Study: Northern Lights may disappear – shrinking protective bubble to put Earth at risk of solar blasts

From the University of Reading:

Britain may lose the magic of the Northern Lights by the middle of the century due to major shifts in solar activity, scientists have discovered.

Space scientists at the University of Reading conclude that plummeting solar activity will shrink the overall size of the sun’s ‘atmosphere’ by a third and weaken its protective influence on the Earth.

This could make the Earth more vulnerable to technology-destroying solar blasts and cancer-causing cosmic radiation, as well as making the aurora less common away from the north and south polar regions for 50 years or more.

Dr Mathew Owens, from the University of Reading’s Meteorology department, led the research. He said: “The magnetic activity of the sun ebbs and flows in predictable cycles, but there is also evidence that it is due to plummet, possibly by the largest amount for 300 years.

“If so, the Northern Lights phenomenon would become a natural show exclusive to the polar regions, due to a lack of solar wind forces that often make it visible at lower latitudes.

“As the sun becomes less active, sunspots and coronal ejections will become less frequent. However, if a mass ejection did hit the Earth, it could be even more damaging to the electronic devices on which society is now so dependent.”

The study, ‘Global solar wind variations over the last four centuries’, published in Scientific Reports, shows how sunspot records can be used to reconstruct what happened the last time the Earth experienced such a dramatic dip in solar activity more than three centuries ago. Combined with updated models and contemporary reports, the researchers were able to predict what could happen during a similar event, likely to occur in the next few decades.

‘PROTECTIVE BUBBLE’

The scientists believe the coming ‘grand minimum’ could be similar to the Maunder Minimum of the 17^thcentury, when sun spot activity almost stopped – another symptom of a less active sun.

Solar wind, made up of electrically charged particles from the sun, travels at around a million miles per hour.

A reduction in solar wind would see the heliosphere – the ‘bubble’ around the solar system maintained by particles emitted by the sun – shrink significantly.

This protective bubble helps shield the Earth from harmful radiation from outer space, but has weakened since the 1950s.

“If the decline in sunspots continues at this rate, we could see these changes occurring as early as the next few decades.” – Professor Mike Lockwood FRS, University of Reading

The scientists predict a rapid reduction in the bubble’s size by around the middle of the 21^st century. The Earth’s own magnetic field deflects some of this radiation, but areas close to the north and south poles are more vulnerable where the Earth’s magnetic field is weakest.

Co-author Professor Mike Lockwood FRS, University of Reading, said: “If the decline in sunspots continues at this rate, and data from the past suggests that it will, we could see these changes occurring as early as the next few decades.

“The Maunder Minimum in solar activity of the 17th century is sometimes mistakenly thought to be the cause of the so-called Little Ice Age, when winter temperatures in Europe, and elsewhere in the world, were lower than average.

“But the Little Ice Age began before the Maunder Minimum and ended after it, and our previous work with the Met Office has shown that  the coming solar minimum will do little to offset the far more significant global heating effects of greenhouse gas emissions.”

Full reference: (open source)

M.J. Owens, M. Lockwood, P. Riley (2017). ‘Global solar wind variations over the last four centuries’. Scientific Reports. DOI: 10.1038/srep41548

h/t to Dr. Leif Svalgaard

Abstract:

The most recent “grand minimum” of solar activity, the Maunder minimum (MM, 1650–1710), is of great interest both for understanding the solar dynamo and providing insight into possible future heliospheric conditions. Here, we use nearly 30 years of output from a data-constrained magnetohydrodynamic model of the solar corona to calibrate heliospheric reconstructions based solely on sunspot observations. Using these empirical relations, we produce the first quantitative estimate of global solar wind variations over the last 400 years. Relative to the modern era, the MM shows a factor 2 reduction in near-Earth heliospheric magnetic field strength and solar wind speed, and up to a factor 4 increase in solar wind Mach number. Thus solar wind energy input into the Earth’s magnetosphere was reduced, resulting in a more Jupiter-like system, in agreement with the dearth of auroral reports from the time. The global heliosphere was both smaller and more symmetric under MM conditions, which has implications for the interpretation of cosmogenic radionuclide data and resulting total solar irradiance estimates during grand minima.

From the conclusions section of the paper:

Firstly, we consider the terrestrial implications. Space weather is primarily the result of rapid changes in the space environment, rather than annual variations reconstructed in this study. Nevertheless, the equilibrium state of the terrestrial magnetospheric system is expected to be very different under MM than modern conditions. This, in turn, will mean a different response to a space weather driver, such as a fast coronal mass ejection. Future work will use a global MHD model of the coupled magnetosphere-ionosphere-thermosphere system to quantitatively investigate this. But even without a numerical model it is possible to draw some qualitative conclusions. The lower P_DYN during the MM would increase the average stand-off distance of the dayside magnetopause⁴³. The width of the far magnetospheric tail, however, is controlled by the solar wind static pressure, P_STA = n_pkT_SW + B²/(2μ_o). As the higher n_p and T_P have a larger effect on P_STA than the reduction in B, the tail would, on average, be somewhat thinner during the MM than in modern times. Thus the magnetosphere would have presented a smaller cross-sectional area to the solar wind, reducing the electric field placed across it by the solar wind and the total solar wind energy that it intersects. A reduction in V_SW and B would mean a reduction in the solar wind electric field, which in turn would combine with the smaller diameter of the magnetosphere to reduce the trans-polar cap potential and polar cap area⁴⁴. Thus the Earth’s magnetosphere would have been somewhat more Jupiter-like, with the part driven by solar wind-driven convection smaller in extent, and the part driven by internal dynamics and co-rotation larger in volume. In addition to an expected reduction in both recurrent and non-recurrent geomagnetic storms during the MM, the expected poleward motion of the nominal auroral oval position may further help explain the dearth of auroral reports from that period for all but the most northerly locations¹⁵. Beyond the magnetopause, the enhanced M_A suggests that the bow shock strength would be enhanced, resulting in more efficient energetic particle acceleration, while the bow shock stand-off distance would be increased on average, resulting in a thicker magnetosheath⁴⁵.

Secondly, we consider the implications for the global heliosphere. Again, a future study will use the reconstructed solar wind parameters with a MHD model of the global heliosphere, but here we consider the first-order implications. Most obviously, a drop in P_DYN will result in an overall smaller heliosphere, though the contribution of pick-up ions to the total solar wind momentum budget⁴⁶ means the P_DYN decrease at large heliocentric distances will be lower than the factor 2 between modern and MM 1-AU values. Any calculation of the heliopause distance under grand solar minima conditions will also need to account for the change in pick-up ion acceleration under the MM reduction in B, particularly out of the ecliptic plane. The shape of the heliosphere is also likely change under MM conditions. For the modern era, P_DYN has been ~2–3 higher at the poles than the solar equator⁴⁷, which results in latitudinal asymmetry in the heliopause stand-off distance and termination shock location⁴⁸. During much of the MM, however, P_DYNbecomes almost uniform with latitude for a greater period of time, suggesting a more spherical heliosphere and termination shock.

In turn, there will also be a number of implications for cosmic ray intensity in near-Earth space, with potential knock-on effects for long-term heliospheric reconstructions on the basis of cosmogenic radionuclide records in ice cores and tree trunks^23,49,50. The relative abundance of radioisotopes such as ¹⁰Be and ¹⁴C can be used to determine the effective shielding of heliosphere from the interstellar cosmic ray spectrum, referred to as the heliospheric modulation potential. Interpreting the modulation potential in terms of heliospheric parameters, such as OSF, necessitates a number of assumptions about the size of the heliosphere, the solar wind speed and the scaling of cosmic ray scattering centers with the HMF intensity²⁰. During grand minima, all of these properties will change, to some degree. As already discussed, we expect a smaller heliosphere, with lower and more symmetric solar wind speeds. The lack of latitudinal solar wind speed structure suggests reduced corotating interaction region formation and hence reduced cosmic ray scattering (even for the same OSF). Furthermore, we note that enhanced V_A during the MM would increase the termination shock strength and may affect the efficiency of anomalous cosmic ray acceleration⁴⁶. While the effect of changing size/shape of the heliosphere is expected to be small on GeV (and greater) energy particles which are largely responsible for cosmogenic isotope production, and hence radionuclide reconstructions of the heliosphere and total solar irradiance⁵¹, it needs to be fully quantified via a galactic cosmic ray transport model and a cosmogenic isotope production model.