importance of understanding three-dimensional magnetic reconnection on the solar surface. For this purpose, I consider non-coplanar magnetic reconnection, a simple case of three-dimensional reconnection driven by a collision of two straight flux tubes which are not on the same plane initially. The relative angle heta between the two tubes characterizes such reconnection, and can be regarded as a measure of magnetic shear. The observable characteristics of non-coplanar reconnection are compared between the two cases of small and large angles. An important feature of the non-coplanar reconnection is that magnetic twist can be produced via the re-ordering of field lines. This is a consequence of the conversion of mutual helicity into self helicities by reconnection. It is shown that the principle of energy conservation when combined with the production of magnetic twist puts a low limit on the relative angle between two flux tubes for reconnection to occur. I provide several observations supporting the magnetic twist generation by reconnection, and discuss its physical implications for the origin of magnetic twist on the solar surface and the problem of coronal heating.

Authors: Chae, J.
Projects:

Publication Status: 1999, J. Kor. Astr. Soc. 32, 137
Last Modified: 2000-02-02 16:04
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Comparison of Transient Network Brightenings and Explosive Events in the Solar Transition Region  

Jongchul Chae   Submitted: 2000-02-02 16:03

The relation between transient network brightenings, known as blinkers, and explosive events is examined based on coordinated quiet Sun observations in the transition region line O V lambda630 recorded by the Coronal Diagnostic Spectrometer (CDS), and in the transition region line Si IVlambda1402 recorded by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument, and in photospheric magnetograms taken by the Big Bear Solar Observatory videomagnetograph. From these observations, we find (1) that explosive events, which are traditionally defined as features with very broad UV line profiles, tend to keep away from the centers of network brightenings, and are mostly located at the edges of such brightenings; (2) that CDS blinkers consist of many small-scale, short-lived SUMER ``unit brightening events'' with a size of a few arc seconds and a lifetime of a few minutes; and most importantly (3) each SUMER ``unit brightening event'' is characterized by a UV line profile which is not as broad as those of explosive events, but still has significantly enhanced wings. Our results imply that individual unit brightening events involve as high velocities as explosive events do, and, hence, blinkers may have the same physical origin as explosive events. It is likely that transient network brightenings and explosive events are both due to magnetic reconnection -- but with different magnetic geometries.

Authors: Chae, J., Wang, H., Goode, P. R., Fludra, A., and Schuehle, U.
Projects:

Publication Status: 2000, ApJ. 528, L119
Last Modified: 2000-02-02 16:03
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Abstracts by Author
Determination of Magnetic Helicity Content of Solar Active Regions from SOHO/MDI Magnetograms
Observational Determination of Rate of Magnetic Helicity Transport through the Solar Surface via Horizontal Motion of Field Line Footpoints
The Magnetic Helicity Sign of Filament Chirality
Active Region Loops Observed with SUMER on board SOHO
Small-scale Magnetic Reconnection in the Quiet Sun
Extreme-Ultraviolet Jets and H alpha Surges in Solar Microflares
Non-Coplanar Magnetic Reconnection as a Magnetic Twist Origin
Comparison of Transient Network Brightenings and Explosive Events in the Solar Transition Region

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