Published in Geophys. Res. Lett., 28, 2353-2356, 2001. Solar Illumination as Cause of the Equinoctial Preference for Geomagnetic Activity

W. Lyatsky1,2, P. T. Newell3 , and A. Hamza1

1Physics Department, University of New Brunswick,

Fredericton, N.B., E3B 5A3 Canada 2Also: Polar Geophysical Institute, Apatity, Russia 3 Johns Hopkins University Applied Physics Laboratory, Laurel,

MD, 20723, USA

Abstract. Geomagnetic and auroral activity vary seasonally with maxima at equinoxes, as has been known for more than a century. The cause remains under debate. The angle made by the Earth’s dipole axis with the typical direction of the interplanetary magnetic field (IMF) can explain a portion (about 17%) of the effect. To explain the majority of the equinoctial effect, we suggest that geomagnetic activity peaks when the nightside auroral zones of both hemispheres are in darkness, as happens at equinox. Under such conditions, no conducting path exists in the ionosphere to complete the currents required by solar wind-magnetosphere-ionosphere coupling, and geomagnetic disturbances maximize. To test this theory, the Universal Time (UT) variation of geomagnetic activity was explored. As our model predicts, geomagnetic activity in December, measured by the Am index, evinces a deep minimum around 0300-0600 UT when the auroral oval of both hemispheres are in darkness and a maximum around 1500-1600 UT when the southern nightside oval is sunlit. In June, complementary effects are predicted and observed. Previous studies using the AE index have shown more ambiguous results. Here we show that if AE is resolved into the AU and AL components, the discrepancy disappears, with the AL component following the same pattern as does Am. We thus conclude that the intensity of global geomagnetic activity is well ordered by whether the nightside auroral oval is sunlit in one hemisphere or neither. 1. Introduction and Background The fact that equinoctial months are favorable for geomagnetic activity is a curious and important observation that may bear on the origin of geomagnetic disturbances. Of the many explanations proposed, research confirms that the following two contribute to the seasonal effect: 1. The axial model [Cortie, 1912] is based on the 7.2° tilt of the solar rotation axis with respect to the ecliptic plane. The Earth

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reaches its highest northern and southern heliographic latitude on September 6 and March 5, respectively. A modern version of this old idea arises because solar wind velocity increases with heliographic latitude. A slight increase in geomagnetic activity is predicted around the dates just cited. 2. A model by Russell and McPherron [1973] assumes that when the interplanetary magnetic field (IMF), lying in its typical configuration (“the Parker spiral”), has the maximum southward component in the geomagnetic-solar (GSM) coordinate system, geomagnetic activity maximizes. Theoretically this should occur on April 5 and on October 5. Many years of research have amply confirmed that Russell and McPherron are correct, in the sense that geomagnetic activity maximizes for southward IMF. However observations also show that the average southward IMF component has only very weak maxima around equinoxes [e.g., Gonzalez et al., 1994]. Because it is so well established that a southward component of the IMF maximizes geomagnetic activity, we will refer to the Russell-McPherron effect in a more limited sense, namely the extent to which it explains seasonal variation (which depends on the extent to which the IMF is actually more southward around equinox). There have been several attempts to test these models [McIntosh, 1959; Berthelier, 1976; Green, 1984; Orlando, 1995; Clua de Gonzalez et al., 1993; Cliver et al., 2000]. For instance, Green [1984] extensively studied magnetic indices and found a spring maximum on March 24, which is clearly more consistent with an equinoctial maxima than the models above. Similar results, also showing the statistical geomagnetic maxima to occur closer to equinox than the Parker spiral or heliographic latitude models imply, have been found by others [Cliver et al., 2000]. Nonetheless, there is no doubt that these effects do contribute to the equinoctial effect. For example, whether the IMF has a sector structure pointing toward or away from the Earth affects the seasonal behavior, as would be expected from the Russell-McPherron effect. The effect of seasonal variations in the solar wind velocity are small (~50 km/s), but this effect also clearly contributes. Regression analysis indicates that, under the entirety of solar wind conditions, only a minority of the equinoctial effect (at most 34%) can be attributed to the combined heliographic and Russell-McPherron effects [Cliver et al., 2000], with each alone contributing perhaps 17%. The empirical view that geomagnetic activity maximizes when the Earth’s magnetic dipole axis is perpendicular to the Earth-Sun line, as happens at equinox, is often called the “McIntosh” (or “equinoctial”) model [McIntosh, 1959]. No widely accepted physical mechanism is associated with the McIntosh model however. Regression analysis shows that the majority of seasonal variability is attributable to a generalized equinoctial effect; that is, that geomagnetic activity correlates well with the angle between the Earth’s dipole axis and the Earth-Sun line [Orlando et al, 1995; Cliver et al., 2000]. Because the Earth’s magnetic dipole axis is offset from the spin axis, the angle the dipole makes with the solar wind varies over the course of a day. There have been a few attempts to study the diurnal variation of geomagnetic activity. However, these earlier results of studies of geomagnetic activity dependence on universal time were not found convincing, perhaps because of an observed discrepancy between the diurnal

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variation in geomagnetic activity with predictions from the Russell-McPherron effect [Berthelier, 1976]. Recently Cliver et al. [2000] reported finding stronger evidence of UT variation in some geomagnetic activity indices. Specifically, a prominent feature of the Am variation is a deep minimum around 0300-0600 UT and a wider maximum around 1600 UT in winter; while in summer the Am variation is smaller and opposite. However Cliver et al. [2000] found that the AE index did not conform to any of the expected patterns. A conflict exists between the clear Am patterns and the muddled AE patterns. Here we study the diurnal and seasonal variations of geomagnetic activity, confirming the diurnal effect. Resolving the AE index into its components, AU and AL, is shown to remove the discrepancy between the Am and AE patterns. We also present a new and simple explanation which accounts both for the diurnal effect, and a significant portion of the equinoctial effect. Our proposal is that geomagnetic activity varies with solar illumination of the nightside auroral ovals, with maximal activity when neither hemisphere is sunlit and minimal activity when the nightside oval of one hemisphere is sunlit. Ionospheric conductivity thus emerges as a major determinant of global magnetospheric events. Our proposal extends several recent theoretical and observational studies. Theoretical work emphasizing the importance of ionospheric conductivity in determining geomagnetic and auroral activity includes Kan [1993], Lysak [1991], and Lyatsky and Hamza [2001]. Observational support includes Newell et al. [1996], Ahn et al. [2000], and Petrinec et al. [2000]. Unlike individual auroral arc formation, geomagnetic activity is a global phenomenon. At equinox, both nightside auroral ovals are in darkness, independent of UT. Near a solstice, one nightside oval is always in darkness, while the other may be in sunlight, depending on UT. Thus we consider combined seasonal and diurnal dependencies. 2. Data Presentation The indices used here are Am and AE. Am is an index of geomagnetic disturbance based on a fairly dense chain of mid-latitude magnetometers. AE is the auroral electrojet index, based on a sparser chain of high-latitude magnetometers. AE can be broken down into the difference between an upper, AU, and lower, AL, component (AE = AU-AL). The former is widely believed to respond directly to solar wind driving, while the AL component responds more to internal magnetosphere dynamics; specifically to auroral substorms. The present paper will show that very different seasonal/diurnal behavior occur in AU (directly driven disturbances) and AL (internal magnetospheric dynamics). The one-hour AE index is available for 1958-1987, but this index is not definitely calculated for later years. The Am index is available from 1959, but, only as a 3-hour averaged index. The models for the seasonal variation in geomagnetic activity mentioned above give different predictions for UT variation. The heliographic latitude model predicts no UT variation in geomagnetic activity and we shall discuss it no further. The diurnal variation expected from the ionospheric conductivity and Russell-McPherron models depend upon season. Expected

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dependencies of the diurnal variation on season from the ionospheric conductivity and Russell-McPherron models are schematically shown in Figure 1. In creating Figure 1(a), we assumed that the midnight auroral oval is located at 70o magnetic latitude (MLAT) (the magnetic coordinate model used is available at http://nssdc.gsfc.nasa. gov/space/cgm). Plotted is the smaller of the two solar zenith angles (one for each hemisphere) at 70o MLAT at midnight. A solar zenith angle greater than 90o means the sun is below the horizon. Ionospheric conductivity is least (highest) when the zenith angle is largest (smallest), although probably any angle greater than about 110o is equivalent. At least one nightside oval is always deep in darkness, which is why only the smaller of the two zenith angles is plotted. We propose that geomagnetic activity will be maximal when the nightside auroral oval is in darkness even in the more sunlit hemisphere. Our model predicts larger UT variations during winter and summer, since near equinox, both hemisphere’s nightside ovals are in constant darkness. In December there should be a minimum near 0400-0500 UT and a maximum near 1600-1700 UT, while in June the geomagnetic activity should have the reverse variation. From the Russell-McPherron model, the maximum UT variation in geomagnetic activity should be observed near the equinoctial months, with smaller UT variation at solstices. For example, in April a maximum is expected around 2230 UT and a minimum around 1030 UT; in October maximum activity is predicted for 1030 UT and minimum for 2230 UT. Figure 2 shows the observed seasonal/UT variations for the Am and AE indices. A strong UT variation in both indices with deep minima around 0300-0600 UT and maxima around 1200-2000 UT in winter is evident. However, in summer the UT variation in Figure 2 is different for the Am and AE indices. To clarify this difference we examine the seasonal/UT variation for the AU and AL indices separately. Figure 3 shows the seasonal/UT variation for the AU and AL indices individually. Among the striking features evident is that AU and AL indices display far different variations. The AU index shows two summer maxima around 1000 and 2000 UT (the right times of the day, but not the right season, as expected from the Russell-McPherron effect). The plot for the AL index is close to that for the Am index: again we see in winter a deep minimum at about 0300-0600 UT and a maximum around 1100-2000 UT, and an opposite (but smaller) variation in summer with a modest maximum around 0000-0400 UT and a wide minimum in the range of 0800-2200 UT. Of particular note is that the maximum in the AL around 1500-1800 UT is not located at equinoxes but is shifted to northern hemisphere winter (a UT and season when both nightside ovals are in darkness). The lower plot in Figure 3 represents the seasonal/UT variation of the occurrence of the most intense events (instances with AL <-300 nT). A very clear deep minimum in winter is evident at about 0400-0700 UT and a maximum around 1400-1900 UT. The opposite variation occurs in June, with a modest maximum around 0000-0500 UT and a wide minimum around 0800-2200 UT. The difference between minimum and maximum geomagnetic disturbance frequency is as much as 6-7 fold. The diurnal effect is extraordinarily strong in December: Figure 3

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shows that the probability of observing AL < -300 nT is as much as 5 times lower at 0400-0700 UT than at 1400-1900 UT. Some anomalous features may result from the non-homogeneous location of magnetic observatories. This might explain the 1400 UT minimum in the AU index in Figure 3, which occurs where stations are sparse. Most other variations in the AL index (notably the deep minimum around 0300-0600 UT and the maximum around 1200-2000 UT in winter), are observed in the sectors with adequate sampling. Moreover, the same variations are observed in the Am index, which is calculated from a more dense network of lower latitude stations. The UT variations emphasized here are real. 3. Discussion and Summary Considerations of solar illumination and hence ionospheric conductivity can explain both seasonal and diurnal variations nicely. The solar wind coupling to the magnetosphere requires that currents be driven into, through, and back out of the ionosphere. When both hemispheres are sunlit, a conducting path exists in the ionosphere which makes this readily possible. Moreover, the ionizing UV creates large numbers of low-energy electrons along the field-line to serve as charge carriers. As a result, under conditions in which one nightside auroral oval is sunlit, currents flow smoothly and stably. When no conducting path exists in the ionosphere, or insufficient charge carriers are available, conditions are unstable to geomagnetic disruptions. Any auroral activity enhances conductivity and the availability of charge carriers along magnetic field lines through impact ionization of the upper atmosphere. That enhanced conductivity leads to higher currents, and hence more aurora, creating a positive feedback. Instead of the graceful smooth current closings possible under sunlit conditions, a more explosive discharge occurs. Thus, around equinox, both auroral zones are in darkness on the nightside, and geomagnetic activity maximizes. In December, minimum geomagnetic activity should take place around 0400-0500 UT when the Canadian Hudson Bay meridian is at midnight; at this time the nightside southern auroral zone is sunlit reducing substorm generation. Maximum activity in January is expected at 1600-1700 UT, when the midnight auroral oval is over the 120o E Siberian meridian, and the nightside oval is in darkness in both hemispheres. In June the situation is opposite, with minimum activity is shifted to 1600-1700 UT with maximum activity at 0400-0500 UT. The current results are fully consistent with recent findings that intense aurora are suppressed by sunlight [Newell et al., 1996; Petrinec et al., 2000]. The behavior of AU is still somewhat unclear, although it plainly is higher in June, under sunlit conditions. Ahn et al. [2000] have previously investigated the seasonal effects on AU and AL separately, partly to disentangle such effects. The explanation given here for the equinoctial geomagnetic activity maxima is not an alternative to the Russell-McPherron effect, but rather complementary. Doubtless the angle that the Earth’s magnetic dipole makes with the Parker spiral does affect geomagnetic activity. The sum of these phenomena, namely the extent to which the auroral ovals are sunlit, the Russell-

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McPherron effect, together with the slightly higher heliographic latitude occurring around equinox, probably accounts for most, if not all, seasonal variation. Recall that regression analysis [Cliver et al., 2000] indicates that about 2/3 of the seasonal variation can be attributed to an unknown equinoctial (or McIntosh) model; we argue that solar illumination is the underlying physical cause. Background conductivity is controlled by solar UV illumination, which affects whether currents have a conducting path through the ionosphere. This model explains why the geomagnetic maxima statistically occur around the equinoxes themselves, and not somewhat shifted as alternatives imply. Clearly the UT variation of nightside auroral activity as represented by the AL index is best explained if substorm activity is most likely when the nightside auroral oval of both hemispheres are in darkness. Acknowledgments. Research at JHU/APL was funded by NSF grant ATM-9909258 and by AFOSR F49620-00-1-0172.

References Ahn, B.-H., H. W. Kroehl, Y. Kamide, and E. A. Kihn, Universal time

variations in the auroral electrojet indices, J. Geophys. Res., 105, 267, 2000.

Berthelier, A., Influence of the polarity of the interplanetary magnetic field on the annual and diurnal variations of magnetic activity, J. Geophys. Res., 81, 4546, 1976.

Cliver, E. W., Y. Kamide, and A. G. Ling, Mountains versus valleys: Semiannual variation of geomagnetic activity, J. Geophys. Res., 105, 2413, 2000.

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Lyatsky, W., and A. M. Hamza, Possible role of ion demagnetization in the plasma sheet in auroral arc and substorm generation, Space Sci. Rev., 95, 373, 200.

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Orlando, M., G. Moreno, M. Parisi, and M. Storini, Diurnal modulation of the geomagnetic activity induced by the southward component of the interplanetary magnetic field, J. Geophys. Res., 100, 19,565, 1995.

Petrinec, S. M., W. L. Imhof, D. L. Chenette, J. Mobilia, and T. J. Rosenberg, Dayside/nightside auroral X ray emission differences – implications for ionospheric conductance, Geophys. Res. Lett., 20, 3277, 2000.

(Received January 8, 2001; Revised March 23, 2001; Accepted March 28, 2001.) _________________

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W. Lyatsky and A. Hamza , Physics Department, University of New Brunswick, Fredericton, N.B., E3B 5A3 Canada (e-mail: lyatsky@unb.ca). P. T. Newell, Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD 20723. (e-mail: Patrick.Newell@jhuapl.edu) Figure 1. The seasonal and diurnal variation of geomagnetic activity predicted from (a) the ionospheric conductivity effect. The auroral oval is assumed to coincide with ±70° geomagnetic latitude. Plotted is the solar zenith angle at the midnight oval of the more sunlit hemisphere. (b) The dependence expected from the Russell-McPherron effect. Figure 2. Seasonal and diurnal variation of Am and AE. Figure 3. Seasonal and diurnal variation in the AU and AL indices and in the occurrence of AL < -300 nT events.

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Figure 2. Seasonal and diurnal variation of the Am and AE indices.

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Figure 3. Seasonal and diurnal variation in the AU and AL geomagnetic activity indices and in

the occurrence of AL < -300 nT events. Note the good correspondence between the AL

variations and those predicted from the solar illumination model (Figure 1a).

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