T. Asikainen

Annual fractions of high‐speed streams from principal component analysis of local geomagnetic activity

We study the latitudinal distribution of geomagnetic activity in 1966–2009 with local geomagnetic activity indices at 26 magnetic observatories. Using the principal component analysis method we find that more than 97% of the variance in annually averaged geomagnetic activity can be described by the two first principal components. The first component describes the evolution of the global geomagnetic activity, and has excellent correlation with, e.g., the Kp/Ap index. The second component describes the leading pattern by which the latitudinal distribution of geomagnetic activity deviates from the global average. We show that the second component is highly correlated with the relative (annual) fraction of high-speed streams (HSS) in solar wind. The latitudinal distribution of the second mode has a high maximum at auroral latitudes, a local minimum at subauroral latitudes and a low maximum at midlatitudes. We show that this distribution is related to the difference in the average location and intensity between substorms related to coronal mass ejections (CMEs) and HSSs. This paper demonstrates a new way to extract useful, quantitative information about the solar wind from local indices of geomagnetic activity over a latitudinally extensive network.

Spatial distribution of Northern Hemisphere winter temperatures during different phases of the solar cycle

Several recent studies have found variability in the Northern Hemisphere winter climate related to different parameters of solar activity. While these results consistently indicate some kind of solar modulation of tropospheric and stratospheric circulation and surface temperature, opinions on the exact mechanism and the solar driver differ. Proposed drivers include, e.g., total solar irradiance (TSI), solar UV radiation, galactic cosmic rays, and magnetospheric energetic particles. While some of these drivers are difficult to distinguish because of their closely similar variation over the solar cycle, other suggested drivers have clear differences in their solar cycle evolution. For example, geomagnetic activity and magnetospheric particle fluxes peak in the declining phase of the sunspot cycle, in difference to TSI and UV radiation which more closely follow sunspots. Using 13 solar cycles (1869–2009), we study winter surface temperatures and North Atlantic oscillation (NAO) during four different phases of the sunspot cycle: minimum, ascending, maximum, and declining phase. We find significant differences in the temperature patterns between the four cycle phases, which indicates a solar cycle modulation of winter surface temperatures. However, the clearest pattern of the temperature anomalies is not found during sunspot maximum or minimum, but during the declining phase, when the temperature pattern closely resembles the pattern found during positive NAO. Moreover, we find the same pattern during the low sunspot activity cycles of 100 years ago, suggesting that the pattern is largely independent of the overall level of solar activity.

A new method to estimate annual solar wind parameters and contributions of different solar wind structures to geomagnetic activity

In this paper, we study two sets of local geomagnetic indices from 26 stations using the principal component and the independent component (IC) analysis methods. We demonstrate that the annually averaged indices can be accurately represented as linear combinations of two first components with weights systematically depending on latitude. We show that the annual contributions of coronal mass ejections (CMEs) and high-speed streams (HSSs) to geomagnetic activity are highly correlated with the first and second IC. The first and second ICs are also found to be very highly correlated with the strength of the interplanetary magnetic field (IMF) and the solar wind speed, respectively, because solar wind speed is the most important parameter driving geomagnetic activity during HSSs while IMF strength dominates during CMEs. These results help in better understanding the long-term driving of geomagnetic activity and in gaining information about the long-term evolution of solar wind parameters and the different solar wind structures.

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.

Energetic Particles in the Cusp: A Cluster/RAPID View

Energetic particles have been persistently observed in the exterior cusp by different satellite missions such as POLAR, Cluster-II, Viking, ISEE etc. Yet the source and the acceleration mechanism of these particles have remained unclear. In this paper I review our studies of energetic particles in the cusp and the nearby high-latitude region of closed magnetospheric field lines (HLPS, high-latitude dayside plasma sheet) using the data obtained by the RAPID instrument onboard the Cluster-II satellites. We conducted a large scale statistical study to examine the dependence of the energetic particle fluxes in the cusp and HLPS on solar wind/IMF conditions as well as on geomagnetic activity. The study showed that energetic ion fluxes in the HLPS correlate strongly with substorm activity and electron fluxes with solar wind speed and geomagnetic activity. In the exterior cusp a clear correlation between lower energy ions (E 75 keV) correlated with substorm activity. Our case studies have shown that when IMF By dominates reconnection can take place near the cusp and release energetic particles from closed field lines to the cusp. Coupled with these detailed observations the statistical results imply that the energetic particles in the HLPS and the cusp originate in the near-Earth magnetotail from where they can drift to the HLPS region. From the HLPS the higher energy particles diffuse more or less directly into the cusp while the lower energy particles are released into the cusp by reconnection. These observations provide a consistent explanation for the cusp energetic particles without a need for significant local acceleration of shocked solar wind plasma to MeV energies. While some energy transfer from the electromagnetic waves to plasma particles is known to occur in the cusp it cannot explain the observations discussed here.