On the long term change in the geomagnetic activity during the 20th century

Abstract

Abstract. The analysis of the aa index series presented in this paper clearly shows that during the last century (1900 to 2000) the number of quiet days (Aa<20 nT) drastically diminished from a mean annual value greater than 270 days per year at the end of the nineteenth century to a mean value of 160 quiet days per year one hundred years later. This decrease is mainly due to the decrease of the number of very quiet days (Aa<13 nT). We show that the so-evidenced decrease in the number of quiet days cannot be accounted for by drift in the aa baseline resulting in a systematic underestimation of aa during the first quarter of the century: a 2-3 nT overestimation in the aa increase during the 20th century would lead to a 20-40% overestimation in the decrease of the number of quiet days during the same period.The quiet days and very quiet days correspond to periods during which the Earth encounters slow solar wind streams flowing in the heliosheet during the period where the solar magnetic field has a dipolar geometry. Therefore, the observed change in the number of quiet days is the signature of a long term evolution of the solar coronal field topology. It may be interpreted in terms of an increase in the magnitude of the solar dipole, the associated decrease of the heliosheet thickness accounting for the observed decrease in the number of quiet days.

Solar Activity and Meteorological Fluctuations in West Africa: Temperatures and Pluviometry in Burkina Faso, 1970-2012

“…Placing now our study in the context of solar cycle phases (Minimum, Increasing, Maximum, and Decreasing) and geomagnetic classification proposed by Zerbo et al [7], we made interesting analysis: 1) During Minimum phase where Quiet activity (QA) is predominant [6][7][8], precipitations increase (Figure 1) and temperatures decreases (Figure 2). This phase is characterized by slow solar wind speed and high density of solar plasma 2) During Maximum phase where Shock Activity (SA) is predominant [6][7][8], precipitations increase (Figure 1) and temperatures fluctuates (Figure 2). Here Coronal mass ejections (CMEs) are the most important solar events with important energy dissipated in interplanetary medium modifying atmospheric circulation and chemistry (overpressure or depression).…”

“…These periods are characterized by high stream solar wind speed flowing from coronal holes [6][7][8] most disturbed magnetic field with important solar dynamo and joule effect. All that implies that there is no zero climate response to the non constant energy receive from sun (G. Tsiropoula [13]).…”

“…This heat radiation from the Sun is believed to be the primary factor of Earth climate conditions. Several studies have been made over the last decade on geomagnetic activities and weather variations (Delouis and Mayaud [1], Courtillot and Le Mouel [2]; Legrand and Simon [3], Simon and Legrand [4], Gonzalez et al [5], Ouattara et al [6], Zerbo et al [7,8]) and local magnetic observations (Patel. [9], Courtillot and Le Mouel [10]).…”

Aurorae Observed at the Canary Islands

“…Not much solar information is available for the first episode, but more exists for the second. In this context it is worth commenting that some studies indicate a pronounced change in solar rotation around 1900 (Balthasar, Vázquez, and Wöhl, 1986) that surely also affected the heliosphere (Echer et al, 2004;Ouattara et al, 2009) and therefore the number of auroral events (see Silverman and Blanchard, 1983). Echer et al (2004) suggests that OMF structures have increased their activity relative to CMF structures during the last 130 years.…”

The Contribution of a Geophysical Data Service: The International Service of Geomagnetic Indices

“…Geomagnetic activity indices bear more than 140 years of information on the evolution during the solar wind's (see, e.g., Rouillard et al, 2007) coupling with the magnetosphere and on the state of the ionized environment of the Earth (see, e.g., Ouattara et al, 2009). Geomagnetic indices have many other applications.…”