Characteristics of the storm-induced big bubbles (SIBBs)

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111,, doi: /2006ja011743, 2006 Characteristics of the storm-induced big bubbles (SIBBs) Hyosub Kil, 1 Larry J. Paxton, 1 Shin-Yi Su, 2 Yongliang Zhang, 1 and Hweyching Yeh 2 Received 24 March 2006; revised 2 June 2006; accepted 15 August 2006; published 19 October [1] Large equatorial plasma depletions, referred to as storm-induced big bubbles (SIBBs), are detected from the Defense Meteorological Satellite Program F15 and from the first Republic of China Satellite during the large magnetic storms of 31 March 2001, 29 October 2003, and 20 November They occur in the equatorial region at night, are elongated in the north-south direction, have steep walls, and always coexist with plasma bubbles. These observations are consistent with the SIBB characteristics described in the companion paper by Kil and Paxton [2006] and corroborate that the SIBBs are associated with bubbles. We discuss the common characteristics of the SIBBs and the role of the E B drift for the formation of the SIBBs. Citation: Kil, H., L. J. Paxton, S.-Y. Su, Y. Zhang, and H. Yeh (2006), Characteristics of the storm-induced big bubbles (SIBBs), J. Geophys. Res., 111,, doi: /2006ja Introduction [2] The formation of the large equatorial plasma density depletions during large magnetic storms has been interpreted in terms of the effects of perturbation electric fields. Greenspan et al. [1991] reported large ion density depletions (minimum density of cm 3 that extended out to ±20 magnetic latitudes) observed during the magnetic storm of March 1989 from the Defense Meteorological Satellite Program (DMSP) F9. Burke et al. [2000] observed high-speed upward drift of plasma at the locations of deep plasma depletions during the storm of 4 6 June 1991 from DMSP F9 and F10. Basu et al. [2001] and Lee et al. [2002] reported much deeper depletions observed during the magnetic storm of 15 July 2000 from DMSP F14 and F15 spacecraft and from the Korea Multipurpose Satellite-1, respectively. The minimum ion density was as low as 10 3 cm 3 at the bottom of the depletion and extended out to ±15 magnetic latitudes. During this storm, severe depletions of the total electron content (TEC) occurred in the equatorial region on the global TEC maps produced by the global positioning system [Kil et al., 2003; Vlasov et al., 2003]. Su et al. [2002] observed plasma depletions of wide in longitude during the storm of 6 7 April 2000 from the first Republic of China Satellite (ROCSAT-1). While these studies commonly considered the storminduced electric fields as the driver of this phenomenon, their interpretation was not consistent. The proposed mechanisms are (1) poleward plasma transport by fountain effect [Tanaka, 1986; Batista et al., 1991; Greenspan et al., 1991; Basu et al., 2001], (2) high-speed bubbles induced by the penetration electric field [Burke et al., 2000], (3) uplift of the 1 Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA. 2 Institute of Space Science, National Central University, Chung-Li, Taiwan. Copyright 2006 by the American Geophysical Union /06/2006JA bottomside ionosphere [Su et al., 2002], and (4) sunward convection of the nighttime plasma [Vlasov et al., 2003]. [3] The discovery of the colocations of the large plasma depletions with plasma bubbles by H. Kil and L. J. Paxton (Ionospheric disturbances during the magnetic storm of 15 July 2000: Role of the fountain effect and plasma bubbles for the formation of large equatorial plasma density depletions, submitted to Journal of Geophysical Research, 2006, hereinafter referred to as Kil and Paxton, submitted manuscript, 2006) directs our attention to the role of bubbles for the creation of the large plasma depletions. Kil and Paxton called the large plasma depletions that occur during storm time and associated with bubbles the storm-induced big bubbles (SIBBs). In this study, we solidify the connection between SIBBs and bubbles and discuss the formation mechanisms of SIBBs based on the common SIBB characteristics. For this purpose, we searched for the SIBBs using the in situ ion density measurements from DMSP F13 and F15 spacecraft and from ROCSAT-1 during the large storms of 31 March 2001, 29 October 2003, and 20 November In section 2, observations are presented during the selected storm periods. In section 3, we summarize the characteristics of the SIBBs and discuss the formation mechanisms of the SIBBs. Conclusions are given in section Observations [4] In this section, we present the in situ ion density measurements from ROCSAT-1 and DMSP F15 (2130 LT orbit) and F13 (0600 LT orbit). The altitudes of DMSP and ROCSAT-1 satellites are 840 km and 600 km, respectively. DMSP satellites have high-inclination (98 ) orbit and their measurements provide latitudinal density profiles. ROC- SAT-1 has a low-inclination (35 ) orbit and provides longitudinal density profiles. The SIBBs are detected during the magnetic storms shown in Figure 1. The times of SIBB detection are indicated by the thick vertical lines. For the 1of7

2 Figure 1. Kp and Dst indices during the geomagnetic storms of 15 July 2000, 31 March 2001, 29 October 2003, and 20 November The vertical thick lines indicate the times when the storm-induced big bubbles (SIBBs) are detected. analysis of the ionospheric disturbances during the 15 July 2000 storm, refer to the companion paper Magnetic Storm on 31 March 2001 [5] Figure 2 shows the satellite passes (top left) and measurements of ion density from F15 (bottom left) and ROCSAT-1 (right) on 31 March to 1 April The F15 pass moves from the south to the north and the ROCSAT-1 pass moves from the west to the east as time progresses. The locations of SIBBs on the F15 passes and the locations of bubbles and SIBBs on the ROCSAT-1 passes are indicated by thick lines on the map. The intervals between the vertical dotted lines in the density plots correspond to the locations of the thick lines on the map. The universal times (UTs) in the DMSP and ROCSAT-1 plots are the times of the beginning of the plots. Each plot presents about 35-min worth of data. The local times are also given on the plots of ROCSAT-1 data. Only a few bubbles are observed over E from F15. A SIBB-type plasma depletion is detected at 0832 UT over 195 E (B in Figure 2b). Around the longitude of SIBB B, ROCSAT-1 detects bubbles on the passes f and g (F and G in Figures 2f 2g). On the ROCSAT-1 passes e i, the locations of bubbles are displaced westward on later orbits. This displacement is related to the westward shift of local sunset time following the Earth s rotation. The first detection of bubbles on each orbit occurred around 1900 LT, the onset time of bubbles. [6] Figure 3 is in the same format as Figure 2 in the longitude range E. These observations are made early in the morning and here we present data from DMSP F LT orbit. F13 crosses the equator from the north to the south in the morning sector. The observations on the ROCSAT-1 passes e i show an occurrence of bubbles in broad longitude regions in the west of 305 E. A SIBB is detected on the ROCSAT-1 pass f (F1 in Figure 3f). A steep wall is formed at 260 E, the bottom is flat between E, and the plasma density increases smoothly to east of 275 E. SIBBs might be present at the longitude of F2 before local sunrise. F LT passes detect the remnants of the depletions on the passes b d. F13 passes by the longitude of F2 about 15 min later and observes a flat ion density (B in Figure 3b). The F13 pass c also detects the flat plasma densities C1 and C2. We note that the flat density is not produced by any instrumental artifact. We identified this 2of7

3 Figure 2. In situ ion density measurements during the magnetic storm of 31 March The left top panel shows the F LT and the first Republic of China Satellite (ROCSAT-1) orbits. The F15 passes cross the equator from the south to the north and the ROCSAT-1 passes proceed eastward. The left bottom panels show the measurements of ion density from F15 and the right panels show the measurements from ROCSAT-1. The universal times (UTs) in the DMSP and ROCSAT-1 plots are the times of the beginning of the plots. About 35-min worth of data are presented in each plot. The locations of bubbles and SIBBs are indicated by vertical dotted lines in the density plots and these intervals are marked by thick lines on the map. phenomenon in the plasma depleted regions in several F LT orbits on 31 March. The observations of F13 pass b and ROCSAT-1 pass f are made at similar UT (1030 UT) but the large plasma depletion is observed only on the ROCSAT-1 pass f. The solar ionization may fill the depletion earlier at 840 km than at 600 km. The ROCSAT-1 pass g detects multiple plasma depletions around 270 E. Comparison of the two passes (f g) shows the creation of larger plasma depletions near the magnetic equator than off the equator. [7] We note that bubbles are detected in the E longitude sector before 0600 UT from ROCSAT-1 and F15. The main phase of the 31 March storm started around 0600 UT. In the hypothesis of Kil and Paxton (submitted manuscript, 2006), the existence of bubbles during the main phase of the storm plays a crucial role for the formation of the SIBBs. The maintenance of the deep and wide depletion until the local sunrise (F1 in Figure 3f) and the reduction of the ion density at the locations of the SIBBs after the sunrise (F13 passes b c and ROCSAT-1 passes g i) may indicate that the plasma depletion mechanism is persistent over the night and effective even after the sunrise Magnetic Storm on 29 October 2003 [8] Two consecutive big storms are recorded on October SIBBs are observed during the first storm period in the American-African sectors from the F orbits. Figure 4 shows the observations of F15 and ROC- SAT-1. SIBBs are detected along the F15 passes a, b, and d. Around 2100 UT, F15 detects a SIBB only in the magnetic north at 2 5 E (A in Figure 4a). The ROCSAT-1 orbit f detects a few isolated plasma depletions at 2 5 E about 3of7

4 Figure 3. The same format as Figure 2 for the data observed by F LT and ROCSAT-1 orbits during the magnetic storm of 31 March The F13 passes cross the equator from the north to the south. 3 hours later (F in Figure 4f). The SIBB A is much larger than the depletions at F. ROCSAT-1 did not detect any plasma depletions in that longitude range when its orbit g passed through the location of the SIBB A at 0132 LT (4.5 hours after the SIBB A detection). There are two possible interpretations: (1) the depletions are decayed away at the time of the ROCSAT-1 pass g or (2) the SIBBs could have formed at 840 km but not at 600 km. The formation of the SIBBs only at 840 km has also been observed during other storms. The SIBB A in Figure 4a is similar to the SIBB B in Figure 2b. Both SIBBs are produced in the longitude regions where a few, small, and isolated depletions occur in the ROCSAT-1 passes. [9] A wide SIBB and deep bubbles are detected by F15 in E (Figure 4b). The SIBB B is seen to be related to the bubbles detected in the E longitude sector on the ROCSAT-1 orbits e h. The observation of bubbles at 37 S magnetic latitude (pass e) indicates that the bubbles in these longitude regions extend to very high altitudes and latitudes. Comparing SIBB A and SIBB B, SIBB B is produced in the place where bubbles are elongated to higher latitudes over a broad range of longitudes, whereas the SIBB A is produced in the place where bubbles are confined to a narrow range in longitude and latitude. The ROCSAT-1 pass g passes under the location of SIBB B about 2.5 hours later. The longitudinal width of SIBB B is 7 and the depletion depth is cm 3. The bubbles encountered on the ROCSAT-1 pass g in 330 E 0 are 1 3 wide in longitude and the depletion depth is 10 4 cm 3. The depletion is much more significant at 840 km than at 600 km. ROCSAT-1 pass h is close to the magnetic equator and larger depletions are detected. The ROCSAT-1 passes e i cross the magnetic equator but no pass evidences the formation of an ionization trough that can be attributed to the uplift of the bottomside ionosphere or poleward plasma transport. The background plasma density on either side of the locations of the bubbles is greater than 10 5 cm 3. The westward shift of the locations of the depletions on the later ROCSAT-1 orbits in the Atlantic sector is related to the westward tilt of the magnetic lines with respect to latitude. [10] ROCSAT-1 detects another bubble group near 300 E (Figure 4f i). ROCSAT-1 pass e did not detect bubbles 4of7

5 Figure 4. The same format as Figure 2 for the data observed by F LT and ROCSAT-1 orbits during the magnetic storm of 29 October around 300 E because its local time was 1750 LT in that longitude. The SIBB D at 840 km (Figure 4d) is created exactly at the longitudes where the ROCSAT-1 passes f i detected bubbles at 600 km. This observation provides clear evidence that the SIBB detected at 840 km is associated with the bubbles detected at 600 km. There is only an hour difference between the detection of SIBB D and the observation of the depletions at 295 E on the ROCSAT-1 pass h. The depletion at 840 km is deeper and wider than the depletions at 600 km. The ROCSAT-1 observations show that the depletions get wider and deeper as the satellite passes closer to the geomagnetic equator Magnetic Storm on 20 November 2003 [11] Figure 5 shows the observations from F15 (2130 LT) and ROCSAT-1 during the storm of 20 November F15 detects a SIBB in the E longitude at 2020 UT (B in Figure 5b). The ROCSAT-1 pass h detects bubbles in these longitudes at 2350 UT (H4 in Figure 5h). The earlier ROCSAT-1 orbits f g did not detect any bubbles in the 0 30 E longitude sector during UT. The small depletions are observed about 3.5 hours later on the ROC- SAT-1 pass h at the location of SIBB B. The depletion depth and longitudinal width of SIBB B is much larger than those of bubbles at H4. The situation is very similar to the SIBB B in Figure 2b and the SIBB A in Figure 4a. The F15 pass c did not detect a SIBB-type plasma depletion. The depletion depth between C1 and C2 in Figure 5c is not as deep as that of SIBB B in Figure 5b. The F15 pass c seems to occur when the SIBB was being formed. ROCSAT-1 passes f and h detect deep depletions and passes i and j detect SIBBs in the longitudes of F15 pass c (347 E). The plasma depletions F and K (Figures 5f and 5k) are not that large but are wider and deeper than normal bubbles.their locations are well aligned with the longitudes of the depletions H2, I2, and J. The zonal plasma drift velocity observed from DMSP F15 and ROCSAT-1 was small on that night. The observations of large depletions I2 and J about 3.5 hours later in the longitudes where F15 detected C1 and C2 may indicate that C1 and C2 were in the process of becoming SIBBs at that time. [12] ROCSAT-1 passes h and i detect wide plasma depletions H1 and I1 near 35 N around 300 E. Except for the depth of the depletion their morphology is similar to the 5of7

6 Figure 5. The same format as Figure 2 for the data observed by F LT and ROCSAT-1 orbits during the magnetic storm of 20 November morphology of SIBBs I2 and J. The depletions H1 and I1 did not expand further to lower latitudes. We note that bubbles did not occur around 300 E (see Figures 5d 5e and 5j 5k). The SIBB-type equatorial plasma depletions are created in the longitude sector where bubbles occur (330 E 0 ) but not in the longitude sector where bubbles are absent ( E). 3. Discussion [13] We summarize the common characteristics of the SIBBs as follows:(1) SIBBs are a nighttime phenomenon in the equatorial F region during very large magnetic storms. (2) SIBBs are elongated in the north-south direction and have steep walls and flat bottoms. (3) SIBBs are always observed with equatorial plasma bubbles. (4) Larger SIBBs are created near the magnetic equator than at the EIA. (5) In some cases, SIBBs are detected at 840 km but not at 600 km. (6) SIBBs are persistent over the night. [14] The storm-induced electric fields have been referred to as the source mechanism of the large equatorial plasma depletions [Tanaka, 1986; Batista et al., 1991; Greenspan et al.,1991; Burke et al., 2000; Basu et al., 2001; Su et al., 2002; Vlasov et al., 2003]. In the electricfield-based mechanisms, the important factor that determines the creation of the large depletions is the severity of the perturbation electric fields rather than the storm size. If the severe perturbation electric fields occur during weak and medium-sized storms, large depletions can still be created during these storms. So far, the large plasma depletions were reported only during very large storms (Dst 200 nt). The relationship between the magnitude of the perturbation electric field and the storm size needs to be verified. 6of7

7 [15] Plasma depletion to 10 3 cm 3 at 840 km can be created by the E B drift if the bottomside F layer is lifted beyond 840 km. If the large depletions were created by the fountain effect, a much deeper and wider trough should be created at 600 km than at 840 km. This predicted behavior is not evident in the comparison of ROCSAT-1 and DMSP data. The ROCSAT-1 orbit across the magnetic equator during the storm of 29 October 2003 (Figure 4), for example, did not show any signature of the uplift of the background bottomside F layer beyond 600 km in the American-African sectors where the large depletions were detected at 840 km. The background plasma density along the ROCSAT-1 passes was greater than 10 5 cm 3 and the formation of ionization trough was not evident in the equatorial region. [16] We observed multiple SIBBs divided by thin walls that had normal background density. This phenomenon may be explained if the perturbation electric fields were present discontinuously only at the locations of the SIBBs. However, this assumption is unrealistic and not supported by observations. [17] The SIBBs seem to persist or grow as time progresses. One very interesting observation is the detection of the remnants of SIBBs at 0600 LT. These observations may indicate that the plasma depletion mechanism persists over the night and is effective even after the sunrise. The persistence or the growth of the SIBBs over the night distinguishes SIBBs from the normal bubbles and equatorial ionization troughs. Bubbles and ionization troughs produced by the E B drift decay once they are created. 4. Conclusions [18] We have investigated the characteristics and formation mechanism of the large equatorial plasma depletions or SIBBs observed during large storms. The observations during different storms consistently demonstrate that the SIBBs are closely associated with plasma bubbles. This idea is supported by the characteristics of the SIBBs (elongation in the north-south direction and formation of steep walls) and by the coexistence of the SIBBs with plasma bubbles. Larger SIBBs are produced near the magnetic equator, at higher altitudes, and in the regions where the bubbles are elongated and occur over a broad longitude range. The SIBBs are seen to grow as time progresses and persist until local sunrise. The observed SIBB characteristics are not consistent with the features expected from the action of the E B drift. We plan to model the detailed interaction of the winds, electric fields, and thermospheric composition disturbance on the plasma content in a flux tube and report the results in a future paper. [19] Acknowledgments. H. Kil, L. J. Paxton, and Y. Zhang acknowledge support from NASATIMED program GUVI grant NAG S.-Y. Su and H. C. Yeh together with the ROCSAT-1 program are supported from ROC National Space Program Office with 93-NSPO(B)-IPEI-FA [20] Zuyin Pu thanks the reviewers for their assistance in evaluating this paper. References Basu, S., Su. Basu, K. M. Groves, H.-C. Yeh, S.-Y. Su, F. J. Rich, P. J. Sultan, and M. J. Keskinen (2001), Response of the equatorial ionosphere in the South Atlantic region to the great magnetic storm of July 15, 2000, Geophys. Res. Lett., 28, Batista, I. S., E. R. de Paula, M. A. Abdu, and N. B. Trivedi (1991), Ionospheric effects of the March 13, 1989, magnetic storm at low and equatorial latitudes, J. Geophys. Res., 96, 13,943 13,952. Burke, W. J., A. G. Rubin, N. C. Maynard, L. C. Gentile, P. J. Sultan, F. J. Rich, O. de La Beaujardière, C. Y. Huang, and G. R. Wilson (2000), Ionospheric disturbances observed by DMSP at middle to low latitudes during the magneticstorm of June , J. Geophys. Res., 105, 18,391 18,405. Greenspan, M. E., C. E. Rasmussen, W. J. Burke, and M. A. Abdu (1991), Equatorial density depletion observed at 840 km during the great magnetic storm of March 1989, J. Geophys.Res., 96, 13,931 13,942. Kil, H., L. J. Paxton, X. Pi, M. R. Hairston, and Y. Zhang (2003), Case study of the 15 July 2000 magnetic storm effects on the ionosphere-driver of the positive ionospheric storm in the winter hemisphere, J. Geophys. Res., 108(A11), 1391, doi: /2002ja Lee, J. J., K. W. Min, V. P. Kim, V. V. Hegai, K.-I. Oyama, F. J. Rich, and J. Kim (2002), Large density depletions in the night time upper ionosphere during the magnetic storm of July 15, 2000, Geophys. Res. Lett., 29(3), 1032, doi: /2001gl Su, S.-Y., H. C. Yeh, C. K. Chao, and R. A. Heelis (2002), Observation of a large density drop out across the magnetic field at 600 km altitude during the 6 7 April 2000 magnetic storm, J. Geophys. Res., 107(A11), 1404, doi: /2001ja Tanaka, T. (1986), Low-latitude ionospheric disturbances: Results for March 22, 1979, and their general characteristics, Geophys. Res. Lett., 13, Vlasov, M., M. C. Kelley, and H. Kil (2003), Analysis of ground-based and satellite observations of F-region behavior during the great magnetic storm of July 15, 2000, J. Atmos. Sol. Terr. Phys., 65, H. Kil, L. J. Paxton, and Y. Zhang, Johns Hopkins University Applied Physics Laboratory, Johns Hopkins Road, Laurel, MD 20723, USA. (hyosub.kil@jhuapl.edu; larry.paxton@jhuapl.edu; yongliang.zhang@ jhuapl.edu) S.-Y. Su and H. C. Yeh, Institute of Space Science, National Central University, Chung-Li, Taiwan. (t @ncu865.ncu.edu.tw; hcyeh@ jupiter.ss.ncu.edu.tw) 7of7