Pages 559-568

The WIND Spacecraft and Its Early Scientific Results

K. W. Ogilvie and M.D. Desch

NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, Space Research 1996
E-mail:ysmdd@lepmodd.gsfc.nasa.gov

ABSTRACT

The WIND spacecraft, part of the ISTP program, was launched by NASA on 1 November 1994, to study the interplanetary medium and the effects of changes and disturbances in it upon the magnetosphere. Initially placed in a double-lunar-swingby orbit, which presently has a perigee of ~1.5 and an apogee of ~ 250 earth radii, the spacecraft may be inserted in 1997 into a halo orbit about the foreward libration point, L1 where it will remain for at least a year. WIND carries modern instrumentation to measure the magnetic field, solar wind and hot plasma, energetic particles and low energy cosmic rays, plasma and radio waves and plasma composition, in addition to two Gamma-ray burst detectors. One of the Gamma-ray instruments is the first Russian instrument to fly on a US spacecraft. This paper describes the spacecraft and some new results in various disciplines.

INTRODUCTION

In this paper the WIND orbit and instruments will be briefly described and some of the many new and interesting scientific results will be. reviewed. These include very extensive wave observations, new observations of the wake of the moon, isotopic observations in energetic particle fluxes and in the solar wind. A special issue of the Geophysical Research Letters (vol.23, no.10) contains 28 papers describing initial results of WIND science.

The WIND spacecraft is shown in Figure 1, and below it is a table showing the instruments indicated above, the instrumenters who built the instrument, and their home institutions. Each team consists of many people from more than one institution, and, because of the nature of the open data system, it merges into the rest of the science community. The data has been made available to everyone, the "key parameters" in real time, and the zero level data somewhat later, through the Central Data Handling Facility. This system seems to be working quite well, now that it has shaken down. We do not propose to discuss it further in this paper.

  Fig. 1. The WIND spacecraft showing the instruments and a list of principal investigators and their institutions.

Figure 2 is a schematic description of WIND science, which falls under three headings. "ISTP", science directly related to the aims of the ISTP project, deals with the input function to the magnetosphere. WIND may be at the foreward libration point, L1, in late 1997, but has already spent considerable time beyond the orbit of the moon in the solar direction. While there, it has made observations on particles reflected from the bow shock and magnetopause, and on particles probably escaping from within the magnetosphere. Quantitative measurements are stressed, since the energy and momentum exchange between the solar wind and the magnetosphere are stressed by ISTP.

Fig. 2. WIND science - schematic distribution of categories.

Measurements related to the heliosphere are those on the more detailed properties of the solar wind (turbulence, wave motions, heat flux, shocks and the attendant particle acceleration). The separation of isotopes in the solar wind plasma is being achieved for the first time on WIND.

Astrophysical studies by WIND include measurements made by the two complementary Gamma-ray burst instruments, shocks and solar particles, as well as the interesting encounters with the moon, our "local small body", brought about as the result of the neccessity of trajectory adjustments.

Soon after launch, the spacecraft was placed in "phasing loops" to provide for its first encounter with the moon, which took place on 27 December, 1994. This cycle of phasing loops, encounters with the moon, and excursions to L1 will continue, as shown in Figures 3, and 4, until WIND is inserted into orbit about L1. (Figure 5).

Fig. 3. WIND trajectory, November 1994 to November1995, in GSE coordinates.

Fig. 4. WIND trajectory, January 1996 to July 1996, in GSE coordinates.

Fig. 5. WIND trajectory, July 1996 to January 1997, in GSE coordinates.

One can see that this complicated trajectory carries WIND into interesting regions where measurements can be made which exploit the wide capabilities of the spacecraft. A more complete description of WIND and its instruments is contained in Space Science Reviews (Russell, 1995).

In the rest of this paper we will present some of the many new results with which WIND is changing our ideas of the sun-Earth connection, the nature of the heliosphere and cosmic Gamma-ray bursts.

ISTP RELATED MEASUREMENTS

From the ISTP point of view, WIND characterizes the input to the magnetosphere, while the other ISTP spacecraft measure the effects produced by this input of energy, momentum, particles, etc. It was earlier thought that recurrent high speed flows led to recurrent geomagnetic storms, while transient flows resulted in non-recurrent storms. In 1974, at a similar point in solar cycle 20 to the one where we presently are in cyle 22, there were two streams from sectors having opposite polarity. This resulted from a tilted heliomagnetic equator, Figure 6a.

Fig. 6a. Solar configuration during (a) 1974 and (b) 1995. Axes with arrowheads mark the direction of the sun's dipole moment, perpendicular to the average position of the heliomagnetic equator (heavy curve). The heliomagnetic equator is tilted with respect to the heliographic equator (thin line) in (a) but not in (b). The dashed lines mark the projection of Earth's trajectory at March equinox (not to scale).

Fig. 6b. 27-day recurrence plots of the hourly Dst index of geomagnetic activity. The filled triangles mark sudden commencements. 

Data from WIND taken in late 1994 and interpreted by Crooker et al. (1996) show a four sector structure in the interplanetary medium as in Fig. 6b; these measurements cover a period later in cycle 22 than the 1974 measurements in cycle 20. What happened as far as the geoeffectiveness of these two situations is concerned?

Geoeffectiveness requires a southward component of the interplanetary magnetic field in the geomagnetic coordinate system. Rosenberg and Coleman (1969) showed that at the equinoxes, the magnetosphere spends more time in interplanetary field with polarity corresponding to the appropriate polar magnetic pole, Figure 6a. Russell and McPherron (1973) showed that geoeffective interplanetary fields (having a southward component in the magnetospheric coordinate system) occur when the interplanetary field points toward the sun in March and away from the sun in September. These two effects combine to provide more geoeffective fields in even solar cycles. Crooker et al. (1996) extend this idea to suggest that not only longer immersion, but more access to high speed flow in favored polarity sectors near equinox provides the 22 year geoactivity cycle.

The data from WIND, Figure 6b, show the Russell-McPherron effect very clearly. The depressions of Dst occur in the shaded regions of toward polarity. This is the kind of observation allowing us to understand the functioning of the sun-Earth connection in some detail.

Quantitative measurements of the energy input to the magnetosphere require that reflections of particles be properly taken into account. Thus, for this reason alone, study of the foreshock is an essential part of the work of ISTP. Both the SWE and 3-D plasma instruments have made quantitative and detailed measurements of the electron and ion foreshocks. These measurements are improvements over those made by the ISEE spacecraft and have disclosed new phenomena. Another important advance by the WIND spacecraft, Figure 7, is the 11 orders of magnitude dynamic range of the 3-D plasma instrument. This instrument has detected bursts of ions with up to 2 MeV energy, both upstream of the bow shock and in the magnetosheath, using its solid state telescopes. Particles having such high energies cannot easily be produced by Fermi acceleration of singly charged ions at the curved bow shock. In a paper by Skoug et al. (1996), these matters have been discussed; the bursts of ions last for a few minutes, shorter than the bursts observed at lower energies.

Fig. 7. Composite proton spectrum measured by the PESA and SST during the event of 13 August 1995 (day 225). The dashed lines are spectra measured at 14:00 UT (before the event) while the solid lines are spectra measured at 14:20 UT (during the event). The PESA traces show ions coming from the Earth, dawn and dusk directions. The 4 SST spectra are spin averaged. The spectra measured during the event show evidence of a flattening and a possible turnover at around 1 keV.

The energy spectrum has the usual power law form, with an exponent close to 4. In the magnetosheath, the bursts are similar, suggesting a magnetospheric origin or ions with higher charge states accelerated at the bow shock; modelling of the bursts tends to support a magnetosheath, the bursts are similar, suggesting a magnetospheric origin or ions with higher charge states accelerated at the bow shock; modelling of the bursts tends to support a magnetospheric origin (Winglee et al., 1996). Using the Step instrument, Mason et al. (1996) have suggested that heavy ions observed upstream of the bow shock are accelerated out of a population associated with Corotating Interaction regions.

At lower energies, the SWE instrument has recorded many crossings back and forth over the leading edge of the electron foreshock, identified by the occurrence of backstreaming electrons and reversal of the electron heat flux. Both reflection from the shock and leakage through the shock occur. The effects of velocity selection and backstreaming electrons cause the formation of positive slopes on the reduced velocity distribution, giving rise to intense Langmuir waves at the foreshock. These waves interact with and scatter the pitch angles of the electrons. Altogether one can say that the ISTP aims of WIND - to make quantitative, accurate, measurements of the interplanetary parameters upstream of the earth are being well fulfilled. These numbers, in the form of key parameters, are available to everyone in near real time, for any purpose.

Heliospheric - Related Measurements

Some measurements made on the WIND spacecraft having applications to Heliospheric physics concern the composition, distribution functions and charge state distributions of solar wind ions. This is the first time, except for 3He observed on ISEE-3, that isotopes in the solar wind have been separated and measured continuously. In addition to the abundances (Table 1), the reduced distribution functions of solar wind ions are also measured (Collier, 1996), Figure 8. A more complete set will be of importance for knowledge of the history of the sun and perhaps for understanding the heating mechanism of the solar wind. The very pronounced suprathermal tails on the velocity distribution functions are a good fit to a kappa distribution, with K in the range 2.5-5.0. The temperature ratios vary with conditions in the solar wind, increasing with increase of solar wind speed in a similar way to the behavior of THe/TH.

Fig. 8. 4He+2, 16O+6, 20Ne solar wind ionic distrubution functions from the MASS instrument on the WIND spacecraft. Note the non-Maxwellian shapes of each.

Type III radio bursts are produced by electrons which travel along the interplanetary magnetic field lines, where they excite oscillations at the local plasma frequency. Simultaneous observations of the same Type III bursts on WIND and Ulysses have been described by Dulk et al. (1996); Figure 9. The paper by Dulk et al. is a study of frequency cut-offs for the theory of type III bursts, but another important use of such observations is that, by using the radio instruments on the WIND and Ulysses spacecraft (WAVES and URAP, respectively), it is possible now to track in 3D the radio trajectories of solar Type III bursts from the sun to Earth, and hence the interplanetary magnetic field lines. The geometry is shown in Figure 10. With WIND in the ecliptic plane and Ulysses at high heliographic latitudes, Type III bursts are tracked simultaneously using the direction-finding capability of each instrument. The resultant path for the event on 7 April 1995 can be seen in the figure. The burst at first tracks, then departs from the idealized Archimedean field line spiral, opening up the possibility of realistically characterizing IMF field lines in 3D for the first time. Further events are being analyzed.

Fig. 9. Solar radio burst observed simultaneously by the Ulysses and WIND spacecraft.

Fig. 10. Triangulation from the WIND and Ulysses spacecraft to yield the motion of a radio burst along the interplanetary magnetic field.

Interesting new measurements at the moon are made possible by the necessity to adjust the trajectory of WIND by close lunar encounters. These are the only lunar wake measurements made since those performed by Explorer 35 and the Apollo subsatellites between 1967 and 1972.

The superior instruments on WIND have made possible studies at high time, and therefore high spatial, resolution of magnetic field (Owen et al., 1996); plasma (Ogilvie 1996; Bosqued, 1996); and plasma waves (Kellogg, 1996; Farrell, 1996). Measurements have been published so far for only one encounter, December 27, 1994, but others will follow. The December, 1994 wake crossing took place at a downstream distance of 6.5 lunar radii (RL), and is illustrated schematically in Figure 11. The absorption of the solar wind which hits the sunward side of the moon leaves a tenuous wake downstream. Subsonic electrons moving into the wake provide an electric field which accelerates ions along the magnetic field direction from both sides of the wake, leading to two ion beams, one moving faster and one slower than the solar wind. The length and the plasma characteristics of the wake thus depend strongly upon the magnetic field direction, and the wake may be longer than 10 RL in the radial field case.

Fig. 11. Observations on the lunar wake were made on December 27, 1994. This is a cartoon showing the way in which electrostatic acceleration along the magnetic field direction produces two ion beams of different velocities as were observed by the SWE instrument on the WIND spacecraft.

The findings of the WIND instrumenters, analyzing only one encounter, on December 27, 1994, are set out in Table II. Earth’s moon is on the borderline of being a "small" body in the solar wind, and studies of its signature at some distance may hold interest for the study of other, much smaller bodies.

Astrophysical Results

Gamma ray burst, GRB, spectroscopy is believed to be one of the keys to understanding high-energy burst sources, whose nature and even distance scale have remained unknown ever since their discovery in the early 70’s. They are believed to be associated with neutron stars. The initial measurements have not been confirmed by the BATSE instrument on the GRO spacecraft, and no evidence for galactic anisotropy of GRB’s has been found. In this situation, measurements of the same event by more than one instrument would be very valuable.

 

In its first year of operation TGRS has triggered on 62 GRBs, 33 of which are bright enough for spectroscopy. Most of the TGRS GRBs were simultaneously observed by WIND/KONUS and approximately half by CGRO/BATSE. Figure 12a shows the TGRS, KONUS, and BATSE energy loss count spectra for GRB 950325-63393 for a subinterval of this burst. After adjusting for geometry, the spectra are consistent with each other in shape and normalization above 35 keV. Figure 12b compares spectra for GRB 950425-00924 from the three instruments. The count spectra for the three instruments are consistent with each other in both shape and normalization above 200 keV, with a 25% TGRS geometry enhancement from 40-150 keV.

Fig. 12. Left. Energy loss spectra for Gamma-ray burst 950325-63393, TGRS and KONUS(WIND) and BATSE(GRO). Right. Spectra for Gamma-ray burst 950425-00924. Spectra have been separated by an order of magnitude for clarity.

This comparison indicates that the three instruments produce similar count spectra, after adjusting for geometry. None of the TGRS GRB spectra that have been examined to date show significant absorption or emission line features. TGRS does detect lines in the background spectrum with its expected resolution, and very good spectra of the Galactic Center 511 keV line were obtained, which demonstrates the ability to detect lines in variable sources and shows that the TGRS instrument is working properly.

If a candidate line feature is reported in a GRB spectrum by one of the instruments currently operating, it is now likely going to be seen by one or more additional instruments, allowing the candidate to be either confirmed or refuted. If TGRS sees a confirmed line, its high resolution spectra may provide insight into the physical processes of a GRB.

Conclusions

With a modern, versatile payload of instruments, the WIND spacecraft is fulfilling its scientific aims as set out in Figure 2, above. In addition to being an essential part of ISTP, it is able to provide new results in heliospheric science and astrophysics, and further investigations of the sun-Earth connection.

ACKNOWLEDGEMENT

The authors wish to thank the authors of publications quoted, and to apologize to the many people whose new work was not described because of limitations of space.

REFERENCES

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