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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Pervasive faint Fe XIX emission from a solar active region observed with EUNIS-13: Evidence for nanoflare heating

We present spatially resolved EUV spectroscopic measurements of pervasive, faint Fe XIX 592.2 Å line emission in an active region observed during the 2013 April 23 flight of the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS-13) sounding rocket instrument. With cooled detectors, high sensitivity, and high spectral resolution, EUNIS-13 resolves the lines of Fe XIX at 592.2 Å (formed at temperature T ≈ 8.9 MK) and Fe XII at 592.6 Å (T ≈ 1.6 MK). The Fe XIX line emission, observed over an area in excess of 4920 arcsec{sup 2} (2.58 × 10{sup 9} km{sup 2}, more than 60% of the active region), provides strong evidence for the nanoflare heating model of the solar corona. No GOES events occurred in the region less than 2 hr before the rocket flight, but a microflare was observed north and east of the region with RHESSI and EUNIS during the flight. The absence of significant upward velocities anywhere in the region, particularly the microflare, indicates that the pervasive Fe XIX emission is not propelled outward from the microflare site, but is most likely attributed to localized heating (not necessarily due to reconnection) consistent with the nanoflare heating model of the solar corona. Assuming ionization equilibrium we estimate Fe XIX/Fe XII emission measure ratios of ∼0.076 just outside the AR core and ∼0.59 in the core.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1989

Time variability of solar active regions at centimeter wavelengths

An analysis is presented of the variability of centimeter wavelength emission originating from the solar transition region and lower corona, and the variability is compared with the sun's quiescent emission. Sequential dual wavelength observations are presented and used to put constraints on a possible model of the variability of active region emissions and to better understand the dynamics of active regions. It is shown that gyroresonance emission contributes significantly to the active region emission at 6 cm, whereas all the quiet-sun emission can be attributed to free-free emission. From comparison of the quiet-sun and active region models, it is shown that the density at the base of the transition region is higher in active regions.

Shevgaonkar, R. K.↗
NASA NTRS1991

Pre- and main-sequence evolution of solar activity

The magnetic activity on single solarlike stars declines with stellar age. This has important consequences for the influence of the sun on the early solar system. What is meant by stellar activity, and how it is measured, is reviewed. Stellar activity on the premain-sequence phase of evolution is discussed; the classical T Tauri stars do not exhibit solarlike activity, while the naked T Tauri stars do. The emission surface fluxes of the naked T Tauri stars are similar to those of the youngest main-sequence G stars. The best representation for solarlike stars is a decay proportional to exp(A x t exp 0.5), where A is a function of line excitation temperature. From these decay laws, one can determine the interdependences of the activity, age, and rotation periods. The fluxes of ionizing photons at the earth early in its history are discussed; there was sufficient fluence to account for the observed isotopic ratios of the noble gases.

Walter, Frederick M.↗
DOE OSTI2016

HOMOLOGOUS JET-DRIVEN CORONAL MASS EJECTIONS FROM SOLAR ACTIVE REGION 12192

We report observations of homologous coronal jets and their coronal mass ejections (CMEs) observed by instruments onboard the Solar Dynamics Observatory (SDO) and the Solar and Heliospheric Observatory (SOHO) spacecraft. The homologous jets originated from a location with emerging and canceling magnetic field at the southeastern edge of the giant active region (AR) of 2014 October, NOAA 12192. This AR produced in its interior many non-jet major flare eruptions (X- and M- class) that made no CME. During October 20 to 27, in contrast to the major flare eruptions in the interior, six of the homologous jets from the edge resulted in CMEs. Each jet-driven CME (∼200–300 km s{sup −1}) was slower-moving than most CMEs, with angular widths (20°–50°) comparable to that of the base of a coronal streamer straddling the AR and were of the “streamer-puff” variety, whereby the preexisting streamer was transiently inflated but not destroyed by the passage of the CME. Much of the transition-region-temperature plasma in the CME-producing jets escaped from the Sun, whereas relatively more of the transition-region plasma in non-CME-producing jets fell back to the solar surface. Also, the CME-producing jets tended to be faster and longer-lasting than the non-CME-producing jets. Our observations imply that each jet and CME resulted from reconnection opening of twisted field that erupted from the jet base and that the erupting field did not become a plasmoid as previously envisioned for streamer-puff CMEs, but instead the jet-guiding streamer-base loop was blown out by the loop’s twist from the reconnection.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE NAKED EMERGENCE OF SOLAR ACTIVE REGIONS OBSERVED WITH SDO/HMI

We take advantage of the HMI/SDO instrument to study the naked emergence of active regions (ARs) from the first imprints of the magnetic field on the solar surface. To this end, we followed the first 24 hr in the life of two rather isolated ARs that appeared on the surface when they were about to cross the central meridian. We analyze the correlations between Doppler velocities and the orientation of the vector magnetic field, consistent finding that the horizontal fields connecting the main polarities are dragged to the surface by relatively strong upflows and are associated with elongated granulation that is, on average, brighter than its surroundings. The main magnetic footpoints, on the other hand, are dominated by vertical fields and downflowing plasma. The appearance of moving dipolar features (MDFs, of opposite polarity to that of the AR) in between the main footpoints is a rather common occurrence once the AR reaches a certain size. The buoyancy of the fields is insufficient to lift up the magnetic arcade as a whole. Instead, weighted by the plasma that it carries, the field is pinned down to the photosphere at several places in between the main footpoints, giving life to the MDFs and enabling channels of downflowing plasma. MDF poles tend to drift toward each other, merge and disappear. This is likely to be the signature of a reconnection process in the dipped field lines, which relieves some of the weight allowing the magnetic arcade to finally rise beyond the detection layer of the Helioseismic and Magnetic Imager spectral line.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

Long duration solar active regions

Nimbus 3 and 4 observations of ultraviolet active regions of the sun have been investigated. It is not clear what is producing these UV-active regions, however, it could be changes in the overall intensity of the solar continuum near the region of the temperature minimum, or the solar flux variations that have been seen could be due to a continuous type of absorption like that associated with the ionization limits of elements such as aluminum, silicon, or magnesium.

Heath, D.↗
NASA NTRS2004

Solar Activity and its Impact on Earth's Climate

The Sun's activity is now approaching an expected 2006 minimum, following the dramatic maximum of Solar Cycle 23, that included events such as the 2001 "Bastille Day" Coronal Mass Ejection, and the record-setting Oct-Nov 2003 solar flares, with their associated sunspots and variations in Total Solar Irradiance, or TSI. On Nov 4,2003 the largest X-ray flare ever detected (X-28) was observed in detail. We discuss recent satellite measurements of TSI by ACRIM 2 and 3 and Virgo, and new precision observations of TSI and SSI (Solar Spectral Irradiance) from the SORCE mission, that launched on January 25,2003. TSI variations recorded during the June 8,2004 transit of Venus show the unprecedented precision of the SORCE Total Irradiance Monitor (TIM) instrument, the first of its kind to employ phase-sensitive detection. The SORCE spectral instruments, XPS, Solstice, and SIM, record the Sun's changes over a wide range of wavelengths, from 1 to more than 2000 nanometers, for the first time covering the peak of the solar spectrum, including spectral components that provide energy inputs to key components of the climate system - ultraviolet (UV) into the upper atmospheric ozone layer, infrared (IR) into the lower atmosphere and clouds, and Visible into the Oceans and biosphere. Succeeding satellite missions are planned to monitor both TSI and SSI through Cycle 24. We summarize current ideas about decadal and longer solar variability, and associated potential impacts on Earth's climate on time scales from decades to centuries, especially highlighting the role of feedbacks in the climate system.

Cahalan, Robert F.↗
NASA NTRS2016

Clasp/SJ Observation of Time Variations of Lyman-Alpha Emissions in a Solar Active Region

The Chromospheric Lyman-alpha SpectroPolarimeter (CLASP) is a sounding rocket experiment launched on September 3, 2015 to investigate the solar chromosphere, and the slit-jaw (SJ) optical system took Lyα images with the high time cadence of 0.6 s. By the CLASP/SJ observation, many time variations in the solar chromosphere with the time scale of <1 minute were discovered (see the poster by Kubo et al., Pa-13). We focused on an active region and investigated the short (<30 s) time variations and relation to the coronal structure observed by SDO/AIA. We compared the Ly(alpha) time variations at footpoints of coronal magnetic fields observed by AIA 211 Å (approx.2 MK) and AIA 171 Å (0.6 MK), and non-loop regions. As the result, we found the <30 s Ly(alpha) time variations had more in the footpoint regions. On the other hand, the <30 s time variations had no dependency on the temperature of the loop.

CLASP↗
NASA NTRS1994

VLA stereoscopy of solar active regions. 1: Method and tests

We develop a new technique for extracting three-dimensional information from multiday solar Very Large Array (VLA) observations. While standard stereoscopic methods provide a three-dimensional view of an object by combining simultaneous observations from two different aspect angles, we relax the condition of simultaneity and exploit solar rotation to vary the aspect angle. The solar radio images are decomposed into Gaussian source components, which are then cross-correlated in maps from preceding and following days. This provides measurements of the three-dimensional position of correlated source centroids. In this first paper, we describe the stereoscopic method and perform tests with simulated and real radio maps (from the VLA at 20 cm), in order to study the accuracy of altitude measurements, and the limitations introduced by (i) source confusion, (ii) source motion, and (iii) the assumed differential rotation rate. The tests demonstrate that (i) the information content of a VLA map relevant for stereoscopic correlation can be conveniently represented in terms of a small number of Gaussian components; (ii) the fitting of the three-dimensional source position is stable within a numerical accuracy of less than or approximately equal to 0.02 map pixels, (iii) the relative accuracy of the altitude determination is uniform over the solar disk, and (iv) source confusion does not affect the accuracy of stereoscopic position measurements for sources with a signal-to-noise ratio of greater than or approximately equal to 36.

Aschwanden, Markus J.↗
NASA NTRS1991

Simultaneous Solar Maximum Mission (SMM) and Very Large Array (VLA) observations of solar active regions

Very Large Array observations at 20 cm wavelength can detect the hot coronal plasma previously observed at soft x ray wavelengths. Thermal cyclotron line emission was detected at the apex of coronal loops where the magnetic field strength is relatively constant. Detailed comparison of simultaneous Solar Maximum Mission (SMM) Satellite and VLA data indicate that physical parameters such as electron temperature, electron density, and magnetic field strength can be obtained, but that some coronal loops remain invisible in either spectral domain. The unprecedent spatial resolution of the VLA at 20 cm wavelength showed that the precursor, impulsive, and post-flare components of solar bursts originate in nearby, but separate loops or systems of loops.. In some cases preburst heating and magnetic changes are observed from loops tens of minutes prior to the impulsive phase. Comparisons with soft x ray images and spectra and with hard x ray data specify the magnetic field strength and emission mechanism of flaring coronal loops. At the longer 91 cm wavelength, the VLA detected extensive emission interpreted as a hot 10(exp 5) K interface between cool, dense H alpha filaments and the surrounding hotter, rarefield corona. Observations at 91 cm also provide evidence for time-correlated bursts in active regions on opposite sides of the solar equator; they are attributed to flare triggering by relativistic particles that move along large-scale, otherwise-invisible, magnetic conduits that link active regions in opposite hemispheres of the Sun.

Willson, Robert F.↗
NASA NTRS1988

Correlations between solar activity and operationally determined satellite drag variation parameters

Operational orbit determination of the Earth Radiation Budget Satellite (ERBS) and the Solar Maximum Mission (SMM) spacecraft using the Goddard Trajectory Determination System (GTDS) in the Flight Dynamics Facility (FDF) of the Goddard Space Flight Center (GSFC) has yielded an orbit solution data base of 3 years for ERBS and 8 years for SMM. One of the parameters in each data base is the drag variation parameter used in the GTDS atmospheric drag model; this parameter is solved routinely to accommodate the different atmospheric densities as they are encountered solution to solution. These two data bases of the drag variation parameter solutions are analyzed to evaluate correlations in the variations of the parameter with changes in the 10.7-cm wavelength solar flux, F10.7, and the geomagnetic index. The data for SMM span a wider range of solar flux values and show a stronger correlation. The data for ERBS, which is at a higher altitude and inclination than SMM, show a significant degree of scatter. For both satellites, the data indicate that changes in the drag variation parameter are more strongly correlated with the F10.7 solar flux than with the geomagnetic index. Correlations with the geomagnetic index are apparent only for severe geomagnetic storm conditions. Results from this analysis enhance the understanding of the drag model and the accommodation density variations in operational orbit determination support.

Smith, E. A.↗
NASA NTRS2017

The MSFC Solar Activity Future Estimation (MSAFE) Model

The Natural Environments Branch of the Engineering Directorate at Marshall Space Flight Center (MSFC) provides solar cycle forecasts for NASA space flight programs and the aerospace community. These forecasts provide future statistical estimates of sunspot number, solar radio 10.7 cm flux (F10.7), and the geomagnetic planetary index, Ap, for input to various space environment models. For example, many thermosphere density computer models used in spacecraft operations, orbital lifetime analysis, and the planning of future spacecraft missions require as inputs the F10.7 and Ap. The solar forecast is updated each month by executing MSAFE using historical and the latest month's observed solar indices to provide estimates for the balance of the current solar cycle. The forecasted solar indices represent the 13-month smoothed values consisting of a best estimate value stated as a 50 percentile value along with approximate +/- 2 sigma values stated as 95 and 5 percentile statistical values. This presentation will give an overview of the MSAFE model and the forecast for the current solar cycle.

Suggs, Ron↗
NASA NTRS2017

The MSFC Solar Activity Future Estimation (MSAFE) Model

The Natural Environments Branch of the Engineering Directorate at Marshall Space Flight Center (MSFC) provides solar cycle forecasts for NASA space flight programs and the aerospace community. These forecasts provide future statistical estimates of sunspot number, solar radio 10.7 cm flux (F10.7), and the geomagnetic planetary index, Ap, for input to various space environment models. For example, many thermosphere density computer models used in spacecraft operations, orbital lifetime analysis, and the planning of future spacecraft missions require as inputs the F10.7 and Ap. The solar forecast is updated each month by executing MSAFE using historical and the latest month's observed solar indices to provide estimates for the balance of the current solar cycle. The forecasted solar indices represent the 13-month smoothed values consisting of a best estimate value stated as a 50 percentile value along with approximate +/- 2 sigma values stated as 95 and 5 percentile statistical values. This presentation will give an overview of the MSAFE model and the forecast for the current solar cycle.

Suggs, Ron↗
NASA NTRS2017

The MSFC Solar Activity Future Estimation (MSAFE) Model

The Natural Environments Branch of the Engineering Directorate at Marshall Space Flight Center (MSFC) provides solar cycle forecasts for NASA space flight programs and the aerospace community. These forecasts provide future statistical estimates of sunspot number, solar radio 10.7 cm flux (F10.7), and the geomagnetic planetary index, Ap, for input to various space environment models. For example, many thermosphere density computer models used in spacecraft operations, orbital lifetime analysis, and the planning of future spacecraft missions require as inputs the F10.7 and Ap. The solar forecast is updated each month by executing MSAFE using historical and the latest month's observed solar indices to provide estimates for the balance of the current solar cycle. The forecasted solar indices represent the 13-month smoothed values consisting of a best estimate value stated as a 50 percentile value along with approximate +/- 2 sigma values stated as 95 and 5 percentile statistical values. This presentation will give an overview of the MSAFE model and the forecast for the current solar cycle.

Suggs, Ron↗
NASA NTRS1975

High energy gamma-ray radiation above 300 keV associated with solar activity

The present state of knowledge about the production of gamma-ray lines and continuum during the impulsive phase of solar flares is reviewed. It is noted that the only definite evidence for gamma rays associated with solar flares was obtained by OSO 7 during the events of August 4 and 7, 1972. This experimental evidence is examined, and gamma-ray line emission mechanisms are discussed, particularly positron annihilation through positronium formation. It is shown that if all the data are taken into account, neither the preflare nor the postflare accumulation model for gamma-ray emission can be ruled out.

Chupp, E. L.↗
NASA NTRS1991

Abundance variations in solar active regions

The diversity in the published values of coronal abundances is unsettling, especially as the range of results seems to be beyond the quoted uncertainties. Measurements of the relative abundance of iron and neon derived from soft X-ray spectra of active regions are presented. From a data base of over 200 spectra taken by the Solar Maximum Mission Flat Crystal Spectrometer, it is found that the relative abundance can vary by as much as a factor of about 7 and can change on timescales of less than 1 h.

Strong, K. T.↗