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Wilcox, J. M.

Publications and source records attributed to Wilcox, J. M..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Relationships between a potential field-source surface model of the coronal magnetic field and properties of the solar wind at 1 AU

A comparison is made between the properties at the source surface of a potential field model of the solar corona and solar wind properties observed with ISEE 3 at 1 AU and extrapolated back to the sun. This is done for three consecutive Carrington rotations in the summer of 1979. The already known properties of a velocity minimum at the interplanetary current sheet and a tendency for the average velocity to increase with longitudinal distance from the current sheet are recovered. A better correlation between source surface properties and velocity is demonstrated by using the field strength on the source surface rather than longitudinal distance from the current sheet. Applying a simple algorithm to distinguish between transient and corotating interplanetary variations substantially increases the correlation. However, given a reasonable estimate of the number of degrees of freedom for the sample, only the correlation between source surface field strength and flow speed after the transients have been eliminated is statistically significant at the 1 percent level.

Suess, S. T.↗
NASA NTRS1984

Solstitial and hemispherical asymmetry in the response of geomagnetic field

It is shown that the geomagnetic field is more prone to disturbances around the June solstice than around the December solstice, as evidenced from a larger enhancement in geomagnetic activity indices, ap, an, and as, following the onset of transient solar disturbances occurring in the thee-month period around June solstice than in the interval around the December solstice. Further, an asymmetry between the northern and southern hemisphere geomagnetic activity is shown to exist, independent of the level of the activity. This asymmetry, represented by (an - as)/(an + as)/2 shows a regular annual variation with a maximum of 60 percent around the June solstice and is almost absent around the December solstice.

Shah, G. N.↗
NASA NTRS1983

The structure of the heliospheric current sheet - 1978-1982

Continuing the study near solar minimum in 1976-1977 (Hoeksema et al., 1982), the configuration of the heliospheric magnetic field for the period 1976-1982 is calculated using a potential field model. Particular attention is given to the structure during the rising phase, maximum, and early decline of sunspot cycle 21, from 1978 to 1982. Four warps in the current sheet (the boundary between interplanetary magnetic field toward and away from the sun) are seen early in this interval; these give rise to a four-sector structure in the interplanetary magnetic field observed at earth. It is noted that the location of the current sheet changes slowly and extends to a heliographic latitude of approximately 50 deg. The strength of the polar field correction throughout this period is determined and included in the model calculations. The lower latitude magnetic fields become much stronger as the polar fields weaken and reverse polarity near maximum, decreasing the effect of the polar field correction.

Hoeksema, J. T.↗
NASA NTRS1983

Comparison of heliospheric current sheet structure obtained from potential magnetic field computations and from observed polarization coronal brightness

A comparison is conducted of the structure of the heliospheric current sheet early in sunspot cycle 21 as computed from the observed photospheric magnetic field with a potential field approximation and as inferred from synoptic maps of the observed coronal polarization brightness. On most of the solar rotations compared, the two methods give essentially the same results; the basic shape of the warped current sheet and the amplitude (in solar latitude) of the displacements of the sheet from the solar equator are similar. On one rotation the current sheet computed with the potential field approximation appears to be distorted by a large photospheric region of unbalanced magnetic flux.

Wilcox, J. M.↗
NASA NTRS1983

The structure of the heliospheric current sheet, 1978 - 1982

The structure of the heliospheric magnetic field changes substantially during the 11 year sunspot cycle. Its configuration for the period 1976 through 1982 using a potential field model was calculated. The structure during the rising phase, maximum, and early decline of sunspot cycle 21, from 1978 to 1982 is considered.

Hoeksema, J. T.↗
NASA NTRS1983

Interplanetary magnetic field and tropospheric circulation

The relation between interplanetary magnetic sector boundary crossings and areas of high vorticity in the troposphere that was reported during 1963-1973 cannot be investigated in the years after 1973 because of changes in the processing of the 500 mb height grids prepared by the National Meteorological Center. In particular, we cannot say that the effect disappeared. The same applies to vorticity computed from NMC winds grids. The Limited Area Fine Mesh grid has a large noise in computed vorticity after December 3, 1974. Therefore the interesting analysis of Larsen and Kelley cannot be extended. They had found that forecasts of Vorticity Area Index were significantly poorer after a sector boundary. Previously announced in STAR as N83-25251

Wilcox, J. M.↗
NASA NTRS1982

Structure of the heliospheric current sheet in the early portion of sunspot cycle 21

The structure of the heliospheric current sheet on a spherical source surface of radius 2.35 solar radii has been computed via the use of a potential field model during the first year and a half after the last sunspot minimum. The solar polar magnetic field that is not fully observed in conventional magnetograph scans was included in the computation. The computed heliospheric current sheet had a quasi-stationary structure consisting of two northward and two southward maxima in latitude per solar rotation. The extent in latitude slowly increased from about 15 deg near the start of the interval to about 45 deg near the end. The magnetic field polarity (away from the sun or toward the sun) at the subterrestrial latitude on the source surface agreed with the interplanetary magnetic field polarity observed or inferred at the earth on 82 percent of the days. The interplanetary field structure observed at the earth at this time is finely tuned to the structure of low-latitude fields on the source surface.

Hoeksema, J. T.↗
NASA NTRS1982

Observation of additional low-degree 5-min modes of solar oscillation

High-order solar oscillations with degrees l=3, 4, and 5 could be detected. The observations were made by measuring the difference between the shifts in the Fe 5,124 spectrum line from light integrated from a central circular portion of the solar disk and from an annular portion exterior to it. The frequencies of the octupole modes agree well with the values obtained from whole-disk measurements at the South Pole. A least-squares fit of the observed frequencies to values interpolated between and extrapolated from the predictions of a sequence of solar models with different chemical compositions selects two models. One, a helium-rich solution, agrees with that of similar analyses of whole-disk data. The extrapolated solution has a relatively deep convection zone, and is thus consistent with analyses of 5-min oscillations of high degree.

Scherrer, P. H.↗
NASA NTRS1982

Structure of the solar oscillation with period near 160 minutes

The solar oscillation with period near 160 minutes is found to be unique in a spectrum computed over the range of periods from about 71 to 278 minutes. A best estimate of the period is 160.0095 + or - 0.001 minutes, which is different from 160 minutes (one ninth of a day) by a highly significant amount. The width of the peak is approximately equal to the limiting resolution that can be obtained from an observation lasting 6 years, which suggests that the damping time of the oscillations is considerably longer than 6 years. A suggestion that this peak might be the result of a beating phenomenon between the five minute data averages and a solar oscillation with period near five minutes is shown to be incorrect by recomputing a portion of the spectrum using 15 second data averages.

Scherrer, P. H.↗
NASA NTRS1981

On the nature of the apparent response of the vorticity area index to the solar magnetic field

The apparent response of the vorticity area index to the solar magnetic field is confined to tropospheric regions of intense circulation. Discussions and calculations that include larger volumes of the troposphere would not be expected to show a significant sun-weather effect. Analysis of the effect in time intervals outside the original 1963-73 is also discussed. An assessment of this sun-weather effect at the present time is given.

Wilcox, J. M.↗
NASA NTRS1981

What causes the warp in the heliospheric current sheet

A comparative discussion of the warp in the heliospheric current sheet is presented. Pioneer 10 and 11 data of the interplanetary magnetic field compared with earlier data (Helios 1 and 2) show a good agreement on the phenomenon of the warp; however, the interpretations differ. One theory (Thomas and Smith, 1980) proposes that fast solar wind streams associated with interaction regions may move the current sheet higher to heliospheric latitudes, thus causing the warp; while the earlier theory (1976) adequately explained the phenomenon by using the observed photospheric magnetic field and the Zeeman effect but omitted the solar wind dynamical considerations as part of the computations. It is shown that the Helios data of the polarity of the interplanetary magnetic field are in good agreement with the computed location of the current sheet, confirming the earlier theory.

Wilcox, J. M.↗
NASA NTRS1981

Solar flare acceleration of solar wind - Influence of active region magnetic field

The direction of the photospheric magnetic field at the site of a solar flare is a good predictor of whether the flare will accelerate solar wind plasma. If the field has a southward component, high-speed solar wind plasma is usually observed near the earth about 4 days later. If the field has a northward component, such high-speed solar wind is almost never observed. Southward-field flares may then be expected to have much larger terrestrial effects than northward flares.

Lundstedt, H.↗
NASA NTRS1981

Solar wind helium and hydrogen structure near the heliospheric current sheet - A signal of coronal streamers at 1 AU

Solar wind flow properties associated with very low helium to hydrogen abundance ratios have been observed with Los Alamos instruments on IMP 6, 7, and 8 during 1971-1978. A characteristic pattern has been discovered consisting of correlated interplanetary field reversals, high plasma density, low and nearly identical H(+) and He(2+) bulk velocities, low H(+) and He(2+) kinetic temperatures, and minima in their ratios. Because coronal streamers straddle the current sheet close to the sun, the pattern discovered is the 'signal' of a coronal streamer at 1 AU. A superposed epoch analysis of 74 well-defined sector boundary crossings provides verification of the above correlation featuring a pronounced minimum in the helium to hydrogen abundance ratio at the sector boundary passage.

Borrini, G.↗
NASA NTRS1981

Geomagnetic activity and Hale sector boundaries

The variation of the geomagnetic activity index Ap at the IMF sector boundaries (+ to - and - to +) has been studied for three solar cycles, separating data into vernal and autumnal equinoxes. It was found that a reported increase in Ap as an effect of a Hale boundary can be better attributed to the occurrence of a negative IMF Bz component in the geocentric solar magnetospheric coordinate system and to the occurrence of high speed solar wind streams.

Lundstedt, H.↗
NASA NTRS1980

The rotation of the sun - Observations at Stanford

Daily observations of the photospheric rotation rate using the Doppler effect have been made at the Stanford Solar Observatory since May 1976. These observations show no daily or long-period variations in the rotation rate that exceed the observational error of about 1%. The average rotation rate is the same as that of the sunspots and the large-scale magnetic field structures.

Scherrer, P. H.↗
NASA NTRS1980

Origin of the warped heliospheric current sheet

The warped heliospheric current sheet for early 1976 is calculated from the observed photospheric magnetic field by a potential field method. Comparisons with measurements of the interplanetary magnetic field polarity for early 1976 obtained at several locations in the heliosphere by Helios 1, Helios 2, Pioneer 11, and at the earth show a rather detailed agreement between the computed current sheet and the observations. It appears that the large-scale structure of the warped heliospheric current sheet is determined by the structure of the photospheric magnetic field and that 'ballerina skirt' effects may add small scale ripples.

Wilcox, J. M.↗
NASA NTRS1980

Doppler observations of solar rotation

Daily observations of the photospheric equatorial rotation rate using the Doppler effect are made at the Stanford Solar Observatory. These observations show no variations in the rotation rate that exceed the observational error of about 1%. The average rotation rate is indistinguishable from that of sunspots and large-scale magnetic field structures.

Scherrer, P. H.↗