The giant, complex sunspot group observed on Sept. 10, 1941.
In September, 1941, one of the most prominent geomagnetic storms in history was triggered by a mammoth sunspot group of complex geometry. (See image) The monster group, first observed on Sept. 10, 1941, occurring at low heliographic latitude and at the eastern limb of the Sun. As I noted in previous posts on such large spots, they form via the gradual assembly of multiple, single flux tubes via convective downdrafts, ultimately leading to the buoyant emergence of a concentrated magnetic field structure with distinct features (umbra, umbral dots, penumbra).
The multiple flux tube sunspot model was originally advanced by Eugene Parker (Astrophys. J, 1979) from the University of Chicago. A simple sketch showing the geometry of the model is illustrated below:
Where v d represents the downdraft velocity and 'x' is the Wilson depression, denoting the gap between the spot 'surface' and the field -free regions. Parker estimated this to be about 1150 km. The downdraft velocity, meanwhile, he calculated to be roughly 2 km/sec.
In more detail, the concentration of hundreds of flux elements or tubes at the solar surface prevented the underlying hot solar plasma from reaching the surface. This also accounted for why sunspots are darker than the surrounding photosphere, because they are some 1500 K cooler,
Observations made over the next week (Sept. 10- 17, 1941) disclosed the spot group growing even as the solar rotation brought it near the center of the solar disk as seen from Earth. As we know today, this is also the configuration for meridian-centered CME (coronal mass ejection). By this stage, the sunspot group was large enough to be seen using the naked eye (using an appropriate filter, of course)
Then at 8h 38m Universal time on Sept. 17th, a spectrohelioscope at the Royal Greenwich Observatory recorded a solar flare above this sunspot group. The instrument detected emissions of ultraviolet and x-ray radiation which - within 8 minutes (the time taken for radiant energy to reach Earth) enhanced the ionization of the Earth's atmosphere causing a sharp perturbation known as a "crochet".
According to a subsequent paper published in The Astrophysical Journal (July 1, 1958) this is a "relatively minor disturbance of Earth's magnetic field which occurs concurrently with certain flares". It is associated with dayside perturbations of the geomagnetic field and affects high frequency radio communications. In this respect, it is analogous to a certain class of sudden ionospheric disturbances.
On the basis of daily sunspot numbers supplied by the U.S. Naval Observatory the Dept. of Terrestrial Magnetism at the Carnegie Institution of Washington formally issued a warning to radio operators to expect significant disturbances in ionosopheric and geomagnetic conditions. beginning on September 18th. This prediction turned out to be accurate so that within 20 hours of the flare a magnetic storm began at 4h 2m UT on Sept. 18th with the arrival of a CME. The latter abruptly compressed the magnetopause generating a magnetic impulse recorded by observatories around the world.
In addition, a magnetic "superstorm" followed which was intense and of long duration. To fix ideas and perspective at least one magnetic observatory (run by U.S. Coast and Geodetic survey) registered six separate magnetic storms with a "K index" of 9 the most severe possible. Five occurred over a 24 hour period.
The more direct physical effects followed including spectacular auroral displays as far south as New Mexico with some citizens wondering if an anti-aircraft search battery had been triggered. (Bear in mind the nation was on edge and this was barely three months before Pearl Harbor). Meanwhile, magnetic activity was found to abruptly increase by 19h 45 m on Sept. 18th and within 5 minutes the Pennsylvania Water and Power Company recorded uncontrolled voltage variations in transmission lines beginning two hours after the magnetic storm commenced.
By Sept. 18-19 widespread interference in radio transmission was reported around the world.
The events of September, 1941 are instructive in that they could occur again, say in the next solar cycle. Will we be ready?
At least at that time, as my mother noted (she was then attending Wisconsin State Teachers' College) the newspapers regularly published brief scientific accounts such as that below *:
This appeared on Sept, 21st, days after the geomagnetic events, and in The Milwaukee Sentinel. Though mom was not big on physics or astronomy, she'd save these clippings to be able to use later on, perhaps even to show to her future kids - if she ever got married and had any. (Five years later, she would) Happily, some ten years later I was the chief beneficiary. Those clippings, along with the associated material in The Book Of Knowledge, drove me to pursue astronomy as a hobby by age 11 and later, a career in space and solar physics.
Because of these same information- bearing cartoons, citizens of the 1940s and 50s were generally informed about natural events even if they lacked advanced degrees. Can the same be said of most citizens today? One wonders.
The more critical question is whether we will be prepared when the next severe geomagnetic disturbance and magnetic superstorms erupt.
* Of course, in 1941 the basic model of the sunspot was of a magnetic "vortex" or "whirling magnet". Even Harvard astronomer Donald Menzel in his wonderful monograph, ''Mathematical Physics' (1961, pp. 274-75) asserted: "Observations suggest sunspots are vortices. He then set out to try to quantify this vortex model as a spinning disk with n electrons per cm2 and which rotates with uniform angular velocity..
n = 2 H/ e v o
where v o is the tangential velocity at the periphery of the vortex-disk.. From these basics, Menzel computed:
n = 10 15 electrons / cm2
Menzel then concluded that given the above, "an excess of protons would produce forces 6 x 10 15 as great" and "a sunspot with such an excess would break up with explosive violence".
"An excess of one proton per sixty square centimeters on the solar surface would produce sufficient positive potential just to overcome the solar attraction by electrostatic repulsion of an electron"
Thereby concluding "the Sun is practically neutral electrically"
What about the magnetic aspect?
"No alignment of the individual atom magnets could possibly be maintained in the presence of the turbulent motion and high temperatures existing on and in the Sun, The effect is undoubtedly electrical..."
One can only marvel at how our understanding of sunspots has progressed since Menzel's book.