E ff ects of solar activity and geomagnetic field on noise in CALIOP profiles above the South Atlantic Anomaly

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Introduction
The South Atlantic Anomaly (SAA) refers to an irregular area, roughly centered on 30 • W 30 • S, extending over South America and the South Atlantic Ocean, where the Earth's geomagnetic field is weakest and the inner radiation belt comes closest to the surface.These conditions, due to the misalignment between the Earth's magnetic and rotational axes, lead to the trapping of ionized particles between 200 and 1600 km above the surface (Ginet et al., 2007) -mostly protons and electrons but also antiprotons created by the interaction with cosmic rays (Adriani et al., 2011).The energetic particles lead to an increase in outgoing radiation, affecting spaceborne electronics and detectors overflying the area.
The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP, Winker et al., 2009) operates from within the A-Train constellation, which orbits the Earth at 705 km of altitude (Stephens et al., 2002).Its altitude and orbit make it travel through the SAA several times a day.It has been obvious since the early stages of CALIOP operation that Introduction

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Full the SAA strongly affected its nighttime observations of backscatter coefficients (Hunt et al., 2009, H09 hereafter), leading to increased levels of random fluctuations in the observed signal.Due to energetic radiations, dark noise levels in the 532 nm channel are 30 times larger in the SAA than elsewhere, leading to a decrease of the nighttime Signal-to-Noise ratio by a factor of 5 [H09].
The noise increase is especially noticeable in clear-sky areas at stratospheric altitudes, where backscatter returns are generally weak due to the absence of particles and low molecular densities compared to the troposphere.We performed a quick analysis of CALIOP data for 2008 and found that the standard deviation of stratospheric backscatter at 30-34 km is consistently 3.2 times larger over the SAA than over the rest of the globe.This has been proved particularly bothersome for the calibration of CALIOP signals based on stratospheric signals, which eventually had to ignore measurements over the area (Powell et al., 2009).Studies based on stratospheric CALIOP measurements require a careful attention to the area (e.g.Vernier et al., 2011;Lopes et al., 2013).
Here we document how changes in the intensity and spatial extent of the SAA across six and a half years (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013) affect levels of dark noise in CALIOP observations at 532 nm.We show that within those noise levels are information about the evolution of the SAA with time.
2 Average extent of the SAA H09 first described how telluric radiations in the SAA induce frequent current spikes and a significant amount of dark noise in CALIOP nighttime measurements.They mapped the profile-level dark noise at 532 nm during September and October 2006.They showed noise is also significant above the polar regions, approximately along the aurora ovals, where charged electrons precipitate from the magnetotail, flowing with the geomagnetic field.Finally, noise also increases at high latitudes, near the edges of nighttime orbits, as sunlight gets scattered above the horizon when the satellite Introduction

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Full transitions from and to daytime.In those regions, noise levels are however much weaker than above the SAA.Dark noise in CALIOP profiles can be tracked via the Parallel RMS Baseline at 532 nm variable, present in CALIOP Level 1 data files, which reports in digitizer counts the root mean square noise of 1000 15 m samples, calculated onboard from the 532 nm parallel channel in the 60.3-75.3km altitude range (Hostetler et al., 2006).H09 showed digitizer counts generally stay below 100 in most areas, and reach their highest levels (> 2000) above the SAA. Figure 1   In stark contrast to the clean geographical patterns of nighttime dark noise, we could not find meaning in the geographical distribution of daytime dark noise, which at first seem rather randomly distributed (not shown).This suggests that daytime dark noise levels are primarily affected by sunlight reflection on bright surface and low-level clouds, or reflection and scattering on ice crystals [H09].In this configuration, increased daytime dark noise levels are triggered by favorable sun-cloud-detector geometries, which are quite random.

Evolution of the SAA
Figure 2 shows how the percentage of nighttime profiles affected by high noise levels (as defined in Sect.2) deviate from the mean (57.5 %) across 15 days windows between June 2006 and December 2012 in the SAA region (red box in Fig. 1).CALIOP data come from Level 1 v.3.01 and 3.02 files, as available.The noisy fraction of profiles is minimum in 2006 (54 %), increases steadily to reach 60 % in 2010, then undergoes a rather abrupt drop in 2011 that brings it back to 2007 levels in early 2012 and onwards.Literature teaches us that the amount of radiation emitted from the SAA, which impacts the noise in CALIOP observations, is anticorrelated with the 11 yr cycle of solar activity (with 1 yr lag) (Furst et al., 2009).This anticorrelation is due to heating of the exo-and thermosphere during maximum solar activity, which leads to a higher neutral density in the altitudes below affected by the SAA (e.g.Qian et al., 2006).This increases the absorption and deflection of trapped particles, resulting in a lower particle flux compared to when the solar activity is minimum (Dachev et al., 1999).Thus the flux of energetic particles emitting radiations below CALIOP is smaller than during minimum solar activity.The 1 yr lag reflects the time needed for the atmosphere to react to incoming solar energy.
CALIOP to the 1 yr lag.Since then, solar cycle 24 has begun and solar activity has been increasing again, while noise in the SAA region decreased accordingly.The solar cycle is not symmetric: the increase in solar activity is faster (∼ 3 yr) than the decrease, which shows as an opposite influence on noise levels (Fig. 2).We should thus expect the SAA noise to reach its minimum in the near future (probably in 2014), and resume a slow increase until the year following the next minimum of solar activity, in 2020.CALIOP will unfortunately be inoperative at this point, but it seems safe to assume that future spaceborne lidar missions will be similarly affected.Since all profiles are noisy in the SAA center (Fig. 1) and the total number of noisy profiles in the SAA region changes with time (Fig. 2), the affected surface must change with time.Figure 3

Noise levels in clear areas
In areas supposedly unaffected by sources of additional noise (non-SAA boxes in Fig. 1), the noisy fraction of profiles is extremely low on average (2 % ± 0.1 %) compared to inside the SAA.Even then, Fig. 4 (top) reveals the influence of the 11 yr solar cycle is quite noticeable in these regions, for the same reasons as over the SAA.Introduction

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Full Intense solar activity warms up the high atmosphere, which leads to less charged particles overall and thus less noise, as in e.g. the relatively low-noise 2006-2008 period.Conversely, noise levels increase in periods of weak solar activity (2009)(2010).Here, the increase is however extremely weak (less than a percent) compared to above the SAA. Figure 4 also reveals that noise levels in all three regions are affected by an underlying yearly cycle, which is minimum in July and maximum in January.Surprisingly, this cycle affects similarly both hemispheres with the season, and thus appears unrelated to the amount of incoming solar light reaching a particular hemisphere.This is contrary to our expectations for noise to be opposite in NH and SH (i.e. to follow the seasonal cycle of solar illumination).Peaks appear however more pronounced in the SH (green) and smoother in the NH (orange).In the latitude band where nighttime observations never directly experience solar illumination (30 • S to 30 • N), all longitudes outside of the SAA region show the same noise cycle.Noise levels in the SAA region are probably also affected, but the effect is too weak compared to the impact of the 11 yr solar cycle to register on Fig. 2.
A possible explanation for this cycle lies in the evolution of the geomagnetic field, which affects the amounts of charged particles that create background dark noise spikes in CALIOP data.The bottom pane of Fig. 4 shows the relative evolution of the deviation shown in the top pane (i.e.divided by the mean), detrended of solar cycle influence by subtracting the SAA equivalent (as in Fig. 2).Over-plotted in dark are relative deviations from the mean of the geomagnetic field intensity along the Earthward vector (He) measured by the magnetometers part of the Space Environment Monitor on the GOES-11 geostationary satellite (Singer et al., 1996).Note that their y-axis (right) is inverted to simplify visual comparison.CALIOP dark noise levels appear well anticorrelated with the geomagnetic field activity documented by GOES.Maximum anticorrelation (−0.67) is obtained when He measurements are delayed by 30 days.We have not identified the particular mechanism through which the geomagnetic field activity affects CALIOP noise levels.

Summary
By studying the evolution of noise in CALIOP nighttime profiles above the South Atlantic Anomaly region, we have found it influenced by the 11 yr cycle of solar activity.We documented how this area has been widening since 2006, shrunk since 2011 and overall shifted slightly to the West, in agreement with previous studies and predictions.
Studies using CALIOP observations in the stratosphere and upper troposphere close to Central and South America should be mindful of the increased noise levels occurring in 2010-2011.We predict that SAA noise levels will soon start increasing again, and will affect future spaceborne lidar missions most near 2020.Moreover, we have shown that all CALIOP nighttime noise levels are influenced by the same 11 yr solar cycle, even in areas totally devoid of solar illumination.We have in addition identified an asyet unexplained yearly noise cycle that appears unrelated to the seasonal change in solar illumination, but is anticorrelated with the geomagnetic field activity.
Acknowledgements.The authors wish to thank the NASA Langley CALIOP team for the CALIOP data, the ICARE data center for access to the data, and ClimServ for providing computing facilities.Data analysis was conducted using Python and Numpy (Oliphant, 2007).Figures were created using Matplotlib (Hunter, 2007).

The publication of this article is financed by CNRS-INSU. Introduction Conclusions References
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began operating in 2006, during the downward section of solar cycle 23, when solar activity was already quite low compared to its 2000 maximum.It kept decreasing until 2009, while CALIOP noise levels over the SAA increased until 2010 due Discussion Paper | Discussion Paper | Discussion Paper | shows the evolution with time of noisy profiles as a function of latitude (top) and longitude (bottom), in bands roughly centered on the SAA.The top figure shows that the SAA stayed roughly centered around 25 • S, and more-or-less symmetrical except its north boundary shifted towards the Equator as the number of noisy profiles increased.The south boundary meanwhile appears constant, although it is weakly disturbed by the seasonal irruption (in December-January) of sunlight scatter.As a result, the SAA area zonally widened by roughly 2-3 • after 2006 to reach a maximum extent in 2010.It quickly narrowed back to 2006 levels after 2011.The bottom figure shows that meridionally, the SAA region extended primarily westwards by roughly 5 • until 2010, but narrowed primarily from the East after 2011.This translates to a slight westwards shift of the SAA between 2006 and 2013, which is consistent with predictions from the literature (e.g.Furst et al., 2009).
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Lopes, F. J. S., Landulfo, E., and Vaughan, M. A.: Assessment of the CALIPSO Lidar 532 nm version 3 lidar ratio models using a ground-based lidar and AERONET sun photometers in Brazil, Atmos.Meas.Tech.Discuss.
Fig.1: Percentage of 333m profiles with dark noise above 200 in 2.5°x2.5°bins for all 2007 CALIOP nighttime observations.The red box delimits a region affected by the SAA but not directly affected by solar noise (top left: 90°W 0°S, bottom right: 0°W 35°S), The three boxes between 45°E and 135°E delimit regions unaffected by any recognized source of increased noise levels, in the South hemisphere (35°S to 15°S, green), the North hemisphere (15°N to 35°N, orange) and around the equator (15°S to 15°N, cyan).

Fig. 1 .
Fig. 1.Fraction (percents) of 333 m profiles with dark noise above 200 in 2.5 • × 2.5 • bins for all 2007 CALIOP nighttime observations.The red box delimits a region affected by the SAA but not directly affected by solar noise (top left: 90 • W 0 • S, bottom right: 0 • W 35 • S), The three boxes between 45 • E and 135 • E delimit regions unaffected by any recognized source of increased noise levels, in the South hemisphere (35 • S to 15 • S, green), the North hemisphere (15 • N to 35 • N, orange) and around the equator (15 • S to 15 • N, cyan).

Fig. 3 :
Fig. 3: Evolution with time of the fraction of noisy 333m profiles as a function of latitude between 70°W and 30°W (top) and longitude between 40°S and 10°S (bottom).White boxes in the bottom figure are due to missing data.

Fig. 3 .
Fig. 3. Evolution with time of the fraction of noisy 333 m profiles as a function of latitude between 70 • W and 30 • W (top) and longitude between 40 • S and 10 • S (bottom).White boxes in the bottom figure are due to missing data.

Fig. 4 :
Fig. 4 : (top) Same as Fig. 2, in areas supposedly unaffected by sources of additional noise (orange, green and cyan boxes in Fig. 1).(bottom) Same as top, but divided by the mean and detrended by subtracting the SAA relative deviation.The dark line shows the deviation of the geomagnetic field from the mean as documented by GOES (He field), delayed by 30 days.Note the y-axis (for GOES) is inverted.

Fig. 4 .
Fig. 4. (top) Same as Fig. 2, in areas supposedly unaffected by sources of additional noise (orange, green and cyan boxes in Fig. 1).(bottom) Same as top, but divided by the mean and detrended by subtracting the SAA relative deviation.The dark line shows the deviation of the geomagnetic field from the mean as documented by GOES (He field), delayed by 30 days.Note the y-axis (for GOES) is inverted.