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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Measured response of the equatorial thermospheric temperature to geomagnetic activity and solar flux changes

Fabry-Perot inteferometer determinations of thermospheric temperatures from 630.0-nm nightglow line-width measurements have been carried out for the period April-August 1983 from Arequipa, Peru (16.4-deg S, 71.5-deg W geographic; 4.4-deg S magnetic). The nightly variation of the thermospheric temperature T(n) measured on 62 nights is compared with MSIS model predictions and found to agree occasionally with the model but, on average, to exceed model predictions by about 180 K. The largest differences, 400-500 K, often occur during strongly increasing geomagnetic activity such as sudden commencements. The rapid increases in T(n) may result from energetic neutrals precipitating at low latitudes from the ring current or from energy carried to equatorial regions from high-latitude (auroral oval) heat sources by gravity waves and equatorward neutral winds.

Biondi, M. A.↗
DOE OSTI2017

Variation of the Solar Microwave Spectrum in the Last Half Century

The total solar fluxes at 1, 2, 3.75, and 9.4 GHz were observed continuously from 1957 to 1994 at Toyokawa, Japan, and from 1994 until now at Nobeyama, Japan, with the current Nobeyama Radio Polarimeters. We examined the multi-frequency and long-term data sets, and found that not only the microwave solar flux but also its monthly standard deviation indicate the long-term variation of solar activity. Furthermore, we found that the microwave spectra at the solar minima of Cycles 20–24 agree with each other. These results show that the average atmospheric structure above the upper chromosphere in the quiet-Sun has not varied for half a century, and suggest that the energy input for atmospheric heating from the sub-photosphere to the corona have not changed in the quiet-Sun despite significantly differing strengths of magnetic activity in the last five solar cycles.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1994

The downward flux of O(+) over the nightside of Venus

We have constructed a map of the downward flux of O(+) over the nightside of Venus at high and low solar activities through a combination of modeling and analysis of Pioneer Venus ion mass spectrometer data. O(+) density profiles were obtained for almost 40 inbound or outbound segments of orbits from the first 2 years of the mission and about 40 more in the recent reentry phase of the mission. We have determined the nearly linear relationship between the 0(+) maximum density and the downward O(+) flux for several solar zenith angles and local times by constructing models of the nightside ionosphere of Venus for a range of downward ion fluxes at the upper boundaries. We find that the largest downward fluxes occur near the terminators, and the fluxes fall off sharply toward the antisolar point. Although the standard deviations in the data are large, there is a suggestion of a local maximum near 155 deg, and the location of this maximum correlates fairly well with structure in the peak electron density as a function of solar zenith angle reported for the Pioneer Venus radio occulation experiment. The average downward ion flux is inferred to be about 1.7 x 10(exp 8)/sq cm/sec over the nightside hemisphere at solar maximum and a factor of 7 less at moderately low solar activity.

Brannon, J. J.↗
NASA NTRS2003

Validation of Spacecraft Active Cavity Radiometer Total Solar Irradiance (TSI) Long Term Measurement Trends Using Proxy TSI Least Squares Analyses

Long-term, incoming total solar irradiance (TSI) measurement trends were validated using proxy TSI values, derived from indices of solar magnetic activity. Spacecraft active cavity radiometers (ACR) are being used to measure longterm TSI variability, which may trigger global climate changes. The TSI, typically referred to as the solar constant, was normalized to the mean earth-sun distance. Studies of spacecraft TSI data sets confirmed the existence of a 0.1 %, long-term TSI variability component within a 10-year period. The 0.1% TSI variability component is clearly present in the spacecraft data sets from the 1984-2004 time frame. Typically, three overlapping spacecraft data sets were used to validate long-term TSI variability trends. However, during the years of 1978-1984, 1989-1991, and 1993-1996, three overlapping spacecraft data sets were not available in order to validate TSI trends. The TSI was found to vary with indices of solar magnetic activity associated with recent 10-year sunspot cycles. Proxy TSI values were derived from least squares analyses of the measured TSI variability with the solar indices of 10.7-cm solar fluxes, and with limb-darked sunspot fluxes. The resulting proxy TSI values were compared to the spacecraft ACR measurements of TSI variability to detect ACR instrument degradation, which may be interpreted as TSI variability. Analyses of ACR measurements and TSI proxies are presented primarily for the 1984-2004, Earth Radiation Budget Experiment (ERBE) ACR solar monitor data set. Differences in proxy and spacecraft measurement data sets suggest the existence of another TSI variability component with an amplitude greater than or equal to 0.5 Wm-2 (0.04%), and with a cycle of 20 years or more.

Lee, Robert Benjamin, III↗
NASA NTRS1973

A study of the diurnal variation in the thermosphere as derived by satellite drag

An analysis was performed on 29,574 densities derived from the drag of 10 satellites to determine simultaneously the parameters of the solar-activity effect in the thermosphere on the one hand, and the amplitude and shape of the diurnal-variation curve on the other. This paper reports on the study of the diurnal variation only. Although a considerable amount of smoothing is inherent in the drag method, it seemed useful to see whether any change could be detected in the shape of the diurnal-variation curve with height, latitude and solar activity. None was detected; the curve remains remarkably stable, symmetric, with a maximum at 14 hr 20 min L.S.T. and a minimum at 2 hr 20 min L.S.T. A systematic variation of the temperature range with height is observed when static models are used to derive it.

Jacchia, L. G.↗
NASA NTRS2022

Plasma Composition Measurements in an Active Region from Solar Orbiter/SPICE and Hinode/EIS

A key goal of the Solar Orbiter mission is to connect elemental abundance measurements of the solar wind enveloping the spacecraft with extreme-UV (EUV) spectroscopic observations of their solar sources, but this is not an easy exercise. Observations from previous missions have revealed a highly complex picture of spatial and temporal variations of elemental abundances in the solar corona. We have used coordinated observations from Hinode and Solar Orbiter to attempt new abundance measurements with the Spectral Imaging of the Coronal Environment (SPICE) instrument, and benchmark them against standard analyses from the EUV Imaging Spectrometer (EIS). We use observations of several solar features in active region (AR) 12781 taken from an Earth-facing view by EIS on 2020 November 10, and SPICE data obtained one week later on 2020 November 17, when the AR had rotated into the Solar Orbiter field of view. We identify a range of spectral lines that are useful for determining the transition region and low-coronal-temperature structure with SPICE, and demonstrate that SPICE measurements are able to differentiate between photospheric and coronal magnesium/neon abundances. The combination of SPICE and EIS is able to establish the atmospheric composition structure of a fan loop/outflow area at the AR edge. We also discuss the problem of resolving the degree of elemental fractionation with SPICE, which is more challenging without further constraints on the temperature structure, and comment on what that can tell us about the sources of the solar wind and solar energetic particles.

Solar physics↗
NASA NTRS1978

The diurnal and solar cycle variation of the charge exchange induced hydrogen escape flux

On the basis of ion temperature and density data at specific points and times in June 1969 provided by the OGO 6 satellite, and altitude profiles of the ion and electron temperature and concentration provided by the Arecibo radar facility over the period February 1972-April 1974, the diurnal and solar cycle variation of the charge-exchange-induced hydrogen escape flux was investigated. It was calculated that for low to moderate solar activity at Arecibo, the diurnal ratio of the maximum-to-minimum charge-exchange-induced hydrogen escape flux was approximately 6 with a peak around noon and a minimum somewhere between 0100 and 0300 h LT. This study of a limited amount of OGO 6 and Arecibo data seems to indicate that the charge-exchange-induced hydrogen escape flux increases as the F(10.7) flux increases for low to moderate solar activity.

Maher, L. J.↗
NASA NTRS1977

The 1976 Standard Atmosphere and its relationship to earlier standards

The 1976 U.S. Standard Atmosphere, representing a mid-latitude atmosphere for moderate solar activity, is compared to earlier standards. For heights of 51 km and below, this standard is identical with its immediate predecessor, the 1962 U.S. Standard Atmosphere. When the density-height profile of each of five earlier model atmospheres is compared with that of the 1976 standard, an oscillation of this parameter around the currently accepted average value is observed, which is partly the result of true density changes related to the 11-year cycle of solar activity and partly the result of earlier uncertainties. The development of knowledge is also elucidated by comparing the temperature-height profiles of each of the important standards used during the preceding century. Number densities of each of six atmospheric species computed for the 1976 U.S. standard are compared over the height region of 0 to 1000 km.

Minzner, R. A.↗
NASA NTRS1996

Evolving Magnetic Structures and Their Relation to Coronal Mass Ejections

Solar activity regions are frequently concentrated into cluster which persist for many solar rotations. These activity complexes are associated with weak dispersed magnetic fields which are most apparent after the activity itself has ceased. We call this combination of persistent activity and dispersed Evolving Magnetic Structures (EMS). Here we show examples of EMSs and describe the evolution of an EMS associated with major Coronal Mass Ejections (CME) and other solar and magnetic disturbances.

magnetic activity↗
NASA NTRS2000

Interhemispheric Plasma Flows in the Equatorial Topside Ionosphere

Latitudinal, longitudinal, and seasonal variations in the field-aligned and perpendicular flows measured at an altitude near 830 km, by the Defense Meteorological Satellite Program F10 satellite are examined. These profiles are studied during the nighttime (2100 magnetic local time) for solstice periods in 1991 when the solar activity is high. Latitude and longitude variations show the influence of F region winds in modulating the observed field-aligned flows. At night, large downward field-aligned flows of the order 400-600/ ms are observed in the winter hemisphere and coincide in longitude with the location of previously identified adiabatic heating effects studied by Venkatraman and Heelis. Interhemispheric flows at 2100 hours local time are seen to extend up to apex heights of approx. 1000 km during times of high solar activity. Perpendicular drifts, however are relatively invariant with latitude as expected.

Venkatraman, Sarita↗
NASA NTRS1988

Interplanetary protons (Ep of about 1 MeV) 1973-1986 and out to 22.4 AU

This paper uses annual mean counting rate data from detectors on two long-lived spacecraft, Pioneer 11 and IMP 8, to study the temporal and heliocentric radial distance variations of the intensity of interplanetary protons (Ep of about 1 MeV) over solar activity cycle 21. The Pioneer 11 data cover the time period April 1973 through 1986 and the heliocentric radial distance range r of between 1.0 and 2.4 AU. IMP 8, in an approximately circular geocentric orbit of semimajor axis 35 earth radii, provides comparable data at 1 AU over the time period 1974-1986. The combination of the two bodies of data shows that the annual mean intensity of such protons varies as the inverse square of the distance from the sun, irrespective of solar activity as measured by the annual mean sunspot number S. Also it is found that the annual intensity at 1 AU is approximately proportional to S, except for anomalously low values in 1979 and 1980, and that the product of the annual mean intensity at Pioneer 11 by r-squared is also approximately proportional to S, except for anomalously low values in 1979, 1980 (in particular), and 1981. The common 1980 'anomaly' is attributed to gross changes in interplanetary conditions associated with the reversal of the polarity of the sun's polar magnetic field.

Van Allen, J. A.↗
NASA NTRS1977

Solar-generated quasi-biennial geomagnetic variation

The existence of highly correlated quasi-biennial variations in the geomagnetic field and in solar activity is demonstrated. The analysis uses a numerical filter technique applied to monthly averages of the geomagnetic horizontal component and of the Zurich relative sunspot number. Striking correlations are found between the quasi-biennial geomagnetic variations determined from several magnetic observatories located at widely different longitudes, indicating the worldwide nature of the obtained variation. The correlation coefficient between the filtered Dst index and the filtered relative sunspot number is found to be -0.79 at confidence level greater than 99% with a time lag of 4 months, solar activity preceding the Dst variation. The correlation between the unfiltered data of Dst and those of the sunspot number is also high with a similar time lag. Such a time lag has not been discussed in the literature, and a further study is required to establish the mode of sun-earth relationship that gives this time delay.

Sugiura, M.↗
NASA NTRS1985

Interplanetary cosmic ray intensity - 1972-1984 and out to 32 AU

The interplanetary cosmic-ray intensity was measured over a complete solar activity cycle for heliocentric distances of up to 32 AU, using the Geiger-Mueller tubes on Pioneer 10 and 11. The study uses the revised background rates derived from observations during the Saturn encounter of Pioneer 11 (September 1, 1979). Time profiles of the intensity for the 1972-1984 period have a flat and relatively constant maximum for 1975 through 1977, a brief (8-month) minimum in mid-1981, and a maximum centered around early January, 1983. The maximum-to-minimum intensity ratio for the solar activity cycle is approximately 2.7. The correlation among the Pioneer 10, Pioneer 11, and Deep River observation results is found to be fairly good. The radius of the heliospheric modulation region is noted to be greater than 32 AU; certain arguments indicate that it lies between 50 and 100 AU.

Van Allen, J. A.↗
NASA NTRS1977

Solar generated quasi-biennial geomagnetic variation

The existence of highly correlated quasi-biennial variations in the geomagnetic field and in solar activity is demonstrated. The analysis uses a numerical filter technique applied to monthly averages of the geomagnetic horizontal component and of the Zurich relative sunspot number. Striking correlations are found between the quasi-biennial geomagnetic variations determined from several magnetic observatories located at widely different longitudes, indicating a worldwide nature of the obtained variation. The correlation coefficient between the filtered Dst index and the filtered relative sunspot number is found to be -0.79 at confidence level greater than 99% with a time-lag of 4 months, with solar activity preceding the Dst variation. The correlation between the unfiltered data of Dst and of the sunspot number is also high with a similar time-lag. Such a timelag has not been discussed in the literature, and a further study is required to establish the mode of sun-earth relationship that gives this time delay.

Sugiura, M.↗
NASA NTRS2004

A Correction for the IRI Topside Electron Density Model Based on Alouette/ISIS Topside Sounder Data

The topside segment of the International Reference Ionosphere (IRI) electron density model (and also of the Bent model) is based on the limited amount of topside data available at the time (40,OOO Alouette 1 profiles). Being established from such a small database it is therefore not surprising that the models have well-known shortcomings, for example, at high solar activities. Meanwhile a large data base of close to 200,000 topside profiles from Alouette 1,2, and ISIS I, 2 has become available online. A program of automated scaling and inversion of a large volume of digitized ionograms adds continuously to this data pool. We have used the currently available ISIs/Alouette topside profiles to evaluate the IRI topside model and to investigate ways of improving the model. The IRI model performs generally well at middle latitudes and shows discrepancies at low and high latitudes and these discrepancies are largest during high solar activity. In the upper topside IRI consistently overestimates the measurements. Based on averages of the data-model ratios we have established correction factors for the IRI model. These factors vary with altitude, modified dip latitude, and local time.

Bilitza, D.↗
NASA NTRS1975

The 1973 solar occultation of the Crab Nebula pulsar

The mean electron density of the solar corona was determined by measuring the dispersion of radiofrequency pulses from pulsar NP 0532 during the June 1973 solar occultation. Trends continued which were noticed in 1971 as solar activity declined. Model fitting results suggest that the corona continued to become even more concentrated toward the equator in 1973 than in 1971. The number density of electrons in most regions decreased. The best model of the distribution of corona electrons is suggested to be one with zero density at the poles. K-corona isophotes and contours of equal path-integrated density are presented for several models. Electron density versus date and position in the corona are tabulated. It is seen that there is no simple relationship between the onset of major solar activity and density or scattering enhancements.

Weisberg, J. M.↗
NASA NTRS1992

The simulated features of heliospheric cosmic-ray modulation with a time-dependent drift model. IV - The role of heliospheric neutral sheet deformation

Previous calculations with a time-dependent drift model revealed the model to be less successful in describing time-dependent modulation during periods of moderate to large solar activity. In this paper, it is argued that a major reason for this is that the previously used wavy heliospheric neutral sheet (HNS) description was based on an idealized HNS not subject to any spatial evolution while propagating radially outward. It is suggested that the deformation and compression of HNS wave peaks will lead to significant increases in the crossfield diffusion across these peaks (short-circuiting). The cosmic rays will effectively experience reduced tilt angles and therefore a reduction in the integrated HNS modulation effect between an observer and the heliospheric boundary. During periods of moderate to large solar activity these HNS deformation processes are progressively more frequent and should lead to a significant reduction in time-dependent modulation as predicted by drift models. Calculations done with radially propagating tilt angles that effectively decrease with radial distance give the expected reduction which improves the general description of modulation from 1987-1988.

Le Roux, J. A.↗
NASA NTRS2009

The Sodium Tail of the Moon

During the few days centered about new Moon, the lunar surface is optically hidden from Earth-based observers. However, the Moon still offers an observable: an extended sodium tail. The lunar sodium tail is the escaping "hot" component of a coma-like exosphere of sodium generated by photon-stimulated desorption, solar wind sputtering and meteoroid impact. Neutral sodium atoms escaping lunar gravity experience solar radiation pressure that drives them into the anti-solar direction forming a comet-like tail. During new Moon time, the geometry of the Sun, Moon and Earth is such that the anti-sunward sodium flux is perturbed by the terrestrial gravitational field resulting in its focusing into a dense core that extends beyond the Earth. An all-sky camera situated at the El Leoncito Observatory (CASLEO) in Argentina has been successfully imaging this tail through a sodium filter at each lunation since April 2006. This paper reports on the results of the brightness of the lunar sodium tail spanning 31 lunations between April 2006 and September 2008. Brightness variability trends are compared with both sporadic and shower meteor activity, solar wind proton energy flux and solar near ultra violet (NUV) patterns for possible correlations. Results suggest minimal variability in the brightness of the observed lunar sodium tail, generally uncorrelated with any single source, yet consistent with a multi-year period of minimal solar activity and non-intense meteoric fluxes.

Matta, M.↗