- Open Access
Unique examples of solar flare effects in geomagnetic H field during partial counter electrojet along CPMN longitude sector
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
- Received: 29 November 2012
- Accepted: 18 April 2013
- Published: 9 October 2013
Yamazaki et al. (2009) reported two strong negative crochets during midday (solar flares on 8 June 2000, 3 July 2002), along the Circum-pan pacific magnetometers network (CPMN). The association of these with equatorial counter electrojet was doubted and suggestion was made to investigate the cause of these unique events. Present investigations were motivated by their paper. In this paper, detailed examination of geomagnetic data for the two events is made at all stations within 75°E (Indian) and 160°E (western Pacific) longitude sectors. Latitudinal variations of ΔH on 18 June 2000 showed negative gradient towards the dip equator suggesting a partial counter electrojet both in the Indian and CPMN sectors. A partial counter electrojet also existed from morning to 1300 LT in the western Pacific sector on 3 July 2002. There are two current sheets over the equatorial electrojet region, one at higher level flowing eastward associated with global Sq current system and another intense current layer at 107 km, flowing eastward during normal and westward during partial/full counter electrojet periods. Solar flares are likely to affect the electrojet current more strongly as a result of the absorption, in the lower E-region, of the shorter wavelength solar X-rays flare spectrum.
- Solar flare
- space weather
- counter electrojet
- geomagnetic field
1.1 Equatorial Electrojet Current (EEJ)
The first ground survey of the geomagnetic horizontal field from a chain of fourteen stations in Peru by Giesecke Chapman 1951) revealed that the daily range of H slowly increased from 7°S geographic latitude to a peak at 13°S (close to magnetic dip equator) by a factor of 2. This was explained by him as due to a narrow band of thin current sheet flowing eastward during the day time hours in the ionosphere (100 km) and named it Equatorial Electrojet (EEJ). Baker and Martyn (1953) pointed out that in the equatorial (magnetic) region the geomagnetic field, electric field and the vertical electron density gradient are mutually orthogonal. With the primary electric field eastward during the daytime, the electrons within 70–140 km altitude drift upward relative to the ions and give rise to a vertical Hall polarization field, that gives rise to an additional eastward Hall current. The effective conductivity is known as Cowling conductivity. Beyond 3° from the magnetic equator the polarization field leaks away along the inclined geomagnetic field lines and the enhanced Cowling conductivity is confined within a narrow range of latitude 3° north and south of the magnetic equator. The equatorial enhancement of Range H was observed later in Nigeria (Onwumechili, 1959), Peru (Forbush and Casaverde, 1961), India (Yacob and Khanna, 1963) and Tschad (Godivier and Crenn, 1965). Extensive observations during the IGY-IGC (1957–59) enabled Rastogi (1962) to detect a longitudinal variation of EEJ, being strongest in American and weakest in Indian longitudes.
1.2 Equatorial sporadic E layer (Es-q)
Matsushita (1951) found that in the dip equatorial region, the latitudinal variation of the top frequency reflected from the sporadic layer, fEs is very similar to that of ΔH. With number of ionospheric stations operating during the IGY within a narrow region of latitudes in Peru, Knecht (1959) identified the characteristics of Es associated with EEJ and called it Equatorial sporadic-E layer or Es-q. Knecht and Schlitt (1961) and Knecht and McDuffie (1962) showed the belt of Es-q is only about 700 km in latitude (α5° dip) coinciding with the belt of EEJ.
1.3 Equatorial Counter Electrojet (CEJ)
Bartels and Johnston (1940a), b) demonstrated an abnormally large lunar variation on the daily variation of H (SqH) at Huancayo such that on certain days the daytime values of H decreased below the nighttime value suggesting the reversed EEJ current direction. Cohen et al. (1962) showed an association between the disappearance of Es-q and a decrease of the H field. Gouin (1962) reported a depression of H on magnetogram at local noon at Addis-Ababa well below the night level on a very quiet day. Later Gouin and Mayaud (1967) named such events as Counter Equatorial Electroject (CEJ). Hutton and Oyinloye (1970) described the disappearance of Es-q during CEJ at Ibadan. Fambitakoye (1971) described the diurnal developments of the latitudinal profiles of ΔH and ΔZ along Central African longitudes on normal and counter electrojet days. On CEJ day ΔH showed a minimum over the magnetic equator and the latitudinal profile of ΔZ was reversed with respect to that during an EEJ day, with a maximum in northern and a minimum in southern fringe region of the electrojet. Rastogi (1972) showed number of examples of the disappearances of Es-q at Huancayo and Kodaikanal during the depressions of ΔH on quiet as well as on disturbed days. Thus, the presence (or absence) of equatorial Es-q during daytime was found to be an alternate parameter to the presence of EEJ (or CEJ).
Based on the manual examination of the magnetograms at a number of equatorial stations Rastogi (1974) showed that CEJ identified by the depression of H field below the mean nighttime level are seen during the evening or morning hours and practically absent during the midday hours. These events were shown to be associated with the disappearance of Es-q on ionograms. Examining the ionograms at Kodaikanal for a period of 20 years (1956–1975) for no Es-q condition, Rastogi (1997) showed that CEJ during midday hours is a very rare phenomenon, occurring on about 1% of time. Further, most of these events were associated with geomagnetic active periods.
1.4 Ionospheric drifts within EEJ region
Spaced antenna drift measurements at the equatorial electrojet station Thumba, India from November 1963 to December 1969 revealed some new features of EEJ (Chandra and Rastogi, 1970; Chandra et al., 1971). Doppler shift of VHF backscatter echoes by Jicamarca radar provided more precise E and F region drifts, from which the ionospheric electric field were estimated (Balsley, 1973). Rastogi et al. (1971) showed that the disappearance of Es-q and the CEJ at Kodaikanal were concurrent with the reversal of ionospheric drift at Thumba, confirming the reversal of ionospheric electric field during CEJ. Fambitakoye et al. (1973) showed that the disappearance of Es-q is related to the inverted latitudinal profiles of ΔH and ΔZ and not necessarily when ΔH at the equatorial station alone decreases below the night time value, discovering the phenomenon of partial counter electrojet P-CEJ event.
Rastogi (1975) suggested that the ΔH at any ground EEJ station is the combined effect of an eastward ionospheric current associated with the global Sq current system together with a narrow band of electrojet current flowing eastward during normal and westward during counter electrojet period. Carter et al. (1976) did detect simultaneous opposite flowing electric current at different altitudes during the Doppler radar measurements in Western Africa.
The correct estimate of the EEJ or CEJ is not when ΔH at the station close to the dip equator is above or below the night time level, but it is estimated by the difference of ΔH at the equatorial and another off equatorial station both being along the same longitude sector (Rastogi, 1974; Rastogi et al., 1977).
1.5 Solar Flare Effect on EEJ and CEJ
The first recorded observation of the white light solar flare and simultaneous disturbances in geomagnetic field (crochet) were independently reported by Carrington (1859) and Hodgson (1859). Later, it was discovered that chromospheric eruptions, geomagnetic crochet and short wave fadeouts (SWF) were concomitant phenomena (Dellinger, 1935; Torroson et al., 1936; Fleming, 1937; Richardson, 1937). McNish (1937) examined the effects of above mentioned three events in 1936 on the magnetic recordings at 18 stations in Western Europe, America and southern Pacific areas. He concluded that the solar flare effect (SFE) is an augmentation of the normal daily variation over the sunlit hemisphere both in direction and magnitude supposedly due to increased atmospheric ionization by ultra violet light from the solar eruption. Richmond and Venkateswaran (1971) suggested that crochets are composite of a fast component presumably produced by solar UV radiation (100-1000 Å and a slow component produced by soft X rays (1–100 Å).
Earlier studies at different latitudes suggested occurrence of SFE to be maximum around local noon, however possibility of SFE in the night time has been reported (Ohshio, 1964; Ohshio et al., 1963; Sastri, 1975a, b). The current layer responsible for SFE has been pointed to be below the normal Sq layer (Volland and Taubenheim, 1958; Veldkamp and Van Sabben, 1960; Pinter, 1967). This is because the photo-ionization due to flare associated X-rays is at a lower altitude than that due to regular radiation.
Nagata (1952) compared the SFE at the equatorial stations, Huancayo, Peru, Kakioka, and Watheroo. He found that the magnitude of crochet is abnormally large at Huancayo than at Kakioka or Watheroo. However, the ratio ΔH(SFE)/Range H(Sq) were not significantly different at the three places. He suggested that there may be a narrow zone of the ionosphere of high electrical conductivity over the magnetic equator. Forbush and Casaverde (1961) showed that the enhancement of SFE in H over the dip equator in Peru varied in a manner similar to the Sq (H) variation in equatorial latitudes. Rastogi (1965) showed that the equatorial variations of enhancements in SFE (H) and Sq (H) were more pronounced in the American than in Indian longitude, corresponding to similar longitudinal variation of the EEJ itself (Rastogi, 1962). Kuwashima and Umai (1985) described SFEs at Japanese stations Memanbetsu, Kakioka-Kanoya and Chichijima. It was shown that SFE in H field were always positive at lower latitude stations Chichijima and very small at Kakioka and negative at northern stations Kanoya, Kakioka or Memambetsu on different occasions. This was due to changing latitude of Sq focus.
Rangarajan and Rastogi (1981) described the effect of a solar flare at 0120 UT on 21 June 1980 in ΔH at EEJ stations in Indian and Pacific longitudes. At Trivandrum (0708 LT) and Annamalainagar (0719 LT) a negative crochet was recorded while at Davao (1022 LT) and Guam (1139 LT) a positive crochet was recorded. Thus, the same solar flare can produce different effects at EEJ stations separated in longitude due to the condition of pre-flare ΔH situation. Rastogi (2003) described the study of SFE on all the three components of the geomagnetic field at Huancayo for the period 1957–1977. Examples of positive and negative crochet were shown during the EEJ and partial or full CEJ events, respectively.
Rastogi et al. (1999) have described SFEs at Indo-USSR chain of stations extending from magnetic latitudes 0° to 45°N, during normal electrojet, partial counter electrojet and full counter electrojet periods. The solar flare at 1250 LT produced negative ΔH at equatorial stations Trivan-drum, Etyiapuram and Kodaikanal, practically no impulse at Annamalainagar, positive impulse at off equatorial stations Hyderabad, Alibag, Ujjain, Sabhawala, again zero impulse at stations close to Sq focus, Tashkent, Alma Ata and Karazande but again negative excursion at northern stations. This clearly showed that the SFE is similar to the EEJ current at equatorial station and to the Sq current at low, middle and high latitudes.
Yamazaki et al. (2009) described two unique SFEs at stations of Circum-pan Pacific Magnetometer Network (CPMN) with negative impulse at equatorial electrojet stations during the local noon time hours which they denoted by SFE*. They observed the counter-Sq SFE in limited longitude (local time) and latitude sectors (i.e. LT ~ 1030–1300, Dip Lat. ≤ 1.5°). The deviations from the night time base level were positive so they felt that this type of SFE cannot be explained by the same mechanism as the morning or evening counter electrojet and its generation mechanism is not well understood. In this paper we have reexamined the geomagnetic data at Indian and western Pacific longitude sectors during the two solar flare events.
3.1 Geomagnetic variations during June 2000 events
At Kirimati (CRI) the daily variations of ΔZ were of opposite phase to that of ΔH. Kirimati has been shown to be close to the northern fringe region of EEJ and ΔZ is expected to show large negative variation during midday hours (Rastogi, 2002). On 18–19 June the daily peak of ΔH was greatly reduced and a partial counter electrojet event was also present. At Ancon (ANC), ΔH showed close daily peaks and ΔZ showed small minimum on any of the days, characteristic of an EEJ station slightly north of the dip equator. No disturbance in the electrojet currents seems to be present on any of the days at Ancon.
In Fig. 4(b) are shown the ΔH on 18 June 2000 minus SqH (June 2000) for stations TIR, ABG and SAB. Corresponding variations of Dst index and hourly mean IMF-Bz on 18 June 2000 are also shown. It is seen that Dst indices were very low on 18 June 2000 indicating a magnetically quiet day. The semi diurnal character of ΔH is clearly seen at TIR and ABG while ΔH at Sabhawala (SAB) a station close to the Sq focus region is primarily diurnal with a peak at noon. There was a single depressed value of IMF-Bz at 14–15 hr LT which does not seem to suggest causing the counter electrojet.
3.2 SFE on 17 June 2000
3.3 SFE on 18 June 2000
Thus, the solar flare effects in the H field at low and equatorial latitudes should be viewed keeping in mind the proportions of the amplitudes of the electrojet and the Sq components of the total current at the time of solar flare.
3.4 SFE on 3 July 2002
Yamazaki et al. (2009) examined SFE at CPMN stations for the period 1998–2005 and out of the 117 cases of SFE identified the two events of negative SFE of 18 June 2000 and 3 July 2002 that occurred near local noon. The negative SFE or SFE* were found to occur in limited longitude/time (≈1030–1300 LT) and latitude (≤1.5° dip latitude). The amplitude of the SFE* (H) for the 18 June 2000 event was -15.9 nT at PON and at nearby stations DAV and CRI the amplitudes were 10.1 and 17.8 nT respectively. For the 3 Jul 2002 event the SFE* (H) amplitudes were −28.4, −25.9 and −22.9 nT at DAV, YAP and PON respectively. The current vector for the SFE* event was found to be opposite to that of Sq around noon. However the current vector for SFE and Sq were same for the latitudes outside equatorial region. For non equatorial stations linear relationship was shown between amplitude of SFE* (H) and pre-flare Sq(H). Points of the stations where SFE(H) were observed were near the best-fit line but for the stations with SFE* (H) significant departure from the best-fit line was observed. They also reported that for both the events rapid northward turning of IMF-Bz was seen and the decrease of H at CRI and PON on 18 June 2000 and at PON on 3 July 2002, could be due to the prompt penetration of electric field (over-shielding). However the events occurred under geomagnetic quiet conditions, and so they concluded that the origin of westward electric field remained to be understood.
The present analysis shows that on 18 June 2000 a partial counter electrojet was observed in both the Indian and CPMN longitude sectors. Also a partial counter electrojet existed in the CPMN sector on 3 July 2002. The stations close to dip equator thus showed negative SFE. Also the plot of the amplitude of SFE(H) on 18 June 2000 against pre-flare ΔH, corrected for Sq(H) component showed a linear relationship. Similar plot for 3 July 2002 again showed a linear relationship though the points for YAP and DAV showed departure from the best-fit line.
Yamazaki et al. (2009) pointed that the local current system, other than the global Sq current system might be responsible for the generation of SFE*. The equatorial electrojet current peak is considered to be around 107 km in contrast to the global Sq current peak around 115–120 km.
Onwumechili (1992) studied 76 rocket-borne magnetometer flights conducted at different latitude sectors that showed daytime peak eastward current around 107 km in the electrojet region or two layers of eastward current, one at 107 km and another (much weaker) at 136 km. Between 2° dip latitude and the Sq focus there are two layers of eastward current centered around 105 km and 123 km. The lower layer is not seen at stations beyond the Sq focus. Further the lower layer flows westward between 4° and 7° dip latitude. Thus the lower layer of intense eastward current flowing within 2–4° from dip equator with westward current peaking around 4° to 7° dip latitude is the electrojet current system. The two current systems, global Sq and equatorial electrojet are coupled and some times overlapping. The appearance of Es-q at 100 km is associated with eastward electric field and its disappearance during counter electrojet with westward electric field. This has been confirmed both by spaced receiver drift measurements at Thumba in India and later by VHF radar measurements from Jicamarca. Daytime reversal of current from eastward to westward in the electrojet region, on some occasions, do not give rise to depression in ground geomagnetic H field. Rastogi (1975) suggested that the ΔH at any ground EEJ station is the combined effect of an eastward ionospheric current associated with the global Sq current system together with a narrow band of electrojet current flowing eastward during normal and westward during counter electrojet period. Thus the ground geomagnetic field variation in the electrojet region results from an eastward flowing Sq current peaking around 115–120 km and an eastward or westward flowing current around 107 km. Solar flares would affect the electrojet current more than the global current as shorter wavelengths of the flare spectrum are generally absorbed in the lower regions of ionosphere.
Solar flare effects (SFEs) are examined in the Indian and western Pacific longitude sectors for the two events of negative SFE observed around midday at equatorial stations of CPMN network on 18 June 2000 and 3 July 2002. Both the midday events of negative SFE(H) at equatorial CPMN stations were associated with partial counter electrojet.
The amplitude of SFE(H) showed a linear relationship with pre-flare ΔH after correction is made for the Sq(H) component at electrojet stations.
The crochets in H during a normal electrojet period are positive at equatorial as well as other low latitude stations. During a partial counter electrojet ΔH at electrojet stations could be negative in comparison to positive impulse at low latitude stations outside the electrojet region.
The equatorial electrojet current peak is around 107 km in contrast to the global Sq current peak around 115–120 km. Global Sq current flows eastward during the day time and the equatorial electrojet current flows eastward during daytime on normal electrojet and westward during counter electrojet conditions. It is likely that solar flares affect the eletrojet current more than the global current as shorter wavelengths of the flare spectrum are generally absorbed in the lower regions of ionosphere.
Thanks are due to Indian Institute of Geomagnetism for the copies of magnetograms from Indian stations. Thanks are also due to World Data Center at Kyoto, SPIDR at Boulder for other data used here. Thanks are due to Physical Research Laboratory for the facilities provided.
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