Local impact of solar variation on NO<sub>2</sub> in the lower mesosphere and upper stratosphere from 2007–2011

Abstract

MIPAS/ENVISAT data of nighttime NO 2 volume mixing ratios (VMR) from 2007 until 2012 between 40 km and 62 km altitude are compared with the geomagnetic Ap index and solar Lyman-α radiation. The local impact of variations in geomagnetic activity and solar radiation on the VMR of NO 2 in the lower mesosphere and upper stratosphere in the Northern Hemisphere is investigated by means of superposed epoch analysis. Observations in the Northern Hemisphere show a clear 27-day period of the NO 2 VMR. This is positively correlated with the geomagnetic Ap index at 60-70 • N geomagnetic latitude but also partially correlated with the solar Lyman-α radiation. However, the dependency of NO 2 VMR on geomagnetic activity can be distinguished from the impact of solar radiation. This indicates a direct response of NO x (NO + NO 2 ) to geomagnetic activity, probably due to precipitating particles. The response is detected in the range between 46 km and 52 km altitude. The NO 2 VMR epoch maxima due to geomagnetic activity is altitude-dependent and can reach up to 0.4 ppb, leading to mean production rates of 0.029 ppb (Ap d) −1 . Observations in the Southern Hemisphere do not have the same significance due to a worse sampling of geomagnetic storm occurances. Variabilities due to solar variation occur at the same altitudes at 60-70 • S geomagnetic latitude but cannot be analyzed as in the Northern Hemisphere. This is the first study showing the direct impact of electron precipitation on NO x at those altitudes in the spring/summer/autumn hemisphere.

Observation of 27 day solar cycles in the production and mesospheric descent of EPP‐produced NO

“…Since we are interested in the production of NO by particle precipitation, we need to remove the constant buildup that occurs especially during the winter to focus on the short‐term response. The variation (ΔNO, Δ AE ) is thus calculated by subtracting a smooth filter of 27 days (i.e., a box‐car running mean) from the observations, a methodology similar to that of Friederich et al []. In order to check if this filter introduces artificial signals, a second high pass filter of 45 days was tested: the results remained unchanged.…”