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At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
NASA NTRS1964

Aurora

Visual auroras - morphology, correlation with geophysical and solar activity, and energetic particles associated with auroras

GEOPHYSICS↗
NASA NTRS2007

Solar Cycle 24 and the Solar Dynamo

We will discuss the polar field precursor method for solar activity prediction, which predicts cycle 24 will be significantly lower than recent activity cycles, and some new ideas rejuvenating Babcock's shallow surface dynamo. The polar field precursor method is based on Babcock and Leighton's dynamo models wherein the polar field at solar minimum plays a major role in generating the next cycle's toroidal field and sunspots. Thus, by examining the polar fields of the Sun near solar minimum, a forecast for the next cycle's activity is obtained. With the current low value for the Sun's polar fields, this method predicts solar cycle 24 will be one of the lowest in recent times, with smoothed F10.7 radio flux values peaking near 135 plus or minus 35 (2 sigma), in the 2012-2013 timeframe (equivalent to smoothed Rz near 80 plus or minus 35 [2 sigma]). One may have to consider solar activity as far back as the early 20th century to find a cycle of comparable magnitude. We discuss unusual behavior in the Sun's polar fields that support this prediction. Normally, the solar precursor method is consistent with the geomagnetic precursor method, wherein geomagnetic variations are thought to be a good measure of the Sun's polar field strength. Because of the unusual polar field, the Earth does not appear to be currently bathed in the Sun's extended polar field (the interplanetary field), hence negating the primal cause behind the geomagnetic precursor technique. We also discuss how percolation may support Babcock's original shallow solar dynamo. In this process ephemeral regions from the solar magnetic carpet, guided by shallow surface fields, may collect to form pores and sunspots.

Pesnell, W. D.↗
NASA NTRS1984

Temporal Variations of Solar UV Spectral Irradiance Caused by Solar Rotation and Active Region Evolution

Variations in the solar 100 to 400 nm UV spectral irradiance caused by solar rotation and active region evolution, are discussed as a function of UV wavelength, CMD dependence, and in relation to the temporal variations in the total solar irradiance, 10.7 cm radio flux, sunspot number and Ca K plage data. Active region radiation at cm wavelengths includes a component proportional to the magnetic field. Active region evolution involves a more rapid growth, peak and decay of sunspots and their strong magnetic fields than the Ca K plages and their related UV enhancements. Major plages often last a rotation or more longer than the active region's sunspots. Large active regions, including those associated with major dips in the total solar irradiance, tend to produce the strongest peaks in 10.7 cm and sunspot numbers on their first rotation, while the Ca K plages and UV enhancements peak on the next rotation and decay more slowly on subsequent rotations. Differences in CMD dependencies cause temporal differences including the stronger presence of 13 day variations in the UV flux.

Donnelly, R. F.↗
NASA NTRS1992

SUSIM/UARS observations of the 120 to 300 nm flux variations during the maximum of the solar cycle - Inferences for the 11-year cycle

Since October 1991, the Solar Ultraviolet Spectral Irradiance Monitor on board the Upper Atmosphere Research Satellite has been measuring the solar spectral irradiance from 120 to 400 nm with low (5 nm) and moderate (1.1 nm) resolution on a daily basis. By scaling the modulation measured over four solar rotations to proxy indicators of solar activity, we estimate the amplitude of the solar ultraviolet irradiance variation during the 11-year activity cycle, extending the wavelength coverage of existing empirical variability models to 120 nm and distinguishing the variability of the emission lines from that of the underlying solar continuum. These estimates are compared with results from another empirical variability model and from direct measurements.

Lean, Judith↗
NASA NTRS1995

Upper Thermosphere Winds and Temperatures in the Geomagnetic Polar Cap: Solar Cycle, Geomagnetic Activity, and Interplanetary Magnetic Field Dependencies

Ground-based Fabry-Perot interferometers located at Thule, Greenland (76.5 deg. N, 69.0 deg. W, lambda = 86 deg.) and at Sondre Stromfjord, Greenland (67.0 deg. N, 50.9 deg. W, lambda = 74 deg.) have monitored the upper thermospheric (approx. 240-km altitude) neutral wind and temperature over the northern hemisphere geomagnetic polar cap since 1983 and 1985, respectively. The thermospheric observations are obtained by determining the Doppler characteristics of the (OI) 15,867-K (630.0-nm) emission of atomic oxygen. The instruments operate on a routine, automatic, (mostly) untended basis during the winter observing seasons, with data coverage limited only by cloud cover and (occasional) instrument failures. This unique database of geomagnetic polar cap measurements now extends over the complete range of solar activity. We present an analysis of the measurements made between 1985 (near solar minimum) and 1991 (near solar maximum), as part of a long-term study of geomagnetic polar cap thermospheric climatology. The measurements from a total of 902 nights of observations are compared with the predictions of two semiempirical models: the Vector Spherical Harmonic (VSH) model of Killeen et al. (1987) and the Horizontal Wind Model (HWM) of Hedin et al. (1991). The results are also analyzed using calculations of thermospheric momentum forcing terms from the Thermosphere-ionosphere General Circulation Model TGCM) of the National Center for Atmospheric Research (NCAR). The experimental results show that upper thermospheric winds in the geomagnetic polar cap have a fundamental diurnal character, with typical wind speeds of about 200 m/s at solar minimum, rising to up to about 800 m/s at solar maximum, depending on geomagnetic activity level. These winds generally blow in the antisunward direction, but are interrupted by episodes of modified wind velocity and altered direction often associated with changes in the orientation of the Interplanetary Magnetic Field (IMF). The central polar cap (greater than approx. 80 magnetic latitude) antisunward wind speed is found to be a strong function of both solar and geomagnetic activity. The polar cap temperatures show variations in both solar and geomagnetic activity, with temperatures near 800 K for low K(sub p) and F(sub 10.7) and greater than about 2000 K for high K(sub p) and F(sub 10.7). The observed temperatures are significantly greater than those predicted by the mass spectrometer/incoherent scatter model for high activity conditions. Theoretical analysis based on the NCAR TIGCM indicates that the antisunward upper thermospheric winds, driven by upstream ion drag, basically 'coast' across the polar cap. The relatively small changes in wind velocity and direction within the polar cap are induced by a combination of forcing terms of commensurate magnitude, including the nonlinear advection term, the Coriolis term, and the pressure gradient force term. The polar cap thennospheric thermal balance is dominated by horizontal advection, and adiabatic and thermal conduction terms.

Killeen, T. L.↗
NASA NTRS1983

The observed relationships between some solar rotation parameters and the activity cycle

Several parameters of the solar rotation show variations which appear to relate to the phase of the solar-activity cycle. The latitude gradient of the differential rotation, as seen in the coefficients of the sin2 and sin4 terms in the latitude expansion, shows marked variations with the cycle. One of these variations may be described as a one-cycle-per-hemisphere torsional oscillation with a period of 11 years, where the high latitudes rotate faster at solar-activity maximum and slower at minimum, and the low latitudes rotate faster at solar-activity minimum and slower at maximum. Another variation is a periodic oscillation of the fractional difference in the low-latitude rotation between north and south hemispheres. The possibility of a variation in the absolute rotational velocity of the sun in phase with the solar cycle remains an open question. The two-cycle-per-hemisphere torsional waves in the solar rotation also represent an aspect of the rotation which varies with the cycle. It is shown that the amplitude of the fast flowing zone rises a year before the rise to activity maximum. The fast zone seems to be physically the more significant of the two zones.

Howard, R.↗
NASA NTRS1966

The problem of atomic hydrogen.

Lyman alpha airglow observations indicate diurnal variation of atomic hydrogen in thermosphere and exosphere with abundance change with solar activity

AIRGLOW↗
NASA NTRS1979

The forecasting center of Meudon, France

Main features of solar activity are described in relation to solar and geophysical forecasting. Spectroheliograms, radio and X-ray data, white light coronal observations, particles data, photospheric images, and photospheric magnetic fields are among the types of data used to identify the active centers and flares of the Sun. Forecasting and identification of geomagnetic activity is also discussed. The forecasting technique is described along with the types of users.

Simon, P.↗