Wilcox Solar Observatory Research Articles

FLOW SOURCES AND FORMATION LAWS OF SOLAR WIND STREAMS

The large-scale stream structure of the solar wind flow is studied in the main acceleration zone from 10 to 40 solar radii from the Sun. Three independent sets of experimental data were used: radio astronomical observations of radio wave scattering using the large radio telescopes of the Lebedev Physical Institute; dual-frequency Doppler solar wind speed measurements from the Ulysses Solar Corona Experiment during the spacecraft's two solar conjunctions in summer 1991 and winter 1995; solar magnetic field strength and configuration computed from Wilcox Solar Observatory data. Both the experimental data on the position of the transonic region of the solar wind flow and the solar wind speed estimates were used as parameters reflecting the intensity of the solar wind acceleration process. Correlation studies of these data with the magnetic field strength in the solar corona revealed several types of solar wind flow differing in their velocities and the location of their primary acceleration region.

Three‐dimensional numerical simulation of MHD waves observed by the Extreme Ultraviolet Imaging Telescope

We investigate the global large amplitude waves propagating across the solar disk as observed by the SOHO/Extreme Ultraviolet Imaging Telescope (EIT). These waves appear to be similar to those observed in Hα in the chromosphere and which are known as “Moreton waves,” associated with large solar flares [Moreton, 1960, 1964]. Uchida [1968] interpreted these Moreton waves as the propagation of a hydromagnetic disturbance in the corona with its wavefront intersecting the chromosphere to produce the Moreton wave as observed in movie sequences of Hα images. To search for an understanding of the physical characteristics of these newly observed EIT waves, we constructed a three‐dimensional, time‐dependent, numerical magnetohydrodynamic (MHD) model. Measured global magnetic fields, obtained from the Wilcox Solar Observatory (WSO) at Stanford University, are used as the initial magnetic field to investigate hydromagnetic wave propagation in a three‐dimensional spherical geometry. Using magnetohydrodynamic wave theory together with simulation, we are able to identify these observed EIT waves as fast mode MHD waves dominated by the acoustic mode, called magnetosonic waves. The results to be presented include the following: (1) comparison of observed and simulated morphology projected on the disk and the distance‐time curves on the solar disk; (2) three‐dimensional evolution of the disturbed magnetic field lines at various viewing angles; (3) evolution of the plasma density profile at a specific location as a function of latitude; and (4) computed Friedrich's diagrams to identify the MHD wave characteristics.

Preferred solar longitudes with signatures in the solar wind

Using spacecraft data collected over three solar cycles, Neugebauer et al. [2000] found a persistent dependence of the solar wind speed and the radial component of the interplanetary magnetic field on solar longitude, defined in a coordinate system with a rotation period of 27.03 days. Here we investigate the solar source of this period using observations of the photospheric magnetic field. We study the lowest‐order nonaxisymmetric modes of the solar field extracted from synoptic charts of the Wilcox Solar Observatory. We find there is a robust magnetic structure on the Sun, which rotates with the period found by Neugebauer et al. [2000] in the solar wind. This rotation is more rapid than the solar equatorial and core rotations. We discuss the association of this nonaxisymmetric mode with the magnetic field generated by the solar mean field dynamo.

Improvement in the prediction of solar wind conditions using near‐real time solar magnetic field updates

The Wang‐Sheeley model is an empirical model that can predict the background solar wind speed and interplanetary magnetic field (IMF) polarity. We make a number of modifications to the basic technique that greatly improve the performance and reliability of the model. First, we establish a continuous empirical function that relates magnetic expansion factor to solar wind velocity at the source surface. Second, we propagate the wind from the source surface to the Earth using the assumption of radial streams and a simple scheme to account for their interactions. Third, we develop and apply a method for identifying and removing problematic magnetograms from the Wilcox Solar Observatory (WSO). Fourth, we correct WSO line‐of‐sight magnetograms for polar field strength modulation effects that result from the annual variation in the solar b angle. Fifth, we explore a number of techniques to optimize construction of daily updated synoptic maps from the WSO magnetograms. We report on a comprehensive statistical analysis comparing Wang‐Sheeley model predictions with the WIND satellite data set during a 3‐year period centered about the May 1996 solar minimum. The predicted and observed solar wind speeds have a statistically significant correlation (∼0.4) and an average fractional deviation of 0.15. When a single (6‐month) period with large data gaps is excluded from the comparison, the solar wind speed is correctly predicted to within 10–15%. The IMF polarity is correctly predicted ∼75% of the time. The solar wind prediction technique presented here has direct applications to space weather research and forecasting.

A neural network study of the mapping from solar magnetic fields to the daily average solar wind velocity

Predictions of the daily solar wind velocity (V) at 1 AU from the flux tube expansion factor ƒs are examined with radial basis function neural networks. The flux tube expansion factor is calculated from the potential field model, using Wilcox Solar Observatory magnetograms, with the source surface placed at 2.5 solar radii. The time series extend over 20 years from 1976 to 1995 and consist of approximately 3000 daily values of ƒs and V. The correlation between monthly averages of 1/ƒs and V is 0.57, independent of the assumed Sun‐Earth solar wind travel time τ. However, for daily averages the correlation drops to 0.38 with τ = 5 days. Even adjusting τ to match the observed velocity does not improve on the overall correlation. A time series of ƒs(t) extending over t − 4 to t is used as input to the neural network. The network is trained to predict the solar wind velocity V(t + 2) 2 days ahead. The overall correlation on a test set, not included in the training, is 0.53, and the root‐mean‐square error is 85 km/s. Although the increase is significant, the correlation is still low. However, by studying a number of test cases it is seen that high‐speed streams originating from coronal holes are well predicted, while transient structures related to coronal mass ejections are not predicted. To go further, a more detailed description of the solar magnetic fields must be included. The potential field model does not describe the currents in the corona, and changes of the photospheric magnetic field from day to day are smoothed out. By examining the relative error of the calculated photospheric magnetic field and the observed field, it is shown that the correlation between 1/ƒs(t) and V(t + 5) increases to 0.47 for errors smaller than 20% and drops to 0.3 for errors larger than 34%.

The effect of the tilt of the HCS on the solar wind speed in the outer heliosphere

The flow of solar wind from the Sun is bimodal. High speed, low density plasma comes from high magnetic latitude regions and low speed, high density flow from near the heliospheric current sheet (HCS). We investigate whether this bimodal flow combined with the HCS tilt controls the solar wind velocities in the outer heliosphere near solar minimum when the Sun's magnetic field is roughly dipolar. We present a simple model of radial solar wind flow with regions of fast and slow flow and a HCS tilt provided by the Wilcox Solar Observatory. This is the first time a model has been used to make quantitative predictions of the solar wind speed in the outer heliosphere based on solar observations. A reasonable fit to the main features of the observed velocity profile at Voyager 2 is obtained if the full‐width of the slow speed region is 30°. We conclude that the main physical parameter determining the solar wind speed observed by Voyager 2 near solar minimum in the outer heliosphere is the tilt of the HCS.

Outstanding student paper awards

The Space Physics and Aeronomy (SPA) Section presented the following outstanding student paper awards at the 1997 AGU Fall Meeting in San Francisco, California, last December.Peter M. Giles presented a paper titled “Time‐Distance Measurement of Solar Meridional Circulation and Rotation from MDI.” After receiving his B.Sc. in physics from Dalhousie University in Halifax, Canada, Peter studied for several years at Simon Fraser University in Vancouver. In 1993 he was accepted into the Ph.D. program in applied physics at Stanford. He received his M.S. from Stanford in 1995. As a research assistant with the Sollar Oscillations Investigation, Peter has been using time‐distance helioseismology to measure large‐scale flows beneath the solar surface. The results presented at the Fall Meeting will form part of his doctoral thesis. He has also served as the resident observer at the Wilcox Solar Observatory.

Magnetic Field of the Sun as a Star: The Mount Wilson Observatory Catalog 1970–1982

Measurements of the mean magnetic field of the Sun (MMFS) seen as a star were regularly conducted at the Mount Wilson Observatory from 1970 October through 1982 December. A listing is presented of all these data (2457 daily values) suitable for comparison with similar data of other observatories and for studies of magnetic variability and rotation of the Sun. The scatter-plot diagrams and power spectra of the Mount Wilson data and also of the total data 1968-1991 (collected from three observatories: Crimean Astrophysical Observatory, Mount Wilson Observatory, and Wilcox Solar Observatory) are also presented. Time variations of the MMFS connected with solar rotation at periods ≈ 27-28 days and also an enigmatic 1 yr variation are briefly discussed. The power spectrum of the 24 yr data set shows that the most significant and phase-coherent synodic periods of the MMFS variations are 26.92 ± 0.02 and 27.13 ± 0.02 days (both are thought to be associated with rotation of the large-scale surface magnetic field near equator of the Sun) and 28.13 ± 0.02 days. It is suggested that the latter period reflects rigid rotation of the global magnetic field concentrated under the bottom of the solar convection zone. The arguments are given in favor of reality and high confidence level of major periodicities exhibited by MMFS variations.

The relationship between large‐scale solar magnetic field evolution and coronal mass ejections

The idea that coronal mass ejections (CMEs) originate from the evolution of the large‐scale solar field and in particular arise from regions of freshly opening coronal flux is reexamined using a new approach. Potential field models constructed from Wilcox Solar Observatory magnetograms are applied to study both the rate of “opening” of coronal fields with time and the locations of observed CMEs compared to the inferred newly open field regions on the Sun. Case studies are drawn from SMM coronagraph data, Yohkoh soft X ray images, and counterstreaming electron events observed on the ICE spacecraft. The results suggest that the large‐scale field evolution paradigm of CMEs deserves further attention and has potential for applications to “space weather” forecasting.

AN INVESTIGATION OF SOLAR FACTORS GOVERNING CORONAL MASS EJECTION CHARACTERISTICS

This paper deals with the influence of the distance of the apparent axes of coronal mass ejections (CMEs) from a neutral line (NL) on the source surface and of coronal hole (CH) boundaries upon apparent characteristics of CMEs: e.g., the structure, the velocity of individual features, and the width. (a) It is found that the chance of measuring a CME velocity of ascent appears to decrease with increasing distance from a neutral line or coronal hole. (b) The apparent velocity of a CME appears to depend on the distance of its core from a neutral line or coronal hole boundary. CME speeds for events within 15 deg of a ‘surface’ neutral line are significantly higher than those apparently much farther from ‘surface’ neutral lines. (c) CME spans tend to be wider when they are more closely associated with ‘surface’ neutral lines. It is shown that the contribution of CMEs in the neighbourhood of the NL (the heliomagnetic latitude of the CME apparent axis L < 15 deg) decreases with increasing length of the chain of coronal streamers separating the CH of like polarity of the magnetic field and depends on the character of the relationship between CMEs and other forms of activity. The study revealed a concentration of the apparent axes of CMEs toward zero lines of the photospheric magnetic field from the J. M. Wilcox Solar Observatory at Stanford.

Zero-level problem of solar magnetographs and observations of large-scale magnetic fields

This paper discusses some aspects of the zero-level problem of solar magnetographs which is particularly important for observations of large-scale magnetic fields on the Sun. Experiments at the STOP telescope of the Sayan Solar Observatory (SSO) showed that in addition to the adjustment errors of the polarization analyzer, the focusing errors of the spectrograph, and the linear polarization of the light (these mechanisms were known previously [5]), “spurious” signals of the magnetograph are brought about by polarization effects in the optical details preceding the polarization analyzer (coelostat mirrors and the objective) and aberration errors of the spectrograph. Disadvantages of the method of monitoring the zero level from the nonmagnetic line λ 512.37 nm FeI are pointed out. A correlation was made between the observations of the solar mean magnetic field in the SSO and WSO (Wilcox Solar Observatory, Stanford, USA) — the observatories which use the different methods of zero-level monitoring.

The latitudinal structure of the heliospheric current sheet during solar activity cycles 21 and 22

Source surface heliomagnetograms from the Wilcox Solar Observatory from 1976 to 1994 were used to determine the evolution of the heliospheric current sheet (HCS) latitudinal extent during solar cycles 21 and 22. We divided the data into four periods, two during the ascending (1976–1978, 1987–1989) and two during the descending phases (1982–84, 1991–94) of the activity cycles. Time intervals near solar maximum activity when the HCS cannot be uniquely determined for every day were not included. We observed a marked difference in the HCS temporal evolution for the ascending and descending periods. A different latitudinal distribution is also present in all four intervals used. For the descending phases of the cycles analysed a two‐sector structure is clearly predominant, whereas for one of the ascending phases (1976–1978) a four‐sector structure is more prominent. Results clearly demonstrate that the evolution of the HCS is far from being uniform during consecutive cycles, nor is it the same before and after the polar field reversals.

Solar coronal structure: A comparison of NSO/SP ground-based coronal emission line intensities and temperatures with Yohkoh SXT and WSO magnetic data

The large-scale structure of the solar corona is investigated using synoptic maps produced from Fe XIV (530.3 nm), Fe X (637.4 nm) and Ca XV (569.4 nm) data obtained at NSO/SP, Yohkoh/SXT X-ray data and Wilcox Solar Observatory (WSO) ‘source surface’ maps. We find that the Fe XIV data are an excellent proxy for spatially-averaged Yohkoh/SXT data. Isolated emission features and large-scale structures are nearly identical in SXT and Fe XIV maps. In addition, coronal holes and other low-emission regions are very similar. Synoptic temperature maps, calculated from the Fe X Fe XIV ratio, show a tendency for the highest temperatures to occur where the large-scale magnetic fields change polarity at high latitudes (cf. /1/), while lower-latitude features, including active regions, have lower apparent temperatures. Regions of enhanced temperature generally follow the heliospheric current sheet (HCS) as defined by the WSO maps. Further, emission in Ca XV (formet at T ≈ 3 MK), generally occurs only over low-latitude regions that are bright in both Fe X ( T ≈ 1 MK) and Fe XIV ( T ≈ 2 MK). Thus, there is evidence for low (≈1 MK), moderate (≈2 MK) and high (≈3 MK) temperatures in close proximity in the low corona.

The magnetic reversal of the 21st solar cycle and LDE-type flares

The spatial and temporal distributions of LDE-type flaring regions are investigated for 42 solar rotations between the years 1977 and 1980 using synoptic maps of the coronal magnetic field at the source surface, prepared at Wilcox Solar Observatory. 14 outstanding LDE-type flaring centers are found to be distributed not at random but mostly inside the large-scale solar sectors of reversed magnetic polarity (10 centers), as well as at their boundaries (4 centers).

Double magnetic cycle of solar activity

The source of the poloidal magnetic field was fixed using a uniform series of surface low-resolution magnetic field observations begun at Wilcox Solar Observatory at Stanford. The results obtained confirm the idea that low-frequency dynamo waves with a period approximately equal to 22 years and a high-frequency wave of a quasi-two-year period can coexist. It seems that an interaction between these components in the convection zone takes place on the Sun. Surface large-scale solar magnetic fields are analyzed using a two-dimensional Fourier method technique to study the poloidal field distribution. The first harmonic approximately equals the period of the magnetic cycle, appears at all latitudes, and reaches its the maximum value in the polar regions. Moreover, spectral analyses of axisymmetric magnetic field derivative in time found that the second important harmonic of a period approximately equal to two years appears at all latitudes. This second high-frequency harmonic dominates the polar latitude regions at the same time as the low-frequency one.

Large‐scale magnetic field and density distribution in the solar minimum corona

We seek a quantitative description of the large‐scale structure of magnetic field and density in the solar minimum corona that is consistent with observations of both white light intensity and the magnetic field at the photosphere. We use white light images from NASA's Solar Maximum Mission (SMM) Coronagraph/Polarimeter and the High‐Altitude Observatory Mark III (MkIII) K‐coronameter, along with photospheric field measurements from Stanford's Wilcox Solar Observatory (WSO), as constraints on the magnetostatic model of Bogdan and Low [1986] (B&amp;L). We find a family of solutions to the B&amp;L model that reproduce observations of white light quite well, each with a different magnetic field structure. We show that the observed photospheric field cannot be used as an exact boundary condition on the B&amp;L model, but we can limit the white light solutions by matching the total observed photospheric magnetic flux. We find a set of model parameters that reproduces white light and photospheric field to within quantifiable model and observational limits and calculate the physical plasma properties corresponding to these parameters. We conclude that this fit represents a self‐consistent description of the solar minimum coronal magnetic field and density.

A coronal magnetic field model with horizontal volume and sheet currents

When globally mapping the observed photospheric magnetic field into the corona, the interaction of the solar wind and magnetic field has been treated either by imposing source surface boundary conditions that tacitly require volume currents outside the source surface (Schatten, Wilcox, and Ness, 1969) or by limiting the interaction to thin current sheets between oppositely directed field regions (Wolfson, 1985). Yet observations and numerical MHD calculations suggest the presence of non-force-free volume currents throughout the corona as well as thin current sheets in the neighborhoods of the interfaces between closed and open field lines or between oppositely directed open field lines surrounding coronal helmet-streamer structures. This work presents a model including both horizontal volume currents and streamer sheet currents. The present model builds on the magnetostatic equilibria developed by Bogdan and Low (1986) and the current-sheet modeling technique developed by Schatten (1971). The calculation uses synoptic charts of the line-of-sight component of the photospheric magnetic field measured at the Wilcox Solar Observatory. Comparison of an MHD model with the calculated model results for the case of a dipole field and comparison of eclipse observations with calculations for CR 1647 (near solar minimum) show that this horizontal current-current-sheet model reproduces polar plumes and axes of corona streamers better than the source-surface model and reproduces coronal helmet structures better than the current-sheet model.

The shapes of galactic cosmic ray intensity maxima and the evolution of the heliospheric current sheet

The observation of a triangular‐shaped galactic cosmic ray (GCR) intensity maximum during the mid‐1980s solar minimum was successfully predicted by drift models of cosmic ray modulation. The assumption underlying this prediction was that the evolution of solar activity, represented in drift models by the time development of the ”tilt” angle of the heliospheric current sheet (HCS), does not vary greatly from cycle to cycle. Tilt angles derived from coronal brightness distributions in the 1970s show that this assumption, seemingly supported by the successful prediction, may not be valid. The evolution of the HCS during the 1970s, when the 11‐year GCR maximum exhibited a broad peak, may have been significantly different (with a broader and less regular period of low tilt angles) from that inferred for the preceding mid‐1960s solar minimum and from that determined by the Wilcox Solar Observatory during the following mid‐1980s minimum. Had the 1970s corresponded to an A &lt; 0 epoch (in the drift formulation), it appears that the resultant GCR intensity maximum would have been double peaked, with a deep rift corresponding to the 1974 minicycle. The different sensitivities of GCR intensity to tilt angle changes in A positive and A negative solar cycles, as demonstrated by other authors, provide support for drift models of modulation. The point we make here is that nonsystematic evolution of the HCS from one cycle to the next can be an additional source of variation in the shapes of consecutive GCR intensity maxima.

E-W motions of large-scale magnetic field structures of the sun

By applying a new method of processing daily full-disk magnetograms obtained at the Wilcox Solar Observatory at Stanford University, it has become possible to reveal the pattern of global E-W motions of field structures which appears to reflect large-scale convective plasma motions beneath the photosphere. Structures of E-W velocity of different sign extend from north to south, traversing the equator. The extent of the structures in longitude is 25°–45°, and the velocity amplitude reaches 0°.4–0°.5 day-1 (60–70 m s-1 at the equator). Boundaries of E-W flows of different sign correlate with strong, large-scale magnetic field hills. The lifetime of the velocity structures is comparable with that of magnetic field structures.

The IRIS network site at the Wilcox Solar Observatory

The Wilcox Solar Observatory at Stanford University houses one of the International Research on the Interior of the Sun (IRIS) network observing stations. The instrument has observed the global oscillations of the Sun continually since it was installed in August 1987. Each site and instrument are different; here we report the details unique to the Stanford site.