qs003.harvey_k01 Posted: 26-Jan-92 Updated: 06-May-92, 15-Sep-92, 01-Nov-92, 11-Mar-93, 06-Jun-93, 13-Dec-93, 21-Jan-94, 17-Sep-94, 10-JUN-95, 3-Feb-96, 20-Sep-96 Events specified:
Collaboration: K. Harvey, Nitta, Strong, Shimizu, Tsuneta, Hara,
Zirin, Brunner, Hudson, Martin, Tang, Acton, Parnell.
Collaboration with any other Yohkoh scientists, contributions
and/or suggestions are welcome.
Question: What are the physical parameters that characterize coronal bright points?
Motivation: Our current knowledge of x-ray bright points (BP) originates from Skylab x-ray images and a few sounding rockets. These data show that BPs have spatial scales <30 arcsec, a typical lifetime of around 8 hours, and vary in number anti-correlated with the sunspot cycle. The SXT with a lower scatter and higher dynamic range should be capable of studying these phenomena in better detail at a higher cadence and over a longer period than has possible to date. It is the first opportunity to obtain ground-based, time-sequence data simultaneously with a time-series of x-ray images and should result in significant progress in understanding BPs and their relation to the magnetic field and other phenomena.
Required Observations/Analysis Techniques: The goal of this initial study of BPs makes use of existing SXT full-Sun images to investigate some of the basic properties of BP. This is the first in a logical series of observational steps we feel are necessary to improve our understanding of BPs. We hope to determine (1) their spatial distribution and relation to magnetic fields, and (2) their size and lifetime distribution. An important goal will be the determination of an objective definition of a BP. We will measure (a) location and position relative to other coronal and photospheric features. (b) size, temperature and emission measure and (c) lifetime - using extended sequence of SXT images. For longer-term goals we need to build up a uniform data base of full disk SXT images (say 5 arcsec pixel, Al1400 filter, dual exposure at a cadence of 10-15 minutes for every orbit during non-flaring times).
Update 20-Sep-96
Progress and Results on Analysis of Data Taken during XBP Campaigns:
23 days of data have been reviewed for which both SXT PFIs and NSO/KP magnetograms or HeI 10830 observations exist. The total number of XBPs in this data set is 239. The general character- istics of the XBPs are as follows: 133 (53%) show gradual intensity changes, 91 (37%) flared, 9 (4%) showed some suggestion of ejecta or jets. The size range of the XBPs is 10-46 arc-seconds with a lifetime range of 5 min to >5 hours, where 26 (10%) were observed for a period of less than 15 minutes. Intensity changes during XBP flares occur in seconds.
For 52 XBPs observed on 7 days, we find that 85% of the XBPs are associated with one of the following magnetic field changes:
Canceling Bipole = Observed mutual decrease of magnetic flux during
encounters of network elements of opposite polarity.
Emerging Bipole = Magnetic elements of opposite polarity that form
and separate with time.
Emerging/Canceling Bipole = An emerging bipole where one or both poles
encounter and cancel with opposite polarity network.
Eight (15%) of 52 XBPs were associated with no bipole (unipolar
magnetic network), no measurable field, or little change in the field.
Canceling Magnetic Bipoles 64% 33
Emerging Bipoles 8 4
Emerging then Canceling Bipoles 13 7
No Bipole, No measurable B , or
Little change in B 15 8
Association of Magnetic Bipoles to XBPs
All XBPs and have associated HeI 10830 dark points, even the short-lived XBPs. However, not all HeI 10830 dark points have XBP counterparts.
>From this analysis, we conclude that:
The emergence or cancelation of photospheric magnetic flux is not in itself a necessary and sufficient condition for the occurrence of an XBP, and the occurrence and variability of XBPs result from the interaction and reconnection (at times sporadic) of the emerging and canceling magnetic fields with existing, local magnetic fields.This analysis will continue during the next few months and incorporate the data from BBSO and MSO.
The results of the above analysis were presented at a meeting on `Observations of Magnetic Reconnection in the Solar Atmosphere' in March 1996 "Observations of X-ray Bright Points" and submitted as a paper in the Proceedings of this meeting.
New Campaigns in 1996:
In April and May 1996, we aquired new observations of XBPs in collaboration with the Yohkoh/SXT, the MDI, EIT, SUMER, and EIT instruments on SOHO, and several ground-based observatories. During this run, at NSO/Kitt Peak, we recorded the full line profile of HeI 10830 and a nearby SiI line at every pixel in the scan area (512 x 512 arc-sec) using the NSO/KP spectromagnetograph. The cadence of these data is 5 minutes. We are in the process of analyzing these data in collaboration with Matt Penn and Harry Jones. Such data allow us to derive for both the HeI and SiI lines the velocity (measured relative to a terrestrial water vapor line), equivalent width, line depth.
Update 3-Feb-96
Revised Title and Authors
Fundamental Characteristics of Coronal Bright Points
Collaboration: K. Harvey, Nitta, Strong, Shimizu, Tsuneta, Hara,
Zirin, Brunner, Hudson, Martin, Tang, Acton, Parnell.
Collaboration with any other Yohkoh scientists, contributions
and/or suggestions are welcome.
We successfully completed two additional collaborative observing
runs targeting XBPs during the last 6 months. The first took
place 11-16 June 1995 and the second 15, 17-23 September 1995.
Our objective as in the previous runs was to obtain observations
in He I 10830 (at NSO/KP) to study the association and time variability
of He I dark points with the X-ray bright points observed with the SXT.
MSO in both runs and BBSO in the second provided important magnetic
field observations of the target area. Of note was additional
support during September of the full-disk He I 10830 imager at
HAO. On learning of our run, the HAO team re-assembled the imager
and installed it on the roof of the HAO building. Despite bad
weather, they obtained a good long run of observations on one
day of the collaboration.
It was decided not to request additional XBP collaborations until at least the spring when the weather prospects are better and the days are longer, although the Sun is providing many opportunities for observing XBPs without active regions. We are now completing a detailed study of the data obtained to date during the several collaborations accomplished in the last 3 years. The results of this analysis, in addition to publication, will be presented at the Reconnection meeting to be held in Bath, England in March 1996.
Late in 1996, Clare Parnell was invited to join our XBP group. She is interested in models of XBPs and, in particular, testing her model that suggests what type of bright point would be related to a particular type of cancelling magnetic feature. She is studying the data from our first XBP run of 9 December 1992.
Update 10-JUN-95
In the last 6 months, we have continued our attempts at acquiring simultaneous observations of quiet regions of the Sun between the SXT and three observatories, NSO/KP, BBSO, MSO. The purpose of these collaborations was to obtain observations in He I 10830 (at NSO/KP) to study the association and time variability of He I dark points with the X-ray bright points observed with the SXT. BBSO and MSO provided crucial magnetic field observations of the target regions. With generally poor weather conditions most of the winter in the southwest US, the first of two scheduled runs produced few collaborative observations. The second run in April 1995 was delayed by both high solar activity and poor weather conditions. We hope to try this campaign again during June 9-16, 1995.
Keith Strong has been carrying on an XBP statistical survey and found a result that is somewhat surprising. The daily sfm images, de-compressed and aligned, were examined for the first year of SXT operation to determine the variation in the number of XBPs during the decline of Cycle 22. The number of XBPs (corrected for available area of the disk free from bright active regions) were followed from the start of mission through December 1992. Initially, as expected, the number of XBPs increases steadily until about May 1992. Their numbers then level off at about 100 (+/- 25) on the Sun at any given time. This in contrast to the continued decrease in larger-scaled activity.
XBPs were also identified in the polar regions to see if we could detect a hemispheric difference in their numbers as is seen with the lower-latitude active regions. XBPs, observed in the thin Al filter images, were counted in the polar regions separately for each pole, taking into account local variations in brightness that might affect the visibility of fainter XBPs. The XBPs were identified and assigned a visibility rating:
Disappointingly, this did not make the counting decisions any less arbitrary. Though they are at least consistently arbitrary (i.e. is that an XBP or something else? How small a brightening can be considered an XBP? and so on...). To overcome the statistics of small numbers, 30 day sums (corrected for deadtime) were made.
- unobstructed view (>90%) of pole
- partially visible (>50%) or thinly veiled
- poor visibility (<50%) or thickly veiled
- bad visibility - bright coronal emission veiling pole
- Un-usable data (e.g., heavy SAA contamination)
For the Visibility 1 data:
1) There was no significant difference in the total monthly averaged
XBP count between the N & S poles. The data hint at a seasonal difference which is reassuring because it seems to correspond to the change in B angle (hence visibility).2) In the first year of the mission, when the total number of XBPs on the Sun Initially rose significantly, the number of XBPs near the poles remained essentially constant (within 1 sigma error bars).
3) The same trends are seen in the Visibility = 2 & 3 data, but, of course, the numbers are much lower.
Questions we are examining to understand the results found:
1) Could this mean that the apparent anti-correlation of XBP numbers is
wrong? Or, it is merely a visibility problem? That is, as the quiet sun intensity drops as the Sun dips towards minimum the XBPs that are actually there all the time become more visible (emerge from the fog).2) Is there something special about the poles (or coronal holes) that could possibly invalidate this statistical approach used?
The results from this work will be presented at the 1995 Solar Physics Division Meeting held in Memphis, Tennessee June 4-8. Abstract submitted:
Contrasting the Solar Cycle Effects in Large- and Small-Scale Magnetic Structures: K. T. Strong and K. L. Harvey
We present data from the Soft X-ray Telescope (SXT) on Yohkoh on the statistics of coronal magnetic structures from September 1991 to April 1995. The SXT provides a unique probe of solar cycle effects on coronal structures from a long-term, uniform, full-disk data base of soft X-ray images of the Sun. We use this data base to look at the variation in the numbers of various types of magnetic features. In particular, we look at the change in numbers of X-ray bright points during the decline of Solar Cycle 22, not only for the Sun as a whole but as a function of location on the Sun. For the overall Sun we find, as expected, that Initially the number of XBPs increases steadily until about May 1992. Their numbers then stabilize at about 100 (+/- 25) on the Sun at any given time. The behaviour of these small bipolar structures is in direct contrast to that of their larger brethren, active regions, which have declined in number, size, and longevity since launch. The number of large-scale, weak-field eruptions has also steadily decreased throughout the mission lifetime of Yohkoh. It will be interesting to see how these numbers recover after solar minimum.
This work was supported by NASA contract NAS8-37334 with the Marshall Space Flight Center and the Lockheed Independent Research Program.
Update 17-Sep-94
Since the last update, we have attempted four XBP campaigns with mixed results. The campaign in Jan 1994 was cancelled due to high solar activity (at least for a quiet Sun program). We were, however, successful in early March, late April and mid-June 1994 in proceedingwith the collaborative observations of XBPs between the Yohkoh SXT and with three ground-based observatories - NSO/KP, MSO, and BBSO. Inter- mittent cloudy weather resulted in these runs being only relatively successful. Loren Acton was the SXT Chief Observer during the March run; he selected two very good areas with a great many XBPs. During a visit to Japan in late March, we were able to put together an effective IDL package that produces clean registered PFIs of the target areas. About 20 XBPs, some with durations as short a few minutes, were observed during this period.
In the April and June campaigns, NSO/KP choose to take area scans in He I 10803. We relied on MSO and BBSO for the photospheric magnetic field observations. The target selection was done by K. L. Harvey. The data from the April run indicates many XBPs, unlike the June period. Analyis of these data show that all of the XBPs have a signature in He I 10830, and that there are considerably more He I `dark points' than XBPs. Poor weather resulted in NSO/KP being able to acquire observations only a couple of days during each of the spring runs. We plan to try this observing program again this fall when the summer rainy season is finished in Arizona.
A paper on the X-ray, magnetic field, and He I 10830 comparisons in the quiet Sun and in XBPs was presented at the AGU/SPD meeting in May 1994. The title of this paper is `X-Ray Bright Points: The Signature of Magnetic Field Reconnection' with the following authors: Karen L Harvey, Frances Tang, Hal Zirin, Sara Martin, Hugh Hudson, Don Mickey, Marilyn Bruner, James Lemen, Keith Strong, Nariaki Nitta, Saku Tsuneta. Loren Acton should be added to this list.
Update 21-Jan-94
Requested in early January 94 a repeat of the XBP campaign to take place between Yohkoh and the ground-based observatories, NSO/KP, BBSO, and Mees beginning Jan 19. Conditions for proceeding with the coordinated observations rests on solar activity levels being low. The objective of this campaign is to obtain the high possible time resolution observations of XBPs with simultaneous magnetic field observations.
Paper, entitled `Are X-ray Bright Points the Signature of Magnetic Field Reconnection?', submitted by K. L. Harvey, K. S. Strong, N. Nitta, and S. Tsuneta for publication in the Proceedings of the National Solar Observatory/Sacramento Peak Summer Workshop Series on `Solar Active Evolution -- Comparing Models with Observations'. This paper discusses with specific examples (from observations obtained during our collaborative campaigns) the complex association of XBPs and their formation and variation with the underlying magnetic field. These associations are with (1) the emergence of magnetic bipoles, (2) the cancellation of magnetic bipoles, (3) with both the emergence and subsequent cancellation of magnetic bipoles, (4) with unchanging bipoles, and (5) with no magnetic bipole. Despite the large variety of associations, most XBP are related to the cancellation of magnetic flux at the solar surface, though only about half of these cancellation sites have observed XBPs.
Update 13-Dec-93
A third XBP campaign took place 29 Nov, and 2-3 Dec 1993. This campaign was a collaboration between the SXT on Yohkoh, NSO/KP, BBSO, MSO. Our target was a long-lived coronal hole in the northern hemisphere. The primary objective was to observe XBPs with high time-resolution, full spatial resolution PFIs of the target: a cadence of 8 sec was obtained on 29 November and 16 sec on the last two days of the run. A quick look at the data shows several bright points, many of which show rapid variations at the time scale of the observations. The three ground-based observatories had a mixture of weather, but we were able to acquire magnetograms of the target on all three days.
Analysis continues on the May 1993 successful collaborative effort. We plan to present the results of both of the runs (and future ones) at the SPD/AGU meeting in Baltimore, MD in May 1994. With the current lower solar activity levels, we plan to request additional collaborative observations of XBPs to increase the data base in order to address the nature of these structures.
Update 06-Jun-93
With the quiet conditions currently on the Sun, we are initiating another XBP campaign to take place 17 through 20 May 1993. Our campaign, a collaboration between the SXT on Yohkoh, NSO/KP, BBSO, MSO, is aimed at repeating the December effort with the following changes: the addition of dark frames to the sequence and longer exposures.
Update 11-Mar-93
Analysis of data obtained during collaborative observing program between the SXT on Yohkoh and the NSO/KP magnetograph on 9 and 10 December 1992. This was a repeat of the program attempted 19/20 October 1992, which was unsuccessful due to cloudy weather at Kitt Peak and higher solar activity. High temporal and spatial resolution PFI observations of were obtained to determine the scale of XBP intensity and structural variations. SXT observations consisted of 2 x 2 full-resolution map, 5x5 arcmin area, a time resolution of 32 seconds, and maximum exposure of 5 seconds during 4 orbits. Coordinated magnetic field observations were taken by NSO/KP spectromagnetograph at a cadence of 7.5 minutes and spatial resolution of 2.3 arc-sec. The analysis to date has concentrated on the data acquired on December 9, the best of the days in terms of coverage and clear weather at Kitt Peak. In total, 19 XBPs were observed in the field of view. All were associated with magnetic bipoles; 4 were static, 12 were cancelling, and 3 were emerging. 17 of the XBPs showed rapid intensity variations; the strongest was an increase of 200 counts, a factor of 2 increase, within 32 seconds, indicating that we are not resolving fully the temporal variations in the intensity of XBPs. The intensity variations of XBPs occurred over a significantly shorter time than observed changes in the associated magnetic field. It is concluded that XBP are the result of reconnection of magnetic fields, and not specifically to the cancellation or emergence of magnetic flux. The results from this analysis were presented at a Yohkoh meeting at ISAS in February 1993, and will be published in the Proceedings of that meeting.
Based on the results of this study, we propose changes in future collaborative programs to improve the time resolution of the SXT partial frame images and the simultaneous magnetograms. Additional observational support from Hawaii, Japan, and China have been planned to extend the magnetic field time base.
Statistical Study of XBPs: We are developing a detailed plan of research using current SXT data to sample XBPs during the year and a half of operation of Yohkoh. We are considering the specific properties and methods of analysis of the data to determine time variation in the counts of BPs, their association with magnetic fields and He I 10830 dark points, lifetimes, temperatures, temporal variations, solar rotation rates.
Update 14-Oct-92
On October 19 and 20, 1992, we plan a trial observing run targeting X-ray bright points, pending solar activity. The observations will be made during two orbits to determine the optimum cadence and spatial resolution for studying the intensity and structural changes in bright points. Collaborative observations will be made of the target with the spectromagnetograph at National Solar Observatory/Kitt Peak. The requested program is as follows:
High Temporal and Spatial Resolution PFI Observations of X-ray Bright Points
To best utilize the PFIs for study of BPs to determine the scale of BP intensity and structural variations, we propose the following observational parameters for this program.
Time resolution: : 32 seconds
Size of Observing Area : 2 (NS) x 2 (EW) map, 5 x 5 arcmin
Pixel summation : Full-resolution
Maximum Exposure : 5 seconds
Location on Sun : Coordinates to be provided by K.L. Harvey
Oct. 19 and Oct. 20, 1992
Type of feature : XBPs
Filters : Mixture of several Thin Al.1 and one AlMg,
the several to be decided
Aspect images desired : FFIs
Coordinated observations: yes, if clear, with NSO/Kitt Peak
Suggested number of
repeats of experiment : repeat for two orbits
Special constraints : low activity
This program is to run in conjunction with the established movie FFI sequence
and is to be set up to revert to a flare mode if one of interest should occur.
To assist in the selection of the target, we request that, if possible, one FFI Al.1 image from first orbit be placed on an isass disk as quickly as possible for comparison with He I spectroheliogram and magnetogram. Once observations are completed, we also would appreciate it if the reformatted SXT data were available on the disk to bring over to Tucson and Lockheed for analysis.
Should it be cloudy at Kitt Peak and activity remains low, this experiment should continue, with the selection of the BP target to be determined at the discretion of the Tobans or by previous day's NSO observation
Update 15-Sep-92
Keith Strong presented a well-received paper at the COSPAR Meeting late August 1992 entitled `Lifetimes and Distribution of Coronal Bright Points Observed with Yohkoh'; authors K. L. Harvey, K. T. Strong, N. Nitta, S. Tsuneta. This paper will be published as part of the Proceedings of this meeting.
Update 06-May-92
A paper is being presented on this topic at the AAS/SPD meeting 92/5/5. A paper, entitled `Observations of the Variability of Coronal Bright Points' has been drafted for inclusion in the special issue of PASJ in October 1992. Authors: K. Strong, K. Harvey, T. Hirayama, N. Nitta, t. Shimizu, S. Tsuneta.