Eccentric dipole of the geomagnetic field during the last reversal, last excursions, and the most significant Holocene anomalies

<p>The geomagnetic field is commonly approximated to a geocentric tilted dipole. However, a next step in the approach of the geomagnetic field is the eccentric dipole which takes the first and second terms of the spherical harmonic representation of the geomagnetic field. In this work, we analyze the behavior of the eccentric dipole during the last reversal (Matuyama – Brunhes, 780 ka), the last excursions (Laschamp, 41 ka, and Mono Lake, 34 ka), and during two interesting features of the geomagnetic field observed during the Holocene (the South Atlantic Anomaly, from 1840 AD or older, and the Levantine Iron Age Anomaly, around 1000 BC). The last reversal and excursions are studied by using the IMMAB4 and LSMOD2 paleoreconstructions, respectively. We found that for these events the center of the eccentric dipole follows a common longitude path. The Holocene anomalies have been analyzed by using two of the most up-to-date paleoreconstructions for the last 3 millennia: the SHAWQ2k and the SHAWQ Iron Age paleoreconstructions. A common longitude path has not been observed between these anomalies.</p>

UV core dimming in coronal streamer belt and the projection effects

During solar minimum activity, the coronal structure is dominated by a tilted streamer belt, associated with the sources of the slow solar wind. It is known that some UV coronal spectral observations show a quite evident core dimming in heavy ions emission in quiescent streamers. In this paper, our purpose is to investigate this phenomenon by comparing observed and simulated UV coronal ion spectral line intensities. First, we computed the emissivities and the intensities of HI Lyα and OVI spectral lines starting from the physical parameters of a time-dependent 3D three-fluid MHD model of the coronal streamer belt. The model is applied to a tilted dipole (10°) solar minimum magnetic structure. Next, we compared the results obtained from the model in the extended corona (from 1.5 to 4 R⊙) to the UV spectroscopic data from the Ultraviolet Coronagraph Spectrometer (UVCS) onboard SOHO during the minimum of solar activity (1996). We investigate the line-of-sight integration and projection effects in the UV spectroscopic observations, disentangled by the 3D multifluid model. The results demonstrate that the core dimming in heavy ions is produced by the physical processes included in the model (i.e., combination of the effects of heavy ion gravitational settling, and energy exchange of the preferentially heated heavy ions through the interaction with electrons and protons) but it is visible only in some cases where the magnetic structure is simple, such as a (tilted) dipole.

The Positive North Atlantic Oscillation with Downstream Blocking and Middle East Snowstorms: The Large-Scale Environment

In this study, the atmospheric conditions for the December 2013 Middle East snowstorm are examined from a case study perspective and by performing a composite analysis of extreme winter events from 1950 to 2013 using reanalysis data. It is revealed that this snowstorm arises from the occurrence of an omega (Ω)-type European blocking (EB) with a strong downstream trough that is associated with a southward-displaced positive-phase North Atlantic Oscillation (NAO+) event. In the anomaly field, the EB exhibits a northeast–southwest (NE–SW)-tilted dipole structure. The Ω-type EB transports cold air into the Middle East and produces snowfall within the trough over the Middle East. The composite analysis shows that the location of cold temperatures depends strongly on the tilting direction and strength of the EB dipole anomaly. The NE–SW [northwest–southeast (NW–SE)]-tilted EB dipole occurs with a southward (northward)-displaced NAO+ event. The NE–SW-tilted EB dipole anomaly is associated with an arching-type low-frequency wave train that spans the North Atlantic, Europe, and the Middle East. This tilting has the most favorable structure for cold air outbreaks over the Middle East and southeastern Europe because this tilting leads to an intense downstream trough over this region. In contrast, a NW–SE-tilted EB dipole anomaly leads to cold temperatures over northwestern Africa and southwestern Europe. The analyses herein also suggest that a strong jet over the North Atlantic may be a precursor for a southward-displaced NAO+ event that is usually associated with an Ω-type EB with a NE–SW-tilted dipole in the anomaly height field that favors a cold air outbreak over the Middle East.