摘要:

This is the first in a series of papers examining large‐scale gravity waves in the thermosphere and their ability to transfer energy from high to low latitudes during magnetic disturbances. The gravity wave source is assumed to be either the Lorentz force of auroral electrojet currents or else a heat input due to energetic particle precipitation or to Joule heating. It is pointed out that the characteristic vertical width of the gravity wave source should usually lie between 2 and 4 pressure scale heights, placing constraints on the vertical wavelengths and horizontal velocities of the generated waves. A simplified analytic model of small‐amplitude wave generation by a current source shows how wave energy production depends on the temporal and spatial dimensions of the source, on the electric field strength, and on the electron density enhancement. The steep thermospheric temperature gradient in the vicinity of the source altitude strongly influences the properties of upward and downward propagating waves compared with waves generated in an isothermal atmosphere. Waves produced by the Lorentz force of Hall currents, by the Lorentz force of Pedersen currents, and by Joule heating are influenced quite differently by this temperature gradient. Because upgoing waves above the source are combinations of waves originally launched upward and waves originally launched downward but reflected around 110 km altitude, the mean effective source altitude is about 110 km for the far field response in the thermosphere. Large‐scale traveling ionospheric disturbances observable at middle latitudes are most likely produced primarily by Pedersen, rather than Hall, currents. The temperature structure of the thermosphere generally causes gravity wave packets to refract upward; waves traveling with a horizontal component of velocity faster than 250 m/s and with an initial downward component of group velocity will always be reflected upward in the lower thermosphere. The effects of viscosity, heat conduction, and Joule dissipation tend to filter out shorter‐period and slower moving waves from observation points at some distance from the source, so that only long‐period fast moving waves can reach low latitudes from an auroral source. For example, a wave with a 94‐min period moving horizontally at 605 m/s is largely dissipated by the time it has traveled 4000 km from a typical auroral source. A numerical simulation using a fairly realistic thermospheric model illustrates many of the points described from analytic con

ISSN:0148-0227    

DOI:10.1029/JA083iA09p04131

年代:1978

数据来源: WILEY

摘要:

A survey is presented of all of the electron plasma oscillation events found to date in association with low‐frequency type III solar radio bursts using approximately 9 years of observations from the Imp 6 and 8, Helios 1 and 2, and Voyager 1 and 2 spacecraft. Plasma oscillation events associated with type III radio bursts show a pronounced increase in both the intensity and the frequency of occurrence with decreasing heliocentric radial distance. This radial dependence explains why intense electron plasma oscillations are seldom observed in association with type III radio bursts at the orbit of the earth. Possible interpretations of the observed radial variation in the plasma oscillation intensity are considere

ISSN:0148-0227    

DOI:10.1029/JA083iA09p04147

年代:1978

数据来源: WILEY

摘要:

Synoptic interplanetary scintillation (IPS) index measurements were taken at 34.3 MHz during May–December 1974 using the University of Iowa Cocoa Cross radiotelescope on a ‘grid’ of 150 selected radio sources covering solar elongation angles up to 180°. Over 80 of these sources displayed definite IPS. The solar elongation dependence of the 34.3‐MHz IPS index is consistent with the elongation angle dependence measured at higher frequencies. Large enhancements (factors of ≳2) of the IPS index are found to coincide with the solar wind (proton) density increases greater than 10 cm−3as measured by Imp 7 and 8 for nearly all observed IPS sources throughout the sky. These ‘all‐sky’ IPS enhancements appear to be caused by increased contributions to the scintillation power by turbulent plasma in regions close to the earth (≲0.3 AU) in all directions. Correlation analysis confirms the IPS response to solar wind density and indicates that the events are due primarily to the corotating solar wind turbulent plasma structures which dominated the interplanetary medium during 1974. The expected IPS space‐time signature for a simple model of an approaching corotating turbulent structure is not apparent in our observations. In some cases, the enhancement variations can be attributed to structural differences in the solar wind density turbulence in a

ISSN:0148-0227    

DOI:10.1029/JA083iA09p04153

年代:1978

数据来源: WILEY

摘要:

The radial dependences of the distant interplanetary magnetic field (IMF) between 1 and 5 AU have been investigated by using Pioneer 10 vector helium magnetometer data acquired in 1972–1973 during Bartels solar rotations 1896–1918. Least squares fits were made of the radial dependences of the averages of the magnitudes of IMF components and total field and plane projections. Radial fits were also made of the standard deviations of these variables over the solar rotation, 1 day, and 3‐hour intervals. Results which emerge from the study include the following: (1) Weighted averages of the radial component of the field are found to vary with heliocentric distance R as R−2.10±0.030, in significant agreement with the Parker model. (2) For the entire data set the tangential component of the field is found to vary as R−1.29±0.06. (3) Further analysis was carried out on a subset of the data for which the average solar wind velocity was relatively low. Within this subset of data obtained during solar rotation periods with low average daily ΣKp the tangential field is found to decrease as R−1.23±0.05and thus to deviate significantly from the Parker model. (4) The deviation between the tangential field variation observed within this subset of the data and the predictions of the model appears to be associated with compressional interaction regions at the leading edges of high‐velocity streams; outside these regions the variations are approximately in accord with the Parker model. (5) The radial decrease of the IMF composite variance (the sum of the variances for all three field components) in a subset of the data obtained during times when the average solar wind velocity was low is consistent with the theory that nonlocally generated Alfvén waves are being swept out from the sun undamped in the interval 1.0‐3.3 AU. (6) The power spectrum in the frequency range 1–8 cycles/d is found to have a slope of approximately 3/2, consistent

ISSN:0148-0227    

DOI:10.1029/JA083iA09p04165

年代:1978

数据来源: WILEY

摘要:

We examined the sources of magnetic fields in recurrent streams observed by the Imp 8 and Heos spacecraft at 1 AU and by Mariner 10 en route to Mercury between October 31, 1973, and February 9, 1974, during Carrington rotations 1607–1610. Most fields and plasmas at 1 AU were related to coronal holes, and the magnetic field lines were open in those holes. However, some of the magnetic fields and plasmas at 1 AU were related to open field line regions on the sun which were not associated with known coronal holes, indicating that open field lines may be more basic than coronal holes as sources of the solar wind. Magnetic field intensities in five equatorial coronal holes, estimated by projecting the measured interplanetary magnetic fields back to the sun by using the principle of flux conservation, ranged from 2 to 18 G with an average of 9 G. Average measured photospheric magnetic fields along the footprints of the corresponding unipolar fields on circular equatorial arcs at 2.5RShad a similar range and average, but in two cases the intensities were approximately 3 times higher than the projected intensities. The coronal footprints of the sector boundaries on the source surface at 2.5RS, determined by a potential field extrapolation of the measured photospheric fields, meandered between −45° and +45° latitude, and their inclination with respect to the solar equator ranged from near 0° at some longitudes to near 90° at others. It is possible that sector boundaries are related to convergence surfaces of the flow near the sun. The high densities observed near sector boundaries between streams might be due in part to the convergence of flows from adjacent corona

ISSN:0148-0227    

DOI:10.1029/JA083iA09p04177

年代:1978

数据来源: WILEY

摘要:

The three‐dimensional structure of the solar wind is often studied by searching for solar latitude variations in observed physical properties that have been averaged over solar longitude. The effects of longitude averaging are examined here by using general qualitative arguments and quantitative calculations for a simple class of assumed spatial variations in the solar wind speed. We show that unless the solar wind spatial structure is simply organized about the solar equator, its presence is extremely difficult to infer from longitude averages unless the observations extend to very high solar latitudes. Thus the absence of semiannual variations in ecliptic observations or the absence of large latitude gradients in radio scintillation or comet tail results do not constitute evidence against the presence of strong spatial variations (or large spatial gradients) in the solar win

ISSN:0148-0227    

DOI:10.1029/JA083iA09p04186

年代:1978

数据来源: WILEY

摘要:

A study has been made of solar wind observations and models of the low‐coronal magnetic field of the sun during the Skylab mission. Potential sources of solar wind plasma have been identified and in most cases systematically associated with observed solar wind flow features. The study pinpoints the problems of associating significant portions of the observed solar wind with sources outside coronal holes, although such associations are strongly suggested by the models. The phenomenological relationship between observed high solar wind speed at 1 AU and small coronal field divergence near the sun is examined and found to be well established but causally inconclusive in light of models of the outer heliospher

ISSN:0148-0227    

DOI:10.1029/JA083iA09p04193

年代:1978

数据来源: WILEY

摘要:

Synoptic observations of the temporal power spectrum of interplanetary scintillations were made in the summer of 1976 on the University of Iowa's Cocoa Cross radio telescope and the University of Maryland's TPT array. Most of the observations were at large (>40°) solar elongations. It was found that the power spectrum broadens when enhanced turbulence at the front of a high‐speed solar wind stream sweeps over the earth. In the case with sufficient synoptic coverage the broadening appeared to result from the relative closeness of the enhanced scattering region rather than from an increase in velocity. A small percentage of the spectra displayed two power law components with a plateau between them. These may result from a flattening in the spatial turbulence spectrum such as has been observed by spacecra

ISSN:0148-0227    

DOI:10.1029/JA083iA09p04200

年代:1978

数据来源: WILEY

摘要:

We have obtained detailed observations of the onset of auroral breakup using a variety of instruments with time resolution of some tens of seconds. Rapid sequences of all‐sky photographs and fast meridian scans by photometers show that breakup is usually preceded by moderate brightening, followed by fading of the auroral brightness lasting 1 or 2 min, before the actual breakup itself. At the time of the fading there is a brief darkening of the poleward sky. Often the breakup is preceded by one or more rapid intensifications, each one preceded by local fading. Pseudobreakups may also occur without the development of a major event. A bona fide breakup may begin on the fading arc, on an adjacent arc, or in an entirely new region nearby. This optical activity is closely correlated with the development of auroral radar echoes, a feature suggesting that variations in the ionospheric and magnetospheric electric and magnetic fields are responsible for the observed auroral variations. Data from the International Magnetospheric Study magnetometer network provide some indication of a correlated response by the local auroral and ionospheric currents, although this could be partly due to changes in conductivity. Riometer recordings show a slow decrease in ionospheric radio wave absorption over a period of about 10 min prior to breakup, with the largest decrease essentially to quiet time values in the region of auroral fading and subsequent breakup. The implications of these observations regarding the trigger mechanism for the expansion phase of a magnetospheric substorm are discusse

ISSN:0148-0227    

DOI:10.1029/JA083iA09p04207

年代:1978

数据来源: WILEY

摘要:

Nighttime multifrequency scintillation and 50‐MHz radar backscatter observations simultaneously performed over a nearly common ionospheric volume at the dip equator in Peru during March 1977 were used to study the relationship between the large‐scale irregularities (∼0.1–1 km) giving rise to scintillations and small‐scale irregularities (3 m) causing 50‐MHz backscatter. It is shown that during the generation phase of equatorial irregularities in the evening hours, the kilometer‐ and meter‐scale irregularities coexist, whereas in the later phase, approximately an hour after the onset, the meter‐scale irregularities decay but the large‐scale ones continue to retain their high spectral intensities. Further, multistation scintillation observations from a host of geostationary satellites as well as from the Wideband satellite indicate that eastward‐drifting irregularity structures detected around midnight cause significant scintillations at UHF andLband but generally fail to give rise to appreciable backscatter. Thus, contrary to expectations, it is possible to have evenLband scintillations without any plume structure on backscatter maps. This indicates that at later local time a cutoff of the spectral intensity probably occurs at some scale length between 100 and 3 m. These observational results are discussed in the context of current theories of plasma instability in the

ISSN:0148-0227    

DOI:10.1029/JA083iA09p04219

年代:1978

数据来源: WILEY