Published in Eos Transactions of the American Geophysical Union, volume 78, number 30, pages 309-310, July 29, 1997.

DO LOCALIZED ELECTRIC FIELDS CAUSE THE STRUCTURE OF THE AURORA?

W. Calvert
Center for Atmospheric Research
University of Massachusetts Lowell
Lowell, Massachusetts

The aurora which has fascinated mankind for centuries is characterized by thin discrete auroral arcs, also referred to as auroral curtains and draperies, having a latitudinal thickness which is often only a few hundred meters or less, as measured by Akasofu [1961], Maggs and Davis [1968], and Borovsky et al. [1991]. Explaining this thin structure still remains one of the major mysteries of auroral research. Borovsky [1993] has recently shown that no currently-accepted theory can account for this remarkable feature of the aurora. It is therefore relevant to reexamine the underlying assumptions for these theories, virtually all of which are based upon the assumption that localized electric fields in the high-altitude auroral zone cause the structure of the aurora.

The critical issue is whether the electric potential of the auroral electron acceleration region increases significantly inside discrete arcs. This potential can be measured by satellites from the energy of the electrons which cause the aurora, and the latitudinal region where these potentials occur is called an inverted-V electron event. According to Newell et al. [1995], the latitudinal width of these inverted-V events is found to be Poisson distributed with an average projected width in the ionosphere of 28 to 35 kilometers. Since this is nearly two orders of magnitude larger than the apparent width of an arc, these measurements therefore cannot account for the structure of discrete arcs more than about 1% of the time. Lin and Hoffman [1982] have also found intense electron precipitation peaks corresponding to discrete arcs inside these inverted-V events, thereby also suggesting that electric fields accompany the aurora without causing its structure.

These observations thus demand a new explanation for the aurora which is not based upon the as sumption that electric fields cause the structure of the aurora. New observations by the Oedipus C rocket which was flown in 1995 also support this conclusion, as discussed below and reported at the 1996 Fall AGU Meeting.

Although it has been argued that such measurements cannot resolve the potential structure inside an arc, these measurements are taken by sweeping a deflection voltage inside the instrument and mea suring the number of electrons which are detected during a short time at each energy step. The spa tial resolution which is necessary in order to define the variation of potential inside an arc is therefore dictated by the energy sweep, whereas the resolution which is necessary to detect a change in poten tial is dictated by the dwell time at a given energy. As a consequence, although the quoted resolution for these measurements is a few kilometers, the actual resolution for detecting a change in potential is only a few hundred meters, and therefore quite adequate to detect the predicted change in potential which should occur as a satellite passes through a sequence of arcs. As shown in Plate 1 of Newell et al. [1995], the lack of any significant jumps or missing pixels in these measurements therefore clearly demonstrates that such measurements measure the actual potential structure of the aurora.

Although it is always hard to challenge an existing theory, at issue is the explanation for the aurora which has fascinated mankind for centuries. It is therefore relevant to debate this topic in the open literature. The original purpose of this article was to point out that such observations contradict all existing theories which are based upon the assumption that electric fields cause the structure of the aurora. What started out as a debate, however, has ended up as a discovery, and this new discovery should not be confused with the original discovery that these observations contradict the widespread assumption that electric fields cause the structure of the aurora.

For the past thirty years the basis for this assumption has been the assumption that electric fields are necessary to cause the aurora. As pointed out by Mark Twain, however, being necessary is hardly sufficient, and the same thing applies to auroral research. The critical issue is whether the observed electric fields are both necessary and sufficient to cause the aurora, and not whether these electric fields often accompany the aurora and sometimes account for its structure.

A mountain of evidence is worth less than one contradiction. This is the essence of the scientific method, and the critical error which has plagued auroral research for the past thirty years has been to believe in such theories at the expense of any observation which contradicts the predictions of those theories. Borovsky has shown that previous models are inadequate to account for the structure of discrete arcs, whereas these observations establish that these models are incorrect. It is therefore not a matter of needing to patch up these theories. These theories need to be discarded and a new model is needed for the structure of the aurora which is not based upon the assumption that electric fields cause the structure of discrete arcs.

New observations by the Oedipus C rocket which was flown in 1995 also confirm this conclusion, as shown in Figure 1. This rocket was flown into the auroral zone during an expansive phase of a multiple-onset substorm. As shown in this figure, this rocket detected the electrons which cause dis crete arcs, while at the same time also measuring the potential of the electron acceleration region from the electron energy outside the loss cone during this event.

Fig. 1. Electrons detected inside discrete auroral arcs by the Oedipus-C rocket at an altitude of 524 km in the auroral zone. The upper panels show the electron energy inside and outside the loss cone for three distinct arcs, and the bottom panel shows the corresponding auroral electron precipitation flux. The lack of any significant increase in the electron energy in the middle panel during this event conclusively establishes that localized electric fields do not cause the structure of the aurora.

These arcs were found to occur inside a broad inverted-V electron event, and the latitudinal resolu tion was about 100 meters. There is therefore no uncertainty about these measurements pertaining to the discrete aurora and resolving the potential structure inside these arcs. Moreover, the accompanying auroral observations also showed no evidence for any kind of temporal variation which could account for the observed electron precipitation flux.

Under the assumption that electric fields cause the structure of the aurora, the observed electron energy in the middle panel of this figure should exactly match the electron precipitation flux which is shown in the bottom panel. This figure, which had not yet been analyzed for this purpose before this article was written, thus constitutes a breakthrough for auroral research, since these measure ments clearly settle this issue and conclusively establish that localized electric fields do not cause the structure of the aurora.

It is also relevant to point out that no flaws have been found in the theories which are discussed by Borovsky, except for the tacit assumption that the loss cone for electron precipitation into the iono sphere is uniformly filled at the top of the electron acceleration region. As pointed out in the author's theory for the structure and behavior of the aurora [Calvert, 1995], these measurements therefore also establish that the loss cone must be normally empty or nearly empty at the top of the acceleration region, and hence that scattering into the loss cone is necessary in order to account for the aurora.

The observations in figure 1 were provided courtesy of D. A. Hardy, Phillips Laboratory, Hanscom AFB, Bedford, Massachusetts. This work was supported in part by NASA contract NAS5-96020. The author also thanks J. L. Horwitz for the opportunity to discuss this important issue in the open literature. -- W. Calvert, 219 Friendship Street, Iowa City, IA

References

Akasofu, S-I., Thickness of an active auroral curtain, J. Atmos. Terr. Phys., 21, 287, 1961.

Borovsky, J. E., Auroral arc thicknesses as predicted by various theories, J. Geophys. Res., 98, 6101- 6138, 1993.

Borovsky, J. E., D. M. Suszcynsky, M. I. Buchwald, and H. V. DeHaven, Measuring the thickness of auroral curtains, Arctic, 44, 231, 1991.

Calvert, W., An explanation for auroral structure and the triggering of auroral kilometric radiation, J. Geophys. Res., 100, 14,887-14,894, 1995.

Lin, C. S., and R. A. Hoffman, Narrow bursts of intense electron precipitation fluxes within inverted- V events, Geophys. Res. Lett., 9, 211-214, 1982.

Maggs, J. E., and T. N. Davis, Measurements of the thicknesses of auroral structures, Planet. Space Sci., 16, 205-209, 1968.

Newell, P. T., K. M. Lyons, and C-I. Meng, A large survey of electron acceleration events, J. Geo phys. Res., 101, 2599, 1996.