Comment on “Westward electric field penetration to the dayside equatorial ionosphere during the main phase of the geomagnetic storm on 22 July 2009” by V. Sreeja et al.

Abstract

[1] Sreeja et al. [2011] have claimed that the “geomagnetic storm that occurred during the period from 21 to 25 July 2009 is anomalous because the storm main phase developed during northward interplanetary magnetic field (IMF)” and “westward electric field penetration to the dayside equatorial ionosphere during the main phase of the storm.” However, as discussed subsequently, the so‐called “anomalous” feature of geomagnetic storm is ill‐fully claimed due to their incorrect IMF Bz data. Since Sreeja et al. [2011] have made serious claims such as the geomagnetic storm of 21–25 July 2009 is anomalous which would have adverse impact on the future scientific investigations, it is highly necessary to clarify. [2] For example, Figures 1a–1d show the variation of interplanetary magnetic field (IMF Bz), interplanetary electric field (IEF) (IEFy = −Vsw × Bz), symmetric ring current (Sym‐H) index and the strength of equatorial electrojet (EEJ), respectively. The 1 min time resolution data of IMF Bz and IEFy are obtained from the Atmospheric Composition Explorer (ACE). The Sym‐H index (1 min values) data is obtained from the World data center for Geomagnetism, Kyoto University (http://wdc.kugi.kyoto‐u.ac.jp/). The strength of equatorial electrojet (1 min values) is obtained from the ground‐based magnetometers at Tirunelveli (8.7°N geog.latitude, 77.8°E geog.longitude and 0.6°N dip.latitude) and Alibagh (18.5°N geog.latitude, 72.9°N geog.longitude and 12.9°N dip.latitude) in India. The IMF Bz and IEFy parameters have been shifted to account for the delay from ACE location to the bow shock (http://omniweb.gsfc.nasa. gov/html/HROdocum.html). It could be clearly seen from Figure 1 that the IMF Bz is southward continuously from several hours before the onset of main phase at 0154 UT (0724 IST) of 22 July 2009. However, unlike Sreeja et al. [2011] claimed, the IMF Bz continues to be southward till 0544 UT (1114 IST). During the same period (0154 to 0544 UT) the ring current continues to be strengthened as evidenced by the variation of Sym‐H index which reached to a maximum negative excursion of −89 nT at 0544 UT. This clearly shows that the main phase of the storm is developed mainly under the southward orientation of the IMF Bz which is a well‐known phenomenon. Therefore, the principal finding of anomalous geomagnetic storm where the main phase is claimed to be developed during the northward IMF Bz is not true. While comparing the time variations of IMF Bz and IEFy with those from Sreeja et al. [2011, Figure 1], it can be clearly noticed that the IMF Bz and IEFy data shown in their figure is incorrect where the IMF Bz is turned northward around ∼0700 IST. Also in section 3.1 of Sreeja et al. [2011], they have claimed that “the IMF Bz is observed to be remain northward for a very long duration (∼8 h) from 0700 IST to 1600 IST.” This is incorrect as can be seen from Figure 1 that the IMF Bz is northward only for a period of 83 min from 0544 to 0707 UT (1114–1237 IST) and later turned southward from 0707 to 0849 UT (1237–1419 IST). Therefore, the anomalous main phase of the geomagnetic storm which developed under northward IMF Bz is not true and primarily arise due to incorrect data of IMF Bz used by Sreeja et al. [2011]. Though the authors have stated in sections 2 and 3.1 that the 1 min data of IMF Bz obtained from the Website of Atmospheric Composition Explorer spacecraft, it is observed that the data shown by Sreeja et al. [2011, Figure 1] do not match with the IMF data of ACE. Further, Sreeja et al. [2011] have stated that they have estimated a convection time delay of ∼40 min from an average solar wind velocity of 560 km/s. However, it is noticed that solar wind velocity never reached to a value of 560 km/s during the entire period of this storm event. Instead, the solar wind velocity varies between ∼300 and 370 km/s during the main phase and varies between ∼370 and 500 km/s during the recovery phase of the storm period. These publicly available data of ACE solar wind and magnetic parameters were also published in earlier studies [e.g., Fok et al. 2010]. In their reply to our comment, Sreeja et al. [2011] insisted that they have estimated the time delay based on the solar wind speed of ∼560 km/s on 24 July. However, their estimation of time delay is inappropriate because the geomagnetic storm is actually developed on 22 July. Research Institute for Sustainable Humanosphere, Kyoto University, Kyoto, Japan. Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan. Space Environment Research Center, Kyushu University, Fukuoka, Japan. Indian Institute of Geomagnetism, Navi Mumbai, India. Solar‐Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan.