Ionospheric total electron contents (TECs) as indicators of solar EUV changes during the last two solar minima

Abstract

During the extended solar minimum from 2007 to 2009 anomalously low densities were observed in the Earth's thermosphere. Solar activity for the minimum of Solar Cycles 23 and 24 was lower than for Solar Cycles 22 and 23. However, a comparison of global daily averaged Total Electron Content (TEC) for these two solar minima does not show any significant difference. Why these significant changes between the 2008 and 1996 minima were observed in the He II EUV absolute irradiance and not in the global TEC for which EUV irradiance is one of the primary drivers is not well understood and may be interpreted as an effect of EUV instrument degradation. In this study, we analyze TEC data from 1995 to 2013 from the Center for Orbit Determination in Europe (CODE) database and compare global daily averaged TEC signal with TEC sectorial spherical harmonics which reflect a narrower temporal and spatial integration of the TEC in the Earth's ionosphere. Our analyses of both TEC data and errors show significant decreases of sectorial TECs for the 2008 and 2009 minimum compared to the minimum of 1996. We also compare the most recent version of Solar and Heliospheric Observatory/Solar EUV Monitor (SOHO/SEM) He II absolute EUV irradiance with the measurements from the Solar Dynamics Observatory/Extreme Ultraviolet Variability Experiment channels. This comparison shows that the EUV He II irradiance was about 12 ± 4% lower for the minimum of 2008 and 2009 compared to the minimum of 1996 when a low-pass filter with a 365 day window was used.

Attribution of interminima changes in the global thermosphere and ionosphere

“…The adjusted time series is shown in Figures and ; it exhibits an interminima change of −1%. Didkovsky and Wieman [] also revised downward the interminima change of −15 ± 6% reported by Didkovsky et al . [], based on five additional sounding rocket calibrations.…”

“…Didkovsky and Wieman [] used the sectorial spherical harmonics in the CODE data to estimate the interminima change in the amplitude of the latitudinally averaged diurnal TEC variation. They found a 31% decrease in the amplitude of the (1,1) harmonic, but they did not explicitly examine daytime versus nighttime interminima TEC changes.…”

“…[], a few other studies have examined the interminima differences in TEC and electron densities. Two recent studies confirm the lack of change in interminima global TEC [ Jee et al ., ; Didkovsky and Wieman , ], but most other results are restricted to a few ground‐based stations and to daytime conditions, so that their implications for the global ionosphere are limited. Liu et al .…”

Comparison ofF1layer critical frequency between recent two solar minimums

“…The solar extreme ultraviolet (EUV) irradiance of Solar EUV Monitor (SEM) was 15 ± 6% lower during the 23/24 minimum than during the preceding minimum [ Didkovsky et al ., ]. After recalibration, Didkovsky and Wieman [] reported a reduction in EUV irradiance of 12 ± 4% from the 22/23 to 23/24 minimums. Since the solar activity is the major factor for the ionospheric variations, the ionospheric behaviors under the extremely low solar activities would appeal to scientists.…”

New 3‐D simulations of climate change in the thermosphere

“…The magnitude of thermospheric density/temperature trends is further complicated by the possible role of anomalously low solar and geomagnetic activity during the minimum period between solar cycles 23 and 24 [ Emmert et al ., ; Solomon et al ., , , ; Saunders et al ., ; Chen et al ., ; Deng et al ., ; Emmert et al ., ; Didkovsky and Wieman , ]. Since the subsequent development of solar cycle 24 has been very weak, this raises the possibility of gradual change in solar forcing, both from energetic photon radiation and from magnetic variation that could confound attempts to quantify anthropogenic change.…”