Comment on 'Interplanetary Magnetic Sector Structure, 1926-1971' by L. Svalgaard and 'Correspondence of Solar Field Sector Direction and Polar Cap Geomagnetic Field Changes for 1965' by W. H. Campbell and S. Matsushita


PAUL F. FOUGERE

Air Force Cambridge Research Laboratories, Hanscom Air Force Base, Bedford Massachusetts 01731

C. T. RUSSELL


Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90024

Originally Published In: J. Geophys. Res., 80(10),1376-1377, 1975.

Recently, Svalgaard [1972] has derived an index of interplanetary polarity based on the dependence of the diurnal variation of the polar cap geomagnetic field on the solar magnetospheric dawn-dusk component of the interplanetary magnetic field [Friis- Christensen et al., 1972]. Svalgaard did not discuss the details of his derivation of this index, e.g., stations used, components examined, and the quality of the data, nor did he discuss the possible limitations of the index. Nevertheless, Campbell and Matsushita [1973] have attempted 'to automate Svalgaard's technique and evaluate his conclusions.' It is the primary purpose of this note to point out that Campbell and Matsushita's analysis for 1965 was quite different from the analysis used by Svalgaard for the same year's data, thus invalidating any evaluation of the relative accuracy of visual versus automatic techniques. Secondarily, this note is a request for more complete documentation of the Svalgaard derivation.

CAMPBELL- MATSUSHITA TECHNIQUE

Campbell and Matsushita made it clear that they attempted to match Svalgaard's analysis. They state, 'Readers should review his [Svalgaard's] paper first to see that we have attempted to make the best possible match of this method' and later, 'We remove one value monthly rather than linearly because this seems to be Svalgaard's method.' In order to evaluate their automatic technique against Svalgaard's visual method they, of course, had to use the same station and component. They assumed that Svalgaard used Godhavn H, stating that,

'Svalgaard used the Godhavn H component records to determine for the UT day the A or C type of polar field, which he states identifies the away or toward (respectively) solar field direction. In Figure 5 we illustrate the disagreement between our computerized identification of average daily plus and minus field residual and Svalgaard's visual evaluation.'

Again in the caption to their Figure 6 they state, 'Second line shows A-C assignment by Svalgaard [1972] from the H component of Godhavn GO(H) magnetograms.' However, Svalgaard did not use the Godhavn magnetograms for 1965.

SVALGAARD TECHNIQUE

Svalgaard [1972] implies that he has used solely the Godhavn H component magnetograms with the statement 'The magnetic observatory at Godhavn has been operating continuously since 1926. This unique series of magnetograms is the basis of the investigation of the long-term behavior of the sector structure presented in the last part of this paper.' In fact, though Godhavn was often used, it was frequently replaced with other stations (L. Svalgaard, personal communication, 1973). In particular, during 1965, Svalgaard used the Canadian IQSY stations, for he states on page 4028, 'Figure 3 compares the observed sector structure with the A-C classification as derived from Canadian IQSY stations for 1964-1965.' Inspection of Figure 3 and the A-C index table for the same period reveals that they are identical.

DISCUSSION

Figure 5 of Campbell and Matsushita must now be reinterpreted as a comparison of Svalgaard's visual results from Z at Canadian IQSY stations with the automated results from H at Godhavn. Thus there has been no test of whether the automatic technique is as accurate as the visual. It is possible that the automatic technique is better than the visual. On the other hand, the estimates of the absolute accuracy of the technique made by comparing the automated results with satellite observations of the interplanetary magnetic field are still valid, and these comparisons suggest that when Thule Z is used rather than Godhavn H, reasonable accuracy is obtained, approaching 85% under the most favorable circumstances.

Since the technique is so promising and since it is useful to have ground measures of interplanetary conditions, especially ones that can be created in retrospect from existing data, it is important to know the details of creation of the existing index. It might be claimed that the existing index has been tested [Friis-Christensen et al., 1971] and proved accurate, and hence these details are irrelevant. Even if this were so, it is important to publish these details so that others might try to improve on the technique by automating it as in the Campbell and Matsushita technique or by using alternative or additional stations, etc.

Furthermore, the index derived by Svalgaard is not as accurate as it appears from tests using modern data, for while the A/C index is independent of geomagnetic activity during the satellite era, it becomes highly dependent on the level of geomagnetic activity prior to 1962 [Fougere, 1974; Russell et al., 1975]. The paper of Svalgaard gives us no clues about why this anomaly occurs.

CONCLUSIONS

In summary, Campbell and Matsushita have not duplicated Svalgaard's visual technique used in deriving the A/C index for 1965. Thus their automatic technique may or may not be better than the visual technique. The lack of documentation in the Svalgaard paper will continue to prevent accurate evaluation of its limitations. The discovery of changes of its properties with time only reinforces the need for such documentation. We recommend that no scientific conclusions be drawn from the use of this index at least until such documentation is forthcoming.

Acknowledgments. One of us (C.T.R.) was supported by the National Science Foundation under NSF grant GA341448-X and by the National Aeronautics and Space Administration under NASA grant NGR 05-007-004.

REFERENCES

Campbell, W. H., and S. Matsushita, Correspondence of solar field sector direction and polar cap geomagnetic field changes for 1965, J. Geophys. Res., 78, 2079-2087, 1973.

Fougere, P. F., Dependence of inferred magnetic sector structure upon geomagnetic and solar activity, Planet.Space Sci., 22, 1173-1184, 1974.

Friis-Christensen, E., K. Lassen, J. M. Wilcox, W. Gonzalez, and D. S. Colburn, Interplanetary magnetic sector polarity from polar geomagnetic field observations, Nature Phys. Sci., 233, 48-50, 197 1.

Friis-Christensen, E., K. Lassen, J. Wilhjelm, J. M. Wilcox, W. Gonzalez, and D. S. Colburn, Critical component of the interplanetary magnetic field responsible for large geomagnetic effects in the polar cap, J. Geophys. Res., 77, 3371-3376, 1972.

Russell, C. T., R. K. Burton, and R. L. McPherron, Some properties of the Svalgaard A/C index, J. Geophys. Res., 80, this issue, 1975.

Svalgaard, L., Interplanetary magnetic sector structure, 1926-1971, J. Geophys. Res- 77, 4027-4034, 1972.


Back to CT Russell's page More On-line Resources
Back to the SSC Home Page