The effects of the pre-reversal

during solar maximum

W. S. Chen¹, J. Y. Liu¹, C. C. Lee¹, Bodo W. Reinisch², F. D. Chu¹
(submit to Annales Geophysicae at 23 July 2004)

¹Institute of Space Science, National Central University, NO. 300 Jung-da Rd., Chung-Li 320, Taiwan


²Center for Atmospheric Research, University of Massachusetts Lowell, 600 Suffolk Street, Lowell, MA 01854, United States


Abstract.
We use a digisonde at Jicamarca and a chain of GPS receivers on the west side of South America to investigate the effects of the pre-reversal enhancement (PRE) in EB drift, the asymmetry (I_a) of equatorial ionization anomaly (EIA), and the magnetic activity (Kp) on the generation of equatorial spread F (ESF). Results show that the ESF appears frequently in summer and equinoctial months, but rarely in winter months. The seasonal variation in the ESF is associated with those in the PRE EB drift and I_a. The larger EB drift (>20 m/s) and smaller |I_a| (<0.3) in summer and equinoctial months provide a preferable conditions to development the ESF. Moreover, the late reversal time of upward drift in summer also help the ESF onset. Conversely, the smaller EB drift and larger |I_a| are responsible for the lowest ESF occurrence in winter months. Regarding the effects of magnetic activity, the ESF occurrence decreases with increasing Kp in the E (March, April, September, and October) and J (May-August) months, but not in the D (November, December, January, and February) months. Furthermore, the larger and smaller EB drifts are presented under the quiet (Kp<3) and disturbed (Kp3) conditions, respectively. These results show that the suppression in ESF and the decrease in EB drifts are mainly caused by the decrease in the eastward electric field.
Key words. equatorial Spread F, pre-reversal enhancement in EB drift, the asymmetry of equatorial ionization anomaly (EIA), magnetic activity

1. Introduction

Equatorial spread F (ESF) takes its name from the range-dispersed ionogram at stations near the dip equator (Booker and Well, 1938). The ESF usually occurs just after local sunset and the scale sizes of irregularities range from a few centimeters to a few hundred kilometers. Based on the previous investigations, the basic mechanism of ESF onset is the gravitational Rayleigh-Taylor (RT) instability in conjunction with the EB drift instability (e.g. Kelley, 1989; Abdu, 2001). The growth rate of instabilities crucially depends on the density gradient that becomes steeper in the nighttime equatorial F region. Moreover, due to the non-linear evolution, these instabilities would fully develop into vertically elongated wedges of plasma depletion from bottom-side to topside of the F layer (e.g. Tsunoda et al., 1982; Zalesack et al., 1982; Sultan, 1996).

The ESF studies using ionosonde (Sales et al., 1996; Stephan et al., 2002; Whalen, 2002), the Global Positioning System (GPS) (Aarons et al., 1996; Mendillo et al., 2000, 2001), radars (Woodman and LaHoz, 1976; Hysell and Burcham, 1998), satellite (Huang et al., 2001; Su et al., 2001), and numerical modeling (Maruyama, 1988; Sultan, 1996) have demonstrated the general morphology of the ESF phenomenon. Now, it is known that there are some candidate mechanisms helping the ESF development. These are (1) the pre-reversal enhancement (PRE) in upward EB drift and associated uplifting of the F layer (Fejer et al., 1999; Whalen, 2002), (2) a small or null transequatorial component of the thermospheric winds and associated symmetry in the equatorial ionization anomaly (EIA) (Maruyama and Matuura, 1984; Maruyama, 1988; Mendillo et al., 2000, 2001), (3) a simultaneous decay of the E region conductivity at both ends of the field line (Tsunoda, 1985; Stephan et al., 2002 ), and (4) a sharp gradient at the bottomside of the F layer (Kelley, 1989).

The scope of this paper is to look for the effects of the first and second mechanisms above on the ESF onset. For two decades, the earlier works have investigated the variability of EB drift on various timescales from day-to-day to season, on longitude, and on solar and geomagnetic activities. Fejer et al. (1979) examined the vertical drift measurements of Jicamarca (12S, 76.9W) incoherent scatter radar (ISR) during 1968-1976 to find the associated variations in season and solar cycle. Abdu et al. (1983) pointed out that a necessary condition for the ESF occurrence is the EB drift from analysis of Fortaleza (4S, 38W) ionosonde data during January-April and September-December in 1978. Recently, Fejer et al. (1999) analyzed the data of the Jicamarca ISR during 1968-1992, and found that the EB drift and ESF occurrence depend on solar flux, season, and magnetic activity. Whalen (2002) also proposed the ESF occurrence and EB drift are a function of magnetic activity and season, using an array of ionospheric sounders in the western American sector during 1958.

Among the studies made on the effects of the second mechanism, most studies employed the short-term data to explore the day-to-day variability of the meridional wind and associated EIA asymmetry. Maruyama and Matuura (1984) proposed that the asymmetry in the electron density distribution, which is caused by the meridional winds, could suppress the ESF activity. However, they did not show the seasonal variation in the asymmetry in the density distribution. Furthermore, Mendillo et al (1992) have noted the possible contributions of transequatorial wind in limited samples of ESF day-to-day variability, using an all-sky imaging system at Kwajalen Atoll (9.4N, 167.5E). Sastri et al. (1997) applied two ionosondes in Brazil to examine the correlation between the ESF occurrences and the associated meridional winds in June of 1978-1981. Mendillo et al. (2000, 2001) utilized the GPS data near the equinox to calculate the asymmetry of EIA and to test the day-to-day variability of ESF.

In the present paper, we first attempt to investigate the seasonal effect of the EIA asymmetry on the ESF development, besides the seasonal effect of the PRE EB drift velocity and the magnetic activity. We conducted one-year observation on the equatorial ionosphere during April 1999-March 2000. A digisonde and a chain of GPS receivers on the west side of South America have been applied on this work. The minimum virtual height of F layer (h’F) and the latitudinal distribution of the vertical total electron content (VTEC) are used to derive the PRE EB drift and the asymmetry of EIA, respectively. Further, we utilize these data to study the dependence of ESF on the PRE EB drift velocity, the asymmetry of EIA, and the magnetic activity (Kp).

2. Experiment Setup

Figure 1 shows the locations of a digisonde and all nine GPS receivers. The Jicamarca digisonde (12S, 76.9W, magnetic dip 2N), near the geomagnetic equator, recorded ionograms from April 1999 to March 2000. The presence/absence of ESF and the ionospheric parameters (e.g. h’F, foF₂ (maximum frequency of F layer)) are determined by both the ARTIST program (Reinisch and Haung, 1983; Reinisch, 1996) and manual work. Notably, we focus on the ESF appearing during 1800-2400 LT (LT=UT-5 hours). The upward PRE velocity is derived by the temporal rate of of h’F, dh’F/dt, which is described as the EB drift velocity (Bittencourt and Abdu, 1981). Since the EB velocity generally reaches its maximum before the onset of ESF (e.g. Fejer et al., 1999; Whalen, 2002), we apply the maximum EB velocity and the associated occurring time between 1800 LT and the ESF onset time on the statistical analyses. For example (Figure 2), as the ESF appears from 1915 LT on 15 April 1999, the maximum value of dh’F/dt and the associated occurring time, 1900 LT, are adopted in the following studies.

The data of the nine GPS sites (Figure 1 and Table 1) are retrieved from the International GPS Service (IGS) for Geodynamics. Two of the GPS sites (BOGT and RIOP) are situated north of the geomagnetic equator, and the BOGT site is located near the northern crest of EIA. The other seven GPS receivers (AREQ, UNSA, TUCU, CFAG, CORD, VALP, and SANT) are placed south of the equator. The AREQ is close the equator, while the TUCU is located near the southern crest. These nine GPS sites offer the equivalent VTEC along the common meridian of ~75W (see Mendillo et al., 2000 for reference). Moreover, an asymmetry index of EIA (I_a), which is related to the transequatorial meridional winds, can be derived from the north versus south crest differences. The I_a is represented as (N-S)/((N+S)/2) (shown in Figure 3), where N and S are the VTEC values of the north and south crests, respectively (Mendillo et al., 2000). Similar to the data processes of dh’F/dt, we apply the maximum value of I_a between 1800 LT and the ESF onset time on the statistical analyses. Notably, the I_a is not estimated during December 1999-March 2000, because the BOGT data is not available in these months.

3. Results and Discussion

Figure 4 displays the seasonal variation in the occurrence probability of ESF. This seasonal variation in the occurrence probability is the percentage of days on which at least one pre-midnight ESF event is observed daily. Clearly, the highest occurrence probabilities (100%) of ESF is in January, near the summer solstice; while the lowest values (~32%) is in June, near the winter solstice. During the equinoctial months (April, September, October, and March), the occurrence probabilities are more than 75%. Generally, the occurrence probability is higher in summer/equinoxes than in winter. In this investigation, the ESF is not categorized into three levels proposed by Whalen [2002], because only one ionosonde is used. Thus, the seasonal variation in ESF (Figure 4) should be compared with that in the total bottomside spread F (BSSF) of Whalen (2002) (his Figure 4), who analyzed the ionograms at Huancayo (12S, 75.3W) during 1958. Notice that the total BSSF consists of strong and weak BSSF and bubble (Whalen, 2002), and Huancayo is located at 160 km east of Jicamarca. The comparison result shows that these two seasonal variations are similar: the highest and lowest occurrence probabilities are in summer and winter, respectively.

We examine the seasonal variation in the PRE EB drift by deriving the monthly mean value of the maximum dh’F/dt. Figure 5 shows the seasonal variation in the monthly mean value of dh’F/dt (solid line) and the associated standard deviation (error bar), as well as the occurrence probability of ESF (gray histogram in background). It is apparent that two peak values, ~28 and ~32 m/s, appear in October and March, respectively; while the smallest dh’F/dt, ~14 m/s, is in June. The seasonal variation in dh’F/dt is close to Fejer et al. (1999), who propounded that the largest and smallest vertical drift velocities are at the equinox and the June solstices, respectively. Consequently, the mean dh’F/dt are ~14-18 and ~20-27 m/s during May-August and November-February, respectively. These mean values are also close to Fejer et al. (1999), in which the maximum values are ~18 and ~28-34 m/s respectively in May-August and November-February during high solar activity period. However, in March, April, September, and October, the values of ~25-32 m/s are slightly smaller than the maximum value (~33-48 m/s) of Fejer et al. (1999) during high solar activity period.

In the equinoctial months (April, September, October, and March), the larger dh’F/dt are associated with the higher ESF occurrence. This demonstrates that the larger upward drift lifts the F layer to higher altitudes, which not only results in favorable conditions for the GRT instability but the larger eastward electric field itself causes an EB drift instability (Maruyama, 1988; Kelley, 1989). In contrast, the small dh’F/dt in the winter months (May-August) would not raise the F layer to altitudes where is high enough to generate irregularities. Thus, the ESF occurrences are lower in these months. These results reveal that the PRE EB drift plays an important role in the seeding mechanism of ESF development (Sultan, 1996; Fejer et al., 1999; Kudeki and Bhattacharyya, 1999; Whalen, 2002). Additionally, there are also other seeding mechanisms such as gravity waves (Sultan, 1996; Kudeki and Bhattacharyya, 1999).

During the summer months (November-February), the mean values of dh’F/dt, ~20-27m/s are smaller than that at the equinox, but higher than that in the winter. Notice that the occurrence probabilitie of ESF in January is the highest value (100%) between April 1999 and March 2000. Such kind of correlation between the dh’F/dt and the ESF occurrence indicates that the upward drifts are still large enough to generate the ESF. Besides the large upward velocity, Maruyama (1988) and Fejer et al. (1999) proposed that the late reversal time of upward drift could also help the ESF developments at December solstice. Here, we examine the seasonal variations in the monthly mean occurring time of the maximum dh’F/dt, as shown in Figure 6. The occurring time of the maximum dh’F/dt can be treated as the reversal time of the upward EB drift. It is found that the occurring times near summer solstice (December and January) are later than that in other months. These results demonstrate that that the large upward EB drift and the late occurring time would provide the most favorable condition for the generation of ESF over Jicamarca during summer (Maruyama, 1988; Fejer et al., 1999).

Figure 7 shows the seasonal variation in the monthly mean value of I_a (solid line), the associated standard deviation (error bar), and the occurrence probability of ESF (gray histogram in background). It is found that the I_a is negative and |I_a| is greater than 0.3 during May-August (winter). In April, September, and October (equinox), the I_a is also negative, but |I_a| is smaller than 0.3. In contrast, the I_a is positive and |I_a| is smaller than 0.3 in November (summer). According to Mendillo et al. (2000, 2001), the I_a index is associated with the transequatorial meridional winds that would distort a presumed symmetrical EIA. Thus, the meridional winds are generally southward and northward in April-October and November, respectively. Moreover, the greater values (> 0.3) of |I_a| during May-August indicate that the associated meridional winds in the winter months are stronger than that in the equinoctial (April, September, and October) and summer (November) months. These are similar to the meridional winds used by Maruyama (1988), in which the wind is northward and stronger near the winter solstice (in his Figure 3).

As shown in Figure 7, the greater |I_a| and lower ESF occurrence concurrently appear during May-August. Based on Maruyama and Matuura (1984), Maruyama (1988), and Mendillo et al. (1992), this negative correlation between these two parameters indicates that the asymmetric EIA is related to the inhibition of ESF development. This is because that the asymmetry of EIA would form an asymmetry in distribution of conductivity and recombination rates along the field line. Then, the asymmetry in conductivity and recombination rates further suppresses the RT growth rate (Maruyama and Matuura, 1984; Maruyama, 1988; Mendillo et al., 1992). Besides the suppressive effect of I_a, the smaller dh’F/dt in the winter months (Figure 5) is not helpful in developing the ESF, either. Therefore, the lower ESF occurrence is caused by the greater |I_a| and the smaller dh’F/dt.

On the other hand, the smaller |I_a| and higher ESF occurrence concurrently appear in April, September, and October. This result shows that the |I_a| is negatively correlated with the ESF occurrence, and demonstrates that the asymmetry in conductivity and recombination rates would be not strong enough to suppress the RT growth rate. Recall that, in the equinoctial months (Figure 5), the values of dh’F/dt are larger. Thus, the higher ESF occurrence is due to not only the larger dh’F/dt but the less |I_a| during the equinoctial months. Additionally, the smaller |I_a| and higher ESF occurrence are also simultaneously in November. Similar to the equinoctial months, the higher ESF occurrence is caused by both the smaller |I_a| and larger dh’F/dt.

The ESF is examined on each day in relation to the magnetic activity on that day, which is taken as the average value of Kp recorded during the 6 hours prior to the onset of ESF (e.g. Fejer et al., 1999; Whalen, 2002). Notice that the average Kp is generally derived from the interval of 1800-2400 UT (1300-1900 LT). Furthermore, the months between April 1999 and March 2000 are categorized into E (March, April, September, and October), D (November-February), and J (May-August) months. These definitions of season and of average Kp are same as those used by Fejer et al. (1999) and Whalen (2002).

The numbers of days on ESF are organized by season and plotted versus the average Kp in Figure 8. For the E months (Figure 8a), the ESF occurs frequently and rarely when Kp is 1 and 5, respectively. Except Kp=2, the number of days tends to decrease with increasing Kp in the E months. Moreover, a descending trend from Kp=1 to 5 is obvious in the J month (Figure 8c), but not in the D months (Figure 8b). The decreasing trends in the E and J months indicate that the increasing magnetic activity progressively suppresses the ESF generation in these two seasons (Fejer et al., 1999; Whalen, 2002). However, these distributions are not consistent with that of total BSSF in Whalen (2002) (his Figure 5), which reported that the decreasing trends existed in the E and D months, not in the J months. This might be because that the distributions of Kp day are different for each seasons between 1958 and 1999-2000.

To further reveal the geomagnetic effect on dh’F/dt and I_a, we examine the monthly mean values of dh’F/dt (Figure 9a) and I_a (Figure 9b) under the quiet (Kp<3) and disturbed (Kp3) conditions during April 1999-March 2000. In the E months, the mean dh’F/dt as Kp3 is less than that as Kp<3, and the differences are ~3-8 m/s. For the D months, the dh’F/dt is larger under the quiet condition and the differences are ~2-5 m/s, except February. Furthermore, for the J months, the dh’F/dt as Kp<3 is also larger in May and August and the differences are ~8 and ~3 m/s, respectively. Notice that no dh’F/dt as Kp3 is shown in June, because all Kp is smaller than 3. Furthermore, only one dh’F/dt as Kp3 is in July and this large upward velocity is related to the prompt penetration effect (Fejer and Scherliess, 1997; Fejer et al., 1999). Generally, the dh’F/dt is smaller under the disturbed condition, and these results are similar to Fejer et al. (1999) that proposed the maximum vertical drift is respectively larger and smaller under the quiet and disturbed conditions during high solar activity period (their Figure 8). The decrease in dh’F/dt indicates that the maximum eastward electric field of PRE is reduced by the disturbed magnetic activity (Scherliess and Fejer, 1997). Moreover, this decrease is due to the disturbance dynamo electric fields which decrease the upward drift velocity near sunset (Fejer et al., 1999).

Regarding the I_a (Fgiure 9b), it found that the mean |I_a|s as Kp<3 are larger than that as Kp3 in August and September. In contrast, the mean |I_a|s as Kp<3 are smaller than that as Kp3 in October and November. Moreover, the mean |I_a|s are nearly same between the quiet and disturbed conditions in April, May, and July. These results suggest that the effect of the magnetic activity on I_a seems not to be evident. Since the I_a is related the meridional winds, the disturbed magnetic activity does not obviously affect the strength of the winds.

4. Summary and Conclusion

We have analyzed the ionogram and TEC data to investigate the long-duration effects of the PRE E×B drift, the EIA asymmetry, and the magnetic activity on the ESF generation. The ionogram and TEC data during April 1999-March 2000 are obtained by a digisonde and a chain of GPS receivers, respectively. It is remarkable that this work first attempt to examine the seasonal variation in the EIA asymmetry. The results point out that the dependence of ESF on season and magnetic activity can be explained by the corresponding effects on the E×B drift and EIA asymmetry.

For the seasonal variation, the ESF appears frequently in summer and equinoctial months, but rarely in winter months. Such kind of occurrence variation is evidently correlated to the magnitude of E×B drift. The correlations in summer and equinoctial months demonstrate that the ionosphere is lifted to higher altitudes where the gravitational drift term is dominant in GRT growth rate leading to develop the ESF, when the upward drift velocities are larger than 20 m/s. Further, the late reversal time would help the ESF generation. During summer, the larger E×B velocity and late reversal time, which is a good condition for the ESF development, cause the highest occurrence probability in this one-year observation. Moreover, the influence of the EIA asymmetry (I_a) are also significant to the ESF occurrence. The value of |I_a| is generally anti-correlated to the ESF occurrence. This anti-correlation indicates that the EIA asymmetry (|I_a|＞0.3) could suppress the RT growth rate, because of the asymmetry in distribution of conductivity and recombination rates.

For the magnetic activity, we have displayed that the number of ESF days decreases with increasing Kp in the E and J months, but not in the D months. This descending trend suggests that the magnetic activity progressively suppresses the ESF generation in those two months. Additionally, our results shows that the E×B velocity is larger and smaller under the quiet and disturbed conditions, respectively. This indicate that this ESF suppression would be due to the effect of magnetic activity on the E×B velocity. Contrarily, it is found that the I_a is not obviously related to the magnetic activity.

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