Linking Solar Minimum, Space Weather, and Night Sky Brightness

New observations indicate previously unrecognized significant sources of night sky brightness variations, not involving corresponding changes in the Sun's EUV flux , occur during deep solar minimum. Our data was taken at 5 sites spanning more than 8,500 km during the deep minimum of Solar Cycle 24 into the beginning of Solar Cycle 25. It shows; Our empirical results contribute to a quantitative basis for understanding and predicting sky variations. They


Article
We report significant changes in night sky airglow not involving corresponding changes in the Sun's EUV flux. Observations from five sites separated by more than 8,500 km along the Earth's surface reveal the clear astronomically dark night sky is rarely, if ever, constant in brightness. The natural night sky is a unique laboratory providing a rich tapestry of solar and terrestrial phenomena. The Methods section provides a detailed discussion of quantities used in this paper.
A quantitative understanding of natural night sky airglow is essential to inform the search for Earth approaching asteroids. Observations by telescopes like the Vera C. Rubin Observatory (VRO or LSST)1, astrophotography opportunities, stargazing world wide, and the study of anthropogenic skyglow 2,3 require this information. Empirical evidence is necessary to guide theories and modeling of night sky brightness variations. Changes in natural night sky brightness provide clues about ionospheric and space weather processes. Extreme events in this realm have a significant impact on electrical grids, electronic communication, and navigation satellites4.
The lack of a consistent data set spanning time and geographic location has made understanding night sky airglow variations difficult. Despite this, researchers have correlated natural night airglow with day-time atmospheric photoionization and subsequent night-time recombination's response to changes in the Sun's extreme ultraviolet (EUV) flux5,6.
Satellite observations suggest historical bright nights are the result of zonal wave superposition. 7 Walker, Krisciunas, and others have correlated observed variations in night sky brightness with prior changes in EUV from the Sun as measured by the 10.7cm (2.8 GHz) solar flux5,6. The hypothesis is, changes in photoionization on the Earth's day-side produce subsequent variable night time airglow by re-radiation from atoms and molecules in a complex chemical environment. This concept can be used to relate changes in solar EUV, estimated from the 10.7cm solar flux, to daily and yearly night sky brightness changes throughout a solar cycle.
The night sky brightness changes we observed are not correlated with changes in solar EUV flux. During the period of our observations, the solar EUV measured by the 10.7cm solar flux was at a low, relatively constant, level (average = 69.77 sfu, stdev = 2.45 , 1 sfu = 104 Jy).  Fig. 1 shows an apparent periodicity of the 26.24 day synodic solar rotation cycle relative to the Earth. The interpretation of this temporal spacing is complicated by the lunar cycle and weather events producing the gaps in the data acquisition stream.
Three very broad increases in airglow brightness were measured at all sites. The ΔMC-N(t) MC-N(t) values plotted on the vertical axis of Fig. 1 are the nightly average increases of the measured airglow above its quiescent level. (see Methods) Peaks in ΔMC-N(t) MC-N(t) occurred near JD 2458435 (12 November 2018), JD 2458589 (16 April 2019), and JD 2458786 (29 October 2019). These dates were obtained by fitting the ΔMC-N(t) MC-N(t) data to a quadratic formula extending 1 or 2 lunations either side of the peak. The significance of these night airglow brightness increases are the subject of this paper. Geomagnetic activity has a long history of time series measurements and their analysis. Conversely, interruptions by the Sun, Moon, weather events, and the lack of a wide spread network of suitable measuring stations have left the study of natural night sky airglow brightness variations relatively undeveloped. This paper juxtaposes a limited night sky airglow brightness data set with the rich body of geomagnetic research.

Article (continued)
The semi-annual variation in geomagnetic activity has been known for over 100 years8 . In 1971, Russell and McPherron proposed a model to explain this phenomena in terms of the relationship between the z component of the interplanetary magnetic field, [B(t)z]GSM , and the z component of the Earth's magnetic field9 (both expressed in the Geocentric Solar Magnetospheric [GSM] coordinate system). In the Russell and McPherron model the interaction between these two magnetic fields acts like a rectifier. When [B(t)z]GSM is negative, opposite Earth's magnetic field , charged particles are more likely to penetrate the ionosphere. When [B(t)z]GSM is positive, in the same direction the Earth's magnetic field they are partially blocked. A negative [B(t)z]GSM produces enhanced geomagnetic activity. In 2019, according to their model geomagnetic activity reaches a maximum around 4 April and 7 October. Expressed in fractions of a year, F, these peaks are at F = 0.257 and F=0.769. It should be emphasized this model is based on the statistics of geomagnetic events above a certain threshold. There are broad peaks and dips with a full width at half maximum of many days.
The existence of a seasonal variation in the brightness of the natural night airglow has been debated for years in the literature. Patat reviews this situation and presents new data derived from spectroscopic observations in the UBVRI passbands at the Cassegrain focus of the 8.2 m telescopes at the Paranal Observatory in Chile10. The observations were made during the decline from maximum to minimum of Solar Cycle 23. He reports a clear seasonal variation in the broadband VRI passbands with two broad maxima (April-May and October) and two broad minima (July-August and December-January).   2 suggests night sky airglow is modulated by the changing alignment of the interplanetary magnetic field relative to that of Earth in a way similar to the Russell-McPherron effect for geomagnetic activity. It should be emphasized that simple cause and effect relationships between changes in solar activity and geomagnetic activity, and/or night sky brightness variations are difficult to establish. In the Russell-McPherron effect the chaotic, highly variable, solar wind is modulated by the interplanetary magnetic field to produce a statistically discernible geomagnetic activity pattern using data encompassing a number of years. The data presented in Fig. 2 are suggestive that night sky airglow variations follow the Russell-McPherron effect, however, data from more years will be required to put this hypothesis on a solid statistical basis.   From July to November 2018 the same large coronal hole was observed on the Sun. It pointed in our direction every solar synodic period. After each such alignment, several days later Earth was engulfed in a high speed stream in the solar wind 11. This set of circumstances produced airglow and geomagnetic events world wide and was imaged by astronauts on the International Space Station12. This series of events produced the broad peak in night sky brightness near JD 2458435 (12 November 2018), see Fig.1 The wide vertical distribution of points in Fig. 1 is the result of real changes in the night airglow.

Article (continued)
Zoltán Kolláth's image ( Fig. 4) was captured on 20.10.2019 at 03:53 UT (2458776.66181 JD) at the beginning of an extended night sky brightness episode13,14 . Fig. 3 shows the image's temporal relationship to the other data. Green (558nm) oxygen and orange (589nm) sodium airglow are visible over the entire sky. The R,G, and B channels in the digital camera data provide estimates of the strength and spatial structure of the oxygen and sodium lines14. Fig. 3 shows this period of time was characterized by predominately negative [B(t)z]GSM and NKE(t) which varied significantly above its median value. Near 2458780.75 JD (24 October 2019) a high speed stream in the solar wind produced a shock wave at Earth's bow shock nose. This produced a dramatic increase in geomagnetic activity, NAp(t). A pulse in NKE(t) deposited energy into the Earth's magnetosphere. Meanwhile, the solar EUV as indicated by the N10.7cm(t) flux was low and constant and is uncorrelated with airglow brightness changes. The period of negative [B(t)z]GSM which followed allowed energetic charged particles to penetrate deep into Earth's ionosphere. The shock wave triggered large variations in airglow brightness during the night reaching peak brighrtnesses near local midnight.
We observed several other episodes of substantial brightness variations in night airglow. In some cases, after a maximum near local midnight, a decline in airglow brightness was followed by a substantial brightening several hours later. Insufficient data prevented us from determining if these features were the result of atmospheric airglow waves15 , energy being released from Earth's magnetospheric tail 16,17,18 , or an unknown physical process. Bottom Panel: The night airglow above the quiescent level, ΔMC-N(t) MC(t), for each site is plotted versus Julian Date (JD). It is interesting to note airglow is increasing in brightness before the arrival of the shockwave. This suggests preconditioning of the magnetosphere-ionosphere system may allow a shock wave to trigger a night sky brightness increase event.
- 6-215  216  217  218  219  220  221  222  223  224  225  226  227  228  229  230  231  232  233  234  235  236  237   239  240  241  242  243  244  245  246  247  248  249  250 Article (continued) A key element in their research is an estimation of the power input into the magnetosphere, P α . They calculate P α , employing interplanetary measurements, with the formula originally derived, theoretically, by Vasyliunas et al.19. P α has only 1 free parameter, the coupling factor α. It is driven by the speed, number density, ion mass measures of the solar wind, and modulated by the interplanetary magnetic field's strength and orientation. In general, geomagnetic parameters have fractional variational amplitudes larger than the corresponding temporal fractional changes in P α . This amplification can be seen by comparing the am geomagnetic index with P α 17. This research group shows the Russell-McPherron Effect is the principal driver of semiannual geomagnetic activity even though it has a small impact on P α . Interestingly, they report the intensity of geomagnetic activity produced by the Russell-McPherron Effect is apparently amplified by the release of energy stored in the Earth's magnetospheric tail.

Article (continued)
It is plausible the brightness of night side airglow, like geomagnetic indices, is driven by P α . Airglow is likely to possess an amplitude of variation and delayed response produced by non-linear processes in the Earth's magnetosphere. To investigate this possibility, Fig. 5 is a graph of 948 nightly airglow averages obtained at five sites, 4 September 2018 through 30 April 2020. On the vertical axis, each point is the nightly average of airglow brightness, ΔMC-N(t) MC-N(t) . The celestial and anthropogenic sources have been removed as outlined in the Methods section of this paper. The horizontal axis is the time in fractions of a year (F). The data were sorted into 36 bins with an F width of 0.0278. The smoothed data curve was obtained from the 36 data bins using a 5 point triangular weighting function. The smoothed, binned, data curve of Fig. 5 shows a semi-annual variation in airglow brightness with broad peaks near 0.273 F and 0.837 F. The peak near 0.837 F has an amplitude and location strongly influenced by high speed streams in the solar wind from coronal holes on the face of the Sun. The wave form of the smoothed binned data of Fig. 5 is similar to the am geomagnetic index amplified response to P α as calculated by Lockwood et al. 17 . This similarity suggests the night side airglow is coupled to the energy input into the Earth's magnetosphere in a way similar to the am geomagnetic index.

Article (continued)
In conclusion, during solar minimum, significant episodes of increased night sky airglow are not produced by changes in solar EUV flux. They appear to be the result of two processes; 1) Changing orientation of the interplanetary magnetic field relative to Earth's magnetic field and 2) Earth entering streams of energetic solar wind.
Some night sky brightness events are relatively local. Others extend at least 8,500 km along the Earth's surface (see Methods for a plot).
Our data suggests the terrestrial night airglow responds to the energy input into the Earth's magnetosphere in a fashion similar to the geomagnetic indices.
We strongly advocate the establishment of a global network of photometers ,in places where anthropogenic skyglow is at a minimum. They would track brightness variations of the natural night sky. Established astronomical observatories are the places to start. These measurements will have a significant impact on the studies of astronomy, space weather, light pollution, biology, and recreation.

Methods
We measure changes in terrestrial zenith night airglow relative to celestial sources. This procedure minimizes errors encountered using data from several different instruments and individual instrument drift in sensitivity if it exists. Our photometers are used to measure differences in brightness of the same place on the celestial sphere along the zenith declination.
In the past, most night sky brightness measurements published in the scientific literature were made by astronomical research-grade instruments occasionally scheduled for this task5,6. Our research is enabled by accurate, low cost, scientific quality SQM-LU-DL20 and TESS-W21 photometers. They provide continuous measures of zenith night sky brightness dusk to dawn every night. These two photometers use the same detector, have slightly different fields of view, and different red responses. Their differential photometric measurements produce similar results. In the differential photometry mode we employ, they are more accurate (error < 0.03 mag/arcsec2) compared to when used as calibrated absolute photometers (error ~ 0.1 mag/arcsec2 ).20 The irradiance-to-frequency semiconductor detector employed by both the SQM-LU-DL and TESS-W instruments is calibrated to report the measurements in mag/arcsec2 .22 On clear astronomically dark nights, these single channel photometers, pointed at zenith, measure light accumulated from terrestrial airglow, stars, planets, scattered star light, zodiacal light, nebulae, galaxies, other faint astronomical sources, and anthropogenic skyglow if present.
Methods Table 1: The sites and instrumentation used in this study.
The artificial levels in Methods Table 1 are estimates from satellite data adjusted by SQM observations on the ground24. CCIDSS is a unique standard more than 60 km away from any significant source of artificial light. Analysis of all sky images14 and the satellite estimate, of less than ½ % artificial light24, indicates anthropogenic skyglow is unmeasurable at zenith at CCIDSS . We have developed techniques and software to process the data20. Our software selects individual instrumental measurements , M(t), taken at time t, when the Sun was more than 18 degrees below the horizon, the Moon was more than 10 degrees below the horizon, and the sky was clear. Sky clearness is measured by computing Chi Squared from a straight line fit to the data extending for 45 minutes on either side of the point in question. A Chi Squared of less than 0.009 rejects cloudiness but not the rising Milky Way. As an additional check the TESS-W near IR sensor is employed to estimate cloud cover. 21  Methods Fig. 1 is a plot of the 12,892, M(t), data points, sorted into ½ h bins in R.A., obtained at CCIDSS September 2018 through April 2020. The vertical distribution of sky brightness, at each sky position of R.A., is due to changes in terrestrial airglow.
Methods Fig. 1 and similar plots for the other sites observationally establish a quiescent value of airglow for each location on the celestial sphere. One could, also, establish a value for the quiescent airglow level by adding up the minimum values for all known sources of diffuse night sky brightness.25 Methods Fig. 1: The vertical axis is the instrumental sky brightness, M(t), in mag/arcsec 2 . It shows changes in airglow due to solar activity at each sky position. The horizontal axis is the position in the sky in hours of R.A.( the data being sorted into ½ h bins). The continuous light curve of quiescent airglow was calculated using the faintest 10% of the M(t) in each ½ h R.A. bin. It is the sky brightness when the terrestrial airglow is at minimum.

Methods (Continued)
Along the zenith declination on the celestial sphere, ½ h bins in R.A. , are used as 48 standard candles. Each such standard candle is the average of the 10% faintest M(t) measurements at its location on the celestial sphere. A set of joined polynomials are fitted to the 48 standard candles to produce a continuous light curve of quiescent airglow brightness. Methods Fig. 1 illustrates these concepts with data from CCIDSS. Each site has a unique light curve of quiescent airglow brightness which depends on its latitude and the degree to which it is influenced by anthropogenic light. Each measured point's brightness above the quiescent airglow, ΔMC-N(t) M(t), is obtained by subtracting the continuous light curve of quiescent airglow from the data, point by point. This procedure removes light from the stars, planets, Milky Way, zodiacal light, other celestial sources, and constant anthropogenic light if present. Thus, each ΔMC-N(t) M(t), is the differential photometric night sky brightness at time t, relative to the brightness of the same point on the celestial sphere when the airglow is at minimum. The same procedure is used to produce, ΔMC-N

Methods (Continued)
Methods Fig. 4 indicates some airglow increase events are local while others extend over relatively large geographic regions. CCIDSS and CSSMLS are 209 km apart. The airglow of the nights in common between these two sites scales with a scale factor of 0.91. The data suggest the largest brightness increase events occur over geographical dimensions of more than than 200 km while lesser airglow brightness events are more local. CCIDSS and Stars211 are 8519 km apart. The airglow of the nights in common between these two sites scales with a scale factor of 0.75. This suggests that some airglow increase events have dimensions of 8,500 km or more while others are more local. Widely observed night sky brightness events may be similar to large-scale structures observed on airglow maps33.  16   603  604  605  606  607  608  609  610  611  612  613  614  615   617  618  619  620   621   622   623   624   625   626   627  629  630  631  632  633  634  635  636  637  638  639  640  641  642  643  644  645  646  647  648  649  650  651  652  653  654  655  656  657  658  659  660  661  662  663  664  665  666  667  668  669  670  671  672  673  674  675  676  677 678 679 Figure 1 Please see the Manuscript PDF le for the complete gure caption Figure 2 Please see the Manuscript PDF le for the complete gure caption Please see the Manuscript PDF le for the complete gure caption Please see the Manuscript PDF le for the complete gure caption Figure 5 Please see the Manuscript PDF le for the complete gure caption Figure 6 Please see the Manuscript PDF le for the complete gure caption Please see the Manuscript PDF le for the complete gure caption Figure 8