Terrestrial‐magnetic activity and its relations to solar phenomena

Bartels, J.

doi:10.1029/te037i001p00001

Cited by 313 publications

(181 citation statements)

References 10 publications

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“…In particular, the rate tends to peak during the declining phase of each cycle when recurrent storms generated by corotating high-speed streams are particularly prominent (e.g., Bartels 1932Bartels , 1940Sheeley et al 1976Sheeley et al , 1977Tsurutani et al 1995;Richardson & Cane 2012b; and references therein). .…”

Section: Relationship Between Storm Rate and Cycle Peak Ssnmentioning

confidence: 99%

Geomagnetic activity during the rising phase of solar cycle 24

Richardson

2013

J. Space Weather Space Clim.

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As previous studies have shown, geomagnetic activity during the solar minimum following solar cycle 23 was at low levels unprecedented during the space era, and even since the beginning of the K p index in 1932. Here, we summarize the characteristics of geomagnetic activity during the first 4 years of cycle 24 following smoothed sunspot minimum in December, 2008, and compare these with those of similar periods during earlier cycles going back to the start of K p (cycles 17-23). The most outstanding feature is the continuing low levels of geomagnetic activity that are well below those observed during the rising phases of the other cycles studied. Even 4 years into cycle 24, geomagnetic storm rates are still only comparable to or below the rates observed during activity minima in previous cycles. We note that the storm rate during the rising phases of cycles 17-23 was correlated with the peak sunspot number (SSN) in the cycle. Extrapolating these results to the low storm rates in cycle 24 suggests values of the peak SSN in cycle 24 that are consistent with the NOAA Space Weather Prediction Center prediction of 90 ± 10, indicating that cycle 24 is likely to be the weakest cycle since at least 1932. No severe (Dst < À200 nT) storms have been observed during the first 4 years of cycle 24 compared with 4 in the comparable interval of cycle 23, and only 10 intense (Dst < À100 nT) storms, compared with 21 in cycle 23. These storms were all associated with the passage of Interplanetary Coronal Mass Ejections (ICMEs) and/or their associated sheaths. The lack of strong southward magnetic fields in ICMEs and their sheaths, their lower speeds close to the average solar wind speed, a~20% reduction in the number of ICMEs passing the Earth, and weaker than normal fields in corotating highspeed streams, contribute to the low levels of geomagnetic storm activity in the rise phase of cycle 24. However, the observation of an ICME with strong southward fields at the STEREO A spacecraft on July 24, 2012, which would have been highly geoeffective had it encountered the Earth, demonstrates that strong geomagnetic storms may still occur during weak solar cycles.

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Section: Relationship Between Storm Rate and Cycle Peak Ssnmentioning

confidence: 99%

Geomagnetic activity during the rising phase of solar cycle 24

Richardson

2013

J. Space Weather Space Clim.

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“…[5] According to the Equinoctial hypothesis [Bartels, 1925[Bartels, , 1932Chapman and Bartels, 1940;McIntosh, 1959;Svalgaard, 1977], which is based on the ∼23°tilt of the Earth's equatorial plane to the ecliptic plane and ∼11°offset between the Earth's rotation and dipole axis, the maxima of geomagnetic activity occur near the vernal and autumnal equinox which have a maximum angle (90°) between the solar wind flow direction and the Earth's dipole axis.…”

Section: Introductionmentioning

confidence: 99%

Solar magnetic polarity dependency of geomagnetic storm seasonal occurrence

2011

J. Geophys. Res.

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[1] For nearly a century it has been known that the tendency for geomagnetic activity is, on average, higher at the equinoxes than at the solstices. Previous studies on semiannual geomagnetic activity were performed mainly for geomagnetic indices such as am, aa, U, and AL. Thus, we need to understand the seasonal variation of geomagnetic activity defined by the Dst index over the long term. It is also necessary to test the solar magnetic polarity dependence of geomagnetic activity. This paper is a statistical analysis of the geomagnetic storms defined by the Dst index. Our storm data consists of two sets of storm data for 5 years at each solar minimum during the four solar cycles (19-22) from 1962 through 1998 for two of each solar magnetic polarity. The storms are divided into two groups defined by Dst index (Dst(min) < −50 nT, |Dst| > 70 nT; Dst(min) < −50 nT, |Dst| > 90 nT). Monthly occurrences of these storms are compared. Storms of |Dst| > 90 nT and of |Dst| > 70 nT occurred 153 and 238 times, respectively, during the testing periods. Storms occurred more frequently during the spring and fall seasons for all solar cycle minima, regardless of solar magnetic polarity.Citation: Oh, S. Y., and Y. Yi (2011), Solar magnetic polarity dependency of geomagnetic storm seasonal occurrence,

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“…For want of a specific mechanism they were attributed to hypothetical "M-regions" on the Sun (Bartels , 1932.). Statistical studies (Allen, 1944; Saemundsson, 1962) indicated that M-regions tend to avoid the immediate vicinity of active solar areas but a r e strengthened just outside these areas, 7 especially on the following side.…”

Section: Introductionmentioning

confidence: 99%