• Where is 'where' in the phrase 'I read somewhere'? You can figure it out yourself, there are some pretty good estimates of how big a rock is needed to cause an extinction event ( we can use this one, because a gigaton here or there won't matter https://futurism.com/19943 ".... 96 km (60 miles) wide to completely and utterly wipe out life on our planet." ). Sounds good to me. Let's make it 100 km, to make sure it kills everything in the nooks an crannies, and we need the radius 50km. Take the density of iron ( 7.874 g/cm3 ) and use the formula for volume ( 4/3πR3 ), for simplicity a sphere. 523598 cubic kilometers, wow, seems like lot, so let's check 100km x 100km x 100km = 1,000,000km3 for a box, and it's less than that, so okay. A cubic kilometer has 1e+15 ( or 1,000,000,000,000,000 cubic centimeters. So the mass of our asteroid is 523598 cubic kilometers times 1,000,000,000,000,000 cubit centimeters times the density of iron, 7.874 g/cm3. Divide grams by a thousand to get kilograms.
• Calculate the mass of the earth's core ( 3,485 kilometers in diameter ), about 1700 km in radius.
• Compare the masses - magnetism isn't that simple, and there's rotation involved etc.
• ( somebody will point out here that you only have to compare the cubes of the radii, since the same formulae is used, but that skips all the big number drama ), the ratio of the masses is about 0.0000254427 invert that .. I then use https://www.themeasureofthings.com to find something that masses more than me by the same ratio. 100kg times 39304.
• Basically the same as if I ran headlong into a solid mass of concrete the size of a three story building ( 40ft x 40ft x 40 ft ). The building won't notice.
• I might affect the paint. That's basically the same thickness as the biosphere on earth.
• There is a whole field called paleo-magnetism ( https://en.wikipedia.org/wiki/Paleomagnetism ), with really good data on the chronology of Earth magnetic field reversals. Also some pretty good data dating the time and size of impacts ( https://www.lpi.usra.edu/publications/newsletters/lpib/new/earths-impact-history-through-geochronology/ ), and extinction events ( https://www.pnas.org/content/111/9/3316 ).

The short answer is, IMHO, no. Any magnetic contribution / mass alteration from a meteorite would be minuscule compared to something like the Deccan or Siberian traps. https://en.wikipedia.org/wiki/Siberian_Traps

Dear Professor Jan,

The idea that bolide impact and geomagnetic reversal might be linked goes back at least 50 years (Glass & Heezen 1967) and the premise that magnetic reversals might foster biotic extinction, even further (Uffen 1963). That these themes continue to be actively researched and debated suggests that neither can be considered irrefutably proved or disproved. Also many of the studies in these fields rest on demonstrating a temporal correlation, rather than proving causation.

A notable exception with regard to impact is the paper by Muller & Morris (1986) who suggest a complex chain of causation involving impact winter, rapid mass redistribution in the hydrosphere/cryosphere, modification of the earth's moment of inertia, and consequent disruption, by shear flow, of the dipole field of the core dynamo. It is worth mentioning that part of this mechanism has been invoked more generally in the context of the possible effect of climate change (ice ages, sea level change) on moment of inertia and magnetic reversals.

In terms of more recent studies, Mohazzabi et. al. (2015) investigated the direct kinetic effects of impact on the core and concluded that any effects (on superrotation of the inner core) would be limited and rapidly damped out. Another approach to testing causal relationships between impact and the geodynamo involves palaeomagnetic studies of long-lived impact-produced cooling melt-sheets, (eg Herve et.al.2015, Eitel, 2014, 2019).

I will leave it to others to comment on the biotic effects of polarity reversals.

Hope this helps

Glass, B. I. L. L., and Bruce C. Heezen. "Tektites and geomagnetic reversals." Nature 214.5086 (1967): 372-372.

Uffen, R. (1963) "Influence of the Earth's Core on the Origin and Evolution of Life". Nature 198, 143–144

Muller, Richard A., and Donald E. Morris. "Geomagnetic reversals from impacts on the Earth." Geophysical Research Letters 13.11 (1986): 1177-1180.

Mohazzabi, Pirooz, and John D. Skalbeck. "Superrotation of Earth’s inner core, Extraterrestrial Impacts, and the effective viscosity of outer core." International Journal of Geophysics (2015).

Hervé, Gwenaël, et al. "Paleomagnetic and rock magnetic study of the Mistastin Lake impact structure (Labrador, Canada): Implications for geomagnetic perturbation and shock effects." Earth and Planetary Science Letters 417 (2015): 151-163.

Eitel, Michael, et al. "Rochechouart impact crater melt breccias record no geomagnetic field reversal." Earth and Planetary Science Letters 387 (2014): 97-106.

Eitel (2019): "Geomagnetic response from meteorite impacts" University of Ludwigs-Maximilians, Munich (Dissertation)

I first came across this idea in "The Cosmic Connection" by Carl Sagan (Doubleday, 1973). He pointed out that there were magnetic field reversals at the times of the last two big impacts with the Earth, both of which were on land. However, as three-quarters of the Earth's surface is water, the majority of such impacts would be at sea. That would explain what seemed to be an otherwise mysterious linkage of marine 'megadeaths', simultaneous extinctions of large numbers of species, and magnetic field reversals - which in themselves might cause some depletion of surface phytoplankton, due to increased levels of solar ultraviolet radiation and cosmic rays reaching the ocean surface, but would not be expected to have such big effects on marine life generally.

The magnetic reversals are a record of earth history and sometimes scientists were trying to connect the extinctions with reversals. But these hypothesis couldn't find any firm scientific grounds.

The impact did rotated the planets like the rotation axis of Neptune is almost 90 degree to magnetic axid due to major impact. Therefor, impact may transform the magnetic polarity but it may not completely reverse the dipole. The transformation could turn the system in quadpole or more complex which may weaken the magnetic field and will expose the system to radiation.