Effect of Sudden Solar Wind Dynamic Pressure Changes at Subauroral Latitudes: Change in Magnetic Field

Abstract. The observations obtained during the International Magnetospheric Study (IMS) from the magnetometers of the IGS network extending from Cambridge, England, to Tromso, Norway, are used to study the response of subauroral current systems to sudden changes in solar wind dynamic pressure. Observations show that the response is very strong at subauroral latitudes. The preliminary response in the H component is a brief, small increase in the dayside morning sector and a decrease in the afternoon and night sectors. The main response in the horizontal field (the H and D components) is toward the pole except in the dayside morning sector. The inferred ionospheric current is mainly a circulatory system flowing counterclockwise when viewed from the north pole everywhere at subauroral latitudes except the dayside morning sector.

Recently, the study of the response of the Earth's magnetosphere to solar wind pressure pulses has become of particular interest because the pressure pulse has been postulated to be the source of many dynamic features in the magnetosphere [Friis-Christensen et al., 1988, 1989; Glassmeier et al., 1989; Sibeck et al., 1989a, b; Kivelson and Southwood, 1991]. For example, observations in ground-based magnetic records reveal the existence of a peculiar type of impulsive disturbances. These disturbances are characterized by a bipolar signature in the H component (north-south component) and a unipolar signature in the D component (east-west component). These signatures have been interpreted as traveling current twin vortices in the ionosphere by using observations from an extended magnetometer array [Honisch and Glassmeier, 1986; Friis-Christensen et al., 1988]. There is much speculation and debate about the source of these traveling current twin vortices and other transient ionospheric features in auroral current systems. Some of the observations have been interpreted to be the result of transient reconnection (or flux transfer events [Russell and Elphic, 1979]) on the dayside magnetopause [Lanzerotti et al., 1986]. More recently, they have been related to pressure pulses in the solar wind [Friis-Christensen et al., 1988, 1989].

To test the possible mechanisms, we conduct a controlled study of the response of subauroral current systems to sudden changes in solar wind dynamic pressure. Since dayside reconnection, as evidenced by the erosion of the magnetopause [Aubry et al., 1971; S. M. Petrinec and C. T. Russell, External and internal influences on the terrestrial magnetopause, submitted to Geophysical Research Letters, 1992], by the occurrence of FTEs [Berchem and Russell, 1984] and by oscillations of the magnetopause [Song et al., 1988], occurs only when the interplanetary magnetic field (IMF) is southward, we can avoid the effects of reconnection by examining only periods of northward IMF. To determine what current systems are generated by the response to pressure pulses, we look for times of interplanetary shock passages when the solar wind dynamic pressure suddenly changes.

The interaction of the solar wind with the Earth's magnetosphere is a highly dynamic process which is still not very well understood. When the solar wind dynamic pressure suddenly increases, the dayside magnetosphere is compressed, and complex current systems are set up in the magnetopause, magnetosphere and ionosphere as a result of the compression. Previous work has revealed that the pattern of response depends on the local time and latitude of the observation site [e.g., Ferraro et al., 1951; Matsushita, 1960, 1962; Araki, 1977]. The responses in the H component are classified into several patterns, SC (a main positive impulse), SC* (a preliminary decrease followed by a main increase), inverted SC (a main negative impulse) and inverted SC* (a preliminary increase followed by a main decrease) [Araki, 1977]. Statistically, the SC* or SC pattern occurs in the afternoon high-latitude region and dayside equational region, and the inverted SC* pattern occurs on the morningside of high latitudes [Matsushita, 1960, 1962; Ferraro et al., 1951; Araki, 1977]. Many authors have also studied the quantitative dependence of responses on the change of solar wind dynamic pressure [Siscoe et al., 1968; Ogilvie et al., 1968; Verzariu et al., 1972; Su and Konradi, 1975; Russell et al., 1992]. Russell et al. [1992] have shown that the simplest response of the ground magnetic field to this compression of the magnetosphere occurs at low and mid latitudes (15 to 30 deg), where the effects of equatorial and auroral ionospheric currents are minimal and the response appears mainly to be caused by the change of the Chapman-Ferraro current. This response is an increase in the H component [Araki, 1977], which is roughly proportional to the change in the square root of dynamic pressure, as predicted qualitatively by the Chapman-Ferraro image dipole model of the magnetopause and more quantitatively, by recent more realistic models such as the ellipsoidal magnetosphere model [Tsyganenko, 1989]. The normalized increase in H at these latitudes is typically 15 nT/nPa^1/2. However, there is still a lack of quantitative understanding of the response at higher latitudes.

The magnetosphere is a vacuum in Tsyganenko's ellipsoidal magnetosphere model. Thus the predicted ground response is caused solely by the change of the magnetopause current when the magnetosphere is compressed together with the induced currents in the interior of the Earth. This model also predicts that the ground response to the increase of Chapman-Ferraro current decays at higher latitudes. Figure 1 shows the normalized change in H as a function of latitude from this model. At subauroral latitudes, which are the primary concern of this paper, the ground magnetic field change due to magnetopause currents is very small, about 4 nT per square root of nPa change of dynamic pressure, while the observed change in H is much greater than 4 nT/nPa^1/2. Thus the observed ground level change must be caused mainly by the ionospheric current system which results from the compression of the magnetosphere. From the subauroral ground signatures observed at different local times, we can infer the ionospheric current system corresponding to the compression of the magnetosphere.

The ground stations used in this study are those in the magnetometer chain operated by the Geomagnetism Unit of the Institute of Geological Sciences (IGS) during the International Magnetospheric Study (IMS) from 1977 to 1979 [Stuart, 1982]. The IGS magnetometer chain forms a closely spaced north-south line from Northern Scandinavia to mid latitudes, and there is a local time spread of about 2 hours along this north-south line, as shown in Figure 2. These stations are located at subauroral latitudes covering the range from 55 to 70 deg geomagnetic latitudes. The L value contours are also drawn in Figure 2, which are computed at 120 km altitude from the field model of Barraclough et al. [1975]. The magnetometers of the IGS stations digitally sample magnetic variations every 2.5 s in three orthogonal directions.

For the solar wind monitor, we use measurements from ISEE 3 and IMP-8 at 5-min resolution. We look for sudden impulse events, i.e., times of interplanetary shock passages when the solar wind dynamic pressure suddenly changes, during the years 1978 and 1979. We examine the response of the ground level magnetic field only when the IMF is northward.

The IGS data show that the response of ground level magnetic field to sudden changes of solar wind dynamic pressure is strong at subauroral latitudes under northward IMF conditions. Figure 3a shows the response of horizontal components (H and D) at York, the most equatorward station in the IGS chain for six sudden impulses observed in the morning hours. The solar wind dynamic pressure observed by ISEE 3 or IMP-8 for 4 of the events are shown in Figure 3b, and there are no solar wind dynamic pressure data available for the other two events. The response in the Z component is much smaller than the horizontal components at this station. During the morning hours, the response in the H component includes a very short preliminary increase followed by a main decrease. These signatures are similar to those of the inverted SC* type of geomagnetic sudden commencement [Araki, 1977]. The main response in D is a decrease. The maximum response occurs a few minutes after the preliminary response. The size of the maximum response varies.

Figure 4a shows the response of H and D components at York for six sudden impulses observed in the afternoon hours. The solar wind dynamic pressure variations for these events observed by ISEE 3 or IMP-8 are shown in Figure 4b. Again, the response in the Z component is much smaller than the horizontal components. During afternoon hours, only some of the events have a noticeable preliminary response, which is a short, small decrease in the H component, opposite to the preliminary response in the morning hours. The maximum response occurs a few minutes later and is an increase in H, which is also opposite to that in the morningside. These signatures are similar to those of SC (without preliminary response) and SC* (with preliminary response) types of geomagnetic sudden commencement [Araki, 1977]. The size of the maximum response also varies. York is in the nightside during most of the sudden impulse events, and the response (not shown) is similar to that in the afternoon side. We also note that the responses in H do not appear to be bipolar since the preliminary response is much smaller than the main response.

Since these sudden impulses of solar wind dynamic pressure have different sizes as demonstrated in Figures 3b and 4b, we test whether the variation in the H component can be ordered by the change in the square root of solar wind dynamic pressure, which is true for the response due to magnetopause current as predicted by the Tsyganenko [1989] ellipsoidal magnetosphere model. Figure 5 shows the preliminary and maximum responses in H at York as a function of local time. It contains only sudden impulse events with dynamic pressure data available from ISEE 3 or IMP-8 and the background magnetic field is relatively quiet. These responses have been normalized by the change in the square root of solar wind dynamic pressure. The thin line shows the response due to compression of the Tsyganenko elliptical magnetopause (eccentricity=0.5) around a vacuum magnetosphere for 56.4 deg latitude. As we can see from this figure, the compression of the Tsyganenko model produces an increase in H comparable to the size of preliminary responses. The magnitude of the maximum response is much greater, and appears not to be well normalized by the change in the square root of dynamic pressure. It is not very clear what controls the magnitude of field responses at this latitude from this figure. Qualitatively, a larger change of solar wind dynamic pressure produces a larger variation in the H component. This can be seen, for example, by comparing events at 0957 LT and 0824 LT in Figures 3, and by comparing events at 1424 LT and 1435 LT in Figure 4. However, the events at 1208 LT and 1238 LT in Figure 4 have similar magnitude, but the amount of increase in the dynamic pressure is very different. The event at 1238 occurs in the summer season and has a much smaller dynamic pressure increase than that at 1208 in the winter. It is probably the conductivity of the ionosphere overhead which is the controlling factor.

The response also varies in magnitude with latitude. Figure 6 shows stacked plots of the H component ordered by geomagnetic latitudes from all IGS station for three sudden impulse events. Unfortunately, these are all the events with a quiet background field and with data available from most of the IGS stations. The event on March 8, 1978, in Figure 6a (event A) and the event on January 5, 1978, in Figure 6b (event B) occur in the dayside afternoon sector for all the stations except St. Anthony. The corresponding solar wind dynamic pressure can be found in Figure 4b. Event A has a much greater pressure increase. For event A, we can see that the response at St. Anthony, which is 4 hours earlier in local time is similar to that at York observed during other events at the same local time, i.e., dayside morning hours. All other stations except St. Anthony have responses similar to those at York in the afternoon. The magnitudes of the main response (an increase in H) are similar for all the latitudes below ~ 65 deg, but become greater for latitudes above ~ 65 deg. For event B, the magnitudes of the response in H are also similar below ~ 65 deg and become much greater above ~ 65 deg. Comparing events A and B, the response for event A is much greater than that for event B below ~ 65 deg, which may be produced by the greater increase in the solar wind dynamic pressure. However, the responses above 65 latitude have similar magnitudes for event A and B, although the pressure increases are very different. This enhanced response at higher latitudes may be the result of the auroral electrojet.

The event on April 17, 1978, in Figure 6c (event C) occurs near local midnight. From this event, we see similar magnitudes of the response in H at all stations below ~ 65 deg latitudes, the same behavior as in events A and B. In contrast to the cases of events A and B, however, the magnitudes become smaller above ~ 65 deg latitude. Perhaps at midnight, the poleward stations in the IGS chain are over the polar cap where the ionospheric conductivity is low. This suggests that above ~ 65 deg latitude, the current is not controlled by the amount of dynamic pressure increase, but by the ionospheric conductivity.

Figure 7 shows || versus for all the sudden impulse events in the dayside when solar wind dynamic pressure data are available. Because of the clear latitude dependence in Figure 6, we have divided the data into two subsets: the top panel of Figure 7 shows the responses for stations with magnetic latitudes < 65 deg, and the bottom panel for magnetic latitudes > 65 deg. The solid line corresponds to the median slope, or ||/, of all the data points. Figure 8 shows || versus for all the sudden impulse events in the nightside, in same format as in Figure 7. The median slope in the bottom panel is greater than that in the top panel in the dayside, but less than that in the top panel in the nightside. This is consistent with our observations in Figure 6, where the response is stronger above 65 deg than below 65 deg in the dayside, and vice versa in the nightside. Comparing top panels in Figure 7 and Figure 8, the median slope is nearly the same, suggesting that the response has similar dependence on the change of solar wind dynamic pressure. However, the response is not as well scaled by the change of as in low and mid latitudes. The correlation coefficients are very low.

The data from three stations, York (56.4 deg MLAT), Lerwick (62.2 deg MLAT) and Tromso (66.9 deg MLAT), are used to examine the polarization of horizontal magnetic field during the sudden impulse events. Figure 9 shows the polarization of horizontal magnetic field as viewed from above the Earth's surface, with numbers indicating local times of the onset of sudden impulses. At York, as shown in Figure 9a, the horizontal magnetic field is mainly elliptically polarized in the prenoon hours. The rotation sense is clockwise for both dayside morning sector and nightside morning sector. This is very interesting because, as discussed earlier, the maximum response in the H component is a decrease in the dayside morning sector and an increase in the nightside morning sector. The polarization in the postnoon hours is not as well defined as in the prenoon hours. The horizontal field is either nearly linear or elliptically polarized. When it is elliptically polarized, the rotation sense is mainly counterclockwise. The polarization of horizontal field at Lerwick (Figure 9b) shows a similar result, i.e., clockwise in the prenoon hours and linear or counterclockwise in the postnoon hours. At Tromso (Figure 9c), the higher-latitude station, the polarization follows a similar rotation sense in general. But there are a few events with undefined polarization.

We have examined these sudden impulse events for all the IGS stations. Figure 10 is a polar plot of the maximum responses of the horizontal magnetic field, which contains the available data from all the stations for all the sudden impulse events as long as the field is relatively quiet before the sudden impulses and we can extract the disturbances from the background field. We have normalized the field variation by the change in the square root of solar wind dynamic pressure because of its success at low and mid latitudes and in the absence of alternate scaling laws. We note that the maximum horizontal field responses from all the IGS stations are similar to those at York. This can be seen more clearly in Figure 11, which is identical to Figure 10 except unit vectors are plotted to show the directions of maximum horizontal field responses. It shows that the response is toward the pole except in the dayside morning quadrant.

If we assume that all these field perturbations are generated by a horizontal current layer flowing in the ionosphere, the equivalent current system can be inferred from the field perturbation by rotating the field vectors 90 deg clockwise. In Figure 12, we show the direction of ionospheric current as inferred from Figure 11. At subauroral latitudes, the ionospheric current flows counterclockwise when viewed from the north pole everywhere except the dayside morning sector. This current flows mainly in the dayside above ~ 65 deg latitude, as suggested from events in Figure 6. We should also note that this current system is a transient response to a sudden increase in solar wind dynamic pressure with a time scale of the order of a few minutes. Because of the lack of coverage at higher latitudes, the closure of the current is not resolved in this study.

To test our understanding of the response of the magnetosphere to pressure changes, we have examined the change in the ground magnetic field at subauroral latitudes during times of sudden impulses for northward IMF. For these sudden impulse events, measurements from solar wind monitors (ISEE 3 or IMP-8) show clear, steplike increases in the solar wind dynamic pressure.

The response in H includes a brief and small preliminary decrease and a strong main increase except on the dayside morning sector, where the variation in H is a brief preliminary increase followed by a main decrease. The response in the dayside morning sector is similar to the signature of the inverted SC* type of geomagnetic sudden commencement. These types of geomagnetic signatures were also observed previously to occur preferentially in the morning side of high latitudes [Araki, 1977]. The response in the dayside afternoon and the nightside is similar to the signature of the SC (without preliminary response) and SC* (with preliminary response) types of the geomagnetic sudden commencement [Ferraro et al., 1951; Araki, 1977]. Previous studies also showed that this type of signature occurs statistically in the afternoonside at high latitudes and at the dayside equatorial region [Araki, 1977].

Observations from the IGS magnetometer chain show that the response is very strong at subauroral latitudes (55 - 70 deg). However, the signature at subauroral latitudes is highly variable and is not simply normalized by the dynamic pressure change as in the low-latitudes and mid-latitudes. Qualitatively, the response increases with increasing change of square root of solar wind dynamic pressure. The response in the higher latitudes ( > 65 deg) is stronger than that in the lower latitudes ( < 65 deg) in the dayside but weaker in the nightside.

Although the maximum response in H is opposite in the morning sector and nightside morning sector, the polarization of horizontal field is in the same sense for all prenoon hours, i.e., clockwise as viewed from above the Earth's surface. The polarization of horizontal field is not as well defined in the postnoon hours as in the prenoon hours. In general, it is either nearly linearly polarized or elliptically polarized in a counterclockwise sense as viewed from above the Earth's surface. This result is inconsistent with the earliest statistics on the polarization of the geomagnetic sudden commencement by Wilson and Sugiura [1961]. They showed that the polarization of the sc is counterclockwise in the morning hours (2200-1000 LT) and clockwise in the afternoon hours (1000-2200 LT) from low to high latitudes in the northern hemisphere, and vice versa in the southern hemisphere. Araki and Allen [1982] found a polarization reversal between latitudes 64 deg N to 72 deg N, either from counterclockwise to clockwise or vice versa as the latitude increases. We should note that neither Wilson and Sugiura [1961] nor Araki and Allen [1982] studied the polarization separately for northward IMF and southward IMF.

From the maximum response of horizontal geomagnetic field, the inferred ionospheric current in response to the pressure change is mainly a counterclockwise circulatory system at subauroral latitudes, as described in Figure 12. Iyemori and Araki [1982] have found that the ionospheric current system corresponding to sudden commencements is a counterclockwise single vortex type in the polar region during northward IMF. Our result indicates that the single vortex type of ionospheric current system for sudden impulses extends to subauroral latitudes. However, it is not clear what controls the current density below ~ 65 deg geomagnetic latitudes. Above ~ 65 deg geomagnetic latitudes, the response seems to be caused by the auroral electrojet. The magnitude of the field disturbance seems have little correlation with the amount of the solar wind dynamic pressure increase but is much greater in the dayside than in the nightside. Thus the ionospheric conductivity may be the key to the current density. The three-dimensional MHD model of Lee and Lysak [1991], in which a pressure pulse can be imposed on the magnetopause, allows the response of the magnetosphere and the ionospheric signatures to be calculated [Lysak and Lee, 1992]. We hope to compare the observations with this model in the future.

We have tried to determine if there is any bipolar signature in the H component in response to the clear, steplike solar wind dynamic pressure increase at subauroral latitudes. These bipolar variations have been inferred to be ionospheric traveling convection twin vortices. As described above, the magnitude of the preliminary response is much smaller than that of the main response in H, and the variation does not appear to be bipolar. Thus the steplike dynamic pressure changes are unlikely to be the source of the ionospheric traveling convection twin vortices. Further studies should examine the response to impulsive pressure variations in the solar wind.

Acknowledgments. This research was supported by the National Science Foundation under research grants ATM90-16900 and ATM91-11913. We thank T. Araki for useful comments. We also wish to acknowledge the assistance of W. Stuart, C. Green, D. J. Southwood, and T. J. Odera in the acquisition of the IGS data base and the National Aeronautic and Space Administration under research grant NAGW-2027 which supported the installation of this data base at UCLA.

The Editor thanks D. H. Fairfield and T. J. Rosenburg for their assistance in evaluating this paper.

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(Received July 27, 1992; revised September 28, 1992; accepted October 1, 1992.)

Fig. 1. Changes in the H component as a function of latitude predicted from the compression of the Tsyganenko ellipsoidal magnetopause around a vacuum magnetosphere. The change in the H component is proportional to the change in the square root of dynamic pressure.

Fig. 2. Map of northwestern Europe showing the locations of digitally recorded magnetometer operated by the IGS network during the IMS.

Fig. 3. (a) Responses of horizontal magnetic field (H and D) to sudden increases of solar wind dynamic pressure at York (geomagnetic latitude=56.4 deg) when the station is in the dayside morning sector. (b) Corresponding solar wind dynamic pressure data observed by ISEE 3 or IMP-8.

Fig. 4. (a) Responses of horizontal magnetic field (H and D) to sudden increases of solar wind dynamic pressure at York when the station is in the dayside afternoon sector. (b) Corresponding solar wind dynamic pressure data observed by ISEE 3 or IMP-8.

Fig. 5. Response in the H component at York as a function of local time normalized by the change in the square root of the solar wind dynamic pressure. Shown are the preliminary response (open symbols) and maximum response (solid symbols) in normalized field strength. The thin line is the response predicted by the compression of the Tsyganenko ellipsoidal magnetopause around a vacuum magnetosphere for 56.4 deg geomagnetic latitude.

Fig. 6. Stacked plots of H components from all the IGS stations for three sudden impulse events: (a) March 8, 1978, (b) January 5, 1978, and (c) April 17, 1978.

Fig. 7. Dayside response of the H component as a function of change in square root of solar wind dynamic pressure. The top panel contains data from lower latitude stations (MLAT < 65 deg ) and the bottom panel from higher-latitude stations (MLAT > 65 deg ). The solid line corresponds to median slope.

Fig. 8. Nightside response of the H component as a function of change in square root of solar wind dynamic pressure. The top panel contains data from lower-latitude IGS stations (MLAT < 65 deg ) and the bottom panel from higher latitude IGS stations (MLAT > 65 deg ). The solid line corresponds to median slope.

Fig. 9. Polarization of horizontal magnetic field as viewed from above the Earth's surface for all sudden impulse events: (a) York, (b) Lerwick, and (c) Tromso. The local times of these events are also shown in the figure.

Fig. 10. Polar plot of the maximum response in horizontal magnetic field from all the IGS stations and for all sudden impulse events when the solar wind dynamic pressure data are available. Circles are 10 deg apart in geographic latitude.

Fig. 11. Same as Figure 10 but only the directions of maximum horizontal field response are shown.

Fig. 12. Directions of ionospheric current inferred from the maximum response in horizontal field.