Donald L. Mickey

The morphology of flare phenomena, magnetic fields, and electric currents in active regions. I - Introduction and methods

This paper introduces a study of electric current systems in solar active regions and their spatial relationship to sites of electron precipitation and high pressure in flares. The primary purpose of this study is to provide observational evidence for or against the flare models commonly discussed in the literature. We determine the photospheric distribution of vertical currents from vector magnetograms. We use Hα line profiles to identify sites of intense nonthermal electron precipitation into the chromosphere and of high pressure in the overlying corona. By observing complete Hα spectra rather than just narrow-band images, we are able to distinguish between electron precipitation and high-pressure sites in the observed flares

THE IMAGING VECTOR MAGNETOGRAPH AT HALEAKALA

We describe an instrument we have built and installed at Mees Solar Observatory on Haleakala, Maui, to measure polarization in narrow-band solar images. Observations in Zeemansensitive photospheric lines have been made for nearly all solar active regions since the instrument began operations in 1992. The magnetograph includes a 28-cm aperture telescope, a polarization modulator, a tunable Fabry-Pérot filter, CCD cameras and control electronics. Stokes spectra of a photospheric line are obtained with 7 pm spectral resolution, 1 arc sec spatial resolution over a field 4.7 arc min square, and polarimetric precision of 0.1%. A complete vector magnetogram observation can be made every eight minutes. The flexibility of the instrument encourages diverse observations: besides active region magnetograms we have made, for example, composite vector magnetograms of the full solar disk, and Hα polarization movies of flaring regions.

Magnetic Free Energy in NOAA Active Region 10486 on 2003 October 29

We calculate the total and the free magnetic energy for solar NOAA active region 10486 on 2003 October 29 using chromospheric vector magnetograms observed with the Imaging Vector Magnetograph at Mees Solar Observatory in the Na I λ5896 spectral line. The magnetic energy is derived from the magnetic virial theorem using observations spanning the X10 flare that occurred at 20:39 UT. Although poor atmospheric seeing prevented us from discerning changes in the free energy associated with the flare, there was an unusually large amount of free magnetic energy in NOAA AR 10486: (5.7 ± 1.9) × 1033 ergs, which is consistent with the very high level of activity observed in this active region. It is thus plausible that the extreme activity was powered by the magnetic free energy.

FORCE-FREENESS OF SOLAR MAGNETIC FIELDS IN THE PHOTOSPHERE

It is widely believed that solar magnetic fields are force-free in the solar corona but not in the solar photosphere at all. In order to examine the force-freeness of active region magnetic fields at the photospheric level, we have calculated the integrated magnetic forces for 12 vector magnetograms of three flare-productive active regions. The magnetic field vectors are derived from simultaneous Stokes profiles of the Fe I doublet λλ6301.5 and 6302.5 obtained by the Haleakala Stokes Polarimeter of Mees Solar Observatory, with a nonlinear least-squares method adopted for field calibration. The resulting vertical Lorentz force normalized to the total magnetic pressure force |Fz/Fp| ranges from 0.06 to 0.32 with a median value of 0.13, which is smaller than the values (~0.4) obtained by Metcalf et al., who applied a weak field derivative method to the Stokes profiles of Na I λ5896. Our results indicate that the photospheric magnetic fields are not so far from force-free as conventionally regarded. As a good example of a linear force-free field, NOAA Active Region 5747 is examined. By applying three different methods (a most probable value method, a least-squares fitting method, and comparison with linear force-free solutions), we have derived relatively consistent linear force-free coefficients for NOAA AR 5747. It is found that the scaled downward Lorentz force (|Fz/Fp|) in the solar photosphere decreases with increasing |α|. Our results also show that the force-freeness of photospheric magnetic fields depends not only on the character of the active region but also on its evolutionary status.

The X3 Flare of 2002 July 15

An X3-class flare occurred on 2002 July 15 with white-light emission and a complex filament eruption. Observations were made in the optical continuum, Hα, UV continuum, microwave, and soft X-rays, as well as with high-cadence longitudinal magnetograms. Within the preflare phase, intense heating is observed accompanying upward motion of the filament. At the onset of the impulsive phase, filament Doppler acceleration is increased from -1.5 to -7.0 km s-2. Flare impulsive emission is double-peaked, possibly corresponding to two magnetic reconnection events: the first occurs above the active region in the corona, while the second takes place in a thin current sheet underneath the eruptive filament. It is probable that a twisted helical flux rope, seen in C IV TRACE images, is formed during the second reconnection. The energy released by the white-light flare is ~1033 ergs and dominates the flare emission spectra. Within the flare impulsive phase, the emission profiles show both abrupt and gradual components in white light, UV, and Hα. These variations are independently reflected in the transverse motions of flare kernels: the abrupt emission phase corresponds to a rapid kernel motion, while the gradual phase corresponds to a more modest kernel motion.

Electric currents and coronal heating in NOAA active region 6952

We examine the spatial and temporal relationship between coronal structures observed with the soft X-ray telescope (SXT) on board the Yohkoh spacecraft and the vertical electric current density derived from photospheric vector magnetograms obtained using the Stokes Polarimeter at the Mees Solar Observatory. We focus on a single active region: AR 6952 which we observed on 7 days during 1991 December. For 11 independent maps of the vertical electric current density co-aligned with non-flaring X-ray images, we search for a morphological relationship between sites of high vertical current density in the photosphere and enhanced X-ray emission in the overlying corona. We find no compelling spatial or temporal correlation between the sites of vertical current and the bright X-ray structures in this active region.

THE PROSPECTS FOR ASTEROSEISMOLOGY FROM GROUND-BASED SITES

We reexamine the possibility of detecting p-mode oscillations in Sun-like stars with ground-based telescopes. Previous attempts to make such observations with photometric techniques have been limited to subgiant stars in M67 and have illustrated the great difficulties involved in performing ground-based asteroseismology. Substantial gains in observing efficiency can be realized from new diagnostic techniques and improvements in instrumentation, especially with newer CCD camera systems. We show that for appropriately selected field stars observed with a network of telescopes or at a high duty cycle site, it will be possible to detect p-mode oscillations from the ground. An alternative to a network of telescopes for asteroseismology would be to develop a dedicated observatory for this purpose at a high duty cycle site, i.e., the South Pole. We estimate the scintillation, the main noise source in asteroseismology, at the pole by modeling the index of refraction structure parameter from meterological data. The model results show that at the Pole the variance of the relative intensity fluctuations--i.e., the scintillation--should be a factor of 5 smaller than at at Mauna Kea. Taking into account the improvements possible with target selection and instrumentation, the South Pole would be an excellent site for asteroseismological work on Sun-like stars.

The magnetic evolution of the activity complex AR 7260: A roadmap

The active region NOAA 7260 rotated onto the north solar hemisphere as a mature bipole: a dominant negative-polarity sunspot with trailing plage and scattered small spots in attendance. The dominantp spot itself had strong magnetic fields and covered almost 400 × 10−6 of a solar hemisphere. For a period of seven days beginning 14 August, 1992 this active region displayed rapid and drastic evolution: no fewer than 50 magnetic bipoles emerged in the area trailing the large sunspot, increasing the regions magnetic flux by more than 1022 Mx. This new group of sunspots formed a complexβγδ configuration with twoδ spots and a high degree of magnetic shear.This region was very well observed byYohkoh and various ground-based instruments. It presented opportunities to study new emerging flux, flares, and also the decay of a large sunspot. For the benefit of later studies we present a description of the global characteristics of this active region, a detailed ‘roadmap’ of its evolution during disk passage including the development of the twoδ regions. We compare proper motion trends and flaring activity to observations of otherδ-spots reported in the literature. We also comment on the observed outflow of magnetic elements from the decayingp spot.

OBSERVATIONS OF HELIUM AND HYDROGEN EMISSION IN QUIESCENT PROMINENCES

Observations of a number of helium triplet (λλ 10830, 4713, 4471, 3889, 4026) and hydrogen (Hγ, δ, η, θ) emission line intensities in six quiescent prominences are presented. The regions of prominence and neighboring corona were raster-scanned by the telescope, and all lines were measured concurrently at each point. The instrumental field of view was 5″ × 20″. The results are compared with previous observations and theory. In particular, the intensity of the λ 10830 emission relative to the other triplets is found to differ strongly from the predictions of the recent detailed calculations of Heasley et al. (1974) for model quiescent prominences.

The Mees CCD imaging spectrograph

The Mees CCD (MCCD) instrument is an imaging spectroscopy device which uses the 25 cm coronagraph telescope and the 3.0 m Coudé spectrograph at Mees Solar Observatory (MSO) on Haleakala, Maui. The instrument works with resolving power up to R ≈ 200 000 with significant throughput from λ3934 Å (Caii K) to λ ≈ 10 000 Å. A fast guiding active mirror stabilizes the image during observations. A rapidly writing magnetic tape storage system allows observations to be recorded at 256 kbytes s−1. Currently, the MCCD is used for imaging spectroscopy of solar flares at λ6563 Å (Hα), and velocity measurements of umbral oscillations; future plans include emission line studies of active region coronae, and photospheric studies of solar oscillations.