M. van de Kamp

High-latitude ionospheric equivalent currents during strong space storms: Regional perspective

Geomagnetically induced currents (GIC) are a space weather phenomenon that can interfere with power transmission and even cause blackouts. The primary drivers of GIC can be represented as ionospheric equivalent currents. We used International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer data from 1994–2013 to analyze the extreme behavior of the time derivative of the equivalent current density (|ΔJeq|/Δt) together with the occurrence of modeled GIC in the European high-voltage power grids (1996–2008). Typically, when intense |ΔJeq|/Δt occurred, geomagnetic activity extended to latitudes 60°. In such cases, typically 5≤Kp<8, and modeling suggested that there were no large GIC in the European high-voltage power grids. Intense |ΔJeq|/Δt and GIC occurred preferentially before midnight or at dawn and were rare after noon. There was a seasonal peak in October and a minimum around midsummer. Intense |ΔJeq|/Δt and GIC occurred preferentially in the declining phase of the solar cycle and were rare around solar minima. A longer perspective (1975–2013) was obtained by comparison with the time derivative of the magnetic field from the IMAGE station Nurmijarvi (NUR, MLAT ∼57°). NUR data indicated that the quietness of summer months may have been due to a coincidental lack of intense storms during the shorter period. NUR data agreed with the increased activity in the declining phase but demonstrated that extreme events could also occur during solar minima.

A model providing long‐term data sets of energetic electron precipitation during geomagnetic storms

We would like to thank the geomagnetic data suppliers and the World Data Center for Geomagnetism, Kyoto, for making the Dst index data available via http://wdc.kugi.kyoto-u.ac.jp. The NOAA/POES data used in this study were made available by the National Oceanic and Atmospheric Administration. AARDDVARK data are available from the AARDDVARK Konsortia (see http://www.physics.otago.ac.nz/space/AARDDVARK_homepage.htm). Other data presented in the paper are available from the corresponding author (annika.seppala@fmi.fi).

Solar wind control of ionospheric equivalent currents and their time derivatives

A solid understanding of the solar wind control of ground magnetic field disturbances is essential for utilizing the existing long time series of ground data to obtain information on solar wind-magnetosphere-ionosphere coupling. We have used 20 years of International Monitor for Auroral Geomagnetic Effects magnetometer data (54°–76° magnetic latitude) to study the solar wind control of the ionospheric equivalent current density and its time derivative ( ). We found that peaks at the premidnight and prenoon ends of the westward electrojet. The prenoon peak was most intense during fast solar wind and radial interplanetary magnetic field (IMF). The location of the peak was not affected by the IMF orientation but persisted at 8–10 magnetic local time and 70°–75° latitude, near the boundary between the westward and eastward electrojets. Sensitivity of this boundary to disturbances was suggested as a possible explanation for the persistent prenoon location of the peak. The premidnight peak was most intense during southward IMF orientation. While faster solar wind mainly resulted in more intense in the premidnight sector, stronger IMF caused the region of intense to spread to the postmidnight, dawn, and dusk sectors. A good correspondence was found between development of the nightside intensification and average substorm bulge and oval aurora as determined by Gjerloev et al. (2007). The bulge aurora covered the western end of the westward electrojet where the equivalent current also had a significant poleward component. The substorm oval aurora, on the other hand, extended eastward along the westward electrojet.

Polar Ozone Response to Energetic Particle Precipitation Over Decadal Time Scales: The Role of Medium-Energy Electrons: OZONE AND MEDIUM-ENERGY ELECTRONS

One of the key challenges in polar middle atmosphere research is to quantify the total forcing by energetic particle precipitation (EPP) and assess the related response over solar cycle time scales. This is especially true for electrons having energies between about 30 keV and 1 MeV, so‐called medium‐energy electrons (MEE), where there has been a persistent lack of adequate description of MEE ionization in chemistry‐climate simulations. Here we use the Whole Atmosphere Community Climate Model (WACCM) and include EPP forcing by solar proton events, auroral electron precipitation, and a recently developed model of MEE precipitation. We contrast our results from three ensemble simulations (147 years) in total with those from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) in order to investigate the importance of a more complete description of EPP to the middle atmospheric ozone, odd hydrogen, and odd nitrogen over decadal time scales. Our results indicate average EPP‐induced polar ozone variability of 12–24% in the mesosphere, and 5–7% in the middle and upper stratosphere. This variability is in agreement with previously published observations. Analysis of the simulation results indicate the importance of inclusion of MEE in the total EPP forcing: In addition to the major impact on the mesosphere, MEE enhances the stratospheric ozone response by a factor of 2. In the Northern Hemisphere, where wintertime dynamical variability is larger than in the Southern Hemisphere, longer simulations are needed in order to reach more robust conclusions.