Odd hydrogen response thresholds for indication of solar proton and electron impact in the mesosphere and stratosphere

• Main Authors: T. Häkkilä, P. T. Verronen, L. Millán, M. E. Szeląg, N. Kalakoski, A. Kero
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-12-01
• Series: Annales Geophysicae
• Online Access: https://angeo.copernicus.org/articles/38/1299/2020/angeo-38-1299-2020.pdf
• ISSN: 0992-7689; 1432-0576

<p>Understanding the atmospheric forcing from energetic particle precipitation (EPP) is important for climate simulations on decadal time scales. However, presently there are large uncertainties in energy flux measurements of electron precipitation. One approach to narrowing these uncertainties is by analyses of EPP direct atmospheric impacts and their relation to measured EPP fluxes. Here we use observations from the microwave limb sounder (MLS) and Whole Atmosphere Community Climate Model (WACCM) simulations, together with EPP fluxes from the Geostationary Operational Environmental Satellite (GOES) and Polar-orbiting Operational Environmental Satellite (POES) to determine the <span class="inline-formula">OH</span> and <span class="inline-formula">HO₂</span> response thresholds to solar proton events (SPEs) and radiation belt electron (RBE) precipitation. Because of their better signal-to-noise ratio and extended altitude range, we utilize MLS <span class="inline-formula">HO₂</span> data from an improved offline processing instead of the standard operational product. We consider a range of altitudes in the middle atmosphere and all magnetic latitudes from pole to pole. We find that the nighttime flux limits for day-to-day EPP impact detection using <span class="inline-formula">OH</span> and <span class="inline-formula">HO₂</span> are 50–130 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">protons</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">cm</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">s</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">sr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="103pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="c32eaea4cde19b87c97e6925db12c566"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="angeo-38-1299-2020-ie00001.svg" width="103pt" height="15pt" src="angeo-38-1299-2020-ie00001.png"/></svg:svg></span></span> (<span class="inline-formula"><i>E</i>&gt;10</span> MeV) and 1.0–<span class="inline-formula">2.5×10⁴</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">electrons</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">cm</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">s</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">sr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="111pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="dec8c6d6f7848726d9f171a98f42ac78"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="angeo-38-1299-2020-ie00002.svg" width="111pt" height="13pt" src="angeo-38-1299-2020-ie00002.png"/></svg:svg></span></span> (<span class="inline-formula"><i>E</i></span> <span class="inline-formula">=</span> 100–300 <span class="inline-formula">keV</span>). Based on the WACCM simulations, nighttime <span class="inline-formula">OH</span> and <span class="inline-formula">HO₂</span> are good EPP indicators in the polar regions and provide best coverage in altitude and latitude. Due to larger background concentrations, daytime detection requires larger EPP fluxes and is possible in the mesosphere only. SPE detection is easier than RBE detection because a wider range of polar latitudes is affected, i.e., the SPE impact is rather uniform poleward of <span class="inline-formula">60^∘</span>, while the RBE impact is focused at <span class="inline-formula">60^∘</span>. Altitude-wise, the SPE and RBE detection are possible at <span class="inline-formula">≈</span> 35–80 and <span class="inline-formula">≈</span> 65–75 <span class="inline-formula">km</span>, respectively. We also find that the MLS <span class="inline-formula">OH</span> observations indicate a clear nighttime response to SPE and RBE in the mesosphere, similar to the simulations. However, the MLS <span class="inline-formula">OH</span> data are too noisy for response detection in the stratosphere below 50 <span class="inline-formula">km</span>, and the <span class="inline-formula">HO₂</span> measurements are overall too noisy for confident EPP detection on a day-to-day basis.</p>