Simultaneous observations of SAR arc and its ionospheric response at sub-auroral conjugate points (L~2.5) during the St. Patrick's Day storm in 2015

Abstract

27 During the St. Patrick’s Day storm on 17 March 2015, an all sky imager at King Sejong station 28 (KSS; geo: 62.2 S, 58.8 W; mag: 50.2 S) captured diffuse aurora and a stable auroral red (SAR) 29 arc. Ground-based Global Positioning System (GPS) observations were also simultaneously 30 conducted at KSS but there was no significant increase in the total electron content’s rate-of- 31 change index (ROTI) or the ionospheric scintillation indices around the SAR arc region. Auroral 32 activities including a SAR arc were also detected by the all sky imager at Millstone Hill station 33 (MHS; geo: 42.6 N, 71.4 W; mag: 52.5 N) which is magnetically conjugate to KSS. The ROTI 34 map around MHS, too, indicates that the ionospheric irregularities occurred only near the diffuse 35 aurora and not in the SAR arc. The northern SAR arc is broader than the southern one and also 36 shows a multiplicity pattern, which may be due to latitude structure within the seasonally- 37 dependent mid-latitude ionospheric trough. These conjugate observations, despite their 38 hemispheric differences, validate the classical SAR arc mechanism of emission driven by heat 39 conduction from the inner magnetosphere that does not generate small-scale ionospheric 40 irregularities that can affect GPS radio signals. Previous studies of SAR arcs and VHF radiowave 41 scintillations did show a positive correlation.