qs066.lean01 Posted: 06-Nov-93 Updated: 08-Mar-94, 08-Oct-94, 29-Jun-95, 08-Apr-96 Events specified: N/A
COLLABORATORS: Judith Lean, John Mariska and George Doschek (NRL); Keith Strong (Lockheed); Hugh Hudson, Harry Warren, Loren Acton
KEYWORDS: Full-disk variability, White-light and Coronal Images, Soft X-ray spectrum.
SCIENTIFIC QUESTIONS TO BE INVESTIGATED: What is the amplitude of the temporal variability in the disk-integrated coronal flux? What are the sources of this variability, and how are these sources related to other magnetic phenomena, as seen in white light and CaK images, and magnetograms? What is the variability of the spectrum defined by the SXT coronal filters?
METHOD: The variability of the full disk coronal flux is determined by integrating the SXT images over the entire disk. Temporal changes in this integrated flux arising from solar activity must be separated from instrumental effects such as sensitivity changes. Redundancy is provided by using SXT images with different filters. The variability sources are investigated by constructing (for example) intensity distribution histograms from the images. Statistical comparisons (correlations, periodograms, demodulation) of the disk-integrated signals and the signals from portions of the histograms with other solar activity indicators (Rx, F10. He EQ) that reflect solar activity in different layers of the solar atmosphere, provide clues about the magnetic origins of the variability sources. Comparisons of the SXT images with other solar images can help to identify and characterize common variability sources. NRL compilations of the emission strengths and the temperature sensitivities of coronal lines will be used to determine the spectral variability. Ultimately, this work will lead to improved parameterizations of the solar soft X-ray fluxes for aeronomic applications such as solar forcing of the NO concentrations in the terrestrial thermosphere and mesosphere, and the impact of this on atmospheric ozone concentrations.
DATA AND INSTRUMENT TO BE USED: The SXT coronal and white light images are the relevant Yohkoh data. CaK images from the BBSO will also be used, together with a variety of solar activity indicators.
PUBLICATIONS AND PRESENTATIONS: Plan to present preliminary results at the joint AGU/SPD meeting in Spring 1994.
Update 08-Apr-96
Please add Harry Warren and Loren Acton to this effort, in addition to the original team.
Work on this topic has benefited greatly in the past six months by addition of a post doc - Dr. Harry Warren - to this effort, and by interactions with Dr. Loren Acton about SXT analysis and interpretation. The focus of recent effort is the identification and quantification of the large scale X-ray variability sources to the full disk SXT fluxes. For the purpose of specifying their contributions to the full disk flux, an empirical technique has been developed to identify coronal holes in the Yohkoh SXT images. CaK images from the BBSO are used to provide information about active region sources. These tasks contribute to the on-going development of a model of solar X-ray spectral irradiance variability that utilizes emission measure distributions and the Mewe spectra determined separately for the different variability sources. As well, a preliminary comparison of SXT the GOES X-ray fluxes points to the importance of multiple flaring events to the full disk irradiance.
Preliminary results were presented at the Fall AGU meeting:
H. P. Warren et al., A Physics-based model of solar irradiance variability from 1-300 A, Abstract SH42B-1, Eos, 1995 Fall Meeting, page F469.
J.T. Mariska et al., Application of a physics-based model of solar irradiance variability to the CIV data from SOLSTICE on UARS, Abstract SH51C-17, Eos, 1995 Fall Meeting, page F479.
Update 29-Jun-95
A paper resulting from this work has recently been published in Geophysics Research Letters (Vol 22, p. 655, 1995): "Correlated brightness variations in solar radiative output from the photosphere to the corona" by Lean, Mariska, Strong, Hudson, Acton, Rottman, Woods and Willson. The abstract is as follows:
ABSTRACT. Correlated brightness variations are shown to occur in time series of coronal soft X-rays exclusive of prominent active regions, chromospheric ultraviolet radiation, and the photospheric total solar irradiance corrected for sunspot effects. These temporal correlations suggest that upwardly extending magnetic fields may have a large scale impact on the solar atmosphere in addition to their demonstrable role of generating localized active regions. The correlations have implications for improving and extending solar spectrum variability models.
The focus of this paper was solar variations during 1992. Work is continuing on this topic using data in 1993 and 1994, with the specific goal of determining the solar cycle amplitude of the soft X-ray fluxes for application to aeronomy. Future plans are to utilize Ca K images and magnetograms to better characterize the sources of irradiance variability throughout the entire solar atmosphere, extending into the corona. This may allow tracking of magnetic activity phenomena through different levels of solar activity. Dr. Harry Warren, a recent NRL postdoctoral appointment, has joined this research topic.
Overlap: qs053 (hara), qs081 (acton), mi005 (acton)
Update 08-Oct-94
Coherent Brightness Variations in Solar Radiative Output from the Photosphere to the Corona
J.L. Lean, J.T. Mariska, K.T. Strong, H.S. Hudson, L.W. Acton, G.J. Rottman, T.N. Woods, R.C. Willson for Submission to Geophysical Research Letters (DRAFT-25 August 1994) Abstract. Coherent brightness variations are shown to occur in coronal soft X-rays, chromospheric ultraviolet radiation and total solar irradiance emitted primarily from the photosphere. This suggests the existence of a brightness source common to widely separated regimes of the Sun's atmosphere, possibly associated with vertically contiguous magnetic structures distinct from prominent coronal and photospheric active regions. Since this brightness source includes the "missing" component that causes significant solar cycle irradiance variability, upwardly extending magnetic fields may have a large scale impact on the solar atmosphere in addition to their demonstrable role of generating localized active regions. This has implications for understanding helioseismic mode frequencies and splittings over the solar cycle which are correlated with change in solar photospheric brightness, and for improving and extending solar spectrum variability models.
Update 8-Mar-94
Attached is an abstract submitted to the upcoming Spring AGU meeting: we would like to submit this as an update to team BB entry qs066.
Estimating Solar Soft X-Ray Forcing of the Middle Atmosphere from YOHKOH SXT Data
J. Lean (E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC 20375, 202-767-5116: email: lean@demeter.nrl.navy.mil) J. Mariska (Naval Research Laboratory), K. Strong (Lockheed Palo Alto Research Laboratory), H. Hara (University of Tokyo) and H. Hudson (University of Hawaii).
Solar soft X-rays penetrate to the Earth's lower thermosphere, where they ionize neutral constituents, yielding products such as NO that may be transported downwards to the middle atmosphere and participate in ozone depleting cycles. Variations in solar soft X-ray forcing may contribute to natural variability of the neutral and ionized atmosphere in the vicinity of 100 km. However, the amplitude of the soft X-ray variability during the 11-year cycle remains poorly determined. Empirical irradiance variability models estimate solar cycle changes of a factor of 10 in the 18-50 soft X- ray fluxes. Lower thermosphere model simulations have established the need for solar soft X-ray forcing in order to explain the NO density changes measured by SME but disagree that the solar cycle variation needs to be as large as a factor 50, instead of the factor of 10 estimated by the irradiance variability models.
Images of the Sun in soft-X rays (10 to 60 ) have been collected almost daily by the Soft X-ray Telescope on the Yohkoh mission, launched in mid 1991 near the peak of solar cycle 22. These images allow an evaluation and characterization of the sources of solar soft X-ray flux variations. Using the of SXT data, we investigate how the various sources change during the large decrease in solar activity that occurred in 1992, in comparison with a variety of solar activity
proxies (e.g., F10.7 and the He EW) and estimate the amplitude of the 11-year cycle variability by extrapolating these associations. We compare our estimates of the amplitude of the solar soft X-ray flux variations with predictions by the empirical variability models.