AGU Chapman Conference on Storm-Substorm Relationship

Substorm Current System and the Enhancement of the Ring Currentt Belt During the Main Phase of a Storm

W. Sun and S.-I. Akasofu

Abstract. It is now well established that O ⁺ ions, instead of protons, are often dominant ions in the storm-time ring current belt. The present study demonstrates that upward field-aligned currents (UFAC) are crucial for the development of the ring current belt during the main phase of geomagnetic storm, although the contribution of O⁺ as a current carrier is not certain.

For this study, we use the fact that positive perturbations of the Y component in middle and low latitudes are caused by field-aligned currents [Sun et al., 1984]. In our earlier study, the AF index is produced by using the maximum positive deviation of the Y component. We have improved the earlier AF index by integrating the positive Y component in terms of longitude, providing a better index for the total UFAC. We examine in detail the storm of January 10, 1997 and a number of others. It is shown that there is a good correlation between the AF index thus obtained and Dst values during the main phase for 22 storm events in 1997 and 1998. The correlation coefficient between the AF index and Dst is 0.94, demonstrating that there is a close relation of the enhancement of ring current during the main phase of a storm and UFAC during substorms.

1. Data Analysis

Magnetometer records from 111 stations on January 10, 1997 are used to determine the ionospheric horizontal currents and field-aligned currents by using the KRM algorithm [Kamide et al., 1981]. The upper and middle panels in Figure 1 show the AU, AL and Dst index, respectively, on January 10, 1997 with a time resolution of 5 minutes. The dot curve in the middle panel shows the hourly Dst values from WDC-2. Many substorm events occur during the day based on the AL index. The Dst index indicates a typical magnetic storm with a lowest Dst value of -77.6 nT at 0955UT. Two big substorm events occurred at 0810 UT and 1105 UT with AL values of -1541 nT and -1646 nT, respectively, are adopted to examine the relationship between substorm current system and the ring current. Figure 2 shows the distribution of ionospheric current vectors and field-aligned currents at 0810 UT and 1105 UT. First event at 0810 UT has weaker elcectrojet and field-aligned currents than second one at 1105 UT. However, it is obvious that electrojet and associated field-aligned currents in first event, which constructs so-called substorm current wedge, locate at lower latitude than second one.

As is well known, the Y (E-W) component variations observed at middle and low latitudes (Y-MLL) are mainly caused by field-aligned currents [c.f. Sun et al., 1984]. Thus we attempt to apply observed Y-MLL to monitor field-aligned currents during substorms. The adopted eight stations are uniformly distributed along longitude. Hence the X (N-S) component variations recorded at eight mid- and low-latitude stations (X-MLL) can be also employed to estimete the Dst index with a high time resolution. Meanwhile, nine stations in the auroral zone are adopted to monitor substorm electrojet. Codes, geographic and eccential dipole coordinates of all 17 stations are listed in table 1.

2. Determination of the AF index

Two top panels in Figure 4 show distributions of field-aligned currents on the equatorial plane (X-Y plane in GSM coordinate) for the two substorm events. The field-aligned currents on the equatorial plane are mapped from the ionosphere at latitudes of 50° ~ 65 ° by using Tsyganenko model (1987). Contour of negative values with red-yellow color shows field-aligned currents flowing into the equatorial plane (upward in the ionosphere). Contour of positive values with blue-cran color indicates field-aligned currents flowing out of the equatorial plane (downward in the ionosphere). A clear wedge-like configuration of field-aligned currents, flowing in at pre-midnight sector and flowing out in post-midnight sector, can be found in two panels. The lower two panels show X component variations in auroral zone (X-AUZ), Y and X component variations at middle and low latitudes (Y-MLL, X-MLL) at diferent magnetic local time (MLT) for two events. The Y-MLL shaded by red color shows positive values in the pre-midnight sector and negative values in the post-midnight sector, which is consistent with the wedge-like configuration of field-aligned currents for both events. However, the magnitude of Y-MLL for the first event at 0810 UT is obviously higher than that in second one at 1105 UT, while field-aligned currents on the equatorial plane around midnight at 0810 UT are more intense than that at 1105 UT although the intensity of auroral electrojet at 0810 UT is lower than that at 1105 UT as shown by lowest value of X-AUZ (or AL index). Hence, Y-MLL is a good measure for the field-aligned currents between 50 ° and 65 ° of latitudes. The contribution to Y-MLL from the field-aligned currents at higher than 65° of latitude is negligible.

As to be discussed above, O⁺ ions may be injected into the equatorial. Thus, positive Y-MLL may be a good messure of the amount of the injected ions near earth region (3 ~ 6 RE). Taking into an account of the range of MLT for injected ions, the integrated positive Y-MLL (ie. positive shaded area) is defined as the new AF index, although the bulk of UFAC is likely to be carried by downward electorns.

3. Correlation of AF index to the Dst Index

We adopted 22 storm events during 1997 and 1998 and calculated the new AF index and the Dst index for each events which are similar to lower two panels in Figure 1. Figure 5 shows correlation between the maxmum AF values and minimum of the Dst values for 22 events. There is a high correlation coeficient of 0.94. This result suggests strongly that there is a close relation between UFAC and the enhancement of the ring current during the main phase of a storm.

4. Asymmetry of the ring current belt during the main phase of a storm

Examining X-MLL which indicates the intensity of the ring current, it can be found that the ring current is highly asymmartric in MLT during the main phase of a storm [cf. Akasofu...]. It is significant to note that the intense negative perturbations of X-MLL caused by westward ring current in the afternoon and evening sectors consist of the section of positive Y-MLL. Negative Y-MLL at early morning sector corresponds to positive or weak negative X-MLL (eastward or weak westward ring current).

5. Summary

(1) Field-aligned currents associated with substorms can be monitored by Y-MLL. Thus the new AF index is designed to be the integration of positive perturbations of Y-MLL. The new AF index is a measure of the effect of UFAC in the ionosphere during substorms.

(2) The new AF index includes effects from both intensity and latitude of UFAC. Thus it is not definitely propotional to the AL/AE index which represents the effect of only intensity of electrojets as shown by X-AUZ and Y-MLL in Figure 4.

(3) Assuming that the enhancement of O⁺ ions in the ring current belt during the main phase of a storm are injected through UFAC from the ionosphere during substorms, we made a statistics for the relationship between the minimum Dst index and maximun AF index during the main phase of storms. The result shows a good correlation between the minimum Dst and the maximum AF with a correlation coeficient of 0.94. It suggests strongly the close connection between the enhancement of the ring current belt and UFAC during the main phase of a storm.

(4) The ring current belt in the main phase of a storm is highly asymmatric in MLT. The left panel in Figure 6 shows X-MLL and Y-MLL from 0400UT to 0930UT during the main phase of the storm and the right panel shows those during the recovery phase. It is shown that the westward ring current at 12 - 0 MLT significantely increases when UFAC (indicated by positive Y-MLL) intesified during the main phase. But the ring current belt tends to be symmatric (uniform) in MLT during the recovery phase (particularly after 2000UT). It may be explained by that the injected ions do not form an uniform (symmetric) ring current during the main phase and accumulate at a local region (afternoon and evening) to form an intense partial ring current. A part of ions will be disappeared due to recombination and the others drift continuesly to form an uniform ring current belt during the recovery phase.