efficiency of the solar wind during HILDCAAs varied between 0.3% and 2.8%, with ... dominating energy dissipation mechanism during HILDCAAs compared to ...
Solar Cycle Dependence and Energy Coupling during Continuous Substorm/HILDCAA Events R. Hajra1, E. Echer1, B. T. Tsurutani2, and W. D. Gonzalez1 1Instituto
Nacional de Pesquisas Espaciais, Sao Jose dos Campos, SP, Brazil 2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
Abstract Continuous substorm events called high-intensity, long-duration, continuous AE activity (HILDCAA) events are studied over ~3.5 solar cycle span (1975-2011) using solar wind/interplanetary databases. The overwhelmed majority (94%) of the 99 HILDCAA cases (with interplanetary data available) were associated with high-speed solar wind stream (HSS) events. The remaining 6% of the cases occurred after the passage of interplanetary coronal mass ejections (ICMEs). The HSS-related HILDCAA events were typically associated with large-amplitude interplanetary Alfvén waves with southward IMF Bz components. It will be shown that HILDCAAs can occur during all phases of the solar cycle, with the descending phase approximately three times more likely to have an event than at solar maximum and the ascending phase. Quantitative study on the energetics of the solar wind-magnetosphere-ionosphere system has shown that the coupling efficiency of the solar wind during HILDCAAs varied between 0.3% and 2.8%, with an average value of ~0.9%. This value is much lower than the > 5% coupling efficiency noted for ICME (and sheath)-driven magnetic storms. Joule heating was found to be the dominating energy dissipation mechanism during HILDCAAs compared to minor contributions of auroral precipitation and ring current injection. However, it should be noted that the energy over the polar cap was not captured in this study, an energy component which is particularly characteristic of HILDCAA events. Possible physical interpretations for the statistical results will be discussed.
HILDCAA events (1975-2011) 133 High-Intensity Long-Duration Continuous AE Activity (HILDCAA) events are selected using following criteria of Tsurutani & Gonzalez [PSS 1987]: • Peak AE > 1,000 nT • Duration ≥ 2 dsys • AE never drop below 200 nT for >2 h at a time • Event occurs outside the main phase of geomagnetic storm (Dst 5% coupling efficiency noted for storms driven by ICMEs and their sheaths. • During HILDCAAs, ~67% of the solar wind energy input was dissipated in the auroral ionosphere in form of Joule heating. Only ~11% of the energy went into the ring current. Joule heating was found to be the dominating dissipation channel during HILDCAA events. • Joule dissipation percentage during main phases of CIR-driven geomagnetic storms (~49%) was significantly lower than during HILDCAAs, while the ring current injection values were comparable for the two. Further, ring current injection during HILDCAAs/CIR-storm main phases was about half of the reported value for intense ICME-storms.
Related papers •
Hajra, R., E. Echer, B. T. Tsurutani, and W. D. Gonzalez (2013), Solar cycle dependence of High-Intensity Long-Duration Continuous AE Activity (HILDCAA) events, relativistic electron predictors?, Journal of Geophysical Research, 118, 5626-5638, doi:10.1002/jgra.50530.
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Hajra, R., E. Echer, B. T. Tsurutani, and W. D. Gonzalez (2014), Solar wind-magnetosphere energy coupling efficiency and partitioning: HILDCAAs and preceding CIR-storms during solar cycle 23, Journal of Geophysical Research, 119, 2675-2690, doi:10.1002/2013JA019646.
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Hajra, R., B. T. Tsurutani, E. Echer, and W. D. Gonzalez (2014), Relativistic electron acceleration during High-Intensity Long-duration Continuous AE Activity (HILDCAA) events: solar cycle phase dependences, Geophysical Research Letters, 41, 1876-1881, doi:10.1002/2014GL059383.
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Hajra, R., E. Echer, B. T. Tsurutani, and W. D. Gonzalez (2014), Superposed epoch analyses of HILDCAAs and their interplanetary drivers: solar cycle and seasonal dependences, Journal of Atmospheric and Solar-Terrestrial Physics, 121, 24-31, doi:10.1016/j.jastp.2014.09.012.
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Hajra, R., B. T. Tsurutani, E. Echer, and W. D. Gonzalez (2014), Relativistic (E > 0.6, > 2.0 and > 4.0 MeV) electron acceleration at geosynchronous orbit during High-Intensity Long-Duration Continuous AE Activity (HILDCAA) events, The Astrophysical Journal (accepted).
