Geomagnetic observations and ionospheric response during storm on 14 April 2006

Transcription

1 Indian Journal of Radio & Space Physics Vol 39, April 2010, pp Geomagnetic observations and ionospheric response during storm on 14 April 2006 N O Bakare $,*, V U Chukwuma & B J Adekoya Department of Physics, Olabisi Onabanjo University, Ago-Iwoye, Nigeria $ sesanbakare1@yahoo.com Received 5 February 2009; revised 8 December 2009; accepted 14 December 2009 A study of the intense geomagnetic storm on 14 April 2006 has been done using ionospheric data obtained from ionosonde station located in the American sector. The results indicate appearance of positive storm before the beginning of geomagnetic disturbance in the mid latitudes and occurrence of strong negative phase at the equator. It was observed that magnetospheric process, responsible for Dst decrease, was the enhancement of the plasma sheet and appeared to be caused by the combined effect of large intense proton density at 1400 hrs UT, southward turning of Bz at 1800 hrs UT and enhancement of speed stream at 1400 hrs UT on 13 April. The ionosonde stations located at American sectors are: College (64.9 N), Gakoma (62.4 N), Goose bay (53.3 N), Boulder (40.0 N), Dyess (32.2 N), Eglin AFB (30.4 N) and Puerto Rico (18.5 N). The present results were obtained using critical frequency fof2. The peak height of F2 layer hmf2 showed that the stations had some degree of simultaneity in the depletion of fof2 between 2000 hrs UT on 14 April and 0100 hrs UT on 15 April and that of hmf2 between 1600 and 1900 hrs UT on 15 April. Keywords: Geomagnetic storm, F2 region critical frequency (fof2) depletion, Interplanetary magnetic field, F2 layer peak height (hmf2) variation, Low latitude index Dst, Solar wind flow speed, Ion number density, Proton temperature PACS Nos: Vv; dj 1 Introduction The Earth is surrounded not by vacuum but by a highly dynamic and coupled system of plasma and magnetic field whose complex interplay constitutes the subject of space weather. The most visible manifestation of space weather is intense auroral activity which occurs during magnetic storms and substorms. The term space weather refers to condition on the sun and in the solar wind, magnetosphere, ionosphere and thermosphere that can influence the performance and reliability of space borne and ground base technological systems and can endanger human life or health. Adverse conditions in space environment can cause disruptions of satellite operations, communications, navigations and electric power distribution leading to a variety of socioeconomic losses (ref. 1 and references therein). The above mentioned statement underscores the increasingly worldwide recognition of the importance of space weather research which has led in recent years to the intensification of investigations of geomagnetic storms. The main objective of space weather studies is to understand the solar and interplanetary causes of magnetic storms and the ionospheric phenomena associated with these magnetic storm. In this work, the geomagnetic observations and ionospheric response during the storm on 14 April 2006 have been presented. This is an attempt to contribute to the current understanding of magnetospheric processes that generate intense magnetic storm as well as explain the F2 region response associated with each interplanetary structure and magnetospheric processes. 2 Interplanetary and geomagnetic observations The dominant interplanetary phenomena causing intense magnetic storm are the interplanetary manifestation of fast coronal mass ejections (CMEs) (refs 2,3). However, it has been reported that the continuous ejection of solar flares and CMEs can be used for safe prediction of geomagnetic storm 4. As soon as a strong flare is seen or a side halo CME is seen and its lateral expansion is measured, a general alert may be given that something might happen in the next few tens of hours. Two interplanetary structures are important for the development of such class of storms: the sheath region just behind the forward shock, and the CME ejecta itself. Frequently, these structures lead to the development of intense storms with two-step growth in their main phase. These structures also lead sometimes to the development of

2 72 INDIAN J RADIO & SPACE PHYS, APRIL 2010 very intense storms, especially when an additional interplanetary shock is found in the sheath plasma of the primary structure accompanying another stream 2. It is important to note that heliographic coordinates of an associated flare are used as the solar source location of CMEs. According to Gopalswamy 5, the solar sources of geoeffective CMEs are typically located within ± 30 from the disk centre. Alexander 6 states that the largest solar flares tend to be associated with CME events and by extension are associated with intense storms. However, it does appear that the largest solar flares do not always cause intense storms while moderate flares do (ref. 7 and references therein). The orientation of the interplanetary magnetic field (IMF) carried by the solar wind is also a very important factor. Geomagnetic activity is known to enhance dramatically whenever the IMF is directed southward 8. It is well established that the Bz component of IMF has the most important influence on the magnetosphere and high-latitude ionosphere as it controls the fraction of the energy in the solar wind which is extracted by the magnetosphere. When Bz is strongly negative, magnetic reconnection between IMF and geomagnetic field produces open field lines which allow mass, energy and momentum to be transferred from solar wind to the Earth s magnetosphere 9. It was also suggested that the primary cause of magnetic storm is the intense and long duration of southward interplanetary magnetic field Figure 1 shows interplanetary and geomagnetic observations during April The storms are summarized using the low latitude index Dst, interplanetary magnetic field Bz, solar wind flow speed, ion number density, electric field, proton temperature and plasma beta. The hourly data has been obtained from National Space Science Centre s OMNIWeb Service ( Dst variation show a monotonic decrease of Dst min in the main phase having a minimum peak (-111nT) at 0900 hrs UT on 14 April. Vieira et al. had classified geomagnetic storms with Dst below -100 nt as intense geomagnetic storms 3. It may be noted that the storm main phase (storm onset) occurs in near coincidences with the sharp southward turning of IMF at the magnetic cloud boundary. The storm main phase development is rapid and the decrease is monotonic. There are numerous mechanisms that can lead to southward components fields in the sheath 13. The Bz plot shows that until ~ 0000 hrs UT on 13 April, there was no definite trend in Bz variations. At ~ 0000 hrs UT, there came a sharp southward turning of Bz; at 0400 hrs UT, Bz reached a value of nt; and then attained a peak value of nt at 0800 hrs UT indicating that IMF has experienced about four hours southward component. It is important to note that the southward turning of Bz at ~ 0400 hrs UT appear to have triggered the depression of Dst beginning from 0100 hrs UT. According to Gonzalez & Tsurutani 10, the IMF structure leading to intense magnetic storms have intense (>10 nt) and long direction (>3 h) southward component. The flow speed plot show a moderate stream throughout 12 April. The stream attained a minimum value of 382 km s -1 at 1800 hrs UT on the same day. Thereafter, there is an enhancement in the speed stream which reached a peak value of 676 km s -1 on 1400 hrs UT on 15 April. It may be noted that during April, the solar wind attained the 500 km s -1 and met criterion of fast solar wind. According to Gonzalez et al. 2,14, intense magnetic storms (Dst < nt) occur when solar wind speed is substantially higher than the average speed of 350 km s -1. The proton number density plot presents the proton density increasing from 0900 hrs UT on 13 April. The proton number density attained a peak value of 17.7 cm -3 at 1400 hrs UT. The large increase in the proton number density during this period signals the arrival of a shock in the interplanetary medium 15. The enhanced solar wind density at 1400 hrs UT drove the plasma sheet density leading to the injection of the ring current and this caused sharp depression in Dst in the interval between 0000 and 0900 hrs UT on 14 April. This statement derives the fact that the plasma sheet density is found to correlate well with high solar wind density 16. It has been shown that the solar wind density drives plasma sheet density with the source of ring current particles being the plasma sheet (ref. 16 and references therein). The electric field began a gradual increase and reached a value of 7.14 mv m -1 at 0400 hrs UT. Four hours later, at 0800 hrs UT it attained its peak value of 7.30 mv m -1. These electric field conditions which gave Bz > 10nT are indicative of an intense storm 17. The structure of geomagnetic storm on 13 April is clearer by the proton temperature and plasma beta. The plasma beta plot shows a value of 0.07 at 1700 hrs UT on 13 April, which increased to 0.12 at 0800 hrs UT on 14 April. For the same time, the

3 BAKARE et al.: GEOMAGNETIC OBSERVATIONs & IONOSPHERIC RESPONSE ON 14 APRIL Fig. 1 Composition of interplanetary and geomagnetic observations during April 2006

4 74 INDIAN J RADIO & SPACE PHYS, APRIL 2010 plasma temperature was after the arrival of the shock decrease precipitously to a low temperature of ~ K at 1700 hrs UT on 13 April. It transpired from the low plasma beta and proton temperature value that the shock followed ejecta which were a magnetic cloud Ionospheric data and Method of analysis The ionospheric data used in this study consists of fof2 and hmf2 obtained from some of the National Geophysical Data Center s SPIDR (Space Physics Interactive Data Resource), a network of ionosonde stations located in the American sector: College, Gakoma, Goose bay, Boulder, Dyess, Eglin AFB, and Puerto Rico. These stations are listed in Table 1. The present study is concerned with variations in fof2 due to the intense geomagnetic storm of 14 April However, the F2 region response to geomagnetic storms is most conveniently described in terms of D(foF2) that is the normalized deviations of the critical frequency fof2 (ref. 19): D( fof 2) = fof2 ( fof 2) ave ( fof 2) ave Hence, the data analysis consisted of D(foF2) of respective hourly values of fof2 during April. The reference for each hour is the average value of fof2 for that hour calculated from the five quiet days during April The criterion for chosen references day is the values of Dst -25nT. Similarly, hmf2 due to geomagnetic storm was considered. The term D(hmF2) is the normalized deviations of the critical frequency hmf2 from the reference. hmf2 ( hmf2) ave D( hmf2) = ( hmf 2) ave American sector Table 1 List of ionosonde stations Code Geographic co-ordinates Difference between IST & UT, hrs φ λ College CO N E -10 Gakoma GA N E -10 Goose Bay GSJ N E -4 Boulder BC N E -7 Dyess DS N E -7 Eglin AFB EG N E -6 Puerto Rico PRJ N E -5 The use of D(foF2) rather than fof2, and of D(hmF2) rather than hmf2 provides a first-order correction for temporal, seasonal and solar cycle variations so that geomagnetic storm effects are better identified. Furthermore, the criterion used in selecting the stations is such that storm variations represented real changes in electron density not simply redistribution of the existing plasma Results and Discussion The plots illustrating D(foF2) vs time (hrs UT) during April 2006 for American sector are depicted in Fig 2. As shown, following the storm commencement on 14 April, there is immediate effect on fof2 in the ionosphere at all stations which are preceded by positive storm. There is appearance of positive storm before the beginning of the geomagnetic disturbances at all stations. However, at 1800 hrs UT on 14 April, depletion of fof2 became obvious at all the stations irrespective of their latitude. On 15 April, a definite pattern began to emerge from available data with respect to 0000 hrs UT. As shown, all the stations indicated, on an average, some degree of simultaneity in fof2. D(foF2) for College shows a negative storm that occurred at 0300 hrs UT on 14 April, the ionosphere above this stations immediately show a positive storm with peak values at 0400 and 0600 hrs UT. However, beginning at 0600 hrs UT fof2 decreased sharply reaching a minimum value of 20% from the reference level at 0900 hrs UT. The D(foF2) variation showed fof2 recovering to 25% increase at 1300 hrs UT. But following a sharp increase in the proton density at 1400 hrs UT, fof2 began to decrease again reaching 20 and 24% depletion at 0100 and 0500 hrs UT, respectively on 15 April. fof2 recovered to 29% at 1000 hrs UT on 15 April but thereafter, started decreasing again and reached 22% at 0000 hrs UT on 16 April before reaching peak depletion of 31% at 0600 hrs UT on 16 April. It is convenient to suggest that depletion of fof2 beginning from 0300 hrs UT on 15 April is a consequence of the large increase in the proton number density at 1400 hrs UT on 13 April. The ionosphere at Gakoma showed a moderate negative storm that occurred at 0300 hrs UT on 14 April. The situation was followed by a positive storm reaching peak value of 137 and 148% at 0500 and 1200 hrs UT, respectively. D(foF2) started to decrease rather sluggishly reaching 45% depletion at 0600 hrs UT indicating the commencement of an

5 BAKARE et al.: GEOMAGNETIC OBSERVATIONs & IONOSPHERIC RESPONSE ON 14 APRIL Fig. 2 Variation in D(foF2) in American sector during April 2006

6 76 INDIAN J RADIO & SPACE PHYS, APRIL 2010 intense storm. Thereafter, fof2 recovered to a positive storm at 0800 hrs UT and lasted till 1300 hrs UT on 15 April. And beginning from 1300 hrs UT on same day, fof2 started to decrease but recovered abruptly only to depress sharply at 1400 hrs UT to reach 47% depletion at 0600 hrs UT. By 1300 hrs UT, fof2 had recovered to a weak positive storm. D(foF2) at Goose bay showed a strong negative storm that occurred at 0100 hrs UT on 14 April. The situation followed by a positive storm reaching a peak value of 58% at 0700 hrs UT before steeply decreasing to 22% at 1200 hrs UT which lasted for about 12 h before reaching the peak depletion of 52% at 0100 hrs UT on 15 April. The D(foF2) variation showed fof2 recovering to 80% increase at 0700 hrs UT on 15 April before gradual decrease to 27% at 1600 hrs UT which lasted until 1800 hrs UT and gradually recovered again. The ionosphere at Boulder showed moderate positive storm that occurred during hrs UT on 14 April. At 0500 hrs UT, the ionosphere showed a negative storm that lasted throughout 14 April with peak depletion values of 34, 25, 25 and 35% at 1000, 1800, 2300 and 0800 hrs UT, respectively. The fof2 recovered abruptly reaching a peak value of 67% at 1100 hrs UT on 15 April before sharp reduction to a negative phase storm almost throughout 16 April. The D(foF2) plot for Dyess appear to show positive ionospheric response to the magnetospheric processes during the period hrs UT. The ionosphere registered a negative storm with 16% depletion that lasted until 38% depletion at 1000 hrs UT and the peak electron density. The ionosphere showed gradual increase to a positive phase storm at 1500 hrs UT on same day before fof2 began to decrease rapidly leading to an intense negative storm at 1800 hrs UT on 14 April with 14% depletion of fof2. D(foF2) started to recover gradually before steeply decreased to 28% at 2300 hrs UT on 15 April. D(foF2) started to recover again before it sharply decreased to 31% at 1000 hrs UT on 15 April. The D(foF2) variation for Eglin AFB showed strong ionospheric F2 region response to the ionospheric processes during the period hrs UT on 14 April. At 0500 hrs UT, the station recorded a major negative storm with 44% depletion at 0900 hrs UT which was followed by a positive storm at 1500 hrs UT. But beginning from 2000 hrs UT on 14 April, fof2 started decreasing sharply reaching 35% depletion at 0000 hrs UT and 44% at 0800 hrs UT which lasted until 1200 hrs UT on 15 April indicating the commencement of an intense storm. Beginning at 1200 hrs UT, the fof2 started to recover gradually until it reached peak at 26% at 2000 hrs UT on 15 April before it gradually decreased. The D(foF2) plot for Puerto Rico shows a positive ionospheric response during hrs UT on 14 April. Thereafter, fof2 started to decrease and lead to negative phase at 0600 hrs UT. The fof2 recovered to a positive storm at 0900 hrs UT on the same day. The D(foF2) plot showed that the ionosphere at Puerto Rico is characterized by positive storm during the period under investigation. The D(foF2) variation further showed negative phase at 1000 and 1700 hrs UT with 21 and 12% depletion, respectively on 14 April. Figure 3 illustrates D(hmF2) vs time (hrs UT) during April 2006 for American sector. As shown, a positive storm that occurred during hrs UT on 14 April became obvious at all stations irrespective of their latitude. On the same day, all the stations on an average showed some degree of enhancement in the hmf2 during hrs UT with the exception of high latitude of Gakoma and Goose bay which showed depletion in the hmf2. Starting from 15 April, a definite pattern began to emerge from the available data at ~ 0000 hrs UT and all the stations indicated on an average some degree of enhancement in hmf2. The simultaneous intense depletion of fof2 at all latitudes at 0000 hrs UT on 16 April suggest that during the intense geomagnetic storm on 14 April 2006, the fof2 depletion at all stations may not be mainly due to changes in neutral composition resulting from neutral wind produced predominantly in the region of Joule heating in the aurora zone. According to Prolss (ref. 21 and references therein), during very intense geomagnetic activity soft particle precipitation increases the vibrational excitation of molecular nitrogen which in turn increases the loss of ionization at F2 region heights as shown in Fig. 3. It is important to note that Maih 22 has reported this low energy (soft) particle precipitation at F2 heights in the equatorial zone. The precipitating particles have also been suggested as the source of heating of the lower part of the thermosphere 23, which may lead to thermospheric composition changes. Given that particle precipitation is known to occur at both higher and lower latitudes during very intense geomagnetic

7 BAKARE et al.: GEOMAGNETIC OBSERVATIONs & IONOSPHERIC RESPONSE ON 14 APRIL Fig. 3 Variation in D(hmF2) in American sector during April 2006

8 78 INDIAN J RADIO & SPACE PHYS, APRIL 2010 disturbances (ref. 21 and references therein), particle precipitation as a mechanism may account for the present simultaneous depletion fof2. It is known that the most prominent feature during an ionospheric storm is the depletion of fof2 (ref. 24) and the total electron content. Seaton 25 introduced the concept of the change in neutral composition during the magnetic storms. The ideas were further elaborated by King 26 that atmospheric waves generated by non-uniform heating in the aurora region as a mechanism for increasing the mixing and thereby changing the composition in the mid-latitude station. The studies on the complex processes of interaction between the ionospheric plasma and neutral gas 27,28 have shown that the decrease in [O]/[N2] in the lower latitude triggers a complex chain of reactions in the ionosphere: the heating and cooling rate; increase in electron, ion and neutral temperatures; and a decrease in electron density near the F2 peak. The decrease in electron density is a direct consequence of the increase in [O2] and [N2] without a substantial change in [O] at F region altitudes 29,30. Finally, the 14 April storm showed that the decrease in fof2 was extended to very low latitude of Puerto Rico ( N) and at the same time globally, probably because of the intense nature of the geomagnetic activity. 5 Conclusion In this paper, a picture of F2 region global structure response to geomagnetic storm using the storm of 14 April 2006 has been presented and it has been shown that the arrival of the shock in the interplanetary medium showed the depletion of fof2 in the ionosphere 24. The storm event of 14 April 2006 is a single step storm and the depletion of fof2 was simultaneous. The present result appears to confirm the suggestion of earlier studies The observed simultaneous depletion of fof2 at all latitudes does not appear to support the previously held notion that the depletion of F2 region plasma density is due to changes in neutral composition resulting from neutral wind produced predominantly by Joule heating in the aurora zone, but rather suggests that particle precipitation does contribute to depletion of fof2 at all latitudes during intense magnetic storm. Acknowledgements The daily values of ionosonde data for all stations were taken from the National Geophysical Centre s SPIDR (http//spidr.ngdc.noaa.gov) and from NSSDCs OMNIweb service (http//nssdc.gsfc.nasa.gov/omni web) for which the authors are grateful. References 1 Chukwuma V U & Bakare N O, An investigation into geomagnetic and ionospheric response associated with the storm of April 12-14, 1981, Nigeria, J Phys (Nigeria), 18 (2) (2006) pp Gonzalez W D, Clua de Gonzalez A L, Sobral J H A, Dal lago A & Vieira L E A, Solar and interplanetary causes of very intense storms, J Atmos Sol-Terr Phys (UK), 63 (2001) pp Vieira L E, Gonzalez W D, Clua de Gonzalez A L & Dal Lago A, A study of magnetic storm development in two or more steps and its association with polarity of magnetic clouds, J Atmos Sol-Terr Phys (UK), 63 (2001) pp Kane R P, How good is the relationship of solar interplanetary parameter with geomagnetic storm, J Geophy Res (USA), 110 (2005) pp 1-13, doi: /2004JA Gopalswamy N, Yashiro S, Michalek G, Xie H, Lepping R P & Howar R A, Solar source of the largest geomagnetic storm of cycle 23, Geophys Res Lett (USA), 32 (2005) L12S09, doi: /2004GL Alexander D, Laboratory exploration of solar energetic phenomena, Astrophys Space Sci (UK), 307 (2007) pp 1-3, , doi: /s Chukwuma V U, On heliophysical and geophysical phenomena during October-November 2003, Acta Geophys Pol (Poland), 57 (3) (2009) pp Chaman Lal, Sun Earth geometry, geomagnetic activity and planetary F2 ion density Part 1: Signatures of magnetic reconnection, J Atmos Sol-Terr Phys (UK), 62 (2000) pp Davis C J, Wild M N, Lockwood M & Tulmay Y K, Ionospheric and geomagnetic response to changes in IMF Bz: a superposed epoch study, Ann Geophys (France), 15 (1997) pp Gonzalez W D & Tsurutani B T, Criteria of interplanetary parameters causing intense magnetic storms (D st <100nT), Planet Space Sci (UK), 35 (1987) pp Gonzalez W D, Joselyn J A, Kamide Y, Kroehi H W, Rostoker G, Tsurutani B T & Vasyliunas V M, What is a geomagnetic storm, J Geophys Res (USA), 99 (A4) (1994) pp Kamide Y, Yokoyama N, Gonzalez W D, Tsurutani B T, Brekke A & Masuda S, Two step development of geomagnetic storm, J Geophys Res (USA), 103 (1998) pp Tsurutani B T, Gonzalez. W D, Tang F, Lee Y T, Okada M & Park D, Solar wind rain pressure corrections and an estimation of the efficiency of various interaction, Geophys Res Lett (USA), 19 (1992) pp Gonzalez W D, Tsurutani B T, Lepping R P & Schwenn R, Interplanetary phenomena associated with very intense geomagnetic storms, J Atmos Sol-Terr Phys (UK), 64 (2002) pp Strickland D J, Daniell R E & Craven J D, Negative ionospheric storm coincident with DE 1-observed thermo-

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