Local impact of solar variation on NO<sub>2</sub> in the lower mesosphere and upper stratosphere from 2007 to 2012

Abstract

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.

The response of mesospheric NO to geomagnetic forcing in 2002–2012 as seen by SCIAMACHY

“…To investigate the impact of geomagnetic activity on NO, we perform a superposed epoch analysis (SEA) [ Chree , ] also known as the compositing method [ von Storch and Zwiers , ]. This method is similar to the one described in Friederich et al [] for stratospheric NO 2 . The idea of a superposed epoch is to investigate the average response of a highly variable system to a short, sporadic external forcing by averaging segments of the time series around a number of specified forcing “events.” Event criteria have to be defined in such a way that a sufficiently large number of events is obtained to gain statistically robust estimates of the averaged response.…”

“…However, in this case, the NO enhancement is largest several days after the events, and the authors conclude that downward transport contributed to the observed signal. Friederich et al [] found the 27 day solar rotation cycle in nighttime mesospheric NO 2 data (46–52 km) at geomagnetic latitudes of the radiation belts. They showed that there is a positive correlation between NO 2 and the Ap index, which, however, has a very small (≤0.6 ppb) amplitude, well in the range of the upper limit given by Sinnhuber et al [].…”

Energetic Particle Influence on the Earth’s Atmosphere

“…There are also a few observations available of a chemical impact of magnetospheric electron precipitation into the middle atmosphere: an increase in OH for a number of energetic electron precipiation events above 70 km (Verronen et al 2011a;Andersson et al 2012) clearly mapping into geomagnetic latitudes connecting to the radiation belts, and showing longitudinal inhomogeneities there (Andersson et al 2014a). The observation of a weak but significant increase of NO 2 related to geomagnetic activity in the upper stratosphere at 45-52 km altitude in Northern latitudes connecting to the radiation belts (Friederich et al 2014).…”

“…However the observational evidence so far suggests that relativistic electrons have an impact on the NOx budget of the middle atmosphere below ∼70 km . And there is newer observational evidence for the direct impact of relativistic electrons on the NOx budget of the lower mesosphere and stratosphere Friederich et al 2014) which show that the impact of EEP below 70 km must be low even at periods of very high geomagnetic activity , though there is a significant, but small impact maximizing around 48 km (Friederich et al 2014). There is also clear evidence for an energetic electron precipitating effect above 70 km altitude from OH observations as shown in Verronen et al (2011a); Andersson et al (2012Andersson et al ( , 2014a.…”

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

“…This is consistent with the result of Newell and Gjerloev [2011], investigating the relation of different geomagnetic indices to the auroral power. The square of the correlation coefficient r is called the coefficient of determination (r 2 ) and indicates how well the data fit a linear relationship [Draper and Smith, 1998]. For the southern winter of 2011, the correlation coefficient of NO with the AE index is 0.75 (see Figure 3), meaning that 56% of the NO variation is explained by a linear relationship with the AE index.…”