NO Radiative Cooling and Ionospheric Response to the HILDCAA Events Following Geomagnetic Storms

Nitric oxide infrared (5.3 µm) radiative cooling plays an important role in Earth's upper atmospheric energy balance during space weather events. Its behavior during a HILDCAA event following a geomagnetic storm can be more complicated than what is observed in an isolated geomagnetic storm (without HILDCAA). In order to understand this contrast, two moderate (SYM‐Hmin −80 nT; April 2005 (event‐1) and −65 nT; December 2006 (event‐2)), and an intense (Dstmin −150 nT; May 2000 (event‐3)) geomagnetic storms followed by HILDCAA events are considered for this study. It is found that, despite the larger solar wind‐magnetospheric energy coupling during the main phases of event‐1 and event‐2, the NO radiative cooling (NORC) and energetic electron precipitation (EEP) rates are larger during the HILDCAA phase. TIE‐GCM simulations indicate that the NORC pattern can be mainly attributed to variation in temperature, [O], and [NO] during HILDCAA. A larger rates of EEP due to the continuous substorm activity contributed to an additional ionization in D‐region and E‐region of the northern polar ionosphere during HILDCAA period of event‐2. This work is the first attempt to understand the NORC and EEP fluctuations during HILDCAA events preceded by geomagnetic events like event‐1 and event‐2. This study presents a comprehensive investigation of NORC and energetic electron (>27.93 keV) precipitation rate during HILDCAA event preceded by a moderate geomagnetic storm. An atmospheric chemistry‐based NORC emission model is used to understand the variation of NORC during intense geomagnetic storm (event‐3). Plain Language Summary The enhanced solar wind‐magnetosphere‐thermosphere energetics, and subsequent NO radiative cooling in lower thermosphere during extreme space weather events regulate the energy equilibrium of Earth's upper atmosphere. Enhance Joule heating and energetic particle precipitation in high‐latitude regions during these events result in temperature and compositional changes in mesosphere and lower thermosphere (MLT) region of the Earth's atmosphere. Enhanced production of NO and its subsequent emission of 5.3 μm make it a natural thermostat in lower thermosphere which cools the MLT region during geomagnetic activity. The enhanced EEP during these events can also enhance the D‐region and E‐region ionospheric densities by ionizing the Earth's upper atmosphere. In this work, we use Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observation an