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17 February 2005 | A potentially important discovery has been found buried within the statistical records of space weather data by Dr. E. Echer, and Dr. W.D. Gonzalez et al.[1] at the Geofisica Espacial, Instituto Nacional de Pesquisas Espaciais, Sao Jose dos Campos (Brazil) and colleagues at the Jet Propulsion Laboratory in California.
The Sun produces a regular flow of charged particles that typically stream outward at velocities of between 300 and 400 kilometers per second. This is known as the solar wind. Occasionally, the sun produces outbursts (coronal mass ejections or CMEs) that are often associated with energetic solar flare activity. These CME's typically travel faster than the background solar wind. If the CME attains a sufficiently high velocity, the leading edge of the CME may form a shock front (similar to a blast wave on Earth), where sudden discontinuous jumps in solar wind velocity, density, temperature and magnetic field strength are observed. The impact of solar wind shock fronts at the Earth are responsible for disturbing the Earth's geomagnetic field. The resulting geomagnetic storms can result in significant displays of auroral activity (northern lights) around the world if the CME disturbance is strong enough. Typically, very strong CME's (associated with very strong shock fronts) are capable of producing visible auroral activity well into the tropical regions of the Earth.
This study discovered an odd, yet potentially important cyclic behavior with solar wind shock fronts. Drs. Echer and Ganzalez et al. examined 574 solar wind shock fronts from data from spanning from 1973 to 2000. During an entire year, the researchers reasoned that solar wind shock fronts should occur roughly the same number of times each month. For example, if one shock is observed each month, then the proportion of shocks observed during that month should be 8.3% (1/12=0.083) of the total observed during the year. But this was not the case. Instead, the researchers found that during the month of July, the shock rate occurrence was 10.4%, and during November the percentage jumped to 13.9% of the total annual occurrence. For November, this is two standard deviations higher than the background rate and is statistically significant. In other words, there must be an underlying physical reason for this increase in occurrence in the months of July and November.
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Conception of solar wind flows, with higher solar wind velocities compressing the solar wind (brighter red areas) and increasing the probability of forming shock fronts. Courtesy Brian Grimm and Living Text. |
Examining the available solar wind data and related solar observations during this period, the researchers found that the increased occurrence frequency is very likely the result of increasingly favorable conditions promoting the formation of shock fronts in the latter half year (lower solar wind velocities, higher densities, etc.). This was suspected from the results of research performed as early as 1965, where solar wind parameters (velocity, density, etc.) at the Earth were known to vary with specific periodicities. But those periodicities most strongly favored shock formation during the month of July, not November; yet the strongest occurrence of shock fronts was in November. The researchers hypothesized that these same cyclic fluctuations in solar wind parameters are what give rise to the favorable conditions promoting solar wind shock front development in July. The same cannot be said for November, however. Some other process must be involved to create a higher proportion of shock front arrivals in November. The solar flare H-alpha index (a measure of the optical brightness and duration of solar flares to estimate total energy output) was found to peak during the months of July and November (with the greatest proportion being during November). But it does not provide a firm causal explanation of the variance in shocks observed at the Earth during November. The researchers acknowledge that further research is required. However, it is interesting to note that an earlier study in 2004 by Dr. Gonzalez[2] et al. found that intense geomagnetic storms occurred most frequently during the months of July and November.
These research results were published in the Journal of Geophysical Research / Space Physics, volume 110 on 2 February 2005 (doi:10.1029/2004JA010527, 2005) by the American Geophysical Union.
[1]Echer, E., W.D. Gonzalez, L. E. A. Vieira, M. V. Alves, and A. L. C. Gonzalez, On the preferential occurrence of interplanetary shocks in July and November: Causes (solar wind annual dependence) and consequences (intense magnetic storms), Journal of Geophys. Res., vol. 110, A02101, doi:10.1029/2004JA010527, 2005.
[2]Gonzalez, W. D., B. T. Tsurutani, and
A. L. Clu´ a de Gonzalez (1999), Interplanetary origin of geomagnetic
storms, Space Sci. Rev., 88, 529.
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